藏北商旭金矿床的碎屑锆石U-Pb年龄及其地质意义

Tracing the Provenance of Detrital Zircons from the Shangxu Gold Deposit in Northern Tibet

商旭金矿床处于班公湖—怒江缝合带中段南侧,位于藏北双湖县境内,是班公湖—怒江缝合带已发现的一例造山型金矿。本文选择该矿床木嘎岗日群(J1-2M)的变质砂岩进行碎屑锆石U-Pb定年研究,以期获得碎屑沉积物物源区的丰富信息。随机选取114颗锆石进行分析,可知:(1)锆石颗粒大小在80~150μm之间,绝大多数锆石颗粒的Th/U比值>0.4;(2)年龄分布范围在223~2 615 Ma之间,年龄峰值分别为280 Ma、451 Ma、908 Ma、1 900 Ma和2 430 Ma。将商旭矿区碎屑锆石U-Pb年龄谱图分别与拉萨地块、羌塘地块沉积岩的碎屑锆石年龄谱图对比分析,获得如下初步结论:该区的沉积物的最大沉积年龄为223 Ma,峰值为908 Ma的年龄群,可比于南羌塘沉积岩的碎屑锆石年龄峰值(~950 Ma),明显不同于拉萨地块沉积岩的碎屑锆石年龄峰值(~1 170 Ma)。综合分析表明,该区的沉积物可能为班公湖—怒江洋残余洋盆的产物,暗示班公湖—怒江洋壳在中生代存在北向俯冲。

The Shangxu gold deposit is located on the southern side of the middle part of the Bangong Co–Nujiangsuture zone in northern Tibet.In this paper,the authors conducted in situ U-Pb dating of detrital zircons from theMiddle–Lower Jurassic metasedimentary rocks in Shangxu for tracing their provenance.The analysis of114detritalzircons of varied sizes and shapes,which were selected randomly,shows that:(1)zircons are80~150μm long,andTh/U ratios of most zircons are higher than0.4;(2)ages of detrital zircons range from223Ma to2615Ma,withfive peaks of ca.280Ma,ca.451Ma,ca.908Ma,ca.1900Ma and ca.2430Ma.By contrasting detrital zircon agesfor the Middle–Lower Jurassic metasedimentary rocks in Shangxu with those in Lhasa and southern Qiangtangterranes,an important observation is that the ca.908Ma age population characteristic of detrital zircons in Shangxuis consistent with the distinctive ca.950Ma age peak in detrital zircons from the western Qiangtang terrane,rather than distinctive ca.1170Ma age peak in detrital zircons from the Lhasa terrane.The authors hold that theage(223Ma)is close to the depositional age.The result indicates that the provenance of Middle–Lower JurassicMugagangri Group in Shangxu probably is the remnant of the Bangong Co–Nujiang oceanic basin,implying thenorthward subduction of the Bangong Co–Nujiang Ocean beneath southern Qiangtang terrane.