Exploring the relationships between shear zones and granites: field and microstructural data for contrasting case studies of the Borborema Province (NE Brazil)

Abstract

We explore meso- and microstructural features of granites closely related to strike-slip shear zones in the Borborema Province, NE Brazil. The Riacho do Icó Stock is an en-cornue intrusion aged at ca. 607 Ma. Magmatic fabric is recorded in the core of the granite, whilst progressive deformation is marked by the development of mylonitic fabrics towards the Afogados da Ingazeira shear zone, including foliation and lineation rotation. Early recrystallization of quartz and K-feldspar crystals is widespread in a polygonised fabric, later submitted to higher strain conditions, characterized by the development of lobate subgrain boundaries, heterogeneously deformed lenses, ameboid quartz ribbons, typical of igneous rocks progressively submitted to deformation at deep crustal levels. On the other hand, the Espinho Branco-Santa Luzia leucogranitic belt is hosted along the Patos Lineament, being aged between the ca. 575-565 Ma interval. These rocks show discordant relationships with the host migmatites and the main structural fabric is characterized by a dominant magmatic foliation that is locally overprinted by solid-state flow. Quartz melt pockets and interstitial quartz grains filling fractures in feldspar clasts are widespread. Such characteristics are compatible with granites that were injected in the continental crust along planar anisotropies (i.e., shear zones) in the late-stage partial melting events that originated the migmatites of the area. The case studies are proxies to understand different episodes of magma emplacement along shear zones in this part of West Gondwana.