Petrologia de rochas alcalinas, cálcio-alcalinas e toleíticas ... - CPRM

ABSTRACT
The rocks which are the object of this study are found in the boundary between the São
Francisco Craton and the Ribeira Mobile Belt. They show themselves up as descontinuons bodies
since Matola (westerly) till Ubari (easterly), being limited by a hypothetic polygon with about 115x20km
extention, named here as Matola-Ubari strip. They are alkaline, calc-alkaline and tholeiitic rocks that
have undergone metamorphism in the amplybolite or granulite facies. The alkaline ones consist of
supersaturated syenites mainly, derived from mantelic source, having high REE concentration, high
LREE/HREE ratio and negative Eu anomalies, which are related to the anorogenic environment as
continental uplift or rift types. The calc-alkaline rocks are represented by intermediate/acid granulites,
as enderbites mainly; by a TTG Association, and by a Granitic Association, which have undergone
amplybolite facies metamorphysm. The enderbitic granulites and the TTG Association are formed by
metaluminous plutonic rocks, probably generated by a two-stage parcial melting process, from
metabasaltic source in a subduction related orogenic environment. They show typical REE signatures.
Despite of the chemical features of metaluminous rocks, the Granitic Association rocks show
muscovite and garnet in some of the samples, a flat HREE pattern, and negative Eu anomalies that
suggest the garnet, feldspar and hornblende actuations in the melting process. The REE signatures
are suggestive of parcial melting process from metapelitic/metagreywackes source or the
contamination of a I-type granitic source by a predominant continental crust, in an orogenic
environment related to subduction. The tholeiitic rocks are basic granulites, pyriclasites and pyribolites
mainly, originated from a PMORB and a continental rift related (CRB) basalts type magmas. The interrelations
between these alkaline, calc-alkaline and tholeiitic rocks keep obscure. While the alkalines
show basic granulite xenolites (Acaxio body, Cachoeirinha farm) part of the TTG and Granitic
Associations are closely related, making it difficult their olistinction in the field works. There is an
exposed intrusive contact with basic granulites in the Oliveira Fortes - Aracitaba road. The enderbitic
rocks are complexly related with the basic types predominant.
The relation with the regional banded gneiss (Mantiqueira Complex), which has been
considered as transamazonic age by Rb/Sr dating (the age of the deformation/metamorphism), points
to earlier at sin-transamazonic ages for the target rocks, since they show the same deformation printed
on the country rocks. Considering their strong deformation/metamorphism, it is not probably to admit a
Brazilian age to hese rocks, unless in case of not recalibrating of the Rb/Sr system, in the country
rocks. In order to define these and other questions, U/Pb dating in zircon and titanite are
recommended, as well as geothermobarometric studies and specific works that fully describe each one
of the refered Matola-Ubari bodies. It is also suggested to investigate the Guaxupe Wedge (Sintax)
relationship especially with the Três Corações Transcurrent Zone, which may have its eastward
projection over here.