Granulite formation is driven by magmatic processes in the deep crust

Abstract

Known examples of granulite facies metamorphism span at least 3.5 b.y. of Earth history. Mineralogic geobarometry indicates that such metamorphism has occurred in the deep crust, typically at 20–30 km (6–9 kbar). Geothermometry indicates that peak T = 700–900°C and therefore that T was elevated by at least 200°C over an “anorogenic” geotherm of 15–20°C km −1. Commonly invoked sources of heat include rising magmas, radioactive decay insulated by continent/continent collision, mantle volatiles, or crustal thinning. Present day crustal thicknesses are normal beneath exposed granulite terranes and the common absence of evidence for post-metamorphic underplating suggests synmetamorphic thicknesses of 60–80 km. Thus, granulites form in tectonically active regions of thickened crust and elevated geotherm. Xenolith suites suggest that granulite facies mineralogy persists in the deepest crust after tectonism in spite of declining temperature to greenshist/amphibolite facies conditions.