西昆仑北缘塔尔一带三叠纪S型花岗质岩石特征及成岩构造环境

CHARACTERISTICS OF TRIASSIC S-TYPE GRANITIC ROCKS AND ITS TECTONIC ENVIRONMENT IN TAER AREA,NORTHERN MARGIN OF THE WEST KUNLUN

西昆仑北缘塔尔一带印支期花岗岩较为发育,以贝勒克其岩体为代表,该岩体岩石具有富硅(w(SiO_2)为68.6%~73.9%)、高钾(w(K_2O)为3.58%~4.45%)、高钙(w(CaO)为2.37%~4.85%)和弱过铝质(A/CNK为0.96~1.17)等特征,主体岩石为钙碱性黑云母二长花岗岩,锆石U-Pb谐和年龄为236 Ma±4 Ma,结合其侵位关系厘定为三叠纪。岩石稀土总量略低(ΣREE为130×10^(-6)~205×10^(-6)),富集轻稀土(LREE/HREE为10.3~16.0),轻重稀土分馏明显((La/Yb)N为12.9~22.8)和中等负Eu异常(δEu为0.40~0.71),总体上表现出S型花岗岩特征。据岩石中锆石Ti含量温度计计算,结合Sr和Yb的含量变化及δEu,Rb/Sr,Rb/Ba,Nd/Th,Nb/Ta和La/Sm等值指标特征,认为岩石是在温度约为703℃、压力800 MPa^1 500 MPa(或深度为40km^50km)的条件下,由壳源的杂砂岩或页岩熔融形成,产于陆-陆碰撞带构造环境中,这与当时的区域构造环境较为一致。

The Indo-China epoch granites of Beilekeqi rock mass in Taer Area of northern margin of the west Kunlun are characterized by SiO2-rich （68.6%~73.9%） ,K-high （3.58% ~4. 45%）, Ca-high （2.37%~4.85%）,and weak peraluminous （theratio of A/CNK is 0. 96~1.17）. They belong to calc-alkaline porphyritic biotite monzogranite with zircon U-Pb dating age of 236 Ma±4 Ma, formed in Triassic period and showed characteristics of S-type granites. Geochemical analysis reveals that REE of the rock is slightly lower （EREE is 130 × 10^-6 ~205 × 10^-6）,enriched in LREE （LREE/HREE is 10.3~16.0）,obvious fractionation of LREE/HREE （（La/Yb）N is 12.9 ~22.8） ,and moderate negative Eu anomaly （δEu is 0. 40~0. 71）. According to temperature calculation by Ti content thermometer of zircon,combination with content change of Yb and Sr,and characteristics of equivalent index of δEu, Rb/Sr, Rb/Ba, Nd/Th, Nb/Ta and La/Sm, etc. ,it believes that the rocks are formed by dehydration melting of biotite in greywacke or shale of crust source under conditions of temperature at 703℃ and pressure of 800 MPa^1500 MPa （or depth of 40 km^50 km）. The granites occur in the structural environment of continent-continent collision, consistent with the regional tectonic environment of the time.