Geologia, petrografia e litogeoquímica do batólito trondhjemítico nordestina, Núcleo Serrinha, nordeste da Bahia

Abstract

The Nordestina Batholith (NB; 720 km 2 ) is a Paleoproterozoic (2.1 Ga) intrusion located in the central part of Serrinha Nucleus, Bahia, Brazil. It intrudes the gneissic-migmatitic Archaean basement and the volcano sedimentary sequences of the Rio Itapicuru Greenstone Belt. Its north-south elongated shape, associated with its concentric magmatic foliation, wich become more intense toward the border, is interpreted as a result of a syn-tectonic emplacement and correlated with the compressional Transamazonic event responsible for the closure of the Rio Itapicuru Basin. In this trondhjemitic batholith there are two predominant lithologies: (i) medium-grained phaneritic rocks; and (ii) porphyritic rocks, the latter limited to the central portion of the massif. Granitic dykes and dioritic enclaves are subordinate features of this batholith. The studied trondhjemitic rocks show a narrow range of SiO 2 contents (68-72 wt. %), low FeO, MgO and TiO 2 , high Al 2 O 3 (15-17 wt. %), fractionated REE patterns, weak and variable Eu anomalies, and La and Yb contents similar to those reported for the Archaean trondhjemites of East Finland. Major and trace element petrogenetic modelling for the NB allows explaining the porphyritic facies as a result of fractional crystallization of from magmas compositionally similar to the borders unit. Correlation with available data from Serrinha Nucleus bibliography indicates that this trondhjemitic magma is correlated with the calc-alkaline volcanism of the Rio Itapicuru Greenstone Belt, and the result of partial melting of a Paleoproterozoic subducted ocean slab.