Petrography, textural variations, and magmatic evolution of subalkaline mafic dykes from the Ponta Grossa Arch, Brazil

Petrografia, variações texturais e evolução magmática de diques máficos subalcalinos do Arco de Ponta Grossa, Brasil

Abstract

: The Ponta Grossa Arch (APG), located in the northern portion of the Paraná Basin, represents one of the main tectonic features associated with the Early Cretaceous magmatism in Brazil. This study presents a detailed petrographic analysis of thin sections from subalkaline mafic dykes, sampled along a section perpendicular to the main 33 dykes trend and grouped according to the three structural alignments of the arch: Guapiara (north), São Jerônimo–Curiúva–Rio Alonzo (central), and Piquiri (south). Four main lithotypes were identified — basalts, dolerites, gabbros, and pegmatitic-textured rocks — characterised by predominant subophitic and intergranular textures, and a mineral assemblage composed mainly of plagioclase, augite, pigeonite, opaque minerals, and apatite. Textural and mineralogical variations among the sectors reflect distinct emplacement and cooling conditions. The northern sector shows evidence of rapid cooling and shallow crystallisation, preserving glass; the central sector is marked by greater magmatic complexity, multiple pulses, and in situ differentiation; whereas the southern sector indicates more homogeneous and deeper crystallisation. These results suggest that the observed petrographic differences are primarily related to crustal depth and post-Cretaceous tectonic evolution of the APG.