Stratigraphy and petrogenesis of the Parana continental flood basalts, southern Brazil.
The Early Cretaceous (-130 Ma) Parana 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 highand
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 Parana region, the dominant magma type has evolved from Gramado to
Pitangat o Paranapanemaw ith time. The stratigraphicalp attern 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 Parana region, the composition of the Gramado
parental magma prior to AFC is regionally variable (in terms of Ba/Nb, (87Sr/86Sr);S, i02).
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 sourcesw ithin a heterogeneousli thospheric 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 asthenosphericc omponent,c onsistentw ith such material being available during
the advanced stages of lithospheric attenuation and rifting