摘要:
鹰扬关群在湘—粤—桂三省交界地区广泛发育,为一套以绿片岩相变质的细碧岩、(石英)角斑岩及相关的火山碎屑岩为主,含有细碎屑岩和碳酸盐岩组合。该群的成岩背景过去一直认为是Rodinia超大陆裂解背景下的大陆裂谷环境,时代归属为新元古代。岩石学、地球化学和锆石 LA-ICP-MS U-Pb 定年结果表明,鹰扬关群火山岩主要属于基性-中性火山(碎屑)岩类,在岩石化学成分上表现为明显富集大离子亲石元素(LILE,包括U、Th、Ba、K和Rb等)和轻稀土元素,而Nb、Ta、P和Ti等高场强元素和重稀土元素相对亏损,反映其具有俯冲-消减作用形成的岛弧-弧后盆地型火山岩地球化学特征;获得鹰扬关群中变角斑岩的锆石LA-ICP-MS U-Pb谐和年龄为(415.1±2.1) Ma (n=13, MSWD=1.8),表明其为加里东期海相火山喷发的产物。结合云开地块北缘存在有早古生代MORB型和岛弧型变质基性火山岩的资料表明,扬子板块和华夏板块结合带(称之为钦—杭结合带)西南段有早古生代的古洋盆,鹰扬关群岛弧-弧后盆地型火山岩可能是钦—杭结合带南西段早古生代洋陆俯冲-消减过程的地质记录,钦—杭结合带西南段加里东期的构造格局是俯冲增生造山带而不是陆内造山带。
Abstract:
Yingyangguan Group is widely distributed in the junction area of the Hunan, Guangdong and Guangxi Provinces. It consists dominantly of the green schist facies metamorphic spilite, (quartz-) keratophyre and related to volcanic clastic rocks, with small quantities of fine clastic rocks and carbonate rocks. Previously it had been thought that this group formed in a continental rift environment under the background of Rodinia supercontinent breakup during the Neoproterozoic time. The combined petrology, geochemistry and zircon LA-ICP-MS U-Pb dating study reveal that the volcanic rocks of Yingyangguan Group belong primarily to basic-intermediate volcanic(-clastic) rocks, and petrochemically are pronounced enrichment in large-ion incompatible elements (LILEs, e.g., U, Th, Ba, K and Rb) and LREEs and pronounced depletion in high field strength elements (HFSEs, e.g., Nb, Ta, P and Ti) and HREEs. Such signatures display geochemical characteristics of typical subduction-related arc-back arc basin volcanic rocks. A metamorphic keratophyre sample of Yingyangguan Group yielded a zircon LA-ICPMS U-Pb concordia age of (415.1±2.1) Ma (n=13, MSWD=1.8), indicate that it is to be the product of Marine volcanic eruption during Caledonian period. Together with early Paleozoic MORB-type and arc-type metabasic volcanic rocks in the northern margin of Yunkai block, appears to support a development of an early Paleozoic oceanic basin within the southwestern segment of the joint belt between the Yangtze and Cathaysian blocks (so-called Qinzhou-Hangzhou joint belt). Therefore, we conclude that Yingyangguan arc-back arc basin volcanic rocks were likely an important record of the oceanic subduction and subsequent continental collision in the southwestern segment of Qinzhou-Hangzhou joint belt during Early Paleozoic time, and Caledonian structural pattern of the southwestern segment of Qinzhou-Hangzhou joint belt is a subduction-accretion orogenic belt and not an intracontinental orogenic belt.
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