Chronological Constraints on Late Paleozoic Collision in the Southwest Tianshan Orogenic Belt, China: Evidence from the Baleigong Granites  

作　　者：HUO Hailong CHEN Zhengle ZHANG Qing HAN Fengbin ZHANG Wengao SUN Yue YANG Bin TANG Yanwen 

机构地区：[1]Laboratory of Dynamic Diagenesis and Metallogenesis,Institute of Geomechanics,Chinese Academy of Geological Sciences,Beijing 100081,China [2]Key Laboratory of Paleomagnetism and Tectonic Reconstruct,Ministry of Natural Resources,Beijing 100081,China [3]Faculty of Earth Sciences,East China University of Technology,Nanchang 330013,China [4]College of Earth and Planetary Sciences,University of Chinese Academy of Sciences,Beijing 100049,China [5]Institute of Geochemistry,Chinese Academy of Sciences,Guiyang 550081,China

出　　处：《Acta Geologica Sinica(English Edition)》2019年第5期1188-1204,共17页地质学报（英文版）

基　　金：financially supported by the National Natural Science Foundation of China (Grant Nos.U1403292, 41472196, 41502085, and 41902214);the National Key Technology Research and Development Program of the Ministry of Science and Technology of China (2015BAB05B04, 2018YFC0604005);the China Geological Survey Bureau (JYYWF20183702, JYYWF20180602)

摘　　要：The Baleigong granites, located in the western part of the southwestern Tianshan Orogen(Kokshanyan region, China), records late Paleozoic magmatism during the late stages of convergence between the Tarim Block and the Central Tianshan Arc Terrane. We performed a detailed geochronological and geochemical study of the Baleigong granites to better constrain the nature of collisional processes in the Southwest Tianshan Orogen. The LA-ICP-MS U-Pb zircon isotopic analyses indicate that magmatism commenced in the early Permian(~282 Ma). The granite samples, which are characterized by high contents of SiO2(67.68-69.77 wt%) and Al2O3(13.93-14.76 wt%), are alkali-rich and Mg-poor, corresponding to the high-K calc-alkaline series. The aluminum saturation index(A/CNK) ranges from 0.93 to 1.02, indicating a metaluminous to slightly peraluminous composition. Trace element geochemistry shows depletions in Nb, Ta, and Ti, a moderately negative Eu anomaly(δEu=0.40-0.56), enrichment in LREE, and depletion in HREE((La/Yb)N=7.46-11.78). These geochemical signatures are characteristic of an I-type granite generated from partial melting of a magmatic arc. The I-type nature of the Baleigong granites is also supported by the main mafic minerals being Fe-rich calcic hornblende and biotite. We suggest that the high-K, calc-alkaline I-type granitic magmatism was generated by partial melting of the continental crust, possibly triggered by underplating by basaltic magma. These conditions were likely achieved in a collisional tectonic setting, thus supporting the suggestion that closure of the South Tianshan Ocean was completed prior to the Permian and was followed(in the late Paleozoic) by collision between the Tarim Block and the Central Tianshan Arc Terrane.The Baleigong granites, located in the western part of the southwestern Tianshan Orogen（Kokshanyan region, China）, records late Paleozoic magmatism during the late stages of convergence between the Tarim Block and the Central Tianshan Arc Terrane. We performed a detailed geochronological and geochemical study of the Baleigong granites to better constrain the nature of collisional processes in the Southwest Tianshan Orogen. The LA-ICP-MS U-Pb zircon isotopic analyses indicate that magmatism commenced in the early Permian（～282 Ma）. The granite samples, which are characterized by high contents of SiO2（67.68–69.77 wt%） and Al2O3（13.93–14.76 wt%）, are alkali-rich and Mg-poor, corresponding to the high-K calc-alkaline series. The aluminum saturation index（A/CNK） ranges from 0.93 to 1.02, indicating a metaluminous to slightly peraluminous composition. Trace element geochemistry shows depletions in Nb, Ta, and Ti, a moderately negative Eu anomaly（δEu=0.40–0.56）, enrichment in LREE, and depletion in HREE（（La/Yb）N=7.46–11.78）. These geochemical signatures are characteristic of an I-type granite generated from partial melting of a magmatic arc. The I-type nature of the Baleigong granites is also supported by the main mafic minerals being Fe-rich calcic hornblende and biotite. We suggest that the high-K, calc-alkaline I-type granitic magmatism was generated by partial melting of the continental crust, possibly triggered by underplating by basaltic magma. These conditions were likely achieved in a collisional tectonic setting, thus supporting the suggestion that closure of the South Tianshan Ocean was completed prior to the Permian and was followed（in the late Paleozoic） by collision between the Tarim Block and the Central Tianshan Arc Terrane.

关 键 词：Ⅰ-type granite Late PALEOZOIC ZIRCON U–Pb DATING Baleigong GRANITES SOUTHWEST TIANSHAN 

分 类 号：P58[天文地球—岩石学]