New high-precision 40Ar/39Ar ages for the Serra do Mar alkaline magmatism in the São Sebastião Island, SE Brazil, and implications

Abstract

We present new high-resolution 40 Ar/ 39 Ar ages for alkaline layered gabbros and cross-cutting mafic-ultramafic and felsic dykes from the São Sebastião Island, in the northern sector of the Serra do Mar Alkaline Province, São Paulo, Brazil. Duplicate analysis of single kaersutite crystals yielded plateau ages of 88 ± 1 and 87.9 ± 0.8 Ma for a gabbro and 87 ± 1 and 86.0 ± 0.8 Ma for a picrite dyke. Two biotite aliquots from a trachyte yielded ages of 86.0 ± 0.5 and 86.2 ± 0.5 Ma, while an alkali feldspar concentrate yielded ages of 86.0 ± 0.5 Ma. The similar ages obtained for minerals with contrasting closure temperatures suggest that the results are crystallization ages and that the rocks were emplaced at shallow crustal levels. Our results, along with available high-resolution data, point to a relatively narrow time span (ca. 88-85 ± 0.6 Ma) for the entire alkaline magmatism on the island. Other alkaline occurrences both nearby and in the continent's interior reveal similar ages, reinforcing the hypothesis that mantle decompression and upwelling of heterogeneous mantle sources led to crustal extension and fast ascension of a variety of alkaline magma types in this segment of South America.

The Late Cretaceous alkaline magmatism in the SE Brazilian coast: new paleomagnetic data and age constraints

“…Most dikes are tholeiitic with similar ages to those of the PMP (Deckart et al 1998). The alkaline dikes are of Late Cretaceous to Tertiary ages (Deckart et al 1998, Guedes et al 2005, Giraldo-Arroyave et al 2021. They are mainly related to the alkaline complexes located on São Sebastião Island and neighboring areas (Fig.…”

“…For more than five decades, radiometric ages for this alkaline magmatism have been collected (e.g., Giraldo-Arroyave et al 2021). Despite the significant age collection, there still needs to be more consistency among ages of the same alkaline occurrence.…”