Process and mechanism of mountain-root removal of the Dabie Orogen—Constraints from geochronology and geochemistry of post-collisional igneous rocks

Abstract

Systematical studies of post-collisional igneous rocks in the Dabie orogen suggest that the thickened mafic lower crust of the orogen was partially melted to form low-Mg^# adakitic rocks at 143-131 Ma. Delamination and foundering of the thickened mafic lower crust occurred at 130 Ma, which caused the mantle upwelling and following mafic and granitic magmatic intrusions. Migmatite in the North Dabie zone, coeval with the formation of low-Mg^# adakitic intrusions in the Dabie orogen, was formed by partial melting of exhumed ultrahigh-pressure metamorphic rocks at middle crustal level. This paper argues that the partial melting of thickened lower and middle crust before mountain-root collapse needs lithospheric thinning. Based on the geothermal gradient of 6.6°C/km for lithospheric mantle and initial partial melting temperature of ∼1000°C for the lower mafic crust, it can be estimated that the thickness of lithospheric mantle beneath thickened lower crust has been thinned to <45 km when the thickened lower crust was melting. Thus, a two-stage model for mountain-root removal is proposed. First, the lithospheric mantle keel was partially removal by mantle convection at 145 Ma. Loss of the lower lithosphere would increase heat flow into the base of the crust and would cause middle-lower crustal melting. Second, partial melting of the thickened lower crust has weakened the lower crust and increased its gravity instability, thus triggering delamination and foundering of the thickened mafic lower crust or mountain-root collapse. Therefore, convective removal and delamination of the thickened lower crust as two mechanisms of lithospheric thinning are related to causality.