Mafic Dike Swarms of Quadrilátero Ferrífero and Southern Espinhaço, Minas Gerais, Brazil

Abstract

Mafic dikes are abundant in the Quadrilátero Ferrífero and Southern Espinhaço regions, southeastern State of Minas Gerais. The dikes range from 1,7 Ga to 120 Ma and form at least three swarms that differ in trend, mineralogy, composition and age. The oldest swarnrhas a baddeleyite U/Pb age of 1,7 Ga (Ibirité Gabbro) and K-Ar age of about 1 ,7-1 ,5 Ga (schists), trends north-south, the dikes vary in width from centimeters to meters, and are deformed and metamorphosed to cnlorite-sericke-quartz-plagioclase schist or are some meters wide and contain plagioclase, augite,-ilmenite, quartz and traces of baddeleyite. These dikes are enriched in LREE [(La/Sm)N =3,68-6.39], and depleted to enriched in HREE [(Dy/Yb)N = 0,82-1,29]. These dikes (and sills also) are related to the opening of the Espinhaço Basin. Nprthwest-and northeast-trending mafic dikes have a U/Pb age of about 906 Ma. They are commonly ten meters wide, and are of metagabbro with relict igneous textures and containing altered plagioclase (Ans-52), amphibole, epidote, ilmenite, quartz, and apatite. Samples of this dike swarm are slightly enriched in LREE and HREE with a (La/Sm)N = 1 .32 to 2.2 and (Dy/Yb)N = 1 .36 to 1 .88, respectively, and also show small negative europium anomaly. Sills, stocks, plugs, laccoliths and phaccoliths with compositions similar to dikes are present in the study area. The Sm-Nd data indicate negative ENU (T) (+ 1 .34 and-3.20) and model ages (T DM) of ca. 2.59 and 1 .35 Ga. These dikes may be related the first stages of the Panafrican/Brasiliano event. North-south, east-west, northwest and northeast trending Phanerozoic dikes are of diabase composed of plagioclase (An«)), augite and magnetite. They show moderate LREE (La/Sm)N = 2,53, and slight HREE enrichments, (Dy/Yb)N = 1,47. These dikes have a K/Ar age of about 120 Ma and may be related to the fragmentation of Gondwanaland. The Quadrilátero Ferrífero and Southern Espinhaço dikes are tholeitic and exhibit prominent iron-enrichment. Some trace element distributions are similar to continental rift basalts. They represent significant amounts of mafic melts formed during the beginning of ample extensional tectonic regimes. There are significant variations in incompatible and partially compatible element abundances both within and between dike swarms, which is due not only to heterogeneous mantle sources, but also to crustal contamination. RESUMO ENXAMES DE DIQUES MÁFICOS DO QUADRILÁTERO FERRÍFERO E ESPINHAÇO MERIDIONAL, MINAS GERAIS, BRASIL Enxames de diques máficos são abundantes no Quadrilátero Ferrífero Quadrilátero Ferrífero e Espinhaço Meridional, sudeste de Minas Gerais. Três eventos de magmatismo básico ocorrem nestas regiões, cada qual com feições estruturais, petrográficas, geoquímicas e geocronológicas próprias. O enxame mais antigo tem idade de 1,7-1,5 Ga (U/P badeleíta-Ibirité Gabbro e K-Ar RT-xistos) e ocorre como sills e diques de direção NS, deformados e metamorfisados, ou como diques não deformados com a textura ignea preservada. Os deformados são clorita-sericita...

Magmatism, rifting and sedimentation related to Late Paleoproterozoic mantle plume events of Central and Southeastern Brazil

“…Brito Neves et al, 1979;Pimentel et al, 1991;Dussin, 1994;Silva et al, 1995). Extensive intracontinental rift sedimentary sequences belonging to the basal formations of the Araí Group and southern Espinhaço Supergroup are coeval with this magmatism.…”

Estimativa da espessura elástica efetiva da litosfera do sul do cráton são francisco usando dados da missão Grace

“…E bem conhecido que durante a evolução tectônica daárea em questão, vários episódios de vulcanismo máfico-ultramáfico ocorreram entre o Arqueano e Proterozóico basicamente, sendo bem documentados (Silva et al, 1995;Carneiro et al, 1997;Teixeira et al, 2000). Como conseqüência,é possível que um mecanismo de empobrecimento químico tenha contribuído para uma diminuição paulatina nos valores de densidade do manto litosférico, uma vez que elementos como Fe e Mg foram retirados do mesmo, acarretando no aumento de sua flutuabilidade (Djomani et al, 2001).…”

“…No caso da porção meridional do Cráton São Francisco, o episódio de abertura do Atlântico Sul, responsável tanto pela injeção de material máfico a aproximadamente 120 Ma (Silva et al, 1995) quanto alcalino (Thompson et al, 1998) compatíveis com este cenário de evolução tectônica (Assumpção et al, 2002). Adicionalmente, dados geoquímicos provenientes de xenólitos e geotermas construídas com base em mediadas de fluxo térmico atestam a atuação do mesmo mecanismo nos terrenos cratônicos presentes no sul do continente africano (Bell et al, 2003).…”

Lead isotope constraints on the mantle sources involved in the genesis of Mesozoic high-Ti tholeiite dykes (Urubici type) from the São Francisco Craton (Southern Espinhaço, Brazil)

“…1). These intrusions were interpreted to be related to the huge igneous activity of the PCFB, due to their geochemical similarities (Silva et al 1995;Mazzucchelli et al 2000), as well as by a K-Ar age of 120 Ma for a dyke outcropping in this area (Silva et al 1995). It is important to note that the dykes are not restricted to the Southern Espinhaço, and are also found in the Southern SFC border, nearby its limit with the PCFB, as reported by Pinese et al (1998), who obtained a K-Ar date of 127 ± 3 Ma for a body located nearby Lavras town.…”