Magnetic Petrology of Crust- and Mantle-Derived Mesoarchean Ourilândia Granitoids, Carajás Mineral Province, Brazil

Abstract

ABSTRACT This paper presents a detailed study of magnetic petrology in crust- and mantle-derived Mesoarchean granitoids (2.92–2.88 Ga) from the Ourilândia do Norte area, which is situated in the midwestern Carajás Mineral Province, southeastern Amazonian Craton (northern Brazil). The textural aspects of opaque minerals and their relation to magnetic susceptibility (MS) were combined with the results of previous works that involve whole-rock geochemistry and mineral chemistry data to discuss the formation conditions and to correlate the MS values and opaque mineral content with the crustal input related to the source of these rocks. The Ourilândia granitoids can be divided into the following three lithological associations: (1) potassic granites represented by biotite monzogranites and high-Ti granodiorites, which both host tonalite-trondhjemite-granodiorite (TTG) affinity tonalitic xenoliths; (2) sanukitoids formed by granodiorites (equi- to heterogranular and porphyritic), with minor occurrences of tonalite, quartz monzodiorite, quartz diorite, and mafic microgranular enclaves; and (3) TTG-affinity porphyritic trondhjemite, which is represented by a small, slightly deformed stock. The cumulative frequency curve from the MS data defines three main magnetic populations as follows: (1) population A is characterized by low MS values (0.05 × 10–3 to 0.59 × 10–3 SI) formed by sanukitoid and trondhjemite rocks, which contain rare opaque minerals; (2) population B is defined by moderate MS values (0.70 × 10–3 to 1.24 × 10–3 SI) wherein sanukitoids predominate over the potassic granites while ilmenite prevails in relation to magnetite; (3) population C is represented by high MS values (1.33 × 10–3 to 17.0 × 10–3 SI) in which potassic granites and high-Ti granodiorites are predominant, in addition to the porphyritic and heterogranular sanukitoids. The Fe/(Fe + Mg) ratios in whole rock, biotite, and amphibole indicate high redox conditions for the sanukitoids and potassic granites, which are mostly above the nickel-nickel oxide (NNO) buffer (+0.5 < NNO < +1.9) and at or slightly below the NNO for the TTG-affinity trondhjemite (–0.5 < NNO < +1.0). The variation in the opaque mineral content (especially magnetite) explains in the first instance the magnetic behavior of these rocks. Furthermore, our results not only suggest that the oxidation states recorded in these granitoids are associated with the nature of their sources, but also suggest that unlike the depleted mantle (reduced in nature), the continental crust (monzogranite source) and subcontinental lithospheric mantle (the source of the sanukitoids and high-Ti granodiorite) are oxidized, while the oceanic crust (trondhjemite source) is moderately oxidized. The low MS values and the scarcity of magnetite reported for the equigranular sanukitoids and trondhjemite can be attributed to the variations in crustal input (crustal anatexis and/or mantle enrichment) in magmas that can change the overall fO2 and thereby promote differences in the Fe-Ti oxide mineral assemblage. The frequent presence of magnetite and high MS in the porphyritic sanukitoid reinforces the differences in the enrichment degree of their source in relation to other sanukitoids. The Ourilândia granitoids provided temperatures between 959 and 738 °C, with a higher water content in the sanukitoids (>5%) than in the potassic granites (<4%) and TTG-affinity granitoids (<4%). Except for the TTG-affinity trondhjemite, which returned higher emplacement depths (580–263 MPa), the studied granitoids were emplaced under high redox conditions in the upper crust (297–80 MPa). The textural aspects of the Fe-Ti oxide minerals suggest cooling temperatures of 620 to 550 °C, as indicated by the subsolidus textures in magnetite and ilmenite (trellis ilmenite, external and internal composite ilmenite lamellae, and titanite to ilmenite and pyrite to goethite replacement) in the tardi- to postmagmatic stages.