Geoquímica dos granitos paleoproterozóicos da Suíte Granítica Velho Guilherme, Província Estanífera do Sul do Pará

Abstract

The granitic rocks of the Antonio Vicente, Velho Guilherme, Mocambo, Benedita e Ubim/Sul massifs of the Velho Guilherme Intrusive Suite (South Para Tin-Province) are hololeucocratic to leucocratic, syenogranite to monzogranite with subordinate alkali-feldspar granite. Geochemical data of representative rock samples from the different massifs of the suite indicated a subalkaline nature, metaluminous to peraluminous character, as well as geochemical affinity whit within-plate and A-type granites of the A 2 subgroup. Fractional crystallization was the main petrogenetic process governing the evolution of the granites of the suite. The tin mineralized members are extremely evolved, silica-rich rocks (SiO 2 >75%) which are the product of magmatic fractionation and interaction with F-enriched fluids. These fluids were responsible for Sn 2+ extraction from primary mineral phases, specially biotite. Sn 2+ was then incorporated into the fluids and later oxidized to Sn 4+ and precipitaded as cassiterite. The Th/Ta ratios of the Velho Guilherme (5.06 to 10.46) and Benedita (6.77 to 15.40) granites suggest a dominant contribution of continental upper crustal sources. For the Ubim/Sul granite magma Th/Ta ratios (3.52 to 5.33) indicated comparatively deeper crustal sources. The ultimate source material for the Mocambo granite magma (Th/Ta of 3.03 to 8.26) is more probably a mixture of lower and upper continental crust. The variable Th/Ta (5 to 100) observed in the Antonio Vicente granite is probably the result of a mixture of continental lower crustal sources with subordinate contribution of sedimentary rocks.