南秦岭懒板凳岩体锆石U-Pb年代学、岩石成因及其地质意义

Zircon U-Pb geochronology,petrogenesis and geology of lanbandeng pluton in South Qinling

懒板凳岩体属南秦岭五龙岩体群最东侧胭脂坝岩体组成部分。本文针对懒板凳岩体中部的金盆地区中细粒花岗岩LA-ICP-MS锆石U-Pb年代学研究表明,中细粒花岗岩成岩年龄为211.2±3.5 Ma,指示岩体侵位时代为晚三叠世。岩石地球化学分析显示,中细粒花岗岩SiO_(2)含量为73.63%~75.22%,属于富硅类型的花岗岩;Na_(2)O含量介于3.54%~4.37%之间,K_(2)O含量介于4.19%~6.47%之间,富钾(Na_(2)O/K_(2)O=0.55~1.04)和富碱(Na_(2)O+K_(2)O=8.56~10.01%)的特点,属高钾钙碱性系列弱过铝质岩石,显示具有A型花岗岩地球化学特征,属A_(2)型花岗岩。通过对金盆地区中细粒花岗岩精确成岩时代的限定,暗示懒板凳岩体内外接触带矽卡岩型、伟晶岩型钨矿床的形成与印支晚期岩浆活动关系密切。

Lanbandeng pluton is a part of Yanzhiba rock masses on the east side of the Wulong Rock Group in south Qinling Mountains.LA-ICP-MS zircon U-Pb dating of the meso-fine-grained granite in Jinpen area at the central Lanbandeng Pluton shows that the diagenetic age of the meso-fine-grained Granite is 211.2±3.5 Ma,indicating that the emplacement age of the rock mass is the Late Triassic.Rock geochemical analysis shows that the SiO_(2) content of medium-fine-grained granite ranges between 73.63%and 75.22%,belonging to silicon rich granite;the content of Na_(2)O ranges between3.54%and 4.37%and K2O ranges between4.19%and 6.47%.It is rich in potassium(Na_(2)O/K_(2)O=0.55~1.04)and alkali(Na_(2)O+K_(2)O=8.56~10.01%),belonging to a high potassium-calcium-alkaline series of weak overaluminum rock with the geochemical characteristics of type A granite that should be attributed to type A2 granite.Based on determination of the diagenetic age of medium-fine-grained granite in the Jinpen area,it is suggested that the formation of skarn and pegmatite type tungsten deposits in the internal and external contact zone of the Lanbandeng rock body is closely related to the late Indosinian magmatism.