广西大瑶山东南缘侏罗纪埃达克质花岗岩的特征、成因及构造意义

Characteristics, petrogenesis and tectonic implications of the Jurassic adakitic granites at southeast edge of Dayao Mountain, Guangxi

对钦杭结合带西南段大瑶山东南缘的中晚侏罗世埃达克质花岗岩(165~153 Ma)进行了岩石学、地球化学研究,并探讨了埃达克质和TTG岩类的特征属性。岩石SiO;含量63.76%~72.13%,总体具高Al_(2)O_(3)(Al_(2)O_(3)≥15%)、低MgO(<3%),亏损重稀土元素(HREE)、正Eu异常或弱的负Eu异常,低Y(≤18×10^(-6))和Yb(≤1.9×10^(-6)),高Sr(>300×10^(-6))和Sr/Y值(>20)等埃达克岩独特的地球化学特征。结合区域构造演化分析认为,该侏罗纪埃达克质花岗岩形成于陆内伸展构造背景,为大陆板内加厚(隆起区)的下地壳底部岩石部分熔融的产物,属大陆板内环境I型花岗岩。具有类似于低镁安山岩/闪长岩系列(LMA)和镁安山岩/闪长岩系列(MA)两种高压型TTG亚类的属性,为古俯冲增生带下地壳弧型岩石熔融的继承性特征,与中生代古太平洋俯冲板片熔融过程无关,属非俯冲成因的埃达克/TTG岩类。其空间上与大瑶山东南缘早古生代俯冲增生带高度重合,且与早古生代TTG侵入岩组合紧密相邻,提示它们可能源自于早古生代洋壳俯冲带或大陆边缘弧下地壳玄武质岩石的部分熔融,因而具有洋俯冲成因的特征属性。

This paper presents the petrology and geochemistry of the middle-late Jurassic adakitic granites (165~153 Ma) in the southeast edge of Dayao Mountain in the southwestern part of the Qinhang junction zone,and discusses its characteristic attribute of adakitic and TTG rocks.The SiO;content of the rocks varies from 63.76%to 72.13%.The rocks show adakitic features with high Al_(2)O_(3)(≥15%),low MgO(<3%),depleted HREE,positive Eu anomaly (some of them are weakly negative Eu anomaly),low Y(≤18×10^(-6)) and Yb(≤1.9×10^(-6)),high Sr(>300×10^(-6)) and Sr/Y(>20).Combined with the study of regional tectonic evolution,it is considered that the Jurassic adakitic granite was formed in the intracontinental extensional tectonic setting,which is the product of partial melting of the thickened lower crust in the continental intraplate (uplift area),and belongs to the intracontinental nonarc environment I-type granite.The Jurassic adakitic granites are similar to the two high-pressure TTG subclasses of low-Mg andesite/diorite series (LMA) and Mg andesite/diorite series (MA).They are the inheritancecharacteristics of arc-type rocks melting in the lower crust of the Paleozoic subduction accretion zone,and belong to non-subduction genesis,which has nothing to do with the Mesozoic Paleo-Pacific subduction slab melting process.They are highly coincident with the early Paleozoic subduction accretion zone in the southern margin of Dayao Mountain,and closely adjacent to the early Paleozoic TTG intrusive rock assemblage,suggesting that they may have originated from the partial melting of basaltic rocks in the oceanic subduction zone or the lower crust of the continental margin arc in the early Paleozoic,so they have the characteristic attributes of oceanic subduction.