Uso de badeleíta em geocronologia e geologia dos diques entre Ponto Novo e Queimadas, NE da Bahia

Abstract

Mafic and ultramafic rocks have low abundance or even absence of zircon. An alternative used to date these rocks has been badeleite, a zirconium oxide that has similar chemical and physical properties to zircon as its composition and presents good results on geochronological studies on U-Th-Pb system. The classic method has low efficiency on baddeleyite, to avoid this problem, a new concentrating method was developed on 2002 but was not used in Brazil until then. This paper presents in detail a variation of this methodology, now in routine, that uses low sample volumes and is effective to concentration both baddeleyite and zircon micro crystals. This methodology was used on samples of a dike swarm freshly knew and a gabbro rich in baddeleyite was dated in 911 ± 27 Ma. This swarm is associated to a large regional segmented magnetic line and it extends between the cities of Senhor do Bonfim and Salvador in San Francisco Craton. This study comprehends the dyke swarm mapping and its petrographic characterization between the cities of Ponto Novo and Queimadas in an area of 50 km x 60 km. Geophysical data and multispectral satellite images were used to mapping the dyke swarm. The petrographic study revealed three rock groups: syenites, monzogabbros and gabbros. Syenites are porfiritic formed by albite phenocrystals immersed in a matrix of amphibole / clinopyroxene, potassium feldspar, quartz, ilmenite, biotite and ± apatite. The monzogabbros are formed by labradorite, augite, potassium feldspar, ilmenite ± apatite. The gabbros are formed by labradorite, augite, ilmenite, ± apatite, ± pyrite and ± orthopyroxene. The gabbro main accessory minerals are baddeleyite, zircon and zirconolite. Mineral composition of these rocks is suggestive of a common source on different degrees of fractionation. Similar age and mineral composition is found between these gabbros and the one from Coastal Province in Salvador. It corroborates the hypothesis of extension of Coastal Province to the CSF.