Cordierite-bearing lavas from Jaguarao, southern Brazil: petrological evidence for crustal melts during early rifting of Gondwana.

The Jaguarão stratoid dacites (Rio Grande do Sul, Brazil) are limited in areal extent, are comprised of about 3.2 km3 of preserved erupted material, and outcrop only in areas of the region underlain by mylonitic and ultramylonitic rocks. They are S-type volcanic rocks containing cordierite, orthopyroxene, plagioclase, and ilmenite as liquidus phases, and partially melted granite, gneiss, and migmatite enclaves that are very similar to the Precambrian basement rocks. The Jaguarão lavas have distinct geochemical signatures and Sr–Nd isotopes with respect to other volcanic rocks of the region. Available geochronological data for Jaguarão dacites range between 157 ± 5 Ma and 139.6 ± 7.4 Ma. Considering the errors, the younger ages obtained for Jaguarão lavas overlap the 138–128 Ma age of rocks of the Serra Geral Group, and thus indicate that the dacites were erupted prior to the break-up of Gondwana in this region. Petrographic, mineralogical, and petrochemical data, as well as the tectonic context of the Jaguarão lavas, suggest that magma genesis was linked, at least in part, to friction melts. The dacitic magma was generated by partial melting reactions involving biotite breakdown in a dominantly quartz-feldspathic source terrane, leaving a granulite facies residue in subsurface. These melts were probably generated as a consequence of crustal thinning linked to simple shear extension just prior to Gondwana break-up and rifting of the southern Atlantic Ocean.