Characterisation of the neoproterozoic magmatic suites of the Conselheiro Pena and Galileia region (Minas Gerais State, Brazil). Geochemical and structural studies of the Galileia and Urucum suites and relationship with the associated rare element pegmatites
Caractérisation des suites magmatiques néoprotérozoiques de la région de Conselheiro Pena et Galileia (Minas Gerais, Brésil). Etude géochimique et structurale des suites Galileia et Urucum et relation avec les pegmatites à élements rares associées
Résumé
The Conselheiro Pena and Galiléia region (Doce River valley) is located at the eastern Minas Gerais State (southeastern Brazil). Geologically, this region is included at the northern Mantiqueira structural province and Atlantic shear belt. It is formed by the gneiss of Early Proterozoic age, representing the regional basement, and by supracrustals sequences (micaschists of Rio Doce group and quartzites of Crenaque group) considered of Neoproterozoic age. Two deformational phases and a medium grade metamorphism (low amphibolite facies) affected the region during the Brasiliano cycle. The D1 phase is the main deformational phase of the region and it is associated to movements with an important dextral strike-slip shear. This phase is followed lately by D2 phase, characterized by extensional movements. We can observe two granitoid suites in this region: the Galiléia and the Urucum suites. The ascension and emplacement of these rocks are associated to the regional deformational phase D1. Within the Urucum suite leucogranites, this phase conditioned the trajectories of the magmatic flux foliation and lineation. Despite the lack of structural data in others regions of the Galiléia suite batholite, the obtained data in the studied region show the importance of the D1 deformational phase during the Galiléia batholite evolution .The Galiléia suite granitoids (594 ±6 Ma, zircon U-Pb age) are made of tonalites, granodiorites, granites and micro granites. These rocks frequently include magmatic enclaves. These granitoids are metaluminous to weakly peraluminous. They show calc-alcaline affinity and they were formed under 10 kbar (30-35 km) and 850°C to 700°C. The mineralogical and geochemical data indicate an evolution from tonalite to microgranite. The Ti, Mg, Al, Fe, Ca, Mn, P, Co, Sc, V, Zn and Ni had a compatible behaviour. In the other way, Si, Na and K were incompatibles elements during the mainly evolutive history of these rocks. The mechanical mixing (mingling) and chemical reequilibrages processes observed between the enclaves and their host rocks, and the presence of two zircon populations show the hybrid character of this suite. Isotopic Rb-Sr and Sm-Nd studies indicates an origin from crustal melting rocks (lower crust) for the granitoids and their enclaves and confirm the reequilibrage between enclaves and host rocks. The Urucum suite leucogranites (582 ± 2 Ma; zircon V-Pb age) are represented by potassic megafeldspar granites (Urucum facies), medium to coarse deformed granites (palmital facies), tourmaline granites and pegmatite granites. These rocks are peraluminous and their composition nears the S type granites. They crystallized about 750-600°C and 4 kbar (12 km), within a fluid rich environment. The mineralogical and chemical studies suggest an evolution from the Urucum facies to the pegmatite granites and the compatible comportment of Mg, Ti, Fe, Ba, Zr, Th, Co, V, Zn, Ga, Ni, Cr and rare earth elements. The origin of this suite from the fusion of crustal rocks is confirmed by the isotopic Rb-Sr and Sm-Nd studies, by the zircon composition and typology and by the presence of inherited zircon nucleus. Field observations together with mineralogical and chemical studies show a close relationship between the Urucum suite leucogranite and the rare element pegmatites of the region. The observed pegmatite geometry and kinematics indicate an emplacement controlled by the D1 deformational phase. The magmas that originated the pegmatites are fluid rich (bore). The pegmatites were emplaced about 540°C and 3.2 kbar. Our data favored a genetic and emplacement model supporting a mantle and a crusted melting in a tranpressional tectonic regime associated with the end of the Brasiliano cycle during the Neoproterozoic. Our geochronological data added to others granitoids ones in the Atlantic shear belt indicate that the calc-alcalin magmatism occurred between 620 and 588 Ma and that the collisional crustal peraluminous magmatism occurred between 584 and 570 Ma. The Sm-Nd model ages of the Galiléia and Urucum suites are concentrated between 2300 and 1800 Ma indicating the important volume of the crust formed during the Transamazônico cycle in the genese of these suites.
La région de Conselheiro Pena et Galileia est située dans l'est de l'état de Minas Gerais (sud-est du Brésil). Elle fait partie du nord de la ceinture de cisaillement atlantique et est formée par des gneiss d'âge protérozoique inférieur et par des séquences supracrustales considérées d'âge néoprotérozoique. Deux phases de déformation et un métamorphisme de moyen degré ont affecté la région pendant le cycle Brasiliano. La phase D1 s'associe à des mouvements décrochants dextres. Cette phase a été suivie tardivement par la phase D2, caracterisée par une phase d'extension. La suite Galileia et la suite Urucum ont une ascension et mise en place associées à la déformation D1. Les granitoïdes de la suite Galileia (594±6 Ma) sont constitués par des tonalités, tonalogranodiorites, granodiorites, granites et microgranites. Dans l'ensemble, ces granitoïdes sont métalumineux à peralumineux avec une affinité calco-alcaline. Ils ont eu une origine sous des pressions d'environ 10 kbars et des températures entre 850°C et 700°C. L'étude isotopique indique une origine à partir de la fusion de roches crustales. Les leucogranites de la suite Urucum (582±2 Ma) sont représentés par des granites peralumineux. Elles ont été cristallisées entre 750°C et 600°C et sous des pressions de l'ordre de 4 KBars dans un environnement riche en fluides. L'origine de cette suite à partir de la fusion de roches crustales est indiquée par les études isotopiques, par la composition et typologie des zircons et aussi par la présence commune de curs de zircons hérités. Les observations de terrain et les études minéralogiques et géochimiques montrent qu'il existe une relation étroite entre les leucogranites de la suite Urucum et les pegmatites à éléments rares de la région. Nous pouvons envisager un modèle de genèse évoquant une fusion du manteau et, en suite, de la croûte due à l'épaississement des marges continentales sous un régime tectonique transpressional associé à la fin du cycle Brasiliano.