内蒙古乌拉山地区大桦背岩体SHRIMP锆石U-Pb年代学研究

A Chronological Study of SHRIMP U-Pb of Zircon from the Dahuabei Intrusion in the Wulashan area, Inner Mongolia

大桦背花岗岩体位于内蒙古乌拉山地区,东距哈达门沟大型钾长石—石英金矿床约5km。前人对该岩体曾做过不少同位素年代学研究,所得结果相差较大,但是多数研究者认为大桦背岩体与区内金矿化具有成因联系。本文采用先进的高灵敏、高分辨率的离子探针(SHRIMP)对大桦背岩体进行了锆石U-Ph定点定年研究。结果表明,大桦背岩体的形成年龄为353±7Ma,应为海西旋回早期产物。此年龄比已得到的乌拉山金矿区成矿年龄大至少70Ma,因而推测大桦背岩体与金矿化之间不存在直接的成因联系。

The Dahuabei granitoid intrusion in the Wulashan area, Inner Mongolia in the west part of the northern marginal zone of the North China craton, is spatially associated with gold mineralization. The large-scale Hadamengou gold deposit, characterized by gold-bearing K-feldspar-quartz veins, occurs immediately to the east of the intrusion. Therefore, some researchers thought that they also have temporal and genetic links. The Dahuabei granitoid batholith with an outcrop area of 172 km2 intruded into the Archaean metamorphic rock of the Wulashan Group. The intrusion can be divided into two lithofacies, the medium-coarse-grained central fades and medium-fine-grained marginal facies, and there is a gradational boundary between the two facies. Petrological and geochemical studies suggest that the Dahuabei plutonic intrusion is mainly biotite monzonitic granite, with a mixed origin of crustal and mantle sources. In this study, one sample from the central lithofacies of the Dahuabei pluton was analyzed for U-Pb in zircon using the advanced Sensitive High-Resolution Ion Mi-croprobe (SHRIMP) technique. The analytical result suggests that the Dahuabei granitoid intrusion was formed at 353. 7 Ma i. e. , the early Hercynian epoch. However, the age of the gold mineralization in the area, given by alteration minerals such as sericite and K-feldspar from the Hadamengou gold deposit, ranges from 276 Ma to 139 Ma. These data indicate that the gold mineralization in the area is at least 70 Ma younger than the Dahuabei intrusion. This rules out the possibility of a direct genetic link between gold mineralization and the granitoid intrusion. Furthermore, the age of the Dahuabei granitoid pluton situated in the south of the collision zone between the Angara block and the North China craton provides reliable chronological evidence for the collision , which is believed to take place at the end of the Devonian.