The 3.5 Ga Sao Tome layered mafic-ultramafic intrusion, NE Brazil: Insights into a Paleoarchean Fe-Ti-V oxide mineralization and its reworking during West Gondwana assembly

摘要

The Sao Tome mafic-ultramafic intrusion, located in the Borborema Province, NE Brazil, consists of a group of metamorphosed dismembered intrusive rocks, composed of ol-websterite and clinopyroxenite, which host stratiform meter-sized massive ilmenite-magnetite lenses with up to 1.98 wt% V2O3 in magnetite. The ol-websterite rocks display dark discontinuous olivine bands intercalated with cumulate augite, whereas the clinopyroxenite rocks are composed mainly of anhedral augite. The mineral chemistry analysis of the mafic phases resulted in contents up to Fo(87.28) and 4605 ppm of Ni in the olivine crystals and high-MgO augite (22.59 wt% MgO), suggesting that these rocks formed from the differentiation of a primitive, fertile peridotite melt. The retention of high nickel contents in the olivine crystals, despite the crustal contaminated signature revealed by Sm-Nd isotopes, suggests that there was no early sulfur saturation in the magma; additionally, the magnetite crystallization did not trigger sulfur saturation, inhibiting Ni-Cu sulfide precipitation or even the formation of individual platinum group metals (PGM) grains. Metamorphism at amphibolite-facies conditions resulted in the development of penetrative foliation and mineral assemblages, wherein tschermakite is dominant. Ultimately, the metasomatic imprint included the following stages: (1) a K-P-H2O stage, represented by newly formed phlogopite, apatite, chlorite and hornblende; (2) a calcic stage, with epidote-group minerals, calcite and minor pyrite, and (3) the least potassic alteration forming diablastic phlogopite and microcline. LA-ICP-MS U-Pb zircon data reveal a concordant age of 3506 +/- 29 Ma for the Sao Tome intrusion, interpreted as the igneous crystallization age, and thus, it represents the oldest record of continental crust generation in South America. Ar-40/Ar-39 phlogopite dating, on the other hand, revealed plateau ages of 597 +/- 5 Ma and 517 +/- 6 Ma. These Ediacaran-to-Cambrian ages repres