新疆东天山小白石头黑云母花岗岩年代学、地球化学特征及其地质意义

Geochronology and Geochemical Characteristics of the Biotite-Granite in the Xiaobaishitou Tungsten Deposit,East Tian Shan Mountains of Xinjiang and Their Geological Implications

小白石头黑云母花岗岩位于中天山地块东部南缘。LA-ICP-MS测得该岩体的锆石U-Pb年龄为322±5Ma(MSWD=3.2)。地球化学测试分析表明:小白石头黑云母花岗岩以高硅(73.46%～75.86%)、富碱(Na2O+K2O=7.1%～8.2%)和弱过铝质(A/CNK=1.002～1.02)为特征。稀土元素C1球粒陨石标准化配分曲线右倾,(La/Yb)N=13.92～20.8,LREE/HREE=10.83～14.46,在配分曲线上显示左高右低的特征,K、Rb、Ba、Th等元素富集,Nb、Zr、Hf、Sm、Y、Yb等元素亏损,具有较典型的岛弧火山岩的地球化学特征。原岩恢复显示其为变杂砂岩重熔形成,并经历了高度分异演化。结合区域地质演化综合分析,认为岩体具有岛弧花岗岩地球化学特征是其原岩的印记,低初始87Sr/86Sr同位素组成特征表明,形成花岗岩的原始岩浆来源于下地壳,指示该岩体产出的大地构造背景很有可能是石炭纪北天山洋盆闭合,碰撞过程中造成中天山地块陆壳加厚,造成下地壳重融形成的。

The Xiaobalshitou biotite-granite pluton is situated in the southeastern margin of the central Tian Shah block. LA-ICP-MS U-Pb isotopic dating for zircon separated from the rock yields an average ^206Pb/^238U age of 322±5Ma. Geochemical analysis to the major element, trace element and REE the intrusion indicates that this granite is rich in silicon （73.46% -75.86% Si） , alkali （Na2O＋K2O = 7.1%-8.2% ） and weekly peraluminous （A/CNK = 1. 002 - 1.02 ）. It is relatively enriched in large ion lithophile elements （K, Rb, Ba, Th） and depleted in high field strength elements （Nb, Ta, Zr,Hf） and exhibits LREE-enriched（ （La/Yb） N = 13.92-20.85 ）, and right-inclined REE distribution patterns. The geochemical dating of the granite yields characteristics similar to those VAG, inheriting the characteristics of its original rock. This study suggests that the original rock of the biotite-granite is metamorphic greywacke. Combing chemical characteristics of the granite with regional geology,it is speculated that this biotite-granite formed during late Carboniferous-early Permian when the ancient Tian Shah ocean closed and the crust of the central Tian Shan was thickened, leading to melting of the lower crust.