Constraining timing and tectonic implications of Neoproterozoic metamorphic event in the Cathaysia Block, South China

Abstract

The Cathaysia Block of the South China Craton includes a Proterozoic basement that experienced a prolonged Precambrian crustal evolution but to date lacks evidence of Proterozoic metamorphic ages. At Lichuan and Jianning, in the Wuyi Domain of the eastern Cathaysia Block, Proterozoic rock units include migmatized paragneiss of the Wanyuan Group and minor amphibolite of the Tianjingping Formation, which are enveloped by schist of Mayuan Group, and all are intruded by Paleozoic and Mesozoic igneous rocks. Detrital zircon grains from the Wanyuan paragneiss display metamorphic rims that yield concordant weighted average 206Pb/238U ages of 860±6Ma and 435±5Ma, along with variably disconcordant ages with lower intercept ages of 442±41Ma. The zircon core ages range from 3015Ma to 851Ma, with three major age populations at 930–865Ma, 1850–1200Ma and 2650–2400Ma. Detrital zircon grains from Mayuan schist samples at Jianning generally lack core-rim structures and yield three main age populations at 860–736Ma, 1835–1775Ma and 2720–2500Ma. Metamorphic ages of ca. 860Ma and ca. 435Ma for the Wanyuan paragneiss along with the youngest detrital zircon constrain the depositional age of the protolith to ca. 865–860Ma, whereas the Mayuan Group is younger and probably deposited after ca. 736Ma. Characteristics of detrital zircon age populations along with regional geological data suggest accumulation of the Wanyuan Group in a convergent and/or collisional setting. Metamorphism and a possible subduction-collision process within the Cathaysia Block at around 860Ma suggest it was not a unified block in early Neoproterozoic. The growth of ca. 440Ma metamorphic rims is likely related to granitic magmatism, such as that exposed in the Lichuan region. The sparse evidence for early Neoproterozoic metamorphism likely reflects widespread overprinting by the Paleozoic tectonothermal event at around 440Ma.