Fe isotopic evidence that ?high pressure? TTGs formed at low pressure

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+/ sigma Fe and delta Fe-56 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. (C) 2022 Elsevier B.V. All rights reserved.

Automated summary

This study reveals the petrogenesis of high-pressure rocks in the North China craton through iron isotopic analysis. The results suggest that the high-pressure features in trondhjemites are actually formed by amphibole-dominated fractionation at low pressure, while tonalites are unlikely to be formed within the garnet stability range at high pressure.