Petrological Constraints on Crystallization Conditions of Mesoarchean Sanukitoid Rocks, Southeastern Amazonian Craton, Brazil

Abstract

We report petrological and geochemical data for the 2*87 Ga Rio Maria sanukitoid granodiorite and associated rocks from Mesoarchean granite-greenstone terranes of the eastern Amazonian craton, Brazil. The dominant rocks have granodiorite to subordinate monzogranitic compositions, with minor proportions of intermediate quartz diorites or quartz monzodiorites, in addition to mafic endmembers occurring as layered rocks or as enclaves. The mineral assemblage is dominated by amphibole-plagioclase-biotite and epidote minerals, all of inferred magmatic origin, pyroxenes being notably absent. Textural and compositional criteria indicate that amphibole is a principal mineral on the liquidus of all the Rio Maria rocks. Crystallization conditions have been derived from a comparison between natural phase assemblages, proportions and compositions and experimental studies carried out on similar magma compositions. The comparison shows that the parental magmas were water-rich, with more than 7 wt % dissolved H2O, with crystallization temperatures in the range 950-680°C. The Mg/(Mg + Fe) ratios of both amphibole and biotite indicate fO2 conditions in the range NNO + 0*5 to NNO + 2*5 (where NNO is nickel-nickel oxide buffer), therefore pointing to both water-rich and oxidizing conditions for sanukitoid magmas. Amphibole compositions indicate emplacement at around 200 MPa, and record a high-pressure stage of magma crystallization around 600-900 MPa. Sanukitoid magmas share two of the principal characteristics of modern arc magmas, elevated redox state and volatile contents, which suggest that they may have formed in a geodynamic environment broadly similar to present-day subduction zones.