| Middle Devonian Picrites of the Southern Margin of Altay Orogenic Belt and Implications for the Tectonic Setting and Petrogenesis |
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| 作者单位： | Zhang Zhaochong State Key Laboratory of Geological Processes and Mineral Resources，China University of Geosciences，Beijing 100083，China; Institute of Geology，Chinese Academy of Geological Sciences，Beijing 100037，China Yan Shenghao Institute of Mineral Resources，Chinese Academy of Geological Sciences，Beijing 100037，China Chen Bailin Institute of Geomechanics，Chinese Academy of Geological Sciences，Beijing 100081，China Zhou Gang He Yongkang No. 4 Geological Party，Xinjiang Bureau of Geology and Resources，Altay 836500，China Chai Fengmei State Key Laboratory of Geological Processes and Mineral Resources，China University of Geosciences，Beijing 100083，China He Lixin No. 4 Geological Party，Xinjiang Bureau of Geology and Resources，Altay 836500，China |
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| 基金项目： | 国家"305"科技攻关项目，国家重点基础研究发展计划(973计划) |
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| 摘 要： | INTRODUCTION Inrecentyears,greatprogressonthegeologic tec tonicevolutionandmineralresourcesofXinjianghas beenachieved.However,manyissuesarestilldebated, suchasancienttectonicpatternsandtheclosuretimeof theancientoceanicbasin(LiandXu,2004).Theseis sueshavelimitedourknowledgeoftheformationande volutionofAsiancontinents,aswellastheexploration anddevelopmentofmineralresources. Recently,theHilaketehalasuporphyrycopperde positwasdiscoveredinthestrataoftheMiddleDevoni anBeitashanFormatio…
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| 关 键 词： | 阿尔泰造山带 岩石发展学 新疆自治区 地质构造 |
Middle Devonian Picrites of the Southern Margin of Altay Orogenic Belt and Implications for the Tectonic Setting and Petrogenesis |
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| Authors: | ZHANG Zhaochong YAN Shenghao CHEN Bailin Zhou Gang He Yongkang Chai Fengmei He Lixin |
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| Abstract: | The Altay orogenic belt of Xinjiang in NW China represents one of the important sites of juvenile crustal growth during the Phanerozoic. However, some important issues, e.g., tectonic evolution and petrogenesis, still remain controversial. The picrites in the south margin of the Altay orogenic belt were discovered in the lower part of marine volcanic-sedimentary sequences of the northwest-striking Middle Devonian Beitashan Formation (Fm.), which consists chiefly of intermediate-basic volcanic rocks intercalated minor carbonate, siltstone and siliceous rocks. The picrites are usually highly porphyritic, and contain abundant forsteritic olivine phenocrysts with minor clinopyroxene distributed in the groundmass, which consist of olivine, clinopyroxene and plagioclase with minor Fe-Ti oxides. The MgO contents of the picrites range from 14 wt% to 22 wt% with Mg# (atomic Mg/(Mg Fe) ratio) of 0.75-0.80. They are characterized by slightly negative Ti anomalies, remarkably negative Nb and Ta anomalies and slightly positive P and Sm anomalies with the similar abundances of HFSE as MORB on the MORB-normalized trace element patterns, all of which characterize typical island arc magmas. In combing with the southwestward migration of the magmas of the Beitashan Fm., we propose that the magmas may result from the southwestward subduction of Junggar ocean plate. The Zr/Nb ratios (23-66) of both picrites and basalts resemble the MORB (10-66), suggesting that they were derived from the MORB-like sources. However, the basalts and picrites display some distinguishable element ratios and REE patterns, e.g., Ti/V (23-35) and Zr/Sm (18-23) ratios of basalts are higher than those of picrites (14-17 and 14-15 respectively), and the basalts display flat-type REE-chondrite patterns whereas the picrites are characterized by lower total REE concentrations ((26-34)×10-6) and slight enrichment of light REE. These distinguished geochemical characteristics could be interpreted by different partial melting degrees and mantle sources, i.e., the basalts were generated by lower partial melting of amphibole-bearing spinel peridotite which was metasomatized by fluids released from subducted oceanic crust, and the picrites were resulted from the higher degree of partial melting of metasomatized garnet peridotite under high temperature. In contrast, the andesite with significant LREE and LILE enrichment may be resulted from the partial melting of eclogites. |
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| Keywords: | picrite geochemistry island arc Middle Devonian Altay orogenic belt |
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