东秦岭大蛇沟钨矿区赋矿围岩成因:锆石U-Pb年代学和地球化学证据

大蛇沟钨矿床是东秦岭杨斜—丰北河金钨成矿带一处典型的石英脉型钨矿床,矿体赋存于杨斜片麻岩套的NE—NEE向构造破碎带内。LA-ICP-MS锆石U-Pb定年结果显示,赋矿含斑眼球状斜长片麻岩的加权平均年龄为(434.2±1.6) Ma (MSWD=0.022, n=24),表明原岩形成于早志留世。地球化学研究表明:岩石属于高钾钙碱性准铝质I型花岗岩类,相对富集Rb、Ba、K、Pb、Sr等大离子亲石元素和轻稀土元素(LREE),亏损Nb、Ta、P、Ti等高场强元素和重稀土元素(HREE),具岩浆弧岩浆岩特征。此外,高Sr、高Sr/Y和La/Yb、低Y和低Yb及弱Eu正异常等特征参数,与典型埃达克岩一致;推测岩浆源于加厚下地壳石榴石角闪岩的部分熔融,形成于早古生代商丹洋向北秦岭板块俯冲的活动大陆边缘弧环境。

Petrogenesis of Host Rocks in the Dashegou Tungsten Orefield,East Qinling Orogen: Evidences from Zircon U-Pb Geochronology and Geochemistry

The Dashegou is a typical quartz-vein type tungsten deposit in the Yangxie-Fengbeihe Au-W metallogenic belt, East Qinling Orogen. The orebodies occur in the NE-ENE-striking faults of the Yangxie gneissic suite. LA-ICP-MS zircon U-Pb dating yielded a weighted average age of (434.2±1.6) Ma (MSWD=0.022, n=24) for the porphyroblastic augen plagiogneiss, indicating that the protolith was formed in the Early Silurian. Whole-rock geochemical data show that the plagiogneiss belongs to high-K calc-alkaline peraluminous I-type granitoids, and is relatively enriched in large ion lithophile elements (LILEs, e.g., Rb, Ba, K, Pb, Sr) and LREEs, and depleted in high field strength elements (HFSEs, e.g., Nb, Ta, P, Ti) and HREEs. In addition, the rocks have high Sr, high Sr/Y and La/Yb, low Y and low Yb, and weakly positive Eu anomalies, resembling typical adakites. This indicates that the parental magma was probably derived from partial melting of garnet amphibolite in the thickened lower crust. Considering also the regional tectonic setting, it is inferred that protolith of the porphyroblastic augen plagiogneiss was formed by island arc magmatism during the Early Paleo-zoic north-dipping subduction of the Shangdan Ocean beneath the North Qinling plate.