Abstract

Earth's oldest preserved granitoid crust dates back to the Paleoarchean and consists predominantly of sodic tonalite-trondhjemite-granodiorite (TTG) granitoids that arose through the partial melting of hydrated metabasalts. In contrast, granites (sensu stricto) typically appear relatively late in the plutonic record of the old cratons. However, the existence of Hadean zircons with mineral inclusion suites that are consistent with crystallization from peraluminous granitic magmas indicates that granitic rocks formed part of the earliest felsic crust; although we have direct evidence, this earliest felsic crust is not preserved. Here we present evidence of an unusual variety of markedly low-CaO, K2O-rich, rutile-bearing, peraluminous granite and rhyolite that was produced concurrently with TTG magmas during three magmatic cycles in the Barberton Greenstone Belt (BGB), southern Africa. This material is not preserved as in situ rock units, but occurs as clasts within a younger conglomerate. Within these rocks, plagioclase feldspar is a rare inclusion in zircon, relative to alkali feldspar, and has low anorthite contents (An < 15%), attesting to the primary nature of the low-Ca signature of the magmas. This, along with Eu/Eu* ∼ 1, high K2O and Sr content, as well as the peraluminous character of the magmas, is a consequence of phengite melting in a metagraywacke source at pressures in excess of those of plagioclase stability. This process contributed to each episode of continental crustal growth through the Paleoarchean to Mesoarchean in the BGB, despite leaving no plutonic record at the typical mid-crustal level of exposure that the TTG plutons around the belt represent.

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