2018 Vol. 43, No. 1
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2018, 43(1): 82-98.
doi: 10.3799/dqkx.2018.005
Abstract:
The core of the Himalayan orogen consists of high-grade metamorphic rocks and leucogranites, forming a natural laboratory for studying crustal anatexis and granite origin during the collisional orogeny. Based on recent achievements of the related studies, the condition, type and P-T path of metamorphism, and mechanism, degree and melt composition of anataxis as well as metamorphic and anatectic timing and duration of high-grade metamorphic rocks in the orogenic core are discussed in this paper. The obtained evidence shows that the orogenic core experienced high-pressure granulite-facies to eclogite-facies metamorphism, with a clockwise-type P-T path characterized by increasing temperature and pressure prograde and early retrogression of near-isothermal decompression, and that the high-pressure rocks record a prolonged high-temperature metamorphic and anatectic process. The muscovite-and biotite-dehydration melting of meta-pelitic rocks during the prograde metamorphism resulted in formation of melts with highly variable chemical compositions. In addition, the formation time and geochemical feature of the Himalayan leucogranites are also summarized. Finally, it is concluded that the leucogranites were derived from the dehydration melting of thickened lower crust during the collisional orogeny.
The core of the Himalayan orogen consists of high-grade metamorphic rocks and leucogranites, forming a natural laboratory for studying crustal anatexis and granite origin during the collisional orogeny. Based on recent achievements of the related studies, the condition, type and P-T path of metamorphism, and mechanism, degree and melt composition of anataxis as well as metamorphic and anatectic timing and duration of high-grade metamorphic rocks in the orogenic core are discussed in this paper. The obtained evidence shows that the orogenic core experienced high-pressure granulite-facies to eclogite-facies metamorphism, with a clockwise-type P-T path characterized by increasing temperature and pressure prograde and early retrogression of near-isothermal decompression, and that the high-pressure rocks record a prolonged high-temperature metamorphic and anatectic process. The muscovite-and biotite-dehydration melting of meta-pelitic rocks during the prograde metamorphism resulted in formation of melts with highly variable chemical compositions. In addition, the formation time and geochemical feature of the Himalayan leucogranites are also summarized. Finally, it is concluded that the leucogranites were derived from the dehydration melting of thickened lower crust during the collisional orogeny.
2018, 43(1): 99-108.
doi: 10.3799/dqkx.2018.006
Abstract:
Relationship between metamorphic porphyroblasts and their matrices in the contact aureole of the concentric zoned Fangshan pluton, Beijing, combined with P-T patterns can be used as criteria to distinguish between magmatic diapirism and balloon expansion mechanisms for pluton emplacement in this study. It is found that, pelitic metamorphic rocks in the Xiamaling Formation in the southern part of the aureole display porphyroblast-matrix relationships reflecting intense plane-strain deformation combined with a nearly isobaric heating P-T trajectory towards the contact. Incorporating previously published estimates of final crystallization depths of four successively emplaced magmatic units, the results favor the balloon-inflation intrusion mechanism.
Relationship between metamorphic porphyroblasts and their matrices in the contact aureole of the concentric zoned Fangshan pluton, Beijing, combined with P-T patterns can be used as criteria to distinguish between magmatic diapirism and balloon expansion mechanisms for pluton emplacement in this study. It is found that, pelitic metamorphic rocks in the Xiamaling Formation in the southern part of the aureole display porphyroblast-matrix relationships reflecting intense plane-strain deformation combined with a nearly isobaric heating P-T trajectory towards the contact. Incorporating previously published estimates of final crystallization depths of four successively emplaced magmatic units, the results favor the balloon-inflation intrusion mechanism.
2018, 43(1): 109-126.
doi: 10.3799/dqkx.2018.007
Abstract:
Metamorphic rock series of the Guanghua Group occurs as a block in Archean TTG gneiss in Tonghua area of Jilin Province, whose forming age remains controversial for a long time. Using SHRIMP and LA-ICPMS zircon U-Pb dating techniques, representative rock samples from the Guanghua Group were analyzed, including garnet biotite schist, biotite monzo-gneiss, garnet hornblende schist and potassium granite intruding into the bottom of the Guanghua Group. The results show that four metamorphic rock samples have similar detrital zircon ages (concordant ages are between 2.6 Ga and 2.5 Ga) though they were collected from different strata. Most detrital zircons from four samples yeild concordant mean ages of 2 529±7 Ma, 2 568±4 Ma, 2 526±11 Ma and 2 530±6 Ma, respectively, suggesting a derivation from Neoarchean magmatic unit. Some zircons from metamorphic rocks record 3 stages metamorphic age (2 525±10 Ma, 1 926±40 Ma and 1 878±16 Ma), indicating that the Guanghua Group experienced both Neoarchean and Paleoproterozoic thermo-tectonic events. Zircons from the potassium granite yield a concordant mean age of 2 154±7 Ma, which is interpreted as intruding time of the ploton. The potassium granite has not suffered any deformation, suggesting a intracontinental rift tectonic setting.
Metamorphic rock series of the Guanghua Group occurs as a block in Archean TTG gneiss in Tonghua area of Jilin Province, whose forming age remains controversial for a long time. Using SHRIMP and LA-ICPMS zircon U-Pb dating techniques, representative rock samples from the Guanghua Group were analyzed, including garnet biotite schist, biotite monzo-gneiss, garnet hornblende schist and potassium granite intruding into the bottom of the Guanghua Group. The results show that four metamorphic rock samples have similar detrital zircon ages (concordant ages are between 2.6 Ga and 2.5 Ga) though they were collected from different strata. Most detrital zircons from four samples yeild concordant mean ages of 2 529±7 Ma, 2 568±4 Ma, 2 526±11 Ma and 2 530±6 Ma, respectively, suggesting a derivation from Neoarchean magmatic unit. Some zircons from metamorphic rocks record 3 stages metamorphic age (2 525±10 Ma, 1 926±40 Ma and 1 878±16 Ma), indicating that the Guanghua Group experienced both Neoarchean and Paleoproterozoic thermo-tectonic events. Zircons from the potassium granite yield a concordant mean age of 2 154±7 Ma, which is interpreted as intruding time of the ploton. The potassium granite has not suffered any deformation, suggesting a intracontinental rift tectonic setting.
2018, 43(1): 127-149.
doi: 10.3799/dqkx.2018.008
Abstract:
Granulite is the most important metamorphic rock type for the study of crust evolution. As a common accessary mineral in granulite, rutile provides a new research window for the evolution of the continental crust. Based on fundamental information for granulite-facies rutile (microtextures, trace elements, and cation substitution) and related trace elements in major rock-forming minerals in the crust, this paper discusses the potential trace element behaviors and diffusion effects during granulite-facies metamorphism, which have received much less attention than eclogite ones during the last decades. The Zr contents in granulite-facies rutile may reflect metamorphic temperatures of different metamorphic stages; however, the formation of secondary zircon and/or ilmenite would significantly affect the Zr contents of former rutile via diffusion effects. The isolated rutile that is not equilibrated with zircon and quartz (e.g., rutile boundaries close to the secondary zircon and ilmenite, orientated rutile needles in garnet) cannot be used to calculate temperatures by Zr-in-rutile thermometer. The Nb, Ta, Cr and V contents of granulite-facies rutile are largely influenced by bulk-rock composition and by the formation and breakdown of biotite, ilmenite and kyanite. In addition, the Fe contents of rutile adjacent to garnet may be significantly modified due to the fast diffusion rate of Fe. Understanding the behaviors of rutile trace elements during granulite-facies metamorphism can provide important mineral constraints for the metamorphic evolution and dynamic processes of the continental crust.
Granulite is the most important metamorphic rock type for the study of crust evolution. As a common accessary mineral in granulite, rutile provides a new research window for the evolution of the continental crust. Based on fundamental information for granulite-facies rutile (microtextures, trace elements, and cation substitution) and related trace elements in major rock-forming minerals in the crust, this paper discusses the potential trace element behaviors and diffusion effects during granulite-facies metamorphism, which have received much less attention than eclogite ones during the last decades. The Zr contents in granulite-facies rutile may reflect metamorphic temperatures of different metamorphic stages; however, the formation of secondary zircon and/or ilmenite would significantly affect the Zr contents of former rutile via diffusion effects. The isolated rutile that is not equilibrated with zircon and quartz (e.g., rutile boundaries close to the secondary zircon and ilmenite, orientated rutile needles in garnet) cannot be used to calculate temperatures by Zr-in-rutile thermometer. The Nb, Ta, Cr and V contents of granulite-facies rutile are largely influenced by bulk-rock composition and by the formation and breakdown of biotite, ilmenite and kyanite. In addition, the Fe contents of rutile adjacent to garnet may be significantly modified due to the fast diffusion rate of Fe. Understanding the behaviors of rutile trace elements during granulite-facies metamorphism can provide important mineral constraints for the metamorphic evolution and dynamic processes of the continental crust.
2018, 43(1): 150-163.
doi: 10.3799/dqkx.2018.009
Abstract:
Amphibolite is one of the common metabasic rock types exposed in the the UHP metamorphic belt of southwestern Tianshan. It is found in the field relationship and mineral reaction textures that most of such amphibolites are retrograded after eclogites or blueschists to various extents at albite-epidote amphibolite facies conditions. However, petrological studies are not systematic and sufficient for those with equilibrated textures from homogeneous blocks. In this paper, it is presented a detailed study of petrography, mineral chemistry and thermodynamic modelling on some albite-free garnet amphibolites in the UHP unit at the lower part of the Habutengsu valley. Results show that they largely consist of green amphibole (Ca-or Na-Ca-amphibole), epidote (or zoisite) and garnet (80% to 90% total volume), substantially different from the equivalents that retrograded from UHP eclogites contain albite porphyroblast. Though these garnet amphibolites are similar in bulk compositions, with high CaO and low Na2O and high Mg content, based on contrasting differences in terms of texture, structure and mineral assemblage, two types are divided herein. Type Ⅰ garnet amphibolite is quartz-free and relict or inclusion assemblage of Grt+Omp+Lws+Gln+Rt indicating peak lawsonite eclogite facies conditions. Rutile is the only Ti-rich phase as inclusion or in the matrix. Type Ⅱ garnet amphibolite is strongly foliated shown by oriented green amphibole, epidote, chlorite and banded quartz aggregates. Garnet is characterized by bimodal grain size and the larger ones have lower Ca and Mn. No HP indicator such as omphacite or sodic amphibole has been detected. Ti-rich phase is dominated by titanite and rare rutile and ilmenite are only found in some garnets. Generally, garnet compositions are distinguishable between the two types of amphibolites, Type Ⅰ with higher Ca but lower Mg. P-T pseudosection calculations show that peak metamorphic conditions of both types are 480 to 520℃, 30 to 33 kbar, in the UHP field akin to other eclogites in the lower reach of the Habutengsu River and adjacent areas to the west. This indicates a coherent UHP unit extending tens of kilometers along the southern Central Tianshan fault, rather than tectonic mélange from a fossil subduction channel.
Amphibolite is one of the common metabasic rock types exposed in the the UHP metamorphic belt of southwestern Tianshan. It is found in the field relationship and mineral reaction textures that most of such amphibolites are retrograded after eclogites or blueschists to various extents at albite-epidote amphibolite facies conditions. However, petrological studies are not systematic and sufficient for those with equilibrated textures from homogeneous blocks. In this paper, it is presented a detailed study of petrography, mineral chemistry and thermodynamic modelling on some albite-free garnet amphibolites in the UHP unit at the lower part of the Habutengsu valley. Results show that they largely consist of green amphibole (Ca-or Na-Ca-amphibole), epidote (or zoisite) and garnet (80% to 90% total volume), substantially different from the equivalents that retrograded from UHP eclogites contain albite porphyroblast. Though these garnet amphibolites are similar in bulk compositions, with high CaO and low Na2O and high Mg content, based on contrasting differences in terms of texture, structure and mineral assemblage, two types are divided herein. Type Ⅰ garnet amphibolite is quartz-free and relict or inclusion assemblage of Grt+Omp+Lws+Gln+Rt indicating peak lawsonite eclogite facies conditions. Rutile is the only Ti-rich phase as inclusion or in the matrix. Type Ⅱ garnet amphibolite is strongly foliated shown by oriented green amphibole, epidote, chlorite and banded quartz aggregates. Garnet is characterized by bimodal grain size and the larger ones have lower Ca and Mn. No HP indicator such as omphacite or sodic amphibole has been detected. Ti-rich phase is dominated by titanite and rare rutile and ilmenite are only found in some garnets. Generally, garnet compositions are distinguishable between the two types of amphibolites, Type Ⅰ with higher Ca but lower Mg. P-T pseudosection calculations show that peak metamorphic conditions of both types are 480 to 520℃, 30 to 33 kbar, in the UHP field akin to other eclogites in the lower reach of the Habutengsu River and adjacent areas to the west. This indicates a coherent UHP unit extending tens of kilometers along the southern Central Tianshan fault, rather than tectonic mélange from a fossil subduction channel.
2018, 43(1): 1-23.
doi: 10.3799/dqkx.2018.001
Abstract:
For summarizing the historical experiences of the study of metamorphic rocks in China, this paper presents a review on the advances from metamorphic petrology to metamorphic geology in the past nearly 70 years. These advances are discussed in three stages and eight aspects. On the basis of reviewing a huge number of literatures, we propose that our researches on ultrahigh pressure metamorphism, Precambrian metamorphic geology, metamorphic chronology, and metamorphic phase modeling are internationally advanced, and researches on blueschist, metamorphic fluids and metamorphic chemical kinetics are synchronized with the world, but the researches on very low-grade metamorphism and some other fields have not attracted enough attention, having large gaps with the international advanced level. It is concluded that the study on metamorphic rocks in China has changed from metamorphic petrology to metamorphic geology, that is, from mono-disciplinary petrology study to multi-disciplinary researches of metamorphic rocks, including metamorphic minerals, geochemistry, isotopic geology, tectonic geology and some other disciplines. Although we have some location advantages in the study of metamorphic rocks, independent innovative researches are still required to turn the location advantages into academic advantages. In addition, as the development of various analytical experimental techniques has accelerated scientific researches, metamorphic geology will advance greatly with the advent of new technologies and the application of big data technology.
For summarizing the historical experiences of the study of metamorphic rocks in China, this paper presents a review on the advances from metamorphic petrology to metamorphic geology in the past nearly 70 years. These advances are discussed in three stages and eight aspects. On the basis of reviewing a huge number of literatures, we propose that our researches on ultrahigh pressure metamorphism, Precambrian metamorphic geology, metamorphic chronology, and metamorphic phase modeling are internationally advanced, and researches on blueschist, metamorphic fluids and metamorphic chemical kinetics are synchronized with the world, but the researches on very low-grade metamorphism and some other fields have not attracted enough attention, having large gaps with the international advanced level. It is concluded that the study on metamorphic rocks in China has changed from metamorphic petrology to metamorphic geology, that is, from mono-disciplinary petrology study to multi-disciplinary researches of metamorphic rocks, including metamorphic minerals, geochemistry, isotopic geology, tectonic geology and some other disciplines. Although we have some location advantages in the study of metamorphic rocks, independent innovative researches are still required to turn the location advantages into academic advantages. In addition, as the development of various analytical experimental techniques has accelerated scientific researches, metamorphic geology will advance greatly with the advent of new technologies and the application of big data technology.
2018, 43(1): 24-43.
doi: 10.3799/dqkx.2018.002
Abstract:
The tempo-spatial distribution and tectonic attribute of the Paleoproterozoic orogens of the North China Craton (NCC) are strongly controversial. One model argues that three times of continental collision occurred sequentially during the period of 1.85-1.95 Ga in the NCC, resulted in the formation of three Paleoproterozoic orogens, i.e. the Khondalite belt, Jiao-Liao-Ji belt and the Trans-North China orogen (TNCO). The other model contests that the above three orogens may have experienced the same long-lived evolutional history within the period of 1.80-1.98 Ga, suggesting particular hot subduction and collision in Paleoproterozoic. Summarizing the recent advances of the study on metamorphism in the Wutai-Hengshan region, this paper presents the time and tectonic attribute of the Paleoproterozoic orogeny. The main litho-structural units in the Wutai-Hengshan region include the Hengshan complex, Wutai complex and Hutuo Group. The Hengshan and Wutai complexes are recognized to have undergone two phases of metamorphism. The first phase was identified to be a medium-pressure type, which was attributed to resulting from a crust thickening orogeny related to the closure of back-arc basins due to continental collision. A progressive metamorphic series can be revealed, including, from south to north, the low-amphibolite facies of the lower Wutai Group and the southern part of the south Hengshan complex, the high-amphibolite facies of the northern part of the south Hengshan complex and the high-pressure granulite facies of the north Hengshan complex, with their pressure peak conditions corresponding to a geothermal gradient of~15℃/km. With increasing in metamorphic grade, the peak ages from metamorphic zircons show a decreasing trend, which are~1.95 Ga, ~1.92 Ga and~1.85 Ga for the low-amphibolite, high-amphibolite and granulite facies rocks respectively. This is because zircon growth during metamorphism depends on the behaviors of fluids and melts, which results in that metamorphic zircons tend to record the time of the peak stages for metamorphism under subsolidus conditions, but to date the retrograde stage with melt crystallization during metamorphism under suprasolidus conditions. As a result, the pressure peak stage of the medium-pressure type metamorphism is determined to be~1.95 Ga, also equivalent to the peak stage of the crust-thickening orogeny. Due to gravity isostasy in the thickening crust region, metamorphic rocks may exhume from deep crust to middle crust, displaying as the post-peak isothermal decompression (ITD) to 0.5-0.7 GPa in P-T paths, and suggested from the 'white eye socket' textures formed by the decomposition of peak garnet under fluid-absent conditions. The metamorphic zircon ages of 1.92 Ga from the high-amphibolite facies rocks of the northern part of the south Hengshan complex were interpreted to represent the decompression of medium-pressure rocks, also constraining the termination of the first-phase orogeny. The second phase of metamorphism is inferred to be a low-pressure type, resulted from deformation within-plate, and displaying as the formation of equilibrium assemblages that overprinted the exhumed medium-pressure rocks triggered by compressional shearing and fluid infiltration. The Zhujiafang shearing belt could be a high-strain zone produced in this within-plate deformation, which also records a clockwise P-T path, but indicating limited crustal thickening. The second phase of metamorphism and deformation was constrained to occur mainly at~1.85 Ga. The sinistral strike-slip of the Zhujiafang shearing belt may have resulted in the uplifting of the north Hengshan granulite terranes. Two phases of metamorphism and deformation were proposed to have occurred within the period of 1.80-1.96 Ga in the Wutai-Hengshan region, which could be significant for discussing the Paleoproterozoic orogenic evolution from the other domains in the NCC.
The tempo-spatial distribution and tectonic attribute of the Paleoproterozoic orogens of the North China Craton (NCC) are strongly controversial. One model argues that three times of continental collision occurred sequentially during the period of 1.85-1.95 Ga in the NCC, resulted in the formation of three Paleoproterozoic orogens, i.e. the Khondalite belt, Jiao-Liao-Ji belt and the Trans-North China orogen (TNCO). The other model contests that the above three orogens may have experienced the same long-lived evolutional history within the period of 1.80-1.98 Ga, suggesting particular hot subduction and collision in Paleoproterozoic. Summarizing the recent advances of the study on metamorphism in the Wutai-Hengshan region, this paper presents the time and tectonic attribute of the Paleoproterozoic orogeny. The main litho-structural units in the Wutai-Hengshan region include the Hengshan complex, Wutai complex and Hutuo Group. The Hengshan and Wutai complexes are recognized to have undergone two phases of metamorphism. The first phase was identified to be a medium-pressure type, which was attributed to resulting from a crust thickening orogeny related to the closure of back-arc basins due to continental collision. A progressive metamorphic series can be revealed, including, from south to north, the low-amphibolite facies of the lower Wutai Group and the southern part of the south Hengshan complex, the high-amphibolite facies of the northern part of the south Hengshan complex and the high-pressure granulite facies of the north Hengshan complex, with their pressure peak conditions corresponding to a geothermal gradient of~15℃/km. With increasing in metamorphic grade, the peak ages from metamorphic zircons show a decreasing trend, which are~1.95 Ga, ~1.92 Ga and~1.85 Ga for the low-amphibolite, high-amphibolite and granulite facies rocks respectively. This is because zircon growth during metamorphism depends on the behaviors of fluids and melts, which results in that metamorphic zircons tend to record the time of the peak stages for metamorphism under subsolidus conditions, but to date the retrograde stage with melt crystallization during metamorphism under suprasolidus conditions. As a result, the pressure peak stage of the medium-pressure type metamorphism is determined to be~1.95 Ga, also equivalent to the peak stage of the crust-thickening orogeny. Due to gravity isostasy in the thickening crust region, metamorphic rocks may exhume from deep crust to middle crust, displaying as the post-peak isothermal decompression (ITD) to 0.5-0.7 GPa in P-T paths, and suggested from the 'white eye socket' textures formed by the decomposition of peak garnet under fluid-absent conditions. The metamorphic zircon ages of 1.92 Ga from the high-amphibolite facies rocks of the northern part of the south Hengshan complex were interpreted to represent the decompression of medium-pressure rocks, also constraining the termination of the first-phase orogeny. The second phase of metamorphism is inferred to be a low-pressure type, resulted from deformation within-plate, and displaying as the formation of equilibrium assemblages that overprinted the exhumed medium-pressure rocks triggered by compressional shearing and fluid infiltration. The Zhujiafang shearing belt could be a high-strain zone produced in this within-plate deformation, which also records a clockwise P-T path, but indicating limited crustal thickening. The second phase of metamorphism and deformation was constrained to occur mainly at~1.85 Ga. The sinistral strike-slip of the Zhujiafang shearing belt may have resulted in the uplifting of the north Hengshan granulite terranes. Two phases of metamorphism and deformation were proposed to have occurred within the period of 1.80-1.96 Ga in the Wutai-Hengshan region, which could be significant for discussing the Paleoproterozoic orogenic evolution from the other domains in the NCC.
2018, 43(1): 44-56.
doi: 10.3799/dqkx.2018.003
Abstract:
The eastern Hebei-western Liaoning provinces are the largest exposure regions of Archean basement rocks in the northern part of North China Craton. Comprehensive studies including detailed mapping of lithological assemblages reveal that the Archean basement rocks in the study regions can be subdivided into eight different assemblages, i.e., 2.64-2.60 Ga MORB-type metabasaltic rocks; 2.61-2.52 Ga metamorphosed tholeiitic to calc-alkaline volcanic rocks; 2.52-2.50 Ga calc-alkaline volcanic rocks with low metamorphic grade; 2.54-2.50 Ga tonalitic-trondhjemitic-granodioritic gneisses; 2.54-2.51 Ga dioritic-quartz dioritic-tonalitic-trondhjemitic-granodioritic gneisses; 2.54-2.51 Ga enderbites and charnockites; 2.57-2.52 Ga dioritic-quartz monzodioritic-granodioritic-monzogranitic gneisses; and 2.53-2.51 Ga weakly gneissic to massive monzogranitic to syenogranitic plutonic rocks. The above lithological assemblages show apparently banded spatial distribution patterns from Fuxin area of western Liaoning in the northeast to the Malanyu area of Zunhua in the southwest. Metamorphic studies indicate that high pressure granulites outcropped in the Qinglong-Shangying-Saheqiao areas record typical isothermal decompression (ITD)-type metamorphic P-T-t paths, which are associated with the thrusting structures from NNW-NW to SSE-SE. In contrast, isobaric cooling (IBC)-type metamorphic P-T-t paths are recorded by the high-temperature granulites from Santunying-Taipingzhai areas, which were accompanied by extensional diapiric structures in the Jinzhou-Xingcheng-Anziling-Jiehekou-Taipingzhai-Santunying regions. Accordingly, it is suggested that the Late Archean tectonothermal events in the eastern Hebei-western Liaoning provinces could have been generated under a typical Archean hot orogenic belt, evolving from subduction and back-arc extension to the final collision.
The eastern Hebei-western Liaoning provinces are the largest exposure regions of Archean basement rocks in the northern part of North China Craton. Comprehensive studies including detailed mapping of lithological assemblages reveal that the Archean basement rocks in the study regions can be subdivided into eight different assemblages, i.e., 2.64-2.60 Ga MORB-type metabasaltic rocks; 2.61-2.52 Ga metamorphosed tholeiitic to calc-alkaline volcanic rocks; 2.52-2.50 Ga calc-alkaline volcanic rocks with low metamorphic grade; 2.54-2.50 Ga tonalitic-trondhjemitic-granodioritic gneisses; 2.54-2.51 Ga dioritic-quartz dioritic-tonalitic-trondhjemitic-granodioritic gneisses; 2.54-2.51 Ga enderbites and charnockites; 2.57-2.52 Ga dioritic-quartz monzodioritic-granodioritic-monzogranitic gneisses; and 2.53-2.51 Ga weakly gneissic to massive monzogranitic to syenogranitic plutonic rocks. The above lithological assemblages show apparently banded spatial distribution patterns from Fuxin area of western Liaoning in the northeast to the Malanyu area of Zunhua in the southwest. Metamorphic studies indicate that high pressure granulites outcropped in the Qinglong-Shangying-Saheqiao areas record typical isothermal decompression (ITD)-type metamorphic P-T-t paths, which are associated with the thrusting structures from NNW-NW to SSE-SE. In contrast, isobaric cooling (IBC)-type metamorphic P-T-t paths are recorded by the high-temperature granulites from Santunying-Taipingzhai areas, which were accompanied by extensional diapiric structures in the Jinzhou-Xingcheng-Anziling-Jiehekou-Taipingzhai-Santunying regions. Accordingly, it is suggested that the Late Archean tectonothermal events in the eastern Hebei-western Liaoning provinces could have been generated under a typical Archean hot orogenic belt, evolving from subduction and back-arc extension to the final collision.
2018, 43(1): 57-81.
doi: 10.3799/dqkx.2018.004
Abstract:
Anshan-Benxi is one of the most important BIF concentration areas in the North China Craton, the BIF occurred in the Anshan Group supracrustal rocks. Using SHRIMP U-Pb zircon dating technique, 12 supracrustal samples of the Anshan Group were analyzed. Most samples present 2.50-2.55 Ga zircon ages and 2.7-3.5 Ga detrital or xenocrystal zircons are observed in some samples. Combined with previous studies, main conclusions can be drawn as follows. (1) The widespread BIF-bearing Anshan Group supracrustal rocks in the Anshan-Benxi area were deposited during the Late Neoarchean. (2) The Anshan Group supracrustal rocks were formed over a continental basement. This study confirms that Anshan-Benxi is an important conponent of the huge Late Neoarchean BIF belt along the western margin of the eastern ancient terrane, and stable tectonic environment seems to be a key factor for the formation of huge-scale BIF ore deposit.
Anshan-Benxi is one of the most important BIF concentration areas in the North China Craton, the BIF occurred in the Anshan Group supracrustal rocks. Using SHRIMP U-Pb zircon dating technique, 12 supracrustal samples of the Anshan Group were analyzed. Most samples present 2.50-2.55 Ga zircon ages and 2.7-3.5 Ga detrital or xenocrystal zircons are observed in some samples. Combined with previous studies, main conclusions can be drawn as follows. (1) The widespread BIF-bearing Anshan Group supracrustal rocks in the Anshan-Benxi area were deposited during the Late Neoarchean. (2) The Anshan Group supracrustal rocks were formed over a continental basement. This study confirms that Anshan-Benxi is an important conponent of the huge Late Neoarchean BIF belt along the western margin of the eastern ancient terrane, and stable tectonic environment seems to be a key factor for the formation of huge-scale BIF ore deposit.
2018, 43(1): 164-175.
doi: 10.3799/dqkx.2018.010
Abstract:
The temperature and pressure of eclogite facies metamorphic rocks are very important to understand the formation and evolution of HP-UHP metamorphic belt. However, the pressures calculated by the traditional thermobarometer that involves omphacite are generally lower than those of the phase equilibrium modeling, for eclogites from the UHP metamorphic belt of the Southwest Tianshan. Petrographic and mineral chemical analyses on omphacite in eclogites and HP veins are conducted, inspired by the study ofZhang et al.(2017) . The results show that the omphacite is typically zoned. From core to rim, Fe3+ content continuously decreases, and Al content increases in response; the decrease in Fe3+/Al ratio corresponds to a general decline in the aegirine content and increase in the jadeite content. Phase equilibrium modeling shows that the omphacite rim with the highest jadeite content grew during decompression after the pressure peak. Thus, the maximum in jadeite component does not necessarily represents peak pressure, and the conventional thermobarometers should be used with caution, especially for the low-temperature assemblages with complicated zonation patterns.
The temperature and pressure of eclogite facies metamorphic rocks are very important to understand the formation and evolution of HP-UHP metamorphic belt. However, the pressures calculated by the traditional thermobarometer that involves omphacite are generally lower than those of the phase equilibrium modeling, for eclogites from the UHP metamorphic belt of the Southwest Tianshan. Petrographic and mineral chemical analyses on omphacite in eclogites and HP veins are conducted, inspired by the study of
2018, 43(1): 176-198.
doi: 10.3799/dqkx.2018.011
Abstract:
The Moerdaoga area is situated in the central-northern Erguna block, where a set of low-medium grade metamorphic rocks of Jiageda Formation develops, such as phyllites, mica schists and metamorphic sandstones. However, the origin, source and sedimentary environment of the protolith are not clear yet. The results of the major and trace elements analysis and LA-ICP-MS zircon U-Pb dating in this study show that phyllite and mica schists are enriched in Al and K and metamorphic sandstones are enriched in Si. These three groups exhibit similarity in trace element geochemistry, with strong enrichment in light REEs and negative Eu anomalies. The primitive mantle-normalized trace element pattern shows strong depletion in Sr. Furthermore, Nb, Ta, P, Ti are relatively depleted, and most high field-strength elements are enriched. The protoliths of the studied samples are mainly sandy sedimentary rocks with a small amount of muddy sedimentary rocks. The sediments show low maturity with moderate chemical weathering. The uppercrust felsic sources with possible old sediment component make a contribution to their provenance. The tectonic discrimination diagrams show that the protoliths were deposited in an active continental margin setting and had a large number of clastic rocks deposited in continental island arc setting. Most detrital zircons are of magmatic origin. The detrital zircon ages show great variations, yet mainly concentrated in Neoproterozoic (1.0-0.8 Ga) and Paleoproterozoic (2.0-1.8 Ga), which suggests that they provided the main sedimentary materials.
The Moerdaoga area is situated in the central-northern Erguna block, where a set of low-medium grade metamorphic rocks of Jiageda Formation develops, such as phyllites, mica schists and metamorphic sandstones. However, the origin, source and sedimentary environment of the protolith are not clear yet. The results of the major and trace elements analysis and LA-ICP-MS zircon U-Pb dating in this study show that phyllite and mica schists are enriched in Al and K and metamorphic sandstones are enriched in Si. These three groups exhibit similarity in trace element geochemistry, with strong enrichment in light REEs and negative Eu anomalies. The primitive mantle-normalized trace element pattern shows strong depletion in Sr. Furthermore, Nb, Ta, P, Ti are relatively depleted, and most high field-strength elements are enriched. The protoliths of the studied samples are mainly sandy sedimentary rocks with a small amount of muddy sedimentary rocks. The sediments show low maturity with moderate chemical weathering. The uppercrust felsic sources with possible old sediment component make a contribution to their provenance. The tectonic discrimination diagrams show that the protoliths were deposited in an active continental margin setting and had a large number of clastic rocks deposited in continental island arc setting. Most detrital zircons are of magmatic origin. The detrital zircon ages show great variations, yet mainly concentrated in Neoproterozoic (1.0-0.8 Ga) and Paleoproterozoic (2.0-1.8 Ga), which suggests that they provided the main sedimentary materials.
2018, 43(1): 199-219.
doi: 10.3799/dqkx.2018.012
Abstract:
There is a set of garnet amphibolite monzogneiss which developed typical garnet "white eye socket" structure from the Badu Group in Suichang-Dazhe region, southwestern Zhejiang Province. However, its genesis and metamorphic evolution remain unclear. In this study, mineral X-ray microprobe microanalysis, LA-ICP-MS zircon U-Pb dating and whole-rock major and trace element analyses were carried out to address these questions. Three stages of mineral assemblages have been identified:the prograde metamorphic assemblages (M1) are mineral inclusions garnet+amphibole+plagioclase+quartz (Grt1+Amp1+Pl1+Q) preserved in the garnet porphyroblasts, the peak metamorphic assemblages (M2) are represented by the "garnet-mantles" and the matrix minerals garnet+K-feldspar+plagioclase+biotite+quartz (Grt2+Kf2+Pl2+Bt2+Q), whereas the retrograde assemblages (M3) are composed of amphibole+plagioclase+biotite+ilmenite (Amp3+Pl3+Bt3+Ilm) symplectites rimming the garnet porphyroblasts. Thermobarometric computation and phase equilibrium modeling show that the metamorphic conditions of the garnet amphibolite monzogneiss are 600-700℃/0.60-0.65 GPa for the M1 stage, 800-820℃/0.94-0.96 GPa for the metamorphic peak M2 stage, and 550-700℃/0.56-0.71 GPa for the retrograde M3 stage, respectively. Metamorphic grade reached granulite facies and metamorphism PTt paths are characterized by clockwise pattern involving the nearly isothermal decompression (ITD) segments, indicating crustal thickening and rapid exhumation process. Petrogeochemistry shows that the protolith of monzogneiss is metaluminous A-type granite with some mantle-derived components which formed in continental crust extensional environment during post orogeny. LA-ICP-MS zircon U-Pb dating indicates two-stage age characteristics, in which the diagenetic age of 1.83-1.85 Ga suggests the protolith of monzogneiss formed in the Paleoproterozoic and the metamorphic age of 220-230 Ma shows the reworking of Indosinian metamorphism to Paleoproterozoic granite. It is the response to the subduction of Indo Pacific plate into South China plate during Indosinian, which provides new evidence for Indosinian orogeny in the southwestern Zhejiang.
There is a set of garnet amphibolite monzogneiss which developed typical garnet "white eye socket" structure from the Badu Group in Suichang-Dazhe region, southwestern Zhejiang Province. However, its genesis and metamorphic evolution remain unclear. In this study, mineral X-ray microprobe microanalysis, LA-ICP-MS zircon U-Pb dating and whole-rock major and trace element analyses were carried out to address these questions. Three stages of mineral assemblages have been identified:the prograde metamorphic assemblages (M1) are mineral inclusions garnet+amphibole+plagioclase+quartz (Grt1+Amp1+Pl1+Q) preserved in the garnet porphyroblasts, the peak metamorphic assemblages (M2) are represented by the "garnet-mantles" and the matrix minerals garnet+K-feldspar+plagioclase+biotite+quartz (Grt2+Kf2+Pl2+Bt2+Q), whereas the retrograde assemblages (M3) are composed of amphibole+plagioclase+biotite+ilmenite (Amp3+Pl3+Bt3+Ilm) symplectites rimming the garnet porphyroblasts. Thermobarometric computation and phase equilibrium modeling show that the metamorphic conditions of the garnet amphibolite monzogneiss are 600-700℃/0.60-0.65 GPa for the M1 stage, 800-820℃/0.94-0.96 GPa for the metamorphic peak M2 stage, and 550-700℃/0.56-0.71 GPa for the retrograde M3 stage, respectively. Metamorphic grade reached granulite facies and metamorphism PTt paths are characterized by clockwise pattern involving the nearly isothermal decompression (ITD) segments, indicating crustal thickening and rapid exhumation process. Petrogeochemistry shows that the protolith of monzogneiss is metaluminous A-type granite with some mantle-derived components which formed in continental crust extensional environment during post orogeny. LA-ICP-MS zircon U-Pb dating indicates two-stage age characteristics, in which the diagenetic age of 1.83-1.85 Ga suggests the protolith of monzogneiss formed in the Paleoproterozoic and the metamorphic age of 220-230 Ma shows the reworking of Indosinian metamorphism to Paleoproterozoic granite. It is the response to the subduction of Indo Pacific plate into South China plate during Indosinian, which provides new evidence for Indosinian orogeny in the southwestern Zhejiang.
2018, 43(1): 220-235.
doi: 10.3799/dqkx.2018.013
Abstract:
Dinggye is located in the central part of the Greater Himalayan crystalline complex (GHC) in southern Tibet. It is essential to investigate the metamorphic P-T path of granulite in this area to better understand the collision and uplifting process of the Tibetan plateau. The petrological study of the high-pressure mafic granulite (retrograded eclogite) from the region indicates four stages:(1) peak eclogite facies mineral assemblage (M1) consists of garnet (core)+omphacite (psedomorph)+quartz+rutile; (2) high-pressure granulite facies mineral assemblage (M2) comprises garnet (mantle)+clinopyroxene+plagioclase+ilmenite+amphibole+biotite; (3) medium-pressure granulite facies assemblage (M3) is composed of garnet (rim)+orthopyroxene+plagioclase+biotite; (4) amphibolite facies mineral assemblage (M4) consists of amphibole+plagioclase. Using the THERMOCALC program, the thermodynamic modeling in the NCFMASHTO system has been undertaken for the high-pressure mafic granulite. Combined with the conventional thermobarometers and the average P-T estimates, the P-T conditions of the different metamorphic stages are estimated to be 786-826℃, 0.78-0.96 GPa (M2); 798-850℃, 0.71-0.75 GPa (M3); and 610-666℃, 0.51-0.60 GPa (M4), respectively, indicating a post-peak clockwise P-T path characterized by nearly isothermal decompression. Combined with geological data available, we propose that the high-pressure mafic granulite (retrograded eclogite) of the Dinggye formed during the Himalayan collisional orogeny, and underwent a post-peak tectonic uplift process of nearly isothermal decompression.
Dinggye is located in the central part of the Greater Himalayan crystalline complex (GHC) in southern Tibet. It is essential to investigate the metamorphic P-T path of granulite in this area to better understand the collision and uplifting process of the Tibetan plateau. The petrological study of the high-pressure mafic granulite (retrograded eclogite) from the region indicates four stages:(1) peak eclogite facies mineral assemblage (M1) consists of garnet (core)+omphacite (psedomorph)+quartz+rutile; (2) high-pressure granulite facies mineral assemblage (M2) comprises garnet (mantle)+clinopyroxene+plagioclase+ilmenite+amphibole+biotite; (3) medium-pressure granulite facies assemblage (M3) is composed of garnet (rim)+orthopyroxene+plagioclase+biotite; (4) amphibolite facies mineral assemblage (M4) consists of amphibole+plagioclase. Using the THERMOCALC program, the thermodynamic modeling in the NCFMASHTO system has been undertaken for the high-pressure mafic granulite. Combined with the conventional thermobarometers and the average P-T estimates, the P-T conditions of the different metamorphic stages are estimated to be 786-826℃, 0.78-0.96 GPa (M2); 798-850℃, 0.71-0.75 GPa (M3); and 610-666℃, 0.51-0.60 GPa (M4), respectively, indicating a post-peak clockwise P-T path characterized by nearly isothermal decompression. Combined with geological data available, we propose that the high-pressure mafic granulite (retrograded eclogite) of the Dinggye formed during the Himalayan collisional orogeny, and underwent a post-peak tectonic uplift process of nearly isothermal decompression.
2018, 43(1): 236-246.
doi: 10.3799/dqkx.2018.014
Abstract:
Melt inclusions in high-grade metamorphic rocks are melt drops enclosed in host minerals such as garnet and zircon, which are composed of daughter minerals, glass and other accidently trapped solid minerals. In the last decade, melt inclusions have become a new approach to the study of partial melting process in high-grade metamorphic rocks. It is generally difficult to identify and analyze melt inclusions since they are mostly less than 20 μm in diameter. This paper systematically summarizes the identification methods, microstructural features, and chemical investigation methods. The significance of melt inclusions on the identification of partial melting and constraints of the P-T condition, mechanism and fluid regime of partial melting are also discussed.
Melt inclusions in high-grade metamorphic rocks are melt drops enclosed in host minerals such as garnet and zircon, which are composed of daughter minerals, glass and other accidently trapped solid minerals. In the last decade, melt inclusions have become a new approach to the study of partial melting process in high-grade metamorphic rocks. It is generally difficult to identify and analyze melt inclusions since they are mostly less than 20 μm in diameter. This paper systematically summarizes the identification methods, microstructural features, and chemical investigation methods. The significance of melt inclusions on the identification of partial melting and constraints of the P-T condition, mechanism and fluid regime of partial melting are also discussed.
2018, 43(1): 247-258.
doi: 10.3799/dqkx.2018.015
Abstract:
The Guishan complex exposed to the south of the Songpa-Guimei fault in the Tongbai orogen underwent two-stage metamorphism and multiple deformation. However, the timing of metamorphism and deformation of the complex and its relation to the Wuguan complex occurring in the same tectonic position in the Qinling orogen remain poorly constrained. In this paper, 4 metasedimentary rocks and 2 metavolcanic rocks from the Guishan complex were performed for muscovite 40Ar/39Ar dating. The results suggest that the plateau ages of muscovite range from 306.7±1.7 Ma (well-preserved muscovite porphyroblasts) to 279.2±2.6 Ma (muscovites overprinted by late deformation). Coupled with published U-Pb zircon data, it is infered that two-stage metamorphism and main thrust deformation of the Guishan complex took place during the Carboniferous, and the reworking of sinistral slid-slip shearing occurred in the Permian. The Guishan and Wuguan complexes have similar rock associations and share the same metamorphic and deformational history. The two complexes constitute a >500 km-long Late Paleozoic medium-grade metamorphic belt, which was resulted from the oceanic subduction-accretion prior to the final collision between the North and South China blocks.
The Guishan complex exposed to the south of the Songpa-Guimei fault in the Tongbai orogen underwent two-stage metamorphism and multiple deformation. However, the timing of metamorphism and deformation of the complex and its relation to the Wuguan complex occurring in the same tectonic position in the Qinling orogen remain poorly constrained. In this paper, 4 metasedimentary rocks and 2 metavolcanic rocks from the Guishan complex were performed for muscovite 40Ar/39Ar dating. The results suggest that the plateau ages of muscovite range from 306.7±1.7 Ma (well-preserved muscovite porphyroblasts) to 279.2±2.6 Ma (muscovites overprinted by late deformation). Coupled with published U-Pb zircon data, it is infered that two-stage metamorphism and main thrust deformation of the Guishan complex took place during the Carboniferous, and the reworking of sinistral slid-slip shearing occurred in the Permian. The Guishan and Wuguan complexes have similar rock associations and share the same metamorphic and deformational history. The two complexes constitute a >500 km-long Late Paleozoic medium-grade metamorphic belt, which was resulted from the oceanic subduction-accretion prior to the final collision between the North and South China blocks.
2018, 43(1): 259-277.
doi: 10.3799/dqkx.2018.016
Abstract:
Suichang-Dazhe pelitic granulites, showing distinct decompressional textures, crop out in Badu complex of the southwestern Zhejiang Province, the Cathaysia block. However, its metamorphic evolution characteristics, metamorphism ages and tectonic implications remain unclear. Results of our study of its petrology, mineral chemistry and LA-ICP-MS zircon U-Pb ages indicate that the rocks contain four distinct metamorphic assemblages, namely the early prograde (M1), pressure peak (M2), peak (M3) and the post-peak (M4) stages. The early prograde (M1) assemblage consists of garnet (core)+biotite+quartz. The pressure peak (M2) assemblage consists of garnet (mantle)+sudoite+rutile+kyanite+corundum+biotite+quartz±staurolite, the mineral assemblage may indicate that the rock has undergone the process of ultrahigh pressure metamorphism. The peak (M3) assemblage consists of garnet (rim-mantle)+biotite+sillimanite+quartz±K-feldspar±plagioclase±ilmenite. Post-peak near-isothermal decompressional (M4) stage following the peak stage could be subdivided into two stages M4-1 and M4-2. The M4-1 stage assemblage consists of garnet (rim)+cordierite+biotite+sillimanite+quartz+ilmenite±spinel±plagioclase±K-feldspar, and the M4-2 stage is represented by garnet (rim)+cordierite+sillimanite+plagioclase+biotite+quartz±K-feldspar. Quantitative phase equilibria modeling in the system Na2O-CaO-K2O-FeO-MgO-Al2O3-SiO2-H2O-TiO2, in combination with traditional thermobarometry, was applied to obtain P-T conditions of 780-810℃ and 8.0-9.2 kbar for M3, 780-860℃ and 5.7-6.0 kbar for M4-1, about 700℃ and about 4.4 kbar for M4-2. The combination of the mineral assemblages, mineral compositions, and metamorphic reaction history in the Suichang-Dazhe pelitic granulite defines a clockwise P-T path that involves near-isothermal decompression that followed the peak granulite-facies metamorphism. Analyses of LA-ICP-MS zircon U-Pb dating indicate that metamorphic zircons recorded metamorphic age of the Suichang-Dazhe pelitic granulite is 233.5-238.9 Ma. The clockwise P-T path reveals that the southwestern Zhejiang Province terrane involved in the subduction or collision followed by exhumation and cooling events and experienced the Early Mesozoic (233.5-238.9 Ma) granulite-facies metamorphism with the amalgamation of the Indochina-South China-North China block in the paleo-Tethyan domain.
Suichang-Dazhe pelitic granulites, showing distinct decompressional textures, crop out in Badu complex of the southwestern Zhejiang Province, the Cathaysia block. However, its metamorphic evolution characteristics, metamorphism ages and tectonic implications remain unclear. Results of our study of its petrology, mineral chemistry and LA-ICP-MS zircon U-Pb ages indicate that the rocks contain four distinct metamorphic assemblages, namely the early prograde (M1), pressure peak (M2), peak (M3) and the post-peak (M4) stages. The early prograde (M1) assemblage consists of garnet (core)+biotite+quartz. The pressure peak (M2) assemblage consists of garnet (mantle)+sudoite+rutile+kyanite+corundum+biotite+quartz±staurolite, the mineral assemblage may indicate that the rock has undergone the process of ultrahigh pressure metamorphism. The peak (M3) assemblage consists of garnet (rim-mantle)+biotite+sillimanite+quartz±K-feldspar±plagioclase±ilmenite. Post-peak near-isothermal decompressional (M4) stage following the peak stage could be subdivided into two stages M4-1 and M4-2. The M4-1 stage assemblage consists of garnet (rim)+cordierite+biotite+sillimanite+quartz+ilmenite±spinel±plagioclase±K-feldspar, and the M4-2 stage is represented by garnet (rim)+cordierite+sillimanite+plagioclase+biotite+quartz±K-feldspar. Quantitative phase equilibria modeling in the system Na2O-CaO-K2O-FeO-MgO-Al2O3-SiO2-H2O-TiO2, in combination with traditional thermobarometry, was applied to obtain P-T conditions of 780-810℃ and 8.0-9.2 kbar for M3, 780-860℃ and 5.7-6.0 kbar for M4-1, about 700℃ and about 4.4 kbar for M4-2. The combination of the mineral assemblages, mineral compositions, and metamorphic reaction history in the Suichang-Dazhe pelitic granulite defines a clockwise P-T path that involves near-isothermal decompression that followed the peak granulite-facies metamorphism. Analyses of LA-ICP-MS zircon U-Pb dating indicate that metamorphic zircons recorded metamorphic age of the Suichang-Dazhe pelitic granulite is 233.5-238.9 Ma. The clockwise P-T path reveals that the southwestern Zhejiang Province terrane involved in the subduction or collision followed by exhumation and cooling events and experienced the Early Mesozoic (233.5-238.9 Ma) granulite-facies metamorphism with the amalgamation of the Indochina-South China-North China block in the paleo-Tethyan domain.
2018, 43(1): 278-295.
doi: 10.3799/dqkx.2018.017
Abstract:
Studies of high-grade metamorphic rocks in an ancient orogenic belt can provide important constraints on the formation and tectonic evolution of orogen. The Qinling complex in the northern West Qinling orogen mainly consists of orogneiss, paragneiss, minor metabasite (mafic granulite) and marble. The petrography, mineral chemistry and phase equilibria modelling in this study show that paragneiss has experencied three metamorphic evolution stages. Stage 1 is the prograde stage, characterized by biotite and quartz occurring as inclusions in garnet. Stage 2 is the peak metamorphic stage, and mineral assemblage in this stage is interpreted to be garnet+sillimanite+K-feldspar+plagioclase+biotite+quartz+rutile, which formed in P-T conditions of 793-803℃ and 8.8-9.5 kbar. Stage 3 is the retrograde stage with decrease in temperature and pressure, which is recorded by garnet and plagioclase. Combining with previous data, it is suggested that the Qinling complex in the northern West Qinling orogen has experienced Early Paleozoic metamorphism and anataxis related to collision orogenesis.
Studies of high-grade metamorphic rocks in an ancient orogenic belt can provide important constraints on the formation and tectonic evolution of orogen. The Qinling complex in the northern West Qinling orogen mainly consists of orogneiss, paragneiss, minor metabasite (mafic granulite) and marble. The petrography, mineral chemistry and phase equilibria modelling in this study show that paragneiss has experencied three metamorphic evolution stages. Stage 1 is the prograde stage, characterized by biotite and quartz occurring as inclusions in garnet. Stage 2 is the peak metamorphic stage, and mineral assemblage in this stage is interpreted to be garnet+sillimanite+K-feldspar+plagioclase+biotite+quartz+rutile, which formed in P-T conditions of 793-803℃ and 8.8-9.5 kbar. Stage 3 is the retrograde stage with decrease in temperature and pressure, which is recorded by garnet and plagioclase. Combining with previous data, it is suggested that the Qinling complex in the northern West Qinling orogen has experienced Early Paleozoic metamorphism and anataxis related to collision orogenesis.
2018, 43(1): 296-316.
doi: 10.3799/dqkx.2018.018
Abstract:
Investigations on metamorphic processes of the Wuhe complex provide new insights into the formation and evolution of Precambrian metamorphic basement at the southeastern margin of the North China Craton (NCC). In this paper, three Paleoproterozoic stages of metamorphic assemblages from mafic granulites in the Wuhe complex is recognized and a clockwise P-T-t path characterized by post-peak near-isothermal decompression and subsequent decompression-cooling by conducting detailed petrographic observations, mineral electron microprobe analysis, zircon LA-ICP-MS U-Pb dating and trace element analysis is reconstructed. The peak HP granulite-facies metamorphism (M1) is characterized by high-Ca cores in granet, high-Al cores in clinopyrexene, plagioclase, quartz and rutile, yielding P-T conditions of 850-900℃ and 1.5 GPa. The medium-pressure (MP) granulite facies assemblage (M2) mainly consists of garnet+orthopyrexene+plagioclase±clinopyrexene surrounding the amphibole porphyroblasts, recording P-T conditions of~900℃ and 1.1-1.2 GPa. Symplectites or coronas composed of hornblende+plagioclase (M3) surrounding the garnet porphyroblasts indicate garnet decompressional reactions occurred at 600-680℃ and 0.65-0.75 GPa. The zircon dating results can be categorized into three groups of~1.90 Ga, ~1.85 Ga and~1.78 Ga, corresponding to the time of HP granulite facies, MP granulite facies and amphibolite facies metamorphism. The comparable metamorphic evolution of mafic granulites from the Wuhe complex to Jiaobei terrane, combined with previous studies on petrogenesis and U-Pb ages of 2.1 Ga granitic rocks, suggest that the Wuhe complex is the west extension of the Jiao-Liao-Ji belt (JLJB) and they constitute a Paleoproterozoic collisional orogen as a whole in the eastern block of the NCC.
Investigations on metamorphic processes of the Wuhe complex provide new insights into the formation and evolution of Precambrian metamorphic basement at the southeastern margin of the North China Craton (NCC). In this paper, three Paleoproterozoic stages of metamorphic assemblages from mafic granulites in the Wuhe complex is recognized and a clockwise P-T-t path characterized by post-peak near-isothermal decompression and subsequent decompression-cooling by conducting detailed petrographic observations, mineral electron microprobe analysis, zircon LA-ICP-MS U-Pb dating and trace element analysis is reconstructed. The peak HP granulite-facies metamorphism (M1) is characterized by high-Ca cores in granet, high-Al cores in clinopyrexene, plagioclase, quartz and rutile, yielding P-T conditions of 850-900℃ and 1.5 GPa. The medium-pressure (MP) granulite facies assemblage (M2) mainly consists of garnet+orthopyrexene+plagioclase±clinopyrexene surrounding the amphibole porphyroblasts, recording P-T conditions of~900℃ and 1.1-1.2 GPa. Symplectites or coronas composed of hornblende+plagioclase (M3) surrounding the garnet porphyroblasts indicate garnet decompressional reactions occurred at 600-680℃ and 0.65-0.75 GPa. The zircon dating results can be categorized into three groups of~1.90 Ga, ~1.85 Ga and~1.78 Ga, corresponding to the time of HP granulite facies, MP granulite facies and amphibolite facies metamorphism. The comparable metamorphic evolution of mafic granulites from the Wuhe complex to Jiaobei terrane, combined with previous studies on petrogenesis and U-Pb ages of 2.1 Ga granitic rocks, suggest that the Wuhe complex is the west extension of the Jiao-Liao-Ji belt (JLJB) and they constitute a Paleoproterozoic collisional orogen as a whole in the eastern block of the NCC.
2018, 43(1): 317-332.
doi: 10.3799/dqkx.2018.019
Abstract:
Niobium and tantalum have the same oxidation state (+5) and nearly identical ionic radii (~0.064 nm), thus they are considered as twin elements in geological properties. As high field strength elements, Nb-Ta are usually reserved in the rutile, hornblende, Nb-Ta ores, sphene, mica and other minerals. Nb and Ta have been the hotpot in the geochemistry in the last two decades, particularly since the beginning of the 21st century, with researches focused on the geochemical characters of Nb-Ta, Nb-Ta partition coefficient between mineral and melt/fluid, and the mechanism of Nb-Ta differentiation during geological processes. The studies of Nb and Ta have enhanced the understanding of the mass balance of elements in the Earth, the growth and accretion of the continental crust, the origin of the Archean tonalitic-trondhjemitic-granodioritic magmas (TTGs), the mechanism of elemental differentiation and the source tracking of metamorphic rocks. In this paper, we present the developments of niobium and tantalum in the past two decades, aiming to facilitate the future application of niobium and tantalum in geochemical researches.
Niobium and tantalum have the same oxidation state (+5) and nearly identical ionic radii (~0.064 nm), thus they are considered as twin elements in geological properties. As high field strength elements, Nb-Ta are usually reserved in the rutile, hornblende, Nb-Ta ores, sphene, mica and other minerals. Nb and Ta have been the hotpot in the geochemistry in the last two decades, particularly since the beginning of the 21st century, with researches focused on the geochemical characters of Nb-Ta, Nb-Ta partition coefficient between mineral and melt/fluid, and the mechanism of Nb-Ta differentiation during geological processes. The studies of Nb and Ta have enhanced the understanding of the mass balance of elements in the Earth, the growth and accretion of the continental crust, the origin of the Archean tonalitic-trondhjemitic-granodioritic magmas (TTGs), the mechanism of elemental differentiation and the source tracking of metamorphic rocks. In this paper, we present the developments of niobium and tantalum in the past two decades, aiming to facilitate the future application of niobium and tantalum in geochemical researches.
