印度-亚洲大陆碰撞初期的海沟沉积：藏东南宗卓组沉积岩石学与物源分析

Trench Deposition during the Initial Indian-Asian Collision:Petrologic and Provenance Analysis of the Zongzhuo Formation,Southeastern Tibet

混杂岩是汇聚板块边缘的地质体,主要形成于俯冲和碰撞的背景下。本文对藏东南浪卡子地区宗卓组进行了详细的野外地质调查、岩石学研究和物源分析。野外调查表明,宗卓组与下伏特提斯喜马拉雅地层的原始接触关系为逐渐过渡的沉积接触,后期构造作用多呈断层接触。界线附近,宗卓组滑塌的岩块长轴沿页岩片理方向展布,显示沉积混杂的特征;宗卓组多数地层受后期构造作用的改造。宗卓组的混杂岩由岩块和"基质"组成,岩块包括砂岩、灰岩、硅质岩,"基质"以硅质页岩、泥岩为主。砂岩岩块碎屑成分多为火成岩岩屑和沉积岩岩屑;碎屑锆石U-Pb年龄主要分布在88~140 Ma,中生代的锆石εHf(t)值变化范围大(-20~17)。这些特征表明岩块的物源为亚洲活动大陆边缘。结合宗卓组的基底为印度大陆北缘的特提斯喜马拉雅地层,因此宗卓组沉积混杂岩为印度-亚洲大陆碰撞之后沉积。由于宗卓组砂岩岩块缺乏冈底斯弧中古近纪年轻(<60 Ma)年龄,推测这些砂岩岩块的碎屑并非直接来自冈底斯弧及拉萨地体,而是来自洋壳俯冲时期形成的增生楔修康混杂岩。由此,宗卓组为印度-亚洲板块碰撞初期,深水环境下侧向搬运形成的一套沉积混杂岩,物源主要来自西侧的修康混杂岩。宗卓组代表了印度-亚洲大陆碰撞最早期的海沟沉积,其分布指示了印度-亚洲大陆碰撞初期的缝合带位置。

Melange is mainly developed in convergent boundaries of oceanic subduetion or continental collision. This study carried out detailed field investigations, petrologic study and detrital zircon geochronology for sandstones from the Zongzhuo Formation in the Nagarze area, southeastern Tibet. Field investigation shows that the contact relation of the Zongzhuo Formation to the underlying Tethys Himalayan strata is sedimentary at early stage and transfers to faulting contact due to late tectonic activities. Collapsed fragments of the Zongzhuo Formation are distributed along the direction of schist foliation, presenting features of mixing sedimentation. Most of the the Zongzhuo Formation is characteristic of multiple late- stage tectonic overprinting. The Zongzhuo Formation melange includes fragments and matrix, the former consisting of sandstones, chert, micrite, and the later consisting mainly of shale or mudstone. Fragments of the sandstones are composed mainly of igneous rocks and sedimentary rocks. U-Pb ages of detrital zircons cluster at 88~140 Ma, and eHf（t） of the Mesozoic-Cenozoic zircons varies greatly from --20 to 17. All these features suggest that the provenance derived from active Asian continental margin. Considering that the basement of the Zongzhuo Formation is the Tethys Himalayan strata in the northern margin of the Indian continent, the Zongzhuo Formation is inferred to be developed after the initial Indian-Asian collision. Due to the fact that the sandstone of the Zongzhuo Formation lacks young detrital U-Pb age of 90 Ma which is characterized by the Gangdese arc （~60 Ma）, we suggest that those sandstone blocks were probably recycled from the Xiukang melange in the west, instead of the Xigaze forearc, Gangdese arc or the Lhasa terrane directly. The Zongzhuo Formation is one set of sedimentary melange formed from lateral movement of the Xiukang me1ange in the west at the deep marine environment during the early Indian- Asian continent collision. Thus, the Zongzhuo Formation represents trench deposition at early stage of the Indian-Asian continent collision, whose distribution indicates the position of the suture zone of the early Indian-Asian continent collision.