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Zhang Z.,Chengdu Institute of Geology and Mineral Resources | Zhang Z.,Research Center for Tibetan Plateau Geology | Geng Q.,Chengdu Institute of Geology and Mineral Resources | Peng Z.,Chengdu Institute of Geology and Mineral Resources | And 2 more authors.
Geological Bulletin of China | Year: 2015

A Yanshanian magmatic arc was discovered in Rutog-Duolong area, in the western of Bangong-Nujiang metallogenic belt. It shows good ore-forming geological condition for several large and medium-size minerals had been found in recent years. Fuye pluton, locating in the west of the magmatic arc, is composed of diorite and diorite-porphyrite, with strong alteration. Primitive mantle-normalized spider diagram is loss Ba, Nb, Ta, Sr and Ti, enrichment Th, U and Pb. Chondrite-normalized REE pattern shows Slowly tilted to the right, and negative Eu anomalies. Zircon Lu-Hf isotope results of sample D1046 and D1049 show εHf(t) mainly focused at the range of -5.2 to -12.2, TDM1 model age concentrated in 0.98~1.28 Ga. The 206Pb/238U weighted average age of sample D1046 and D1049 are 119.06±0.48 Ma, and 120.03±0.68 Ma, representing they formed in the middle age of Early Cretaceous. Combined with geochemical characteristics, it is I-type granitoid, and formed with both mantle and lower crust. According to these magmatic bodies' characteristics of this arc, Bangong-Nujiang Tethys ocean at least started northward subduction, in the Middle Jurassic. And it last to Early Cretaceous. ©, 2015, Science Press. All right reserved.


Wang B.,Research Center for Tibetan Plateau Geology | Wang L.,Research Center for Tibetan Plateau Geology | Xu J.,CAS Guangzhou Institute of Geochemistry | Chen L.,Research Center for Tibetan Plateau Geology | And 4 more authors.
Geological Bulletin of China | Year: 2015

A high pressure metamorphic (HP) and ultrahigh pressure metamorphic (UHP) belt is exposed in Baxoi, Amdo and Pengco areas along the middle-eastern Bangongco-Nujiang suture zone (BNSZ). However, the similar HP-UHP metamorphic rocks have not discovered in the western section of BNSZ. In this paper, the authors first report the high-pressure granulite (probably retrograde eclogite) at Dongco area in the western section of the BNSZ. Dongco high-pressure granulite occurs in lenticular, stratoid and massive forms in plagioclase amphibolites and metamorphic gabbros. Based on the intensive study of petrology and mineralogical geochemistry, the authors hold that the HP granulite was made up of the early mineral assemblage of garnet (Grt)+clinopyroxene (Cpx) + plagioclase (Pl) (mostly saussuritized) and the late massive retrograde mineral assemblages amphibole (Hb)+plagioclase (Pl). Garnets in high-pressure granulites are surrounded by plagioclase which forms the corona. LA-ICP-MS zircon U-Pb dating yielded the age of original rock from the high-pressure granulite of 254±2 Ma, indicating that the opening time of Bangong Co-Nujiang River oceanic basin can be traced to the Late Permian. Geological temperature pressure indicator shows that the temperature-pressure conditions of high-pressure granulite phase are 780~900℃, 13~16 kPa, while those of retrogressive metamorphic amphibolite phase are 430~480℃, 4.5~5.2 kPa, respectively, suggesting that the high-pressure granulites were most likely the product of granulite facies (even eclogite facies) high pressure metamorphism of the Bangong Co-Nujiang River Tethyan oceanic crust. The discovery of Dongco high pressure granulites shows that a high-pressure metamorphic belt might exist within the BNSZ, and might have been formed by the deep subduction of the Bangong Co-Nujiang River Tethyan Ocean. Further study of the high-pressure granulites is of great significance for recognizing the deep subduction process, continent-continent collision orogeny and the tectonic evolution of BNSZ in the Tibetan Plateau. ©, 2015, Science Press. All right reserved.


Wang B.,Chengdu Institute of Geology and Mineral Resources | Wang B.,Research Center for Tibetan Plateau Geology | Wang L.,Chengdu Institute of Geology and Mineral Resources | Wang L.,Research Center for Tibetan Plateau Geology | And 7 more authors.
Gondwana Research | Year: 2015

Paleozoic arc-type volcanic rocks in northern Tibet, provide important information on the Paleozoic tectonic evolution of the Longmu Co-Shuanghu Tethyan Ocean (LSTO). In this study, we obtained zircon U-Pb and Hf isotopic data, and major and trace element data for volcanic rocks from the Riwanchaka and Laxiongco areas in northern Tibet. LA-ICP-MS U-Pb analyses of zircons show that volcanic rocks in the Riwanchaka and Laxiongco areas formed at 374-372 and 356-353 Ma, respectively. These calc-alkaline volcanic rocks consist of basalt, andesite, dacite, and rhyolite. The basalt and andesite in the Riwanchaka area exhibit low TiO2 and high MgO and Na2O concentrations, and are enriched in large ion lithophile elements (LILEs) and depleted in high field strength elements (HFSEs). The basalt probably originated from partial melting of a lithospheric mantle source, metasomatized by subduction-related fluids, whereas the andesite was probably derived from hydrous basalts after fractional crystallization. The Riwanchaka and Laxiongco rhyolites are characterized by high Na2O/K2O ratios and low Al2O3 concentrations, are weakly metaluminous to peraluminous (A/CNK=0.97-1.11), and exhibit typical arc-like geochemical signatures (e.g., enrichment in LILEs and strong depletion in HFSEs and Sr). In addition, they show high zircon ε Hf(t) values (+7.61 to +4.45). The geochemical characteristics and zircon ε Hf(t) compositions of the rhyolites indicate that they were derived from partial melting of juvenile basaltic lower crust. These results, combined with previous data, indicate that the LSTO lithosphere was subducted northward beneath the North Qiangtang subterrane (NQT) during the Late Devonian-Early Carboniferous. We suggest that the NQT represents a slice of an active continental margin developed on the southern margin of the Eurasian continent during the Late Devonian-Early Carboniferous. © 2015 International Association for Gondwana Research.


Wang B.-D.,Chengdu Institute of Geology and Mineral Resources | Wang B.-D.,Research Center for Tibetan Plateau Geology | Wang L.-Q.,Chengdu Institute of Geology and Mineral Resources | Wang L.-Q.,Research Center for Tibetan Plateau Geology | And 10 more authors.
Lithos | Year: 2015

The Bangong-Nujiang suture zone (BNSZ) is located in the central Tibetan Plateau, and represents the relict of the Bangong-Nujiang Tethyan Ocean (BNTO). This paper presents zircon U-Pb ages and whole-rock geochemical and Sr-Nd isotope data for the ophiolitic rocks from the Rutog, Dongco, Dongqiao, Amdo, and Dengqen areas (from west to east) in the BNSZ. Zircon U-Pb ages obtained from five gabbros and one leucogabbro from the five areas are 169±2, 167±2, 187±2, 184±2, 177±3, and 164±2Ma, respectively. Mafic rocks (gabbros and basalts) in the BNSZ can be divided into MORB-like and OIB-type, based on geochemical data. The MORB-like rocks are tholeiitic and can be further divided into N- and E-MORB subtypes, marked by depleted and flat REE patterns, respectively. All MORB-like rocks show supra-subduction zone (SSZ) geochemical affinities such as mild enrichment in the large ion lithophile elements and depletion in the high field strength elements, coupled with high and positive whole-rock ε Nd(t) values (+8.1 to +5.4). The OIB-type rocks, which formed in the Early Cretaceous (~132-108Ma), are exposed in the Dongco, Dongqiao, and Tarenben areas, and they consist mostly of alkali basalts with within-plate geochemical characteristics and positive ε Nd(t) values (+5.9 to +4.5). On the basis of these new data, combined with information from the literature, we further support that the BNTO was an intra-oceanic arc-backarc basin that existed between the North Lhasa subterrane (NLT) and the South Qiangtang subterrane (SQT) during the Early-Middle Jurassic. This basin may have been active until the Early Cretaceous, when the OIB-type basalts erupted. © 2015 Elsevier B.V.


Wang B.,Chengdu Institute of Geology and Mineral Resources | Wang B.,Research Center for Tibetan Plateau Geology | Wang L.,Chengdu Institute of Geology and Mineral Resources | Wang L.,Research Center for Tibetan Plateau Geology | And 10 more authors.
Gondwana Research | Year: 2014

Triassic volcanic rocks in the Jiangda-Deqen-Weixi continental margin arc (DWCA) in the Sanjiang area of SW China formed as a result of the closure of the Paleo-Tethys and associated continental collision, and are therefore important for reconstructing the evolution of the Paleo-Tethys. Representative samples of rhyolite and basalt from the Jijiading-Luchun-Yezhi area yield weighted mean 206Pb/238U ages of 245-249Ma and 245Ma, respectively, and exhibit the features of bimodal volcanic rocks. The mafic rocks are enriched in large ion lithophile elements (e.g., Rb, Ba, U), are depleted in high field strength elements (e.g., Nb, Ta, Ti), and have low εNd(t) values (-3.55 to -3.69) and high initial 87Sr/86Sr values (0.70788-0.70804). The mafic rocks were formed by partial melting of an enriched lithospheric mantle source that had been metasomatized by fluids derived from subducted sediments. The felsic rocks are characterized by high K2O (K2O/Na2O=2.16-71.33) and A/CNK (>1.1), and low Mg# (20-45) and εNd(t) values (-9.61 to -10.66), with zircon εHf(t) values of -8.1 to -16.6. Their old Nd model ages are similar to those of ancient Yangtze basement, indicating that they formed by partial melting of this ancient crust as a result of underplating by basaltic magma. We suggest that the bimodal volcanic rocks in the DWCA formed in an extensional setting as a result of slab break-off during a margin of arc-continent collision. The present results, combined with previous data, reveal three stages of collision in the Paleo-Tethys: early collision convergence (~255-250Ma), syn-collision extension (249-237Ma), and late collision orogenesis (236-212Ma). © 2013 International Association for Gondwana Research.

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