江南造山带仙霞复式岩体的成因研究

Genesis of the Xianxia Batholith in the Jiangnan Orogenic Belt and its Geological Significance

仙霞复式岩体位于江南造山带东北部,是该区出露面积最大的复式岩体之一。本文在野外地质调查的基础上,对仙霞复式岩体不同期次岩浆岩开展了锆石U-Pb年代学和全岩地球化学分析,结果显示该复式岩体侵位分为早、中、晚三阶段,岩性分别为二长花岗岩(143.0±3.5 Ma)、花岗闪长岩(140.0±2.0 Ma)和钾长花岗岩(138.6±3.5 Ma),是江南造山带燕山期早期(146~135 Ma)岩浆作用的产物。仙霞复式岩体三阶段岩浆岩均为高碱、弱过铝质岩石,具有相似的稀土元素配分模式,微量元素均呈Rb、Ba、U、K、Pb等大离子亲石元素(LILE)富集,Nb、Ta、P、Ti等高场强元素(HFSE)亏损的特征,锆石εHf(t)同位素为负值(–15.54~–1.03)。综合分析显示,仙霞复式岩体的岩浆起源于下地壳物质为主的壳幔混合源区,在岩浆上升演化过程中经历了显著的结晶分异作用,形成于早白垩世(146~135 Ma)古太平洋板块向西俯冲的背景下。

The Xianxia batholith is located at the boundary of Anhui and Zhejiang provinces,northeastern Jiangnan orogenic belt.A systematic investigation of the geology,geochronology,and geochemistry of the Xianxia batholith demonstrate that(1)the Xianxia batholith can be divided into 3 stages of magmatism based on the geological and geochronological characteristics,which are the early-stage monzogranite(143.0±3.5 Ma),the middle-stage granodiorite(140.0±2.0 Ma),and the late-stage K-feldspar granite(138.6±3.5 Ma);(2)all of the 3 stage granitoids are high calc-alkaline and metaluminous.They were derived from the lower crust with mantle input,moreover,SiO2 and alkaline contents increase,while the Mg,Fe,Al contents decrease from the early to late stages.Combined with the previous studies,it is considered that the Xianxia batholith was formed during the western subduction of the Paleo-Pacific Plate in the Early Cretaceous(146–135 Ma).