Transient hot channels: Perpetrating and regurgitating ultrahigh-pressure, high-temperature crust–mantle associations in collision belts

Abstract

Two-dimensional numerical modeling of early continental collision associated with subduction of the lithospheric mantle shows that the formation and exhumation of coesite- and diamond-bearing rocks metamorphosed at 700 to 900 °C in the presence of dense supercritical silicate fluids and melts may be explained by a transient “hot channel effect”. Anomalously high temperature is caused by intense viscous and radiogenic heating in the channel composed of deeply subducted radiogenic upper-crustal rocks (especially, sediments of passive margin origin) and mantle rocks. Heating is associated with partial melting of crustal rocks caused by pervasive flow of aqueous fluids relieved by rapid dehydration (deserpentinization) of the overriding mantle lithosphere that has been hydrated during earlier subduction stages. The channel penetrates the plate interface to the bottom of the lithosphere of the overriding plate (150–200 km) and is characterized by metamorphic temperatures reaching 700 to 900 °C. Low effective viscosity of rocks subsequent to increased temperature, partial melting and fluid infiltration permits profound mixing of hydrated mantle and crustal rocks. The hot channel exists only during early collision, but rapidly produces large amounts of ultrahigh-pressure, high-temperature rocks. Further collision closes the channel by squeezing rheologically weak, partially molten, buoyant rocks between the strong lithospheric mantles of the two colliding plates. Assemblages of complicated P– T paths with repetitive loops characterize exhumation of ultrahigh-pressure rocks in the convoluted flow pattern of the hot channel.