拉萨地体中部上白垩统达雄组的建立及构造隆升意义

摘要

An E-W-trending late Cretaceous terrestrial molasse with a thickness of 1800 m deposited in the Coqen basin,central Lhasa terrane.Detailed measurement on the well outcropped Xiagezi section near Neidaxiong village in the study area was carried out.This set of the strata can be divided into five members as follow:medium-coarse grained sandstones,massive conglemerates,medium grained sandstones interbedded with siltstones and mudstones,massive siltstones/mudstones,and homogeneous mediumgrained sandstones.The strata unconformabley overlays over the Langshan limestones and are covered by Cenozoic sediments.Analysis of lithogical facies assemblage and sedimentary structures shows that the strata formed under the alluvial fan-braided river systems.Phacinoides occurring in the interbedded volcanic rocks and the underlying Langshan limestones indicate that the strata formed at ～98～91 Ma,and this is consistent with the age of the youngest detrital zircons.Statistic results also show that conglomerate and sandstone modal compositions imply a "magmatic arc" source area.The detrital zircon U-Pb ages and Hf isotopes suggest that the widely spread Early Cretaceous Zenong volcanic rocks on the central Lhasa terrane may be the main source.Based on distribution,composition and the provenance of the Late Cretaceous terrestrial sediments,this study defined the "Daxiong Formation" and compared it with the Jingzhushan Formation in the north of the central Lhasa terrane.The study suggests that the two may represent early tectonic uplifting of the central Lhasa terrane during the early Late Cretaceous (～98～91 Ma),and this provides geological evidence for the uplifting of the Qinghai-Tibet plateau prior to the IndoAsian continental collision.%拉萨地体中部措勤盆地内,呈东西向发育一套厚度巨大(～1800m)的晚白垩世陆相磨拉石沉积建造.通过对研究区内达雄村附近出露较好的虾格子剖面开展详细的测制工作,并将该套地层自底到顶分为5段,依次为中—粗粒砂岩层段、大套砾岩层段、中粒砂岩夹粉砂岩—泥岩层段、大套紫红色与绿色粉砂岩/泥页岩层段以及均质中粒砂岩层段.地层底部与下伏郎山组含圆笠虫灰岩呈沉积不整合关系,顶部被新生代沉积物覆盖.通过岩相组合与沉积结构构造分析研究,认为该套地层形成于冲积扇—辫状河沉积体系.通过地层顶部火山灰夹层和下伏郎山灰岩的圆笠虫化石显示其形成年龄约为98～91 Ma,这与获得的砂岩碎屑最年轻锆石年龄较为一致.砾岩砾石统计结果显示火山岩砾石为其主要成分.砂岩碎屑成分显示以大量火山岩岩屑为主.这些碎屑成分特征表明其主要来自岩浆弧的物源区.结合碎屑锆石U-Pb年代学和Hf同位素数据表明,中拉萨地体上广泛分布的则弄群火山岩可能为其主要的物源区.本文综合该套晚白垩世沉积的时空分布、岩性组合以及物源区特征,将其新命名为“达雄组”,并将其与中拉萨地体北侧的竟柱山组对比表明,两者可能共同代表了中拉萨地体在晚白垩世早期(约98～91 Ma)所经历构造隆升,为青藏高原腹地在印度—亚洲大陆碰撞之前隆升提供了地质证据.