东昆仑河尔格头地区晚三叠世花岗岩成因:年代学、地球化学及Sr-Nd-Pb同位素约束

The petrogenesis of the Late Triassic granites in the Heergetou area, East Kunlun:constraints from geochronology, geochemistry and Sr-Nd-Pb isotopes

河尔格头地区位于东昆仑造山带西段,发育大量花岗岩岩石组合,对该地区晚三叠世花岗岩的研究不仅有助于认识东昆仑造山带印支期构造-岩浆演化历史,还可为古特提斯洋俯冲及洋盆闭合时限提供约束。本文报道了对河尔格头地区花岗岩的LA-MC-ICP-MS锆石U-Pb定年、地球化学、Sr-Nd-Pb同位素的研究结果,为建立完善的年代学格架和构造演化提供了新资料。本次在河尔格头英云闪长岩中获得了230.9±0.7Ma的锆石U-Pb年龄,结合野外接触关系,可以确定该期花岗岩为晚三叠世岩浆活动的产物。全岩地球化学分析结果显示,河尔格头晚三叠世花岗岩为准铝到弱过铝质岩石,属中-高钾钙碱性系列,富集轻稀土和Rb、Th等大离子亲石元素,不同程度亏损Nb、Ta、P、Ti等高场强元素,具负铕异常,属I型花岗岩。从Sr-Nd-Pb同位素分析结果可以看出河尔格头晚三叠世花岗岩具有较高的ISr值(0.705058~0.707582)和负的εNd(t)值(-2.05^-5.14),双阶段模式年龄tDM2为1170~1422Ma,表明岩浆主要来源于地壳。经校正后的(206Pb/204Pb)i值为18.37949~18.58525,(207Pb/204Pb)i值为15.56543~15.59755,(208Pb/204Pb)i值为37.79668~38.27230,Pb同位素组成图解显示其物质来源与壳幔相互作用有关。综合岩体岩石地球化学特征及前人研究成果,河尔格头地区花岗岩显示板块俯冲阶段的特征,可能为阿尼玛卿洋壳消减过程中的产物,古特提斯洋的闭合时限或可推迟至晚三叠世。

The Heergetou area is located in the western part of the East Kunlun orogenic belt. There are a large number of granitic rock combinations. The study of the Late Triassic granites in this area not only helps to understand the Indosinian tectonic-magmatic evolution history of the eastern Kunlun orogenic belt, but also provides constraints on the subduction and the closing time of the Paleo-Tethys Ocean. This paper reports the LA-MC-ICP-MS zircon U-Pb dates, geochemical, and Sr-Nd-Pb isotopic results of granites in the Heergetou area. This dataprovides new information enabling establishment of a complete chronological framework and tectonic evolution of the study area.The zircon U-Pb age of the tonalite in the Heergetou area is 230.9±0.7 Ma, which combined with the field contact relationships,indicates that the granites are the products of Late Triassic magmatic activity.The whole-rock geochemical analyses show that the Late Triassic granites in the Heergetou area lie in the middle-high K calc-alkaline fields.Theiraluminium saturation index shows metalumininous and weak peraluminous features, enrichments of LREE and LILE(Rb, Th etc.), depletions of Nb, Ta, P and Ti(HFSE)and Eu negative anomaly indicating that they belong to the I-type granites. The Sr-Nd-Pb isotopicanalysis results show that the Late Triassic granites in the Heergetou area have high ISr value(0.705058~0.707582) and negative εNd(t) value(-2.05^-5.14), with a two stage Nd mode age(tDM2) of 1170~1422 Ma, indicating that the magma was mainly derived from the crust. The corrected(206Pb/204Pb)i value is 18.37949~18.58525, the(207Pb/204Pb)i value is 15.56543~15.59755, and the(208Pb/204Pb)i value is 37.79668~38.27230. The Pb isotopic composition shows that the material source is related to the crust-mantle interaction. In combination with the geochemical characteristics and previous research, the granites in the Heergetou area show the characteristics of plate subduction stage, which may be the product of the subduction process of the Animaqing oceanic crust. The closing time of the Paleo-Tethys Ocean maybe postponed to the Late Triassic.