Navegando por Assunto "Granito - Bannach (PA)"

Agora exibindo 1 - 1 de 1

Acesso aberto (Open Access)
Geologia e geoquímica dos granitóides Arqueanos da área de Bannach (PA): uma reavaliação das áreas de ocorrência do trondhjemito mogno e granodiorito Rio Maria
(Universidade Federal do Pará, 2018-08-21) MACHADO, Jean Ricardo Mesquita; OLIVEIRA, Davis Carvalho de; http://lattes.cnpq.br/0294264745783506
The Bannach area is located in the center-west of Rio Maria Domain, Carajás Province, Amazonian craton. Based on field relationships, petrography and geochemical behavior, it was possible to identify six varieties that was divided into two groups: (i) high silica and low Mg group, represented by trondhjemitic rocks associated with mogno Trondhjemite (TdhMg), leucogranodiorites and fine grained granitoids and (ii) low silica and high Mg groups, represented by biotite granodiorites, amphibole bearing tonalites and biotite tonalites (enclaves). The trondhjemites, leucogranodiorites are the most extensive batholiths of the region, covering approximately 90% of the work area, presenting medium-to-coarse heterogranular texture and E-W to NW-SE ductile deformation features. The biotite granodiorites occur as a small stock in the north-east, being formed by more enriched in mafic minerals rocks than the predominant granodioritic variety. The amphibole bearing tonalites (± quartz diorites) is emplaced along shear zones that intersect the central portion of the area, being more deformed than the other rocks and the only amphibole bearing granitoid. In smaller proportions, the biotite tonalites represent mega-enclaves of the trondhjemitic rocks, whereas the fine granitoids intersect the other rocks of the region. These varieties are divided into two groups. Geochemically, the group of high silica (SiO2> 70%) presents high levels of Al2O3, CaO and Na2O (especially leucogranodiorites) in detriment of Fe2O3, MgO, Ni and Cr. Moreover, these varieties have high La/Yb and Gd/Er ratios, discrete or absent Eu negative anomalies, and strongly fractionated REE patterns. On the other hand, the low silica granitoids present high content of Fe2O3, MgO, Ni and Y, emphasizing the high K- Ba-Sr content of the biotite granodiorites in relation to the other granitoids in the area, in addition to their moderate to high La/Yb ratios, while the other low silica granitoids present low La/Yb ratios because of the high HREE content, which provides a sub-horizontal REE pattern. These differences trace back distinct formation processes for Bannach granitoids. Although the formation of trondhjemites and leucogranodiorites occurs in the garnet stability zone, its chemical differences go back to sensible differences in its magma. The origin of the trondhjemites is associated with the partial melt of garnet amphibolite, under high pressure conditions in a subduction environment. The leucogranodiorites, due to their higher sodium enrichment, Ba and Sr, have their compositional control associated to different degrees of melting of basalt under different pressure levels and by a source richer in subducted sediments of the oceanic crust to produce magmas of similar composition to the leucogranodiorite. Therefore, it is assumed that these rocks would have originated by the fusion of enriched tholeiite basalts installed below an older TTG crust that would have been assimilated by these melts. The geochemical aspects, such as the high LILE content and even high presence of mantle affinity elements (Mg, Ni and Cr) of the biotite granodiorites and amphibole bearing tonalites may denounce their affinities with the rocks of the Sanukitoid suite of Rio Maria. The ambiguous geochemical character of the biotite granodiorites associated with the highest SiO2 content when compared to the quartz diorites, as well as their higher levels of Rb and Ba, Sr, Y and the La/Y and Sr/Y ratios indicate a more evolved and conditions of formation in higher pressure and that has felsic parent magmas, as well as a mafic component acting in its origin, approaching the Closepet-type granites. In this way, these compositional affinities indicate a strong petrogenetic analogy, with an origin from an enriched mantle or even from a mafic source of high-K. On the other hand, the less evolved character of the amphibole tonalites, as well as the low La/Yb and Sr/Y ratios indicate that these have a higher mantle affinity and that formed at low depths. It is assumed that the origin of these rocks would have been from a metassomatized mantle by slab fluids in a subduction environment.