Ti distribution in quartz across a heterogeneous shear zone within a granodiorite: The effect of deformation mechanism and strain on Ti resetting

Abstract

The study of a heterogeneous ductile shear zone that developed at ~500°C and 0.2GPa during post-magmatic cooling of a granodiorite has allowed the effect of strain and recrystallization on Ti re-equilibration of quartz to be assessed. Understanding this effect is critical for applying Ti-in-quartz thermobarometry to mylonites. Differently strained quartz across the shear zone shows a heterogeneous distribution of Ti concentrations ([Ti]) (measured by Secondary Ion Mass Spectrometry, SIMS) ranging between 2 and 45ppm. Quartz cathodoluminescence (CL) is proven by spectral analysis to be correlated with [Ti], allowing CL images to be calibrated as Ti maps using SIMS measurements. Coarse-grained weakly deformed domains consist of magmatic quartz extensively recrystallized by grain boundary migration (GBM) and mostly (65–75% area) contain 20–38ppm Ti. Resetting to lower [Ti] occurred locally: (i) in haloes surrounding titanite and biotite inclusions ([Ti] as low as 6ppm); (ii) along grain boundaries; and (iii) towards the interface of quartz domains with other mineral domains. With increasing strain, quartz underwent progressive grain size reduction and developed a bimodal microstructure with elongate grains (>100's μm long) surrounded by mantles of new grains (10–30μm in size) recrystallized by subgrain rotation (SGR). Dynamic recrystallization by SGR, associated with prism<a>slip, became increasingly dominant over GBM as strain increased towards the shear zone core. Significant resetting of Ti in quartz only occurred in high strain domains (at shear strain γ probably >>10) in the shear zone core where fine recrystallization amounts to 50–60% by area and coarser cores are strongly sub-structured. These domains are not compositionally homogeneous and still show a range of [Ti] mostly between 2 and 10ppm. In all strain facies of the shear zone quartz-filled pressure shadows associated with feldspar show an almost constant [Ti] of ~2ppm. The pristine Ti content of the magmatic quartz mylonitized in the shear zone core is therefore significantly reset and converges “asymptotically” towards the “equilibrium” 2ppm [Ti] shown by new quartz precipitated in pressure shadows. It is inferred that extensive recrystallization by SGR and repeated cycles of dislocation creep and rearrangement provided fluid access to quartz grain interiors, promoting chemical buffering and leading to partial re-equilibration to low [Ti]. These observations imply limitations on the use of the Ti-in-quartz thermobarometry to constrain ambient conditions of ductile deformation.