大别山北麓白石坡银矿区花岗斑岩锆石U-Pb年龄、地球化学和Sr-Nd-Hf同位素组成及其地质意义

白石坡银矿床是大别山地区具代表性的中型银矿床，已探明银储量219.8 t。银矿体与矿区花岗斑岩空间关系密切。为深化理解其成矿地质背景，对白石坡花岗斑岩进行了详细的岩相学观察、锆石U-Pb同位素测年、岩石地球化学和Sr-Nd-Hf同位素研究。通过激光剥蚀电感耦合等离子体质谱(LA-ICP-MS)分析，获得花岗斑岩锆石U-Pb同位素年龄为(142±2) Ma(MSDW=1.9)，表明其形成于早白垩世。岩石具较高的SiO2含量(73.94%~76.14%)和富K2O(7.46%~9.55%)等特点，总体属于强过铝质(A/CNK=1.07~1.45)、高钾钙碱性系列，具高分异花岗岩特征；岩石富集轻稀土((LREE/HREE)N=10.88~12.89)，具中等Eu负异常，并具有较低的Sr、Y、Yb含量。岩石锶同位素初始比值ISr为0.714 762~0.715 890，钕同位素εNd(t)值约为-13.8，两阶段Nd模式年龄TDM2=2.06~2.05 Ga。锆石εHf(t)值集中于-16.6~-13.5，两阶段Hf模式年龄TDM2=1.98~1.81 Ga。岩石地球化学和Sr-Nd-Hf同位素特征共同揭示，该花岗斑岩可能是扬子陆壳北缘古老地壳物质重熔演化的产物。白石坡花岗斑岩及相关银矿床形成于白垩纪加厚下地壳拆沉之前地壳持续挤压加厚环境，与陈棚组火山活动不存在直接联系。

Geochronology,geochemistry and Sr-Nd-Hf isotopic constraints on the origin of the ore-bearing granite porphyry in the Baishipo silver deposit,Dabie Orogen

The Baishipo silver deposit is located in the northern margin of the Dabie Orogen, central China. Silver mineralization characterized by Ag-Pb-Zn quartz veins is spatially associated with the Baishipo granite porphyry. Elemental, whole-rock Sr-Nd and zircon U-Pb-Hf isotopic analyses have been carried out in this study, in order to understand the mineralizational setting. In situ zircon U-Pb dating yielded an Early Cretaceous crystallization age (142±2 Ma, MSDW=1.9) for the Baishipo granite porphyry. The granite porphyry is characterized by high concentrations of SiO2 and potassium, belongs to the high-K calc-alkaline series, and is generally strongly peraluminous. It is enriched in light rare earth elements (LREEs) and large ion lithophile elements, but depleted in heavy REEs and high field strength elements (Nb, Ta and Ti), and characterized by negative Eu anomalies. They have initial 87Sr/86Sr ratios ranging from 0.714762 to 0.715890 and negative εNd(t) values clustered of -13.8. In situ zircon Hf isotopic analyses show that the εHf(t) values of zircons from the granite porphyry vary from -16.6 to -13.5 with calculated two-stage Hf model ages of 1.81 to 1.98 Ga, suggesting a homogeneous crustal source without significant magma contamination or mixing. The geochemical and Sr-Nd-Hf isotopic signatures suggest that the Baishipo granite porphyry is a highly fractionated granite and is likely to be derived by partial melting of the ancient crust of the Yangtze Block. We propose that the Baishipo granite porphyry and related silver deposit formed in a thickened crust setting, before the Cretaceous tectonic collapse in the western Dabie Orogen.