羌塘雀莫错始新世石英二长岩的年代学、地球化学与岩石成因

摘要

Syenite and alkali-rich quartz-monzonite are generally considered to be derived from partial melting of the enriched mantle. This paper reports quartz-monzonites derived from partial melting of thickened continental crust. The Qoimaco pluton is located at the Qoima Mountain, Northeast of the Qoimaco Lake in the northern Qiangtang (central Tibetan Plateau). The pluton was regarded as mainly consisting of syenite porphyries with formation age of 45 to 23 Ma. Our recent detailed field geological investigation, petrological, geochemical and geochronological studies demonstrated that the pluton mainly consists of quartz-monzonite, and has zircon U-Pb age of 41.71±0.29 Ma, simultaneously with the Eocene volcanic lavas in the area. The rocks have high SiO2(65.12%–66.71%), Al2O3(>14.79%), and alkali (Na2O+K2O, 9.08%–9.71%), K2O (5.50%–5.92%), Sr (1874–2001 μg/g), and Sr/Y (124–136) and La/Yb (67–74) ratios. The rocks are characterized by depleted heavy rare earth elements (e.g., Yb=1.24–1.34 μg/g), Y (14.4–15.7 μg/g) and high field strength elements, enriched large ion lithophile elements, and negligible Eu and Sr anomalies and slightly high Mg#values (47–56). The major and trace element features of the rocks are similar to those of the contemporary adakitic volcanic lavas in the area. In addition, the rocks have very uniform initial 87Sr/86Sr ratios (~0.7069) and εNd(t) values (from ?2.6 to?2.8). Considering the regional geological, petrological and tectonic data, we suggest that the Qoimaco quartz-monzonites were generated through partial melting of the subducted crust during intra-continent subduction, which subsequently interacted with the mantle peridotite during ascent, in the compression setting in the central Tibet triggered by India-Asia continental collision.%正长岩以及富碱的石英二长岩常常被认为起源于富集地幔的熔融.本文报道了起源于增厚陆壳熔融的石英二长岩.雀莫错岩体是分布在羌塘北部(青藏高原中部)的雀莫错(湖)东北部雀莫山上的一个侵入体, 前人认为其由正长斑岩组成, 形成时代不确定(45~23 Ma).近期, 我们对该侵入体进行了详细的野外地质调查和室内岩石学、地球化学以及年代学研究.雀莫错侵入岩主要由石英二长岩组成, 激光锆石U-Pb测年显示, 该岩体的侵位时代为始新世(41.71±0.29 Ma), 与区域上大面积始新世火山岩同期.雀莫错侵入岩高硅(SiO2=65.12%~66.71%)、富碱(Na2O+K2O=9.08%~9.71%)、富钾(K2O=5.50%~5.92%)和高铝(Al2O3>14.79%), 同时高Sr (1874~2001 μg/g), 亏损重稀土Yb (1.24~1.34 μg/g)和Y (14.4~15.7 μg/g), 高Sr/Y(124~136)和 La/Yb(67~74)比值, 富集大离子亲石元素(LILEs), 亏损高场强元素(HFSEs), 具有不明显的 Eu负异常、Sr 正异常和略高的 Mg#(47~56), 与区域上大面积的同期埃达克质火山岩类似.岩石具有非常均一的(87Sr/86Sr)i同位素比值(~0.7069)和 εNd(t)值(?2.6~ ?2.8).结合区域地质、岩石和构造资料, 认为雀莫错侵入岩形成于印度?欧亚大陆汇聚诱发的高原中部挤压阶段: 挤压导致陆内俯冲, 俯冲地壳发生部分熔融, 岩浆在上升的过程中与地幔橄榄岩发生小规模反应,然后岩浆上升侵位形成了该区侵入岩.