Abundant small mafic intrusions occur associated with granitoids along the Gangdise^ magmatic belt. In addition to many discrete gabbro bodies within the granitoid plutons, a gabbro-pyroxenite zone occurs along the so...Abundant small mafic intrusions occur associated with granitoids along the Gangdise^ magmatic belt. In addition to many discrete gabbro bodies within the granitoid plutons, a gabbro-pyroxenite zone occurs along the southern margin of the Gangdise^ belt to the north of the Yarlung Zangbo suture. The mafic intrusion zone spatially corresponds to a strong aeromagnetic anomaly, which extends -1400 km. The mafic intrusions consist of intermittently distributed small bodies and dikes of gabbro and dolerite with accumulates of pyroxenite, olivine pyroxenite, pegmatitic pyroxenite and amphibolite. Much evidence indicates that the Gangdise^ gabbro-pyroxenite assemblage is most likely a result of underplating of mantle-derived magma. Detailed field investigation and systematic sampling of the mafic rocks was conducted at six locations along the Lhasa-Xigaze^ segment of the mafic intrusive zone, and was followed by zircon SHRIMP Ⅱ U-Pb dating. In addition to the ages of two samples previously published （47.0±1 Ma and 48.9±1.1 Ma）, the isotopic ages of the remaining four gabbro samples are 51.6±1.3Ma, 52.5±3.0 Ma, 50.2±4.2Ma and 49.9±1.1Ma. The range of these ages （47-52.5 Ma） provide geochronologic constraints on the Eocene timing of magma underplating beneath the Gangdise^ belt at ca. 50 Ma. This underplating event post-dated the initiation of the India-Eurasia continental collision by 15 million years and was contemporaneous with a process of magma mixing. The SHRIMP Ⅱ U-Pb isotopic analysis also found several old ages from a few zircon grains, mostly in a range of 479-526 Ma （weighted average age 503±10 Ma）, thus yielding information about the pre-existing lower crust when underplating of mafic magma took place. It is believed that magma underplating was one of the major mechanisms for crustal growth during the Indian-Eurasia collision, possibly corresponding in time to the formation of the 14-16 km-thick ＂crnst-mantle transitional zone＂ characterized by Vp=6.85-6.9 km/s.展开更多
The Kejie pluton is located in the north of the Changning-Menglian suture zone. The rock types are mainly biotite-granite. Zircon LA-ICP-MS U-Pb dating indicates that the Kejie pluton emplaced at about 80-77 Ma, Late ...The Kejie pluton is located in the north of the Changning-Menglian suture zone. The rock types are mainly biotite-granite. Zircon LA-ICP-MS U-Pb dating indicates that the Kejie pluton emplaced at about 80-77 Ma, Late Cretaceous. The Kejie pluton samples are characterized by high SiO2 （71.68%-72.47%）, K2O （4.73%-5.54%）, total alkali （K2O ＋ Na2O = 8.21%-8.53%）, K2O/Na2O ratios （1.36-1.94） and low P2O5 （0.13%-0.17%）, with A/CNK of 1.025-1.055; enriched in U, Th, and K, depleted in Ba, Nb, St, Ti, P and Eu. They are highly fractionated, slightly peraluminous 1-type granite. The two samples of the Kejie pluton give a large variation of εHf（t） values （-5.04 to 1.96） and Hf isotope crustal model ages of 1.16-1.5 Ga. Zircon Hf isotopes and zircon saturation temperatures of whole-rock （801℃-823℃） show that the mantle-derived materials maybe have played a vital role in the generation of the Kejie pluton. The Kejie pluton was most likely generated in a setting associated with the eastward subduction of the neo-Tethys ocean, where intrusion of mantle wedge basaltic magmas in the crust caused the anatexis of the latter, forming hybrid melts, which subsequently experienced high-degree fractional crystallization.展开更多
The metamophic peridotite of ophiolite belt in Mt. Ailao is composed of lherzolite and harzburgite. The former shows the charateristics of primary pyrolite and the latter shows those of deleted (relict)pyrolite. By pa...The metamophic peridotite of ophiolite belt in Mt. Ailao is composed of lherzolite and harzburgite. The former shows the charateristics of primary pyrolite and the latter shows those of deleted (relict)pyrolite. By partial melting of lherzolite, two primary magmas: tholeiitic magma and picrite-basalt magma are formed. The former evoluted into gabbro-diabase-pyroxenic basalt rock series and show the characteristics of MORB; while the latter evoluted into gabbro-diorite-albite basalt-picrite basalt one, and show the characteristics of para-MORB.展开更多
The Cretaceous granitoids in the middle and northern Gangdese, Tibet are generally interpreted as the products of anatexis of thickened deep crust genetically associated with the Lhasa-Qiangtang collision. This paper ...The Cretaceous granitoids in the middle and northern Gangdese, Tibet are generally interpreted as the products of anatexis of thickened deep crust genetically associated with the Lhasa-Qiangtang collision. This paper reports bulk-rock major element, trace element and Sr-Nd isotopic data, zircon U-Pb age data, and zircon Hf isotopic data on the Zayu pluton in eastern Gangdese, Tibet. These data shed new light on the petrogenesis of the pluton. Our SHRIMP zircon U-Pb age dates, along with LA-ICPMS zircon U-Pb age dates recently reported in the literature, indicate that the Zayu pluton was emplaced at about 130 Ma, coeval with Early Cretaceous magmatic rocks in other areas of eastern Gangdese (e.g., Rawu, Baxoi areas) and the Middle Gangdese. The Zayu pluton samples lack amphibole and muscovite, and are compositionally characterized by high SiO2 (69.9%–76.8%), K2O (4.4%–5.7%), and low P2O5 (0.05%–0.12%). These samples also have A/CNK values of 1.00–1.05, and are enriched in Rb, Th, U, and Pb, and depleted in Ba, Nb, Ta, Sr, P, Ti, and Eu. These geochemical features suggest that the Zayu pluton samples are metaluminous to slightly peraluminous and are of highly fractionated I-type granite. The Zayu pluton samples have high ? Nd(t) values (?10.9–?7.6) and low initial 87Sr/86Sr ratios (0.7120–0.7179) relative to melts derived from mature continental crust in the Gangdese (e.g., Ningzhong Early Jurassic strongly peraluminous granite). The Zayu pluton samples are heterogeneous in zircon ? Hf(t) values (?12.8–?2.9), yielding ancient zircon Hf crustal model ages of 1.4–2.0 Ga. The data obtained in this study together with the data in the recent literature suggest that the Early Cretaceous granitoids in eastern Gangdese represent the eastward extension of the Early Cretaceous magmatism in the middle Gangdese, and that the Lhasa micro-continent block with ancient basement may extend for ～2000 km from east to west. Zircon Hf isotopic data and bulk-rock zircon saturation temperature (789–821 °C) indicate that mantle-derived materials likely played a role in the generation of the Zayu pluton. We propose that the Zayu pluton was most likely generated in a setting associated with southward subduction of the Bangong-Nujiang ocean floor, where mantle wedge-derived magmas may have provided the heat and material for the anatexis of ancient crust of the Lhasa micro-continent, resulted in hybrid melts (i.e., mantle-derived basaltic magmas + crust-derived felsic magmas). Such hybrid melts with subsequent fractional crystallization are responsible for the highly evolved Zayu pluton (crust thickening is not a prerequisite).展开更多
We report a combined internal and whole-rock Sm-Nd isochron age, and Nd and Pb isotopic data for gabbro dikes of the Luobusha ophiolite in Tibet. The Sm-Nd isochron of data for two whole rocks and plagioclase and clin...We report a combined internal and whole-rock Sm-Nd isochron age, and Nd and Pb isotopic data for gabbro dikes of the Luobusha ophiolite in Tibet. The Sm-Nd isochron of data for two whole rocks and plagioclase and clinopyroxene separates from one of the rocks yields a Middle Jurassic age of (177±31) Ma (with an initial εNd(t) = +8), which provides a significant bound on the time of formation of the Luobusha ophiolite. The Nd and Pb isotopic charac-teristics of the dike indicate an Indian-Ocean-type isotopic affinity, and we conclude that the Luobusha ophiolite formed in an oceanic setting during the Middle Jurassic.展开更多
The electrical conductivities of the dunite from the Qinghai-Xizang (Tibetan) Plateau were measured with the impedance spectra method at 1.0-4.0 GPa and 643-1093 K. The experimental results indicated that activation e...The electrical conductivities of the dunite from the Qinghai-Xizang (Tibetan) Plateau were measured with the impedance spectra method at 1.0-4.0 GPa and 643-1093 K. The experimental results indicated that activation enthalpies of the dunite are smaller than 0.9 eV, the conduction mechanism in dunite may be attributed to the mixed electrical conduction involving grain interiors and boundaries. On the basis of the results of this experiment, we can deduce that there exists cold mantle in the area of Gaize-Lugu in the Qinghai-Xizang (Tibetan) Plateau by reverse methods from the magnetotelluric sounding data (conductivity-depths profile) available for western Tibet. The result provides the present cold mantle viewpoint with strong proof on the basis of high temperature and pressure experiments.展开更多
文摘Abundant small mafic intrusions occur associated with granitoids along the Gangdise^ magmatic belt. In addition to many discrete gabbro bodies within the granitoid plutons, a gabbro-pyroxenite zone occurs along the southern margin of the Gangdise^ belt to the north of the Yarlung Zangbo suture. The mafic intrusion zone spatially corresponds to a strong aeromagnetic anomaly, which extends -1400 km. The mafic intrusions consist of intermittently distributed small bodies and dikes of gabbro and dolerite with accumulates of pyroxenite, olivine pyroxenite, pegmatitic pyroxenite and amphibolite. Much evidence indicates that the Gangdise^ gabbro-pyroxenite assemblage is most likely a result of underplating of mantle-derived magma. Detailed field investigation and systematic sampling of the mafic rocks was conducted at six locations along the Lhasa-Xigaze^ segment of the mafic intrusive zone, and was followed by zircon SHRIMP Ⅱ U-Pb dating. In addition to the ages of two samples previously published （47.0±1 Ma and 48.9±1.1 Ma）, the isotopic ages of the remaining four gabbro samples are 51.6±1.3Ma, 52.5±3.0 Ma, 50.2±4.2Ma and 49.9±1.1Ma. The range of these ages （47-52.5 Ma） provide geochronologic constraints on the Eocene timing of magma underplating beneath the Gangdise^ belt at ca. 50 Ma. This underplating event post-dated the initiation of the India-Eurasia continental collision by 15 million years and was contemporaneous with a process of magma mixing. The SHRIMP Ⅱ U-Pb isotopic analysis also found several old ages from a few zircon grains, mostly in a range of 479-526 Ma （weighted average age 503±10 Ma）, thus yielding information about the pre-existing lower crust when underplating of mafic magma took place. It is believed that magma underplating was one of the major mechanisms for crustal growth during the Indian-Eurasia collision, possibly corresponding in time to the formation of the 14-16 km-thick ＂crnst-mantle transitional zone＂ characterized by Vp=6.85-6.9 km/s.
基金financially supported by the Programme of the China Geological Survey (No.1212011120608, No.1212011220907)the National Key Projects for Basic Research of China (No.2009CB421002, No.2011CB403102)+2 种基金NSF of China (No. 40672044)Program for Changjiang Scholars, Innovative Research Team in University (No.IRT1083)111 project (No.B07011)
文摘The Kejie pluton is located in the north of the Changning-Menglian suture zone. The rock types are mainly biotite-granite. Zircon LA-ICP-MS U-Pb dating indicates that the Kejie pluton emplaced at about 80-77 Ma, Late Cretaceous. The Kejie pluton samples are characterized by high SiO2 （71.68%-72.47%）, K2O （4.73%-5.54%）, total alkali （K2O ＋ Na2O = 8.21%-8.53%）, K2O/Na2O ratios （1.36-1.94） and low P2O5 （0.13%-0.17%）, with A/CNK of 1.025-1.055; enriched in U, Th, and K, depleted in Ba, Nb, St, Ti, P and Eu. They are highly fractionated, slightly peraluminous 1-type granite. The two samples of the Kejie pluton give a large variation of εHf（t） values （-5.04 to 1.96） and Hf isotope crustal model ages of 1.16-1.5 Ga. Zircon Hf isotopes and zircon saturation temperatures of whole-rock （801℃-823℃） show that the mantle-derived materials maybe have played a vital role in the generation of the Kejie pluton. The Kejie pluton was most likely generated in a setting associated with the eastward subduction of the neo-Tethys ocean, where intrusion of mantle wedge basaltic magmas in the crust caused the anatexis of the latter, forming hybrid melts, which subsequently experienced high-degree fractional crystallization.
文摘The metamophic peridotite of ophiolite belt in Mt. Ailao is composed of lherzolite and harzburgite. The former shows the charateristics of primary pyrolite and the latter shows those of deleted (relict)pyrolite. By partial melting of lherzolite, two primary magmas: tholeiitic magma and picrite-basalt magma are formed. The former evoluted into gabbro-diabase-pyroxenic basalt rock series and show the characteristics of MORB; while the latter evoluted into gabbro-diorite-albite basalt-picrite basalt one, and show the characteristics of para-MORB.
基金Supported by National Natural Science Foundation of China (Grant Nos. 40572051, 40830317, 40873023, 40672044)National Basic Research Program of China (Grant No. 2009CB421002), Chinese "111" Project (Grant No. B07011)Programme of the Integrated Study of Basic Geology of Qinghai-Tibetan Plateau of the China Geological Survey
文摘The Cretaceous granitoids in the middle and northern Gangdese, Tibet are generally interpreted as the products of anatexis of thickened deep crust genetically associated with the Lhasa-Qiangtang collision. This paper reports bulk-rock major element, trace element and Sr-Nd isotopic data, zircon U-Pb age data, and zircon Hf isotopic data on the Zayu pluton in eastern Gangdese, Tibet. These data shed new light on the petrogenesis of the pluton. Our SHRIMP zircon U-Pb age dates, along with LA-ICPMS zircon U-Pb age dates recently reported in the literature, indicate that the Zayu pluton was emplaced at about 130 Ma, coeval with Early Cretaceous magmatic rocks in other areas of eastern Gangdese (e.g., Rawu, Baxoi areas) and the Middle Gangdese. The Zayu pluton samples lack amphibole and muscovite, and are compositionally characterized by high SiO2 (69.9%–76.8%), K2O (4.4%–5.7%), and low P2O5 (0.05%–0.12%). These samples also have A/CNK values of 1.00–1.05, and are enriched in Rb, Th, U, and Pb, and depleted in Ba, Nb, Ta, Sr, P, Ti, and Eu. These geochemical features suggest that the Zayu pluton samples are metaluminous to slightly peraluminous and are of highly fractionated I-type granite. The Zayu pluton samples have high ? Nd(t) values (?10.9–?7.6) and low initial 87Sr/86Sr ratios (0.7120–0.7179) relative to melts derived from mature continental crust in the Gangdese (e.g., Ningzhong Early Jurassic strongly peraluminous granite). The Zayu pluton samples are heterogeneous in zircon ? Hf(t) values (?12.8–?2.9), yielding ancient zircon Hf crustal model ages of 1.4–2.0 Ga. The data obtained in this study together with the data in the recent literature suggest that the Early Cretaceous granitoids in eastern Gangdese represent the eastward extension of the Early Cretaceous magmatism in the middle Gangdese, and that the Lhasa micro-continent block with ancient basement may extend for ～2000 km from east to west. Zircon Hf isotopic data and bulk-rock zircon saturation temperature (789–821 °C) indicate that mantle-derived materials likely played a role in the generation of the Zayu pluton. We propose that the Zayu pluton was most likely generated in a setting associated with southward subduction of the Bangong-Nujiang ocean floor, where mantle wedge-derived magmas may have provided the heat and material for the anatexis of ancient crust of the Lhasa micro-continent, resulted in hybrid melts (i.e., mantle-derived basaltic magmas + crust-derived felsic magmas). Such hybrid melts with subsequent fractional crystallization are responsible for the highly evolved Zayu pluton (crust thickening is not a prerequisite).
基金This work was supportedby both the National Natural Science Foundation of China and the National Key Project for Basic Research on the Tibetan Plateau (Grant Nos. 49772107, 49802005 and G1998040800) the U.S. National Science Foundation (EAR-9805318)
文摘We report a combined internal and whole-rock Sm-Nd isochron age, and Nd and Pb isotopic data for gabbro dikes of the Luobusha ophiolite in Tibet. The Sm-Nd isochron of data for two whole rocks and plagioclase and clinopyroxene separates from one of the rocks yields a Middle Jurassic age of (177±31) Ma (with an initial εNd(t) = +8), which provides a significant bound on the time of formation of the Luobusha ophiolite. The Nd and Pb isotopic charac-teristics of the dike indicate an Indian-Ocean-type isotopic affinity, and we conclude that the Luobusha ophiolite formed in an oceanic setting during the Middle Jurassic.
基金This work was supported by the National Climbing Program (Grant No. 95-P-39) under the auspices of the National Key Project on the Tibetan Plateau (Grant No. G1998040800) the National Natu- ral Science Foundation of China (Grant No. 49674221).
文摘The electrical conductivities of the dunite from the Qinghai-Xizang (Tibetan) Plateau were measured with the impedance spectra method at 1.0-4.0 GPa and 643-1093 K. The experimental results indicated that activation enthalpies of the dunite are smaller than 0.9 eV, the conduction mechanism in dunite may be attributed to the mixed electrical conduction involving grain interiors and boundaries. On the basis of the results of this experiment, we can deduce that there exists cold mantle in the area of Gaize-Lugu in the Qinghai-Xizang (Tibetan) Plateau by reverse methods from the magnetotelluric sounding data (conductivity-depths profile) available for western Tibet. The result provides the present cold mantle viewpoint with strong proof on the basis of high temperature and pressure experiments.
