东昆仑鄂拉山岩浆带晚三叠世后碰撞伸展:来自索拉沟高分异I型花岗岩的证据

鄂拉山岩浆带位于东昆仑造山带最东端，为研究该地区晚三叠世的构造背景，选取索拉沟地区钾长花岗岩开展研究。LA-ICP-MS锆石U-Pb定年结果显示，索拉沟钾长花岗岩加权平均年龄为（233±1）Ma，形成于晚三叠世早期。该岩石有很高的w（SiO₂）（75.91%~77.23%）、富K₂O和Na₂O，贫CaO、MgO、TiO₂和P₂O₅，A/CNK介于1.01~1.05，属高钾钙碱性系列，锆石饱和温度733~768℃，具有强烈的Eu负异常（Eu/Eu*=0.09~0.25），明显富集大离子亲石元素（LILE Rb、Th、U、K等）和轻稀土元素（LREE），亏损Ba、Sr及Nb、P、Zr、Ti等高场强元素（HFSE），显示高分异I型花岗岩的特征。锆石Hf同位素初始值（176Hf/177Hf）范围为0.282 487~0.282 611，ε_Hf（t）介于-3.54~-0.56；对应的两阶段模式年龄T_2DM（Hf）为1.16~1.33 Ga。索拉沟钾长花岗岩是新生下地壳部分熔融后经过分离结晶作用形成，新生下地壳是幔源岩浆在特提斯洋俯冲阶段（242~238 Ma）底侵古老地壳形成。结合晚古生代至中生代东昆仑地区的构造演化特征，认为索拉沟钾长花岗岩形成于张性构造背景，与古特提斯洋俯冲结束后巴颜喀拉地体与东昆仑地体后碰撞造山伸展作用有关。 

Late Triassic post-collision extension at Elashan magmatic belt,East Kunlun Orogenic Belt: Insights from Suolagou highly fractionated I-type granite

Studying the Elashan magmatic belt located easternmost of the East Kunlun Orogenic Belt (EKOB), we present zircon U-Pb age and Lu-Hf isotopes and whole-rock chemistry of syenogranite, from Suolagou area, in order to discuss its rock type and tectonic setting. The zircon weighted mean ages of the syenogranite is (223±1) Ma, indicating it was formed in the Late Triassic. The syenogranite is characterized by high silic, enrichment of alkaline, but depletion in calcium, magnesium, titanium and phosphorus. The A/CNK values range from 1.01 to 1.05, showing a peralkaline affinity, with strong negative Eu anomalies (Eu/Eu*=0.09-0.25). Zircon saturation temperatures are 733 to 768℃ and the syenogranite. The rock is obviously enriched in large ion lithophile elements (LILE Rb, Th, U, K, etc.) and light rare earth elements (LREE), and depleted in Ba, Sr and Nb, P, Zr, Ti and other high field strength elements (HFSE), showing characteristics of highly fractionated I-type granites. The initial zircon Hf isotope value (176Hf/177Hf)_i ranges from 0.282 487 to 0.282 611, ε_Hf(t) ranges from -3.54 to -0.56, and two-stage mode age T_2DM(Hf) is 1.16 to 1.33 Ga. The rock is formed by crystallization after partial melting of the juvenile lower crust, and the juvenile lower crust is formed by the mantle-derived magma underlying the ancient crust during the northward subduction stage of the Paleo-Tethys Ocean(242-238 Ma). Combining with the Late Paleozoic to Mesozoic tectonic evolution of the East Kunlun area, we suggest that the syenogranite is formed in the extensional tectonic setting, related to the post-collision extension of the Bayan Kala terrane and the East Kunlun (close of Paleo-Tethys ocean).