Peraluminous leucogranites of the Cordilheira Suite: A record of Neoproterozoic collision and the generation of the Pelotas Batholith, Dom Feliciano Belt, Southern Brazil

The Cordilheira Suite is comprised of peraluminous granites that constitute, together with the Quiteria granite, the beginning of the formation of the Pelotas Batholith in the central portion of the Dom Feliciano Belt. The batholith is composed of seven granitic suites with minor occurrences of gabbro, diorite and subvolcanic rocks. Its evolution between 650 and 550 million years ago is characterised by a ranging from metaluminous to peraluminous rocks and from calc-alkaline to alkaline and peralkaline series. The Cordilheira Suite is composed of the Cordilheira, Arroio Francisquinho, Butid and Tres Figueiras granites, which contain muscovite and/or biotite, with the following accessory minerals: tourmaline, garnet, sillimanite, apatite, zircon, monazite and ilmenite. The granitic bodies are elongate; their ascent and emplacement were controlled by high-angle shear zones oriented at N45-70 degrees E. They have mylonitic structures with magmatic foliation accompanied by a low-angle stretching lineation, indicating that these bodies were emplaced under syn-kinematic conditions during a transcurrent event The granites have high-K calc-alkaline affinity and are peraluminous. The LILE and REE contents are low. The K2O/Na2O and CaO/Na2O ratios are approximately 1 and less than 03, respectively. Pseudosections calculated using the Perple X program suggests that the granites were formed at partial melting temperatures between 740 and 820 degrees C and pressure between 8.5 and 9 kbar. Petrographic and chemical data suggest that the magmas were generated by the partial melting of the migmatitic pelitic gneisses of the Varzea do Capivarita Complex and, to a lesser degree, orthogneisses of the Arroio dos Ratos Complex, which left a granulitic residue. UHT conditions of granulite fades metamorphism were recorded in the Varzea do Capivarita paragneisses by mineral paragenesis and indicate that temperatures between 900 and 1000 degrees C and pressures between 4 and 8 kbar were required for the partial melting. It is likely that these medium-pressure conditions resulted from the thickening of the continental crust when the Rio de La Plata Craton collided with the Kalahari Craton to form southwestern Gondwana at the end of the Neoproterozoic. (C) 2012 Elsevier Ltd. All rights reserved.