Destruction of ancient lower crust through magma underplating beneath Jiaodong Peninsula, North China Craton: U-Pb and Hf isotopic evidence from granulite xenoliths

摘要

Zircons from granulite xenoliths entrained in a Late Cretaceous mafic dike in the Jiaodong Peninsula, North China Craton (NCC), show three distinct U-Pb age populations. Part of the old zircon grains yield discordant data that project to ages of about 2.4 to 2.5Ga, a few grains indicate growth at about 2.0Ga and a third group yield Cretaceous ages with peaks at 120 and 90Ma. The oldest zircons give Hf T_(DM) model ages of 2.6-2.8Ga. These results demonstrate the existence of original Archean lower crust in the Jiaodong region. Zircons of 2.0Ga have similar Hf T_(DM) model ages as the Neoarchean-Paleoproterozoic grains, suggesting that these zircons were products of metamorphic recrystallization due to thermal event without juvenile input. Early Cretaceous zircons yield εHf(t) values of -21 to -12 and Late Cretaceous zircons large variable εHf(t) from +4 to -50. These data suggest that magmatic underplating occurred in the Neoarchean to Earliest Proterozoic lower crust of the NCC, both in the Early and Late Cretaceous. It is suggested that the Mesozoic magma underplating, which also provided the heat source for the voluminous Mesozoic magmatism in the NCC, significantly modified the composition of the Archean to Paleoproterozoic lower crust of the NCC.