Feldspar and orthopyroxene piezometers constrained using quartz–feldspar and olivine–orthopyroxene mineral pairs from natural mylonites

Abstract

Differential stress magnitude is a fundamental parameter in the study of geodynamic processes in the continental lithosphere, and is typically estimated in quartz- and olivine-dominated lithologies using recrystallized grain size piezometers. Here we evaluate the piezometric relationships in natural mylonites with mineral pairs of quartz–feldspar and olivine–orthopyroxene, which commonly coexist in deformed crustal and mantle rocks, respectively. To analyze mineral pairs, we measured dynamically recrystallized grain sizes in both phases that deformed under approximately isoviscous conditions. Using the experimentally calibrated piezometers for quartz and olivine, we develop piezometric relationships for feldspar and orthopyroxene. These new feldspar and orthopyroxene piezometers reasonably predict grain size relationships observed for mineral pairs from naturally deformed lithospheric shear zones. Combining our naturally constrained datasets with the existing experimental datasets for feldspar and orthopyroxene, we also derive piezometers that are consistent with the wattmeter model and with observations from other studies of mylonitic rocks. Our results provide an alternative for estimating stress in rocks such as granulites and pyroxenites, for which the quartz and olivine piezometers are unsuitable.