藏南太昭地区早侏罗世岩浆岩的形成机制和构造动力学过程

藏南地区冈底斯岩基是长期构造岩浆作用的产物，保存着与洋-陆俯冲和陆-陆碰撞作用相关的构造动力学过程的丰富记录，是揭示深部岩石圈物质组成及其长期演化特征的重要岩石探针。太昭地区发育高Na/K比花岗岩（组一）和低Na/K比花岗岩（组二），另外，在距离太昭不远的巴松措地区发育一套辉长岩-闪长岩。本文以组二花岗岩为主要研究对象，开展与其他二套岩体的对比研究：（1）组二花岗岩的形成年龄为190~194Ma，与组一花岗岩为同期岩浆作用；（2）同位素组成方面，组二花岗岩的Nd-Hf同位素组成（ε_Hf（t）=-7.2~-2.5；ε_Nd（t）=-4.1~-3.7）略高于组一花岗岩，Sr同位素（⁸⁷Sr/⁸⁶Sr（t）=0.7061~0.7070）比值略低于组一花岗岩，而与巴松措中基性岩相近，这也暗示组二花岗岩的源区不同于组一花岗岩，是由受熔体交代的弧下地幔物质部分熔融作用产生的基性岩浆分离结晶的产物，而组一来源于中下地壳古老基性岩部分熔融；（3）地球化学组成方面，组二花岗岩的Al₂O₃、CaO、TiO₂、FeO^T、MgO、Sr、Sc等元素组成与SiO₂含量呈负相关关系，但Na₂O、Rb、Nb、Ta等元素组成与SiO₂含量呈正相关关系，表明组二花岗岩是较原始的基性岩浆经历角闪石和斜长石的分离结晶作用的产物，并且在岩浆向高硅演化阶段经历了钾长石的分离结晶作用。综合以上分析，本文认为太昭地区这两组花岗岩形成于新特提洋板片向北俯冲作用相关的构造背景。

Petrogenesis and tectonic dynamics of Early Jurassic magmatic rock in the Taizhao area,southern Tibet

The Gangdese batholith in southern Tibet preserves various magmatic rocks that can be used to infer tectono-magmatic processes associated with the northward subduction of the Neo-Tethyan oceanic lithosphere and with the Indian-Eurasian continental collision. The granitic pluton in Taizhao area in this batholith consists of two groups of granites, i.e., Group Ⅰ with high Na/K ratios and Group Ⅱ with low Na/K ratios, respectively. Furthermore, a set of gabbro and diorite developed in Basum Co area, several kilometers to the east of Taizhao area. The Group Ⅱ granites are treated as the main object in this paper to the studies for comparing with the other two ones. Zircon U-Pb dating results indicate that Group Ⅱ granites formed at 190~194Ma, which is coeval with Group Ⅰ granites. The Group Ⅱ granites are characterized by slightly more enriched Hf (εHf(t)=-7.2~-2.5) and Nd(εNd(t)=-4.1~-3.7) and lower Sr (87Sr/86Sr(t)=0.7061~0.7070) isotope compositions than those in Group Ⅰ granites, which is similar to the isotope compositions of intermediate-mafic rocks. Such features indicate that Group Ⅱ granites represent the product derived from fractional crystallization of intermediate-mafic magmas. The intermediate-mafic magmas originated from melting of melt-modified sub-arc mantle. The contents of SiO2 of Group Ⅱ granites display negative correlations with Al2O3, CaO, TiO2, FeOT, MgO, Sr and Sc, but positive correlations with Na2O, Rb, Nb and Ta. These geochemical features suggest the important role of hornblende and plagioclase and potassium feldspar as fractional crystallization phases during the differentiation of more-primitive mafic magma and the formation of high-Si granites, respectively. Both of them possibly represent the contemporaneous magmatism during the northward subduction of the Neo-Tethyan Oceanic lithosphere.