江南造山带西段新元古代下江群清水江组的凝灰岩锆石U-Pb定年及其构造背景

Zircon U-Pb Dating of Tuff and Its Tectonic Background of Qingshuijiang Group in Xiajang Formation, Neoproterozoic in West Jiangnan Orogenic Belt

江南造山带是扬子地块与华夏地块形成统一华南板块过程中,长期碰撞拼合的产物,区域上,广泛出露最老的前寒武纪时期的地层,岩石主要为一套巨厚陆源碎屑岩夹火山碎屑岩组合。华南板块在前寒武的形成演化,以及前寒武地层的时代归属、地层划分和区域地层对比,一直是地学界的热点与难点。本文在黔东南榕江县朗洞采集了下江群清水江组中的凝灰岩,开展锆石U-Pb年龄和微量元素研究。锆石LA-ICP-MS U-Pb年龄为768.2±4.8 Ma,与黔东南地区已获取的清水江组中凝灰岩或含凝灰质岩石的同位素年龄数据进行分析对比,将清水江组沉积时限大致限制在785~755 Ma。这一年龄与相邻的广西龙胜三门街的辉长辉绿岩(761±8 Ma)、流纹英安岩(765±14 Ma)和湖南通道的超镁铁岩(772±11 Ma)等岩浆岩同位素数据高度吻合,可能同属雪峰期火山-岩浆作用产物。锆石微量元素特征及比值在地球化学图解显示,下江群清水江组凝灰岩中锆石具有岛弧源区锆石特征,暗示该时期黔东南地区可能属于沟-弧-盆大地构造背景,扬子地块与华夏地块在该时期属于弧陆俯冲环境,自此之后,统一的华南板块最终形成,江南造山带进入陆内演化阶段。

Jiangnan orogenic belt is the outcome of long time’s collision during the Yangtze block and Cathaysia block formed unified South China Plate.In some area, the oldest stratum in the Cambrian period exposed widely, the rock mainly is a composition of extremely thick terrigenous clastic rock and pyroclastic rock.The formation and evaluation in Precambrian, the stratum age in Precambrian, the comparison of stratum division and regional stratum of South China Plate always are highlights and difficulties in the academic circle.In this paper, the tuff in Qingshuijiang group of Xiajiang formation in Langdong, Rongjiang county of Qiandongnan was collected, the Zircon U-Pb age and rare element were studied.The Zircon LA-ICP-MS U-Pbage is 768.2±4.8Ma, it compared with the isotope age data of tuff or tuff-bearing rock, the depositional age of Qingshuijiang group are confined to 785~755 Ma.These ages are good agree with gabbro diabase(761±8Ma)and rhyodacite(765±14Ma)in the nearby Sanmenjie in Longsheng of Guangxi, ultramafite(765±14Ma)in Tongdao of Hunan and other magmatic rock isotope date, maybe the outcome of volcanic and magmatic action in Xuefeng period.According to the geochemical diagram of zircon rare element characteristic and ratio, it shows the tuff zircon in Qingshuijiang group of Xiajiang formation has the features of island arc, means it might belong to the arc-trench-basin tectonic background, the Yangtze block and Cathaysia block are in the arc-continent subduction environment.From then on, the unified South China Plate formed and Jiangnan orogenic belt entered the intracontinental evolution period.