Hadean origins of paleoarchean continental crust in the central wyoming province

Abstract

The scarce remnants of Earth's earliest history make it challenging to describe the crust-forming processes that operated during that time, and all evidence that survived subsequent tectonism and recycling deserves to be studied closely. We present geologic, petrologic, geochemical, and isotopic descriptions of Paleoarchean gneisses in the central Wyoming Province. We identify two groups of gneisses: a bimodal suite of amphibolite and tonalite-trondhjemite-granodiorite (TTG) layered gneisses (3385-3450 Ma), and a suite of massive trondhjemite and granite gneisses (3300-3330 Ma). Here, 3.82 Ga inherited zircon components are present in several samples. Negative bulk rock initial εNd values also indicate that older crust was involved. Oxygen isotopic compositions of zircon mainly fall within the range of mantle zircon δ18O values, but several analyses extend up to ~6.5%. Initial Hf isotopic compositions of 3.82 Ga zircon are negative and require derivation from Hadean crustal sources. Our data reveal the record of a 3.82 Ga differentiation event during which Hadean crust was partially melted, and zircon crystallized from those melts. Hadean crust must have persisted in the central Wyoming Province until 3.45-3.37 Ga, when mafic crust partially melted to form the TTG layered gneisses, which also incorporated the 3.82 Ga zircons. Subsequent intracrustal recycling at 3.33-3.30 Ga produced calc-alkalic granites. The central Wyoming Province provides a significant addition to the sparse record of Hadean crust being magmatically reworked to form the abundant quartzofeld-spathic gneisses common in Archean terrains worldwide, which effectively transformed Earth's evolving continental crust from mafic to felsic in composition. Show more