The Serra do Bueno potassic diatreme: a possible hypabyssal equivalent of the ultramafic alkaline volcanics in the Late Cretaceous Alto Paranaίba Igneous Province, SE Brazil

Abstract Cretaceous, strongly alkaline mafic igneous provinces occur around the margins of the Ordovician to Cretaceous Paraná sedimentary basin of southern Brazil. The Serra do Bueno diatreme is situated in the southern portion of the largest of these alkaline provinces, the Alto Paranaíba Igneous Province in Minas Gerais. The well-exposed diatreme crops out close to the south-west surface limit of the São Francisco craton and is adjacent to several other poorly exposed ultramafic alkaline pipes, previously described variously as kimberlites (Barbosa, 1991) and lamproites (Ramsay and Tompkins, in press). The diatreme has two distinct facies: (1) a crater facies dominated by lapilli tuffs; and (2) a magmatic hypabyssal facies formed by a relatively fresh ultramafic (MgO = 15 wt.%) potassic (K 2 O/Na 2 O = ∼ 1.5) intrusion that contains xenoliths of meta-sediments, feldspathic gneiss and dunite. It is massive and porphyritic, with large olivine phenocrysts (Fo 87 ) and smaller crystals of diopside (Ca 50 Mg 44 Fe 6 ), phlogopite, perovskite, ilmenite and zeolites in a fine-grained groundmass that contains altered leucite and up to 20% devitrified glass. The Serra do Bueno intrusion shows a strong enrichment in light relative to heavy rare earth elements, with La/Yb of ∼85. Its initial 87 Sr/ 86 Sr (0.705176) and 143 Nd/ 144 Nd (0.512312) isotopic ratios are similar to those of other intrusions (e.g. Limeira 2) and lavas (e.g. Presidente Olegário) in the Alto Paranaíba Igneous Province. This suggests that these Late Cretaceous alkaline magmas were all derived from a similar source, predominantly within the subcontinental lithospheric mantle. Laser 40 Ar/ 39 Ar analyses have yielded an isochron of 90 ± 4 Ma for the Serra do Bueno intrusion. This age is higher than the corresponding K/Ar bulk-rock age for the same sample but similar to K/Ar ages determined on mica separates from both intrusive and extrusive rocks in the Alto Paranaíba Igneous Province.