Mantle underplating, granite tectonics, and metamorphic P-T-t paths

Abstract

Arrival of mantle-derived magma at the base of the continental crust leads to melting, major crustal reworking, and overturn; these processes can form both counterclockwise and high- T clockwise pressure-temperature-time (P-T-t) paths in the same tectonic episode. Regions with low- P , high- T counterclockwise P-T-t paths form where the upper crust moves down to compensate for the upward ascent of granite. High- T shear zones separate these regions from ascending granite plutons and their migmatitic envelopes, where mediumto high-pressure rocks record rapid uplift along a clockwise P-T-t path. The increase in pressure in counterclockwise P-T-t paths does not require tectonic thickening of the crust but results from ensialic, mantle-heat–initiated overturn of the continental crust. Nor does the attenuation of deep-crustal rocks denote extension; instead, it results from lateral migration of low-density melts in the source region of the ascending granites. As the crustal overturn is gravity driven to create isostatic equilibrium, the high- T metamorphic paths end in isobaric cooling. The mantle-derived magma may have its source in a mantle plume, impinging beneath continental crust or, in a back-arc setting, may result from slab pull-back and replacement by hot asthenosphere.