TERRAS-RARAS EM LITOGEOQUÍMICA: - 1. UM EXEMPLO DE ESQUEMAS DE DISTRIBUIÇÃO EM ROCHAS GRANÍTICAS

CLAUDIO VIEIRA DUTRA, J. H. GROSSI SAD, ANTONIO CARLOS PEDROSA SOARES

Resumo


The geological framework as well as the petrography and geochemístry of some intrusive granítoid plutons (Upper Proterozoic) autcropping in the Coronel Murta-Virgem da Lapa regíon, Northeast of Minas Gerais State, were carefully studied by former authors. This paper presents and díscusses the results of REE (rare earth elements) geochemical analysis by the ICP spectrometry method applied on selected samples of those granitoids. The analytícal procedures are bríefly described. Four varieties of granites are analysed: biotite granite (4 samples), muscovíte granite (3 samples), albite-tourmalíne granite (2 samples) and pegmatoid granite (l sample). The LREE (light rare earth elements) contents are, respectively, (in ppm) 315.86, 26.51, 8.95, and 7.25, while the HREE (heavy rare earth elements) ones are 15.16. 2.94, 2.23, and 2.30. The La/Yb ratios are, respectively, 57.3, 19.6, 4.4, and 3.7. The biotite granites don't show an anomaly for the Eu. It appears in muscovite granites and markedly increases in the albite-tourmaline and pegmatoid granites. The deplection latias in the system biotite granite .... muscovite graníte are: REE-lO, LREE-12, and HREE-5. While in the system muscovíte granite → albite-tourmaline granite those ratios are: REE-3, LREE-3, and HREE-l.3. Based on petrographic evidence, geological relations and major-trace elements geochemistry the system biotite granite → muscovite granite -+ albite-tourmaline granite was interpreted as a metasomatic sequence that may explain the evolution af the anatectíc plutons generated by crustal meltíng. The pegmatoid granítes were considered as magmatic residues emplaced on the granitic cupolas. The REE data here present also point to those evolutionary hypothesis.

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