2019 Vol. 44, No. 12
Display Method:
PDF 311KB
2019, 44(12): 3961-3983.
doi: 10.3799/dqkx.2019.982
Abstract:
The recycling of crustal material into the mantle by subduction is the first-order mechanism of Earth's interior. In order to decipher the crustal-mantle interaction in subduction zones,it is important to distinguish different types of metasomatism by subducting crust-derived fluids such as aqueous solutions and hydrous melts to the mantle wedge. For this purpose,various lines of petrological and geochemical evidence have been used to determine the physicochemical properties of subduction zone fluids at the slab-mantle interface in subduction channels. In doing so,it is critical to determine how crustal rocks underwent metamorphic dehydration and partial melting at mantle depths. After incorporation of subduction zone fluids into the mantle wedge,different compositions of mantle metasomatites were generated in the mantle wedge to result in mantle heterogeneities. As soon as these metasomatites underwent partial melting,mafic igneous rocks were produced with both petrological and geochemical signatures of the subducted crust and the mantle wedge. In this regard,such processes as metamorphism,metasomatism and magmatism in subduction zones are the keys to the recycling of crustal material at convergent plate boundaries. The mantle wedge is the key lithotectonic unit linking the subducting slab and the obducting plate and thus plays an important role in the material transport and energy exchange in the subduction system. The orogenic mantle wedge peridotite directly records different types of crustal metasomatism in subduction zones. Subduction zone magmatism is the manifestation for recycling of subducted oceanic and continental rocks. These rocks witness the processes of magmatic melts from the mantle wedge to crustal levels above subduction zones,providing the natural samples to decode indirectly the crustal recycling at convergent plate boundaries. Although there are many advances in the study of subduction zones with respect to the crust-mantle interaction,such three processes as metamorphism,metasomatism and magmatism in subduction zones are still the most important targets in the deep Earth science. Many key problems cannot be resolved if no sufficient attention is paid to an integrated study of these three aspects. Such problems include the physiochemical properties of subduction zone fluids,the mechanism and process of crust-mantle interaction,the material source and triggering mechanism of mantle-derived magmatism above subduction zones,and the impact of deep mantle process on the shallow crustal environments. The future research needs to focus on the key question of material recycling in subduction zones,and take the metamorphism,metasomatism and magmatism in subduction zones as a whole in the framework of Earth system science. This concerns the transport process and the resource and environmental effects of volatile components,and clarify the coupling mechanism of plate subduction and material recycling in deep Earth by intensive studies of paleo-subduction zones.
The recycling of crustal material into the mantle by subduction is the first-order mechanism of Earth's interior. In order to decipher the crustal-mantle interaction in subduction zones,it is important to distinguish different types of metasomatism by subducting crust-derived fluids such as aqueous solutions and hydrous melts to the mantle wedge. For this purpose,various lines of petrological and geochemical evidence have been used to determine the physicochemical properties of subduction zone fluids at the slab-mantle interface in subduction channels. In doing so,it is critical to determine how crustal rocks underwent metamorphic dehydration and partial melting at mantle depths. After incorporation of subduction zone fluids into the mantle wedge,different compositions of mantle metasomatites were generated in the mantle wedge to result in mantle heterogeneities. As soon as these metasomatites underwent partial melting,mafic igneous rocks were produced with both petrological and geochemical signatures of the subducted crust and the mantle wedge. In this regard,such processes as metamorphism,metasomatism and magmatism in subduction zones are the keys to the recycling of crustal material at convergent plate boundaries. The mantle wedge is the key lithotectonic unit linking the subducting slab and the obducting plate and thus plays an important role in the material transport and energy exchange in the subduction system. The orogenic mantle wedge peridotite directly records different types of crustal metasomatism in subduction zones. Subduction zone magmatism is the manifestation for recycling of subducted oceanic and continental rocks. These rocks witness the processes of magmatic melts from the mantle wedge to crustal levels above subduction zones,providing the natural samples to decode indirectly the crustal recycling at convergent plate boundaries. Although there are many advances in the study of subduction zones with respect to the crust-mantle interaction,such three processes as metamorphism,metasomatism and magmatism in subduction zones are still the most important targets in the deep Earth science. Many key problems cannot be resolved if no sufficient attention is paid to an integrated study of these three aspects. Such problems include the physiochemical properties of subduction zone fluids,the mechanism and process of crust-mantle interaction,the material source and triggering mechanism of mantle-derived magmatism above subduction zones,and the impact of deep mantle process on the shallow crustal environments. The future research needs to focus on the key question of material recycling in subduction zones,and take the metamorphism,metasomatism and magmatism in subduction zones as a whole in the framework of Earth system science. This concerns the transport process and the resource and environmental effects of volatile components,and clarify the coupling mechanism of plate subduction and material recycling in deep Earth by intensive studies of paleo-subduction zones.
2019, 44(12): 3984-3992.
doi: 10.3799/dqkx.2019.232
Abstract:
In order to study the effects of aqueous fluid activity on the subduction channel processes and continental collision dynamics,systematic numerical models were constructed with integrated thermodynamic and thermomechanical methods. The model results indicate that the subducted crustal materials may either exhume along the subduction channel to the surface near the suture zone,or extrude sub-vertically upward through the mantle wedge to the crust of the overriding plate. The contrasting modes are strongly dependent on the thermal structure of subduction zones. The temperature field controls the aqueous fluid and melt activities,which further regulates the weakening of overriding mantle wedge and finally dominates the material transportation in the subduction channel. Meanwhile,the lithospheric deformation during continental subduction and collision is also strongly dependent on the fluid-melt activity and the induced lithospheric weakening. The numerical models contribute significantly to the better understanding of subduction-zone fluid-melt activity and the geodynamic processes.
In order to study the effects of aqueous fluid activity on the subduction channel processes and continental collision dynamics,systematic numerical models were constructed with integrated thermodynamic and thermomechanical methods. The model results indicate that the subducted crustal materials may either exhume along the subduction channel to the surface near the suture zone,or extrude sub-vertically upward through the mantle wedge to the crust of the overriding plate. The contrasting modes are strongly dependent on the thermal structure of subduction zones. The temperature field controls the aqueous fluid and melt activities,which further regulates the weakening of overriding mantle wedge and finally dominates the material transportation in the subduction channel. Meanwhile,the lithospheric deformation during continental subduction and collision is also strongly dependent on the fluid-melt activity and the induced lithospheric weakening. The numerical models contribute significantly to the better understanding of subduction-zone fluid-melt activity and the geodynamic processes.
2019, 44(12): 3993-3997.
doi: 10.3799/dqkx.2019.253
Abstract:
The thermal structure of subduction zone is one of the most important factors controlling the evolution of subduction plate. Previous studies on the thermal structure of oceanic subduction zones have been carried out by establishing analytical and numerical models. It is found that the age and velocity of the subduction plate are the key factors affecting the thermal structure of the subduction zones. In order to further understand the thermal structure of the continental subduction zone,especially to understand the difference between the numerical model results and petrological results,we established two-dimensional numerical kinematic and geodynamic models of the continental subduction zone to study its thermal structure evolution. The model results show that if the subduction velocity and dip angle of the continental plate are the same as those of the oceanic plate,lower initial temperature causes the continental subduction zone to be colder than the oceanic subduction zone. However,when the initial temperature of the continental plate is high,the subduction velocity is super slow and the heat generation of radioactive elements in the continental crust is taken into account,the thermal structure of the continental subduction zone obtained by the model can explain the hot subduction temperature obtained from high-pressure and ultra-high-pressure metamorphic rocks. On the other hand,if there is dynamic decoupling between the subduction plate and the overlying plate,the hot subduction temperature can also be obtained.
The thermal structure of subduction zone is one of the most important factors controlling the evolution of subduction plate. Previous studies on the thermal structure of oceanic subduction zones have been carried out by establishing analytical and numerical models. It is found that the age and velocity of the subduction plate are the key factors affecting the thermal structure of the subduction zones. In order to further understand the thermal structure of the continental subduction zone,especially to understand the difference between the numerical model results and petrological results,we established two-dimensional numerical kinematic and geodynamic models of the continental subduction zone to study its thermal structure evolution. The model results show that if the subduction velocity and dip angle of the continental plate are the same as those of the oceanic plate,lower initial temperature causes the continental subduction zone to be colder than the oceanic subduction zone. However,when the initial temperature of the continental plate is high,the subduction velocity is super slow and the heat generation of radioactive elements in the continental crust is taken into account,the thermal structure of the continental subduction zone obtained by the model can explain the hot subduction temperature obtained from high-pressure and ultra-high-pressure metamorphic rocks. On the other hand,if there is dynamic decoupling between the subduction plate and the overlying plate,the hot subduction temperature can also be obtained.
2019, 44(12): 3998-4003.
doi: 10.3799/dqkx.2019.275
Abstract:
The discovery of kyanite+spinel exsolutions in former stishovite of pelitic gneisses from the South Altyn HP-UHP metamorphic belt suggests an ultra-deep subduction and exhumation of the South Altyn continental rocks to/from mantle depths in the stability field of stishovite (≥ 300 km). Whether such an ultra-deep subducted rock is specific sample or common case is still enigmas to geologists for more than a decade. To solve this problem,through a series of petrological studies,it is found for the first time quartz paramorphs after stishovite in omphacite and garnet of eclogite from Jianggalesayi,South Altyn UHP belt,and the peak metamorphic pressures of Yinggelisayi garnet pyroxenite with clinopyroxene exsolutions in garnet and Songshugou felsic gneiss with quartz exsolutions in garnet are re-estimated to be ≥ 9-10 GPa in the stability field of stishovite. These lines of new evidence,together with the previous discovery of kyanite+spinel exsolutions in the former stishovite of the pelitic gneiss from the South Altyn UHP belt,suggest that ultra-deep subduction of continental materials might be more common and diverse than previous thought. The minimum stability pressure of super-silicon garnet in SiO2-saturated rock system is estimated to be ≥ 9-10 GPa based on high temperature and high pressure experiments,which provides a new method to confirm the ultra-deep subduction of continental materials to mantle depth in stishovite stability field (~300 km).
The discovery of kyanite+spinel exsolutions in former stishovite of pelitic gneisses from the South Altyn HP-UHP metamorphic belt suggests an ultra-deep subduction and exhumation of the South Altyn continental rocks to/from mantle depths in the stability field of stishovite (≥ 300 km). Whether such an ultra-deep subducted rock is specific sample or common case is still enigmas to geologists for more than a decade. To solve this problem,through a series of petrological studies,it is found for the first time quartz paramorphs after stishovite in omphacite and garnet of eclogite from Jianggalesayi,South Altyn UHP belt,and the peak metamorphic pressures of Yinggelisayi garnet pyroxenite with clinopyroxene exsolutions in garnet and Songshugou felsic gneiss with quartz exsolutions in garnet are re-estimated to be ≥ 9-10 GPa in the stability field of stishovite. These lines of new evidence,together with the previous discovery of kyanite+spinel exsolutions in the former stishovite of the pelitic gneiss from the South Altyn UHP belt,suggest that ultra-deep subduction of continental materials might be more common and diverse than previous thought. The minimum stability pressure of super-silicon garnet in SiO2-saturated rock system is estimated to be ≥ 9-10 GPa based on high temperature and high pressure experiments,which provides a new method to confirm the ultra-deep subduction of continental materials to mantle depth in stishovite stability field (~300 km).
2019, 44(12): 4004-4008.
doi: 10.3799/dqkx.2019.233
Abstract:
The South Altyn orogen is a typical continental ultrahigh-pressure (UHP) belt which is claimed to host the deepest subducted crustal rocks. HP-UHT granulites occurred in the belt are of significance for understanding the ultradeep subduction and exhumation of continental crust. This paper presents the results of petrography,mineral chemistry,phase equilibria modelling and zircon U-Pb dating for felsic and mafic granulites from Bashiwake area. Mafic granulites have mainly recorded the exhumation process of ultradeep continental crust,with metamorphic evolution from HP eclogite facies to HP-UHT granulite facies,further to low-pressure (LP)-UHT granulite facies conditions and the subsequent isobaric cooling. In addition to the similar exhumation to that of mafic granulites,felsic granulites have also preserved the early prograde history from amphibolite facies to UHP eclogite facies conditions. Combing previous studies,the peak conditions of UHP eclogite facies is estimated to be > 7-9 GPa and > 1 000℃,covering the stishovite-stability field. Zircon age dating suggests that both felsic and mafic granulites have similar protolith ages of~900 Ma and metamorphic ages of~500 Ma. Metamorphic and dating studies indicate that the South Altyn continental crust was subducted to ultradeep depths of 200-300 km during the Early Paleozoic and exhumed to the bottom of a thicken crust where HP-UHT metamorphism occurred. Then the crust was uplifted rapidly to the shallow crustal level and experienced LP-UHT metamorphism followed by subsequent isobaric cooling.
The South Altyn orogen is a typical continental ultrahigh-pressure (UHP) belt which is claimed to host the deepest subducted crustal rocks. HP-UHT granulites occurred in the belt are of significance for understanding the ultradeep subduction and exhumation of continental crust. This paper presents the results of petrography,mineral chemistry,phase equilibria modelling and zircon U-Pb dating for felsic and mafic granulites from Bashiwake area. Mafic granulites have mainly recorded the exhumation process of ultradeep continental crust,with metamorphic evolution from HP eclogite facies to HP-UHT granulite facies,further to low-pressure (LP)-UHT granulite facies conditions and the subsequent isobaric cooling. In addition to the similar exhumation to that of mafic granulites,felsic granulites have also preserved the early prograde history from amphibolite facies to UHP eclogite facies conditions. Combing previous studies,the peak conditions of UHP eclogite facies is estimated to be > 7-9 GPa and > 1 000℃,covering the stishovite-stability field. Zircon age dating suggests that both felsic and mafic granulites have similar protolith ages of~900 Ma and metamorphic ages of~500 Ma. Metamorphic and dating studies indicate that the South Altyn continental crust was subducted to ultradeep depths of 200-300 km during the Early Paleozoic and exhumed to the bottom of a thicken crust where HP-UHT metamorphism occurred. Then the crust was uplifted rapidly to the shallow crustal level and experienced LP-UHT metamorphism followed by subsequent isobaric cooling.
2019, 44(12): 4009-4016.
doi: 10.3799/dqkx.2019.251
Abstract:
Lawsonite is a representative mineral in cold oceanic subduction zone. It contains high water and trace element contents,such as Sr,Pb,and REE,and thus its formation and breakdown greatly influence fluid activity in subduction channel,deep water and trace element recycling,metasomatism and partial melting of mantle wedge as well as arc volcanism. However,lawsonite-bearing eclogite is rarely exposed because lawsonite is very delicate that seldom survives exhumation. In this paper,we summarize the methods to identify former presence of lawsonite in eclogite and determine that clinozoisite eclogite and kyanite-bearing eclogite in the Yuka terrane,North Qaidam UHPM belt (NQUB) are retrograde products of lawsonite eclogite. This makes the NQUB being the second continental subduction-type orogenic belt which contains lawsonite eclogite. Phase equilibrium modelling obtains clockwise P-T paths for these two eclogites. The P-T path and peak metamorphic conditions of the kyanite-bearing eclogite are similar to the coesite-bearing phengite eclogite in the Yuka terrane,whereas the clinozoisite eclogite has slightly lower peak P-T conditions. Zircon dating yields metamorphic ages of 437 Ma and 436 Ma for the kyanite-bearing eclogite and clinozoisite eclogite,respectively,which are similar to UHP eclogite facies metamorphic ages in the NQUB,and a protolith age of 1 273 Ma for the kyanite-bearing eclogite. The similar P-T path and metamorphic age of the kyanite-bearing eclogite with coesite-bearing eclogite indicate that these two eclogites underwent continental deep subduction together. This results show that lawsonite eclogite is not peculiar to oceanic cold subduction zone. The main factors controlling the formation of lawsonite eclogite are bulk composition and metamorphic conditions. In the Yuka terrane,eclogite with high Mg# favors lawsonite-bearing peak assemblage.
Lawsonite is a representative mineral in cold oceanic subduction zone. It contains high water and trace element contents,such as Sr,Pb,and REE,and thus its formation and breakdown greatly influence fluid activity in subduction channel,deep water and trace element recycling,metasomatism and partial melting of mantle wedge as well as arc volcanism. However,lawsonite-bearing eclogite is rarely exposed because lawsonite is very delicate that seldom survives exhumation. In this paper,we summarize the methods to identify former presence of lawsonite in eclogite and determine that clinozoisite eclogite and kyanite-bearing eclogite in the Yuka terrane,North Qaidam UHPM belt (NQUB) are retrograde products of lawsonite eclogite. This makes the NQUB being the second continental subduction-type orogenic belt which contains lawsonite eclogite. Phase equilibrium modelling obtains clockwise P-T paths for these two eclogites. The P-T path and peak metamorphic conditions of the kyanite-bearing eclogite are similar to the coesite-bearing phengite eclogite in the Yuka terrane,whereas the clinozoisite eclogite has slightly lower peak P-T conditions. Zircon dating yields metamorphic ages of 437 Ma and 436 Ma for the kyanite-bearing eclogite and clinozoisite eclogite,respectively,which are similar to UHP eclogite facies metamorphic ages in the NQUB,and a protolith age of 1 273 Ma for the kyanite-bearing eclogite. The similar P-T path and metamorphic age of the kyanite-bearing eclogite with coesite-bearing eclogite indicate that these two eclogites underwent continental deep subduction together. This results show that lawsonite eclogite is not peculiar to oceanic cold subduction zone. The main factors controlling the formation of lawsonite eclogite are bulk composition and metamorphic conditions. In the Yuka terrane,eclogite with high Mg# favors lawsonite-bearing peak assemblage.
2019, 44(12): 4017-4027.
doi: 10.3799/dqkx.2019.256
Abstract:
The Early Paleozoic tectonic framework and evolution history of the North Qinling orogenic belt (NQOB) is in great dispute and has drawn wide concern. In this contribution,we systematically summarized our recent progresses on Early Paleozoic HP-UHP metamorphism in the NQOB and constrained the Early Paleozoic tectonic evolution of the NQOB from the perspectives of metamorphism. The discovery of coesite in the Danfeng amphibolite provides a conclusive evidence for the UHP metamorphism in the NQOB. The amphibolites,which are widely distributed in Qinling Complex in East Qinling,commonly experienced HP-UHP metamorphism and are products of continental (deep) subduction. HP-UHP eclogites and host gneisses have clockwise P-T-t paths and similar peak metamorphic ages of 500-490 Ma,and experienced two stages retrogression and anataxis at 470-450 Ma and 420-400 Ma,respectively. The times of the two stage retrogression and anataxis are coincident with the magmatism in the NQOB,which occurred at 460-450 Ma and~420 Ma,suggesting that the Early Paleozoic magmatism in the NQOB were formed under the tectonic setting of breaking-off of deeply subducted continental slab and post-orogenic extension-thinning,respectively. The HP-UHP rocks have protolith ages of ca. 800 Ma,and geochemical characteristics similar to Mid-Late Neoproterozoic magmatic rocks from South Qinling,indicating that continental materials from South Qinling were dragged down to mantle depth and experienced UHP metamorphism by the north dipping Shangdan oceanic lithosphere,and the Shangdan Ocean was already closed at 500 Ma. The Qinling Group is a tectonic complex rather than a uniform stratigraphic unit or a microcontinent and only part of it underwent continental deep subduction. The Early Paleozoic orogenisis in North Qinling was ended and the HP-UHP rocks were exhumated up to the surface in Mid-Devonian,and became the main provenance of sedimentary rocks in Liuling group in South Qinling. The Liuling basin is a post-orogenic extensional basin,rather than a fore-arc basin.
The Early Paleozoic tectonic framework and evolution history of the North Qinling orogenic belt (NQOB) is in great dispute and has drawn wide concern. In this contribution,we systematically summarized our recent progresses on Early Paleozoic HP-UHP metamorphism in the NQOB and constrained the Early Paleozoic tectonic evolution of the NQOB from the perspectives of metamorphism. The discovery of coesite in the Danfeng amphibolite provides a conclusive evidence for the UHP metamorphism in the NQOB. The amphibolites,which are widely distributed in Qinling Complex in East Qinling,commonly experienced HP-UHP metamorphism and are products of continental (deep) subduction. HP-UHP eclogites and host gneisses have clockwise P-T-t paths and similar peak metamorphic ages of 500-490 Ma,and experienced two stages retrogression and anataxis at 470-450 Ma and 420-400 Ma,respectively. The times of the two stage retrogression and anataxis are coincident with the magmatism in the NQOB,which occurred at 460-450 Ma and~420 Ma,suggesting that the Early Paleozoic magmatism in the NQOB were formed under the tectonic setting of breaking-off of deeply subducted continental slab and post-orogenic extension-thinning,respectively. The HP-UHP rocks have protolith ages of ca. 800 Ma,and geochemical characteristics similar to Mid-Late Neoproterozoic magmatic rocks from South Qinling,indicating that continental materials from South Qinling were dragged down to mantle depth and experienced UHP metamorphism by the north dipping Shangdan oceanic lithosphere,and the Shangdan Ocean was already closed at 500 Ma. The Qinling Group is a tectonic complex rather than a uniform stratigraphic unit or a microcontinent and only part of it underwent continental deep subduction. The Early Paleozoic orogenisis in North Qinling was ended and the HP-UHP rocks were exhumated up to the surface in Mid-Devonian,and became the main provenance of sedimentary rocks in Liuling group in South Qinling. The Liuling basin is a post-orogenic extensional basin,rather than a fore-arc basin.
2019, 44(12): 4028-4033.
doi: 10.3799/dqkx.2019.231
Abstract:
Insights into the occurrences and preservation mechanisms of coesite bear important implications for the formation and evolution of ultrahigh-pressure (UHP) metamorphic rocks. Early researches reveal that coesite usually appears as inclusions in strong host minerals in UHP rocks. Thus far,interstitial coesite has only been found in the UHP two-mineral eclogite from Yangkou,Sulu orogen. Two major mechanisms have been proposed for the preservation of coesite:the "pressure-vessel" model involving "tectonic overpressure" and dry metamorphic environment. Interstitial coesite and abundant coesite inclusions in dolomite recently discovered in a UHP metasedimentary rock from the Ganjialing area in Dabieshan highlight the role of dry metamorphic environment and undermine the importance of "pressure-vessel" model in preserving coesite.
Insights into the occurrences and preservation mechanisms of coesite bear important implications for the formation and evolution of ultrahigh-pressure (UHP) metamorphic rocks. Early researches reveal that coesite usually appears as inclusions in strong host minerals in UHP rocks. Thus far,interstitial coesite has only been found in the UHP two-mineral eclogite from Yangkou,Sulu orogen. Two major mechanisms have been proposed for the preservation of coesite:the "pressure-vessel" model involving "tectonic overpressure" and dry metamorphic environment. Interstitial coesite and abundant coesite inclusions in dolomite recently discovered in a UHP metasedimentary rock from the Ganjialing area in Dabieshan highlight the role of dry metamorphic environment and undermine the importance of "pressure-vessel" model in preserving coesite.
2019, 44(12): 4034-4041.
doi: 10.3799/dqkx.2019.268
Abstract:
In order to investigate the applicability of Ti-in-zircon thermometry to low temperature and high pressure eclogites,we used the calibration of other scholars to estimate the metamorphic temperatures for four typical low-temperature and high (ultrahigh)-pressure eclogites from North Qilian and western Tianshan,China. Compiled different HP/UHP eclogite samples from the literature,the Ti-in-zircon temperatures are generally higher than the estimations by other thermometers (up to 58%),especially for low-temperature metamorphic zircon. Although temperature exerts the dominant control on Ti content in zircon,other factors (e.g. pressure,TiO2 and SiO2 activity,lattice defect,other trace element substitutions,disequilibrium zircon growth and metamorphic fluids) also influence the calculated temperature results. This study proposes the metamorphic fluids may have contributed to the overestimated Ti-in-zircon temperatures.
In order to investigate the applicability of Ti-in-zircon thermometry to low temperature and high pressure eclogites,we used the calibration of other scholars to estimate the metamorphic temperatures for four typical low-temperature and high (ultrahigh)-pressure eclogites from North Qilian and western Tianshan,China. Compiled different HP/UHP eclogite samples from the literature,the Ti-in-zircon temperatures are generally higher than the estimations by other thermometers (up to 58%),especially for low-temperature metamorphic zircon. Although temperature exerts the dominant control on Ti content in zircon,other factors (e.g. pressure,TiO2 and SiO2 activity,lattice defect,other trace element substitutions,disequilibrium zircon growth and metamorphic fluids) also influence the calculated temperature results. This study proposes the metamorphic fluids may have contributed to the overestimated Ti-in-zircon temperatures.
2019, 44(12): 4042-4049.
doi: 10.3799/dqkx.2019.235
Abstract:
Garnet is one of the most important mineral in high pressure (HP) to ultrahigh pressure (UHP) metamorphic rocks. It is an ideal phase to constrain the P-T-t conditions of metamorphic and anatectic processes during continental subduction zone metamorphism. Garnets from subduction zone metamorphic rocks can be classified into metamorphic,magmatic and peritecitc garnets based on their typical features. Magmatic garnet crystallizes from magmatic melts,shows almandine-spessartine in compositions and contains crystal inclusions such as quartz,feldspar and apatite. Metamorphic garnet forms through metamorphic reactions at subsolidus conditions,shows decreasing spessartine from core to rim,and contains crystal inclusions mainly composed of metamorphic reactants. Peritectic garnet forms through peritectic reactions at supersolidus conditions,and contains crystal inclusions consisting of not only minerals crystallized from peritectic melts but also residual minerals from peritectic reactants. The identification of peritecitc garnet in UHP metamorphic rocks provides unique evidence for partial melting of the deeply subducted continental crust,which is an important progress in crustal anatexis of collisional orogens.
Garnet is one of the most important mineral in high pressure (HP) to ultrahigh pressure (UHP) metamorphic rocks. It is an ideal phase to constrain the P-T-t conditions of metamorphic and anatectic processes during continental subduction zone metamorphism. Garnets from subduction zone metamorphic rocks can be classified into metamorphic,magmatic and peritecitc garnets based on their typical features. Magmatic garnet crystallizes from magmatic melts,shows almandine-spessartine in compositions and contains crystal inclusions such as quartz,feldspar and apatite. Metamorphic garnet forms through metamorphic reactions at subsolidus conditions,shows decreasing spessartine from core to rim,and contains crystal inclusions mainly composed of metamorphic reactants. Peritectic garnet forms through peritectic reactions at supersolidus conditions,and contains crystal inclusions consisting of not only minerals crystallized from peritectic melts but also residual minerals from peritectic reactants. The identification of peritecitc garnet in UHP metamorphic rocks provides unique evidence for partial melting of the deeply subducted continental crust,which is an important progress in crustal anatexis of collisional orogens.
2019, 44(12): 4050-4056.
doi: 10.3799/dqkx.2019.234
Abstract:
The origin of the light Fe and heavy Mg isotope enrichments in arc lavas remains unclear because of the lack of constraints on the Fe-Mg isotope compositions of altered oceanic crust (AOC) and metamorphic fluids in subduction zones. Carbonate-barren AOC has Mg isotope compositions similar to those of fresh mid-ocean ridge basalts, suggesting that low-to-high temperature alteration of oceanic crust by seawater and hydrothermal fluids results in limited Mg isotope fractionation. Fe-Mg isotope measurements show that the early omphacite-epidote veins have higher δ56Fe and δ26Mg compared to the host eclogites and that the δ56Fe and δ26Mg gradually decrease from the early omphacite-epidote through epidote-quartz to the late kyanite-epidote-quartz veins. These results indicate significant Fe-Mg isotope fractionation during fluid-rock interaction and fluid evolution due to the dissolution-precipitation processes of minerals in subduction zones. Compared to mid-ocean ridge basalts, the similar or higher δ56Fe and δ26Mg of AOC and metamorphic veins suggest that AOC-derived fluids are probably enriched in heavy Fe-Mg isotopes. Thus, contribution from AOC-derived fluids is unlikely to explain the light Fe and heavy Mg isotope compositions of arc lavas. We propose that the Fe-Mg isotope anomaly of arc lavas may result from a combination of prior melt depletion and addition of serpentinite-derived 54Fe-26Mg-rich fluids into the overlying mantle wedge.
The origin of the light Fe and heavy Mg isotope enrichments in arc lavas remains unclear because of the lack of constraints on the Fe-Mg isotope compositions of altered oceanic crust (AOC) and metamorphic fluids in subduction zones. Carbonate-barren AOC has Mg isotope compositions similar to those of fresh mid-ocean ridge basalts, suggesting that low-to-high temperature alteration of oceanic crust by seawater and hydrothermal fluids results in limited Mg isotope fractionation. Fe-Mg isotope measurements show that the early omphacite-epidote veins have higher δ56Fe and δ26Mg compared to the host eclogites and that the δ56Fe and δ26Mg gradually decrease from the early omphacite-epidote through epidote-quartz to the late kyanite-epidote-quartz veins. These results indicate significant Fe-Mg isotope fractionation during fluid-rock interaction and fluid evolution due to the dissolution-precipitation processes of minerals in subduction zones. Compared to mid-ocean ridge basalts, the similar or higher δ56Fe and δ26Mg of AOC and metamorphic veins suggest that AOC-derived fluids are probably enriched in heavy Fe-Mg isotopes. Thus, contribution from AOC-derived fluids is unlikely to explain the light Fe and heavy Mg isotope compositions of arc lavas. We propose that the Fe-Mg isotope anomaly of arc lavas may result from a combination of prior melt depletion and addition of serpentinite-derived 54Fe-26Mg-rich fluids into the overlying mantle wedge.
2019, 44(12): 4057-4063.
doi: 10.3799/dqkx.2019.241
Abstract:
It is well known that the mantle wedge metasomatized by fluids derived from the subducting slab serves as the source of arc magmas. However, it is uncertain whether the subducting crust would be metasomatized by fluids released from mantle wedge metasomatites. Now this is firstly illustrated by a detailed study of petrology and geochemistry for whiteschist from the Dora-Maira Massif in the Western Alps.Based on the whole-rock geochemistry and zirconology for the whiteschist and its country rock, it is concluded that the protolith of the whiteschist is a kind of S-type granites, providing a resolution to the long-standing controversy on the protolith nature of the whiteschist in this region. The δ18O values of metamorphic zircon in the whiteschist are significantly lower than those of magmatic zircon, suggesting that the protolith was metasomatized by low δ18O fluids before the peak UHP metamorphism. The whiteschist shows the highest δ26Mg values up to 0.75‰ among high-T silicate rocks, which are much higher than those of the metagranite, suggesting that the metasomatic fluids have heavy Mg isotope compositions.Based on the Mg isotope systematics of major terrestrial silicate reservoirs, it is proposed that such fluids would probably originate from talc-rich serpentinites that were generated at forearc depths by hydration of the mantle wedge peridotite during prograde subduction of the Neotethyan oceanic slab. The mantle wedge serpentinites were then metastably carried by the subducting continental crust to subarc depths, where they underwent dehydration for reversed metasomatism of the deeply subducting continental crust at the slab-mantle interface in the continental subduction channel. The results provide not only the new idea for tracing fluid sources in the continental subduction channel, but also the first example that the deeply subducting continental crust underwent the reversed metasomatism by the fluids derived from dehydration of the mantle wedge metasomatite. This reversed metasomatism would greatly modify the geochemical composition of the deeply subducted continental crust, which has bearing on the origin of arc magmas rich in heavy Mg isotopes.
It is well known that the mantle wedge metasomatized by fluids derived from the subducting slab serves as the source of arc magmas. However, it is uncertain whether the subducting crust would be metasomatized by fluids released from mantle wedge metasomatites. Now this is firstly illustrated by a detailed study of petrology and geochemistry for whiteschist from the Dora-Maira Massif in the Western Alps.Based on the whole-rock geochemistry and zirconology for the whiteschist and its country rock, it is concluded that the protolith of the whiteschist is a kind of S-type granites, providing a resolution to the long-standing controversy on the protolith nature of the whiteschist in this region. The δ18O values of metamorphic zircon in the whiteschist are significantly lower than those of magmatic zircon, suggesting that the protolith was metasomatized by low δ18O fluids before the peak UHP metamorphism. The whiteschist shows the highest δ26Mg values up to 0.75‰ among high-T silicate rocks, which are much higher than those of the metagranite, suggesting that the metasomatic fluids have heavy Mg isotope compositions.Based on the Mg isotope systematics of major terrestrial silicate reservoirs, it is proposed that such fluids would probably originate from talc-rich serpentinites that were generated at forearc depths by hydration of the mantle wedge peridotite during prograde subduction of the Neotethyan oceanic slab. The mantle wedge serpentinites were then metastably carried by the subducting continental crust to subarc depths, where they underwent dehydration for reversed metasomatism of the deeply subducting continental crust at the slab-mantle interface in the continental subduction channel. The results provide not only the new idea for tracing fluid sources in the continental subduction channel, but also the first example that the deeply subducting continental crust underwent the reversed metasomatism by the fluids derived from dehydration of the mantle wedge metasomatite. This reversed metasomatism would greatly modify the geochemical composition of the deeply subducted continental crust, which has bearing on the origin of arc magmas rich in heavy Mg isotopes.
2019, 44(12): 4064-4071.
doi: 10.3799/dqkx.2019.242
Abstract:
Jadeite quartzite is a rare and fluid-related metamorphic rock, usually occurring as tectonic blocks of high-pressure (HP) to ultrahigh-pressure (UHP) metamorphic zones in continental and oceanic subduction zone.A combined study of whole-rock geochemistry, Mg and O isotopes, zircon U-Pb ages and trace elements was carried out for coesite-bearing jadeite quartzites from the Dabie orogen. The results indicate that the Middle Paleoproterozoic protolith of jadeite quartzites was weathered to a kind of sedimentary rocks in a passive continental margin and then underwent significant metasomatism by metamorphic fluids with high δ26Mg values during the continental subduction in the Triassic. The metamorphic fluids were produced by the breakdown of biotite in the metasedimentary rocks. The regional occurrence of jadeite quartzites over an exposure area of about 50 km2 indicates that the metamorphic fluids would have flowed on a large scale in the continental subduction channel. Such widespread fluid flow is evident in the continental subduction zone, with the metamorphic fluids would have flowed on a large scale at the slab-mantle interface in the continental subduction channel.
Jadeite quartzite is a rare and fluid-related metamorphic rock, usually occurring as tectonic blocks of high-pressure (HP) to ultrahigh-pressure (UHP) metamorphic zones in continental and oceanic subduction zone.A combined study of whole-rock geochemistry, Mg and O isotopes, zircon U-Pb ages and trace elements was carried out for coesite-bearing jadeite quartzites from the Dabie orogen. The results indicate that the Middle Paleoproterozoic protolith of jadeite quartzites was weathered to a kind of sedimentary rocks in a passive continental margin and then underwent significant metasomatism by metamorphic fluids with high δ26Mg values during the continental subduction in the Triassic. The metamorphic fluids were produced by the breakdown of biotite in the metasedimentary rocks. The regional occurrence of jadeite quartzites over an exposure area of about 50 km2 indicates that the metamorphic fluids would have flowed on a large scale in the continental subduction channel. Such widespread fluid flow is evident in the continental subduction zone, with the metamorphic fluids would have flowed on a large scale at the slab-mantle interface in the continental subduction channel.
2019, 44(12): 4072-4080.
doi: 10.3799/dqkx.2019.238
Abstract:
The system of ultrahigh-pressure (UHP) metamorphic rocks and veins is a natural laboratory to understand the nature and behavior of metamorphic fluids in subduction zones. This paper presents a review of the studies on three suits of eclogite (amphibolite)-vein system from the Dabie UHP terrane in order to discuss the dissolution and crystallization processes of subduction-zone metamorphic fluids, variation in fluid oxygen fugacity (fO2), and fluid-assisted boron (B) transfer. The study of UHP eclogites and enclosed multiple veins indicates that UHP fluid transferred materials by the dissolution of various components. This solute-rich fluid then experienced a three-stage crystallization process, which produced omphacite-epidote vein, epidote-quartz vein, and kyanite-epidote-quartz vein. La and Cr contents and δEu values of vein epidote are critical geochemical indicators for assessing the precipitating sequence of veins. The investigation on an eclogite-amphibolite-vein system indicates that low-pressure fluids have much higher fO2 conditions than high-pressure (HP) and UHP fluids in continental subduction zones. Such high fO2 conditions also lead to the growth of some unusual minerals (e.g., low pressure retrograde rutile). The investigation on tourmaline-bearing eclogite-vein system indicates that metacarbonate is an important reservoir of isotopically heavy boron (B) in subducted continental crust, and the release of B of metacarbonate at convergent boundary exerts a significant influence on deep B cycling. The studies above provide important insights into the fluid evolution and material cycling in subduction zones.
The system of ultrahigh-pressure (UHP) metamorphic rocks and veins is a natural laboratory to understand the nature and behavior of metamorphic fluids in subduction zones. This paper presents a review of the studies on three suits of eclogite (amphibolite)-vein system from the Dabie UHP terrane in order to discuss the dissolution and crystallization processes of subduction-zone metamorphic fluids, variation in fluid oxygen fugacity (fO2), and fluid-assisted boron (B) transfer. The study of UHP eclogites and enclosed multiple veins indicates that UHP fluid transferred materials by the dissolution of various components. This solute-rich fluid then experienced a three-stage crystallization process, which produced omphacite-epidote vein, epidote-quartz vein, and kyanite-epidote-quartz vein. La and Cr contents and δEu values of vein epidote are critical geochemical indicators for assessing the precipitating sequence of veins. The investigation on an eclogite-amphibolite-vein system indicates that low-pressure fluids have much higher fO2 conditions than high-pressure (HP) and UHP fluids in continental subduction zones. Such high fO2 conditions also lead to the growth of some unusual minerals (e.g., low pressure retrograde rutile). The investigation on tourmaline-bearing eclogite-vein system indicates that metacarbonate is an important reservoir of isotopically heavy boron (B) in subducted continental crust, and the release of B of metacarbonate at convergent boundary exerts a significant influence on deep B cycling. The studies above provide important insights into the fluid evolution and material cycling in subduction zones.
2019, 44(12): 4081-4085.
doi: 10.3799/dqkx.2019.255
Abstract:
In order to investigate mantle heterogeneities induced by subducted crustal plate, many studies have been focused on geochemical characteristics of stable metal isotopic systematics in subduction zone in recent years. Limited Mg isotopic fractionation occurred during subduction and exhumation, thus lighter Mg isotopic composition of continental basalts might be caused by carbonatite metasomatism. Instead of inherited Li isotopes of amphibolite, eclogite has lighter Li isotopies, which are attributed to kinetic diffusion, dehydration, or rehydration. Isotopes of Fe and Cr have insignificant fractionation during formation of eclogite, however, Fe and Cr isotopic composition of serpentinite are related to oxidation index, suggesting dehydration or alteration of fluids during serpentinization could shift Fe and Cr isotopic composition of serpentinite.
In order to investigate mantle heterogeneities induced by subducted crustal plate, many studies have been focused on geochemical characteristics of stable metal isotopic systematics in subduction zone in recent years. Limited Mg isotopic fractionation occurred during subduction and exhumation, thus lighter Mg isotopic composition of continental basalts might be caused by carbonatite metasomatism. Instead of inherited Li isotopes of amphibolite, eclogite has lighter Li isotopies, which are attributed to kinetic diffusion, dehydration, or rehydration. Isotopes of Fe and Cr have insignificant fractionation during formation of eclogite, however, Fe and Cr isotopic composition of serpentinite are related to oxidation index, suggesting dehydration or alteration of fluids during serpentinization could shift Fe and Cr isotopic composition of serpentinite.
2019, 44(12): 4086-4094.
doi: 10.3799/dqkx.2019.262
Abstract:
Metasomatism of the mantle wedge by crust-derived fluids is a crucial mechanism responsible for subduction zone magmatism. However, how crust-mantle interactions proceed in continental subduction zones is poorly understood. Orogenic peridotite is a direct, ideal sample to resolve this issue. Through combined studies of mineralogy, petrology and geochemistry for on Dabie-Sulu orogenic peridotites, it is found that the Ni/Co ratio of olivine can successfully discriminate between mantle and crustal peridotites, and that mantle-derived peridotites originated from the subcontinental lithospheric mantle wedge beneath the North China craton. The Lijiatun dunites (Sulu) derived from such lithospheric mantle wedge were likely modified by carbonatitic melts prior to their incorporation into the subduction channel. The Maowu (Dabie) and Jiangzhuang (Sulu) peridotites and their metasomatic veins record slab-mantle interaction processes at depths of~170-200 km. Deeply subducted continental crust would release Si-Al-rich melts to heterogeneously modify the lower margin of the mantle wedge, which could result in forming various garnet-and pyroxene-rich metasomatites and in generating significant Os isotope fractionation. Such processes can not only modify lithological and geochemical compositions of the mantle wedge but also affect crust-derived melt compositions. Recycled heterogeneous peridotites and metasomatites could contribute to the mantle sources of various subduction-related mantle magmas. Therefore, deep continental subduction is a crucial mechanism responsible for the mantle heterogeneity.
Metasomatism of the mantle wedge by crust-derived fluids is a crucial mechanism responsible for subduction zone magmatism. However, how crust-mantle interactions proceed in continental subduction zones is poorly understood. Orogenic peridotite is a direct, ideal sample to resolve this issue. Through combined studies of mineralogy, petrology and geochemistry for on Dabie-Sulu orogenic peridotites, it is found that the Ni/Co ratio of olivine can successfully discriminate between mantle and crustal peridotites, and that mantle-derived peridotites originated from the subcontinental lithospheric mantle wedge beneath the North China craton. The Lijiatun dunites (Sulu) derived from such lithospheric mantle wedge were likely modified by carbonatitic melts prior to their incorporation into the subduction channel. The Maowu (Dabie) and Jiangzhuang (Sulu) peridotites and their metasomatic veins record slab-mantle interaction processes at depths of~170-200 km. Deeply subducted continental crust would release Si-Al-rich melts to heterogeneously modify the lower margin of the mantle wedge, which could result in forming various garnet-and pyroxene-rich metasomatites and in generating significant Os isotope fractionation. Such processes can not only modify lithological and geochemical compositions of the mantle wedge but also affect crust-derived melt compositions. Recycled heterogeneous peridotites and metasomatites could contribute to the mantle sources of various subduction-related mantle magmas. Therefore, deep continental subduction is a crucial mechanism responsible for the mantle heterogeneity.
2019, 44(12): 4095-4101.
doi: 10.3799/dqkx.2019.254
Abstract:
Subduction zones are the major sites for mass exchange between crust and mantle. Although a great deal of studies have devoted to the crust-mantle interaction in oceanic subduction zones, it is still not clear what are physicochemical processes and mechanisms for the crust-mantle interactions in subduction zones. Orogenic peridotites are widely exposed in collisional subduction zones and they were originally located in the mantle wedge above the subducting continental slab, providing us excellent samples to resolve this issue. Through a systematic study of petrology and geochemistry for orogenic peridotites from the Dabie-Sulu and North Qaidam orogens, it is found that crustal metasomatism results in the occurrence of both newly grown zircon and relict zircon. The two types of zircons provide important constraints on not only the timing of crustal metasomatism but also the origin, property and composition of metasomatic agents in the mantle wedge. The mantle wedge peridotites in the continental subduction zones underwent multiple episodes of crustal metasomatism by different properties of fluids derived from the deeply subducted continental crust during continental collision. They were also metasomatized by fluids derived from precedingly subducted oceanic crust. They reacted with subducted continental crust-derived melts to generate garnet pyroxenites, which have high water contents and thus can serve as the mantle source of mafic igneous rocks with high water contents.
Subduction zones are the major sites for mass exchange between crust and mantle. Although a great deal of studies have devoted to the crust-mantle interaction in oceanic subduction zones, it is still not clear what are physicochemical processes and mechanisms for the crust-mantle interactions in subduction zones. Orogenic peridotites are widely exposed in collisional subduction zones and they were originally located in the mantle wedge above the subducting continental slab, providing us excellent samples to resolve this issue. Through a systematic study of petrology and geochemistry for orogenic peridotites from the Dabie-Sulu and North Qaidam orogens, it is found that crustal metasomatism results in the occurrence of both newly grown zircon and relict zircon. The two types of zircons provide important constraints on not only the timing of crustal metasomatism but also the origin, property and composition of metasomatic agents in the mantle wedge. The mantle wedge peridotites in the continental subduction zones underwent multiple episodes of crustal metasomatism by different properties of fluids derived from the deeply subducted continental crust during continental collision. They were also metasomatized by fluids derived from precedingly subducted oceanic crust. They reacted with subducted continental crust-derived melts to generate garnet pyroxenites, which have high water contents and thus can serve as the mantle source of mafic igneous rocks with high water contents.
2019, 44(12): 4102-4111.
doi: 10.3799/dqkx.2019.286
Abstract:
At different depths, the subducted slabs could release melts/fluids with distinct chemical components from different reservoirs into the subduction channel. Such melts/fluids may then affect the geochemical compositions of the overlying mantle wedge and the island arc magmas. However, how to identify the sources of melts/fluids at different depths in the subduction channels remains a challenging issue in studies of the subduction zones. Based on the Mg isotope studies on the ultramafic rocks derived from the mantle wedge at different depths, Mg isotopes are proposed to be a useful tool to distinguish the sources of melts/fluids in the subduction channel.A set of metamorphic ultramafic rocks from the Franciscan complex in California that have undergone multiple stages of metasomatism at the shallow depth (< ~60 km) in the subduction channel was studied. During the dehydration reactions that produced talc from serpentine, light Mg isotopes were preferentially released into fluids whereas heavy Mg isotopes were retained in talc. The tremolite-dominated samples that metamorphosed further by slab-derived fluids have high CaO contents and light Mg isotopic compositions, implying that a certain amount of Mg-bearing calcites could be dissolved into fluids and participated in metamorphism of mantle wedge peridotites. The petrographic and elemental geochemical studies of the ultramafic rocks from the Maowu complex of the Dabie orogenic belt, which were derived from the deep mantle wedge (~160 km), confirmed the metasomatism genesis. Combined with multiphase inclusions, element geochemistry, and peak P-T condition, we speculate that the metasomatic fluid was supercritical. Zircon geochronology studies revealed that the metasomatism mainly occurred during the oceanic crust subduction at Paleozoic (454±58 Ma), and the ultra-high pressure metamorphism occurred during the continental crust subduction at Triassic (232.8±7.9 Ma). The large amount of carbonate mineral inclusions and heavy oxygen isotope characteristics of the Paleozoic zircon indicate the incorporations of carbonate components during the Paleozoic metasomatism. The lighter Mg isotope composition of whole rocks and individual minerals than that of the mantle and the Dabie eclogite, indicates that the carbonate components should be sedimentary Mg-rich carbonates, which was dissolved in the supercritical fluid.Due to that the sedimentary carbonate has a unique and significantly enriched light Mg isotope feature, the metasomatism will cause heterogeneous Mg isotopic compositions of the mantle wedge, which may account for the observed Mg isotope characteristics of the arc lavas. Magnesium isotopes thus could be a potentially useful tracer of crust-mantle interactions at subduction zones.
At different depths, the subducted slabs could release melts/fluids with distinct chemical components from different reservoirs into the subduction channel. Such melts/fluids may then affect the geochemical compositions of the overlying mantle wedge and the island arc magmas. However, how to identify the sources of melts/fluids at different depths in the subduction channels remains a challenging issue in studies of the subduction zones. Based on the Mg isotope studies on the ultramafic rocks derived from the mantle wedge at different depths, Mg isotopes are proposed to be a useful tool to distinguish the sources of melts/fluids in the subduction channel.A set of metamorphic ultramafic rocks from the Franciscan complex in California that have undergone multiple stages of metasomatism at the shallow depth (< ~60 km) in the subduction channel was studied. During the dehydration reactions that produced talc from serpentine, light Mg isotopes were preferentially released into fluids whereas heavy Mg isotopes were retained in talc. The tremolite-dominated samples that metamorphosed further by slab-derived fluids have high CaO contents and light Mg isotopic compositions, implying that a certain amount of Mg-bearing calcites could be dissolved into fluids and participated in metamorphism of mantle wedge peridotites. The petrographic and elemental geochemical studies of the ultramafic rocks from the Maowu complex of the Dabie orogenic belt, which were derived from the deep mantle wedge (~160 km), confirmed the metasomatism genesis. Combined with multiphase inclusions, element geochemistry, and peak P-T condition, we speculate that the metasomatic fluid was supercritical. Zircon geochronology studies revealed that the metasomatism mainly occurred during the oceanic crust subduction at Paleozoic (454±58 Ma), and the ultra-high pressure metamorphism occurred during the continental crust subduction at Triassic (232.8±7.9 Ma). The large amount of carbonate mineral inclusions and heavy oxygen isotope characteristics of the Paleozoic zircon indicate the incorporations of carbonate components during the Paleozoic metasomatism. The lighter Mg isotope composition of whole rocks and individual minerals than that of the mantle and the Dabie eclogite, indicates that the carbonate components should be sedimentary Mg-rich carbonates, which was dissolved in the supercritical fluid.Due to that the sedimentary carbonate has a unique and significantly enriched light Mg isotope feature, the metasomatism will cause heterogeneous Mg isotopic compositions of the mantle wedge, which may account for the observed Mg isotope characteristics of the arc lavas. Magnesium isotopes thus could be a potentially useful tracer of crust-mantle interactions at subduction zones.
2019, 44(12): 4112-4118.
doi: 10.3799/dqkx.2019.230
Abstract:
A series of experiments reacting peridotite with melts derived from partial melting of eclogites was accomplished in order to better understand factors that control crust-mantle interaction in subduction zones. The experiments were conducted using the reaction couple method at 0.8-3.0 GPa and 1 200-1 425℃. The experimental results show that kinetics and consequence of melt-rock reaction are controlled by factors including major element composition and H2O in reacting melt, temperature, pressure, and physical state of reacting peridotite. Orthopyroxene enrichment in mantle beneath subduction zones is a result of interaction between melt derived from recycling continental crust and overlaying mantle. Formation of orthopyroxenite veins in mantle rocks is related to hydrous mantle metasomatism. Garnet-bearing and garnet-rich lithologies in mantle rocks were likely formed by melt-rock reaction in the low-temperature regime.
A series of experiments reacting peridotite with melts derived from partial melting of eclogites was accomplished in order to better understand factors that control crust-mantle interaction in subduction zones. The experiments were conducted using the reaction couple method at 0.8-3.0 GPa and 1 200-1 425℃. The experimental results show that kinetics and consequence of melt-rock reaction are controlled by factors including major element composition and H2O in reacting melt, temperature, pressure, and physical state of reacting peridotite. Orthopyroxene enrichment in mantle beneath subduction zones is a result of interaction between melt derived from recycling continental crust and overlaying mantle. Formation of orthopyroxenite veins in mantle rocks is related to hydrous mantle metasomatism. Garnet-bearing and garnet-rich lithologies in mantle rocks were likely formed by melt-rock reaction in the low-temperature regime.
2019, 44(12): 4119-4127.
doi: 10.3799/dqkx.2019.244
Abstract:
Crustal material subducted to mantle depths inevitably interacted with the mantle wedge at the slab-mantle interface. This may generate a variety of ultramafic metasomatites that served as the mantle source of mafic igneous rocks in collisional orogens. Therefore, mafic igneous rocks in collisional orogens are the important target to study the recycling of subdcuted crustal materials and its associated crust-mantle interaction. In order to decipher the mechanism and processes of the recycling of subducted continental crustal materials, a combined study of element and isotope geochemistry was performed for post-collisional andesitic volcanics from the Dabie orogen, China. SIMS zircon U-Pb ages for these volcanic rocks are 124±3 to 130±2 Ma, indicating that they formed at Early Cretaceous. In addition, the relict zircons have Middle Neoproterozoic and Triassic U-Pb ages, respectively, corresponding to the ages of protolith formation and ultrahigh-pressure metamorphism (UHP) for UHP metaigenous rocks in the Dabie-Sulu orogenic belt. They have island-arc basalts (IAB)-like trace-element patterns, enriched Sr-Nd-Hf isotope compositions, and variable zircon δ18O values mostly different from the normal mantle. These element and isotope features indicate that the post-collisional andesitic volcanics are the products of partial melting of metasomatically enriched orogenic lithospheric mantle. During the Triassic subduction of the South China block (SCB) beneath the North China block (NCB), the overlying NCB lithospheric mantle wedge peridotite was metasomatized by felsic melts originated from the subdcuted SCB continental crust, the melt-peridotite reaction in the continental subduction channel generated fertile and enriched metasomatites of mafic composition. Partial melting of such metasomatites in the Early Cretaceous gave rise to these andesitic volcanics. Therefore, the mantle sources for post-collisional mafic igneous rocks in collisional orogens would be generated by the slab-mantle interaction in continental subduction channel, and the lithochemical and geochemical composition of these mafic rocks is dictated by the proportion of felsic melts incorporating into the mantle wedge.
Crustal material subducted to mantle depths inevitably interacted with the mantle wedge at the slab-mantle interface. This may generate a variety of ultramafic metasomatites that served as the mantle source of mafic igneous rocks in collisional orogens. Therefore, mafic igneous rocks in collisional orogens are the important target to study the recycling of subdcuted crustal materials and its associated crust-mantle interaction. In order to decipher the mechanism and processes of the recycling of subducted continental crustal materials, a combined study of element and isotope geochemistry was performed for post-collisional andesitic volcanics from the Dabie orogen, China. SIMS zircon U-Pb ages for these volcanic rocks are 124±3 to 130±2 Ma, indicating that they formed at Early Cretaceous. In addition, the relict zircons have Middle Neoproterozoic and Triassic U-Pb ages, respectively, corresponding to the ages of protolith formation and ultrahigh-pressure metamorphism (UHP) for UHP metaigenous rocks in the Dabie-Sulu orogenic belt. They have island-arc basalts (IAB)-like trace-element patterns, enriched Sr-Nd-Hf isotope compositions, and variable zircon δ18O values mostly different from the normal mantle. These element and isotope features indicate that the post-collisional andesitic volcanics are the products of partial melting of metasomatically enriched orogenic lithospheric mantle. During the Triassic subduction of the South China block (SCB) beneath the North China block (NCB), the overlying NCB lithospheric mantle wedge peridotite was metasomatized by felsic melts originated from the subdcuted SCB continental crust, the melt-peridotite reaction in the continental subduction channel generated fertile and enriched metasomatites of mafic composition. Partial melting of such metasomatites in the Early Cretaceous gave rise to these andesitic volcanics. Therefore, the mantle sources for post-collisional mafic igneous rocks in collisional orogens would be generated by the slab-mantle interaction in continental subduction channel, and the lithochemical and geochemical composition of these mafic rocks is dictated by the proportion of felsic melts incorporating into the mantle wedge.
2019, 44(12): 4203-4221.
doi: 10.3799/dqkx.2019.225
Abstract:
A large number of small-scale dykes are located in the eastern part of the West Qinling, but they are lack of detailed study. Based on the detailed field work and petrological research, zircon U-Pb geochronology, geochemistry and zircon Lu-Hf isotope studies were carried out for the quartz syenites in the Tietang Gorge from Tianshui, West Qinling. The zircon U-Pb dating shows the crystallization ages of the quartz syenites are~250 Ma, indicating that they were formed in the Early Indosinian. Geochemistry research shows that the Tietang Gorge quartz syenites belong to the series of high-alkaline calc-alkaline rocks with weakly peraluminous. They have some adakitic characteristics with high Mg#, enrichment in Ba, Sr, K and strong depletion in Nb and Ta, and no significant Eu anomalies. The zircon εHf(t) for the quartz syenites ranges from -1.44 to +3.17, and the one-stage Hf model ages (tDM1) vary from 765 to 945 Ma. The Tietang Gorge quartz syenites formed in the continental marginal arc environment of the northward subduction of the Animaqing Ocean. They derived from the partial melting of the subduction oceanic crust with its overlying sediment interacted with the mantle wedge, and a slight fractional crystallization occurred in their shallow magma chamber.
A large number of small-scale dykes are located in the eastern part of the West Qinling, but they are lack of detailed study. Based on the detailed field work and petrological research, zircon U-Pb geochronology, geochemistry and zircon Lu-Hf isotope studies were carried out for the quartz syenites in the Tietang Gorge from Tianshui, West Qinling. The zircon U-Pb dating shows the crystallization ages of the quartz syenites are~250 Ma, indicating that they were formed in the Early Indosinian. Geochemistry research shows that the Tietang Gorge quartz syenites belong to the series of high-alkaline calc-alkaline rocks with weakly peraluminous. They have some adakitic characteristics with high Mg#, enrichment in Ba, Sr, K and strong depletion in Nb and Ta, and no significant Eu anomalies. The zircon εHf(t) for the quartz syenites ranges from -1.44 to +3.17, and the one-stage Hf model ages (tDM1) vary from 765 to 945 Ma. The Tietang Gorge quartz syenites formed in the continental marginal arc environment of the northward subduction of the Animaqing Ocean. They derived from the partial melting of the subduction oceanic crust with its overlying sediment interacted with the mantle wedge, and a slight fractional crystallization occurred in their shallow magma chamber.
2019, 44(12): 4222-4234.
doi: 10.3799/dqkx.2019.169
Abstract:
The Barkol basin is sandwiched between the Maqinwula Island Arc and the Harlic Island Arc. Until now, its internal stratigraphic distribution, fault structure, hydrocarbon accumulation and basin-mountain relationship are still not clear. In this paper, geophysical and geological methods such as magnetotelluric sounding, aeromagnetic, and seismic methods are adopted to carry out comprehensive research. The research results show that the Cenozoic sediments are relatively thin, and the Mesozoic and Upper Permian strata have the characteristics of thinning gradually from the center of the basin to the periphery. The Lower Permian and Carboniferous strata are characterized by thick Piedmont deposits and thinning southward. There are 12 faults in the basin, the strike of which is nearly east-west, most of which are thrust faults. These faults constitute the basic tectonic framework of the study area. The basin is divided into three secondary structural units:the northern slope, the Harlic thrust belt and the Barkol depression. The Dahe structure in the Barkol depression is a favorable area for hydrocarbon accumulation. The southern Mount Harlic is thrusting northwest into the basin that is bounded by the Harlic fault. The basin and the northern Mt. Maqinwula are bounded by the Mt. Maqinwula fault, and the western Mt. Bogda fault is bounded by the Dongwu fault.
The Barkol basin is sandwiched between the Maqinwula Island Arc and the Harlic Island Arc. Until now, its internal stratigraphic distribution, fault structure, hydrocarbon accumulation and basin-mountain relationship are still not clear. In this paper, geophysical and geological methods such as magnetotelluric sounding, aeromagnetic, and seismic methods are adopted to carry out comprehensive research. The research results show that the Cenozoic sediments are relatively thin, and the Mesozoic and Upper Permian strata have the characteristics of thinning gradually from the center of the basin to the periphery. The Lower Permian and Carboniferous strata are characterized by thick Piedmont deposits and thinning southward. There are 12 faults in the basin, the strike of which is nearly east-west, most of which are thrust faults. These faults constitute the basic tectonic framework of the study area. The basin is divided into three secondary structural units:the northern slope, the Harlic thrust belt and the Barkol depression. The Dahe structure in the Barkol depression is a favorable area for hydrocarbon accumulation. The southern Mount Harlic is thrusting northwest into the basin that is bounded by the Harlic fault. The basin and the northern Mt. Maqinwula are bounded by the Mt. Maqinwula fault, and the western Mt. Bogda fault is bounded by the Dongwu fault.
2019, 44(12): 4235-4251.
doi: 10.3799/dqkx.2019.045
Abstract:
The geochemical composition of sandstones in sedimentary basin plays an important role in the study of sedimentary provenance and tectonic settings. In this paper, detailed petrography and geochemical analyses were carried out on the sandstones of the Yaojia Formation from 4 drilling cores in the study area. All sandstone samples have the highest content of quartz (Q), followed by feldspar (F), and the lowest amount of lithic fragments (L), with an average of 42%, 37% and 21% respectively, featuring with heavy mineral assemblage of zircon-titanium magnetite-garnet, which suggests an acidic or metamorphic source. Dickinson discrimination diagrams show provenance mainly from continental block provenance. The REE distribution patterns are uniform, with LREE enrichment, flat HREE, and moderate negative Eu anomalies (average 0.63). Chemical index of alteration CIA (52.02-60.16, average 56.69) of the sandstones displays that they have experienced low grade of chemical weathering and alteration under arid paleoclimate condition. The discrimination diagrams for provenance attribute indicate a mixed source material composition of old sedimentary rocks and felsic igneous rocks. Based on major elements, trace and rare earth elements tectonic setting discrimination diagrams, suggested that source materials of the Yaojia Formation sandstones were from passive margin environment, and its provenances were from Yanshan intracontinental orogenic belts of the northern margin of Huabei Craton.
The geochemical composition of sandstones in sedimentary basin plays an important role in the study of sedimentary provenance and tectonic settings. In this paper, detailed petrography and geochemical analyses were carried out on the sandstones of the Yaojia Formation from 4 drilling cores in the study area. All sandstone samples have the highest content of quartz (Q), followed by feldspar (F), and the lowest amount of lithic fragments (L), with an average of 42%, 37% and 21% respectively, featuring with heavy mineral assemblage of zircon-titanium magnetite-garnet, which suggests an acidic or metamorphic source. Dickinson discrimination diagrams show provenance mainly from continental block provenance. The REE distribution patterns are uniform, with LREE enrichment, flat HREE, and moderate negative Eu anomalies (average 0.63). Chemical index of alteration CIA (52.02-60.16, average 56.69) of the sandstones displays that they have experienced low grade of chemical weathering and alteration under arid paleoclimate condition. The discrimination diagrams for provenance attribute indicate a mixed source material composition of old sedimentary rocks and felsic igneous rocks. Based on major elements, trace and rare earth elements tectonic setting discrimination diagrams, suggested that source materials of the Yaojia Formation sandstones were from passive margin environment, and its provenances were from Yanshan intracontinental orogenic belts of the northern margin of Huabei Craton.
2019, 44(12): 4252-4263.
doi: 10.3799/dqkx.2018.199
Abstract:
The tight sandstone reservoir of Yanchang Formation in Ordos basin has a high content of feldspar and a wide range of brittle fractures. At present, there is still a lack of understanding of the fluid filling rule in intra-granular pores of feldspar and the fractal characteristics. In this study, the concept of "fluid filling in intra-granular pores" of feldspar is proposed by means of casting thin section, field emission scanning electron microscope, image processing and fractal dimension calculation. The micro filling process of feldspar in Yanchang Formation reservoir is simulated quantitatively, and the specialty of the intra-granular pores in feldspar compared with inter-granular pores is pointed out. According to time, the filling process could be divided into two stages, one is unsteady filling in the early stage and the other is steady filling in the later stage. According to the distribution characteristics of filling rate value, all the filling space in the inner-granular pores of feldspar could be divided into three filling areas:high-speed filling area, medium speed filling area and low-speed filling area, and the power function curve of the sweep efficiency is established and the physical significance of the fractal dimension of the flow path is also clarified. The conclusion could provide important enlightenment for the recovery of reservoir forming process of Yanchang Formation in Ordos basin.
The tight sandstone reservoir of Yanchang Formation in Ordos basin has a high content of feldspar and a wide range of brittle fractures. At present, there is still a lack of understanding of the fluid filling rule in intra-granular pores of feldspar and the fractal characteristics. In this study, the concept of "fluid filling in intra-granular pores" of feldspar is proposed by means of casting thin section, field emission scanning electron microscope, image processing and fractal dimension calculation. The micro filling process of feldspar in Yanchang Formation reservoir is simulated quantitatively, and the specialty of the intra-granular pores in feldspar compared with inter-granular pores is pointed out. According to time, the filling process could be divided into two stages, one is unsteady filling in the early stage and the other is steady filling in the later stage. According to the distribution characteristics of filling rate value, all the filling space in the inner-granular pores of feldspar could be divided into three filling areas:high-speed filling area, medium speed filling area and low-speed filling area, and the power function curve of the sweep efficiency is established and the physical significance of the fractal dimension of the flow path is also clarified. The conclusion could provide important enlightenment for the recovery of reservoir forming process of Yanchang Formation in Ordos basin.
2019, 44(12): 4264-4274.
doi: 10.3799/dqkx.2019.002
Abstract:
The NW-trending Chengbei fault was formed in the Mesozoic and located at the central part of the Bohai bay basin. In order to clarify the fault segmentation characteristics and its control over the framework of the Chengbei sag during the Cenozoic evolution, we integrate 3D seismic exploration data, well logging data from Bohai oil field and Shengli oil field, and analyzes the geometric features, activity rate of the fault and the contact relationship and distribution characteristics of the strata in the sag. The results show that the Cenozoic Chengbei fault is connected by four segments in NWW-and NNW-trending with an 'M' shape on the plan view; the Chengbei fault experienced 3 stages during its evolution in the Cenozoic:reactivation in segment during Ek-Es4 (~65-42 Ma), connection stage during Es3-Ed (~42-23 Ma), and strike-slip transformation stage during N-Q (~23-0 Ma); the evolution of the four segments in the Cenozoic dominated the basement morphology, strata onlap, transverse anticline development, and the evolution of the sub-sag, by the control of the Chengbei fault segments the Chengbei sag evolution can be divided into three stages:the horizontal expansion stage during Ek-Es4, the vertical deepening stage during Es3-Ed and the strike-slip reconstruction stage during N-Q.
The NW-trending Chengbei fault was formed in the Mesozoic and located at the central part of the Bohai bay basin. In order to clarify the fault segmentation characteristics and its control over the framework of the Chengbei sag during the Cenozoic evolution, we integrate 3D seismic exploration data, well logging data from Bohai oil field and Shengli oil field, and analyzes the geometric features, activity rate of the fault and the contact relationship and distribution characteristics of the strata in the sag. The results show that the Cenozoic Chengbei fault is connected by four segments in NWW-and NNW-trending with an 'M' shape on the plan view; the Chengbei fault experienced 3 stages during its evolution in the Cenozoic:reactivation in segment during Ek-Es4 (~65-42 Ma), connection stage during Es3-Ed (~42-23 Ma), and strike-slip transformation stage during N-Q (~23-0 Ma); the evolution of the four segments in the Cenozoic dominated the basement morphology, strata onlap, transverse anticline development, and the evolution of the sub-sag, by the control of the Chengbei fault segments the Chengbei sag evolution can be divided into three stages:the horizontal expansion stage during Ek-Es4, the vertical deepening stage during Es3-Ed and the strike-slip reconstruction stage during N-Q.
2019, 44(12): 4275-4283.
doi: 10.3799/dqkx.2019.182
Abstract:
Based on the multi-platform satellite remote sensing data of 128 time-phases, the variation characteristics and causes of the beach in Dongting Lake before and after the operation of the Three Gorges Reservoir (TGR) during 1994-2016 were analyzed in this study. The results show that after the operation of the TGR, the variation range of Dongting Lake water level and beach is smaller than that before the operation, and presents linear relationship. The linear trend of Dongting Lake beach is different in different time periods, showing the characteristics of first expansion and then contraction. Compared with before TGR operation, Dongting Lake beach under the same water level is larger after TGR operation. The higher the water level, the greater the increase. Before the operation of the TGR, Dongting Lake was continuously silted up. After the operation, the sediment deposition has decreased to a negative, and the elevation of beach decreases at a rate of 1.59 mm/a. The operation of TGR and lake sand mining are important influence factors for the change of Dongting Lake beach.
Based on the multi-platform satellite remote sensing data of 128 time-phases, the variation characteristics and causes of the beach in Dongting Lake before and after the operation of the Three Gorges Reservoir (TGR) during 1994-2016 were analyzed in this study. The results show that after the operation of the TGR, the variation range of Dongting Lake water level and beach is smaller than that before the operation, and presents linear relationship. The linear trend of Dongting Lake beach is different in different time periods, showing the characteristics of first expansion and then contraction. Compared with before TGR operation, Dongting Lake beach under the same water level is larger after TGR operation. The higher the water level, the greater the increase. Before the operation of the TGR, Dongting Lake was continuously silted up. After the operation, the sediment deposition has decreased to a negative, and the elevation of beach decreases at a rate of 1.59 mm/a. The operation of TGR and lake sand mining are important influence factors for the change of Dongting Lake beach.
2019, 44(12): 4284-4292.
doi: 10.3799/dqkx.2019.180
Abstract:
Slope displacement is the most direct embodiment of slope stability. Thus, it is of great significance to monitor the known landslides and detect the unknown landslides by routine time series displacements of landslide prone areas. Synthetic Aperture Radar (SAR) images with its wide coverage and capability of high presicion displacement monitoring play more and more important roles in landslide identification and detection. In this study, time series InSAR analysis method combining distributed scatterers and point-like targets is introduced. Then, we investigate the stability of the Outang landslide and surrounding slopes with 19 ALOS PALSAR images from 2007 to 2011 and 47 Sentinel-1 images from 2015 to 2018. Three new active slopes were identified with the Sentinel-1 datasets compared with the results from ALOS PALSAR datasets. Time series displacement analysis indicate the rainfall and water level fluctuation seriously affect the stability of slopes in the Three Gorges area. As a result, time series InSAR analysis can be carried out routinely to monitor and detect potential landslides.
Slope displacement is the most direct embodiment of slope stability. Thus, it is of great significance to monitor the known landslides and detect the unknown landslides by routine time series displacements of landslide prone areas. Synthetic Aperture Radar (SAR) images with its wide coverage and capability of high presicion displacement monitoring play more and more important roles in landslide identification and detection. In this study, time series InSAR analysis method combining distributed scatterers and point-like targets is introduced. Then, we investigate the stability of the Outang landslide and surrounding slopes with 19 ALOS PALSAR images from 2007 to 2011 and 47 Sentinel-1 images from 2015 to 2018. Three new active slopes were identified with the Sentinel-1 datasets compared with the results from ALOS PALSAR datasets. Time series displacement analysis indicate the rainfall and water level fluctuation seriously affect the stability of slopes in the Three Gorges area. As a result, time series InSAR analysis can be carried out routinely to monitor and detect potential landslides.
2019, 44(12): 4293-4298.
doi: 10.3799/dqkx.2019.269
Abstract:
Through the prediction methods such as experience, theoretical analysis and numerical simulation, combined with the measured data, the ground settlement caused by the earth pressure shield construction is analyzed. The analysis results show that the maximum settlement estimated by the prediction method is always higher than the measured settlement value; the curve shape of the Oteo method is more optimized; when the tunnel depth is shallow, the Loganathan-Poulos, Sagaseta, Peck and Verruijt-Booker methods all overestimate the maximum settlement, and the settling troughs given by the Loganathan-Poulos, Sagaseta and Verruijt-Booker methods are wide; the numerical simulation method, which is more effective than the analytical and empirical method, can simulate the construction process effectively. According to the research results, the experience, theoretical analysis and numerical simulation methods have a certain safety reserve space, and these three methords can be used for ground subsidence prediction caused by earth pressure shield construction in soft soil layer.
Through the prediction methods such as experience, theoretical analysis and numerical simulation, combined with the measured data, the ground settlement caused by the earth pressure shield construction is analyzed. The analysis results show that the maximum settlement estimated by the prediction method is always higher than the measured settlement value; the curve shape of the Oteo method is more optimized; when the tunnel depth is shallow, the Loganathan-Poulos, Sagaseta, Peck and Verruijt-Booker methods all overestimate the maximum settlement, and the settling troughs given by the Loganathan-Poulos, Sagaseta and Verruijt-Booker methods are wide; the numerical simulation method, which is more effective than the analytical and empirical method, can simulate the construction process effectively. According to the research results, the experience, theoretical analysis and numerical simulation methods have a certain safety reserve space, and these three methords can be used for ground subsidence prediction caused by earth pressure shield construction in soft soil layer.
2019, 44(12): 4299-4312.
doi: 10.3799/dqkx.2018.555
Abstract:
Susceptibility assessment of region landslides plays an important role in geological hazard risk management. In previous studies, few of them applied the combination of multivariate statistic model and machine learning method to assess landslide susceptibility. Taking Wanzhou District of Three Gorges reservoir as an example, nine index factors including slope angle, slope direction, curvature, terrain surface texture, stratum lithology, slope structure, geological structure, water distribution and land use, were selected as the evaluation indexes of landslide susceptibility. The state of each index was graded based on the contrast values calculated by weights of evidence (WOE) model, landslide area ratio and grading area ratio firstly. Then the BP neural network model optimized by particle swarm optimization (PSO-BP) was applied to obtain the weight of each index. The landslide susceptibility index (LSI) was calculated by the combining weight of states and weight of indexes determined by these two models (WOE-BP) and landslide susceptibility mapping was obtained based on the GIS platform. The results indicate that water distribution, stratum lithology and geological structure are the main index factors influencing the development of landslides in Wanzhou District. The accuracy of the WOE-BP model reaches 80.8%, better than 73.1% of WOE model and 71.6% of BP neural network model. The proposed model provides an effective approach for calculating the weight of index quantificationally and optimizing the landslide susceptibility evaluation.
Susceptibility assessment of region landslides plays an important role in geological hazard risk management. In previous studies, few of them applied the combination of multivariate statistic model and machine learning method to assess landslide susceptibility. Taking Wanzhou District of Three Gorges reservoir as an example, nine index factors including slope angle, slope direction, curvature, terrain surface texture, stratum lithology, slope structure, geological structure, water distribution and land use, were selected as the evaluation indexes of landslide susceptibility. The state of each index was graded based on the contrast values calculated by weights of evidence (WOE) model, landslide area ratio and grading area ratio firstly. Then the BP neural network model optimized by particle swarm optimization (PSO-BP) was applied to obtain the weight of each index. The landslide susceptibility index (LSI) was calculated by the combining weight of states and weight of indexes determined by these two models (WOE-BP) and landslide susceptibility mapping was obtained based on the GIS platform. The results indicate that water distribution, stratum lithology and geological structure are the main index factors influencing the development of landslides in Wanzhou District. The accuracy of the WOE-BP model reaches 80.8%, better than 73.1% of WOE model and 71.6% of BP neural network model. The proposed model provides an effective approach for calculating the weight of index quantificationally and optimizing the landslide susceptibility evaluation.
2019, 44(12): 4128-4134.
doi: 10.3799/dqkx.2019.240
Abstract:
It is of great significance to search for the evidence of paleo-oceanic crust recycling in collisional orogens to understand the geodynamic transition from oceanic subduction to continental subduction, and also the development of plate tectonics.This is illustrated by the petrology and geochemistry of Late Paleozoic and Late Mesozoic mafic magmatic rocks in the Tongbai-Hong'an orogens. The fluid metasomatism of subducted oceanic crust at sub-arc depth (80-160 km) was recorded by the Late Paleozoic mafic rocks, which are characterized by the arc-like trace element features and depleted radiogenic isotopes, while the melt metasomatism of subducted oceanic crust at post-arc depth (>200 km) was recorded by Late Mesozoic mafic rocks, which are characterized by the OIB-like trace element features and depleted-weakly enriched radiogenic isotopes. These qualitative interpretations are further confirmed by quantitative calculations, which indicates that the content of incompatible elements and the enrichment degree of radiogenic isotopes in mafic igneous rocks are mainly controlled by the nature and proportion of crustal components in the mantle sources. Therefore, the recycling of the subducted paleo-oceanic crust at sub-arc and post-arc depths, are confirmed by arc-like and OIB-like mafic igneous rocks in the collisional orogenic belt, respectively.
It is of great significance to search for the evidence of paleo-oceanic crust recycling in collisional orogens to understand the geodynamic transition from oceanic subduction to continental subduction, and also the development of plate tectonics.This is illustrated by the petrology and geochemistry of Late Paleozoic and Late Mesozoic mafic magmatic rocks in the Tongbai-Hong'an orogens. The fluid metasomatism of subducted oceanic crust at sub-arc depth (80-160 km) was recorded by the Late Paleozoic mafic rocks, which are characterized by the arc-like trace element features and depleted radiogenic isotopes, while the melt metasomatism of subducted oceanic crust at post-arc depth (>200 km) was recorded by Late Mesozoic mafic rocks, which are characterized by the OIB-like trace element features and depleted-weakly enriched radiogenic isotopes. These qualitative interpretations are further confirmed by quantitative calculations, which indicates that the content of incompatible elements and the enrichment degree of radiogenic isotopes in mafic igneous rocks are mainly controlled by the nature and proportion of crustal components in the mantle sources. Therefore, the recycling of the subducted paleo-oceanic crust at sub-arc and post-arc depths, are confirmed by arc-like and OIB-like mafic igneous rocks in the collisional orogenic belt, respectively.
2019, 44(12): 4135-4143.
doi: 10.3799/dqkx.2019.273
Abstract:
Continental basalts,erupted in either rift or flood mode,usually show oceanic island basalt (OIB)-like geochemical compositions,with the presence of crustal components in the mantle sources of OIB. However,it is uncertain for OIB when and how the crustal components were incorporated into the mantle sources. In comparison,the formation of continental basalts is associated with specific settings for subduction of oceanic slabs and its interaction with the mantle. In this regard,a geochemical study of continental basalts can provide insights into the crust-mantle interaction in oceanic subduction channels. This is illustrated for the geochemistry of Cenozoic continental basalts in eastern China. These basalts are characterized by OIB-like trace element compositions and depleted to weakly enriched Sr-Nd isotope compositions. After precluding the effect of crustal contamination,the geochemical features of these basalts can be interpreted by recycling of crustal components into their mantle sources. The subducting oceanic crust is the major source of crustal components. In addition to both igneous oceanic crust and seafloor sediment,the lower continental crust of the overriding continental margin was offscrapped into the mantle sources. There are a series of differences in Pb isotope compositions between the Cenozoic basalts in North and South China,suggesting a difference in the nature of crustal components between these two areas. The mantle sources of Cenozoic continental basalts were produced by the crust-mantle interaction in the oceanic subduction zone during the Mesozoic subduction of paleo-Pacific slab beneath the Euroasian continent. Hydrous melts were produced by partial melting of the subducting oceanic crust at postarc depths. They were then reacted with the overlying mantle wedge to generate metasomatic domains,whose partial melting in the Cenozoic give rise to the continental basalts.
Continental basalts,erupted in either rift or flood mode,usually show oceanic island basalt (OIB)-like geochemical compositions,with the presence of crustal components in the mantle sources of OIB. However,it is uncertain for OIB when and how the crustal components were incorporated into the mantle sources. In comparison,the formation of continental basalts is associated with specific settings for subduction of oceanic slabs and its interaction with the mantle. In this regard,a geochemical study of continental basalts can provide insights into the crust-mantle interaction in oceanic subduction channels. This is illustrated for the geochemistry of Cenozoic continental basalts in eastern China. These basalts are characterized by OIB-like trace element compositions and depleted to weakly enriched Sr-Nd isotope compositions. After precluding the effect of crustal contamination,the geochemical features of these basalts can be interpreted by recycling of crustal components into their mantle sources. The subducting oceanic crust is the major source of crustal components. In addition to both igneous oceanic crust and seafloor sediment,the lower continental crust of the overriding continental margin was offscrapped into the mantle sources. There are a series of differences in Pb isotope compositions between the Cenozoic basalts in North and South China,suggesting a difference in the nature of crustal components between these two areas. The mantle sources of Cenozoic continental basalts were produced by the crust-mantle interaction in the oceanic subduction zone during the Mesozoic subduction of paleo-Pacific slab beneath the Euroasian continent. Hydrous melts were produced by partial melting of the subducting oceanic crust at postarc depths. They were then reacted with the overlying mantle wedge to generate metasomatic domains,whose partial melting in the Cenozoic give rise to the continental basalts.
2019, 44(12): 4144-4151.
doi: 10.3799/dqkx.2019.243
Abstract:
The generation of continental arc andesites is generally attributed to subduction of oceanic slabs beneath continental margins, but the petrogenetic processes of andesites remain widely debated. In order to address this problem, a series of integrated geochemical studies were performed for Mesozoic andesitic volcanics and associated basaltic and dacitic volcanics from the Middle and Lower Yangtze Valley, South China. The results lead to proposition of a new model for the generation of andesites. Laser-ablation inductively coupled mass spectroscopy (LA-ICPMS) zircon U-Pb dating yields consistent ages of Early Cretaceous for the formation of these volcanics, which are characterized by arc-like trace element distribution patterns showing significant enrichment in large ion lithophile element (LILE), Pb and light rare earth element (LREE) but depletion in high field strength element (HFSE) and heavy rare earth element (HREE). They also exhibit relatively enriched Sr-Nd-Hf isotope compositions, high radiogenic Pb isotope compositions and high zircon O isotope composition. Crustal contamination and magma mixing had insignificant contributions to the enriched compositions of these andesites. Instead, the enriched compositions were imparted by incorporating the subducted crust-derived materials into their magma sources. Despite their formation in the Late Mesozoic, their magma sources were generated through the crust-mantle interaction when the Cathaysian oceanic crust was subducted beneath the Yangtze craton in the Early Neoproterozoic. There are large amounts of subducted sediment-derived hydrous melts in the magma sources of continental arc andesites, in contrast to the limited amounts of aqueous solutions and hydrous melts in the magma sources of oceanic arc basalts. It is the hydrous melts that would chemically react with the overlying mantle wedge peridotite to generate mafic-ultramafic metasomatites. In the Early Cretaceous, these metasomatites underwent partial melting due to remote backarc extension owing to westward subduction of the Paleo-Pacific slab beneath the eastern China continent. Whereas partial melting of the ultramafic metasomatite produced basaltic melts, partial melting of the mafic metasomatite produced andesitic melts. In this regard, petrogenesis of both continental arc andesites and oceanic arc basalts shares two-stage processes, in which the first is the generation of mantle sources through subduction zone metasomatism and the second is the partial melting of mantle sources for mafic magmatism, with the first stage corresponds to the slab-mantle interaction in oceanic subduction zones.
The generation of continental arc andesites is generally attributed to subduction of oceanic slabs beneath continental margins, but the petrogenetic processes of andesites remain widely debated. In order to address this problem, a series of integrated geochemical studies were performed for Mesozoic andesitic volcanics and associated basaltic and dacitic volcanics from the Middle and Lower Yangtze Valley, South China. The results lead to proposition of a new model for the generation of andesites. Laser-ablation inductively coupled mass spectroscopy (LA-ICPMS) zircon U-Pb dating yields consistent ages of Early Cretaceous for the formation of these volcanics, which are characterized by arc-like trace element distribution patterns showing significant enrichment in large ion lithophile element (LILE), Pb and light rare earth element (LREE) but depletion in high field strength element (HFSE) and heavy rare earth element (HREE). They also exhibit relatively enriched Sr-Nd-Hf isotope compositions, high radiogenic Pb isotope compositions and high zircon O isotope composition. Crustal contamination and magma mixing had insignificant contributions to the enriched compositions of these andesites. Instead, the enriched compositions were imparted by incorporating the subducted crust-derived materials into their magma sources. Despite their formation in the Late Mesozoic, their magma sources were generated through the crust-mantle interaction when the Cathaysian oceanic crust was subducted beneath the Yangtze craton in the Early Neoproterozoic. There are large amounts of subducted sediment-derived hydrous melts in the magma sources of continental arc andesites, in contrast to the limited amounts of aqueous solutions and hydrous melts in the magma sources of oceanic arc basalts. It is the hydrous melts that would chemically react with the overlying mantle wedge peridotite to generate mafic-ultramafic metasomatites. In the Early Cretaceous, these metasomatites underwent partial melting due to remote backarc extension owing to westward subduction of the Paleo-Pacific slab beneath the eastern China continent. Whereas partial melting of the ultramafic metasomatite produced basaltic melts, partial melting of the mafic metasomatite produced andesitic melts. In this regard, petrogenesis of both continental arc andesites and oceanic arc basalts shares two-stage processes, in which the first is the generation of mantle sources through subduction zone metasomatism and the second is the partial melting of mantle sources for mafic magmatism, with the first stage corresponds to the slab-mantle interaction in oceanic subduction zones.
2019, 44(12): 4152-4156.
doi: 10.3799/dqkx.2019.223
Abstract:
Crustal anatexis during the tectonic evolution of collisional orogenic belts has a fundamental impact on physical properties and chemical compositions of the deep crust. As the direct witness to crustal anatexis, nanogranite inclusions, which formed during partial melting of hosted crustal rocks, are key to determine the compositions of the natural melts as well as melting mechanism. Nanogranite inclusions within garnet and zircon were identified in pelitic gneisses and felsic gneisses from the Namche Barwa complex of the eastern Himalayan syntaxis. They have a typical granitic mineral assemblage of K-feldspar, plagioclase, and quartz with or without biotite. These minerals represent the former melt as a result of dehydration melting of biotite in gneisses, which is captured by peritectic minerals (like garnet). Homogenization experiments on the nanogranite inclusions were conducted under high temperature and pressure and high temperature and room pressure conditions in order to obtain homogenized glasses. Chemical analyses show that these homogenized glasses are dominated by peraluminous granites. The major-and trace-element compositions of these glasses can be used to trace the melting processes of the host rocks. A combination of natural observation and experimental investigation of nanogranite inclusions from collisional orogenic belts is crucial to gain further insights into the crustal anatexis and associated melt compositions.
Crustal anatexis during the tectonic evolution of collisional orogenic belts has a fundamental impact on physical properties and chemical compositions of the deep crust. As the direct witness to crustal anatexis, nanogranite inclusions, which formed during partial melting of hosted crustal rocks, are key to determine the compositions of the natural melts as well as melting mechanism. Nanogranite inclusions within garnet and zircon were identified in pelitic gneisses and felsic gneisses from the Namche Barwa complex of the eastern Himalayan syntaxis. They have a typical granitic mineral assemblage of K-feldspar, plagioclase, and quartz with or without biotite. These minerals represent the former melt as a result of dehydration melting of biotite in gneisses, which is captured by peritectic minerals (like garnet). Homogenization experiments on the nanogranite inclusions were conducted under high temperature and pressure and high temperature and room pressure conditions in order to obtain homogenized glasses. Chemical analyses show that these homogenized glasses are dominated by peraluminous granites. The major-and trace-element compositions of these glasses can be used to trace the melting processes of the host rocks. A combination of natural observation and experimental investigation of nanogranite inclusions from collisional orogenic belts is crucial to gain further insights into the crustal anatexis and associated melt compositions.
2019, 44(12): 4157-4166.
doi: 10.3799/dqkx.2019.252
Abstract:
The amalgamation of supercontinents is associated with a series of orogenic processes during plate convergence from oceanic subduction, arc-continent collision and continent-continent collision. These processes are recorded in different types of magmatic rocks. The South China block is one of the most important continents in supercontinent Rodinia, whose amalgamation is caused by Grenvillian subduction of oceanic slabs with considerable crust-mantle interaction. This paper presents a summary of magmatic records in the northern margin of the South China block during the Rodinia amalgamation. The 900-950 Ma magmatic rocks are mainly of mafic to intermediate compositions with a few plagiogranites, and they are the products of intraoceanic subduction. As the subduction style evolved into Andean type, ancient terrigenous sediments were carried into subduction zones to undergo dehydration melting, giving rise to hydrous felsic melts which would react with the overlying mantle wedge. This results in the formation of highly enriched mantle sources, whose partial melting in the Middle Neoproterozoic to produced mafic magmatic rocks with very negative zircon εHf(t) values. In this regard, mafic to ultramafic rocks were generated in the mantle wedge through crustal metasomatism by subducting oceanic crust-derived fluids during the Rodinia amalgamation. Some of these rocks were partially melted in the subduction stage to form mafic arc volcanics along convergent plate boundaries, and the other parts were partially melted together with the overlying crust during lithospheric extension by continental rifting at a later time for bimodal magmatism.
The amalgamation of supercontinents is associated with a series of orogenic processes during plate convergence from oceanic subduction, arc-continent collision and continent-continent collision. These processes are recorded in different types of magmatic rocks. The South China block is one of the most important continents in supercontinent Rodinia, whose amalgamation is caused by Grenvillian subduction of oceanic slabs with considerable crust-mantle interaction. This paper presents a summary of magmatic records in the northern margin of the South China block during the Rodinia amalgamation. The 900-950 Ma magmatic rocks are mainly of mafic to intermediate compositions with a few plagiogranites, and they are the products of intraoceanic subduction. As the subduction style evolved into Andean type, ancient terrigenous sediments were carried into subduction zones to undergo dehydration melting, giving rise to hydrous felsic melts which would react with the overlying mantle wedge. This results in the formation of highly enriched mantle sources, whose partial melting in the Middle Neoproterozoic to produced mafic magmatic rocks with very negative zircon εHf(t) values. In this regard, mafic to ultramafic rocks were generated in the mantle wedge through crustal metasomatism by subducting oceanic crust-derived fluids during the Rodinia amalgamation. Some of these rocks were partially melted in the subduction stage to form mafic arc volcanics along convergent plate boundaries, and the other parts were partially melted together with the overlying crust during lithospheric extension by continental rifting at a later time for bimodal magmatism.
2019, 44(12): 4167-4172.
doi: 10.3799/dqkx.2019.276
Abstract:
Formation and evolution of the island/continental arcs are keys to understanding the plate tectonics and continent growth. In the Qilian and West Qinling, NW China, two different types of arc magmatic belts have been recognized. One is the North Qilian accretionary belt in the north, which consists of ophiolites, high-pressure metamorphic complex, and continental-type arc magmatic belt with ages of 520-440 Ma. The magmas are mainly intermediate to acid volcanics and intrusions. In the south is the Qi-Qin accretionary belt, which consists of Cambrian ophiolites (525-490 Ma) and Izu-Bonin-Mariana (IBM)-type intra-oceanic arc volcanic complexes (470-440 Ma). The Cambrian ophiolites are meanly composed of (1) the Hawaii-type picrite, (2) the within-plate alkaline basalt with intra-plate ocean-island-basalt (OIB) compositions, and (3) the within-plate tholeiitic basalt with enriched mid-ocean-ridge-basalt (E-MORB) compositions. The rock assemblage is best interpreted as an oceanic plateau of mantle plume within the Proto-Tethys ocean plate, obducted as ophiolitic fragments in the QQAB. The intraoceanic arc volcanic complexes are composed of high-Mg basalts, basaltic andesite, high-Al andesite, boninite and sanukite. Therefore, trench jam and subduction cessation caused by the arrival of an oceanic plateau are the primitive trigger for initiation of a new subduction zone with the development of younger volcanic sequence (~460-440 Ma).
Formation and evolution of the island/continental arcs are keys to understanding the plate tectonics and continent growth. In the Qilian and West Qinling, NW China, two different types of arc magmatic belts have been recognized. One is the North Qilian accretionary belt in the north, which consists of ophiolites, high-pressure metamorphic complex, and continental-type arc magmatic belt with ages of 520-440 Ma. The magmas are mainly intermediate to acid volcanics and intrusions. In the south is the Qi-Qin accretionary belt, which consists of Cambrian ophiolites (525-490 Ma) and Izu-Bonin-Mariana (IBM)-type intra-oceanic arc volcanic complexes (470-440 Ma). The Cambrian ophiolites are meanly composed of (1) the Hawaii-type picrite, (2) the within-plate alkaline basalt with intra-plate ocean-island-basalt (OIB) compositions, and (3) the within-plate tholeiitic basalt with enriched mid-ocean-ridge-basalt (E-MORB) compositions. The rock assemblage is best interpreted as an oceanic plateau of mantle plume within the Proto-Tethys ocean plate, obducted as ophiolitic fragments in the QQAB. The intraoceanic arc volcanic complexes are composed of high-Mg basalts, basaltic andesite, high-Al andesite, boninite and sanukite. Therefore, trench jam and subduction cessation caused by the arrival of an oceanic plateau are the primitive trigger for initiation of a new subduction zone with the development of younger volcanic sequence (~460-440 Ma).
2019, 44(12): 4173-4177.
doi: 10.3799/dqkx.2019.266
Abstract:
The Qinling orogen records the whole amalgamation of the North China and South China blocks. The Paleozoic magmatism in the Qinling orogen registers crust-mantle interaction and geodynamics of the orogeny. In this paper, we give a brief review of the Paleozoic magmatism.Intermediate-mafic magmatic rocks derived from the metasomatic mantle reveal fluid activities in the subducted channel. The Fushui mafic complex is enriched in K, and its mantle source has been metasomatized by subducted oceanic sediment. The Kanfenggou pluton belongs to high-Mg diorite. It is inferred by the geochemical characteristics that the mantle sources of the Kanfenggou pluton have experienced subducted fluid metasomatism. The results show that they exhibit a clear spatio-temporal variation, which might be caused by the subduction, retreat, advancement, and rollback of the Proto-Tethys oceanic plate.Therefore we suggest that the Paleozoic crust-mantle interaction was induced by the subduction of the Proto-Tethys ocean.
The Qinling orogen records the whole amalgamation of the North China and South China blocks. The Paleozoic magmatism in the Qinling orogen registers crust-mantle interaction and geodynamics of the orogeny. In this paper, we give a brief review of the Paleozoic magmatism.Intermediate-mafic magmatic rocks derived from the metasomatic mantle reveal fluid activities in the subducted channel. The Fushui mafic complex is enriched in K, and its mantle source has been metasomatized by subducted oceanic sediment. The Kanfenggou pluton belongs to high-Mg diorite. It is inferred by the geochemical characteristics that the mantle sources of the Kanfenggou pluton have experienced subducted fluid metasomatism. The results show that they exhibit a clear spatio-temporal variation, which might be caused by the subduction, retreat, advancement, and rollback of the Proto-Tethys oceanic plate.Therefore we suggest that the Paleozoic crust-mantle interaction was induced by the subduction of the Proto-Tethys ocean.
2019, 44(12): 4178-4185.
doi: 10.3799/dqkx.2019.236
Abstract:
The Foping gneiss dome in the South Qinling tectonic belt is one of the most typical gneiss domes. The gneiss dome has records the multi-stage tectonic evolution from syn-collision orogeny to post-orogenic extension, and therefore is critical to understanding the Early Mesozoic tectonic evolution of the Qinling orogen. The studies on field work, petrography, mineral chemistry, and conventional geothermobarometry show that Foping gneiss dome has typical metamorphic zoning, with metamorphic temperature gradually decreasing from the Precambrian basement in the core to the sedimentary covers in the north and south flanks. Zircon LA-ICP-MS U-Pb dating of different types of rocks and monazite SHRIMP U-Pb dating of the metapelites give a consistent metamorphic age of ca. 206-196 Ma. In combination with the investigation of Early Mesozoic granitoids in the South Qinling tectonic belt, the authors infer that the metamorphism of the Foping gneiss dome was induced by Late Triassic magmatic diapir in a post-collisional extensional setting.
The Foping gneiss dome in the South Qinling tectonic belt is one of the most typical gneiss domes. The gneiss dome has records the multi-stage tectonic evolution from syn-collision orogeny to post-orogenic extension, and therefore is critical to understanding the Early Mesozoic tectonic evolution of the Qinling orogen. The studies on field work, petrography, mineral chemistry, and conventional geothermobarometry show that Foping gneiss dome has typical metamorphic zoning, with metamorphic temperature gradually decreasing from the Precambrian basement in the core to the sedimentary covers in the north and south flanks. Zircon LA-ICP-MS U-Pb dating of different types of rocks and monazite SHRIMP U-Pb dating of the metapelites give a consistent metamorphic age of ca. 206-196 Ma. In combination with the investigation of Early Mesozoic granitoids in the South Qinling tectonic belt, the authors infer that the metamorphism of the Foping gneiss dome was induced by Late Triassic magmatic diapir in a post-collisional extensional setting.
2019, 44(12): 4186-4194.
doi: 10.3799/dqkx.2019.267
Abstract:
The formation of high-temperature (HT)/low-pressure (LP) metamorphic rocks requires high thermal gradients of > 30℃/km. It is intriguing which tectonic setting is responsible for such geological processes. This paper presents a summary of our petrological and geochemical studies on metagranite and metabasalt from the northern margin of the South China block, which were formed during breakup of Rodinia supercontinent in the middle Neoproterozoic. The results demonstrate that continental rifts are the most plausible setting for the production of HT/LP metamorphic rocks. The HT/LP metamorphism is mainly recorded in alumino silicates-bearing metagranites, in which metamorphic andalusite and sillimanite were produced by muscovite dehydration reaction. Metamorphic P-T conditions of 1.0-3.5 kbar and 560-660℃ were obtained from the petrology of aluminosilicates-bearing peak mineral assemblages in combination with pseudosection calculations. The metamorphic andalusite shows very negative δ18O values in O isotope disequilibrium with magmatic zircon, further demonstrating that it is the metamorphic product after magma crystallization. The U-Pb dating of metamorphic titanite yields concordant ages of 751±11 Ma for the HT/LP metamorphism, consistent with the peak age of the Rodinia breakup. The metabasalt shows island arc basalts-like trace element distribution patterns, indicating that its source was generated by metasomatic reaction of the mantle wedge peridotite with fluids derived from the subducting oceanic crust. Therefore, the mantle source was formed during the Grenvillian assembly of Rodinia supercontinent. In this regard, the continental rifting that resulted in the supercontinental breakup was developed in the former subduction zone. By comparing heat flow required to form the metamorphic peak mineral assemblages with that provided by heat producing elements in the metagranites, it appears that anomalously high heat flow was indeed delivered from the asthenospheric mantle to the continental rift, leading to the HT/LP metamorphism during the Rodinia breakup.
The formation of high-temperature (HT)/low-pressure (LP) metamorphic rocks requires high thermal gradients of > 30℃/km. It is intriguing which tectonic setting is responsible for such geological processes. This paper presents a summary of our petrological and geochemical studies on metagranite and metabasalt from the northern margin of the South China block, which were formed during breakup of Rodinia supercontinent in the middle Neoproterozoic. The results demonstrate that continental rifts are the most plausible setting for the production of HT/LP metamorphic rocks. The HT/LP metamorphism is mainly recorded in alumino silicates-bearing metagranites, in which metamorphic andalusite and sillimanite were produced by muscovite dehydration reaction. Metamorphic P-T conditions of 1.0-3.5 kbar and 560-660℃ were obtained from the petrology of aluminosilicates-bearing peak mineral assemblages in combination with pseudosection calculations. The metamorphic andalusite shows very negative δ18O values in O isotope disequilibrium with magmatic zircon, further demonstrating that it is the metamorphic product after magma crystallization. The U-Pb dating of metamorphic titanite yields concordant ages of 751±11 Ma for the HT/LP metamorphism, consistent with the peak age of the Rodinia breakup. The metabasalt shows island arc basalts-like trace element distribution patterns, indicating that its source was generated by metasomatic reaction of the mantle wedge peridotite with fluids derived from the subducting oceanic crust. Therefore, the mantle source was formed during the Grenvillian assembly of Rodinia supercontinent. In this regard, the continental rifting that resulted in the supercontinental breakup was developed in the former subduction zone. By comparing heat flow required to form the metamorphic peak mineral assemblages with that provided by heat producing elements in the metagranites, it appears that anomalously high heat flow was indeed delivered from the asthenospheric mantle to the continental rift, leading to the HT/LP metamorphism during the Rodinia breakup.
2019, 44(12): 4195-4202.
doi: 10.3799/dqkx.2019.237
Abstract:
The Dabie orogenic belt consists of a series of fault-bounded lithotectonic units with various metamorphic grades and evolutional histories related to continental subduction and exhumation. Based on the existing controversy and problem in the region, this study performed the detailed field investigation, and petrological, element-isotope geochemical and zircon geochronological researches on the granitic rocks from different subducted slices such as the Susong metamorphic zone (SZ), Central Dabie ultrahigh-pressure metamorphic zone (CDZ) and North Dabie complex zone (NDZ). The results suggest that:(1) the precursor ages of the granitic gneisses in the SZ can be divided into two types, i.e. the Late Archean (2.5-2.7 Ga) and the Neoproterozoic (770-830 Ma), and Neoproterozoic protoliths of them were derived from remelting of the Late Archean rocks with~2.0 Ga metamorphic overprinting during the Neoproterozoic continental rifting; (2) the granitic orthogneisses in the CDZ at least contain two kinds of precursor ages of~750 Ma and 780-800 Ma with different petrogenesis, which underwent two episodes of partial melting at~230 Ma and~220 Ma during the Triassic subduction and exhumation; (3) there are a variety of leucosomes, formed by high-T decompression-induced biotite dehydration melting during the initial stage of exhumation at~209 Ma and water-fluxed heating melting corresponding to the mountain-root collapse at 110-145 Ma, within the migmatites in the NDZ; (4) the meta-diorites from the NDZ were discovered and are documented to be resulted from partial melting of the Triassic deep-subducted mafic lower-crustal rocks during the mountain-root collapse in the Early Cretaceous. Thus, all these provide new constraints on diverse partial melting during the Neoproterozoic continental rifting, the Triassic crustal subduction-exhumation and the Cretaceous mountain-root collapse in the Dabie orogen, Central China.
The Dabie orogenic belt consists of a series of fault-bounded lithotectonic units with various metamorphic grades and evolutional histories related to continental subduction and exhumation. Based on the existing controversy and problem in the region, this study performed the detailed field investigation, and petrological, element-isotope geochemical and zircon geochronological researches on the granitic rocks from different subducted slices such as the Susong metamorphic zone (SZ), Central Dabie ultrahigh-pressure metamorphic zone (CDZ) and North Dabie complex zone (NDZ). The results suggest that:(1) the precursor ages of the granitic gneisses in the SZ can be divided into two types, i.e. the Late Archean (2.5-2.7 Ga) and the Neoproterozoic (770-830 Ma), and Neoproterozoic protoliths of them were derived from remelting of the Late Archean rocks with~2.0 Ga metamorphic overprinting during the Neoproterozoic continental rifting; (2) the granitic orthogneisses in the CDZ at least contain two kinds of precursor ages of~750 Ma and 780-800 Ma with different petrogenesis, which underwent two episodes of partial melting at~230 Ma and~220 Ma during the Triassic subduction and exhumation; (3) there are a variety of leucosomes, formed by high-T decompression-induced biotite dehydration melting during the initial stage of exhumation at~209 Ma and water-fluxed heating melting corresponding to the mountain-root collapse at 110-145 Ma, within the migmatites in the NDZ; (4) the meta-diorites from the NDZ were discovered and are documented to be resulted from partial melting of the Triassic deep-subducted mafic lower-crustal rocks during the mountain-root collapse in the Early Cretaceous. Thus, all these provide new constraints on diverse partial melting during the Neoproterozoic continental rifting, the Triassic crustal subduction-exhumation and the Cretaceous mountain-root collapse in the Dabie orogen, Central China.
