Petrografia e geoquímica do granodiorito Rio Maria da região de bannach e comparações com as demais ocorrências no terreno granito-greenstone de Rio Maria – Pará

Abstract

The Archean Rio Maria Granodiorite (RMGD) is exposed in large domains of the Rio Maria Granite-Greenstone Terrane (RMGGT), southeastern Amazonian craton. New occurrences of the Archean Rio Maria Granodiorite situated to the east of Bannach town, immediately to the west of the type-area, have been studied. In the Bannach area, the RMGD is intrusive in the greenstone belts of the Andorinhas Supergroup. The field relationships between the RMGD and Archean tonalites and trondhjemites (TTGs) are not conclusive and it is admitted that both have similar ages. Leucogranites correlated with the Archean Xinguara and Mata Surrao granites and the Paleoproterozoic Musa and Bannach granites of the Jamon Suite are intrusive in the RMGD. The rocks of the RMGD exposed in the studied area are quite uniform, displaying medium- coarse-evengrained texture, a generally weak or WNW-ESE striking foliation, and including systematically mafic enclaves, a feature observed in all occurrences of the Rio Maria Granodiorite. The Rio Maria Granodiorite in the Bannach area is formed dominantly of epidote-biotite-hornblende granodiorite with subordinate monzogranite. Intermediate and layered mafic rocks are associated with the granodiorite assemblage, but they will not be discussed in the present paper. The RMGD display metaluminous character and some geochemical characteristics in common with calc-alkaline series, however it has lower Al 2 O 3 and CaO and higher MgO, Cr and Ni contents compared to calc-alkaline series, approaching geochemically the sanukitoid series. Rare earth element patterns are characterized by a pronounced enrichment in light rare earth elements (LREE) and strong to moderate fractionation of heavy rare earth elements (HREE) (La/Yb n =15,52 a 44,38). Field aspects and petrographic and geochemical characteristics of the RMGD of the studied area are very similar to those of other RMGD occurrences elsewhere. Some subtle differences are the more restricted distribution of monzogranitic rocks in the Bannach domain and the higher quartz content in the granodiorite of the type-area. The major geochemical characteristics of the rocks of different occurrences of the RMGD are very similar, but compared to the studied area, in the type-area and other domains, the dominant granodiorites are more evolved, showing higher SiO 2 , Al 2 O 3 , K 2 O and Rb and lower Na 2 O, CaO, and #Mg.