Thermal structure of the crust during granulite metamorphism: petrological speculations and geodynamic implications

It is shown on a basis of generalization of lateral and vertical distribution of P-T parameters in granulite-gneiss belts of various ages that the thermal gradient within the lower crust during granulite metamorphism is rather low, ca. 5-10℃/km. Two extreme types of granulitic crustal sections are recognized. First type (GM1) corresponds to normal and especially to thinned (25-40km) crust. High and ultra-high temperatures (up to 1050℃) are characteristic for the deepest granulites of this type. Second type (GM2) corresponds to thickened (50-55km) crust. It is characterized by moderately high temperatures in the lower crust (from 700-800℃ at crust-mantle boundary). GM1 is generally connected with settings related to the hot mantle upwelling to crust-mantle boundary accompanied by heat and penetrating fluid fluxes and crustal extension or crustal thinning. Maximal heating is characteristic for the most thinned crust, i.e., for riftogenic environment including extension zones in back areas of active continental margins and in the crust of epi-continental sedimentary basins. Metamorphism of GM2 type is related to compression conditions and collision crustal stacking