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

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 approximate to 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. 970 degrees C (close-to-liquidus) to 755 +/- 45 degrees C (close-to-solidus), under oxidizing conditions, compatible with 0 <= Delta NNO <= +1. Estimated water and halogen fugacity ratios are f(H2O)/f(HF) >= 10(5.8), 10(3.5) <= f(H2O)/f(HCl) <= 10(3.8) and f(HF)/f(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.