Objective The Precambrian metamorphic basement is widely exposed in the Huai'an area of Hebei Province. Previous studies have proposed different schemes for the stratigraphic division of metamorphic rocks in the Huai'an area. However, the formation and metamorphic ages of these metamorphic rocks remain controversial.

Methods Through the 1:50,000 regional geological survey in Huai'an area, Hebei Province, the Mashikou Formation and Yousuopu Formation of the Sanggan Group in the Huai'an Complex Belt have been redefined and subdivided based on rock association. The zircon U−Pb age dating of granulites and gneiss of the Sanggan Group was carried out based on in−situ LA−ICP−MS analysis.

Results The Mashikou Formation is mainly composed of granulites, with occurrences of hypersthene and augite. The rock mass of Mashikou Formation is overall grayish yellow in color, with locally interlayers of magnetite quartzite. Compared to the Mashikou Formation, there is a significant increase in layers of plagioclase amphibolite in the Yousuopu Formation, containing higher proportions of biotite and hornblende, locally interbedded with a small amount of grayish−colored marble veins. Accroding to characteristics of rock association and protolith reconstruction, the protolith of Sanggan Group is mainly composed of ultra−basic and basic volcanic rocks, with a small amount of intermediate acidic volcanic rocks interbedded with muddy clastic, ferro−siliceous, and carbonate sedimentary rocks. It belongs to volcano−sedimentary series which were formed at extensional and passive continental margin basins. The zircon U−Pb dating of granulites and gneiss from Sanggan Group show two age peaks between 2.6～2.2 Ga and 2.0～1.7 Ga.

Conclusions The rock association and zircon U−Pb age distribution peaks collectively indicate that the protoliths of Sanggan Group in the Huai'an area were formed in the late Neoarchean (～2.5 Ga) and underwent amphibolite−facies to granulite−facies metamorphism in the Late Paleoproterozoic (～1.8 Ga).