UnB - Instituto de Geociências - Produção Científica
RESUMOS/ABSTRACTS 

Low pressure corundum crystallization in the Peixe Alkaline Complex (Tocantins, Brazil) as evidenced by fluid inclusion characterization.

Flávio H. Freitas-Silva 
José Carlos Gaspar
Geraldo F. Andrade
Luiz F. W. Kitajima

In: Brazilian Geology Congress, 40^th,Belo Horizonte, 1998, SBG. Anais: 174-174. 

ABSTRACT

            The syenitic suites comprise an intrigated rock group which do not have a consensuous about their magmatic petrogenesis. The most obscurous feature often observed in this alkaline rocks are the presence of corundum in their paragenesis. The occurrence of corundum in igneous rocks has been interpreted in ways: as xenocrystal formed in the mantle/crust interface and sampled during the ascention of primary alkaline magma; contamination in the deeper crust of na alkaline basaltic magma by a sialic/aluminosous crust and due to the mixture of granitic and carbonatitic magmas.

            In the Peixe Alkaline Complex corundum occurs as disseminated microscopic to centimetric phenocrystals in nepheline syenite and centimetric to decimetric crystals in related syenitic pegmatites. The fluid inclusions observed in this mineral are disposed along rombohedral crystal planes or isolated. They display estriated negative crystal to anedral morphologies, pointing to their primary origin.

            The fluid inclusions are caracteristically biphasic at room temperature (about 25^o  C) na display a very constant Vg near 50%. The microthermometric data obtained for these fluid inclusions showed a pure CO2 composition which presented melting temperatures between –54.5 and –56.7^o C, mainly around –54.4^o C. A few isolated inclusions melted around –62C. Their homogeneization occurred between +28.5 and +31.1^o C with an expressive number in the +30^o C. Characteristically, the homogeneization occurred in the critical phase with some inclusions homogeinizing in the vapour and liquid phases.

            The microthermometric data established a moderate to low density supercritical fluid, composed by pure CO2, although low water content (<10% molar fraction) could be present. The resulted isochores indicated a low pressure trapping crystallization conditions. For na irrealistic highest trapping temperature of the 1000^o C the pression would not exceed 2.5 to 3 kbar, for the more probable 700-775^o C (independent geothermometry) the trapping pressures are situated around 1.5 to 2.2 kbar. This is in accordance to the low pressure sillimanite-cordierite contact metamorphic aureole observed in the country rocks.

            These results preclude the xenocrystal hypothese and indicate a primary, near surface, corundum crystallization from the syenitic magma. The surprising, but persistent, CO2 metaestable melting temperature does not have a conclusive interpretation yet, but in analogy with a metaestable ice melting, has been attributed to a supercooling stage of a mgma system: a sudden isotherm lowering and corundum nucleation followed by na isothermal crystallization and corundum growth. This is possible, among other alternatives, by a fast ascention of a mantelic alkaline magma (>1000^o  C) and its emplacement in the upper crust (around 700-750^o  C). These facts do not constrain the sienitic magma can be formed between a mantle derived CO2 rich carbonatitic, or a similar Si-poor magma, with na upper crust stationary granitic melt. Thus, independently of the magma origin, the results indicate that the cinematic history of crystallization may be more important than original peraluminous nature of the magma in the petrogenesis of corundum-bearing alkaline rocks.