北秦岭早古生代宽坪岩体两期花岗质岩浆锆石U-Pb年代学、地球化学及其地质意义

摘要

宽坪岩体位于北秦岭造山带东部,岩体主要由片麻状黑云母二长花岗岩和似斑状黑云母二长花岗岩组成.锆石LA-ICP MS U-Pb定年结果表明宽坪岩体可划分为两期:早期为片麻状黑云母二长花岗岩,其锆石U-Pb年龄为448.3±4.5 Ma,该期花岗岩富硅、富碱,属准铝质和高钾钙碱性系列,形成压力较低;晚期为似斑状黑云母二长花岗岩,其锆石U-Pb年龄为421.4±2.5 Ma,该期花岗岩属强过铝质和高钾钙碱性—钙碱性系列.系统的地球化学研究表明,早期的片麻状二长花岗起源于中上地壳(贫斜长石的源区)在H20饱和条件下发生部分熔融形成的贫Al2O3、富SiO2花岗质岩浆;晚期的似斑状二长花岗岩代表较高温的中—下地壳(杂砂岩或是花岗质片麻岩)发生部分脱水熔融形成的岩浆,这表明北秦岭地区从450～420 Ma经历了从早期的同碰撞阶段到后期的后碰撞阶段.%Objectives:The Early Paleozoic Kuanping granite pluton in Danfeng area have important geological significance for constraining the tectonic evolution of the Qinling Mountains.Kuanping granite pluton is composed by gneissic biotite monzogranite and porphyraceous biotite granite.Methods:Based on the field work,through the microscope observation,the whole rock chemical analysis,the ziron LA-ICP MS U-Pb dating of the gneissic biotite monzogranite and porphyraceous biotite monzogranite.Results:The gneissic biotite monzogranite and porphyraceous biotite monzogranite have ages of 448.3 4.5Ma and 421.4 2.5Ma respectively.The gneissic biotite monzogranite display high SiO2 and (Na2O+K2O) contents,they belong to metaluminous and high-K calc-alkaline series,it is considered to be formed by fluxing melting of plagioclase-poor pelitic rocks in middle—upper crust.The porphyraceous biotite granite is peraluminous,they are considered to be formed by dehydration melting of metagerywacks or granitic gnesis in middle—lower crust.Conclusions:The geochemical evolution of the Paleozoic granites in the northern Qinling from 450 to 420Ma reveals a tectonic evolution from syn-collisional to post-collisional process.