鄂西走马地区大塘坡组顶部泥岩碎屑锆石LA-ICP-MS U-Pb 年龄及其地质意义

Mudstones from the topmost part of the Datangpo Formation in the Zouma area, western Hubei: LA-ICP-MS zircon U-Pb dating and its geological implications

关键词:  LA-ICP-MS  U-Pb定年  碎屑锆石  Marinoan冰期  Rodinia超大陆  大塘坡组  走马地区

Key words:  LA-ICP-MS  U-Pb dating  detrital zircon  Marinoan  Rodinia supercontinent  Datangpo Formation  Zouma area

摘要:

地球在新元古代经历了Rodinia超大陆的聚合和解体，并发生了Sturtian和Marinoan两期全球规模的冰川事件。华南地区南华系古城组和南沱组冰碛岩分别对应Sturtian冰期和Marinoan冰期沉积记录，大塘坡组属于其间冰期沉积。对鄂西走马地区大塘坡组顶部泥岩中的碎屑锆石进行了LA-ICP-MS U-Pb定年，锆石具有高Th/U比值（0.47～1.69），并显示明显的振荡环带，为岩浆成因；62颗锆石的62个数据点分析，获得的60个谐和年龄分布在651～2435Ma，主要峰值为～794Ma。最年轻单颗锆石U-Pb年龄651±7.7Ma，结合前人研究认为华南地区Marinoan冰期启动时间应晚于651Ma；碎屑锆石为亲扬子型，可能来自鄂中古陆和扬子北缘；分布最集中的751～851Ma年龄段内～850Ma峰值记录了扬子地台与华夏褶皱带碰撞拼合事件，而～820Ma、～800Ma、～760Ma等峰值则揭示了与Rodinia超大陆解体有关的幕式岩浆活动。

Abstract:

In the geological history of the assembly and breakup of the supercontinent Rodinia on the Earth during the Neoproterozoic, there once occurred two phases of global glacial events in the Sturtian and Marinoan epoches. Previous studies of isotope ages show that the initiation and termination of the Sturtian glacial epoch are dated at 717 Ma and 660 Ma, while the termination of the Marinoan glacial epoch is dated at 635 Ma. However, the initiation of the Marinoan glacial epoch remains uncertain. The tillites from the Nanhuan Gucheng Formation and Nantuo Formation in southern China respectively represent the depositional records of the Sturtian and Marinoan glacial deposits, and the Datangpo Formation is represented by the interglacial deposits. The present paper reports the LA-ICP-MS zircon U-Pb age determinations of the mudstones from the topmost part of the Datangpo Formation in the Bajiaohe section, Zouma area, western Hubei. 62 spots of 62 zircon grains give 60 concordia ages (concordency >90%) ranging between 651 Ma and 2435 Ma, of which 52 zircon U-Pb ages vary from 751 Ma to 851 Ma, with the peak age of about 794 Ma. The youngest zircon U-Pb age is in general agreement with that reported by the predecessors in the adjacent areas, and thus the initiation of the Marinoan glacial epoch may be deduced to be later than 651 Ma. The detrital zircons have the Yangtze affinity, and probably originated from the central Hubei palaeocontinent and northern Yangtze landmass. The peak age of about 850 Ma within the main age range of 751 Ma to 851 Ma may record the collision and amalgamation events occurred in the Yangtze platform and Cathaysia fold zone, while the peak ages of about 820 Ma, 800 Ma and 760 Ma may record the episodic magmatism associated with the breakup of the supercontinent Rodinia.