Growth and metamorphic evolution of the Precambrian lower crust at the southeastern margin of the North China Block

The Precambrian lower-crustal rocks at the southeastern margin of the North China Block( NCB) occur as high-grade metamorphic or granulite terrains and xenoliths in the Mesozoic dioritic porphyry. These rocks provide an excellent natural laboratory to study formation and evolution of the Precambrian lower crust in the region. The metamorphic terrains distribute over the and Bengbu areas,mainly composed of Huoqiu Group ( complex) and Group and Fengyang Group ( Wuhe complex). The complex comprises muscovite-plagioclase gneiss,quartzite,mica schist,marble,meta-sandstone,banded iron formations( BIF) and amphibolite,which are mostly covered by Quaternary. In comparison,the Wuhe complex is chiefly exposed on the surface, and consists of the metamorphosed igneous mafic and felsic rocks and supracrustal rocks associated with the Paleoproterozoic foliated K-feldspar granites and Mesozoic granitoids,constituting the Bengbu uplift. Its main rock types comprise a variety of lithologies,including garnet amphibolite,garnet granulite,rodingite,garnet-plagioclase-amphibole gneiss,granitoid gneiss,mica schist,marble,quartzite and meta-sandstone. These various Precambrian metamorphic rocks generally contain similar peak minerals such as garnet,clinopyroxene,plagioclase,rutile and quartz. As a result,combined with their zircon U-Pb dating,it is suggested that they mostly suffered from the HP granulite-facies metamorphism at Palaeoproterozoic. Integrated investigations on metamorphic petrology,geochemistry,Hf-isotope and zircon U-Pb geochronology of the rocks suggest that the Precambrian lower crust beneath the region experienced an episodic growth and multiple metamorphic evolution and modification. Intensive tectono-thermo events and metamorphic overprinting for them mainly occurred at several peaks such as 2. 7 ~ 2. 8Ga,2. 5 ~ 2. 6Ga,2. 1Ga,1. 8 ~1. 9Ga,390 Ma and 176 Ma in the southeastern margin of the NCB. However,the formation time of the Precambrian lower crust was≥2. 1Ga. Based on the related investigations,we propose the present scientific problems on the Precambrian lower-crustal metamorphic rocks at the southeastern margin of the NCB and their related prospective projects.