四川乡城-稻城地区花岗岩地球化学特征及构造背景探讨

GEOCHEMICAL CHARACTERISTICS AND TECTONIC IMPLICATION OF GRANITES IN THE XIANGCHENG-DAOCHENG,SICHUAN PROVINCE

四川乡城-稻城地区发育4套花岗岩体：包括石英闪长玢岩、花岗细晶岩、二长花岗（细晶）岩和花岗闪长岩，形成时代为印支期到燕山晚期。花岗岩的地球化学研究表明：w（SiO2）=57．82％～81．66％，σ=0．06～2．45，w（K2O＋Na2O）=1．38％～7．76％，AR=1．26～2．93，属于亚碱性岩体；EREE含量较低，轻稀土相对富集，δEu=0．09～1．28，（La／Yb）N=1．88～11．96，总体表现为右倾型。各套花岗岩微量元素地球化学特征差异明显，相对富集大离子亲石元素，贫化高场强元素，Rb／Sr=0．04～6．18。根据花岗岩的R1-R2，Rb—Yb＋Ta，Ta—Yb构造判别图解和区域构造演化史研究，认为二长花岗岩形成于板内构造环境，花岗细晶岩、闪长玢岩和花岗闪长岩形成于岛弧构造环境，其形成与义敦岛弧带碰撞造山活动有关。

Granites in Xiangheng-Daohen areas were mainly composed of quartz-diorite porphyrite,granite aplite , monzonitic granite （aplite） and granodiorite. The age of the granite was from Indosinian to late-Yanshanian. Based on the study of main elements, REE elements and trace elements of the Granites,it was showed that the w（SiOe）=57.82%-81. 66% ,the Rittmann index （a） = 0.06 - 2.45, the AR = 1.26 - 2.93, suggesting that the granites belonged to subalkaline. The gross of REE content was low, LREE was relatively rich,8Eo = 0.09- 1.28, （La/Yb）N = 1.88 -11.96, and the shape of ehondrite-normalized REE patterns inclined towards the right. The granites were of great difference in trace elements, relative LILE-enrichment and HFSE-depletion,Rb/Sr= 0.04-6.18. The discrimination diagrams of tectonic setting about R1-R2, Rb-Yb4- Ta, Ta-Yb and regional tectonic setting evolution revealed that monzonitie granites were formed in continental environment;granite aplite,quartz-diorite porphyrite and granodiorite were foemed in island arc condition. They were resulted from the collision orogenic activities of Yidun island arc.