Ronaldo Pierosan

Geochemistry of Palaeoproterozoic volcanic rocks of the Iricoumé Group, Pitinga Mining District, Amazonian craton, Brazil

The Iricoumé Group includes 1.88 Ga volcanic units of the Iricoumé–Mapuera volcano-plutonic association, part of the Uatumã magmatic series in the Guyana shield portion of the Amazonian craton. In the Pitinga Mining District, these rocks consist dominantly of felsic trachyte to rhyolite, associated with voluminous ignimbrite and minor ash-fall tuffs and surge deposits. Mafic rocks are present as basaltic clasts within volcanic breccias, and mostly as mafic microgranular enclaves in the associated Mapuera plutonic rocks. The felsic rocks have high contents of SiO2, FeOt, K2O, Rb, and alkalis; low TiO2, CaO, Sr, Ba, Nb, Ta, and Eu; and show metaluminous to weakly peraluminous bulk-rock compositions. They exhibit alkaline geochemical features, expressed by Na2O + K2O averages of 8.8 wt.%, FeOt/(FeOt + MgO) ≥ 0.8, and high Ga/Al ratios, compatible with A-type magmas. The studied samples plot in the field of within-plate or post-collisional rocks in a (Nb + Y) versus Rb diagram. Nb/Y ratios indicate that they are comparable to A2-type rocks which, allied with their high LREE/Nb ratios, suggest that they were produced from mantle sources modified by previous subduction in a post-collisional setting. Two compositional populations of Ca-amphibole, a Mg-rich (actinolite to Mg-hornblende) and a Fe-rich one (Fe-edenite to Fe-pargasite, Fe-hornblende and Fe-actinolite), characterize the Iricoumé Group volcanics. The Fe-rich amphiboles crystallized under lower fO2 and higher pressure conditions compared with the Mg-rich amphiboles, indicating different levels of crystallization or re-equilibration during ascent of the magmas. Zircons from rhyolites show trace-element compositions typical of magmatic crystals with high Th/U ratios, and REE patterns compatible with zircon-melt partition coefficients for silicic magma compositions. Their relatively lower zircon/rock partition coefficients are due to early apatite crystallization. Fractional crystallization mainly of plagioclase-hornblende and biotite-alkali feldspar with minor amounts of apatite explains the geochemical trends observed in the felsic Iricoumé volcanic rocks.

Paleoproterozoic (~1.88Ga) felsic volcanism of the Iricoumé Group in the Pitinga Mining District area, Amazonian Craton, Brazil: insights in ancient volcanic processes from field and petrologic data

O Grupo Iricoume corresponde ao mais expressivo vulcanismo Paleoproterozoico do Escudo das Guianas, craton Amazonico. As rochas vulcânicas sao coexistentes com os granitoides Mapuera, e pertencem ao magmatismo Uatuma. Possuem idades U-Pb em torno 1888 Ma, e assinaturas geoquimicas de magmas tipo-A. As vulcânicas do Iricoume consistem de traquitos a riolitos porfiriticos, associados a ignimbritos ricos em cristal e tufos co-ignimbriticos de queda e surge. A quantidade e a morfologia dos fenocristais podem ser utilizadas para distinguir lava (fluxo e domo) de unidades hipabissais. A morfologia dos cristais em ignimbritos permite a distincao entre unidades efusivas e ignimbritos, quando os piroclastos estao obliterados. Tufos co-ignimbriticos sao macicos e alguns exibem estratificacoes que sugerem deposicao por correntes de tracao. Temperaturas de cristalizacao de zircao e apatita variam de 799°C a 980°C, sao compativeis com temperaturas de liquidos tipo-A e podem ser interpretadas como temperatura liquidus minima. Estimativas de viscosidade para composicoes rioliticas e traquiticas fornecem valores proximos a de liquidos determinadas experimentalmente e ilustram curvas tipicas de decaimento exponencial, com a adicao de agua. O posicionamento das vulcânicas Iricoume e de parte dos granitoides Mapuera foi controlado por falhas anelares em ambiente intracratonico. Uma genese relacionada a ambiente de complexo de caldeiras pode ser assumida para a associacao vulcano-plutonica Iricoume-Mapuera no Distrito Mineiro de Pitinga.

Paleoproterozoic Domo of Lava from Iriri Group – Sonho Meu Farm – Northeast of Mato Grosso, Amazon Craton: geology, geochemistry, and geochronology

The Iriri Group is composed of acid to intermediate effusive and pyroclastic rocks, which crop out in Northeastern Mato Grosso state, Western Vila Rica. In the vicinity of Sonho Meu Farm, there is some part of an acid to intermediate lava dome, which is 2 km long and 250 m wide. The dome has a hypabyssal core that changes laterally to an autoclastic facies rich in pumice and shards with mingling textures, and subsequently to a facies with flow structures. The core has porphyritic texture and phenocrysts of plagioclase and biotite embedded in a medium-grained matrix of the same composition. The pumice-rich facies has a texture similar to ignimbrites and reoignimbrites. In this facies, a porphyritic to glomeroporphyritic texture is observed, with evidence of pumice collapse. The magma mixture is characterized by clusters of hornblende and plagioclase, in the form of millimetric to decimetric enclaves. Geochemical data on the rocks with no evidence of mingling indicate metaluminous to peraluminous high-K calcalkaline compositions. Rare earth elements patterns show a slight enrichment in LREE over HREE. Geochemical data show an enrichment in Rb, Sr, and K2O and a negative Ta-Nb anomaly, which is common in calcalkaline rocks. Concentrations of trace elements are compatible with post-collisional tectonic environments. U-Pb zircon geochronology yields an age of 1.87 Ga. Sm-Nd isotopic studies report negative €Nd values and Archean Nd model ages, indicating contamination by crustal material. This paper suggested that such dome was formed by melting of the lithospheric mantle, modified by previous subduction events, with subsequent magmatic differentiation.

Caracterização Petrográfica e Geoquímica da Parte Leste do Granito Europa, Distrito Mineiro de Pitinga, AM

The important mineral deposits of the Pitinga Mine, in the Amazonian region are related to A-type granites intruded in the Iricoume Group. The Europa granite is one of these A-type rocks, intruded in the Iricoume Group, which is represented by subaerial vulcanoclastic rocks (crystal-rich ignimbrites, thin massive tuffs and siltic tufaceous arenites) and minor hipabissal rhyolites. The volcanic rocks were probably generated in a caldera environment. The Europa granite is an alkali-feldspar peralkaline granite (hipersolvus) without genetic relationship with to the volcanic rocks of the Iricoume Group, but it could have been generated during the resurgence stages. The petrographic and geochemical data attest that fractional crystallization process was the principal mechanism during the crystallization, which led to the generation of two different granitic facies. The Nb soil anomalies overprinted on the more differentiated facies are related to the astrophillite weathering.