大兴安岭中部塔尔气中侏罗世碰撞后花岗质岩石的确定及地质意义

Determination and geological implication of the Middle Jurassic post-collisional granitoids in Taerqi area,central Great Xing’an Range

大兴安岭地区以发育巨量显生宙花岗岩为特征,而且通过对这些花岗质岩浆作用的期次、特征以及成因进行研究对于探讨中国东北地区构造-岩浆演化历史具有重要意义。本文通过对大兴安岭中部塔尔气杂岩体进行锆石U-Pb年代学、全岩主量元素和微量元素以及锆石Hf同位素组成分析,确定这些花岗质岩石的形成时代、成因以及构造背景。塔尔气杂岩体是早石炭世、中侏罗世和早白垩世三期岩浆作用的产物,且中侏罗世花岗质岩石是塔尔气杂岩体的主体,其主要岩石类型为花岗闪长岩、二长花岗岩和正长花岗岩。这些花岗质岩石的锆石均呈自形-半自形,具有典型的震荡生长环带和高Th/U(0.35~2.02)比值,表明其为岩浆成因锆石。LA-ICP-MS锆石U-Pb年龄结果显示中侏罗世花岗质岩石主要形成于173~166Ma。它们的Si O2含量为70.54%~77.96%,Na2O/K2O比值为0.86~1.28,Al2O3含量为11.67%~14.83%,Ti O2含量为0.07%~0.60%,具有较低的Mg O含量(0.02%~0.74%),A/CNK值介于0.9~1.1,表明这些花岗质岩石属于高钾钙碱性I型花岗岩,具有准铝质-弱过铝质特征。塔尔气中侏罗世花岗质岩石富集Rb、Th、U、Zr和轻稀土元素,亏损Ba、Nb、Ta、Sr、Ti和重稀土元素,且具有中等到强烈的铕负异常。此外,它们的锆石176Hf/177Hf值为0.282874~0.282985,εHf(t)值为+7.01^+10.9,二阶段模式年龄为768~516Ma。上述地球化学特征暗示其原始岩浆源于新元古代-显生宙期间新增生陆壳物质的部分熔融,源区残留相主要为斜长石和角闪石,并在岩浆演化过程中经历了斜长石和钾长石等矿物的分离结晶作用。结合前人研究成果,认为研究区中侏罗世花岗质岩石形成于碰撞后伸展环境,可能与蒙古-鄂霍茨克洋闭合后的岩石圈拆沉作用有关。

The Great Xing’an Range is characterized by voluminous Phanerozoic granites,and the studies of the stages,characteristics and genesis of the granitic magmatism play important roles in the exploration of tectonic-magmatic evolution in the Northeast China.In this study,we undertook zircon U-Pb dating,whole rock major elements and trace elements,as well as zircon Hf isotopic analyses of the Taerqi complex in the central Great Xing’an Range,with an aim to determine their formation timing,petrogenesis and tectonic settings.The Taerqi complex is the product of Early Carboniferous,Middle Jurassic and Early Cretaceous magmatisms,and the Middle Jurassic granitoids are the major part of the Taerqi complex,which are composed of granodiorite,monzogranite and syenogranite.Zircons from the dated granitoids are euhedral to subhedral in shape,and are characterized by typical oscillatory growth zoning and high Th/U ratios(0.35~2.02),indicating a magmatic origin.LA-ICP-MS zircon U-Pb data indicates that the Middle Jurassic granitoids were emplaced in 173~166 Ma.These granitoid samples have Si O2 contents of 70.54%~77.96%,Na2 O/K2 O ratios of 0.86~1.28,Al2 O3 contents of 11.67%~14.83%,and Ti O2 contents of 0.07%~0.60%,with low Mg O contents(0.02%~0.74%),and A/CNK values of 0.9~1.1,indicating they belong to high-K calc-alkaline I-type granite,with metaluminous to weakly peraluminous affinities.These granitoid samples are enriched in Rb,Th,U,Zr and light rare earth elements,and depleted in Ba,Nb,Ta,Sr,Ti and heavy rare earth elements,with medium to strong negative Eu anomalies.In addition,zircons from the Middle Jurassic granitoids have initial176 Hf/177 Hf ratios of 0.282874 to 0.282985,and theirεHf(t)values range in+7.01^+10.9,with corresponding two-stage model age of 768~516 Ma.The above geochemical characteristics suggest that the primary magma was derived from partial melting of juvenile crustal material accreted during the Neoproterozoic to Phanerozoic with plagioclase and hornblende in the residue,of which,the magma then experienced fractional crystallization of plagioclase and K-feldspar during magmatic evolution.Combined with previous studies,we conclude that the studied Middle Jurassic granitoids were emplaced in a postcollisional extensional setting,which was likely related to the lithospheric delamination caused by the closure of the Mongo-Okhotsk Ocean.