Análise petrográfica dos ignimbritos bem preservados da formação cachoeira da ilha, centro-norte do cráton amazônico, estado de Roraima
DOI:
https://doi.org/10.18190/1980-8208/estudosgeologicos.v30n1p100-114Keywords:
Volcanism, Paleoproterozoic, Guyana ShieldAbstract
The Cachoeira da Ilha Formation (FCI) is located in the north central portion of the Amazonian Craton, northeast of Roraima. It is found by ignimbritic rocks with about 1990 Ma. Acid pyroclastic rocks in the region of the municipality of Amajari were classified as ignimbrites: solid, moderately welded and rheomorphic from variations in the degree in the welding process, proportion and type of pyroclasts and textural aspects. The crystalloclastsare made of quartz, sanidine and plagioclase and are dispersed in a glass-matrix. They are usually found with corrosion, dentate and intensely sericitized and/or saussuritized gulfs, showing low temperature hydrothermal alteration. It is also possible to distinguish mafic, microporphyritic and cognate lithitic fragments.
References
Gibbs, A.K.; Barron, C.N. 1993. The Geology of the Guiana Shield. New York, Claredon Press, Oxford, 245 p.
Le maitre, R. W. 2002. Igneous rocks: A classification and glossary of terms. 2. ed. Cambridge: University Press, 232 p.
Santos, J.O.S. 2003. Geotectônica do Escudo das Guianas e Brasil-Central. In: Geologia, Tectônica e Recursos Minerais do Brasil, Bizzi, L.A., Schobbenhaus, C., Vidotti, R.M., Gonçalves, J.H. editores. CPRM, Brasília, p. 169-226.
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