Tectonics and heat sources for granulite metamorphism of supracrustal-bearing terranes

Abstract

There are generally three important processes in the granulite-facies metamorphism of supracrustal rocks: (1) transport from the surface to 15–30+ km, (2) heating to 700–800+ °C, and (3) transport back to the surface and re-exposure at the top of a crust of “normal” thickness (35–40 km). Mantle-derived magmatism, and/or magmatic and/or metasomatic fluid flow, can transport the necessary heat for granulite metamorphism, but many granulite terranes do not contain sufficient volumes of mafic intrusive rocks, or evidence of extensive fluid flow, to readily explain heating by this mechanism. One possible process for the formation of large granulite terranes involves continental collision and crustal doubling by simple-shear thrusting. Heating of the underthrust supracrustal rocks would occur by thermal relaxation, but, if granulite-facies temperatures are to be attained at mid-crustal levels, this mechanism necessarily produces extensive melting of the lower crustal plate, suggesting that granulite terranes formed in this way should contain substantial syn- or post-metamorphic crustally-derived intrusions. In this situation, subsequent uplift and re-exposure at the surface is a natural consequence of erosion and isostatic readjustment. An alternative process which may allow the heating of supracrustal rocks to granulite-facies temperatures is imbricate thrusting in which thin slivers or “flakes” of supracrustal rocks are thrust down to the lowermost crust. This mechanism requires a separate tectonic event, such as a second thrusting event, to transport the granulite-facies rocks back to the surface. Other possible heat sources for supracrustal granulite terranes include preheating of the overthrust crust by arc or pre-orogenic magmatism, and shear heating on thrust planes. Models indicate that pre-heating of the overthrust plate has only minor thermal effects upon the thermal history of the underthrust crust, although shear heating may be significant. Heat sources for granulite metamorphism of the Archean Limpopo Belt of South Africa are considered in terms of possible tectonic mechanisms. The metamorphic grade and presence of abundant supracrustal rocks in this terrane demonstrate that it was once at the Earth's surface, buried to ∼ 8.5 kbar (∼ 28 km) and heated to ∼ 850°C, then re-exposed at the surface. There are no recognizable mantle-derived igneous rocks of suitable age, and no evidence for extensive fluid flow that could have provided a magmatic heat source for the metamorphism. Shear heating is unlikely because the pattern of metamorphic isograds is not related to faults or shear zones. Imbricate thrusting of thin silvers of supracrustal rocks into the lowermost crust to allow heating without magmatism would require a separate tectonic event to transport the granulites to the surface; there is no evidence for such a later event. The most satisfactory and self-consistent model for Limpopo metamorphism involves thermal relaxation of a perturbed temperature profile produced by Tibetan-style continental collision of the Zimbabwe and Kaapvaal Cratons, with consequent crustal doubling to at least 65 km thick, and the generation of crustal melts which advected heat into the zone of granulite metamorphism. There is no evidence for significant pre-collisional heating of either crustal plate; both probably had typical “shield-type” pre-collisional geothermal gradients. Crustally-derived magmas, therefore, must have been produced during thermal and structural equilibration of the Limpopo collision. Abundant granitoid rocks of similar age to the metamorphism occur both as migmatites and as discrete plutons.