Thermal imprints of late Permian Emeishan basalt effusion: Evidence from zircon fission-track thermochronology

Abstract

Late Permian Emeishan large igneous province (LIP) developed in response to an ascending mantle plume beneath the southwestern Yangtze Craton, China. Mantle plume models predict two types of thermal effect on the lithosphere: (1) high lithospheric heat flow anomalies due to the ascending magma, and (2) heating of underlying country rocks by erupted lava flow. Evidence for these two effects of the Emeishan LIP may be documented in pre-LIP strata. As such, we determined the maximum temperature of sedimentary rocks deposited before and after the LIP using zircon fission-track (ZFT) thermochronology, which is sensitive to temperatures of 150–250 °C. ZFT ages of post-LIP sedimentary rocks are variable and overlap with depositional ages, reflecting post-LIP burial heating should be lower than the ZFT closure temperature. Pre-LIP samples yield scattered ZFT ages younger than their depositional ages, and contain an older age component that increases with depth. This indicates that the upper part of the pre-LIP strata should have experienced higher temperature than lower ones. Such a heating pattern cannot be explained by pre- or post-LIP burial heating, which predict higher temperatures and thus younger ages for stratigraphically lower samples. We propose that the downward increasing age pattern is evidence for massive top-down lava heating. Forward thermal modelling suggests that a >1.5-km-thick effusive basalts is required to reset the ZFT ages of rocks that are ∼3 km beneath the LIP. Our results support the existence of massive effusive basalts in the intermediate zone of the Late Permian Emeishan LIP.