湖南七宝山铜多金属矿床石英斑岩时代与成因:锆石U-Pb定年及Hf同位素与稀土元素证据

湖南浏阳七宝山铜多金属矿床位于钦杭成矿带西段,是湘东北规模最大的铜多金属矿床。矿床的形成与区内的石英斑岩关系密切。石英斑岩内锆石具有岩浆锆石特征,LA-ICP-MS锆石U-Pb定年结果为155~153 Ma,代表其形成年龄,属晚侏罗世岩浆活动产物。岩浆锆石的~(176)Hf/~(177)Hf=0.282296~0.282603,εHf(t)=–12~–2.7,平均地壳模式年龄tDM2=1377~2056 Ma;锆石ΣREE=496~4162μg/g,(Yb/Nd)N=71.9~3133.8,HREE强烈富集,具有强烈Ce正异常(δCe=1.68~203.13)和强烈至中等Eu负异常(δEu=0.05~0.67),表明石英斑岩的岩浆源区具有明显壳源特征,来自于古元古代至中元古代地壳的部分熔融。结合岩石学研究,七宝山矿区石英斑岩的形成除了中下地壳冷家溪群或更古老的基底物质的部分熔融外,还有幔源组分加入,这一期岩浆与成矿作用与岩石圈拆离和软流圈物质上涌及随后的玄武岩底侵作用有关。

Zircon U-Pb Dating,Hf Isotope and REE Geochemistry of the Quartz-porphyry in the Qibaoshan Cu-polymetallic Deposit in Hunan

The Qibaoshan Cu-polymetallic deposit in Liuyang belongs to the west section of the Qinzhou-Hangzhou metallogenic belt,and it is the largest Cu-polymetallic deposit in northeastern Hunan province.The mineralization of the Qibaoshan Cu-polymetallic deposit is closely related to the quartz-porphyry.The zircons from quartz-porphyry are typical magmatic zircon,and the LA-ICP-MS U-Pb dating yielded age of 155-153 Ma,indicative of an Early Mesozoic intrusion.The magmatic zircons have 176Hf/177Hf values of 0.282296-0.282603,εHf(t) of-12--2.7,and crustal model ages (tDM2) from 1377 to 2056 Ma.The zircons have high ΣREE concentrations of 496×10-6-4162×10-6,enriched in HREE with high (Yb/Nd)N=71.9-3133.8.The strong positive Ce anomalies (δCe =1.68-203.13) and strong to moderate negative Eu anomalies (δEu =0.05-0.67) of the zircons indicate that the rocks were formed of the partial melting of the Paleoproterozoic to Mesoproterozoic crust.The porphyry related mineralization in the period of 175-145 Ma in the region may related to the northwestward subduction of the Izanagi plate.All these data suggest the magma from the partial meilting material of the Lengjiaqi group in the lower crust or even much older crust was in the magma chamber already,the asthenospheric mantle fluid upwelled into the magma chamber and they mixed inhomogeneously by the effects of the delamination of lithosphere and asthenosphere upwelling.And then the magma intruded upward to form the quartz porphyry.