Petrological evolution of silica-undersaturated sapphirine-bearing granulite in the Paleoproterozoic Salvador–Curaçá Belt, Bahia, Brazil

Abstract

The Salvador-Curaçá Belt, located in São Francisco Craton, Brazil, was subjected to granulite facies metamorphism during the Paleoproterozoic orogeny (c. 2.0 Ga). Well preserved in enclaves of silicaundersaturated sapphirine-bearing granulite occur in a charnockite outcrop located along a kilometric-scale shear zone. The sapphirine-bearing granulite preserves domains with distinct mineral assemblages that record interactions between melt and peritectic phases (orthopyroxene 1 + spinel 1 + biotite 1). Sapphirine was crystallized in the Si-poor cores of the enclaves, sillimanite and spinel-cordierite symplectites in the intermediate Si-rich domains between cores and margins, and garnet and quartz-bearing cordierite/biotite symplectites in Si-rich margins of the enclaves. Melt-rock interactions and metamorphism occurred at ultrahigh temperatures of 900-950°C at 7.0-8.0 kbar pressures. The mineralogical evolution of the domains reflects not only the influence of changes in bulk composition in the equilibrium volume of the reactions but also P-T changes during orogeny evolution. Electron microprobe dating of monazite both in the sapphirinebearing granulite and charnockite indicates UHT metamorphism timing at c. 2.08-2.05 Ga that is related to global Paleoproterozoic UHT metamorphic events that occurred during the Columbia supercontinent assembly.

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

“…Pressures of 7-8 kbar and temperatures of o C (Barbosa, 1986;Barbosa and Fonteilles, 1989) were obtained, in agreement with the expected metamorphic evolution of the granulitic terranes (Bohlen, 1987). Nevertheless, in some places temperatures of about 1000 o C are recorded in sapphirine-bearing granulites (Leite, 2002;Pinho, 2005;Leite et al, 2009) pointing to UltraHigh-Temperature (UHT) conditions (Harley, 1998).…”

“…The metamorphic peak reached conditions of 900-1000 °C at 8.4-11.1 kbar. The Al 2 O 3 contents (>10 wt.%) and TiO 2 (>5 wt.%) in the orthopyroxene and biotite porphyroblast cores attest to the UHT peak conditions (Leite et al, 2009). Sapphirine-bearing granulites are found as small relics in high-grade metamorphic belts.…”

“…Bohlen, 1987;Harley, 1989). The thrusting of the Itabuna Belt over the Jequié Block Sabaté, 2002, 2004) was part of a collisional event, and the associated metamorphism that took place in the region close to 2081 ± 20 Ma (Silva et al, 2002;Pinho, 2005;Leite et al, 2009) reached UHT conditions, leading to the crystallization of the sapphirine-bearing granulite.…”

“…In the heterogeneous granulites, granulitized supracrustal rocks, including mafic and felsic metavolcanic rocks, quartzites, iron formations and aluminous-magnesian granulites, are present and are occasionally crosscut by anatectic garnet-and cordierite-bearing granites Sabaté, 2002, 2004;Pinho, 2005;Leite, 2002;Leite et al, 2009).…”

“…In this paper, our main goal is to present microstructures and mineral compositions for sapphirine-garnet-orthopyroxenesillimanite-quartz-bearing granulites that occur in the southern part of ISCB (Leite et al, 2009). From the mineral chemistry data, the evolution of UHT metamorphism will be discussed and the P-T-time path will be characterized in order to understand the tectonic setting in which such extreme conditions were reached (e.g.…”