Neoproterozoic High-K calc-alkaline granites forming the large Agudos Grandes and Socorro batholiths, SE Brazil: Crystallization conditions and tectonic implications

Abstract

I-type, high-K calc-alkaline (HKCA) granites are the main components of two large Neoproterozoic batholiths that occur in different tectonic domains in SE Brazil (Agudos Grandes (emplaced in the central Ribeira Fold Belt) and Socorro (emplaced in the Socorro-Guaxupé Nappe). We investigated representative samples from both batholiths using detailed petrography by optical and electronic microscopy and chemical analysis of main minerals (amphibole, biotite and plagioclase) by EPMA aiming to obtain reliable estimates of magmatic temperatures and pressures. Granites from both batholiths show broadly similar whole-rock compositions and crystallized from oxidized magmas at ∼ NNO+1 to +2. However, they were emplaced at different depths (Agudos Grandes: 3–5 kbar; Socorro: 5–6 kbar; Al-in-hornblende geobarometry) and crystallized at slightly, but consistently different temperatures (respectively, lower and greater than 800 °C). The lower temperatures of the Agudos Grandes granites may account for the presence of titanite as a main accessory mineral, in contrast with its absence, or occurrence as thin rims in ilmenite in the Socorro granites. The pressure estimates are consistent with the metamorphic conditions of the country rocks and suggest that magma emplacement was influenced by a combination of density control and evolving tectonic stress regimes, with the dominantly more mafic rocks of Socorro emplaced at greater depths. On the other hand, higher Sr/Y ratios and REE fractionation shown by the Agudos Grandes whole-rocks may result from magma equilibration in a thicker crust (>60 km, as contrasted with ∼50 km for Socorro) at the time of magma generation. However, the possible presence of cumulate components (as suggested, for instance, by high apatite and hornblende saturation temperatures) requires that these contrasts are tested in the future by proxies that are independent of whole-rock composition (e.g., trace-element chemistry of minerals).