Metamorfismo de temperatura ultra-alta de granulitos com safirina de 2,0 Ga do Bloco Itabuna-Salvador-Cura?§??, Bahia, Brasil

摘要

The study area is located in the S?￡o Francisco Craton, in the southern part of the State of Bahia, Brazil. In this region, the??Palaeoproterozoic Itabuna-Salvador-Cura?§?? Block (ISCB) comprises supracrustal rocks associated with tonalite/trondhjemite??and monzonite orthogneisses together with subordinate metamafic rocks of Archean and Palaeoproterozoic ages. All exposed??rocks are strongly deformed and were re-equilibrated under granulite facies conditions at 2.07-2.08 Ga. The supracrustal??rocks include a sapphirine-bearing granulite that contains sapphirine, quartz, orthopyroxene, sillimanite, garnet, mesoperthite,??plagioclase, biotite and cordierite. Based on microstructural relations, the metamorphic peak paragenesis of the sapphirinea??bearing??granulite is Grt 1 + Opx 1 + Bt 1 + Qtz + Sil 1 + Spr 1 + Mp. Mineral reaction microstructures, such as symplectitic coronas??around porphyroblasts, helped to identify the following retrograde parageneses, interpreted as formed during the tectonic??rise of these rocks Grt 1 ?± Qtz = Opx 2 + Sil 2 , Grt 1 = Opx 2 + Spr 2 (+ Sil 2 ), Opx 1 + Sil 1 + Qtz = Crd and Grt 1 + Qtz = Opx 3 +??Crd. In turn, Bt 2 + Qtz symplectites formed by the reaction Opx 1-2 + Mp + H 2 O = Bt 2 + Qtz record the retrograde cooling??path. Thermobarometry calculations were performed by means of the multiequilibria method (e.g. THERMOCALC) and??classic thermobarometeres, which indicated P = 7-11 kbar and T 900 ?°C for the sapphirine-bearing granulite. One of the??mechanisms associated with the ultrahigh-temperature metamorphism is the addition of radioactive material and the presence??of basaltic magmas during the formation of ISBC. Fractional crystallization of basaltic magma produced tonalite intrusions,??dated approximately 2.15-2.13 Ga, close to the metamorphic peak at 2.07-2.08 Ga.