藏北羌塘南缘早白垩世青草山强过铝质S型花岗岩的成因:来自地球化学和锆石U-Pb年代学的约束

Petrogenesis of the Early Cretaceous Qingcaoshan Strongly Peraluminous S-Type Granitic Pluton,Southern Qiangtang,Northern Tibet:Constraints from Whole-Rock Geochemistry and Zircon U-Pb Geochronology

藏北羌塘南缘扎普-多不杂岩浆弧内的青草山花岗岩体由花岗斑岩和石英二长斑岩组成,目前缺乏地球化学和年代学数据来约束其成因和形成时代。用LA-ICP-MS方法测得石英二长斑岩锆石206Pb/238U年龄加权平均值为122±1 Ma（MSWD=3.9）,花岗斑岩的锆石SHRIMP U-Pb年龄为114.6±1.2 Ma（MSWD=1.1）,表明岩体形成于早白垩世。岩体含白云母、堇青石,无角闪石,具有富铝（A12O3含量：14.81%~15.86%）、贫钙（CaO含量：1.10%~2.44%）、总碱含量高（K_2O＋Na_2O含量：6.86%~8.80%）的特征,铝饱和指数A/CNK为1.06~1.20,在CIPW标准矿物计算中出现刚玉分子（1.20%~2.86%）,未出现透辉石,表明该岩体为一套强过铝质亚碱性S型花岗岩。（La/Yb）N=3.24~16.20,LREE/HREE=4.37~12.4,在配分曲线上显示左高右低的特征,富集Rb,Th,U,K,La,Ce等大离子亲石元素,亏损Ta,Nb,P,Ti,Y等高场强元素,具有较典型的岛弧岩浆岩地球化学特征。研究结果揭示,青草山花岗岩为班公湖-怒江洋壳北向俯冲背景下,上地壳杂砂岩质成分发生部分熔融作用的产物。

The Qingcaoshan granitic pluton, located in the Zhapu-Duobuza magmatic arc in southern Qiangtang, consists of granite porphyry and quartz monzonitic porphyry. However, the age and genesis are not studied due to absence of geochemical and geochronological data. The weighted average ^206pb/^238U age of 122± Ma （MSWD= 3.9） for quartz monzonitic porphyry and 114.6±1.2 Ma （MSWD= 1.1） for granite porphyry is present by LA-ICP- MS and SHRIMP zircon U-Pb dating. The intrusion have minor muscovite and cordierite, no hornblende. The granitic pluton is characterized by enriched Al （Al2O3： 14.81%-15.86%）, depleted Ca （CaO： 1.10%-2.44%）, and high content of alkali （K2O±Na2O： 6.86%-8.80%）. Father more, it is also characterized by ASI of 1.1 （A/CNK： 1.06-1.20）, corundum （1.20%-2.86%） and absence of diopside showed through CIPW calculation, which are indicative of peraluminous sub-alkaline S-type graniten. The intrusive rock is relatively enriched in large ion lithophile elements （Rb, Th, U, K, La, Ce） and depleted in high field strength elements （To, Nb, P, Ti, Y）, and exhibits LREE-enriched （（La/Yb）N： 3.24-16.20） with right-inclined REE distribution patterns. A synthesis of all these characteristics indicates that the Qingcaoshan granitic pluton was probably derived from partial melting of greywacke components in the upper crust as a result of norward subduction of the Bangong Co-Nujiang oceanic crust towards the Qiangtang block.