Geochemistry and age of cassiterite tonalite/trondhjemite, southernmost São Francisco Craton, Minas Gerais, Brazil

Abstract

The Cassiterite Tonalite/Trondhjemite outcrops at the southernmost São Francisco Craton and belongs to the tectonic context of the Paleoproterozoic Mineiro Belt. This body is elongated according to the ENE-WSW direction and has an exposed area of 300km2. In terms of modal composition, its varies from tonalite to trondhjemite and the main mineralogy includes plagioclase, quartz, hornblende, biotite, microcline with zircon, apatite, allanite, magnetite, ilmenite, pyrite, rutile, epidote, sphene, white mica, carbonate and chlorite as accessory and secondary minerals. The Cassiterite rocks have normative corundum, peraluminous character, calc-alkaline affinity and Na2O/K2O values ranging from 3.15 to 4.43. They can be correlated to the high-Al2O3 trondhjemites formed in a continental volcanic margin. The rare earth element condrite normalized pattern is characterized by heavy-REE depletion which is accompanied by the enrichment in SiO2. The Cassiterite Tonalite/Trondhjemite yielded an age of 2162 ± 10 Ma and Sm-Nd model age (TDM) of 2.47 Ga, in agreement with a Paleoproterozoic mantle source. A possible model for the origin of this body is proposed. It involves (1) the formation of an oceanic crust that related to the ascent of a mantelic plume with an age close to 2.47 Ga; and (2) subduction, metamorphism and partial melting of this basaltic crust generating a trondhjemitic liquid that further evolved through hornblende and plagioclase fractionation in a continental volcanic margin.