Structures, petrology and geochemistry of Florianópolis Dike Swarm, Paraná-Etendeka Magmatic Province, in the northern part of Santa Catarina Island

Abstract:

The eocretaceous (~134-130 Ma) Florianópolis Dike Swarm (FDS) in south Brazil exposes a variety of dominantly basaltic dikes interpreted to be feeders of volcanic sequences of Paraná-Etnedeka Magmatic Province (PEMP). This study aims to identify the variety of rocks and their temporal relationships connecting structural, petrographic, geochemical and geochronological work in FDS dikes from the northern portion of the Santa Catarina Island.

The predominant basalt dikes are mostly high-Ti (>3%), Sr (~640-780 ppm) and P₂O₅ (0.5-0.6%), and equivalent to the Urubici (-Khumib) magma-type of the PEMP. Varieties with lower Sr (~440-550 ppm) and P₂O₅ (~0.4%) are subordinate, and tend to be isotopically slightly more primitive (e.g., lower ⁸⁷Sr/⁸⁶Sr(t), 0.704-0.705; less negative εNd(t), -1 to -2).

Field and petrographic evidence of assimilation and mixing with products of local melting of host highly evolved Neoproterozoic granites is observed in some dikes, especially those of intermediate composition (mostly trachydacites) which show high ⁸⁷Sr/⁸⁶Sr(t) (0.709) and more negative εNd(t) (-4.0). AFC quantitative modelling involving a high-Sr-Ti-P basalt magma and an evolved high silica, high Rb/Sr granite provides a reference for the generation of felsic volcanics in the volcanic pile.

The FDS dykes may be up to 70 m thick, and are mostly oriented in a NNE-trend, but a subordinate set of younger, much thinner (often <2 m thick) NW dikes is present. Their compositions may be similar to those of basalts from the main trend, but a peculiar set of low-Ti, low-Sr dikes occurs exclusively with this direction, and may represent a distinct, younger event whose age is as yet unconstrained.