湘东南万洋山岩体的锆石SHRIMP U-Pb年龄、成因及构造意义

Zircon SHRIMP U-Pb Age,Petrogenesis and Tectonic Implication of Wanyangshan Pluton in Southeast Hunan Province

本文对湘赣交界地区发育的万洋山岩体进行锆石SHRIMP U-Pb定年和岩石学、地球化学分析。该岩体由英云闪长岩、花岗闪长岩和二长花岗岩组成,本次主要讨论英云闪长岩及其中发育的石英闪长岩包体,并获得英云闪长岩的锆石U-Pb年龄为438.0±3.0 Ma,石英闪长岩包体的锆石U-Pb年龄为425.6±3.1 Ma,为晚志留世。英云闪长岩矿物组合为斜长石、钾长石、黑云母、石英以及少量的角闪石、磁铁矿和榍石;地球化学特征显示为低硅、准铝质及钙碱性的花岗岩,在岩石类型判别图解中为I型花岗岩。石英闪长岩包体为细粒结构,矿物组合为角闪石、斜长石、黑云母、石英及少量辉石,表明岩石包体是岩浆成因的;包体中存在异常共生矿物斜长石斑晶、针状磷灰石,CIPW标准矿物计算中未出现刚玉分子,地球化学组成显示其具有低硅、低碱、准铝质的钙碱性岩类特征;包体还表现为富Mg、Fe以及高Mg#值（4550）,显示出包体高镁、偏基性的特征;包体与寄主岩稀土元素配分模式图和微量元素蛛网图分布特征基本一致,表明二者在成因上相关联。石英闪长岩包体分异指数DI=4548与辉长岩接近,SiO2含量略高于辉长岩,表明石英闪长岩包体源于上地幔基性辉长质岩浆、经岩浆混合演化形成。英云闪长岩显示为岛弧岩浆岩、具有活动大陆边缘岩浆岩特征,结合英云闪长岩的岩石类型、岩石包体成因认为：万洋山岩体可能是扬子板块与华夏板块在俯冲消减的地球动力学背景下,软流圈地幔上涌,诱发岩石圈地幔和上覆的古老地壳物质重熔,形成以壳源为主、壳幔混合成因的花岗岩。

This paper analyze the zircon SHRIMP U-Pb dating, petrology and geochemistry of Wanyangshan pluton developed in the boundary of Hunan and Jiangxi, which is composed of tonalite, granodiorite and monzonitic granite. Tonalite and quartz diorite enclave developed in it are mostly discussed, the age of tonalite is 438.0±3 Ma and the age of quartz diorite enclave is 425.6±3.1 Ma, which suggest these rocks are granite in Late Silurian Epoch. Tonalite is composed of plagioclase, potash feldspar, biotite, quartz, with a little of amphibolite, magnetite and sphene, considereing to be low Silicon, quasi-aluminous and calc-alkali granite, and showing to be I-type granite in the rock type discriminated diagrams. Quartz diorite enclave is fine-grained, which comprised of hornblende, plagioclase, biotite, quartz and a little pyroxene, suggesting rock enclave is magmatic. Also there are abnormal paragenetic mineral plagioclase phenocryst and acicular apatite in enclave, without corundum molecule in CIPW standard mineral calculation. Geochemically, the enclaves are showed to be low Silicon, low alkali, quasi-aluminous and cal-alkali rocks, which are also rich in Mg, Fe and with a high value of Mg#（45-50）, suggesting these enclaves have the characteristic of high-Mg and partial-bacic. The distributing characteristic of enclave and parasitic rock is basicly identical in the rare element distributed patterns and trace element spider diagram, which suggest their genesis are relative. Combined with the characteristic of petrology, petrogeochemistry, zircon U-Pb age, and by means of SiO2-A/CNK, MgO-FeOT, covariant sketch, quartz diorite enclaves are magmatic mixing. The characteristic of zircon crystal in quartz diorite enclave is showed to be simple platelike annule, which is the same as that of gabbro, the differentiation index of enclaves （DI=45-48） is close to gabbro, while the value of SiO2 is a little higher than gabbro, suggesting quartz diorite enclaves come from mantle basic gabbroic magma which were evolved by magmatic mixing. Tonalite has the characteristic of island-arc magma and active marginal continental magma, combined with the rock type of tonalite and the genesis of enclave, we consider that Wanyangshan Pluton are probably fromed mostly by crust source and with crust-mantle mixed, when asthenosphere mantle upwelling, causing remelting of lithosphere mantle and old earth matter under the geodynamical background of subduction between Yangzi Plate and Huaxia Plate.