APPARENT MECHANICAL HARDENING OF HIGHLY STRAINED QUARTZITE IN THE INNER AUREOLE OF THE PAPOOSE FLAT PLUTON, EASTERN CALIFORNIA

The Papoose Flat pluton of the White-Inyo Range (eastern California) is a late Cretaceous quartz monzonite laccolith that forcefully intruded into Cambrian metasedimentary rocks at a depth of 12‒15 km. Magma emplacement led to penetrative deformation and vertical translation of the pluton margin, resulting in ~90% attenuation of stratigraphic units around the western pluton margin and heating of the inner aureole (150 m wide) rocks to 500‒550 °C. Conductive heating models suggests emplacement and cooling of the pluton did not exceed 30 kyrs, indicating strain rates on the order of 10^-12 s^-1 for the western part of the pluton’s aureole.

Here we present average grain size data of dynamically recrystallized quartz preserved in the relatively pure quartzite from the inner aureole to estimate differential stress and strain rates associated with the forceful emplacement of the Papoose Flat pluton. Quartz grain sizes range from 98‒225 µm, corresponding to differential stresses of 6.7‒12.9 MPa, with the majority of grain sizes being 119 ± 16 µm (15.1 +1.9/-1.5 MPa). Using the quartz flow law parameterization of Hirth et al. (2001), strain rate estimates for the western margin of the Papoose Flat aureole are 10^-13‒10^-15 s^-1 — 1 to 3 orders of magnitude slower than previous estimates based on stratigraphic attenuation and conductive heat modeling. If both analytical methods are valid, this suggests either an apparent mechanical hardening of the quartzites during later stages of deformation (possibly due to the presence of carbonic fluids), or strain partitioning between mechanically stronger (quartzite) and weaker (shales, limestones) layers in the pluton’s aureole. The presence of CO₂ in the fluid phase results in a decreased water activity and therefore fugacity, acting as a non-wetting agent that hardens quartz. However, even significant fractions of CO₂ would only nominally adjust the calculated strain rates, and would further enhance the offset from field-based measurements by making the quartzite more viscous. Therefore, unless the effects of CO₂ on quartzite rheology extends beyond perturbing water fugacity, it is likely the offset is the result of strain partitioning to weaker rock types within the aureole. This study provides an evaluation of quartz flow laws when applied to natural samples with relatively simple P-T-D histories.