对湖北大洪山地区一套紫红色砂-砾岩系沉积年代的再认识--碎屑锆石U-Pb年龄及其地质意义

摘要

湖北大洪山地区花山群六房咀组之上不整合覆盖了一套紫红色砂-砾岩。这套岩石的沉积时代存在争议，一种观点认为是南华纪早期；另一种观点认为是晚侏罗世。利用LA-ICP-MS分析技术，对这套碎屑沉积岩进行了碎屑锆石U-Pb测年，获得有效年龄数据125组，年龄值变化范围较大（3223~771Ma），主要集中于新太古代末—古元古代早期（约2500Ma）、古元古代（约2000Ma）和新元古代（约800Ma）3个时间段。对比鄂东南地区和三峡地区莲沱组岩石学特征和碎屑锆石年龄分布特征，认为其具有相似的物源，这套岩系沉积时代应该为南华纪早期而非晚侏罗世。综合分析扬子地块的前寒武纪年龄，认为扬子地块可能存在大量未岀露的新太古代末—古元古代早期（约2500Ma）基底，约2500Ma的岩石应该是扬子地块深部基底的重要组成部分。扬子地块存在广泛的古元古代（约2000Ma）岩浆活动，这期岩浆活动可能是Columbia超大陆聚合在扬子地块的响应。新元古代（约800Ma）扬子地块北缘存在Rodinia超大陆裂解同期裂谷，裂谷内基性岩可能为大洪山地区南华纪地层中基性岩砾石的物源。%The sedimentary age of a series of purple-red colored glutenite sedimentary rocks in Dahong Mountain area of northern Hubei Province is controversial. Some researchers hold that it is of late Jurassic period, while others believe it is of early Nanhua peri⁃od. In this paper, the authors studied detrital zircon geochronology of this layer and obtained 125 groups of effective age data which range from 3223Ma to 771Ma, and the age data are focused on three periods: late Neo-Archean to early Paleoproterozoic (~2500Ma), Paleoproterozoic (~2000Ma), and Neoproterozoic (~800Ma). The authors compared the petrological features and the detri⁃tal zircon age distribution of Liantuo Formation in southeastern Hubei, Three Gorges area, and the purple-red colored glutenite. It is considered that the Liantuo Formation in southeastern Hubei, Three Gorges area and the purple-red colored glutenite have similar sources. The sedimentary age of this series of purple-red colored glutenite sedimentary rocks in Dahong Mountain area of northern Hubei Province is early Nanhua period rather than the late Jurassic period. Based on a comprehensive analysis of the Precambrian age of Yangtze block, the authors hold that there may exist lots of Neo-Archean to early Paleoproterozoic concealed rocks, and the rocks around 2500Ma may be an important part of the basement of Yangtze block. There existed a magmatic activity around 2000Ma in Yangtze block, and it may be connect to the aggregate event of Columbia supercontinent. There is a Neoproterozoic rift on the northern margin of Yangtze block, which might be connected to the breakup of Rodinia supercontinent, and the basic rock was probably the source of the mafic gravel in the purple-red colored glutenite sedimentary rocks.