ULTRAHIGH-PRESSURE METAMORPHISM AND CONTINENTAL SUBDUCTION TO DEPTHS OF 200 KM: IMPLICATIONS FOR INTRA-CONTINENTAL ULTRAPOTASSIC MAGMATISM

Recent petrological data for exhumed relicts of continental subduction from the Kokchetav massif (Kazakhstan), Sulu/Dabie terranes (China), WGR (Norway) and the Saxonian Erzgebirge indicate that some sections of continental lithosphere were recovered from depths up to 200 km. Relationships between oceanic subduction and arc magma petrogenesis are relatively well understood; the magmatic legacy of continental subduction is far less clear. Correlation of spatial/temporal associations of continental subduction-collision orogens containing UHP metamorphic relicts with broadly coeval or younger, ultrapotassic lavas, may indicate a direct petrogenetic relationship between UHPM and potassic magmatism. Slab break-off at depths of ~ 200 km provides a simple mechanism for the generation of small degrees of partial melts of the leading edge of the slab, and ultrapotassic magmatic products located at ~ 200-300 km lateral distance from the collisional suture. Derivation of an interpolated geotherm for continental subduction from petrological data indicates that at depths of ~ 200 km P-T conditions of crustal rocks approach the solidus, and require only a small thermal perturbation to undergo partial melting. This can be provided by slab break-off and asthenospheric circulation. Partial melts resulting from elimination of Kfs and K-wadeite in recrystallized continental crust may have surface expressions as high-K lavas lamproites and possibly group II kimberlites), and/or hybridization and metasomatism of the overlying lithospheric mantle. Isotopic signatures have indicated continental crust source components in some lamproites and other ultrapotassic rocks spatially and temporally related to UHPM-bearing collisional orogens. Kimberlites are generally considered to be derived from partial melting of Phl-Mgs garnet lherzolite at pressures of > 5 GPa; carbonate may be generated in mantle rocks by carbonation reactions. The Kokchetav massif contains diamondiferous marbles recrystallized at ~ 7 GPa, which have suffered decarbonation reactions deep in the collision zone. This demonstrates that supracrustal platform carbonates could be subducted to depths of ~ 200 km in the Proterozoic. and that metasomatised mantle above the collision zone could constitute a source region for kimberlite magmatism.