大别山皇城山银矿区及外围陈棚组火山岩U-Pb年代学、地球化学和成矿构造背景

皇城山银矿床是西大别山地区代表性的浅成低温热液矿床，受陈棚组火山机构的枝叉状裂隙构造控制.目前人们对该矿床成因、成矿时间和构造环境的理解尚不深刻，因此对相关火山岩进行了激光等离子质谱（LA-ICP-MS）锆石U-Pb测年、元素地球化学和Sr-Nd-Hf同位素研究，探讨了其岩石成因和构造属性.结果表明，皇城山矿区陈棚组熔结凝灰岩及外围流纹岩锆石U-Pb年龄分别为133.4±1.5 Ma（MSWD=1.3）和133.1±0.8 Ma（MSWD=0.59），形成于早白垩世.陈棚组火山岩在较短时间内喷发形成，其喷发时限基本代表了皇城山银矿床形成时间.陈棚组流纹岩具高硅（SiO₂=73.83%~77.38%）、高钾（K₂O=4.65%~9.31%）特征，属于过铝质（A/CNK=1.09~1.31）和高钾钙碱性岩石系列.岩石具轻稀土富集，重稀土平坦，大离子亲石元素相对富集，Ba、Nb、Ta、P、Ti明显负异常和负Eu异常（δEu=0.33~0.88）特点.岩石锶同位素初始比值I_Sr为0.708 220~0.708 741，ε_Nd（t）值为-17.2~-18.5，Nd两阶段模式年龄T_DM2=2.32~2.43 Ga.锆石ε_Hf（t）值范围为-22.3~-26.1，Hf两阶段模式年龄T_DM2=2.29~2.49 Ga.主、微量元素和Sr-Nd-Hf同位素特征共同揭示陈棚组流纹岩为扬子陆壳北缘古老地壳物质重熔演化的产物.结合区域地质特征，陈棚组火山岩及其相关皇城山银矿床形成于大别山早白垩世加厚下地壳拆沉作用初期、挤压向伸展转化环境. 

Geochronology and Geochemistry of Volcanic Rocks from the Huangchengshan Volcanogenic Epithermal Silver Deposit,Dabie Orogen,China: Implications for Tectonic Setting

The Huangchengshan deposit is the largest high-sulfidation epithermal silver deposit in the Western Dabie orogen. Silver mineralization characterized by argentiferous quartz veins is controlled by fractures of volcanic apparatus, and is spatially associated with volcanic rocks of Chenpeng Formation. In situ zircon U-Pb dating yielded 133.4±1.5 Ma (1σ, MSWD=1.3) and 133.1±0.8 Ma (1σ, MSWD=0.59) for the tuff and rhyolite from Chenpeng Formation, respectively, indicating an Early Cretaceous volcanism. The data also suggest a syngenetic volcanogenic epithermal mineralization age of ca.133 Ma for the Huangchengshan silver deposit. Rhyolites of the Chenpeng Formation display high contents of SiO₂ (73.83% to 77.38%) and K₂O (4.65% to 9.31%), belonging to high-K calc-alkaline series. Their A/CNK ratios range from 1.09 to 1.31, exhibiting peraluminous characteristics. Rare earth elements show light rare earth elements (LREEs) enrichment and flat heavy rare earth elements (HREEs) patterns, with moderate negative Eu anomalies (δEu=0.33-0.88). They are enriched in large ion lithophile elements (LILEs), but depleted in Ba, Nb, Ta, P and Ti. These geochemical characteristics show affinity to highly fractionated granites. The rhyolites have high I_Sr values (0.708 220 to 0.708 741) and negative ε_Nd(t) values (-17.2 to -18.5), with two-stage Nd model ages of 2.32 to 2.43 Ga. Zircon ε_Hf(t) values are negative (-22.3 to -26.1) with an calculated two-stage Hf model ages of 2.29 to 2.49 Ga. The geochemical and Sr-Nd-Hf isotopic signatures suggest that the rhyolite is likely to be derived by partial melting of the ancient lower crust of the Yangtze Block. We propose that this volcanism and related Huangchengshan silver deposit formed in a transitional stage from crustal compression to extension driven by delamination of thickened lower crust during the Early Cretaceous.