西秦岭造山带三叠纪大规模成矿作用背景: 来自恰冬铜矿高镁安山岩的证据

西秦岭造山带广泛发育与三叠纪岩浆活动密切相关的金、铜、钼等多金属矿床, 表明该地区存在一期三叠纪大规模成矿事件, 但其成矿地球动力学背景一直存在争议。恰冬铜矿是西秦岭造山带内与早三叠世高镁安山岩有关的铜矿床。本文对高镁安山岩开展了锆石U-Pb年代学、岩石地球化学和Sr-Nd-Hf同位素分析, 对其岩石成因及构造环境进行了探讨, 以进一步约束西秦岭造山带三叠纪大规模成矿作用的地球动力学背景。安山岩的锆石U-Pb年龄为246.1±1.6Ma, 形成于早三叠世。岩石属于高钾钙碱性-钙碱性系列, SiO₂含量为54.60% ~56.21%, 具有较高的MgO含量(3.78%~4.57%)和Mg^#(49.7~62.1), 与日本Setouchi火山岩带中典型赞岐岩相类似。安山岩富集轻稀土元素(LREE)和Cs、Rb、U、Sr等大离子亲石元素(LILEs), 显著亏损Nb、Ta、Ti、P等高场强元素(HFSEs), 其初始⁸⁷Sr/⁸⁶Sr比值、ε_Nd(t)值和锆石ε_Hf(t)值分别为0.7073~0.7090、-9.72~-11.03和-2.1~-18.6, 是俯冲沉积物熔融产生的熔体与地幔楔发生交代反应的产物, 并经历了一定程度的结晶分异。以上研究表明, 恰冬铜矿高镁安山岩形成于早三叠世古特提斯洋俯冲背景下的活动大陆边缘环境。结合区域成矿资料分析, 西秦岭造山带在早-中三叠世处于洋壳俯冲环境, 发育岩浆-热液成矿系统。

The geodynamic setting of Triassic large-scale mineralization event in the West Qinling Orogen: Evidence from high-Mg andesite in the Qiadong copper deposit

A large-scale mineralization event happened in Triassic in West Qinling Orogen which resulted in the formation of a large number of gold, copper, molybdenum and polymetal deposits. However, the metallogenic geodynamic setting of this event remains debating. Qiadong copper deposit is located in the western portion of West Qinling Orogen with an identified Cu reserve of 50000t, whose mineralization is correlated to the Early Triassic high-Mg andesites. This paper presents zircon U-Pb geochronology, whole-rock geochemistry and the Sr-Nd-Hf isotope compositions of high-Mg andesites from the Qiadong copper deposit. The petrogenesis and tectonic implications of andesites are discussed, aiming to provide further constraints on the geodynamic backgrounds of the Triassic large-scale mineralization in this area. The andesites yield a LA-ICP-MS zircon U-Pb weighted mean age of 246.1±1.6Ma, suggesting its formation of Early Triassic. Geochemical analyses show that the andesites belong to high-K calc-alkaline to calc-alkaline series, with high SiO2 (54.60%~56.21%), MgO (3.78%~4.57%) and Mg# (49.7~62.1), which are similar to those of typical sanukitoids within the Setouchi volcanic belt in Japan. The high-Mg andesites display enrichment in light rare earth elements (LREE) and large-ion lithophile elements (LILEs, e.g., Cs, Rb, U, Sr), depletion in high field strength elements (HFSEs, e.g., Nb, Ta, Ti, P), with whole-rock initial 87Sr/86Sr, εNd(t) and zircon εHf(t) values of 0.7073~0.7090, -9.72~-11.03 and -2.1~-18.6 respectively, indicating that they are products of metasomatic interaction between the subduction sediment-derived melts and mantle wedge which experienced a certain degree of crystallization differentiation. To be more concrete, the high-Mg andesites of the Qiadong copper deposit were formed in an active continental margin setting due to the subduction of Paleo-Tethys Ocean in the Early Triassic. Combined with regional mineralization data, the Early-Middle Triassic magmatic-hydrothermal mineralization system in the West Qinling Orogen was formed in the subduction setting.