A Thermotectonic Model for Preservation of Ultrahigh-Pressure Phases in Metamorphosed Continental Crust

Abstract

Continental rocks subjected to ultrahigh-pressure (UHP) metamorphism contain relict minerals indicating formation at mantle depths approaching or exceeding 100-125 km. Thermobarometric calculations and phase-equilibrium constraints indicate peak metamorphic conditions of approximately 700-900 °C and 28-40 kbar. These extremely high pressures and relatively modest temperarures can be explained through the deep subduction of coherent tracts of continental lithosphere, and reflect the low thermal conductivities of geologic materials. Relatively rapid return to midcrustal levels after detachment from the downgoing slab may result from buoyancy of the UHP continental crust. Incomplete prograde conversion to the ultrahigh-pressure mineral assemblage at depth could enhance the exhumation process. Adiabatic decompression of UHP metamorphic rocks would result in P-T paths that enter the granulite and pyroxene homfels metamorphic facies; under these conditions, rising. intensely metamorphosed units would remain hot enough to ensure obliteration of precursor UHP phase assemblages. Numerical experiments simulating Pacific-type subduction and continent collision followed by exhumation suggest that survival of traces of UHP metamorphic phases during ascent requires that the terranes migrate toward the surface as relatively thin sheets. Cooling during decompression may occur if (I) sulxluction continues outboard at tectonically lower levels, allowing heat conduction downward into the refrigerating subducting plate, (2) unroofing occurs along an inboard, extensional fault or shear zone, allowing heat conduction upward into the cooler hanging wall, or (3) a combination of both processes. Documented coesite ± diamond-bearing lithotectonic units from western Europe and central + eastern Asia range in thickness to a maximum of about 10 ± 5 km, but most resurrected ultrahigh-pressure complexes are substantially thinner. Thus. although large, coherent masses of relatively cold material may move down subduction zones resulting in UHP metamorphism at profound depths, ascent toward the surface of thin slabs of such complexes, with cooling across both upper and lower surfaces, may be required for the partial preservation of ultrahigh-pressure mineral assemblages.