山西支家地银多金属矿区火山–次火山岩锆石U-Pb年代学研究及其地质意义

本文在对山西支家地银多金属矿床地质特征研究的基础上,对矿区出露的火山-次火山岩(流纹斑岩、石英斑岩、花岗岩)中的锆石做了系统的LA-ICP-MS U-Pb年龄测定:石英斑岩成岩年龄为135.4±0.8 Ma,流纹斑岩成岩年龄为136.2±0.6 Ma,花岗岩成岩年龄为136.2±0.7 Ma,其时代均为早白垩世,表明该区出露的火山–次火山岩属于同一期岩浆活动的产物。石英斑岩与成矿关系密切,其成岩年龄135 Ma可以近似作为支家地矿床的成矿年龄。流纹岩和石英斑岩在化学成分上表现为富硅(SiO_2=72.43%~78.48%)、高钾(K_2O/Na_2O值平均为25.17)、强过铝质(A/CNK=1.24~1.95),二者稀土元素和微量元素特征非常类似,微量元素明显富Rb、Th、K、La、Nd、Zr等、相对亏损大离子亲石元素Ba、Sr、Eu等元素,Ti含量较低,属轻稀土元素富集型的钾玄岩系列过铝质花岗岩类,花岗岩仅在P和K元素化学成分上与前2者相差比较大(石英斑岩和流纹岩富K,亏损P,而花岗岩反之)。矿区三类岩石地球化学特征显示其具后碰撞花岗岩的特征,推测其可能形成于后碰撞伸展环境。本次所获锆石U-Pb年龄,不仅丰富了研究区火山-次火山岩类的同位素年龄资料,也为建立中生代构造–岩浆–成矿事件提供了重要信息。

Zircon U-Pb Geochronology of the Volcanic-subvolcanic Rocks and its Geological Implications on the Zhijiadi Silver Polymetallic Mineralization

LA-ICP-MS zircon U-Pb dating of the volcanic-subvolcanic rocks (rhyolite porphyry, quartz porphyry and granite) from the Zhijiadi deposit yields ages of 135.4±0.8 Ma, 136.2±0.6 Ma, and 136.2±0.7 Ma for the quartz porphyry, rhyolite porphyry, and granite, respectively. Obviously, they formed simultaneously in Early Cretaceous. Since the quartz porphyry is closely related with the mineralization, its age (about 135 Ma) can be considered to be the age of the mineralization. The rhyolite and quartz porphyry are silicon-rich (SiO2=72.43%-78.48%), highly potassic (K2O/Na2O average value is 25.17) and strong peraluminous (A/CNK=1.24-1.95). Moreover, they have similar rare earth element patterns and trace element characteristics, such as enrichments of Rb, Th, La, K, Nd, Zr, other large ion lithophile elements, depletions of Ba, Sr, Eu, and Ti. So, they should belong to the shoshonite series peraluminous granite. However, the granite has different phosphorus and potassium from the quartz porphyry and the rhyolite (granites have high phosphorus and lower potassium, but the quartz porphyry and the rhyolite have high potassium and lower phosphorus). Their geochemistry and formation environment reveal that they have characteristics of postcollisional granites, and thus mostly like to have been formed post collisional extensional environment.