Palaeozoic metamorphism of the Neoproterozoic basement in NE Cathaysia: zircon U-Pb ages, Hf isotope and whole-rock geochemistry from the Chencai Group

Abstract

The protoliths of the metamorphic suite of the Chencai Group in the northeastern margin of the Cathaysia Block in South China are composed of Neoproterozoic argillaceous–arenaceous rocks of turbidite facies, marine carbonates, gabbro and basalt. Here, we present zircon U–Pb ages, Hf isotopes, and whole-rock geochemical data on migmatites and metagabbros from the Chencai Group and the surrounding hornblende gneisses. The gabbroic enclaves in the migmatites and hornblende gneiss are characterized by subduction-related geochemical characteristics, which differentiate them from the migmatites. Zircons from the hornblende gneiss, migmatite and gabbro yielded weighted mean 206Pb/238U ages of 879 ± 10 Ma, 438 ± 3 Ma and 453.2 ± 3.5 Ma, respectively. Hf isotopic analysis reveals ϵHf(t) values of +0.02 to −2.55 for zircons from the migmatite, whereas those from the hornblende gneisses and gabbros show positive ϵHf(t) values (+3.41 to +13.36 and +1.80 to +5.25, respectively). Our data correlate the protoliths of the hornblende gneiss and metagabbros to subduction-related magmatism prior to the assembly of the Yangtze and Cathaysia blocks. The migmatite and gabbro record a Palaeozoic tectonothermal event that led to migmatization and resetting of Neoproterozoic ages. Our results suggest that the Palaeozoic event was initiated before c. 453 Ma with metamorphism at c. 438 Ma.