Elemental and Sr-Nd-Pb isotope geochemistry of the Florianópolis Dyke Swarm (Paraná Magmatic Province): crustal contamination and mantle source constraints

Abstract

The Florianópolis Dyke Swarm is located in Santa Catarina Island, comprising also the adjacent continental area, and belongs to the Paraná Magmatic Province (PMP). The dyke outcrops in the island are 0.1–70m thick and most of them are coast-parallel (NE-SW trending), with subordinate NW-SE trending. The vast majority of the dykes has SiO2 varying from 50 to 55wt% and relatively high-Ti (TiO2>3wt%) contents and these rocks were divided using the criteria commonly used to distinguish the different magma-types identified in the volcanic rocks from the PMP. The Urubici dykes (Sr>550μg/g) are the most abundant and some of them experienced crustal contamination reaching to 10%, as evidenced by low P2O5/K2O (0.30–0.21), high (Rb/Ba)PM (1.0–2.2), and radiogenic Sr and Pb isotope compositions (87Sr/86Sri up to 0.70716 (back to 125Ma) and 206Pb/204Pbm up to 19.093). The Pitanga (Sr<550μg/g) and the basaltic trachyandesite dykes are less abundant and almost all of them were also substantially affected by at least 15% of crustal assimilation, evidenced by high (Rb/Ba)PM (up to 2.6) and Sr (87Sr/86Sri=0.70737–0.71758) and Pb (206Pb/204Pbm=18.446–19.441) isotope ratios, as well as low P2O5/K2O values (0.30–0.18). The low-Ti (TiO2<2wt%) dykes are scarce and show a large compositional variability (SiO2: 50.4–64.5wt%), with similar geochemical characteristics of the low-Ti volcanic rocks (Gramado-Palmas) from southern PMP, although the most primitive dykes show hybrid characteristics of Ribeira and Esmeralda magmas. The presence of granitic xenoliths with border reactions and dykes with diffuse contacts indicate that crustal contamination probably occurred by assimilation from re-melted the host rocks. Considering only the high-Ti Urubici dykes that were not affected by crustal contamination, the Sr, Nd and Pb isotope mixing modelling indicates the participation of a heterogeneous metasomatized (refertilized) subcontinental lithospheric mantle (SCLM). This mantle source was originated by partial melting of a depleted sublithospheric mantle (DMM – Depleted Mantle MORB), which was hybridized by addition of pyroxenite (<5%) and carbonatite (up to 2%) melts. The isotope mixing modelling also points to a significant participation (up to 50%) of Archean SCLM, not evidenced in the mantle sources of the northern PMP high-Ti Pitanga flows (dominated by Neoproterozoic SCLM).