Geologia, petrografia e geoquímica do Trondhjemito Mogno e rochas arqueanas associadas, Terreno Granito-Greenstone de Rio Maria - SE do Pará

Abstract

Geological mapping alongside petrographical and geochemical studies in the occurrence area of the Mogno Trondhjemite in the Rio Maria Granite Greenstone Terrain (RMGGT) led to the identification of two distinct TTG facies. The Mogno Trondhjemite designation was maintained for the dominant facies with NW-SE to E-W structural style and distributed over east and west through the mapped area. The association identified on the central-west portion of the area, with NE-SW and N-S dominantly foliation was defined as a new stratigraphic unit called Mariazinha Tonalite. In the mapped area it was also possible to individualize another unit, the Grotão Granodiorite which intrudes Mariazinha Tonalite and it was proposed the redefinition of Parazônia Tonalite which now is to be called Parazônia Quartz-diorite. Geochronological data (Almeida in prep.) revealed that Mogno Trondhjemite and Mariazinha Tonalite are not part of the second generation of TTG rocks from the RMGGT because they yield ages older than 2.87 Ga. Also, the Parazônia Quartz-diorite is similar in age with the sanukitoid association of the RMGGT. Mogno Trondhjemite and Mariazinha Tonalite are mainly composed by quartz and plagioclase with biotite and magmatic epidote as the main ferro-magnesian minerals. Their geochemical characteristics are compatible with Archaean TTG granitoids with high Al2O3 contents, relatively poor in ferromagnesian elements and display ETR patterns with moderate to strong fractioning of ETRP and smooth Eu anomalies. The Grotão Granodiorite is a leucogranite composed by plagioclase, quartz and potassic feldspar with biotite and epidote as the main ferromagnesian minerals. This leucogranite has metaluminous to peraluminous character and has a distinct geochemical behaviour in comparison with RMGGT TTG granitoids and the Xinguara Granite (Leite 2001, Leite et al. 2004). In K-Na4 Ca digrams the Grotão Granodiorite escapes from the trondhjemitic trend as it shows enrichment in K in relation to TTG granitoids, but with minor K contents related to the Xinguara Granite and the biotite-granodiorite studied by Medeiros (1987). Parazônia Quartz-diorite shows compositions varying from quartz-diorite to granodiorite. The Parazônia Quartz-diorite is very similar to the sanukitoid rocks from the RMGGT (Oliveira et al. 2006, 2009), even though the Parazônia Quartz-diorite also displays some remarkable differences in relation to Bannach intermediate rocks, such as minor values of silica and Mg#.