Stratigraphy and petrogenesis of the Parana continental flood basalts, southern Brazil

Peate, David William (1990). Stratigraphy and petrogenesis of the Parana continental flood basalts, southern Brazil. PhD thesis The Open University.

DOI: https://doi.org/10.21954/ou.ro.0000dfc2

Abstract

The Early Cretaceous (~130 Ma) Paraná lavas of South America represent the largest preserved example of a continental flood basalt (CFB) province, with a present area in excess of 1.2 x 106 km2. Magmatism is dominated by tholeiitic basalts (> 90 %), with minor rhyolites as a late-stage feature along the continental margin. The basaltic magmas show a wide diversity in trace element and isotope composition, and a revised scheme of distinct magma types is proposed, based on new geochemical data plus a review of literature analyses, which retains the previously established overall classification into high and low-Ti varieties; viz. high-Ti (> 2 wt%) - (Urubici, Pitanga, Paranapanema}, low-Ti (< 2 wt%) - (Gramado, Esmeralda, Ribeira).

A preliminary regional picture of the internal north-south structure of the lava pile is presented, constrained largely by analyses of borehole drilicore chippings which have provided a window to the otherwise inaccessible deeper stratigraphical levels. Within this central/north Paraná region, the dominant magma type has evolved from Gramado to Pitangato Paranapanema with time. The stratigraphical pattern of overlapping geochemical units which dip towards the north suggests a northward migrating source for the Parana magmatism, and this was probably related to the rapid northward propagation of initial rifting of the South Atlantic ocean rather than any motion relative to an underlying hot-spot. Detailed stratigraphical investigations on the coastal Serra Geral escarpment (Santa Catarina state, Brazil) showed complex interbedding of Urubici- and Granzado- type flows. Division of the Urubici-type flows into locally correlated sub-units has allowed the effects of polybaric fractionation to be resolved. The Gramado magma type is dominated by AFC (assimilation / fractional crystallisation), and the geochemical variations are consistent with taking an average Palmas rhyolite composition (the best estimate of a local crustal melt) to be the contaminant. Within the southern Paraná region, the composition of the Gramado parental magma prior to AFC is regionally variable (in terms of Ba/Nb, (87Sr/86Sr)i, SiO2).

The Urubici and Gramado magma types can not be related to a single mantle source by varying degrees of partial melting, and require distinct source regions. The 'enriched' isotopic and trace element signatures inferred for uncontaminated Gramado- and Urubicitype magmas are distinct from MORB-OIB and it is suggested that at least the trace elements (and therefore the isotopic characteristics) of the Paranä CFB were largely derived from sources within a heterogeneous lithospheric mantle.

The sharp stratigraphical transition from the lithospheric-dominated Gramado magma type to more 'depleted' trace element and isotope compositions (the minor Esmeralda magma type) can not be accommodated on trace element grounds by a decrease in the amount of crustal assimilation. Instead, these late-stages of magmatism show a significant asthenospheric component, consistent with such material being available during the advanced stages of lithospheric attenuation and rifting.

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