南秦岭勉略构造带火神庙地区关家沟组变质沉积岩系碎屑锆石U-Pb年龄及地质意义

Detrital Zircon U-Pb Ages and Geological Significance of Metasedimentary Rocks from Guanjiagou Formation in Huoshenmiao Area,Mianlue Tectonic Belt of South Qinling

南秦岭勉略构造带南缘的略阳火神庙地区发育一套灰绿色绢云绿泥石英千枚岩、灰白色绢云钠长石英千枚岩和灰色黑云石英千枚岩的岩石组合.该套变质沉积岩系被前人划归关家沟组,但其原岩成岩时代和形成的构造环境都存在争议.采用LA-ICP-MS锆石U-Pb测年方法对从略阳火神庙地区关家沟组变质沉积岩中获得的碎屑锆石进行了研究,结果表明碎屑锆石的年龄介于932～723Ma,主要年龄谱分别为727～723Ma、760～758Ma、897～809Ma和932Ma,主要峰值年龄为848Ma,次要峰值年龄为725Ma、758Ma和932Ma,最年轻的一组年龄为723～727Ma(平均年龄为725Ma),表明该地层的沉积时代应不早于南华纪.结合区域地质资料认为其碎屑物质的来源比较明确,主要来自勉略构造带和南侧碧口微地块内的岩浆岩,扬子板块北缘汉南地区出露的岩浆岩也为该地层提供了有限的物质.此次获得的碎屑锆石年龄较为集中,反映的皆为青白口纪到南华纪的年龄信息,结合前人对该区沉积地层的研究资料,认为其可能为一套裂谷环境的沉积地层.

A suit of gray-green sericite-chlorite-quartz phyllite,gray-white sericite-albite-quartz phyllite and gray biotite-quartzphyllite is exposed in the Huoshenmiao area,Lueyang County.The sequence is previously regarded as the Guanjiagou Formation,but the depositional age and tectonic setting of the protolith are still in dispute.In this paper,detrital zircons from the metasedimentary rocks of the Guanjiagou Formation in the Huoshenmiao area were tested by LA-ICP-MS zircon U-Pb dating.The zircon U-Pb ages range from 932 to 723 Ma,and the main age groups are 727 to 723 Ma,760 to 758 Ma,897 to 809 Ma and 932 Ma,respectively;apeak at 848 Ma and the secondary peak ages at 725 Ma,758 Ma and 932 Ma.The youngest age group is 723 Ma to 727 Ma(725 Ma on average),which indicates that the depositional age of this formation was not earlier than the Nanhua period.Combined with the regional geological data,it is proposed that the detrital material is mainly sourced from magmatic rocks within the Mianlue tectonic belt and Bikou microblock,and less from magmatic rocks exposed in the Hannan area and the northern margin of the Yangtze plate.It should be noticed that the ages of the detrital zircons are fairly concentrated,from Qingbaikou to Nanhua period.Combined with data of the sedimentary strata in this area,it is suggested that this sedimentary sequences might be deposited in a rift environment.