The Temporal Relationship Between Alkaline and Tholeiitic Magmatism in the Paraná-Etendeka Igneous Province: ID-TIMS U-Pb Ages of the José Fernandes Gabbro and Dykes of the Ponta Grossa Arch, Brazil

Abstract:

The Ponta Grossa Arch (PGA) region in S-SE Brazil hosts prominent NW-oriented lineaments with hundreds of tholeiitic dykes of the Ponta Grossa Dyke Swarm (PGDS) and also alkaline intrusions concentrated between the Guapiara and São Jerônimo-Curiúva lineaments. Many of these intrusions lack more robust geochronological data; the alkaline intrusions appear to be both coeval with (e.g. Jacupiranga, Juquiá with ~130 Ma) and much younger than (e.g. Tunas, Cananéia with ~85 Ma) the adjacent Paraná basaltic lavas.

We present in this work the first ID-TIMS U-Pb baddeleyite-zircon ages for diabase dykes of the Guapiara Lineament and for an alkaline intrusion (José Fernandes Gabbro) in order to determine more precisely the space-time relation between alkaline and tholeiitic magmatism in the PGA.

The dated diabase dykes show high TiO₂ (up to 4.5 wt%) and variable Sr (405-890 ppm). Baddeleyite-zircon concordia ages are 130.3 ± 0.5 Ma (all uncertainties reported at 2σ) and 131.3 ± 0.7 Ma, within the range of previously reported step-heating ⁴⁰Ar/³⁹Ar ages (133.1 ± 0.5 to 130.8 ± 0.4 Ma), confirming a good coherency between crystallization and cooling ages.

Three distinct samples of the José Fernandes Gabbro were investigated (melagabbro, banded gabbro and quartz monzogabbro). Preliminary baddeleyite U-Pb ages obtained for a crust-contaminated quartz monzogabbro (52-56% SiO₂; ~4% K₂O; δ¹⁸O= +6.7 to +7.5‰; eNd(T)= -10) yielded a weighted mean ²⁰⁶Pb/²³⁸U data of 133.56 ± 0.31 Ma as the age for the intrusion. The age is about 2 m.y. older than the U-Pb ages obtained for the diabase dykes of the PGA.

FAPESP proc. 2012/06082-6; CNPq 202043/2014-2