Magnetic petrology of the Neoarchean granitoids in the Vila Jussara Suite, Carajás Province, Amazonian Craton

Abstract

The Vila Jussara Suite (VJS) is formed by Neoarchean (~ 2.75 Ga) granites that are located in the Sapucaia Domain of the Carajás Province (CP), Amazonian Craton. Four petrographic varieties were identified in the VJS: biotite-hornblende monzogranite (BHMzG); biotite-hornblende tonalite (BHTnl); biotite monzogranite (BMzG); and hornblende-biotite granodiorite (HBGd). In terms of magnetic signature, BHMzG has two subgroups: the first subgroup has low magnetic susceptibility (MS) values (0.16 × 10^-3 to 0.81 × 10^-3) and more commonly contains ilmenite with titanite rims; the second subgroup shows moderate to high MS (1.91 × 10^-3 to 6.02 × 10^-3) and magnetite dominant over ilmenite. BHTnl has moderate MS (0.85 × 10^-3 to 1.36 × 10^-3) and dominance of pyrite followed by magnetite. BMzG and HBGd have comparatively high MS (3.35 × 10-³ to 19.3 × 10^-3 and 2.14 × 10^-3 to 25.0 × 10^-3, respectively), with magnetite dominant over pyrite. The granite varieties of the VJS were formed under different oxygen fugacity (fO2) conditions, varying from reducing (< fayalite-magnetite-quartz (FMQ)) to oxidizing conditions (nickel-nickel oxide (NNO) to NNO+1). In addition, biotite-hornblende syenogranite occurs subordinately and shows high MS values and extremely high FeO_t/(FeO_t + MgO) ratios, both in whole-rock and amphibole and biotite. The granites of the VJS are similar to other Neoarchean granites of the Carajás province. The reduced VJS granites are akin to the ferroan granites of Planalto suite, Estrela Complex and Vila União area and the magnesian granites of VJS approach the magnesian granites of Vila União area.

Keywords:
Neoarchean; magnetic petrology; degree of oxidation; Carajás Province; ferroan and magnesian granitoids