西南三江甘孜-理塘洋晚古生代构造演化:来自理塘蛇绿混杂岩堆晶辉长岩U-Pb年龄的约束

    Late Paleozoic tectonic evolution of Ganzi-Litang Ocean in Sanjiang region, Southwest China: Constraints from U-Pb geochronology of cumulate gabbro in Litang ophiolitic mélange belt

    • 摘要: 理塘县高城镇西北的擦岗隆洼岩组是一套以玄武岩为基质的蛇绿混杂岩,通过该蛇绿混杂岩中堆晶辉长岩的LA-ICP-MS 锆石U-Pb定年,显示其年龄范围介于349 ~ 274 Ma之间,形成2个年龄组:第一组斜锆石206Pb/238U年龄加权平均值为346±17 Ma,代表堆晶辉长岩早期岩浆活动的时代;第二组斜锆石206Pb/238U年龄加权平均值为286.2±5.1 Ma,代表了堆晶辉长岩晚期岩浆活动的结晶年龄。擦岗隆洼岩组的超基性—基性岩具有N-MORB(正常洋中脊玄武岩)和E-MORB(富集型洋中脊玄武岩)组合特征,指示这些岩石可能形成于洋中脊环境,岩浆物质源自N-MORB所代表的亏损上地幔源区受地幔柱交代混染后而形成的混合源区。研究认为,甘孜−理塘洋最早可能在中泥盆世,受区域地幔柱活动影响开始拉张,导致中咱地块从扬子陆块西缘裂离,并在早石炭世发育形成洋盆。在中晚三叠世甘孜−理塘洋壳开始向西俯冲,并于晚三叠世末闭合,区域进入弧陆碰撞造山阶段。

       

      Abstract: Located in the northwest of Gaocheng Town, Litang County, the Caganglongwa rock Formation is a set of ophiolitic mélange with basalt as the matrix. LA-ICP-MS zircon U-Pb dating of the cumulate gabbro in the ophiolitic mélange shows an age distribution range from 274 Ma to 349 Ma, indicating two age groups. The first group of baddeleyite 206Pb/238U has a weighted average age of 346 ± 17 Ma, which represents the age of early magmatism of cumulate gabbro; the second group of baddeleyite 206Pb/238U has a weighted average age of 286.2 ± 5.1 Ma, which represents the crystallization age for late magmatism of cumulate gabbro. The ultrabasic and basic rocks in the Caganglongwa rock Formation display combined characteristics of N-MORB and E-MORB, indicating that these rocks may be formed in the mid-ocean ridge environment, and the magma material comes from a mixed source region formed by depleted upper mantle source region represented by N-MORB and metasomatically contaminated by the mantle plume. The research finds that the Ganzi-Litang Ocean may begin to stretch as early as the Middle Devonian due to the regional mantle plume activity, resulting in the split of the Zhongza block from the western margin of the Yangtze block and the Formation of an ocean basin in the Early Carboniferous. By the Mid-Late Triassic, the Ganzi-Litang oceanic crust was subducted westward and closed at the end of the Late Triassic, and the region entered the arc-continent collision orogeny stage.

       

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