长江中下游地区武山中酸性岩石Li-Sr-Nd同位素及地球化学

Li⁃Sr⁃Nd Isotopes and Geochemistry of Wushan Intermediate⁃Acidic Magmatic Rocks in Middle⁃Lower Yangtze River Region

为确定长江中下游地区中酸性岩石的成因,对武山花岗闪长斑岩、石英闪长玢岩和基性岩脉进行Li-Sr-Nd同位素和地球化学分析测试.武山花岗闪长斑岩样品以高SiO_(2)、MgO、K_(2)O和低Na_(2)O的含量,富集大离子亲石元素和轻稀土元素,Y和Yb相对亏损为特征.花岗闪长斑岩的全岩(^(87)Sr/^(86)Sr)_(i)、ε_(Nd)(t)和δ^(7)Li值分别为0.7030~0.7074、-5.1~-4.6和+0.9‰~+3.2‰,均介于地壳和地幔端元的同位素值之间,揭示出年轻下地壳起源的长英质熔体和富集地幔起源的基性熔体之间的壳幔岩浆混合过程.岩浆在上升至侵位过程中经历了轻微的以辉石和角闪石为主的分离结晶和轻微的地壳混染作用.长江中下游地区早白垩世中酸性岩及相关矿床与古太平洋板块俯冲引起的软流圈地幔上涌及其引起的岩石圈活化有关.

To provide important insights into the petrogenesis of the Early Cretaceous intermediate-acidic magmatic rocks in middle-lower Yangtze River region,it carried out a detailed investigation of Li-Sr-Nd isotopes and geochemistry for the Wushan granodiorite porphyry,quartz diorite porphyry and the basic dyke.The Wushan granodiorite porphyry samples are characterized by the high concentrations of SiO_(2),MgO,and K_(2)O and low concentration of Na_(2)O,respectively,enrichment in LREEs and LILEs,and relative depletion in Y and Yb.They exhibit isotopic values of(^(87)Sr/^(86)Sr)_(i)=0.7030-0.7074,ε_(Nd)(t)=-5.1--4.6,andδ^(7)Li=+0.9‰-+3.2‰,respectively,which are between the isotopic values of crustal and mantle end-members,revealing a crustmantle mixing process between felsic melts from a juvenile lower crust and mafic melts from an enriched mantle source.They were subsequently subjected to weakly fractional crystallization of pyroxene and hornblende and minor crustal contamination during the ascent and emplacement.The Early Cretaceous intermediate-acidic magmatic rocks and related deposits in middlelower Yangtze River region could be related to upwelling of asthenospheric mantle caused by the subduction of the Paleo-Pacific plate and its induced reactivation of sub-continental lithosphere.