REFINED CALCULATIONS OF STRAIN RATES FROM SPIRAL INCLUSION TRAILS IN GARNET: PASSO DEL SOLE, CENTRAL SWISS ALPS

Strain rates may be estimated from crystals with curved inclusion trails if information is available on both growth rates and rotation rates. The approach pioneered by Biermeier and Stüwe (2003, JMG 21:253) is elegant and powerful, but their analysis relied upon assumptions about nucleation and growth histories for sub-populations of garnet crystals that are difficult to verify. In this study, garnets collected from pelitic schist at Passo del Sole, central Swiss Alps, have proven to be especially advantageous for such calculations of strain rates during the Alpine orogeny, because special features in them eliminate the need for some assumptions that are normally required.

Complex, concentric zoning patterns in garnets from this locality can be correlated among porphyroblasts on the scale of a hand sample, based on both compositional similarities (e.g., intricate yet identical variations in Ca concentration; equivalent Mn concentrations at zone boundaries) and textural similarities (e.g. initiation of curvature of inclusion trails). Thus, each zone serves as a time marker during garnet growth, and any single garnet porphyroblast can represent the growth history of garnet within the sample. In addition, the mechanism governing growth rates for these crystals has been identified as size-proportional, diffusion-controlled growth, providing a further constraint on crystallization kinetics. Together, these constraints greatly increase the reliability of estimates of garnet growth rates, and the strain-rate calculations that depend on them.

Conditions of garnet growth during prograde metamorphism were modeled using THERMOCALC in a series of MnNCKFMASH pseudosections, modified to account for changes in the effective bulk composition during progressive growth of garnet. Assuming a constant heating rate, the time intervals for each stage of garnet growth, and thus the garnet growth rates in each interval, were determined from the intersections of garnet isopleths in successive pseudosections, which combine to produce a P-T-t path. Preliminary strain-rate calculations based on the rates of garnet growth, combined with the rate of rotation of the inclusion trails, indicate that the strain rate at this locality during Alpine metamorphism was on the order of 10^-14 s^-1.