Fe isotopic evidence that “high pressure” TTGs formed at low pressure

Abstract

Tonalite–trondhjemite–granodiorite (TTG) rocks consitute the dominant felsic lithology of the Archean crust in most cratons, but their petrogenesis and geodynamic setting are still matters of great debate. Here, we report new Fe isotopic analyses for ca. 2.5 Ga diorites and TTGs with compositions typically interpreted to reflect a high-pressure affinity in eastern Hebei Province of the North China craton. Whole-rock Fe3+/∑Fe and δ56Fe of the trondhjemites show strong correlations with MgO and Fe2O3T, both indices of igneous differentiation, supporting the radical reinterpretation that the apparent “high pressure” signatures in the trondhjemites instead formed by amphibole-dominated fractionation at low pressure. Likewise, for the tonalites, their light Fe isotopes make them unlikely to be the melting products of enriched mafic rocks at high pressure within the garnet stability range. Thus, Fe isotopes support that “high pressure” TTGs in verity formed at lower pressure with little involvement of residual garnets. Our results thus challenge the evidence once purported to infer Archean subduction.