内蒙古小井子北铜钼矿区黑云母二长花岗岩年代学、地球化学特征及地质意义

Chronology of U-Pb zircon and geochemistry of the biotite monzogranite and its geological implication of the north Xiaojingzi copper-molybdenum deposit in Inner Mongolia

巴林左旗小井子北铜钼矿位于大兴安岭南段铅锌—银—铜—锡—铁成矿带内,是近年来发现的一个小型矿床,成矿与中粗粒黑云母二长花岗岩密切相关。黑云母二长花岗岩的SiO_2含量为69.7%~71.15%,富钾(K_2O=3.65%~3.93%),富铝(Al_2O_3=14.34%~14.64%),属高钾钙碱性系列。A/NK值为1.36~1.39,A/CNK值为0.98~1.04,属于准铝质—过铝质岩石。利用LA-ICP-MS锆石U-Pb同位素测年方法测得黑云母二长花岗岩年龄是(131.79±0.64)Ma(2σ,N=24,MSWD=0.35)^(134.16±0.55)Ma(2σ,N=30,MSWD=0.47),其年龄基本可以代表小井子北铜钼矿的成矿年龄,表明该矿床是早白垩世强烈构造—岩浆活动的产物。黑云母二长花岗岩花岗岩具有典型的A型花岗岩地球化学特征,属于低Sr高Yb型,暗示了在成岩过程中存在着斜长石的分异作用,源区的残留相为角闪岩相,形成深度较浅(约30 km),形成位置可能为中上地壳。黑云母二长花岗岩形成在板内岩石圈伸展环境下,可能与幔源岩浆底侵作用促使上覆先存地壳发生部分熔融有关。

The north Xiaojingzi copper-molybdenum deposit is a newly found deposit in the southern Da Hinggan mountains area, the mineralization is closely related to the biotite monzogranite. The biotite monzogranite is rich in silica （SiO2：69.7%～71.15%）, potassium （K2O：3.65%～3.93%） and Al （Al2O3 ：14.34%～14.64%）, which belongs to high-potassium calc-alkaline series. The biotite monzogranite is characteristic with A/NK values of 1.36～1.39, A/CNK value of 0.98～1.04, which belongs to metaluminous-weak peraluminous series. The dating result of zircon U-Pb showed that the formation age of the biotite monzogranites ranged from （131.79 ± 0.64） Ma （2σ, N=24, MSWD = 0.35） to （134.16 ± 0.55） Ma （2σ, N=30, MSWD = 0.47）. It is concluded that the mineralization of the north Xiaojingzi copper-molybdenum deposit is in the same period as that of the rock formation, and it shows that this deposit is the product of the early Cretaceous tectonic-magmatic activities. The geochemical features of biotite monzogranite define typical A-type affinities, belong to low Sr and high Yb granite. It is predicted that the parental magmas were derived from middle to upper continental crust with variable fractionation of plagioclase.The genesis of the biotite monzogranite might be ascribed to the under plating of the mantle-derived magma which caused younger crust partial melting to form granitic magma within the lithosphere extension environment.