EXHUMED RECORD OF CRUST-MANTLE INTERACTION AS A MECHANISM FOR UHT METAMORPHISM IN A THICKENED DEEP CRUSTAL HOT ZONE

Rare exposures of deeply exhumed continental lithosphere provide critical constraints on the dynamic evolution of the lower crust and crust-mantle boundary. A >400 km² region of high pressure (1.4-1.6 GPa) Archean gneiss in the western Canadian Shield is interpreted to provide a view of a deep continental crustal hot zone. The region contains abundant mafic sills with MORB-like chemistry, interpreted to have been emplaced in a backarc setting. Upper crustal sedimentary rocks and MORB-like gabbros were emplaced in a region characterized by an elevated geothermal gradient and high mantle heat flux across thin lithosphere. Prograde and ultrahigh temperature (UHT) metamorphism occurred during closure and thickening of a hot backarc environment and coincident with tectonic juxtaposition of the backarc with a continental arc, culminating at T >950 °C and P>1.4 GPa. Further intraplating and underplating of the already hot deep crust led to extensive melting and development of a garnet-rich densified orogenic root, with densities locally in excess of 3.0-3.4 g/cm³. Parts of this root may have experienced foundering into the upper mantle. The results suggest that lower continental crust can be the locus of significant crust-mantle interaction. This interaction occurs in two directions: (1) from the mantle to the crust as mantle-derived magmas are emplaced into the lower crust during hot zone development, and (2) from the crust to the mantle during foundering of dense lower crustal rocks into the upper mantle. Thus, this region records a lower crustal view of the transition from back arc to continental arc and an ideal setting for UHT metamorphism and granite genesis.