Silicic magmatism in the Paraná-Etendeka Province in Brazil: a progress report on stratigraphy, petrogenesis and eruptive styles

Abstract:

Although a subordinate component of the Paraná-Etendeka (PE) CFB Province (~3% of the volcanic package), silicic volcanics make up an appreciable preserved volume of 15,000 km³, among the largest of such rocks in a CFB. Moreover, they have wide compositional variation, and are positioned at different stratigraphic levels in the lava pile, constituting important markers of timing and magma generation processes in mantle and crust, and bringing information on the composition of the crust hidden beneath the Paraná Basin. Two main types are recognized. The Palmas-type (134.6-134.2 Ma; Ar-Ar dating) occurs at the upper portion of the first PE volcanic cycle dominated by low-Ti basalts, evolving from dacite to rhyolite. High ⁸⁷Sr/⁸⁶Sr(i) (0.713-0.726) and moderately negative εNd(i) (-8) can be modeled by AFC involving contamination of low-Ti basalt with relatively young, non-depleted, high-Rb/Sr crust; a predominant role for a cratonic crust of >2.0 Ga age can be discarded. The younger Chapecó-type (134.3 Ma, baddeleyite U-Pb dating) corresponds to the lower portion of the second PE volcanic cycle, dominated by high-Ti basalts which became progressively more primitive (implying higher melting temperatures in the mantle) over a ~3 Ma time span. Chapecó-type volcanics are exclusively trachydacites, thought to be produced by remelting of high-Ti basalt underplates with minor crustal contamination, as indicated by ⁸⁷Sr/⁸⁶Sr(i) (0.705) and εNd(i) (-5) just slightly more evolved than associated basalts. Detailed structural investigations supported by ASM allow identify that both types of PE silicic rocks, characterized by high-temperature (liquidus T~1000^oC) and low H₂O (1-2 wt%), erupted predominantly as lavas and lava-domes, and more restrictly as rheoignimbrites, from local linear feeder systems as conduits and dikes. Shallow-level evolutive processes seem restricted to contamination with host crystalline rocks in dikes and mixing/mingling in conduits. Evidence for shallow magma chambers is scant; high-silica rhyolites are remarkably absent, and in most cases these magmas seem to have raised directly from sources at lower to middle crust carrying a small phenocryst cargo (An-rich plagioclase, clinopyroxene and apatite) that occasionally shows signs of Sr isotope disequilibrium.

Financing: Fapesp