QUÍMICA MINERAL DE ANFIBÓLIOS E BIOTITAS E CONDIÇÕES DE CRISTALIZAÇÃO DE GRANITÓIDES PALEOPROTEROZÓICOS DA REGIÃO DE VILA RIOZINHO, PROVÍNCIA AURÍFERA DO TAPAJÓS, CRÁTON AMAZÔNICO

Abstract

The Old São Jorge, Younger São Jorge, Jardim do Ouro, porphyry granites, and the Maloquinha suite granitoids found in the Vila Riozinho region, eastern portion of the Tapajós Gold Province, are representative of the Paleoproterozoic magmatism (1.98-1.88 Ga). The quartz monzodioritic rocks of the Older São Jorge granite have amphiboles with Mg-hastingsite, tschermakite and Mg-hornblende composition. The dominant monzogranitic facies and the less frequent quartz syenitic rocks have homogeneous Mg-hornblende with rare borders of actinolite. The high Mg/(Mg+Fe 2 ) ratios and the Al 1 contents in amphibole indicate crystallization under oxidizing conditions at pressures between 3-1 kbar for the monzogranitic facies and <1.7 kbar for the quartz-syenitic facies. The Younger São Jorge granite contains Mg-hornblende and actinolite with slightly higher Mg/(Mg+Fe 2 ) ratios but with inferior Al 1 , content compared to equivalent rocks of the Older São Jorge granite. Al-in amphibole indicate crystallization between 2-1 kbar. The Jardim do Ouro granite amphibole varies from Fe-hornblende to Fe-edenite with comparatively lower Mg/(Mg+Fe 2 ) ratios, indicating crystallization in a somewhat less oxidizing setting. In the porphyry granite associated with Older São Jorge granite, an homogeneous Mg-hornblende with Mg/(Mg+Fe 2 ) ratios, alcalis and Al 1 contents similar to the amphiboles of the monzogranitic rocks of that pluton predominate, suggesting that both rocks are cogenetic. A porphyry granite, associated with ignimbrites of the Moraes Almeida Formation and Maloquinha suite, contains Fe-hornblende similar in composition to the amphiboles of the Jardim do Ouro granite, indicating a possible relationship between these rocks. The biotite of the Older São Jorge granite, Younger São Jorge granite and porphyry granite associated with the former is a Mg-biotite with evidence of post-magmatic reequilibrium. The biotite of the Jardim do Ouro granite is comparatively enriched in iron and does not show evidence of post-magmatic reequilibrium.