Geochronology of the medium- and low-grade metamorphosed supracrustal rocks from northern Hebei: Constraint on the Late Neoarchean tectonic evolution of the North China Craton

The Late Neoarchean witnessed the cratonization of the North China Craton( NCC) through amalgamation of several micro-blocks to form a coherent basement. The Archean rocks in this craton have experienced varied grades of regional metamorphism ranging up to upper amphibolite and granulite facies at ~ 2500 Ma. However,medium- and low-grade metamorphosed( greenschist to lower amphibolite facies) volcanic-sedimentary rocks are widely distributed in northern Hebei Province. It includes Hongqiyingzi supracrustal rocks in the Humaying district and the low-grade metamorphosed part of the Dantazi supracrustal rocks in the Dayinshan district. The Hongqiyingzi supracrustal rocks in the Humaying district consist mainly of meta-mafic volcanic rocks, epidote amphibolite,amphibolite,garnet amphibolite,hornblende-plagioclase gneiss,biotite-plagioclase gneiss,biotite monzonitic gneiss,quartz schist and magnetite quartzite. SIMS zircon U-Pb ages of 2486 ± 18Ma( MSWD = 1. 4) and 2507 ± 37Ma( MSWD = 2. 0) are interpreted as the formation age of the amphibolite and biotite-plagioclase gneiss,repectively. The Dantazi supracrustal rocks in the Dayinshan district consist mainly of low-grade metamorphosed volcanic rocks,biotite-quartz schist,amphibolite,magnetite quartzite and marble. SHRIMP zircon U-Pb ages of 2490 ± 19Ma( MSWD = 2. 0) and 2502 ± 8Ma( MSWD = 0. 83) are interpreted as the formation age of the meta-baslt and meta-dacite,respectively. Therefore,the medium- and low-grade metamorphosed volcanicsedimentary rocks in the northern Hebei region were formed in the Late Neoarchean. Our new data,combined with the 2511 ~ 2503 Ma Qinglong low-grade volcanic-sedimentary rocks in the eastern part of the NCC suggest that the medium- and low-grade metamorphosed volcanic-sedimentary rocks distributed not only in the central area but also in the margin of the NCC. The protoliths of these mediumand low-grade metamorphosed supracrustal rocks were deposited on Archean high-grade metamorphosed complex within the craton as a cover sequence,marking the cratonization of the NCC.