Moderate overthickening of thinned sialic crust and the origin of granitic magmatism and regional metamorphism in low-P-high-T terranes

Abstract

Formation of low-pressure ( P )-high-temperature ( T ) metamorphic terranes by homogeneous shortening and thickening of tectonically thinned sialic crust and overlying sedimentary basins at geologically reasonable strain rates avoids the shortcomings of hypotheses linking the characteristically high geothermal gradients to doubling of normal-thickness crust, crustal extension alone, intrusion of granitoids, or extensive basic magmatism. On the basis of geological and P-T constraints from the Archean Slave province, the P-T histories of three points in a hypothetical block of crust are presented in P-T , P -distance, and P -time diagrams. The relations between the rock that ends up at the surface, base of the basin, base of the crust, isotherms, and depth determine the type of metamorphism and the degree of partial melting required to produce abundant granitoids. This simple qualitative framework can account for the characteristic features of low- P -high- T metamorphic terranes and reconcile those who believe granites are the cause of the metamorphism with those who favor the contrary.