Abstract
This paper reviews general aspects of alkaline-carbonatitic rocks of Brazilian, Paraguayan and Bolivian terrains. Although 30 such occurrences are known in literature, only the major ones have been thoroughly investigated. The carbonatites are of Cretaceous age, with two well-defined Lower Cretaceous and Upper Cretaceous generation episodes. A clear tectonic control by ancient structural features such as archs, lineaments and faults characterizes most cases. The rocks exhibit a large compositional variation, in decreasing orders of abundance from calciocarbonatites to magnesiocarbonatites to ferrocarbonatites. In some complexes, they form multistage intrusions. C-O isotopes indicate that, in general, the carbonatites were affected by post-magmatic processes associated with the topographic level of emplacement and low-temperature H 2 O-CO 2 rich fluids responsible for the increased amount of heavy carbon and oxygen. Sr-Nd isotopic compositions similar to those of coeval alkaline silicate rocks, ranging from depleted to enriched mantle sources, have been influenced by two distinct metasomatic events in Proterozoic at 2,0-1.4 Ga and 1.0-0.5 Ga. Sr-Nd-Pb-Os data seem related to an isotopically enriched source, their chemical heterogeneities reflecting a depleted mantle that was metasomatized by small-volume melts and by fluids rich in incompatible elements. Fractional crystallization and liquid immiscibility are believed to be the most effective processes in the formation of the Cretaceous carbonatites, with minor contribution of crustal contamination. Pb isotopic ratios yield evidence that HIMU and EM I mantle components played an important role in the genesis of the carbonatitic magma.
Highlights
A few review papers dealing with Cretaceous carbonatites of the southeastern Brazilian Platform are known in literature
Petrogenetic studies performed on a selected number of prominent Cretaceous complexes, as exemplified by Barra do Itapirapuã (Ruberti et al 2002), Ipanema (Guarino et al 2011), Juquiá (Beccaluva et al 1992, Azzone et al 2013) and Lages (Traversa et al 1996) in Brazil and by Cerro Chiriguelo (Castorina et al 1996, 1997) and Cerro Sarambí (Gomes et al 2011b) in Paraguay indicate that processes of fractional crystallization and liquid immiscibility from parental alkaline mafic magmas are the main responsible for the generation of carbonatitic liquids, as suggested by field relationships and geochemical characteristics
In the southeastern Brazilian Platform, Lower Cretaceous and Upper Cretaceous episodes of alkaline-carbonatite magmatism took place along tectonic lineaments genetically related to regional structural features like the Ponta Grossa Arch and the Alto Paranaíba Uplift in Brazil and the Ponta Porã Arch in Paraguay
Summary
A few review papers dealing with Cretaceous carbonatites of the southeastern Brazilian Platform are known in literature. Considering the occurrences reviewed in the present study, this is the most abundant association, being represented mainly by the Lower Cretaceous complexes of the Ribeira Valley (Ipanema, Itapirapuã, Jacupiranga, Juquiá), Santa Catarina (Anitápolis) and Paraguay (Cerro Sarambí) and the Upper Cretaceous intrusions of Mato Preto, in the Ribeira Valley and, apparently, Caiapó and Morro do Engenho in Goiás.
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