BRAZILIAN KAMAFUGITES

PATRICIA BARBOSA DE ALBUQUERQUE SGARB, JOSÉ CARLOS GASPAR, JOEL GOMES VALENÇA

Resumo


Brazilian kamafugites occur in the Mata da Corda (Minas Gerais state) and Santo Antonio da Barra (Goias state) sub-provinces, which belong to Minas-Goias Alkaline Province, one of the Cretaceous alkaline provinces that are located around the Parana Basin in Brazil and Paraguay. The MC and SAB kamafugites are mafurites and ugandites. The rocks are all feldspar-free, with abundant clinopyroxene (mostly diopside) and Ti-magnetite in very fine- to medium-grained porphyritic to seriated textures. Perovskite is abundant in all MC kamafugites but in SAB kamafugites this mineral is accessory or absent. Kalsilite is generally interstitial, often altered. Mineralogically, the ugandites are similar to the mafurites, with the exception that they contain leucite (subhedral pseudomorphs) as the major felsic phase. Also, the mafurites have much more olivine phenocrysts (5-19% modal) than the ugandites (0-3% modal) while the ugandites have much more feldspathoid. The Brazilian kamafugites here studied are all undersaturated in SiO2 (36.6 to 44.9 wt% in MC and 39.9 to 44.2 wt% in SAB), the alkaline characteristic of the rocks being reflected in the contents of TiO (4.1 to 7.8 wt% in MC and 2.8 to 4.1 wt% in SAB), K2O (1.2 to 6.8 wt.% in MC and 0.5 to 4.1 wt.% in SAB), and Na2O (0.49 to 2.0 wt% in MC and 1.0 to 4.3 wt.% in SAB). The presence of normative feldspathoid and olivine and lack of normative quartz are significant. MgO contents (6.0 to 18.0 wt% in MC and 5.6 to 15.2 wt% in SAB) are low when compared to those of other ultrabasic rocks, the highest values being associated with samples of high modal olivine content. The clinopyroxene/magnetite isotopic temperature data of the MC and SAB kamafugites are in the range of 830-1060°C (Sgarbi et al. 1998). The U-Pb perovskite ages of Brazilian kamafugites are in the range of 68-89.6 Ma (Sgarbi et al. 2000).

Palavras-chave


Kamafugites; Mafic alkaline volcanism; Cretaceous; Brasil.

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