LOWER CRUSTAL MAGMA GENESIS AND PRESERVATION: A STOCHASTIC FRAMEWORK FOR THE EVALUATION OF BASALT-CRUST INTERACTION

Abstract

We present a quantitative assessment of the thermal and dynamic response of an amphibolitic lower crust to the intrusion of basaltic dike swarms in an arc setting. We consider the effect of variable intrusion geometry, depth of intrusion, and basalt flux on the production, persistence, and interaction of basaltic and crustal melt in a stochastic computational framework. Distinct melting and mixing environments are predicted as a result of the crustal thickness and age of the arc system. Shallow crustal (?30?km) environments and arc settings with low fluxes of mantle-derived basalt are likely repositories of isolated pods of mantle and crustal melts in the lower crust, both converging on dacitic to rhyodacitic composition. These may be preferentially rejuvenated in subsequent intrusive episodes. Mature arc systems with thicker crust (?50?km) produce higher crustal and residual basaltic melt fractions, reaching ?0·4 for geologically reasonable basalt fluxes. The basaltic to basaltic andesite composition of both crustal and mantle melts will facilitate mixing as the network of dikes collapses, and Reynolds numbers reach 10?4–1·0 in the interiors of dikes that have been breached by ascending crustal melts. This may provide one mechanism for melting, assimilation, storage and homogenization (MASH)-like processes. Residual mineral assemblages of crust thickened by repeated intrusion are predicted to be garnet pyroxenitic, which are denser than mantle peridotite and also generate convective instabilities where some of the crustal material is lost to the mantle. This reconciles the thinner than predicted crust in regions that have undergone a large flux of mantle basalt for a prolonged period of time, and helps explain the enrichment of incompatible elements such as K2O, typical of mature arc settings, without the associated mass balance problem.