Magmatic and hydrothermal evolution of mafic pegmatites and their host basalts, Paraná Large Igneous Province, Brazil

Abstract The high-Ti Paranapanema sequence in the Parana Large Igneous Province consists of a succession of pahoehoe lava flows. Some of these flows host mafic segregation structures, such as horizontal pegmatitic sheet layers and vesicular cylinders. Geochronology, whole-rock and mineral chemistry of two pegmatite occurrences and their host basalts permit characterizing crystal-melt segregation processes that occur during magmatic differentiation after lava flow emplacement. The mineralogy of the pegmatites is similar to that of the host basalts, and it includes plagioclase, pyroxene and titanomagnetite-ilmenite crystals; the only observable difference is the coarser grain size of primary phases and an increase in the volume fraction of clay minerals derived from glass devitrification and vesicle infilling in the pegmatites (~15 vol% clay in the pegmatites vs. ~6 vol% clay in the basalts). The pegmatites from both sites are more fractionated than the enclosing lavas and are enriched in TiO2, FeOtotal, K2O, P2O5 and incompatible trace elements (e.g., Ba, Rb, Nb, Hf, Y, Zr, La, Ce, Yb). Thermodynamic modeling of whole-rock compositional data suggests that the Barracao pegmatites were formed after ~8–27 wt% fractional crystallization of the host basalt, whereas the Capanema pegmatites formed after ~38–43 wt% fractional crystallization of the host basalt. 40Ar/39Ar geochronology indicates slight yet significant age differences between plagioclase phenocrysts from the pegmatites and whole-rock fragments from the enclosing lavas (Capanema: 133.56 ± 0.52 pegmatite vs. 134.30 ± 0.46 Ma enclosing lava; Barracao: 133.23 ± 0.88 pegmatite vs. 134.49 ± 0.31 Ma enclosing lava). These age differences likely result from hydrothermal alteration of the plagioclase phenocrysts (e.g., secondary albite and K-feldspar), and they are not due to resolvable differences in crystallization age. Smectite-celadonite aggregates recovered from the Capanema basalt suggest that hydrothermalism occurred at 123.18 ± 0.61 Ma, approximately ten million years after rock crystallization.