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



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 (An30Ab70), 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 ƒH2OHF ≥ 105.8, 103.5 ≤ ƒH2OHCl ≤ 103.8 and ƒHFHCl ≤ 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.


Intensive crystallization parameters; High-K calc-alkaline granites; Bragança Paulista-type granites; Neoproterozoic magmatism; Socorro- Guaxupé Nappe.

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DOI: http://dx.doi.org/10.1590/2317-4889201820180033


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