Dike transport of granitoid magmas

Abstract

Thermal and fluid-dynamical analyses suggest that for viscosities and density contrasts spanning the range considered typical for many calc-alkalic granitoids, dike ascent is a viable mechanism for the transport of large volumes of granitoid melt through the continental crust. We present calculations showing that a granitoid melt with calculated viscosity of the order of 10 6 Pa {dot operator} s and a density contrast between magma and crust of 200 kg/m 3 can be transported 30 km through the crust in ∼1 month, corresponding to a mean ascent velocity of 1 cm/s. Using analysis modified from numerical studies of the flow of basaltic magmas in dikes, we also present an expression that allows the calculation of the critical (minimum) dike or fault width required for granitic magma to ascend without freezing. For all reasonable estimates of Cordilleran granitoid viscosity and density contrast, the critical dike width is determined to be between ∼2 and 7 m. Calculated peak batholith-filling rates are orders of magnitude greater than mean cavity-opening rates based on estimated fault slippage, which is consistent with chemical evidence for intermittent supply of magma pulses. © 1993 Geological Society of America.