西秦岭糜署岭花岗岩体年代学、地球化学特征及其构造意义

摘要

An EW trending Indosinian granitoide belt distributes near the Mianlue Suture Zone in the southern margin of the Qinling Orogenic Belt.The Mishuling intrution is one of the Indosinian granitoids located in the western Qingling and north to the Mianlue Suture Zone.This article studies the LA-ICP-MS zircon U-Pb ages and geochemistry characteristics of the Mishuling biotite quartz monzonite and monzodiorite enclaves.The results show that the crystallization age of the Mishuling biotite quartz monzonite is 214.5 ±1.6Ma(MSWD =0.24).The A/CNK ratios of the Mishuling biotite quartz monzonite and monzodiorite enclaves range from 0.64 to 0.97,indicating that the granitoids are metaluminous rocks.The Σ REE of the granitoides are 197.3 × 10-6 ～ 246.6 × 10-6.The REE patterns show rightward incline and moderate negative Eu anomaly.The trace element geochemistry is characterized evidently by negative anomaly of Ta,Nb,P,Ti,Ba,Sr,etc.and positive anomaly of Rb,U,La,Zr,Hf,Nd,Y,etc.The host rock biotite quartz monzonite and enclaves monzodiorite are Ⅰ-type granitoids,which may originate from a single magma.The magma was formed by partial melting of basic rocks.The Mishuling intrusions are post-orogenic granitoids which were formed in a transitional tectonic setting from syn-(compressional setting) to post-collision (extensional setting).They were products formed by partial melting of thicken lower crust resulting from the collision between the North China plate and Yangtze plate.%秦岭造山带南缘勉略缝合带附近发育东西向展布的印支期花岗岩带,位于西秦岭地区勉略缝合带北侧的糜署岭岩体是该花岗岩带的一部分.本文对糜署岭岩体寄主岩及二长闪长质包体进行LA-ICP-MS锆石U-Pb定年和地球化学特征研究.结果表明,糜署岭岩体的结晶年龄为214.5±1.6Ma(MSWD =0.24).糜署岭岩体寄主岩和二长闪长质包体A/CNK在0.64～0.97之间,具有准铝质特征.稀土元素总量(ΣREE)为197.3×10-6～246.6×10-6,在稀土元素配分图上显示为右倾型,Eu具有中等的负异常.高场强元素Ta、Nb、P、Ti和大离子亲石元素Sr、Ba明显亏损,而Rb、U、La、Zr、Hf、Nd、Y等元素具有明显的正异常.寄主岩和包体是同源岩浆演化而来,是以基性岩为主的源岩部分熔融形成的花岗质岩浆上升侵位形成的,为Ⅰ型花岗岩.糜署岭岩体形成于同碰撞(挤压环境)向碰撞后(伸展环境)转化阶段,为后造山花岗岩类,是印支期扬子与华北地块全面碰撞导致的地壳增厚下地壳部分熔融的产物.