西藏沙让斑岩钼矿床锆石SHRIMP定年和角闪石Ar-Ar定年及其地质意义

SHRIMP zircon U-Pb and amphibole ^(40)Ar-^(39)Ar dating of amphibole diorite from Sharang porphyry molybdenum deposit in Gongbo'gyamda County, Tibet, and its geological implications

采用SHRIMP锆石微区U-Pb测年及角闪石40Ar-39Ar测年对冈底斯成矿带东段工布江达县沙让斑岩钼矿区的角闪石闪长岩体进行了年代学研究。对角闪石闪长岩中单颗粒锆石11个样品点分析,其206Pb/238U年龄范围在(45.39±0.77)～(52.7±3.3)Ma之间,加权平均值为(47.17±0.41)Ma,(n=11,MSWD=2.0)。对角闪石样品采用40Ar-39Ar阶段升温测年,年龄谱呈马鞍形,说明角闪石样品存在过剩氩,加权平均年龄(55.10±0.79)Ma不具地质意义,最低坪年龄(53.25±0.60)Ma接近但大于角闪石的结晶年龄。综合前人研究结果,笔者认为:①角闪石闪长岩体形成年龄为(53.25±0.60)Ma,属于喜马拉雅期,相当于始新世,岩体侵入时间早于含矿主岩体斜长花岗斑岩的形成时间。②受后期花岗斑岩体侵入热事件的影响,锆石的蜕晶化作用导致年龄偏小,角闪石Ar-Ar法最低坪年龄基本代表成岩年龄。③斑岩钼矿的形成与主碰撞期岩浆底侵作用有关,始新世早期,冈底斯成矿带的地壳已经增厚到可以产生地壳熔融的厚度,形成斑岩钼矿。④冈底斯成矿带存在40～65Ma的主碰撞期,显示出念青唐古拉成矿带具有良好的寻找斑岩型钼矿-矽卡岩型多金属矿的潜力。

Located in Gongbo＇gyamda County of Tibet, the Sharang ore deposit is a porphyry Mo deposit belonging to the eastern part of the Gangdese metallogenic belt. This study provides the high-precision magmatic zircon SHRIMP and ^40Ar-^39Ar dating of the amphibole diorite from the Sharang Mo deposit. SHRIMP U-Pb dating data of 11 zircon grains from the amphibole diorite present ^206Pb/^238U concordant ages from （45.39±0.77）Ma to （52.7±3.3） Ma with a weighted mean age of （47.17±0.41） Ma（n=11, MSWD = 2.0）. The amphibole samples yielded a ^40Ar-^39Ar saddle-shaped age spectrum after the incremental heating from 800℃ to 1 200℃, showing the existence of excess argon. The minimum ^40Ar-^39Ar plateau age is （53.25±0.60） Ma. Based on previous stu-dies and the authors＇ dating results, some conclusions have been reached： ① crystallization of the amphibole diorite occurred at （53.25±0.60） Ma during the Himalayan period（early Eocene）;② the later magmatic activity resulted in the metamictization of zircons, which has caused the incorrect test result, i.e., the data obtained are lower than the zircon crystallization age; the minimum Ar-Ar plateau age （53.25±0.60） Ma represents the rock-forming age of the amphibole diorite;③ in early Eocene, crust thickening beneath the Gangdese belt led to the melting of the crust, which contributed to the formation of the porphyry Mo deposits; ④ there existed mineralization during the main Indian-Asian collision period （40～65 Ma）in the Gangdese belt, which is of great significance for the ore-prospecting work of this region.