西秦岭江里沟花岗岩体地球化学特征、年代学及地质意义

Geochemical Characteristics and Chronology of the Jiangligou Granitic Pluton in West Qinling and Their Geological Significance

江里沟岩体出露于西秦岭造山带北段,侵入于大关山组之中,岩性为二长花岗岩,主要矿物组成为斜长石、石英、钾长石、黑云母等。该岩体SiO_2=71.76%～75.86%,Al_2O_3=11.67%～14.84%,MgO=0.35%～0.70%,Sr=58×10^-6～223×10^-6,Y=1.2×10^-6～16.1×10^-6,Yb=1.21×10^-6～1.76^-6,里特曼指数介于1.42～2.08,在K_2O-SiO_2关系图上投入高钾钙碱性系列,A/CNK介于1.05～1.1,属过铝质花岗岩。在原始地幔标准化的微量元素分布图中,大离子亲石元素(LILE)K、Th、Rb、Ba等富集,高场强元素(HFSE)Nb、P、Ti、Y、Yb等相对亏损。稀土元素总量(∑REE=62×10^-6～170×10^-6)变化较大,稀土元素球粒陨石标准化分配型式表现为轻稀土强烈富集,重稀土相对亏损的右倾型且Eu负异常较明显(LREE/HREE=6.3～16.2,(La/Yb)N=4.9～17.4,δEu=0.38～0.66),总体显示出喜马拉雅型花岗岩的岩石地球化学特点。LA-ICP-MS锆石U-Pb测年表明其形成年龄为264.0±1.4Ma,属中二叠世晚期。结合前人研究和本次的工作分析认为,江里沟岩体形成于加厚地壳的构造环境,可能是由于受到阿尼玛卿洋盆在闭合过程中俯冲碰撞远程效应的影响,使得本区发生挤压作用,地壳加厚增温,给该地区地壳深部物质重熔侵位提供了动力学条件和岩浆活动空间。

The Jiangligou granitic pluton intruding the Daguanshan Formation outcrops in the northern segment of West Qinling, and consists mainly of monzonitic granite mineralogically containing plagioclase, quartz, potassium feldspar and biotite. The pluton is characterized by typical Himalaya granite features with Si02 =71.76%～75.86%, Al203 =11.67%～14.84%, MgO=0.35%～0.70%, Sr=58×10-6 ～ 223×10-6, Y=l.2×10-6～16.1×10-6, Yb=l.21×10-6～1.76×10-6. Rittmann index ranging between 1.42 and 2.08 is plotted in high K calc-alkaline series in the K20-SiO2 diagram and A/CNK is 1.05～1.1, suggesting that the Jiangligou pluton belongs to the peraluminous granites. Primitive mantle normalized trace elements patterns reveal that large ion lithophile elements （LILE） K, Th, Rb, Ba are enriched, and strength element （HFSE） Nb, P, Ti, Y, Yb are depleted. Total REE content varies greatly with ∑REE = 62×10-6 ～ 170×10-6. Chondrite-normalized REE distribution pattern show strong enrichment in LREE and relative depletion in HREE, with obviously Eu negative anomaly （LREE/HREE = 6.3～16.2, （La/Yb）N=4.9～17.4, 3Eu=0.38～0.66）. LA-ICP-MS isotopic dating show a zircon U-Pb isotopic age of 264.0±1.4 Ma for the Jiangligou granitic pluton, indicating it belongs to middle Permian. Previous studies and this study reveal that the Jiangligou granitic pluton was formed in the thickened crust environment likely affected in distance by subduction and collision resulting from closing process of A＇ nyemaqen oceanic basin, resulting in compression and extrusion effect in this area. Crust thickening and temperature increasing provide dynamic conditions and magmatic activity space for deep crustal material remelting emplacement in the area.