湘东邓阜仙花岗岩成因及对成矿的制约:锆石/锡石U-Pb年代学、锆石Hf-O同位素及全岩地球化学特征

摘要

南岭地区晚三叠世成矿规模较小且零散,而晚侏罗世成矿规模巨大,以往的研究较少关注两期花岗岩之间的成因联系及与成矿的关系.湘东邓阜仙花岗岩体位于南岭地区中段,是具有晚三叠世、晚侏罗世两期岩浆活动的复式岩体,并发育有钨矿床.本文对区内花岗岩及蚀变岩型矿体进行了结石/锡石U-Pb定年、锆石Hf-O同位素与全岩主微量分析,以期揭示两期花岗岩的地球化学特征、成因联系和与成矿的关系.锆石SIMS U-Pb定年结果表明,区内花岗岩的成岩年龄分别为～228Ma和～ 153Ma.湘东钨矿的锡石U-Pb定年结果为154.4±2.1Ma,表明钨锡矿化形成于晚侏罗世.两期花岗岩均具有高硅、高钾、弱过铝质至强过铝质的特征,结合锆石的Hf-O同位素组成,指示它们具有相似的成因,均起源于中-古元古代基底地壳物质的部分熔融,晚三叠世花岗岩混入了更多的成熟地壳物质,区域的富矿地层对于湘东钨矿形成的直接影响不大.邓阜仙晚侏罗世花岗岩更高的Fe含量和Rb/Sr、Rb/Ba比值,更显著的Eu、Ce负异常,指示其经历了更高程度的分异演化,两期成岩过程中的差异可能与区域上的晚侏罗世发生的太平洋俯冲作用相关.%In the Nanling Range,the mineralization was tremendous in Late Jurassic,but was scattered and small in scale in Triassic.Previous studies paid little attention to the genetic link between the two stages of granites and their relationship with the mineralization.The Dengfuxian composite granitic pluton from eastern Hunan Province was formed by magmatism in Late Triassic and Late Jurassic in the Nanling Range,and contains tungsten deposits.This paper presents zircon and cassiterite U-Pb geochronology,zircon Hf-O isotopes and whole-rock major and trace elements for the granites to investigate their geochemistry,petrogenesis and relationship with the mineralization.Zircon SIMS U-Pb dating results show that the granites formed at ～ 228Ma and ～ 153Ma.Cassiterite U-Pb dating result indicates that the mineralization occurred at 154.4 ± 2.1Ma.Both stages of granites are enriched in SiO2 and K2 O,and are weak to strong peraluminous.In combination with the Hf-O isotopes,the geochemical characteristics indicate that the two stages of granites were similarly derived by partial melting of crustal rocks from the Mesoproterozoic-Paleoproterozoic basement,while the Late Triassic granites involved more mature crustal materials.It indicates that the sedimentary strata enriched in ore have little contribution to the deposits.The Late Jurassic granites have higher Fe contents and Rb/Sr and Rb/Ba ratios,as well as stronger Eu and Ce depletions,indicating higher degrees of differentiation.The geochemical differences of the two stages of granites probably resulted from different tectonic environments,in as much as the Late Jurassic granites formed in a regime related with the subduction of the Pacific plate.