Geologia, petrografia e geoquímica do Granodiorito Sanukitoide Arqueano Rio Maria e rochas máficas associadas, leste de Bannach-PA

Abstract

New occurrences of the Archean Rio Maria Granodiorite (RMGD) situated to the east of Bannach town, immediately to the west of it’s type-area, in the Rio Maria Granite-Greenstone Terrain (RMGGT), southeastern Amazonian craton, were studied. The typical granodiorites, wich are dominant in the RMGD, include systematically mafic enclaves, as observed in other areas. However, in the studied area there are also mafic and intermediate rocks (IMR) with the RMGD. The Rio Maria Granodiorite is intrusive in the greenstone belts of the Andorinhas Supergroup. Field relationships between the RMGD and Archeans tonalites and trondhjemites (TTGs) are not conclusive being admitted that both have similar ages. Leucogranites correlated with the Xinguara and Mata Surrão Archean granites and the Paleoproterozoics Musa and Bannach granites of the Jamon Suite are intrusive in the RMGD. The dominant rocks of the RMGD are quite uniform, with, medium-to coarse-evengrained texture, and granodioritic or subordinate monzogranitic composition. The GDrm shows generally a gray color with greenish shades due to it’s strongly saussuritized plagioclase, white the monzogranites displays a rose gray color. The Rio Maria Granodiorite display generally a weak or striking WNW-ESE foliation due to mafic minerals and, sometimes, enclaves orientation. Two domains of IMR were identified: In the main domain, located near Bannach town, are exposed mostly quartz diorites and quartz monzodiorites; in the second domain situated in the center of the area, a minor occurrence of layered rocks was described. The dominant rocks in the larger body are mesocratic, dark-green, sometimes with rose tones rocks, with fine-to coarseevengrained texture. The layered rocks, interpreted as cumulatic rocks are inequigranular with a remarkable concentration of generally quadratic or short prismatic coarse amphibole crystals, enveloped by leucocratic intercumulus material. The RMGD and IMR rocks show similar textural and mineralogical aspects. The RMGD is formed dominantly by epidote-biotite-hornblende granodiorite (EpBtHbGd) with subordinate epidote-hornblende-biotite granodiorite (EpHbBtGd), and epidote-biotite-hornblende monzogranite (EpBtHbMzG). The mafic enclaves included in the RMGD are epidote-biotitehornblende diorites (EpBtHbDr) transitioning to monzodiorities. In the main mafic body epidotebiotite-hornblende quartz diorite (EpBtHbQzD) varying to epidote-biotite-hornblende quartz monzodiorite (EpBtHbQzMzD) are dominant. The layered rocks are enriched in mafic minerals, mostly amphibole, compared to monzodiorites and granodiorites. The RMGD and IMR follow the calc-alkaline series trend in some diagrams. However, they display lower Al2O3 and CaO and larger MgO, Cr and Ni contents compared to calc-alkaline series, approaching geochemically the sanukitoids series. The patterns of rare earth elements of different rocks are similar, with pronounced enrichment in light rare earth elements (LREE) and strong to moderate fractionation of heavy rare earth elements (HREE) (La/Ybn=11,92 a 44,38). However, the (La/Yb)n ratio is lower in the EpBtHbQzD and EpBtHbQzMzD (La/Ybn=17,20 a 22,81), compared to the RMGD (La/Ybn=15,52 a 44,38). Compared to the RMGD and IMR, the layered rocks are relatively enriched in HREE (La/Ybn=11,92 a 14,37), probably in response to amphibole accumulation. The RMGD and IMR display some geochemical affinities, but also significant difference, and are interpreted as cogenetic but not comagmatic rocks. Geochemical data show that there is a ompositional gap between both the RMGD and IMR and the layered rocks and mafic enclaves, suggesting distinct processes for the origin of the latter group of rocks. Field aspects and petrographic and geochemical caracteristics denote that the RMGD and IMR are cogenetic rocks. However, geochemical data suggest that the intermediate rocks and the RMGD are not related by a fractional cristalization process. The wide distribution of granodiorities and relatively local occurrence of do not also favor this hypothesis. It is concluded that the intermediate rocks derived from similar sources than those of RMGD, but probably result of a higher degree of melting, being both cogenetic, but not comagmatic rocks. The layered rocks are genetically related to the sanukitoid association, but they had a particular magmatic evolved involving participation of crystal accumulation processes.