辽东半岛早白垩世三股流岩体岩石成因:微量元素模拟和Sr-Nd同位素的制约

The petrogenesis of the Early Cretaceous Sanguliu pluton in the Liaodong Peninsula,NE China:Constrained from the trace-element modelling and Sr-Nd isotopes

辽东半岛在中生代发生大规模的岩浆活动和金成矿作用,其中尤以白垩纪岩浆作用最为强烈。本文对早白垩世三股流岩体进行了锆石U-Pb年代学、全岩地球化学和Sr-Nd同位素研究。三股流岩体主要由似斑状角闪黑云花岗岩和细粒黑云母花岗岩组成,分别形成于125.1±1.2Ma和128.7±2.2Ma。穿切三股流岩体的闪长岩脉形成于125.1±2.4Ma。角闪黑云花岗岩具有准铝质、富水、高氧逸度特征,为高钾钙碱性I型花岗岩。微量元素模拟和Sr-Nd同位素表明角闪黑云花岗岩可能来源于古元古代变基性岩和辽河群变中性岩的部分熔融。细粒黑云母花岗岩并非直接来源于中下地壳岩石的部分熔融,而是角闪黑云花岗岩分异演化的结果。岩相学和微量元素模拟表明,分离相主要为钾长石+斜长石+角闪石+黑云母+石英。细粒黑云母花岗岩亏损中稀土的特征表明榍石也是重要的分离相。随着岩浆的演化,粒间高硅熔体由于发生水饱和而快速结晶形成具有细粒结构的高硅黑云母花岗岩。闪长岩脉具有高Mg#(0.51~0.52)和高Sr/Y(83.3~85.5)特征,属于埃达克质高镁闪长岩。相对于三股流花岗岩,较低的87Sr/86Sr初始值(0.708723)、负的εNd(t)值(-14.1)指示闪长岩脉可能形成于幔源岩浆和壳源岩浆的混合作用。由于太平洋板块的俯冲造成华北克拉通的减薄,早白垩世辽东半岛处于强烈伸展体制,富集岩浆圈地幔部分形成的玄武质岩浆底侵下地壳,引起中下地壳发生部分熔融并发生强烈的结晶分异,形成三股流岩体。同时,玄武质岩浆在上升过程中与壳源花岗质岩浆发生一定程度的混合作用,形成埃达克质高镁闪长岩脉。

Large-scale magmatic activity and gold mineralization occurred in the Liaodong Peninsula during the Mesozoic,especially in the Cretaceous Era.In this paper,we report LA-ICPMS U-Pb ages,whole-rock geochemistry and Sr-Nd isotopic data for the Early Cretaceous Sanguliu pluton.The Sanguliu pluton is composed of porphyritic coarse-grained amphibole biotite granite,the fine-grained biotite granite and the late diorite dyke that cut through the amphibole biotite granite.They emplaced at 125.1±1.2 Ma,128.7±2.2 Ma and 125.1±2.4 Ma,respectively.The amphibole biotite granite is metaluminous high-K2 O calc-alkaline I-type granite with high H2 O content and high fO2.The trace-element modelling and Sr-Nd isotopes indicate that the amphibole biotite granite was derived from the partial melting of the Paleo-Proterozoic meta-mafic rocks and the meta-andesite of the Liaohe Group.The fine-grained biotite granite was derived from the amphibole biotite granite through intensive fractional crystallization with separate phases of potassium feldspar,plagioclase,amphibole,biotite and quartz.Depletion in MREE of fine-grained biotite granite indicates titanite as an important additional separate phase.The diorite dyke have high Mg#(0.51~0.52)and high Sr/Y ratios(83.3~85.5),and belongs to high-Mg adakitic diorite.They have low initial87 Sr/86 Sr ratio(0.708723)and negativeεNd(t)values(-14.1),which indicate that the diorite dyke derived from a process of magma mixing between siliceous crustal melts and basaltic magma from enriched lithospheric mantle.In the Early Cretaceous,the Liaodong Peninsula was in an extensional environment related to the subduction of the Paleo-Pacific Plate.The underplated hot basalt derived from the enriched lithospheric mantle deduced the partial melting of the lower crust and the intensive fraction crystallization,which formed the Sanguliu pluton.Simultaneously,the basaltic magma mixed with a certain portion of the granitic melt during the ascending process,forming the adakitic high Mg diorite dyke.