CONDIÇÕES DE METAMORFISMO NO COMPLEXO GRANULÍTICO SANTA MARIA CHICO, RS

Abstract

METAMORPHIC CONDITIONS IN THE SANTA MARIA CHICO GRANULITE COMPLEX, RS. Granulite-facies rocks froin Western Rio Grande do Sul in southernmosl Brazil are bimodal acid and basic in composition, wilh sinaller ainounls of interinediate and ultrabasic compositions; supracrustal relics include quartzites, marbles and sillimanite gneisses. The first granulite-facies event is only registered as inlensely exsolved megacrysts of pyroxenes, plagioclase and other minerais. A third metamorphic episode was caused by the intrusion of abundant granites, general i ng actinolite, hornblende, clorite and tale from the mafic minerais. Microprobe analyses show the a.i.phiboles as pargasite in the M 2 high-grade event and the plagioclase as andesine, labradorite, bylownile and anorthite. This event was dated at 2.1 Ga by Sm/Nd isofopes and the intrusion of granites occurred at 550 Ma. Temperatura and pressure of the M2 metamorphic event were estimated, using several thermometers and barometers. The temperature is estimated at 800-850°C and the pressure at 9.4 kbar.