TitaniQ temperatures and textural analysis as a record of the deformation history in a major continental shear zone system, Borborema Province, Brazil

Abstract

We combine quartz textural analysis with Titanium-in-quartz (TitaniQ) geothermometry to investigate the late stages of deformation in mylonitic rocks from the Sucuru dike swarm in the Borborema Province. The wide range in TitaniQ temperatures (>340 to 740 °C) closely related to grain sizes are attributed to inefficient Ti resetting during bulging (BLG) and subgrain rotation (SGR) recrystallization. The highest temperature values (>500–740 °C) occur in the core of quartz ribbons and porphyroclasts. They are interpreted as the record of early-stage plastic deformation, likely during the intrusion of magmatic bodies, and are progressively overprinted during a lower-temperature stage (>340–500 °C) associated with intense dynamic recrystallization and production of a large amount of fine-grained matrix. The dominantly weak crystallographic preferred orientations (CPOs) can be attributed to a large variety in crystallographic orientations of parent grains, which control to a large extent the CPO evolution of daughter grains, and to activation of multiple slip systems in the <a > direction (basal, rhomb and prism) due to synkinematic temperature decrease. The CPOs are consistent with strain localized by operation of dislocation creep-accommodated grain boundary sliding (DisGBS) during the low-temperature stage. DisGBS seems to be less important in coarse pure quartz domains, where moderate to strong CPOs indicate the dominance of dislocation creep. Several evidence of intracrystalline deformation in quartz ribbons and porphyroclasts point to dominant activation of dislocation creep at the early high-temperature stage of deformation, with activation of prism <c>. The dike orientations, geometry and evidence of solid-state deformation in their margins, fit well with progressive E-W dextral shearing, where they intrude and crystallize in a NW-SE direction and rotate clockwise during shearing. This implies that the dextral shear regime that dominated the Borborema province from ca. 600 Ma was still in effect into the Cambrian some 60–70 million years later.