J. metamorphic Geol., 2012 doi:10.1111/jmg.12017 Strain rates at high temporal resolution from curved inclusion trails in garnet, Passo del Sole, Central Swiss Alps C. A. BERG,* W. D. CARLSON AND J. N. CONNELLY† 1Department of Geological Sciences, The University of Texas at Austin, Austin, TX 78712, USA (
[email protected]) ABSTRACT Quantitative strain rates at outcrop scale are very difficult to obtain, but they may be estimated from crystals with curved inclusion trails by calculating rotation rates from growth rates and corresponding deflections of the internal foliation. Garnet in a quartzose pelite at Passo del Sole in the central Swiss Alps is extraordinarily valuable for calculation of strain rates during Alpine orogenesis, because the unusual zoning patterns clearly define the kinetics of its nucleation and growth. Complex concentric zoning patterns can be correlated from one crystal to another in a hand sample, based on compositional and microstructural similarities; the ubiquity of these features demonstrates that all garnet crystals nucleated at nearly the same time. Compositional bands whose radial widths are proportional to crystal size provide evidence for growth governed by the kinetics of intergranular diffusion of locally sourced nutrients. Together, these constraints increase the reliability of estimates of rates of garnet growth, and the strain-rate calculations that depend on them. To obtain growth rates, P–T conditions during garnet crystallization were modelled in a series of pseudosections, and compositional evolution was connected to rates of garnet growth by means of an independently determined heating rate. These growth rates, combined with measured amounts of curvature of inclusion trails, indicate that the time-averaged strain rate at Passo del Sole during Alpine metamorphism was on the order of 10)14 s)1.