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Insights on the origin of the Graciosa A-type granites and syenites (Southern Brazil) from zircon U-Pb geochronology, chemistry, and Hf and O isotope compositions

2019; Elsevier BV; Volume: 340-341; Linguagem: Inglês

10.1016/j.lithos.2019.05.001

1872-6143

Frederico Castro Jobim Vilalva, Antonio Simonetti, Sílvio Roberto Farias Vlach,

Geochemistry and Geologic Mapping

The Neoproterozoic, post-collisional, Graciosa Province (southern Brazil) comprises several A-type intrusions and related volcanics of alkaline and subalkaline (or aluminous) petrographic associations. U-Pb age determinations, trace element, and Hf and O isotopic compositions were obtained for zircons from six distinct A-type granitic and syenitic plutons from both alkaline and subalkaline associations from the province. Geochronological results indicate that their emplacement, petrogenetic evolution and crystallization spanned over a ~9 million years interval, with peak magmatism at ~580 Ma. Plutons were emplaced in a post-collisional extensional regime related to the geodynamic evolution of the south-southeastern part of the Gondwana supercontinent. Hf isotopic compositions are highly variable, with εHf(t) values between +1.4 to −31, whereas δ18O signatures are mainly between +4.7 and +6.0‰. These features, along with trace element signatures, especially for Hf, U, Nb, and rare-earth elements, suggest that generation of the Graciosa rocks occurred via partial melting of a lithospheric mantle previously metasomatized by slab-derived fluids, followed by variable degrees of crustal contamination at depth.