西藏达金砾岩的沉积时代、物源及构造背景  

Age,provenance and tectonic setting of Dajin conglomerate,Tibet

作　　者：李顺[1,2] 丁林[1,3] 付佳俊[1,2] 岳雅慧[1] 

机构地区：[1]中国科学院青藏高原研究所,北京100101 [2]中国科学院大学,北京100049 [3]中国科学院青藏高原地球科学卓越创新中心,北京100101

出　　处：《岩石学报》2016年第11期3537-3546,共10页Acta Petrologica Sinica

基　　金：国家自然科学基金项目(41490615);中国科学院先导专项(XDB03010401)联合资助

摘　　要：达金砾岩出露于西藏阿里地区冈仁波齐峰南麓，因其地层中发现深海有孔虫化石而受到广泛关注，然而由于达金砾岩的沉积时代不确定，对其沉积的大地构造背景也没有统一的认识。本文中，根据最年轻的碎屑锆石U—Pb年龄限定达金砾岩的最大沉积时代为渐新世末．中新世初（24—27Ma）。综合野外观察，砂岩碎屑统计和碎屑锆石U—Ph年龄分析，本文认为达金砾岩的底部和顶部地层中碎屑物质直接来源于北侧的冈底斯弧，而其中部地层中碎屑物质（包括有孔虫化石）应再旋回自西侧的弧前盆地错江顶群地层。达金砾岩与上覆的冈底斯砾岩连续沉积，且其沉积时代，源区分析结果都与冈底斯砾岩一致，因此本文认为达金砾岩应隶属于冈底斯砾岩。达金砾岩地层变形较弱，产状近乎水平，应沉积于拉张应力背景下，可能与渐新世末一中新世初俯冲的印度板片的“折返”和“断离”有关。The Dajin conglomerate that crops out along the southern foot of Mt. Kailas is a matter of concern because of the presence of larger benthic foraminifera in the strata. As the uncertainty of the depositional age, the tectonic setting of this unit is still in debate. In this study, the youngest age cluster of detrital zircons from Dajin conglomerate constrains the maximum depositional age of this unit to be Late Oligoeene to Early Miocene （24 -27Ma）. Provenance data （field observation, sandstone petrology and detrital zircon U-Pb ages ） indicate that the basal and top sections of Dajin conglomerate were derived directly from Gangdese are, but the middle part of this unit （including the foraminiferal assemblages ） was recycled from the Cuojiangding Group in the west. Dajin conglomerate is in conformable contact with the overlying Gangdese conglomerate. The age and provenance results of Dajin conglomerate are also consistent with Gangdese conglomerate. Therefore, we conclude that Dajin conglomerate is part of the Gangdese conglomerate. The strata of Dajin conglomerate are weakly deformed and mostly planar. This unit should be deposited in extensional tectonic regime, which might be related to the rollback and break-off of the down-going Indian plate during Late Oligocene to Early Miocene.

关 键 词：达金砾岩 冈底斯砾岩 源区分析 锆石U-PB年龄 西藏 

分 类 号：P588.21[天文地球—岩石学]