扬子板块西北缘碧口微地块南华系碎屑锆石U-Pb年龄及其物源示踪

On the Hongyanggou area of the Bikou micro-block on the northwestern margin of the Yangtze plate, there develops a Nanhua - Sinian sedimentary cover above the volcanic rock series of the Bikou Group. However, the sedimentary age of the Nanhua System still lacks evidence, and its provenance and tectonic background remain inconclusive. In this paper, the LA - ICP - MS zircon U - Pb dating method was used to conduct a detrital zircon dating study on the arkose in the upper conglomerate-bearing series of the Nanhua System. The results show that the ages of detrital zircons can be divided into two groups: (1) The late Neoproterozoic age group (750 - 800Ma), with a peak age of 795Ma; (2) The early - middle Neoproterozoic age group (820 - 920Ma), this group has an obvious peak, with a peak age of 850Ma. The average age of the youngest age group of detrital zircons is 750Ma, that is, the sedimentary age of the upper conglomerate-bearing series of the Nanhua System is no older than 750Ma. Combined with the stratigraphic sequence relationship, it is considered that the sedimentary age of this stratum may be the Late Nanhua Epoch. Comprehensive studies suggest that the provenance of the upper conglomerate-bearing series of the Nanhua System is multi - sourced, mainly from the Neoproterozoic intermediate - acidic magmatic rocks inside the Bikou micro - block and in the southern HouLongmenshan tectonic belt and the Hannan - Micangshan micro - block on the northwestern margin of the Yangtze plate. The magmatic rocks exposed in the Mianlue tectonic belt on the north side may also have provided a small amount of provenance for this stratum, and its sedimentary event corresponds to the post - collision - breakup stage of the Bikou micro - block and the northwestern margin of the Yangtze plate in the middle - late Neoproterozoic (~ 810Ma).

扬子板块西北缘碧口微地块红岩沟地区碧口群火山岩系之上发育有南华—震旦纪沉积盖层,但南华系的沉积时代尚缺乏依据,其物源及构造背景也仍无定论。本文采用LA-ICP-MS锆石U-Pb测年方法,对南华系上部含砾岩系中的长石砂岩进行了碎屑锆石测年研究,结果表明碎屑锆石的年龄可以分为两组: (1)新元古代晚期年龄组(750 ~ 800Ma),峰值年龄为795Ma; (2)新元古代早中期年龄组(820 ~ 920Ma),该组有明显峰值,峰值年龄为850Ma。碎屑锆石最小年龄组平均年龄为750Ma,即南华系上部含砾岩系沉积时代不老于750Ma,结合地层序列中的层位关系,认为该地层的沉积时代可能为晚南华世。综合研究认为,南华系上部含砾岩系物源具多源性,主要来自扬子板块西北缘碧口微地块内部和南侧后龙门山构造带、汉南-米仓山微地块的新元古代中酸性岩浆岩,北侧勉略构造带出露的岩浆岩可能也为该地层提供了少量的物源,其沉积事件对应于新元古代中晚期(~ 810Ma)碧口微地块及扬子板块西北缘后碰撞-裂解阶段。