On the crystallization conditions of the Neoproterozoic, high-K calc-alkaline, Bragança Paulista-type magmatism, southern Brasília Orogen, SE Brazil

Abstract

Syn-orogenic Neoproterozoic granitic magmatism (ca. 630–600 Ma) in southern Brasilia Orogen includes large batholiths of compositionally expanded high-K calc-alkaline granites (lato sensu) and small occurrences constituted by anatetic leuco-granites associated to the regional metamorphism. We present petrographic, mineralogical and geochemical data for a representative sample of the Braganca Paulista-type granite and estimates for magmatic crystallization conditions. The sample corresponds to a porphyritic hornblende-biotite monzogranite (CI ≈ 20) comprising perthitic microcline megacrysts within a medium-grained matrix with plagioclase (An 30 Ab 70 ), quartz, microcline and mafic minerals. The main mafic minerals are biotite (0.51 ≤ mg# ≤ 0.57) and Ca-amphibole (potassic-magnesio-hastingsite, 0.47 ≤ mg# ≤ 0.52), with similar volumes; accessory phases (ca. 2 vol.%) are Fe-Ti oxides (ferrian ilmenite, titanohematite, hematite and magnetite), allanite, apatite, zircon, titanite and sulphides. Our results indicate crystallization at 510 ± 60 MPa and temperatures from ca. 970oC (close-to-liquidus) to 755 ± 45oC (close-to-solidus), under oxidizing conditions, compatible with 0 ≤ ΔNNO ≤ +1. Estimated water and halogen fugacity ratios are ƒ H2O /ƒ HF ≥ 10 5.8 , 10 3.5 ≤ ƒ H2O /ƒ HCl ≤ 10 3.8 and ƒ HF /ƒ HCl ≤ 10 -2.2 . The obtained pressure, integrated with available regional geobarometric data, suggests a relative uplift rate about 0.2–0.3 (km/Ma) from the main regional metamorphism to the post-orogenic magmatism in the area.