西秦岭北缘达加勒埃达克质花岗岩地球化学特征及构造意义

GEOCHEMICAL CHARACTERISTICS AND TECTONIC SIGNIFICANCE OF DAGALE ADAKITIC GRANITE IN THE NORTHERN MARGIN OF WEST QINLING

西秦岭北部广泛分布中酸性岩体,时代多属于印支期和燕山期,并多具有埃达克岩的特点。本次研究分析了达加勒花岗岩的岩石学和岩石地球化学特征,发现该岩体主要是以花岗岩、二长花岗岩为主,属于高钾钙碱性系列;ACNK值在1.1~1.25之间,平均1.17,属于过铝质;岩石富SiO_(2)(67.98%~74.89%)和Al_(2)O_(3)(13.27%~15.50%),高Sr(773.70×10^(-6)~824.30×10^(-6)),低Yb(0.48~10^(-6)~1.32×10^(-6))、Y(2.79×10^(-6)~14.24×10^(-6)),(La/Yb)N比值24.39-45.73,无负Eu异常(δEu=0.57~1.08),具有典型的埃达克岩地球化学特征。研究认为花岗岩浆起源于下地壳的部分熔融,源岩主要是变质杂砂岩,有少量的泥质岩,其形成时代可能为燕山早期。这与研究区印支一燕山期存在陆陆碰撞或者陆陆俯冲导致地壳加厚,加厚的下地壳发生部分熔融结果一致。

Intermediate-acid rock mass is widely distributed in the northern part of West Qinling,and most of them belong to the Indo-Chinese epoch and The Yanshan Epoch.In this study,the petrological and petrogeochemical characteristics of dagale granite are analyzed,and it is found that the rock mass is mainly composed of granite and monzogranite,belonging to the series of high potassium calcium alkaline.A CNK value ranged from 1.1 to 1.25,with an average of 1.17,which was peraluminous.This granite has typical geochemical characteristics of adakite,including rich SiO_(2)(67.98%~74.89%)and Al_(2)O_(3)(13.27%~15.50%),high Sr(773.70×10^(-6)~824.30×10^(-6)),low Yb(0.48×10^(-6)~1.32×10^(-6)),Y(2.79×10^(-6)~14.24×10^(-6)),(La/Yb)N ratio 24.39~45.73,no negative Eu anomaly(Eu=0.57~1.08).Granitic magma originated from partial melting of the lower crust,and the source rock was mainly metamorphic complex sandstone with a small amount of arglitic rock,which may have formed in the early yanshanian,which is consistent with the thickening of the crust caused by land-land collision or land-land subduction during the Indochina yanshanian period in the study area,and the partial melting of the thickened lower crust.