藏南泽当岛弧花岗岩成因限定——来自角闪石岩锆石U-Pb、地球化学和Sr-Nd同位素的证据

Petrogenesis of the Zetang arc granite in southern Tibet: Evidence from zircon U-Pb dating,geochemistry and Sr-Nd isotopes of amphibolite

泽当岛弧位于藏南乃东县境内,主要由英云闪长岩、花岗闪长岩、奥长花岗岩和角闪石岩组成,是晚侏罗世洋壳俯冲的产物。本文对该地区出露的角闪石岩开展了野外地质调查,通过室内SHRIMP锆石U-Pb定年、全岩地球化学和Sr-Nd同位素等研究,探讨了泽当角闪石岩的成因、源区及其与花岗岩的成因关系,获得以下认识:角闪石岩成岩年龄为159.1±7.2 Ma;角闪石岩具有较低Sr含量、较高的Y含量和较低的Sr/Y值,富集MREE;角闪石岩与花岗岩Sr-Nd同位素特征一致,角闪石岩87Sr/86Sr(t)值(0.7040~0.7045)较低,εNd(t)值变化范围为+5.5~+6.1。结合前人研究认为,泽当岛弧的英云闪长岩、花岗闪长岩、奥长花岗岩和角闪石岩均来自地幔楔部分熔融作用。英云闪长岩为相对原始的岩浆,在角闪石分离结晶作用未结束前,先从岩浆房中分离出来;残余岩浆继续发生角闪石分离结晶作用,导致Sr/Y值进一步升高和Cr、Ni、MREE含量进一步降低。因此,角闪石的分离结晶作用是泽当岛弧高Sr/Y花岗岩形成的关键因素。

The Zetang island arc,located in Nedong County,southern Tibet,mainly consists of tonalite,trondhjemite,granodiorite and amphibolite.The Zetang island arc has been identified as a late Jurassic intra-oceanic arc.In this paper,integrated field geological survey,SHRIMP zircon U-Pb dating,whole rock geochemistry and Sr-Nd isotope study were carried out on the Zedang island arc to discuss magma source of amphibolite and its relationship with coeval granites.Some conclusions have been reached:①SHRIMP zircon206 Pb/238 U dating of amphibolite shows a crystallization age of 159.1±7.2 Ma;②Sr-Nd isotopic characteristics of amphibolite and granites are similar,with 87Sr/86Sr(t)ratios being 0.7040~0.7045 andεNd(t)values being+5.5~+6.1.③geochemical characteristics show low Sr,high Y,low Sr/Y radio,and enrichment of MREE.A comparison with other published data shows that tonalite,trondhjemite,granodiorite and amphibolite of Zetang arc were likely derived from the partial melting of the mantle wedge.Tonalite is a relatively primitive magma,and had been separated from the magma chamber before the full completion of amphibolite fractional crystallization.With the ongoing crystallization of amphibolite,the Sr/Y ratios,Cr,Ni and MREE values of residual magma increased step by step.Therefore,the fractional crystallization of amphibolite seems to have been the key factor for forming the high Sr/Y granites of Zetang island arc.