Did the character of subduction change at the end of the Archean? Constraints from convergent-margin granitoids

Abstract

Large ion lithophile and high field strength element distributions in juvenile upper continental crust are controlled chiefly by the abundance of tonalite-trondhjemite-granodiorite (TTG) in the Archean shifting to a combination of TTG, calc-alkaline granitoid, and graywacke control thereafter. Geochemical differences between TTG and high-silica adakites do not require production of most TTG magmas in descending slabs. Changes in the ratio of TTG to calc-alkaline granitoids after 2.5 Ga indicate that Archean subduction zones must have differed from younger subduction zones in two very important ways: (1) a deep mafic crust served as a TTG magma source (either as thickened crust or in descending slabs), and (2) they did not give rise to significant volumes of calc-alkaline magma. Thickened mafic crust in the Late Archean may have resulted from plate jams in subduction zones caused by thicker oceanic crust and oceanic plateaus produced during Late Archean mantle thermal events.