MANTLE CONTRIBUTION TO POST-COLLISIONAL ULTRA-HIGH TEMPERATURE METAMORPHISM

A number of tectonic settings have been proposed for the development of long-hot orogens during which granulite to ultra-high temperature (UHT) metamorphism persists for 30-100 Myrs. Long-hot orogens are typically associated with continental collision and supercontinent assembly, driving extreme crustal anatexis and developing large volumes of granitic melt. Clockwise pressure-temperature metamorphic paths have been identified for the Mesoproterozoic Grenville Orogen and active Himalayan Orogen; whilst counter-clockwise paths have been determined for the Archean Naipier Complex in Antarctica and Paleoproterozoic Musgraves Province in Australia. Clockwise pressure-temperature paths are thought to reflect near isothermal decompression during a channel flow type model, where crust under-plating an orogen at depths in excess of 80 km undergo radiogenic heating resulting in the lateral extrusion of the melt. Counter-clockwise pressure-temperature metamorphism is typically associated with post-collisional extension or supercontinent stability where highly radiogenic mid- to lower- crustal sediments are insulated, driving melting at temperatures that can exceed 1000°C. Both the channel flow and post-collisional models for the development of long-hot orogens require high radiogenic heat-production in the mid to lower crust to increase temperatures and do not expect a significant mantle contribution to melting. Conversely magmatism in the Musgraves Province shows a strong mantle signature over 50 Myrs of bimodal alkali-calcic magmatism suggesting the mantle may contribute significantly to post-collisional UHT metamorphism. Through 2D numerical models we explore how the mantle may contribute to the temperature evolution and melt generation in a long hot-orogen. To minimise mantle contribution, we model purely advective heat transfer, and contrast these results with temperature conditions during active convection. During purely advective heat transfer partial melting takes longer to initiate, generates smaller volumes of melt and ends earlier than when convective heat transfer is included. These results suggest that convection in the upper mantle can decrease the required contribution of crustal radiogenic heat production during post-collisional high temperature metamorphism.