【摘要】 对采自宜昌三峡地区南华纪沉积岩中碎屑锆石进行了U-Pb定年,新元古代锆石U-Pb年龄在833Ma、785Ma出现高峰,说明此时期有两期大规模岩浆活动.结合前人的Hf同位素结果,新元古代锆石U-Pb年龄与εHf(t)值关系图表明:910～890Ma之间锆石εHf(t)值表现为高正值(≈10,接近亏损地幔演化值),890～840Ma锆石εHf(t)值明显降低,并有负值出现,另外在890Ma处有年龄峰出现.笔者认为扬子和华夏板块的拼合可能在890Ma发生了由俯冲到陆-陆或陆-弧的碰撞,之前的高εHf(t)值由洋壳俯冲造成,之后碰撞作用陆壳物质熔融造成了εHf(t)值的降低;840～800Ma的锆石εHf(t)值有正也有负,800～780Ma的锆石εHf(t)值小于0,780～750Ma的锆石εHf(t)值大于0.这些数据与830～795Ma、780～745Ma两期地幔柱事件吻合.更多还原

【Abstract】 This paper reports LA-ICP-MS U-Pb dates of detrital zircons from the Nanhua clastic sedimentary rocks in the Yangtze gorges.Neoproterozoic U-Pb ages show two peaks at 833Ma and 785Ma,assumably corresponding to two large-scale periods of magmatism.εHf(t)values for the 910-890Ma zircons are positive(≈10,similar to the value of the coeval depleted mantle),while those for the 890-840Ma zircons tend to decrease to negative values,and there shows age peaks at 890Ma.These data allow us to infer that transformation of oceanic subduction into continental collision or continental-arc collision occurred at about 890Ma via amalgamation of the Yangtze and Cathaysia blocks.The high εHf(t)values prior to 890Ma resulted from the subduction of oceanic crust.The subsequent drop of εHf(t)values was caused by the crust melting and crustal collision.εHf(t)values for the 840-800Ma zircons are either negative or positive,whereas εHf(t)values for the 800-780Ma zircons are all negative and εHf(t)values for the 780-750Ma zircons are mostly positive.These data are in coincidence with two stages of the mantle plume beneath the Yangtze craton at 830-795Ma and 780-745Ma.更多还原