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    甘肃北山地区榴辉岩的变质年龄:来自锆石的U-Pb同位素定年证据

    Neoproterozoic eclogitic metamorphic age of the Beishan eclogite of Gansu, China:Evidence from SHRIMP U-Pb isotope dating

    • 摘要: 提要:北山造山带地处塔里木—中朝板块与哈萨克斯坦板块的交汇部分,主造山时期被认为是早古生代。榴辉岩产在北山造山带的南带,带中榴辉岩与大量基性超基性洋壳岩石和俯冲碰撞有关的花岗岩等伴生,反映该带有可能代表板块的边界。北山榴辉岩的锆石SHRIMP U-Pb同位素定年表明,榴辉岩相的变质年龄为(819±21)Ma,原岩年龄为(1007±20)Ma,表明北山地区存在新元古代一次重要的板块裂解和俯冲碰撞事件。与北山相邻的祁连山南部柴北缘地区近年也报道存在新元古代板块裂解和蛇绿岩洋壳形成事件,表明中国西部存在区域上的新元古代洋盆裂解事件,或称之为罗德尼亚大陆裂解事件。需强调的是,北山榴辉岩相变质事件发生在新元古代,柴北缘榴辉岩的原岩虽为新元古代洋壳岩石,但榴辉岩相变质作用则发生在早古生代,反映北山新元古代俯冲碰撞事件之后的又一次俯冲碰撞事件。

       

      Abstract: Abstract:The Beishan orogen is located at the junction between the Tarim-Sino-Korean and Kazakhstan plates, and its main orogenic event occurred in the Early Paleozoic. Eclogites are associated with ultramafic-mafic rocks and collisional granites in the south belt of the Beishan orogen, which suggests that this belt might represent a plate boundary. SHRIMP U-Pb isotope dating of zircons from the Beishan eclogite yielded an eclogite-facies metamorphic age of 819 ± 21 Ma and a protolith age of 1007 ± 20 Ma, indicating an important plate breakup and subduction-collision event in the Neoproterozoic. Recently an event of Neoproterozoic plate breakup and ophiolitic oceanic crust formation has also been reported in the North Qaidam marginal area in the southern Qilian Mountains adjacent to the Beishan, suggesting the occurrence of a regional breakup event in a Neoproterozoic oceanic basin in western China, which is called the Rodinian Supercontinent breakup event. However, it should be emphasized that the eclogitic-facies metamorphism in the Beishan took place in the Neoproterozoic, while that for the North Qaidam marginal eclogite occurred in the Early Paleozoic (500-440 Ma) though its protolith is the Neoproterozoic oceanic crustal rock, suggesting another subduction-collision event after the Neoproterozoic subduction-collision event in the Beishan.

       

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