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Wang Z.-M.,CAS Institute of Geology and Geophysics | Wang Z.-M.,University of Chinese Academy of Sciences | Han C.-M.,CAS Institute of Geology and Geophysics | Han C.-M.,Xinjiang Research Center for Mineral Resources | And 7 more authors.
Precambrian Research | Year: 2014

The Dunhuang Block, located in the eastern margin of the Tarim Craton, possesses a wealth of information about the geological evolution of the Tarim Craton. In this study, we conducted detailed petrological, geochemical and geochronological investigations on garnet-bearing amphibolites as well as granites in the Dunhuang Block, which record important metamorphic and magmatic events that are poorly constrained. Based on field occurrence, geochronological and geochemical characteristics, the garnet-bearing amphibolites can be divided into two groups. Group I garnet-bearing amphibolite displays typical features of island arc basalts, which may formed at ~1.83Ga in a collisional setting during the assembly of the Columbia supercontinent, whereas group II has geochemical characteristics of ocean island basalt which may formed at ~1.61Ga in an ocean island setting. The ca. 1.83Ga amphibolite has positive e{open}Hf(t) values (+4.4 to +8.9), with TDM1 model ages of 2.02-1.83Ga, indicating an important addition of juvenile mantle-derived materials during the formation of its protolith. The ca. 1.61Ga amphibolite has e{open}Hf(t) values ranging from -0.6 to +5.6 and corresponding TDM1 model ages varying from 2.03Ga to 1.84Ga, which suggest that juvenile mantle-derived materials were dominantly contributed to the formation of its protolith but were later contaminated by crustal materials. The U-Pb ages obtained from the rims of the zircon grains in amphibolites and granites recorded a tectonothermal event at ca. 400Ma, which is coeval with the widespread magmatism in the Beishan orogen. We thus propose that these two groups of garnet-bearing amphibolites were possibly not related to each other during Paleoproterozoic time, and a late Paleozoic orogenic event brought them together during which both of them underwent metamorphism. © 2014 Elsevier B.V. Source

Wang Z.M.,CAS Institute of Geology and Geophysics | Wang Z.M.,Xinjiang Research Center for Mineral Resources | Wang Z.M.,University of Chinese Academy of Sciences | Xiao W.J.,CAS Institute of Geology and Geophysics | And 6 more authors.
Acta Petrologica Sinica | Year: 2013

The protolith of the Hongliuxia amphibolites is tholeiite formed in immature arc, and the typical mineral assemblages are garnet (Grt), amphibole (Amp), diopside (Di) and plagioclase (PI). The coarse-grain garnet porphyroblasts usually display obvious zonings. Petrographic and mineral chemistry indicate that the amphibolites have undergone four successive stages: prograde, peak, isothermal decompression and isobaric cooling. The prograde metamorphic assemblages (M1) are the minerals preserved in garnet porphyroblasts (Grt I + Amp I + Pl I + Qtz), and the P-T metamorphic conditions are estimated to be 550°C to 600°C and 0. 3 Gpa to 0. 5 Gpa; the peak metamorphic assemblages (M2) are the mantle part of garnet and the minerals in the matrix (Grt II + Amp II + Di + Pl II + Qtz), P-T estimates give peak metamorphic conditions of 650°C to 780°C and 0. 8 Gpa to 0. 9Gpa; the M3 stage is characterized by the Pl III + Amp III corona around garnet porphyroblasts, which infer an obvious decompression process, and the temperature of metamorphism is estimated to be 630°C to 700°C; the temperature of the M4 stage drop a lot, which is inferred from the breakdown of diopside to produce AmpIV + PlIV - The estimated P-T conditions indicate a clockwise P-T-t path. The tectonic evolution history is: plate subduction cause high-temperature metamorphism, after that the crust tends to be thin, then the deep rock cool down to the normal geothermal gradient. The crystallization age of garnet-bearing amphibolite is 1611 ±6Ma, which is contemporary to the breakup Columbia Supercontinent, so the Tarim Craton may connect with the breakup of Columbia Supercontinent. Source

Song D.,CAS Institute of Geology and Geophysics | Song D.,University of Chinese Academy of Sciences | Xiao W.,CAS Institute of Geology and Geophysics | Xiao W.,Xinjiang Research Center for Mineral Resources | And 6 more authors.
Gondwana Research | Year: 2013

The origin and tectonic settings of metasedimentary sequences in the Central Asian Orogenic Belt have been a matter of debate regarding their contributions with some proposals of being microcontinents or accreted material, largely due to a lack of high resolution geochronological data. This paper reports detrital zircon U-Pb age and Hf isotopic data for the previously mapped Precambrian metasedimentary rocks from the Beishan orogenic collage, southern Altaids. Our data show that Precambrian ages dominate all the analyzed samples, but two samples yield Paleozoic zircons which suggest that they were not deposited in the Precambrian. The late Paleoproterozoic-early Mesoproterozoic group (~. 2000-1300. Ma) is the largest age population among the six samples analyzed. This age population (~. 2000-1300. Ma) corresponds to the assembly and subsequent break-up of the Columbia supercontinent. Only one sample (11SYS01) yields Neoproterozoic ages (with two peaks at 930 and 785. Ma), which shows a possible affinity with the Tarim Craton. Hence, the age spectra presented here are generally different from that of the Tarim Craton and the metasedimentary rocks from the Central Tienshan. Our data show that the Tarim Craton is not the main source area for the metasedimentary rocks from the Beishan orogenic collage, but instead multiple source areas may have contributed to the Beishan collage. Combining our new results with published data, we favor an allochthonous origin for the metasedimentary sequences which may be associated with major thrust tectonics. Therefore, a long-lived arc accretionary model is proposed for the tectonic evolution of the Beishan orogenic collage. © 2013 International Association for Gondwana Research. Source

Song D.,Chinese Academy of Sciences | Song D.,University of Chinese Academy of Sciences | Xiao W.,Chinese Academy of Sciences | Xiao W.,Xinjiang Research Center for Mineral Resources | And 4 more authors.
International Geology Review | Year: 2013

The tectonic nature of metamorphic terranes and their role in orogenesis are problematic. Here we present new U-Pb ages and geochemical data for widespread metamorphic rocks and associated granitoids from Northwest China. Orthogneisses from the metamorphic complexes have crystallization ages of ∼457, ∼452, and ∼526 Ma. One paragneiss (schist) has a maximum depositional age of 312 ± 7 Ma. Three foliated granites were emplaced at ∼450, ∼349, and ∼410 Ma, and all lack inherited Precambrian ages. The metamorphic terranes may have undergone multiple petrotectonic events as revealed by the metamorphic ages. Both the orthogneisses and granitoids show enrichment in large ion lithophile elements (LILEs) and light rare Earth elements (LREEs), and depletion in high field strength elements (HFSEs), which indicate that they formed in a subduction-generated accretionary arc setting. Our study demonstrates that the metamorphic terranes in the Beishan area, originally considered as Precambrian basement with suspected Neoarchaean to Palaeoproterozoic ages, are actually parts of early Palaeozoic arcs. The protoliths were probably metamorphosed arc plutonic and sedimentary rocks. Combined with other studies, we speculate that the Beishan Orogen formed by progressive arc accretion during the latest Neoproterozoic to early Palaeozoic time. This new interpretation has implications for other high-grade metamorphic terranes within orogens that have been assumed to represent ancient or pre-existing micro-continental blocks. If so, the importance of collision as a mechanism of mountain building has been overestimated, and the accretionary process as a mechanism of continental growth has been underestimated. Source

Wang Z.-M.,CAS Institute of Geology and Geophysics | Wang Z.-M.,Xinjiang Research Center for Mineral Resources | Wang Z.-M.,University of Chinese Academy of Sciences | Han C.-M.,CAS Institute of Geology and Geophysics | And 10 more authors.
Journal of Asian Earth Sciences | Year: 2014

As part of Central Asian Orogenic Belt (CAOB), the Central Tianshan zone plays a crucial role in the reconstruction of the tectonic evolution of the CAOB. Furthermore, it is bordered by the Tarim Craton to the south, and the comparable evolutionary history between them enables the Central Tianshan zone to provide essential information on the crustal evolution of the Tarim Craton. The eastern segment of the Central Tianshan tectonic zone is characterized by the presence of numerous Precambrian metamorphic rocks, among which the Xingxingxia Group is the most representative one. The granitoids gneisses, intruded into the Xingxingxia Group, consist of two major lithological assemblages: (1) biotite-monzonitic gneisses and (2) biotite-plagioclase gneisses. These metamorphosed granitoid rocks are characterized by enrichment in SiO2, Al2O3 and K2O and depletion in MgO and FeOT. The Rittmann index (σ) spreads between 1.44 and 2.21 and ACNK (Al2O3/(CaO+Na2O+K2O)) ranges from 1.03 to 1.08, indicating that these granitoid gneisses are high-K calc-alkaline and peraluminous. Trace element data indicate that the studied samples are enriched in LREE with moderate REE fractionated patterns ((La/Yb)N=10.5-75.3). The concentrations of HREE of the garnet-bearing gneisses are significantly higher than those of garnet-free gneisses. The former show pronounced negative Eu anomalies (Eu/Eu*=0.32-0.57), while the latter are characterized by negligible negative Eu anomalies to moderate positive Eu anomalies (Eu/Eu*=0.80-1.35). In addition, the enrichment of LILE (Rb, Th, K, Pb) and depletion of HFSE (Ta, Nb, P, Ti) of the examined granitoid gneisses are similar to typical volcanic-arc granites. Zircons U-Pb dating on the biotite monzonitic gneiss yields a weighted mean 206Pb/238U age of 942.4±5.1Ma, suggesting their protoliths were formed in the early Neoproterozoic, which is compatible with the time of the assembly of supercontinent Rodinia. The zircons have a large εHf(t) variation from -5.6 to +3.2, suggesting that both old crust-derived magmas and mantle-derived juvenile materials contributed to the formation of their protoliths. Based on field observation, and petrological, geochemical and geochronological investigations, we infer that the granitoid gneisses from Xingxingxia were probably formed on a continental arc that resulted from the interaction of Australia and the Tarim Craton during the assembly of the Rodinia supercontinent, and that the Central Tianshan zone was a part of the Tarim Craton during that time. Besides, the Grenvillian orogenic events may have developed better in the Tarim Craton than previously expected. © 2014 Elsevier Ltd. Source

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