Constraining the depositional history of the Maricá group through U–Pb zircon geochronology of tuff, Camaquã Basin, Dom Feliciano Belt, Brazil

Abstract

The Maricá Group, located in the Dom Feliciano Belt in southern Brazil, represents the basal unit of the Camaquã Basin; however, its depositional chronology remains a subject of ongoing debate. In this context, a thin layer of intermediate tuff from São Rafael Formation, exposed in the Passo do Salsinho region, was identified and analyzed to constrain the timing of sedimentation. Field observations, petrographic analysis, and X-ray diffraction (XRD) data indicate a thin intermediate tuff layer of andesitic composition, composed predominantly of plagioclase, and K-feldspar, subordinately quartz, embedded in a matrix of devitrified glass now transformed into illite. In situ U–Pb zircon dating by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) yielded a crystallization/deposition age of 618 ± 4 Ma. Inherited zircon grains with ages ranging from 2.9 to 1.7 Ga were also identified and interpreted as xenocrystal components assimilated during magma ascent, likely derived from deep crustal sources of Archean to Paleoproterozoic age. This new geochronological evidence refines the temporal framework of the Maricá Group, suggesting that volcanism in the region may have provided the material interbedded within the sedimentary succession. A potential source area for this tuff may be associated with the Cordilleran-type arc that evolved between 630 and 605 Ma, linked to the closure of the Adamastor Ocean, specifically the Pinheiro Machado Magmatic Arc.