大兴安岭北段漠河地区早古生代花岗质岩体:对额尔古纳-兴安地块碰撞后伸展过程的响应

The Early Paleozoic granites in the Mohe region of northern Great Xing,an Range:Response to post-collision extension of the Erguna-Xing,an blocks

本文报道了大兴安岭北段漠河地区早古生代二长花岗岩的岩相学、锆石U-Pb年龄、Hf同位素组成以及全岩主-微量元素地球化学数据,以揭示古亚洲洋构造域早古生代期间的构造-岩浆演化过程。漠河地区花岗岩中锆石发育典型的岩浆振荡生长环带和高Th/U比值,表明为岩浆成因锆石,其LA-ICP-MS U-Pb同位素测试结果显示漠河地区花岗岩形成于482~456Ma,代表奥陶纪-志留纪岩浆活动的产物。花岗岩主体为二长花岗岩,主要矿物组成为石英、斜长石和碱性长石,及少量黑云母。全岩主-微量元素地球化学分析结果显示,花岗岩具有高SiO_(2)、Al_(2)O_(3)和全碱(Na_(2)O+K_(2)O)含量,低MgO、Mg^(#)、Cr和Ni含量,并显示高钾钙碱性的属性;结合花岗岩准铝质特征,表明其属于高钾钙碱性I型花岗岩。此外,花岗岩样品富集大离子亲石元素(Rb、Ba、K、Sr等)和轻稀土元素,亏损高场强元素(Nb、Ta、P、Ti等)和重稀土元素,显示明显Eu负异常,与地壳组分的地球化学特征较为相似。花岗岩的锆石ε_(Hf)(t)值大多为正值,结合它们相应的Hf同位素二阶段模式年龄(1487~1106Ma),表明花岗岩的岩浆源区以中-新元古代期间新增生的年轻地壳组分为主,并且存在少量古老地壳物质的贡献。结合部分花岗岩样品重稀土元素亏损的特征,表明漠河地区早古生代花岗岩可能起源于石榴石角闪岩相和角闪岩相地壳物质的部分熔融作用,岩浆演化后期的分离结晶作用对岩浆成分的变化影响并不显著。结合额尔古纳地块广泛发育的同时期造山后岩石组合(A型花岗岩,双峰式岩石组合和碱性侵入体),本文认为漠河地区早古生代花岗岩是地幔物质底侵导致下地壳物质重熔的产物,与额尔古纳地块和兴安地块碰撞后的伸展环境下地幔物质上涌密切相关。

In this paper,we present petrography,zircon U-Pb ages,Hf isotopic compositions,and whole-rock major-and trace-element data for the Early Paleozoic monzogranites in the Mohe region of northern Great Xing'an Range,with the aim of revealing the tectono-magmatic evolution of the Paleo-Asian Ocean tectonic domain during the Early Paleozoic.Zircons from the Early Paleozoic monzogranites in the Mohe region display fine-scale oscillatory growth zoning,together with their high Th/U ratios,implying their magmatic origin.Zircon LA-ICP-MS U-Pb dating results indicate that these monzogranites in the Mohe region were formed during the 482~456Ma,representing the products of magmatic activities during Ordovician-Silurian Period.For mineral assemblage,monzogranites are mainly composed of quartz,plagioclase and alkaline feldspar,with minor biotite.Whole-rock major-and trace-element analysis shows that the monzogranite samples in the Mohe region have high SiO_(2),Al_(2)O_(3),and total alkali contents(Na_(2)O+K_(2)O),low MgO,Cr and Ni contents,as well as low Mg^(#)values.These monzogranites belong to sub-alkaline and high-K calc-alkaline series,combined with their metaluminous characteristics,which all point to an I-type affinity.In addition,the monzogranite samples are enriched in large ion lithophile elements(Rb,Ba,K,Sr)and light rare earth elements,and depleted in high field strength elements(Nb,Ta,P,Ti)and heavy rare earth elements,with obvious Eu negative anomalies,similar to those of igneous rocks derived from crustal materials.Most of positive zircon ε_(Hf)(t)values and corresponding two-stage model ages(1487~1106Ma)of the monzogranite samples indicate that they were originated by dominant juvenile crustal components accreted during the Mesoproterozoic-Neoproterozoic,with minor contributions of ancient crustal materials Considering their characteristics of heavy rare earth elements depletions,these granite samples probably originated from partial melting of lower crust of garnet amphibolite facies and amphibolite facies,with insignificant fractional crystallization during magma evolution.Combined with extensive contemporaneous post-orogenic magmatic assemblages(A-type granites,bimodal igneous rock associations and alkaline intrusions),we argue that the formation of Early Paleozoic granites in the Mohe region are triggered by magmatic underplating due to hot asthenospheric upwelling within a post-collision extensional regime after the amalgamation of the Erguna and Xing'an blocks.