西藏狮泉河地区晚侏罗世中性侵入岩锆石U-Pb年龄、地球化学特征及构造意义

摘要

A set of intermediate intrusive rocks have been identified from a late Cretaceous pluton which intruded into the Paleozoic stratigraphy along the south of Shiquanhe belt, during the 1:50000 geological survey in the Gaer area. The intrusive rocks are exhibited as stock and apophyse, with the types of quartz diorite, tonalite and granodiorite. The zircon from quartz diorites and tonalites yield U-Pb ages of 157. 3 ± 1. 5 ( MSWD=0. 96 ) and 159. 3 ± 3. 7 Ma ( MSWD =2. 30 ) with LA-ICP-MS method, indicating late Jurassic magmatic activities. These intermediate-acid intrusive rocks have a SiO2 content of 59. 65% ~64. 57%, K2 O of 1. 95% ~2. 54%, Na2 O of 1. 97% ~2. 54%, K2 O/Na2 O of 0. 77% ~1. 29%, P2 O5 of 0. 085% ~0. 139%, Al2 O3 of 13. 63% ~15. 68%and A/CNK value of 0. 89~1. 04, which could belong to meta-aluminous rocks. The REE content of these rocks range from 107. 37 × 10 -6 to 148. 33 × 10 -6 with high grade fractionation of LREE/HREE (6. 77~9. 67) and some negative anomaly of Eu (0. 63~0. 84). Compared with the primitive mantle, all rocks are enriched in large ion lithophile elements ( Rb and Th et al) and depleted in high field strength elements ( Nb, Ta, Ti and P et al) with geochemical characteristics of the island arc calcalkaline magma. Petrological and geochemical characteristics indicate that the rocks have acidic rule of evolution, with the features of comagmatic evolution and typical I-type granite, and they were formed in the island arc environment with plate subduction. This research also indicates that subduction-type granite already existed in the Shiquanhe area during late Jurassic, and the magmatic activities were likely the product of subducted southward from Northern subzone in Shiquanhe area.%侵位于狮泉河结合带南缘和结合带南侧古生代地层的晚白垩世侵入岩中,解体出一套中性侵入岩,呈岩珠、 岩枝状产出,主要岩石类型为石英闪长岩、 英云闪长岩和花岗闪长岩,其中石英闪长岩和英云闪长岩的LA-ICP-MS锆石U-Pb年龄为157.3±1.5 Ma(MSWD=0.96)和159.3±3.7 Ma(MSWD=2.30),指示岩浆活动时间为晚侏罗世牛津期.该岩体SiO2含量在59.65%~64.57%之间,K2 O为1.95%~2.69%之间,Na2 O为1.97%~3.42%,K2 O/Na2 O在0.74~1.29之间,P2 O5为0.085%~0.139%之间,Al2 O3为13.63%~15.68%之间,A/CNK为0.89~1.07,属于准铝质—弱过铝质花岗岩.岩石稀土元素总量(∑REE)为107.37×10-6~148.33×10-6,LREE/HREE在6.77~9.67之间,轻重稀土分异程度强,δEu为0.63~0.84,铕略具负异常;在原始地幔标准化蛛网图上,富Rb、Th等大离子亲石元素(LILE)而亏损Nb、Ta、Ti和P等高场强元素(HFSE),具有岛弧钙碱性岩浆的地球化学性质.从岩石学、 岩石地球化学特征研究表明,该时期岩石显示同源岩浆演化的特征,具有典型I型花岗岩特征,形成于板块俯冲下的岛弧环境.表明狮泉河一带在晚侏罗世已经存在俯冲型花岗岩,研究认为该时期岩浆活动很可能是狮泉河洋盆北缘(北亚带)持续扩张时,向南缘(南亚带)及中冈底斯带北缘俯冲的产物.