Petrogenese das intrusões graniticas compostas de Teofilandia e Barrocas, Greenstone Belt do Rio Itapicuru, Bahia, Brasil

AUTOR(ES)
FONTE

IBICT - Instituto Brasileiro de Informação em Ciência e Tecnologia

DATA DE PUBLICAÇÃO

26/08/2002

RESUMO

The Rio Itapicuru Greenstone Belt (RIGB) is a Paleoproteozoic sequence situated in the northern portion of the São Francisco Craton, Bahia state, Brazil. Overlying a basement consisting of gnaisse and migmatites, this volcanic-sedimentary belt is intruded by several granitoid intrusions of different chemical compositions. Two of the most important intrusive bodies, the Barrocas (BGr) and the Teofilandia granitoids (TGr), which are early Proterozoic in ages (2130 and 2127 Ma, respectively), are located in the southern portion of the RIGB, and display E-W and NE-S W trending orientation, in contrast with the N-S trend of most of the remaining intrusions related to the supracrustal pile. Even being considered time- and spatial-related granitoids, the field relationships reveal the TGr as the earlier intrusion. It has a granodioritic to quartz-monzodioritic modal composition, and associated with a quartz-porfiritic sub-volcanic rock. The BGr has a granodioritic modal composition and rare transitions to tonalite. Both belong to the calc-alkaline and I-type suites and have metaluminous signatures, characteristic of felsic igneous rocks of orogenic environments. The TGr displays Si02 (64%-71%), Na20 (4,02% -5,92%), CaO (2,32% - 4,44%) and Ti02 (0,28% -0,48%), with low to moderate Rb/Sr, Rb/K and Sr/Y ratios. The BGr has higher Si02 values (up to 73,57%), and lower Rb/Sr, Rb/K and Sr/Y ratios and Na20, CaO, Ti02, Sr, Ba and Y. Although both granitoids have juvenile arc signatures, characterized by low 87Sr/86Sr isotopic ratios (0,69964-0,70177) and positive SNd2I30Ma (>+2.03), their initial isotopic ratios indicate different sources, revealing a likely intra-oceanic tectonic setting for them. Geochemical modellings point to their being the result of partial melting of a basaltic protoliths of the RIGB sequence (oceanic crust) under amphibolite-eclogite facies conditions. All these features, in addition to the whole-rock O isotope data (>7.5%o), preclude the accretion of recycled materials, suggesting a pervasive alteration of their basaltic protoliths before melting. Fine to medium-grained microdioritic enclaves having metaluminous and calc-alkaline compositions are scattered in the TGr, as well as more restricted coarse-grained varieties, formed by mafic mineral accumulation from the host granitoid. Another coarse-grained enclave type resulted from the mixture with more primitive magmas. Neither the TGr nor the BGr has xenoliths of the surrounding regional rocks. Small and irregular granodioritic bodies cut the TGr;, which are characterized by high Rb and K values and narrow negative isotopic variation (ЄNd2130Ma= -8.49 to - 9.38), indicating partial melting of an ancient crustal material. Of the two deformational events documented in RIGB, the Dl tectonic event directly affected the granitoids. This event, characterised by a NW-SE dextral transpression movement, was responsible for an expressive curved fault along the contact of the BGr with the basalt units of the supracrustal RIGB in the north, and for another one along the contact plane of the TGr with the basement in the south, representing a possible suture zone. The ongoing transpression developed E-W planes, E-NE mineral and stretchting lineation trends, as well as kinematic indicators in the TGr, whereas imprinted a high (medium) angle dipping S-C foliation structures in the BGr granitoid. Open upright folds with low NW-trending angle axes nucleated in a northeastern sector within the area determined by these two main faults, which gradually became closed-like folds in the central sectors, with their axes uprighting westward according to an apparent clockwise rotation. The dextral movement culminates in the two expressive upright mylonitic segments which form the wedge of a delta-like megastructure. Notwithstanding, as expected in syn-tectonics granitoids bodies emplaced in transpressional domains, the movement not inferred sigmoidal patterns to the geometric elements of the TGr and BGr intrusions. All these features indicate the folding as the dominant movement instead of rotation. These relationships support the suggestion that both igneous bodies are sheet-like intrusions, and provide evidence for the interplay between the TGr and BGr and the Dl tectonic event. The young D2 event locally affected the TGr and BGr through dominantly brittle features with many parallel N-S trending faults.

ASSUNTO(S)

geologia estrutural - itapicuru rio (ba) petrogenese granito

ACESSO AO ARTIGO

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