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

Cretaceous, strongly alkaline mafic igneous provinces occur around the margins of the Ordovician to Cretaceous Parana sedimentary basin of southern Brazil. The Serra do Bueno diatreme is situated in the southern portion of the largest of these alkaline provinces. 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 potassic intrusion that contains xenoliths of meta-sediments, feldspathic gneiss and dunite. Laser ⁴⁰Ar/³⁹Ar 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 Paranaiba Igneous Province. -Authors