2006 Vol. 31, No. 1
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2006, 31(1): 1-7.
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Abstract:
Four-period calc-alkaline volcanic rocks are widespread in the Yanshan erogenic belt. The components of magma sources were controlled predominantly by the interaction between the continental crust and upper mantle near MOHO surface. The Tiaojishan and Yixian formations are the representative types of volcanic activity. The petrological features of Tiaojishan Formation volcanic rocks are relatively uniform, geochemical parameters varies at a narrow range and the AFC processes of magma are not strong enough, which indicates that the components of the magma source were not very complex. The estimated thickness of the crust was 40 - 45 km during the Early to Middle Jurassic. We infer that Tiaojishan tra-chyandesite magma was melted from amphibole-bearing garnet granulite + underplating basalts at thickening crust condition. A possible model, delaminating lower continental crust into the underlying weakening lithospheric mantle and interaction between them, has been proposed as one means to explain the genesis of basic-middle basic magma in the Yixian Formation. The inferred source of these magmas was lower crust + weakening lithosphere. Rheological behavior of the lithospheric mantle has been heterogeneously translated from rigid to ductile. This translation process was induced by the effects of fluid, melt, water released from mantle minerals, shear stress and increasing temperature in the lithospheric mantle beneath the small extension basin and above the uplifted asthenospheric diapir after approximately 135 Ma These local areas were the preferable sets to hold the delaminated lower crust. The interaction between the foundering continental lower crust and the underlying weakening lithospheric mantle may be compared with the interaction between the subducting ocean crust and mantle wedge at a subduction zone.
Four-period calc-alkaline volcanic rocks are widespread in the Yanshan erogenic belt. The components of magma sources were controlled predominantly by the interaction between the continental crust and upper mantle near MOHO surface. The Tiaojishan and Yixian formations are the representative types of volcanic activity. The petrological features of Tiaojishan Formation volcanic rocks are relatively uniform, geochemical parameters varies at a narrow range and the AFC processes of magma are not strong enough, which indicates that the components of the magma source were not very complex. The estimated thickness of the crust was 40 - 45 km during the Early to Middle Jurassic. We infer that Tiaojishan tra-chyandesite magma was melted from amphibole-bearing garnet granulite + underplating basalts at thickening crust condition. A possible model, delaminating lower continental crust into the underlying weakening lithospheric mantle and interaction between them, has been proposed as one means to explain the genesis of basic-middle basic magma in the Yixian Formation. The inferred source of these magmas was lower crust + weakening lithosphere. Rheological behavior of the lithospheric mantle has been heterogeneously translated from rigid to ductile. This translation process was induced by the effects of fluid, melt, water released from mantle minerals, shear stress and increasing temperature in the lithospheric mantle beneath the small extension basin and above the uplifted asthenospheric diapir after approximately 135 Ma These local areas were the preferable sets to hold the delaminated lower crust. The interaction between the foundering continental lower crust and the underlying weakening lithospheric mantle may be compared with the interaction between the subducting ocean crust and mantle wedge at a subduction zone.
2006, 31(1): 8-13.
Abstract:
This paper presents a review on the issues related to the mechanism and geodynamic regime of the Cenozoic-Mesozoic thinning of the subcontinental lithosphere beneath eastern China. The pattern and causes of highly chemical heterogeneity have been emphasized, and several tectonic implications have been postulated. It has been suggested that the major transformation of the subcontinental lithosphere beneath this region are composed of two separate events, which are regarded as a local response to the Mesozoic global event.
This paper presents a review on the issues related to the mechanism and geodynamic regime of the Cenozoic-Mesozoic thinning of the subcontinental lithosphere beneath eastern China. The pattern and causes of highly chemical heterogeneity have been emphasized, and several tectonic implications have been postulated. It has been suggested that the major transformation of the subcontinental lithosphere beneath this region are composed of two separate events, which are regarded as a local response to the Mesozoic global event.
2006, 31(1): 14-22.
Abstract:
The comparison of Cenozoic basalts and their mantle xenoliths from both sides of the Taihangshan gravity lineament reveals that: (1) Cenozoic basalts from western and eastern North China display opposite temporal trends, suggesting lithospheric thinning in western North China during the Cenozoic and lithospheric thickening in eastern North China; (2) The lithospheric mantle in the west is mostly Late-Archean to Proterozoic in age, whereas that in the east has a modern age with minor Proterozoic age; (3) The crust-mantle transition in the west is considerably thicker than in the east, probably due to different extents of magmatic underplating. The spatial heterogeneity of the lithospheric mantle beneath North China may have resulted from the diachronous thinning processes. Paleogeographic data suggest the first occurrence of the Taihangshan gravity lineament was initially formed during the Early Cretaceous, which is coeval with the peak of Mesozoic mag-matism in this region. Given the fact that Mesozoic magmatism was closely related to the lithospheric thinning, it is proposed that the diachronous lithospheric thinning is the main mechanism by which the Taihangshan gravity lineament was formed.
The comparison of Cenozoic basalts and their mantle xenoliths from both sides of the Taihangshan gravity lineament reveals that: (1) Cenozoic basalts from western and eastern North China display opposite temporal trends, suggesting lithospheric thinning in western North China during the Cenozoic and lithospheric thickening in eastern North China; (2) The lithospheric mantle in the west is mostly Late-Archean to Proterozoic in age, whereas that in the east has a modern age with minor Proterozoic age; (3) The crust-mantle transition in the west is considerably thicker than in the east, probably due to different extents of magmatic underplating. The spatial heterogeneity of the lithospheric mantle beneath North China may have resulted from the diachronous thinning processes. Paleogeographic data suggest the first occurrence of the Taihangshan gravity lineament was initially formed during the Early Cretaceous, which is coeval with the peak of Mesozoic mag-matism in this region. Given the fact that Mesozoic magmatism was closely related to the lithospheric thinning, it is proposed that the diachronous lithospheric thinning is the main mechanism by which the Taihangshan gravity lineament was formed.
2006, 31(1): 23-30.
Abstract:
The Re-Os isotope system is a new technique of geochemical dating and tracing, specially in the basic subject of the composition-structure-evolution of the earth's mantle. The chondritic model of the evolution of Os isotope compositions in the upper mantle is the important background of Re-Os isotope dating for the mantle-derived materials, especially using Re depleted model age and Os proxy isochron methods. The comprehensive studies of various meteorites, mantle-derived peri-dotitic xenoliths and ophiolitic podiform chromites offered strong constraints on the evolution of Os isotope compositions in the convective upper mantle. We offer the Os isotope composition of convective upper mantle in the Late Archean, based on ophiolitic podiform chromitites from Zunhua, Hebei Province, which is 0. 110 2. This is consistent with the chondritic model. The parameters of the chondrite model in common use are: the initial osmium isotope ratio is 0. 095 31 at 4. 558 Ga; the present osmium isotope ratios are 0. 127 0 of carbonaceous chondrite and 0. 129 6 of the primitive upper mantle respectively. The latter is similar to that of ordinary and enstatite chondrites.
The Re-Os isotope system is a new technique of geochemical dating and tracing, specially in the basic subject of the composition-structure-evolution of the earth's mantle. The chondritic model of the evolution of Os isotope compositions in the upper mantle is the important background of Re-Os isotope dating for the mantle-derived materials, especially using Re depleted model age and Os proxy isochron methods. The comprehensive studies of various meteorites, mantle-derived peri-dotitic xenoliths and ophiolitic podiform chromites offered strong constraints on the evolution of Os isotope compositions in the convective upper mantle. We offer the Os isotope composition of convective upper mantle in the Late Archean, based on ophiolitic podiform chromitites from Zunhua, Hebei Province, which is 0. 110 2. This is consistent with the chondritic model. The parameters of the chondrite model in common use are: the initial osmium isotope ratio is 0. 095 31 at 4. 558 Ga; the present osmium isotope ratios are 0. 127 0 of carbonaceous chondrite and 0. 129 6 of the primitive upper mantle respectively. The latter is similar to that of ordinary and enstatite chondrites.
2006, 31(1): 31-37.
Abstract:
Two types of "complex" peridotitic xenoliths exist in the earth mantle and/or lower crust: "polymict" peridotitic xenoliths and "composite" peridotitic xenoliths. Composite peridotitic xenoliths usually comprise two or more components of peridotite and pyroxenite/granulite or of peridotite with pyroxenite veins and can be easily identified in petrography, sometimes in hand samples. But the polymict peridotitic xenoliths are tectonites composed of minerals from various peridotites and/or pyroxenites and melts and brought together by mantle shear zone and difficult to identify in petrography, they must be classified by compositional analyses of constituent minerals. Polymict peridotitic xenoliths have only been found in kim-berlite pipes from the Kaapvaal craton, South Africa so far. These polymict peridotitic xenoliths originated from the mantle shear zone and recorded the whole evolutionary processes of fluid/melt-assisted mantle deformation and metasomatism. Composite peridotitic xenoliths are relatively common and occur in global basalts and kimberlites of all kinds. Composite peridol-itic xenoliths have a variety of origins such as mantle deformation, crust-mantle transitional zone sampling, metasomatism and accumulates, etc.. These composite peridotitic xenoliths also record the intermediate stage of metasomatism and modification processes of the lithospheric mantle and/or lower crust. Therefore, these rare and important complex peridotitic xenoliths provide the most direct samples for the geodynamic evolution of the continental lithosphere.
Two types of "complex" peridotitic xenoliths exist in the earth mantle and/or lower crust: "polymict" peridotitic xenoliths and "composite" peridotitic xenoliths. Composite peridotitic xenoliths usually comprise two or more components of peridotite and pyroxenite/granulite or of peridotite with pyroxenite veins and can be easily identified in petrography, sometimes in hand samples. But the polymict peridotitic xenoliths are tectonites composed of minerals from various peridotites and/or pyroxenites and melts and brought together by mantle shear zone and difficult to identify in petrography, they must be classified by compositional analyses of constituent minerals. Polymict peridotitic xenoliths have only been found in kim-berlite pipes from the Kaapvaal craton, South Africa so far. These polymict peridotitic xenoliths originated from the mantle shear zone and recorded the whole evolutionary processes of fluid/melt-assisted mantle deformation and metasomatism. Composite peridotitic xenoliths are relatively common and occur in global basalts and kimberlites of all kinds. Composite peridol-itic xenoliths have a variety of origins such as mantle deformation, crust-mantle transitional zone sampling, metasomatism and accumulates, etc.. These composite peridotitic xenoliths also record the intermediate stage of metasomatism and modification processes of the lithospheric mantle and/or lower crust. Therefore, these rare and important complex peridotitic xenoliths provide the most direct samples for the geodynamic evolution of the continental lithosphere.
2006, 31(1): 38-48.
Abstract:
The compositions and formation history of basement metamorphic rocks are the key to understanding crust evolution. Based on zircon U-Pb-Hf isotopic studies and the compositions of basement metamorphic rocks in northeastern Guangdong Province and southern Jiangxi Province, this paper indicates that the protoliths of these metamorphic rocks are sedimentary rocks deposited in the Late Neoproterozoic. They have high SiO2, Rb, Zr, Y and transitional metal concentrations and relatively low Al2O3, CaO, Na2O, Sr and Nb contents. They were deposited in an active continental margin basin near a volcanic arc. The basin formation was closely related with the extensional regime during the breakup of the supercontinent Rodinia. Neoproterozoic sediments in the Longchuan area of northeastern Guangdong Province are mainly composed of Neo-archaean and Mesoproterozoic clastic constituents with small amounts of Mesoarchaean and Neoproterozoic fragments, quite distinct from those in southwestern Fujian Province and northern Guangdong Province, showing different provenances. The basement metamorphic rocks in northeastern Guangdong Province were not influenced by the Early Paleozoic Caledonian movement, but underwent strong Indosinian metamorphism and anatexis.
The compositions and formation history of basement metamorphic rocks are the key to understanding crust evolution. Based on zircon U-Pb-Hf isotopic studies and the compositions of basement metamorphic rocks in northeastern Guangdong Province and southern Jiangxi Province, this paper indicates that the protoliths of these metamorphic rocks are sedimentary rocks deposited in the Late Neoproterozoic. They have high SiO2, Rb, Zr, Y and transitional metal concentrations and relatively low Al2O3, CaO, Na2O, Sr and Nb contents. They were deposited in an active continental margin basin near a volcanic arc. The basin formation was closely related with the extensional regime during the breakup of the supercontinent Rodinia. Neoproterozoic sediments in the Longchuan area of northeastern Guangdong Province are mainly composed of Neo-archaean and Mesoproterozoic clastic constituents with small amounts of Mesoarchaean and Neoproterozoic fragments, quite distinct from those in southwestern Fujian Province and northern Guangdong Province, showing different provenances. The basement metamorphic rocks in northeastern Guangdong Province were not influenced by the Early Paleozoic Caledonian movement, but underwent strong Indosinian metamorphism and anatexis.
2006, 31(1): 49-56.
Abstract:
A petrochemical analysis was undertaken of peridotitic xenoliths in volcanic rocks that erupted from the Paleozoic to the Cenozoic within the eastern part of the North China craton, and the peridotites as tectonic intrusive in the Early Meso-zoic from the Sulu orogen. The results show that the cratonic mantle, which was refractory and existed when the kimberlites intruded in the Paleozoic, had almost been replaced by the newly accreted fertile lithospheric mantle during the Mesozoic-Ce-nozoic. The erosion, metasomatism and intermingling caused by the accreted asthenospheric material acting on the craton mantle along the weak zone and deep fault (such as the Tanlu fault) in the existing lithosphere resulted in the lithosphere thinning at a larger scale 100 Ma ago (but later than 178 Ma). The largest thinning would be in the Eogene. The upwelling asthenospheric material transformed into accreted lithospheric mantle due to the asthenospheric temperature falling in the Neogene (leading to relatively slight lithospheric incrassation), and finally accomplished mantle replacement. The peridotitic body in the Sulu orogen represents the products spread from the modified cratonic lithospheric mantle.
A petrochemical analysis was undertaken of peridotitic xenoliths in volcanic rocks that erupted from the Paleozoic to the Cenozoic within the eastern part of the North China craton, and the peridotites as tectonic intrusive in the Early Meso-zoic from the Sulu orogen. The results show that the cratonic mantle, which was refractory and existed when the kimberlites intruded in the Paleozoic, had almost been replaced by the newly accreted fertile lithospheric mantle during the Mesozoic-Ce-nozoic. The erosion, metasomatism and intermingling caused by the accreted asthenospheric material acting on the craton mantle along the weak zone and deep fault (such as the Tanlu fault) in the existing lithosphere resulted in the lithosphere thinning at a larger scale 100 Ma ago (but later than 178 Ma). The largest thinning would be in the Eogene. The upwelling asthenospheric material transformed into accreted lithospheric mantle due to the asthenospheric temperature falling in the Neogene (leading to relatively slight lithospheric incrassation), and finally accomplished mantle replacement. The peridotitic body in the Sulu orogen represents the products spread from the modified cratonic lithospheric mantle.
2006, 31(1): 57-65.
Abstract:
The northwestern boundary of the Yangtze (South China) block has not been efficiently constrained yet. Pb and Nd isotopic compositions of the Qilianshan Precambrian basements and granitoids in the western segment of the Central Oro-genie System (COS) of China provide constraints on the northwestern boundary of the Yangtze block. Most samples from the Qilianshan Precambrian basement have mantle-depleted Nd isotopic model ages (TDM)ranging from 0. 75 to 2. 5 Ga, with a peak value of -2. 1 Ga. The Qilianshan Paleozoic granitoids have TDM values ranging from 1. 07 to 2. 14 Ga. They show that crustal growth of the Qilianshan mainly occurred in the Proterozoic, without significant Archean information. The Precambrian basement rocks and the Paleozoic granitoids are characterized by high radiogenic Pb isotopic composition. Most samples have 206Pb/204Pb > 18. 0, 207Pb/204Pb > 15. 5 and 208Pb/204Pb > 38. 0. The TDM values and Pb isotopic compositions of the Qilianshan basement and granitoids are similar to those of the Yangtze block but distinct from those of the North China block, indicating that the crustal basement of Qilianshan has an affinity with the Yangtze block. It is suggested that the northwestern boundary of the Yangtze block is located along the north side of the Qilianshan belt. Since the Proterozoic, the Qilianshan belt had experienced lithospheric rifting and subsequent Qilian ocean opening. These tectonic events had thus happened within the Yangtze block.
The northwestern boundary of the Yangtze (South China) block has not been efficiently constrained yet. Pb and Nd isotopic compositions of the Qilianshan Precambrian basements and granitoids in the western segment of the Central Oro-genie System (COS) of China provide constraints on the northwestern boundary of the Yangtze block. Most samples from the Qilianshan Precambrian basement have mantle-depleted Nd isotopic model ages (TDM)ranging from 0. 75 to 2. 5 Ga, with a peak value of -2. 1 Ga. The Qilianshan Paleozoic granitoids have TDM values ranging from 1. 07 to 2. 14 Ga. They show that crustal growth of the Qilianshan mainly occurred in the Proterozoic, without significant Archean information. The Precambrian basement rocks and the Paleozoic granitoids are characterized by high radiogenic Pb isotopic composition. Most samples have 206Pb/204Pb > 18. 0, 207Pb/204Pb > 15. 5 and 208Pb/204Pb > 38. 0. The TDM values and Pb isotopic compositions of the Qilianshan basement and granitoids are similar to those of the Yangtze block but distinct from those of the North China block, indicating that the crustal basement of Qilianshan has an affinity with the Yangtze block. It is suggested that the northwestern boundary of the Yangtze block is located along the north side of the Qilianshan belt. Since the Proterozoic, the Qilianshan belt had experienced lithospheric rifting and subsequent Qilian ocean opening. These tectonic events had thus happened within the Yangtze block.
2006, 31(1): 66-70.
Abstract:
Weizhou island is a volcanic island, which is located in Beibu Bay. The base of Weizhou island is constituted by lava. The Nanwan volcano effusive mass forms modern volcanic physiognomy of Weizhou island. Pycroclastics and base surge extend to the north island, covering more than half of the island. Eyuzui and Zhuzidao are two typical sections, which are selected in order to observe and describe the volcanic activity of Nanwan volcano in detail. Special structures were found that belong to the base surge, including large-scale and low-angle cross-bedding, slaty-bedding, climb-bedding, and distal facies accretionary lappili. These structures prove that Nanwan volcano is a multiplex volcano caused by alternation of magma eruption and phreatomagmatic eruption. Phreatomagmatic eruption plays an important role in the formation of Nanwan volcano. The eruptive center is located in the Nanwan sea.
Weizhou island is a volcanic island, which is located in Beibu Bay. The base of Weizhou island is constituted by lava. The Nanwan volcano effusive mass forms modern volcanic physiognomy of Weizhou island. Pycroclastics and base surge extend to the north island, covering more than half of the island. Eyuzui and Zhuzidao are two typical sections, which are selected in order to observe and describe the volcanic activity of Nanwan volcano in detail. Special structures were found that belong to the base surge, including large-scale and low-angle cross-bedding, slaty-bedding, climb-bedding, and distal facies accretionary lappili. These structures prove that Nanwan volcano is a multiplex volcano caused by alternation of magma eruption and phreatomagmatic eruption. Phreatomagmatic eruption plays an important role in the formation of Nanwan volcano. The eruptive center is located in the Nanwan sea.
2006, 31(1): 71-80.
Abstract:
Large-scale volcanic rocks of the Houcheng and Zhangjiakou formations are developed in the Zhangjiakou-Xuanhua (Zhang-Xuan) area. Zircon LA-ICPMS U-Pb dating gives the eruption age of dacite in the Houcheng Formation as (130±1) Ma, indicating that the Houcheng Formation formed during the Early Cretaceous, not during the Late Jurassic as previously thought. Volcanic rocks of the Zhangjiakou Formation have a zircon LA-ICPMS U-Pb age of (126±1)Ma, i. e., Early Cretaceous. In-situ zircon Hf analyses show that zircons from the Houcheng Formation dacite have εHf(t) values of -18. 8 to - 25. 5 and Hf crustal model ages of 2. 78 to 2. 37 Ga with an average of 2. 54 Ga. Their model ages are similar to those of basement rocks in the northern North China craton, representing an important stage of crustal growth in the North China craton. Thus, we suggest that the dacite was mainly derived from the partial melting of Late Archean crustal materials. Whereas, zircons from the rhyolite of the Zhangjiakou Formation have εHf(t) values of -15. 1 to -18. 5 and younger Hf crustal model ages of 2. 34 to 2. 13 Ga, indicating that the rhyolite was derived from Late Archean crustal materials, but with the involvement of mantle materials. The age and Hf isotopic data suggest that the large-scale volcanic rocks in the Zhang-Xuan area were the result of crust-mantle interaction during the Early Cretaceous lithospheric thinning in the eastern North China cratoa.
Large-scale volcanic rocks of the Houcheng and Zhangjiakou formations are developed in the Zhangjiakou-Xuanhua (Zhang-Xuan) area. Zircon LA-ICPMS U-Pb dating gives the eruption age of dacite in the Houcheng Formation as (130±1) Ma, indicating that the Houcheng Formation formed during the Early Cretaceous, not during the Late Jurassic as previously thought. Volcanic rocks of the Zhangjiakou Formation have a zircon LA-ICPMS U-Pb age of (126±1)Ma, i. e., Early Cretaceous. In-situ zircon Hf analyses show that zircons from the Houcheng Formation dacite have εHf(t) values of -18. 8 to - 25. 5 and Hf crustal model ages of 2. 78 to 2. 37 Ga with an average of 2. 54 Ga. Their model ages are similar to those of basement rocks in the northern North China craton, representing an important stage of crustal growth in the North China craton. Thus, we suggest that the dacite was mainly derived from the partial melting of Late Archean crustal materials. Whereas, zircons from the rhyolite of the Zhangjiakou Formation have εHf(t) values of -15. 1 to -18. 5 and younger Hf crustal model ages of 2. 34 to 2. 13 Ga, indicating that the rhyolite was derived from Late Archean crustal materials, but with the involvement of mantle materials. The age and Hf isotopic data suggest that the large-scale volcanic rocks in the Zhang-Xuan area were the result of crust-mantle interaction during the Early Cretaceous lithospheric thinning in the eastern North China cratoa.
2006, 31(1): 81-92.
Abstract:
High-Mg diorites occur widely in western Shandong: the Tietonggou and Jinling intrusions are typically representative. The weighted mean 206pb/238U ages from LA-ICPMS zircon U-Pb dating results for early norite-diorite, late pyroxene-diorite from the Tietonggou intrusion and biotite-diorite from the Jinling intrusion are (131. 4±4. 9)Ma(n = 15), (134. 5±2. 3)Ma(n=13), (132. 8±4. 2)Ma(n=12), respectively, implying that they were formed in the Early Cretaceous. The weighted mean 207Pb/206Pb age for round zircons from late pyroxene-diorite from the Tietonggou intrusion is (2 513±54)Ma(n=8), suggesting that the basement of the North China craton should exist in the research area. The high-Mg diorites are characterized by enrichment in Mg#, Na, and light rare earth elements (LREE). They are poor in heavy rare earth elements (HREE) and high field strength elements (HFSE), being similar to adakite. The occurrence of the mantle peridotite xenoliths and the high-Mg feature for these intrusive rocks imply that their primary magma should be derived from the upper mantle. However, Sr-Nd isolopic compositions (Isr: 0.704 75 - 0. 707 72; εNd(t) values: -3. 95 to - 13. 30), depletion in HFSE, and the occurrence of the Archean inherited zircons suggest that crustal materials be involved in the derivation of primary magma The compositional difference between the diorites from the Tietonggou and Jinling intrusions could be attributed to magma sources and partial melting degrees. It is considered that the Early Cretaceous high-Mg diorites were formed by the mixed melting of the delaminated lithosphere (lithospheric mande+crust) and asthenosphere, based on their geochemistry, the mantle-derived xenoliths, and the Early Mesozoic lithospheric evolutionary history of the eastern North China craton.
High-Mg diorites occur widely in western Shandong: the Tietonggou and Jinling intrusions are typically representative. The weighted mean 206pb/238U ages from LA-ICPMS zircon U-Pb dating results for early norite-diorite, late pyroxene-diorite from the Tietonggou intrusion and biotite-diorite from the Jinling intrusion are (131. 4±4. 9)Ma(n = 15), (134. 5±2. 3)Ma(n=13), (132. 8±4. 2)Ma(n=12), respectively, implying that they were formed in the Early Cretaceous. The weighted mean 207Pb/206Pb age for round zircons from late pyroxene-diorite from the Tietonggou intrusion is (2 513±54)Ma(n=8), suggesting that the basement of the North China craton should exist in the research area. The high-Mg diorites are characterized by enrichment in Mg#, Na, and light rare earth elements (LREE). They are poor in heavy rare earth elements (HREE) and high field strength elements (HFSE), being similar to adakite. The occurrence of the mantle peridotite xenoliths and the high-Mg feature for these intrusive rocks imply that their primary magma should be derived from the upper mantle. However, Sr-Nd isolopic compositions (Isr: 0.704 75 - 0. 707 72; εNd(t) values: -3. 95 to - 13. 30), depletion in HFSE, and the occurrence of the Archean inherited zircons suggest that crustal materials be involved in the derivation of primary magma The compositional difference between the diorites from the Tietonggou and Jinling intrusions could be attributed to magma sources and partial melting degrees. It is considered that the Early Cretaceous high-Mg diorites were formed by the mixed melting of the delaminated lithosphere (lithospheric mande+crust) and asthenosphere, based on their geochemistry, the mantle-derived xenoliths, and the Early Mesozoic lithospheric evolutionary history of the eastern North China craton.
2006, 31(1): 93-100.
Abstract:
The compositions of the whole rocks and trace elements of minerals in peridotites can reflect the characters of lithospheric mantle. The nature and evolution of the Cenozoic lithospheric mantle beneath Hannuoba, which is located on the north edge of the intra-North China orogenic belt, are mainly discussed based on the in situ, LAM-ICPMS detected trace element compositions of clinopyroxenes in the Hannuoba peridotitic xenoliths, combined with detailed petrographic and petrochemical studies. The Hannuoba lithospheric mantle was formed by different partial melting of the primitive mantle Most of the samples reflect a partial melting degree of lower than 5% with a few samples of 15% - 20%. Major element compositions of the whole rocks and geochemical compositions of clinopyroxenes reveal the coexistence of both fertile and depleted mantle underneath the Hannuoba region during the Cenozoic. This was probably caused by the asthenospheric mantle that replaced the aged craton mantle through erosion, intermingling and modification. Our conclusion is further supported by the existence of both carbonatitic magmatic material and silicate melt/fluid metasomatism as magnified by the trace element compositions of the clinopyroxenes from the Hannuoba lithospheric mantle.
The compositions of the whole rocks and trace elements of minerals in peridotites can reflect the characters of lithospheric mantle. The nature and evolution of the Cenozoic lithospheric mantle beneath Hannuoba, which is located on the north edge of the intra-North China orogenic belt, are mainly discussed based on the in situ, LAM-ICPMS detected trace element compositions of clinopyroxenes in the Hannuoba peridotitic xenoliths, combined with detailed petrographic and petrochemical studies. The Hannuoba lithospheric mantle was formed by different partial melting of the primitive mantle Most of the samples reflect a partial melting degree of lower than 5% with a few samples of 15% - 20%. Major element compositions of the whole rocks and geochemical compositions of clinopyroxenes reveal the coexistence of both fertile and depleted mantle underneath the Hannuoba region during the Cenozoic. This was probably caused by the asthenospheric mantle that replaced the aged craton mantle through erosion, intermingling and modification. Our conclusion is further supported by the existence of both carbonatitic magmatic material and silicate melt/fluid metasomatism as magnified by the trace element compositions of the clinopyroxenes from the Hannuoba lithospheric mantle.
2006, 31(1): 101-109.
Abstract:
The North Qilian orogenic belt in northwestern China is an elongate tectonic unit situated between the North China plate in the north and the Qaidam plate in the south. It is a result of the collision of two plates during the Caledonian stage. The Sunan area, from Baiquanmen to Bianmagou, is located in the western sector of the North Qilian mountains, crossing three tectonic units (back-arc basin, island-arc and trench subduction complex) and three ophiolite zones (Baiquanmen-Jiugequan, Dachadaban and Bianmagou). The Yingou Formation is distributed over the Sunan area, and consists of medium-basic volcanic rocks, volcanic clastic rocks interspersed with cherts, marbles, metamorphic mudstone (slate or schist) and sandstone. The geochemistry of the cherts shows that the cherts, coexisting with ophiolite from Bianmagou, Dachadaban, Jiugequan and Baiquanmen, all formed the background of the continental margin basin. The rare earth elements show that these cherts formed in the deep-water basin far from the continent, where terrestrial material had no obvious effect on them. It is inferred that North Qilian orogenic belt was located in the tectonic background of an archipelagic ocean where a wide multi-island ocean existed in the Ordovician.
The North Qilian orogenic belt in northwestern China is an elongate tectonic unit situated between the North China plate in the north and the Qaidam plate in the south. It is a result of the collision of two plates during the Caledonian stage. The Sunan area, from Baiquanmen to Bianmagou, is located in the western sector of the North Qilian mountains, crossing three tectonic units (back-arc basin, island-arc and trench subduction complex) and three ophiolite zones (Baiquanmen-Jiugequan, Dachadaban and Bianmagou). The Yingou Formation is distributed over the Sunan area, and consists of medium-basic volcanic rocks, volcanic clastic rocks interspersed with cherts, marbles, metamorphic mudstone (slate or schist) and sandstone. The geochemistry of the cherts shows that the cherts, coexisting with ophiolite from Bianmagou, Dachadaban, Jiugequan and Baiquanmen, all formed the background of the continental margin basin. The rare earth elements show that these cherts formed in the deep-water basin far from the continent, where terrestrial material had no obvious effect on them. It is inferred that North Qilian orogenic belt was located in the tectonic background of an archipelagic ocean where a wide multi-island ocean existed in the Ordovician.
2006, 31(1): 110-120.
Abstract:
There are different theories about the genesis and age of banded-augen (rapakivi) anatectic granitoids (charnock-ite), which outcrop extensively in Yunkai region, western Guangdong Province. Their petrochemistry, chronology, defor-mational and metamorphic structures were studied. The petrochemical features of most granitoids are: A/CNK > 1. 1, Cao/Na2O=0. 62 - 1. 61 (average 0. 94, > 0.3), Al2O3/TiO=16.6-60.6 (average 23. 68), loss high field strong elements Ta、Nb、Zr, strong peraluminous high-K calc-alkaline and calc-alkaline granitoids in a post-collisional tectonic enviroment of sub-duction-collision orogenic belt in an active-continental margin. The temperatures of charnockite and gneissic garnet-bearing biotite monzonitic granite are obviously higher than that of banded-augen (rapakivi) biotite monzonitic granite, and charnockite and gneissic garnet-bearing biotite monzonitic granite with the evolutional characterics of A-type granites. From banded-granite, augen (rapakivi) biotite monzonitic granite to charnockite and gneissic garnet-bearing biotite monzonitic granite, the forming ages are (465±10)Ma, (467±10)Ma, (435±11)Ma and (413±8)Ma respectively, and become younger. These results show that there were oceanic-continental subduction-collision and post-collisional extension-delamination-underplating between the Yangtze and Cathaysia plates during the Caledonian, and they experienced compressional uplift and extensional exhumation during the Indosinian. It provides important evidence of the oceanic-continental subduction-collision of the Yangtze plate downward to the Cathaysia plate during the Caledonian in South China.
There are different theories about the genesis and age of banded-augen (rapakivi) anatectic granitoids (charnock-ite), which outcrop extensively in Yunkai region, western Guangdong Province. Their petrochemistry, chronology, defor-mational and metamorphic structures were studied. The petrochemical features of most granitoids are: A/CNK > 1. 1, Cao/Na2O=0. 62 - 1. 61 (average 0. 94, > 0.3), Al2O3/TiO=16.6-60.6 (average 23. 68), loss high field strong elements Ta、Nb、Zr, strong peraluminous high-K calc-alkaline and calc-alkaline granitoids in a post-collisional tectonic enviroment of sub-duction-collision orogenic belt in an active-continental margin. The temperatures of charnockite and gneissic garnet-bearing biotite monzonitic granite are obviously higher than that of banded-augen (rapakivi) biotite monzonitic granite, and charnockite and gneissic garnet-bearing biotite monzonitic granite with the evolutional characterics of A-type granites. From banded-granite, augen (rapakivi) biotite monzonitic granite to charnockite and gneissic garnet-bearing biotite monzonitic granite, the forming ages are (465±10)Ma, (467±10)Ma, (435±11)Ma and (413±8)Ma respectively, and become younger. These results show that there were oceanic-continental subduction-collision and post-collisional extension-delamination-underplating between the Yangtze and Cathaysia plates during the Caledonian, and they experienced compressional uplift and extensional exhumation during the Indosinian. It provides important evidence of the oceanic-continental subduction-collision of the Yangtze plate downward to the Cathaysia plate during the Caledonian in South China.
2006, 31(1): 121-128.
Abstract:
This study presents the open and dehydration melting experiments on solid retrograde eclogite from Dongchonghe in Yingshan, Hubei Province. The sample contains hydraous hornblende and biotite. The experiments are at 0. 1 GPa and 750, 800, 850 and 900℃. The duration time for heating is 4 h. The initial melting commences from dehydration melting of hydrous minerals, and melts appear at 850℃. Melts are mainly in the three localized melting systems: they are the grain boundaries of symplectite, one side of sample on which is higher melting degree, and the grain boundaries and cracks of quartz. Because of different compositions of minerals in different localized melting systems, chemical compositions of melts range from basic to acid at the same temperature. And it runs to acid at the same localized melting system as increase of temperature. This study implies, at the same pressure, temperature and different compositions of minerals in different localized melting systems are the two main factors that influence compositions of melts. And it provides experimental evidences to understand dehydration melting behavior of eclogite and to contrast with other basic metamorphic rocks.
This study presents the open and dehydration melting experiments on solid retrograde eclogite from Dongchonghe in Yingshan, Hubei Province. The sample contains hydraous hornblende and biotite. The experiments are at 0. 1 GPa and 750, 800, 850 and 900℃. The duration time for heating is 4 h. The initial melting commences from dehydration melting of hydrous minerals, and melts appear at 850℃. Melts are mainly in the three localized melting systems: they are the grain boundaries of symplectite, one side of sample on which is higher melting degree, and the grain boundaries and cracks of quartz. Because of different compositions of minerals in different localized melting systems, chemical compositions of melts range from basic to acid at the same temperature. And it runs to acid at the same localized melting system as increase of temperature. This study implies, at the same pressure, temperature and different compositions of minerals in different localized melting systems are the two main factors that influence compositions of melts. And it provides experimental evidences to understand dehydration melting behavior of eclogite and to contrast with other basic metamorphic rocks.
2006, 31(1): 129-135.
Abstract:
The Furong tin orefield, located in southern Hunan, China, is a newly-discovered super-large tin orefield. In contrast to most other tin deposits associated with S-type granites, the Furong tin deposit is spatially and temporally associated with the Qitianling A-type granite. The 3He/4He ratios of fluid inclusions in sulfides from this orefield range from 0. 14 to 2. 95 Ra, which are lower than that of the mantle, and higher than that of the crust, indicating that the helium in ore-forming fluids from this orefield is a mixture of two end-member components, mantle-source He and crust-source He. This conclusion is in accordance with the S isotope, Sr isotope and other results, suggesting that there must be some mantle-source substance in mineralization of this orefield. The Qitianling granite belongs to A-type granite which formed in a post-orogenic extensional settings. The Furong tin orefield is located at the contact zone (outer or inner) or within of Qitianling granite, and its main tin-mineralization time is consistent with the intrusion time of Qitianling granite. The relationship reflects that their forming settings of geodynamics might both be related to the Mesozoic lithospheric extension in South China.
The Furong tin orefield, located in southern Hunan, China, is a newly-discovered super-large tin orefield. In contrast to most other tin deposits associated with S-type granites, the Furong tin deposit is spatially and temporally associated with the Qitianling A-type granite. The 3He/4He ratios of fluid inclusions in sulfides from this orefield range from 0. 14 to 2. 95 Ra, which are lower than that of the mantle, and higher than that of the crust, indicating that the helium in ore-forming fluids from this orefield is a mixture of two end-member components, mantle-source He and crust-source He. This conclusion is in accordance with the S isotope, Sr isotope and other results, suggesting that there must be some mantle-source substance in mineralization of this orefield. The Qitianling granite belongs to A-type granite which formed in a post-orogenic extensional settings. The Furong tin orefield is located at the contact zone (outer or inner) or within of Qitianling granite, and its main tin-mineralization time is consistent with the intrusion time of Qitianling granite. The relationship reflects that their forming settings of geodynamics might both be related to the Mesozoic lithospheric extension in South China.
2006, 31(1): 136-142.
Abstract:
Conodonts have received extensive attention in terms of their potential as geochemical tracers, due to their precise stratigraphic age and high REE concentration as well as their relative resistance to diagenetic alternation. The ratios of Sm/Nd of paleoseawater from the Cambrian to the Triassic in the northern of the Yangtze craton were determined using the Isotope Dilution Mass Spectrometry method. The samples exhibit a large variation in Sm/Nd ratio (0. 108-0. 348). It shows that the temporal variability in Sm/Nd ratios of the seawater from the Cambrian to the Triassic is characterized by strongly correlated eustatic changes of sea level and submergence history of old continental in the Three Gorges area, corresponding to the previously published sedimentological and geochemical studies on these areas. In the other hand, the preliminary conclusion revealed that the submarine hydrothermal system (mantle flux) and the Emeishan mantle plume with higher Sm/Nd ratios, which were existing in the subarea adjacent to the Three Gorges area during the Phanerozoic, may likely influence the Sm/Nd values of contemporaneous seawater. It allows us to speculate that the Srn/Nd curve of paleoseawater constructed by using conodonts has a potentially wide range of applications in studies relating to the paleoenvironmental and regional tectonic event.
Conodonts have received extensive attention in terms of their potential as geochemical tracers, due to their precise stratigraphic age and high REE concentration as well as their relative resistance to diagenetic alternation. The ratios of Sm/Nd of paleoseawater from the Cambrian to the Triassic in the northern of the Yangtze craton were determined using the Isotope Dilution Mass Spectrometry method. The samples exhibit a large variation in Sm/Nd ratio (0. 108-0. 348). It shows that the temporal variability in Sm/Nd ratios of the seawater from the Cambrian to the Triassic is characterized by strongly correlated eustatic changes of sea level and submergence history of old continental in the Three Gorges area, corresponding to the previously published sedimentological and geochemical studies on these areas. In the other hand, the preliminary conclusion revealed that the submarine hydrothermal system (mantle flux) and the Emeishan mantle plume with higher Sm/Nd ratios, which were existing in the subarea adjacent to the Three Gorges area during the Phanerozoic, may likely influence the Sm/Nd values of contemporaneous seawater. It allows us to speculate that the Srn/Nd curve of paleoseawater constructed by using conodonts has a potentially wide range of applications in studies relating to the paleoenvironmental and regional tectonic event.
