UnB - Instituto de Geociências - Produção Científica
RESUMOS/ABSTRACTS
Kamafugite affinity of the Tapira alkaline-carbonatite complex (Minas Gerais, Brazil)
Brod,J.A.
Thompson,R.N.
Gibson,S.A.
Junqueira-Brod,T.C.
Boaventura,G.R.
In: International Geological Congress, 31st., Rio de Janeiro.Abstracts( CDROM).
Keywords: APIP, Carbonatite, Kamafugite, Mantle magmas, Phlogopite-picrite, Bebedourite
Together with similar carbonatite-bearing intrusions (Araxá, Serra Negra, Salitre and Catalão), the Tapira complex forms part of the Late-Cretaceous Alto Paranaíba Igneous Province (APIP), which also include kimberlites, lamproites and large volumes of kamafugites. The complex is dominated by alkaline silicate rocks (bebedourites, subordinately dunites, wehrlites, melilitolites and syenites), intruded by carbonatites and ultramafic fine-grained dykes (phlogopite-picrite, bebedourite). Phlogopite-picrites represent parental magmas, giving rise to the other Tapira rock-types through a complex interplay of fractional crystallisation and liquid immiscibility processes. They are typically composed of olivine phenocrysts set in a groundmass of phlogopite, carbonate, perovskite, apatite and chromite. Flow-differentiation and exsolution of carbonate globules are common. Phlogopite-picrites are peralkaline (molecular K2O+Na2O/Al2O3 = 1.04-3.02) ultrapotassic (K2O = 2.68-5.87 wt.%, K2O/Na2O = 2-15) rocks. The high MgO (8.73-21.34 wt.%), Ni (82-932 ppm) and Cr (134-1012 ppm) contents indicate that they represent primitive, mantle-derived liquids. They are strongly SiO2-udersatured (24.3-33.4 wt.%), Al2O3-poor (1.63-4.69 wt.%) and rich in CO2 (3.29-21.9 wt.%), CaO (9.31-19.41 wt.%) and TiO2 (3.24-5.37 wt.%). Th/Yb, Ta/Yb, Th/Zr, and Nb/Zr ratios of phlogopite-picrites are similar to those of Toro-Ankole kamafugites. Their Sr- and Nd-isotope composition is within the range of other APIP magmas and their multi-element diagram patterns are similar to those of APIP kamafugites. This results establish a strong kamafugitic affinity for Tapira magmas and suggest a link between carbonatitic and kamafugitic magmatism in the remainder of the APIP.