内蒙古沙麦岩体:正ε_(Nd)(t)值的过铝质花岗岩  

A peraluminous granite with positive ε_(Nd)(t)values:the Shamai pluton in Inner Mongolia,northeast China

作　　者：胡朋[1] 聂凤军[1] 赫英[2] 刘妍[1] 

机构地区：[1]中国地质科学院矿产资源研究所,北京100037 [2]西北大学地质学系,西安710069

出　　处：《岩石学报》2006年第11期2781-2790,共10页Acta Petrologica Sinica

基　　金：国家"973"项目(2002CB412606);国家自然科学基金项目(40573025)资助

摘　　要：内蒙古沙麦岩体位于大兴安岭中段西坡,是二连浩特-东乌珠穆沁旗花岗岩带的重要组成部分。岩体中脉岩和围岩捕虏体较为发育,岩性组合主要为中细粒黑云母花岗岩和细粒似斑状花岗岩。在化学成分上,沙麦岩体以铝弱过饱和,富硅,全碱含量中等和低磷为特点,标准矿物中刚玉分子大于1%,可归为过铝质 S 型花岗岩。在稀土元素和微量元素组成上,岩体富 Rb、Th、U、Ta,贫 Ba、Sr、P、Ti,Rb/Sr 比值高,K/Rb 比值低,Eu 负异常明显。在 Nd 同位素组成上,岩体 Nd 同位素组成变化较大,除一件样品ε_(Nd)(t)值为负值以外,普遍以具正ε_(Nd)(t)值为特点,T_(2DM)值介于275.79～1048.41Ma 之间。研究结果表明,沙麦岩体形成于晚中生代后造山的岩石圈伸展环境中,是幔源基性岩浆底侵及其诱发的壳-幔混合岩浆,同化中上地壳泥质围岩并经高程度分离结晶的产物。Located in the western slope of central Da Hinggan Mountains, the Shamai pluton is an important parts of the Erlianhot- Dongwuqi granite belt, and has an outcrop area of about 2700km^2. It is composed of fine- to medium-grained biotite granite. Many dike swarms and xenoliths were discovered in the pluton. Chemical analysis show that the granites have high SiO2 （73.55% - 76. 10% ）, moderate Al2O3 （ 12. 86% - 13.98% ） and alkaline （ Na2O ＋ K2O = 6.46% - 9.34% ）, low P2O5 （ 〈 0. 1% ）. They are slightly peraluminous, contain normative corundum （ 〈 1% ）. It can be ascribed to the peraluminous S-type granite. Trace and rare earth elements of the granites are characterized by enriched in Rb, Th, U, Ta, and depleted in Ba, Sr, P, Ti. Chondrite-normalized REE patterns display significant negative Eu anomalies （δEu = 0.02 -0. 13）. The characteristics of the trace and REE elements of the granite suggest that it has been experienced highly fractional crystallization. The granites have broad Nd isotopic composition with initial εNd values ranging from - 1.67 to 7. 80, and young two-stage depleted-mantle model ages （ T2DM =275.79 - 1048. 41Ma）. Based on field geological investigations, combined the geochemical characteristics with the regional tectonic settings, it is suggested that the Shamai pluton was formed in the late Mesozoic post-collision stage in which a rapid lithospheric extension must have occurred, resulting in the underplating of the basaltic magma. The crust-mantle miscible magma induced by the underplating assimilated the upper crust and fractional crystallized （AFC） during ascending at the same time, and finally intruded the Middle-Lower Jurassic volcanic- sedimentary rocks and formed the Shamai pluton.

关 键 词：后造山 伸展作用 过铝质 壳-幔相互作用 沙麦岩体 大兴安岭 

分 类 号：P588.121[天文地球—岩石学] P597.3[天文地球—地质学]