Abstract
The NW-SE elongated Tunas intrusion, occupying an area of 22 km², crops out about 80 km from the city of Curitiba, and it is emplaced into Precambrian metavolcanic-sedimentary rocks of the Acungui Group. Five sub-circular volcanic structures are recognized. Petrographically, the massif mainly consists of syenites and alkali syenites with subordinate alkali gabbros, essexites, syenogabbros and syenodiorites; small late syenitic dykes and magmatic breccias are also found. Previous K-Ar data give a Late Cretaceous mean age of 82.2 Ma for Tunas rocks. In this article we present new U-Pb age determinations on zircon crystals from syenitic rocks (ID-TIMS, 82.7 ± 0.7 Ma; SHRIMP, 84.7 ± 1.2 Ma) which confirm that age, placing the massif into the Late Cretaceous group of alkaline intrusions tectonically associated with the Sao Jeronimo-Curiuva Lineament, a NW-SE-trending branch of the Ponta Grossa Arch in southern Brazil.
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