Compositon, twin laws and structural state of plagioclases of vulcanic rocks of the Parana basin
DOI:
https://doi.org/10.5902/2179460X24865Abstract
Plagioclases of distinct lava flows of the Southern Paraná basin were optically determined through the laws of Albite-CarIsbad, Carlsbad and Albite twin, which are the most frequent.
In the basaltic rocks of uniform granulation of the composition is in the range between An45-An65, with a concentration of values in the interval An50-An55. In the inequigranular types the phenocrysts, in general, display values above An60. In the first case, the plagioclases are of the volcanic and transition types; in the latter, the phenocrysts, commounly, are of the plutonic or transition types, in this way showing the thermic history of the rocks.
The knowledge of the structural state of the plagioclase of volcanic rocks of the Paraná basin favours the precision of the optical measurements for the calculation of anorthite, allowing the selection of the most adequate determining curve (high or low temperature).
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References
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RÜEGG, N.R. Plagioclásios plutônicos e vulcânicos em diabásios da Bacia do Paraná. Rev. Bras. Geoc., São Paulo, 2(J):8-l7, 1972.
SARTORI, P.L.P. & GOMES, C.B. Composição químico.mineralôgica das últimas manifestações vulcânicas na região de Santa Maria, RS. An. acad, bras. ciênc., Rio de Janeiro, 52(1): 125-133, 1980.
SARTORI, P.L.; MACIEL FILHO, C.L. & MENEGOTTO,E. Contribuição ao estudo das rochas vulcânicas da Bacia do Paraná na região de Santa Maria, RS. Rev. Bras. Geoc., São Paulo, 5(3): 141-159, 1975.
SLEMMONS, D.B. Determination of volcanic and plutonic plagioclases using a three- or four-axis universal stage. Geol. Soc.Amer., Special Paper, 69: 1-64, 1964.
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