班公湖-怒江缝合带西段昂龙岗日地区中新世岩浆作用

Miocene magmatism in Anglonggangri area,western Bangong-Nujiang suture zone

拉萨地块中新生代构造岩浆活动一直以来都是青藏高原基础地质的研究热点之一。在该地块西部发育有较多的新生代尤其是中新世岩浆岩。本文以拉萨地块西部昂龙岗日地区新发现的中新世花岗岩为研究对象,对其岩石类型、岩石成因与构造背景进行了探讨。昂龙岗日地区中新世花岗岩由细粒白云母二长花岗岩和中细粒二长花岗岩组成,内部发育少量的暗色包体。细粒白云母二长花岗岩的锆石U-Pb年龄为9.95±0.14 Ma,中细粒二长花岗岩的锆石U-Pb年龄为10.68±0.2 Ma,均为中新世。二者都具有高硅(SiO_(2)=72.35%~74.74%)、富钾(K_(2)O/Na_(2)O=1.15~1.45,均大于1)、高铝(Al2O3=14.35%~14.83%)和弱过铝质-过铝质(A/CNK=1.08~1.14)的特征,CIPW刚玉分子数均大于1%。其轻重稀土元素分异十分明显,具中等负Eu异常,富集Rb、Th、K等大离子亲石元素,亏损Nb高场强元素,相对富集Hf、Zr等高场强元素,Sr、Ba、P、Ti等则表现出明显的负异常,为高钾钙碱性系列的过铝质S型花岗岩。两者具有相对低Sr、高Sr/Y、La/Yb值,极低的Y、Yb含量。研究区花岗岩起源于加厚下地壳环境下,并发生了一定程度的岩浆混合作用。综合区域研究成果,昂龙岗日地区中新世花岗岩的形成,可能与印度板块北向俯冲至北拉萨地块下发生板片撕裂后,高热软流圈物质透过板片断离窗上涌诱发下地壳发生部分熔融有关。

The Mesozoic and Cenozoic tectonic magmatic activity in the Lhasa block has always been one of the research hotspots in the basic geology of the Qinghai-Xizang Plateau.There are many Cenozoic,especially Miocene magmatic rocks developed in the western part of the Lhasa block.This article takes the newly discovered Miocene granite in the Anglonggangri area of the western Lhasa block as the research object,and explores its rock types,rock genesis,and tectonic background.The Miocene granites in the Anglonggangri area are composed of fine-grained muscovite monzonitic granite and medium-fine-grained monzonitic granite,with a small amount of dark inclusions developed internally.The zircon U-Pb age of fine-grained muscovite monzonitic granite is 9.95±0.14 Ma,and the zircon U-Pb age of medium-fine-grained monzonitic granite is 10.68±0.2 Ma,both from the Miocene.Both of them have the characteristics of high silicon(SiO_(2)=72.35%~74.74%),rich potassium(K_(2)O/Na_(2)O=1.15~1.45,all greater than 1),high aluminum(Al2O3=14.35%~14.83%),and weak peraluminum peraluminum(A/CNK=1.08~1.14).The number of CIPW corundum molecules is greater than 1%.The differentiation of light and heavy rare earth elements is very obvious,with a moderate negative Eu anomaly,enriched in large ion lithophile elements such as Rb,Th,K,and depleted in high field strength elements such as Nb,relatively enriched in high field strength elements such as Hf and Zr,Sr,Ba,P,Ti,etc.These characteristics indicate that it is a high potassium calcium alkaline series of peraluminous S-type granite.Both of them have relatively low Sr,high Sr/Y,La/Yb ratios,and extremely low Y and Yb contents.The granite in the research area originated from a thickened lower crustal environment and underwent a certain degree of magma mixing.Based on the comprehensive regional research results,the formation of Miocene granites in the Anglonggangri area may be related to the northward subduction of the Indian plate into the north Lhasa block,where plate tearing occurred and high-temperature asthenosphere material flowed through the plate fragments,causing partial melting of the lower crust.