Tourmalinites of the Brusque Group in the São João Batista-Tijucas area, State of Santa Catarina, Brazil

Abstract

The Dom Feliciano Belt, in the State of Santa Catarina, is represented by the Itajaí and Brusque groups and the Florianópolis Batholith. The basal Rio do Oliveira Formation of the Brusque Group consists of metabasic/calc-silicate, metavolcanic-exhalative, metapelitic and metapsammitic units. A discontinuous tourmalinite sequence integrating the metavolcanic-exhalative unit stretches out from São João Batista to Tijucas. Two types of tourmalinites were distinguished and named Rio do Oliveira and Morro do Carneiro. The first is very fine-grained, banded, and composed of greenish-yellow or brown tourmaline. The second is coarser-grained, massive to slightly foliated, and composed of color-zoned tourmaline. Microprobe and laser ablation-ICPMS analyses showed that the Rio do Oliveira tourmaline is richer in Al, alkalis, and poorer in Ti, Mg, Fe and Ca when compared to the Morro do Carneiro tourmaline. Rio do Oliveira tourmalinites were formed by selective substitution of pelitic and psammitic sediments by percolation of B-rich fluids through the volcanic-sedimentary unit during diagenesis and metamorphism. Morro do Carneiro tourmaline has detrital cores and rims enriched in Mg, Fe, Ti, Mn, Sr, Co and Ca, when compared to the Rio do Oliveira tourmaline. An intermediate dark zone is Ti- and Fe-rich. The abrupt increase in Ca from core to rim may have resulted from the equilibrium with the surrounding Ca-rich environment (underlying calc-silicate rocks). Even if the REE patterns obtained for Rio do Oliveira and Morro do Carneiro tourmalines are practically identical, it is proposed that the fluids that generated the Morro do Carneiro tourmalinites have also been metasomatic/igneous.