MODELING STRAIN RATES AROUND RAPIDLY GROWING PLUTONS

Recently, it has been suggested that felsic magmas commonly ascend to their emplacement level very rapidly in dikes with only a few volume percent of crystals, and then spend perhaps 10,000 to 1,000,000 years crystallizing into plutons. To avoid pre-mature freezing, felsic dikes must have minimum widths and flow rates. Based on published calculations, we consider critical widths and flow rates of 3-16 m and 0.003-8.0 m•s^-1, respectively, and a dike of 1 km in plan length. Considering end-member combinations of these rates and dimensions, we calculate pluton filling rates between 0.3 and 4000 km³•a^-1.

Although a number of observations and theoretical predictions appear to be consistent with this suggestion, the structural and metamorphic ramifications for the wall rocks have not been fully explored. We have constructed several different mathematical/geometrical models to determine strain rates in wall rocks surrounding rapidly growing dike-fed plutons. The calculated filling rates lead to extremely rapid strain rates, regardless of which model approach we use, with specific rates depending on the pluton size, shape, and stage in the filling history. Because of the non-linear relation between radius and volume, these strain rates vary exponentially with time during growth. However, considering central map-view sections through plutons, strain rates on the order of 10^-6•s^-1 to 10^-7•s^-1 might be expected near their margins through much of their growth history, and rates will be fast in general across the entire aureole. Rates are particularly fast for relatively small plutons (e.g. R=5 km), and (in central map-view sections) those with a sheet-like or spheriodal shape.

Can we find evidence for such rapid rates within or around exposed plutons? Are such rapid rates reasonable, given that many middle- to upper-crustal plutons are surrounded by well developed ductile deformation aureoles? Given that emplacement-related deformation in the aureole would easily outpace the thermal flux, should aureoles around post-tectonic plutons appear to have undergone "static" contact metamorphism? How do the model results change when we incorporate stoping? This presentation will explore some of these issues.