Thermal gradient and geochronology of a Paleozoic high-grade terrane in the northeastern Cathaysia block, South China

Abstract

The Chencai terrane in the northeastern Cathaysia Block forms part of the northeast extension of the Paleozoic orogeny in South China. Recent field investigations have revealed the occurrence of high grade rocks in this terrane including granulite facies metapelites (khondalites) and orthopyroxene-bearing granulites. However, the thermal gradients and metamorphic history of this region have not been well constrained, thus impeding a better understanding of the geodynamic history of the Paleozoic orogeny in South China. Here we report results from a systematic study on the metamorphic PT conditions and geochronology of the orthopyroxene granulites and metasedimentary rocks (metapelites) from the Chencai area. The orthopyroxene granulites preserve granulite facies peak assemblage of Opx+Grt+Pl+Kfs+Qtz and a retrograde P-T trajectory. Peak PT conditions are 845–860°C and 0.83–0.86Gpa, corresponding to a thermal gradient of ~30°C/km which is much hotter than the normal thermal gradient of continental crust. Mineral compositions of the metasedimentary rocks re-equilibrated during retrograde metamorphism so that the peak PT conditions recorded by these rocks are much lower than those obtained from the orthopyroxene granulite. Peak PT conditions are 710–720°C, 0.57–0.58Gpa for garnet-biotite gneiss and 760–780°C, 0.60–0.62Gpa for biotite garnetite. Zircon U-Pb dating shows that this episode of granulite facies metamorphism occurred at 430–450Ma and the detrital population shows minor or no contribution from the basement rocks of the Cathaysia Block. The Neoproterozoic rocks occurring along the Jiangnan belt are considered to have provided the sedimentary source as indicated by the two main age peaks of 0.79Ga and 0.83Ga from the detrital zircon cores. Lu-Hf isotope data on the zircons indicates extensive alteration and little new zircon growth during the Paleozoic metamorphism. The εHf(t) values and model ages (TDM1) of zircons suggest that the source rocks involved juvenile crustal components with Hf model ages of 1.1–1.5Ga. Sedimentation of such a continental shelf sequence as well as the high thermal gradient of ~30°C/km confirm the previously proposed continental arc formation and maturation model in the Chencai region.