Unraveling a hidden Rhyacian magmatic arc through provenance of metasedimentary rocks of the Crixás greenstone belt, Central Brazil

Abstract

The Crixás-Goiás Domain, Central Brazil, comprises an association of Archean TTG (tonalite-trondhjemite-granodiorite) and Archean-Paleoproterozoic greenstone belts. The Crixás greenstone belt, located in the northern part of the domain, is constituted of Mesoarchean metakomatiites and metabasalts overlaid by Paleoproterozoic carbonaceous phyllites and metagraywackes that host important gold deposits. In this work, we present new integrated petrographical, geochemical and isotopic data to investigate the provenance and tectonic setting of the metasedimentary rocks of the Crixás greenstone belt. Mineral and textural immaturities, particularly of the metagraywackes, are consistent with a proximal source area; which is also evidenced by the chemical aspects that denote absence of significant sediment sorting and recycling. Geochemical data indicate provenance from mixed mafic and felsic sources. U-Pb detrital zircon geochronology shows unimodal age spectra, with a dominant peak at ca. 2.2–2.1 Ga and minor contribution of ca. 2.7 and 2.4 Ga grains. Concordia diagrams yielded upper intercepts between 2213.4 ± 5.6 and 2162.2 ± 9.2 Ma, interpreted as the age range of the main source area. The rocks present relatively homogeneous Sm-Nd model ages (2.44 to 2.34 Ga) and slightly negative to positive ƐNd, ranging from −0.27 to +1.25. We argue that the original sedimentary protoliths were deposited in a trench or forearc basin related to a ca. 2.2–2.1 Ga island arc formed adjacent to an Archean paleocontinent constituted of TTG-greenstone terranes. Rapid erosion of the rising intraoceanic arc resulted in syn-orogenic sedimentation that started with deep ocean black shale deposition in an anoxic environment followed by episodic turbidite currents of poorly-sorted immature sands. Sedimentary rock layers were tectonically transported to the continent and preserved in the elongated keels of Mesoarchean mafic–ultramafic volcanic rocks. This orogeny can be correlated with the amalgamation of the Goiás Massif during the Siderian-Rhyacian crustal growth related to successive collage of arc-related terranes in the formation of the western margin of the São Francisco paleocontinent. Sediment provenance of the Crixás greenstone belt proved to be crucial to the recognition and characterization of Paleoproterozoic crust that are apparently no longer preserved in outcrops.