The Dongco ophiolite occurred in the middle-western segment of the Bangong-Nujiang suture zone. The thickness of the ophiolite suite is more than 5 km, which is composed, from bottom to top, of the mantle peridotite, ...The Dongco ophiolite occurred in the middle-western segment of the Bangong-Nujiang suture zone. The thickness of the ophiolite suite is more than 5 km, which is composed, from bottom to top, of the mantle peridotite, mafic-ultramafic cumulates, basic sills (dykes) and basic lava and tectoni- cally emplaced in Jurassic strata (Mugagongru Group). The Dongco cumulates consist of dunite- troctolite-olivine-gabbro, being a part of DTG series of mafic-ultramafic cumulates. The basic lavas are characterized by being rich in alkali (Na2O+ K2O), TiO2, P2O5 and a LREE-rich type pattern dip- ping right with [La/Yb]=6.94―16.6 as well as a trace elements spider-diagram with normal anomaly of Th, Nb, Ta, Hf. Therefore, the Dongco basic lavas belong to ocean-island basalt (OIB) and dis- tinctly differ from mid-ocean ridge basalt (MORB) and island-arc basalt (IAB) formed in the plate convergence margin. The basic lavas have higher 87Sr/86Sr (0.704363―0.705007), lower 143Nd/144Nd (0.512708―0.512887) and εNd(t ) from +2.7―+5.8, indicating that they derive from a two-components mixing mantle source of depleted mantle (DM) and enriched mantle (EMI). From above it is ready to see that the Dongco ophiolite forms in oceanic island (OIB) where the mantle source is replaced by a large amount of enriched material, therefore it distinctly differs from these ophiolites formed in island-arc and mid-oecan ridge. Newly obtained SHRIMP U-Pb dating for zircon of the cumulate troctolite is 132 ± 3 Ma and whole-rock dating of ^(39)Ar/^(40)Ar for the basalt is 173.4 ± 2.7 Ma and 140.9 ± 2.8 Ma, indicating that the Dongco ophiolite formed at Early Cretaceous and the middle-western segment of the Bangong-Nujiang oceanic basin was still in the developing and evolving period at Early Cretaceous.展开更多
The petrology and mineral assemblages of blueschists in the Gangmar, central Qiangtang, northern Tibet were examined, which indicates that the metamorphic condition is high-pressure low-temperature. In this note, we r...The petrology and mineral assemblages of blueschists in the Gangmar, central Qiangtang, northern Tibet were examined, which indicates that the metamorphic condition is high-pressure low-temperature. In this note, we reported the 40Ar/39Ar dating results of glaucophane from two blueschist samples. Their apparent ages are (275.0±1.3) Ma and (287.6±2.3) Ma and similar isochron ages are (275.0±0.9) Ma and (282.4±0.8) Ma, respectively. These iso-topic datings show the high-pressure metamorphism occurring in Lower Permian. The ages also coincide with the evolution of Palaeo-Tethys ocean in Late Palaeozoic.展开更多
Xiaoqinling District is an important gold-producing area in China.It ranks second to Jiaodong with regard to gold deposits.The uprising period of the Wenyu granitic pluton and the wall-rocks of the deposit,as well as ...Xiaoqinling District is an important gold-producing area in China.It ranks second to Jiaodong with regard to gold deposits.The uprising period of the Wenyu granitic pluton and the wall-rocks of the deposit,as well as the mineralizing depth and reserved place of gold ore bodies,are significant to ore exploration.Fission-track(FT)analysis of zircons and apatites of granitic rocks from the Wenyu granitic pluton shows that apatite FT(AFT)data modeling indicates a rapid cooling rate of 20°C/Ma from 138 to120 Ma after emplacement at 138 Ma.Thermal evolution and inversion curves suggest a secondary phase of fast cooling and uprising from 45 to 35 Ma,and 35 Ma,respectively,with a cooling rate of 6.7°C/Ma and a denudation quantity of^4.3 km.The last cooling phase took place from<4 Ma,with an average cooling rate of^11.3°C/Ma and a denudation amount of 1.3 km.Total exhumation quantity of 5.6 km and uprising elevation of 7.3 km are similar to the estimated results of fluid inclusions from the Dongtongyu and Wenyu gold deposits.The39Ar/40Ar dating of sericite from the fault planes of the Xunmadao-Xiaohe and Taiyao faults demonstrate two uprising activities of the ore-host metamorphic complex.The Huashan and Wenyu granitic plutons intensively occurred during 77 and 45 Ma,respectively.These data sets are valuable for understanding the uplifting process and for preserving gold ore bodies in the Xiaoqinling area,as well as for further studies on tectonic evolutions of the Taihua Complex and the Qinling-Dabie Orogen.展开更多
A suite of potassium-bearing minerals from the Walgidee Hills lamproite intrusion in the Kimberley region of Western Australia was selected for 39Ar/40Ar dating. These included wadeite, jeppeite, priderite, potassium ...A suite of potassium-bearing minerals from the Walgidee Hills lamproite intrusion in the Kimberley region of Western Australia was selected for 39Ar/40Ar dating. These included wadeite, jeppeite, priderite, potassium richterite, and phlogopite. All recorded excellent plateau ages, with the mean age of the combined data set being 17.3±0.3 Ma. Phlogopite recorded the largest uncertainty, whereas, of the other minerals,wadeite gave the best precision. Although rare to absent in common magmatic rocks, these minerals are widely distributed in alkaline complexes and in lamproite, kimberlite and orangeite intrusions. The results indicate this suite of minerals is excellent for 39Ar/40Ar dating and that they can be used singly or in combination to obtain the precise magmatic crystallization ages of ultra-alkaline rocks. Because of the stability of potassium richterite at mantle depths, 39Ar/40Ar dating of MARID(micaamphibole-rutile-ilmenite-diopside) xenoliths should be a more widely-applied technique to investigating mantle geodynamics.展开更多
基金Supported by China Geological Survey of Ministry of Land and Resources (Grant No. 200313000059)Department of International-Cooperation of Ministry of Land and Resources (Grant No. 2001010205)
文摘The Dongco ophiolite occurred in the middle-western segment of the Bangong-Nujiang suture zone. The thickness of the ophiolite suite is more than 5 km, which is composed, from bottom to top, of the mantle peridotite, mafic-ultramafic cumulates, basic sills (dykes) and basic lava and tectoni- cally emplaced in Jurassic strata (Mugagongru Group). The Dongco cumulates consist of dunite- troctolite-olivine-gabbro, being a part of DTG series of mafic-ultramafic cumulates. The basic lavas are characterized by being rich in alkali (Na2O+ K2O), TiO2, P2O5 and a LREE-rich type pattern dip- ping right with [La/Yb]=6.94―16.6 as well as a trace elements spider-diagram with normal anomaly of Th, Nb, Ta, Hf. Therefore, the Dongco basic lavas belong to ocean-island basalt (OIB) and dis- tinctly differ from mid-ocean ridge basalt (MORB) and island-arc basalt (IAB) formed in the plate convergence margin. The basic lavas have higher 87Sr/86Sr (0.704363―0.705007), lower 143Nd/144Nd (0.512708―0.512887) and εNd(t ) from +2.7―+5.8, indicating that they derive from a two-components mixing mantle source of depleted mantle (DM) and enriched mantle (EMI). From above it is ready to see that the Dongco ophiolite forms in oceanic island (OIB) where the mantle source is replaced by a large amount of enriched material, therefore it distinctly differs from these ophiolites formed in island-arc and mid-oecan ridge. Newly obtained SHRIMP U-Pb dating for zircon of the cumulate troctolite is 132 ± 3 Ma and whole-rock dating of ^(39)Ar/^(40)Ar for the basalt is 173.4 ± 2.7 Ma and 140.9 ± 2.8 Ma, indicating that the Dongco ophiolite formed at Early Cretaceous and the middle-western segment of the Bangong-Nujiang oceanic basin was still in the developing and evolving period at Early Cretaceous.
基金the National Key Basic Research Project (Grant No. G1998040800) the CAS's Key Basic Research Project for the Tibetan Plateau (Grant Nos. KZ951-A1-204 and KZ95T-06).
文摘The petrology and mineral assemblages of blueschists in the Gangmar, central Qiangtang, northern Tibet were examined, which indicates that the metamorphic condition is high-pressure low-temperature. In this note, we reported the 40Ar/39Ar dating results of glaucophane from two blueschist samples. Their apparent ages are (275.0±1.3) Ma and (287.6±2.3) Ma and similar isochron ages are (275.0±0.9) Ma and (282.4±0.8) Ma, respectively. These iso-topic datings show the high-pressure metamorphism occurring in Lower Permian. The ages also coincide with the evolution of Palaeo-Tethys ocean in Late Palaeozoic.
基金supported by the National Natural Science Foundation of China(90814006)
文摘Xiaoqinling District is an important gold-producing area in China.It ranks second to Jiaodong with regard to gold deposits.The uprising period of the Wenyu granitic pluton and the wall-rocks of the deposit,as well as the mineralizing depth and reserved place of gold ore bodies,are significant to ore exploration.Fission-track(FT)analysis of zircons and apatites of granitic rocks from the Wenyu granitic pluton shows that apatite FT(AFT)data modeling indicates a rapid cooling rate of 20°C/Ma from 138 to120 Ma after emplacement at 138 Ma.Thermal evolution and inversion curves suggest a secondary phase of fast cooling and uprising from 45 to 35 Ma,and 35 Ma,respectively,with a cooling rate of 6.7°C/Ma and a denudation quantity of^4.3 km.The last cooling phase took place from<4 Ma,with an average cooling rate of^11.3°C/Ma and a denudation amount of 1.3 km.Total exhumation quantity of 5.6 km and uprising elevation of 7.3 km are similar to the estimated results of fluid inclusions from the Dongtongyu and Wenyu gold deposits.The39Ar/40Ar dating of sericite from the fault planes of the Xunmadao-Xiaohe and Taiyao faults demonstrate two uprising activities of the ore-host metamorphic complex.The Huashan and Wenyu granitic plutons intensively occurred during 77 and 45 Ma,respectively.These data sets are valuable for understanding the uplifting process and for preserving gold ore bodies in the Xiaoqinling area,as well as for further studies on tectonic evolutions of the Taihua Complex and the Qinling-Dabie Orogen.
文摘A suite of potassium-bearing minerals from the Walgidee Hills lamproite intrusion in the Kimberley region of Western Australia was selected for 39Ar/40Ar dating. These included wadeite, jeppeite, priderite, potassium richterite, and phlogopite. All recorded excellent plateau ages, with the mean age of the combined data set being 17.3±0.3 Ma. Phlogopite recorded the largest uncertainty, whereas, of the other minerals,wadeite gave the best precision. Although rare to absent in common magmatic rocks, these minerals are widely distributed in alkaline complexes and in lamproite, kimberlite and orangeite intrusions. The results indicate this suite of minerals is excellent for 39Ar/40Ar dating and that they can be used singly or in combination to obtain the precise magmatic crystallization ages of ultra-alkaline rocks. Because of the stability of potassium richterite at mantle depths, 39Ar/40Ar dating of MARID(micaamphibole-rutile-ilmenite-diopside) xenoliths should be a more widely-applied technique to investigating mantle geodynamics.
