Mesoarchean migmatites of the Carajás Province : from intra-arc melting to collision.

Abstract

In the Carajás Domain, northern Carajás Province, Amazonian Craton, the oldest units encompass the Mesoarchean migmatites of the Xicrim-Cateté Orthogranulite and Xingu Complex. The Xicrim-Cateté Orthogranulite underwent early dehydration and late water-fluxed partial melting. The first process resulted in net-structured and schollen migmatites with a pargasite-bearing mafic granulite paleosome. The F-pargasite breakdown produced a neosome

with peritectic enstatite, diopside, and plagioclase and a residual amphibole-free mafic granulite. The late water- fluxed partial melting generated quartzofeldspathic leucosomes in shear bands of the NW-SE-trending high-angle

transcurrent shear zones. The Xingu Complex is composed of stromatic, net-structured, schollen and schlieren

migmatites developed in the late water-fluxed partial melting event. These migmatites have orthogneiss and am- phibolite paleosome and syn-tectonic quartzofeldspathic leucosome and biotite-rich melanosome oriented along a

low angle NE-SW to NW-SE gneissic foliation. The Xicrim-Cateté paleosome age remains undefined, but the U–Pb zircon ages of the pyroxene-bearing neosome constraint the dehydration-melting to c. 3.06–2.93 Ga. The εHf data (−2.2 to +1.2) of the neosome tie the dehydration-melting of the pargasite-bearing granulite to the underplate of mantle melts. The Zr-Ti-Y content and Ti/V ratios of the pargasite-bearing granulite suggest a compositional shift of their protoliths from MORB to IAT, characterizing a scenario of subduction installation and magmatic evolution. The Xicrim-Cateté pyroxene-bearing neosome is geochemically similar to the Xingu orthogneiss. Both rocks have a TTG affinity and similar εHf values (+0.8 to +1.6) and crystallization ages (2.94 Ga). It indicates a common

source for them and suggests that the dehydration partial melting of the primitive mafic crust produced a signif- icant portion of the TTG felsic continental crust of the Carajás Domain. It probably occurred during the island-arc

setting evolution between 3.06 and 2.93 Ga when supra-subduction mantle melts started to trigger the composi- tional differentiation of the mafic crust. The late water-fluxed partial melting was controlled by fluid influx into the

structures developed during the regional deformation of the Carajás Domain at c. 2.89–2.85 Ga, likely associated with a collisional event in the Carajás Province.