The bimodal basalt-rhyolite suites of the Paranà CFB province: a reappraisal of the petrogenesis and tectonomagmatic significance

The Paranà Continental Flood Basalt (CFB) Large Igneous Province (LIP) of eastern south-America mainly consists of voluminous High-Ti (HT) and low-Ti (LT) basaltic suites erupted as precursors of the continental break up and opening of the South Atlantic Ocean in Lower Cretaceous. A bimodal rhyolite-basalt association, concentrated toward the South Atlantic margins, is observed with rhyolites generally laying at the top of the CFB sequence. Rhyolitic rocks are classically subdivided into predominant low-Ti (LT, dacite-rhyolite Palmas-type) and subordinate high-Ti (HT, trachydacite Chapeco-type) groups, which share the same geochemical features (Ti and other incompatible element) with the underlying basalts. The most popular petrogenetic interpretations for the Paranà rhyolites s.l. deal with: 1) re-melting of underplated CFB (Bellieni et al., 1986); 2) a more complex model with LT rhyolites generated by shallow level assimilation-fractional crystallization processes from LT basalts, whereas for HT rhyolites a genesis by partial melting of underplated HT basalts is preferred (Garland et al., 1995). A critical re-examination of the available data leads us to infer that both LT and HT rhyolitic rocks could be generated by fractional crystallization processes from the respective underlying basalts. Fractionation modes for the two types appear, however, distinctly different: LT dacite-rhyolites could be generated in shallow magma chambers where they underwent significant crustal assimilationù -as indicated by their isotopic signatures- and were erupted mainly as extensive rheoignimbrites and thick lavas (up to 600 m total thickness); HT trachydacites do not show, instead, evidence of crustal assimilation (the isotopic features perfectly overlap those of HT basalts) and their generation is compatible with continuous fractional crystallization during rise through crustal fissures, as indicated by their high degree of porphyricity and emplacement mainly as lavas and dykes. These differences in fractionation styles between LT and HT silicic magmas could reside in the comparatively lower silica saturation and higher temperatures of the HT suite, resulting in a lower viscosity thus preventing significant ponding at crustal levels. The generation of rhyolitic magmas by fractional crystallization from the underlying basalts appears to be a common petrogenetic process in most CFB LIPs, particularly in the Gondwana realm, marking the inversion of the stress regime from regional extension due to lithosphere doming to localized continental rifting as a precursor of oceanization.