Floresta and Bodocó Mafic–Ultramafic Complexes, western Borborema Province, Brazil: Geochemical and isotope constraints for evolution of a Neoproterozoic arc environment and retro-eclogitic hosted Ti-mineralization

Abstract

The retrograde high-pressure metamorphosed Floresta and Bodocó mafic–ultramafic complexes (FMUC and BMUC, respectively) occur as clusters of small bodies, lenses and boudins with close association with granitic, gneissic rocks and metasediments in the southern limits of the Alto Pajeú and Piancó–Alto Brígida domains, Borborema Province, northeast Brazil. Ultramafic lithologies including metadunites, olivine cumulate rocks, metapyroxenites, chromitites and massive ilmenomagnetitites showing evidence of cumulus textures and mafic members include garnet amphibolites and coarse metahornblendites. BMUC rocks include Cr–Fe–Ti chromitites in the Fazenda Esperança deposit, and FMUC rocks show important Fe–Ti–V economic mineralization in the mines of the Serrote das Pedras Pretas and Lagoa dos Angicos and the Riacho da Posse deposit in development (54 Mt @13% Ti). The whole-rock and mineral geochemistry of these complexes are akin to arc cumulate rocks, particularly Alaskan-type complexes, which are widely considered to have formed above subduction zones.A coarse metahornblendite from the Serrote das Pedras Pretas mine was dated by U–Pb zircon through laser ablation-multicollector-inductively coupled plasma-mass spectrometry (LA-MC-ICP-MS). Information obtained from the cores of igneous zircons represent an upper intercept age of crystallization at 1024±18Ma with 4.2 mean square weighted deviation (MSWD), whereas zircon exhibiting coarse rims with low Th/U at 0.008–0.038 have concordia ages of 685±17 and 625±6Ma. According to their zircon textures, these ages may be representative of a high-grade eclogitic or progressive granulitic/retro-eclogitic stage during the Pan-African–Brasiliano event. Additionally, the age of the igneous protolith, 1028±63Ma, is well marked by a Sm–Nd isochron based on 12 samples.Positive ɛNd(t) values between 0.01 and 4.84 suggest a depleted mantle or a source close to the chondritic reservoir for the mafic–ultramafic bodies studied. The TDM model ages between 1.1 and 1.6Ga are consistent with a juvenile Mesoproterozoic source or mixing between Paleoproterozoic and Neoproterozoic sources. The dispersion of initial ratios of 87Sr/86Sr at 0.7016–0.7090 and the high fractionation of light rare earth elements (LREE) relative to heavy rare earth elements (HREE) suggest a depleted mantle source metasomatized by subduction-related melts/fluids.The mafic–ultramafic complexes, volcanic and sedimentary rocks, and some granitic intrusions with post-collisional signature after 975Ma constitute a proto to marginal arc-back-arc basin system that evolved between 1025 and 975Ma. However, FMUC and BMUC which are currently arc cumulate rocks were originally picrite melts in a suprasubduction ophiolite zone.The economic deposits of Fe–Ti associated with retro-eclogitic and ultramafic members of the FMUC and enclosed within the late Ediacaran granite can be linked in part by early accumulation under magmatic conditions, secondary enrichment of rutile up to 12% in the high-pressure (HP) stage, and hydrothermal interactions during granite emplacement.