Magmatic and solid-state flow of a 3.3 Ga rhyolite from the Gavião Block, São Francisco Craton, Brazil

Abstract

The distinction between magmatic and solid-state flow can be complex in some rocks, especially rhyolites, which tend to deform differently than other quartz-feldspathic rocks. Furthermore, this type of study in Paleoarchean rhyolites is extremely rare. In this sense, this work presents field data, microstructural analysis, and electron-backscatter diffraction (EBSD) analyses of quartz porphyroclasts and aggregates from the 3.3 Ga Contendas Rhyolite in the Gavião Block, northeastern Brazil. The Contendas Rhyolite presents the superposition of magmatic and solid-state processes. It displays preserved igneous characteristics (such as euhedral feldspar phenocrysts, feldspars with rapakivi microstructure, and quartz porphyroclasts with primary embayments) coexisting with solid-state deformation microstructures (recrystallized tails, recrystallized elongated aggregates of quartz, and deformation twins). There is evidence that the magmatic flow occurred in the N-S direction, defined by the N-S alignment of feldspar phenocrysts and quartz porphyroclasts, and by the N-S orientation of the [c] axes of the parent grains of the quartz porphyroclasts. On the other hand, quartz porphyroclasts were deformed by dislocation creep, with dominance of the prism \(\left\langle a\right\rangle\) slip, and recrystallize by subgrain-rotation. The approximately N-S trending tectonic foliation and up-dip mineral stretching lineation of the Contendas Rhyolite indicate that solid-state deformation occurred in the E-W direction, which favored the preservation of the magmatic fabric. Moreover, the microstructural and textural characteristics of the Contendas Rhyolite define medium-grade solid-state deformation, with a temporal break between magmatic and solid-state flows.