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 Brasília 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 Bragança 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 Ab₇₀ ), 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.

KEYWORDS:
Intensive crystallization parameters; high-K calc-alkaline granites; Bragança Paulista-type granites, Neoproterozoic magmatism, ­Socorro-Guaxupé Nappe