Using apatite to resolve the age and protoliths of mid-crustal shear zones: A case study from the Taxaquara Shear Zone, SE Brazil

Abstract

Shear zones accommodate strain and facilitate migration of hydrothermal fluid and magma through the crust. Unravelling the deformation history of shear zones requires correspondence between the closure temperature of mineral geochronometers and the temperature of deformation. Here, we adopt apatite U–Pb-trace element analysis as a tool for dating deformation and tracing the protoliths of mid-crustal shear zones through a case study of the Taxaquara Shear Zone (TSZ), a major transpressional shear zone in the southern Ribeira Belt of SE Brazil. Apatite from mylonites in the TSZ yield U–Pb ages of 558–536 Ma, considering uncertiainties, which slightly overlap with 40Ar/39Ar ages of 538 ± 2 Ma from muscovite in the lower limit. The closure temperature of apatite is estimated at 500–460 °C, which is slightly higher than that estimated for syn-kinematic muscovite (445–420 °C). Apatite from shear zone mylonites has Sr/Y and LREE systematics typical of apatite from S- and I-type granitoids, suggesting the adjacent and undeformed Pilar do Sul and Piedade granites are the likely protoliths of the mylonites. This interpretation is supported by new U–Pb ages of ca. 605 Ma from pre-kinematic zircon and titanite from mylonites, which corresponds closely with new U–Pb apatite ages and previously published U–Pb monazites ages from the Pilar do Sul Granite. We suggest the U–Pb system of apatite in the TSZ was reset via volume diffusion during rapid cooling given that it preserves the igneous geochemical signatures. Moreover, this interpretation is consistent with the lower apatite closure temperature (500–460 °C) relatively to the temperature of deformation (530–480 °C). The revised ~560–535 Ma age for the TSZ demonstrates that it post-dates the collisional phase of the Ribeira Belt (620–595 Ma and 595–565 Ma), indicating protracted strain accommodation during the Brasiliano–Pan African orogeny, and supports correlation with the 600–550 Ma and 570–550 Ma transpressional Dom Feliciano and Kaoko Belts. This study demonstrates that apatite is a powerful tool for unravelling the history of mid-crustal shear zones as it is stable in a wide range of lithotypes, has trace element compositions that are sensitive to the environment of formation, and Pb closure temperatures typical of mid-crust conditions. U–Pb-trace element analysis of apatite provides a robust means to date shear zones that can be complimentary to, or independent of, more traditional 40Ar/39Ar analysis of mica or amphibole.