湖南七宝山矿床石英斑岩锆石U-Pb定年及Hf同位素地球化学

Zircon U-Pb dating, Hf isotope of magmatic rocks from Qibaoshan Cu polymetallic deposit, Hunan Province

湖南浏阳七宝山铜多金属矿床位于钦杭成矿带西南段,是湘东北规模最大的铜多金属矿床,矿区内的石英斑岩对成矿贡献非常大。石英斑岩内锆石具有典型岩浆期锆石特征,LA-ICP-MS U-Pb定年结果为154.8±1.8 Ma,代表其形成年龄,属晚侏罗世岩浆活动产物。岩浆期锆石的176Hf/177Hf=0.282120~0.282539,对应的εHf(t)值均为负值,集中在-19.8^-4.9,计算的亏损地幔模式年龄(tDM1)集中在1001~1596 Ma,平均地壳模式年龄(tDM2)集中于1519~2450 Ma,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,七宝山矿区内石英斑岩体成因可能是在华南地块受到伊泽奈奇(Izanagi)板块向西北俯冲的影响下,古元古代至中元古代地壳部分熔融形成岩浆,而后岩石圈拆沉和软流圈物质上涌进入岩浆发生不均匀混合在晚侏罗世上侵形成的。

The Qibaoshan Cu-polymetallic deposit in Liuyang is belong to the west section of Qinzhou-Hangzhou metallogenic belt,and it is the largest Cu-polymetallic deposit in northeastern Hunan Province. The quartz-porphyry is on a great contribution of the mineralization of the Qibaoshan Cu-polymetallic deposit. The zircons from quartz-porphyry are typical magmatic zircons. LA-ICP-MS U-Pb dating of the zircons shows the age of 154.8 ± 1.8 Ma, which represents the crystallization age of quartz-porphyry, and indicate it was Late Mesozoic intrusion. The new born zircons have intermediate176Hf/177 Hf of 0.282120 ~0.282539, εHf(t) of-19.8 ^-4.9, model age of crust tDM2 from 1519 to 2450 Ma, indicating that the rock was formed by the partial melting of the Paleoproterozoic to Mesoproterozoic crust. Through the comprehensive study of crystallization age and Hf isotope model age of the zircons, the crustal magma formed the quartz-porphyry was from a mixed magma from the crust source and the mantle source or form a mixed magma with the characteristics of the depleted mantle, which may containing the very old continental crust(the Hadean crust).In 175 ~ 145 Ma, an extensive rocks related to the porphyry and skarn copper deposit intruded of Qinzhou-hangzhou metallogenic belt. And these rocks were the product of the heyday of the system of temperature increasing and pressure depressing after the Izanagi Plate subducting toward northwest into the South China Block. The study of the geochemical characteristics of the quartz-porphyry before shows that the magma also having the the mantle source. All these data suggest the magma from the partial melting material of Paleoproterozoic to Mesoproterozoic crust or even much older crust was in the magma chamber already, the asthenospheric mantle fluid upwelling into the magma chamber and they mixed inhomogenously by the effects of the delamination of lithosphere and asthenosphere upwelling. And then the magma intruded upward to form the quartz porphyry.