Sedimentology, geochemistry and chronology of Jurassic in the Ganzi-Litang ophiolite mélange belt of Xinlong area, Sichuan and its tectonic setting

Abstract

甘孜-理塘蛇绿混杂岩带作为扬子板块与中咱地块的碰撞结合带，其对探讨西南“三江”多岛弧盆系构造演化具有重要意义。本文选取四川新龙地区瑞环山组进行沉积学、古生物学、地球化学和同位素年代学研究，限定其沉积时代及物质属性，探讨其沉积构造背景。新龙地区瑞环山组为碳酸盐岩夹碎屑岩岩石组合，其碎屑岩物源区为安山质火山岩及陆缘复理石，母岩构造背景为大洋岛弧环境。瑞环山组灰岩中珊瑚化石主要集中于晚侏罗世-早白垩世。碎屑锆石定年结果表明砂岩中锆石主要来源于扬子板块（435Ma、764Ma、1888Ma峰值）以及甘孜-理塘俯冲增生杂岩（229Ma峰值），锆石最新年龄为196±2Ma，指示其应沉积于早侏罗世之后的残留海盆地。研究结果表明，甘孜-理塘洋在晚三叠世末弧陆碰撞造山之后又经历了侏罗纪残留海阶段，在残留海盆地西侧临近甘孜-理塘俯冲增生杂岩浅海环境中沉积了瑞环山组碳酸盐岩夹碎屑岩建造，至早白垩世全面进入造山隆升阶段。;The Ganzi-Litang ophiolite mélange belt, as a collision belt between the Yangtze plate and the Zhongzhan block, is important to explore the tectonic setting of the multi-arc-basin system in the Sanjiang region, southwestern China. In this paper, we studied sedimentology, paleontology, geochemistry and isotopic chronology of the Ruihuanshan Formation in Xinlong area, Sichuan, to limit sedimentary age and material properties, and discuss sedimentary tectonic setting. The Ruihuanshan Formation in Xinlong area is carbonate with clastic rock assemblage. Its source region is andesitic volcanic rocks and flysch, and the tectonic setting of the parent rock is an oceanic island arc. The coral fossils in the limestones of the Ruihuanshan Formation are mainly concentrated in the Late Jurassic-Early Cretaceous. The detrital zircon dating results indicate that the zircons in the sandstone are mainly derived from the Yangtze plate (peaks at ca.435Ma, 764Ma, 1888Ma) and the Ganzi-Litang subduction accretive complex (peak at ca. 229Ma), and the youngest age is 196±2Ma, indicating that it should be deposited in the residual basin after the Early Jurassic. The results show that the Ganzi-Litang ocean basin went experienced the Jurassic residual sea stage after the arc-land collisional orogeny at the end of the Late Triassic, and the Ruihuanshan Formation carbonates with clastic rock formation were deposited in a neritic environment near the Ganzi-Litang subduction accretive complex on the west side of the residual basin, and then entered the full-scale orogenic uplift stage in the Early Cretaceous.