Geology, petrography and geochemistry of Archaean high-magnesium granitoids of the Água Azul do Norte area, southern Carajás Domain, Pará

Abstract

The Mesoarchaean high-Mg granitoids that outcrop in the Água Azul do Norte area occur as two elongated and deformed bodies in the E-W direction. They are bodies composed of mylonitic porphyritic granodiorites and tonalites, with amphibole, biotite, and epidote as the main mafic minerals. Their evolution is marked by fractionation of Fe₂O₃, MgO, TiO₂, and Al₂O₃, an increase of K₂O and K₂O/Na₂O ratio from the amphibole-rich facies toward the more evolved facies with biotite. Sr, Y, Zr, #Mg, Ni, Cr, and Sr/Ba decrease, whereas Rb, Ba, and Rb/Sr increase towards the more differentiated facies. The rare earth elements (REE) patterns show a significant fractionation of heavy rare earth elements (HREE) with moderate LaN/YbN ratio and discrete anomalies of Eu. The studied rocks are predominantly metaluminous, with high values of #Mg, Cr, and Ni, distinct geochemically from calc-alkaline series of active continental margins and Archaean TTG suites and leucogranodiorites. The petrographic and geochemical affinity between the studied rocks and classical occurrences of high-Mg granitoids suggest that they can be part of a similar magmatic suite, identified for the first time in this part of the Carajás Province.