THERMAL AND RHEOLOGICAL CONSTRAINTS ON LOWER CRUSTAL MELTING AND MIXING: A STOCHASTIC EVALUATION OF BASALTIC INTRUSION BY PROGRESSIVE DIKING

Observations of sparse mid-and-lower crustal outcrops illuminates the potential for the injection of basalt in the form of dikes overlapping in space and time, as a mechanism of generating lower crustal melts and compositional diversity. A two-dimensional thermal and fluid-flow model was developed to consider the amount of crustal melt produced, and the dynamics of interaction of basalt and crustal melt for an amphibolitic lower crust. Dike orientation and periodicity are stochastic in order to avoid ad hoc assumptions about dike intrusion size and timing. The compilation of numerous model realizations can then be combined to develop a statistical measure of the likelihood of different melt and rheological regimes.

Basalt flux consistent with estimates from island arcs (~.001 m³/m²yr ) produces subsolidus mingling as the initially buoyant basalt melt solidifies and becomes denser than the country-rock. Crustal melt fraction is locally up to 0.2 with this flux, and rarely exceed the critical melt fraction, i.e. the crystalline framework remains intact. The small melt fraction of lower crustal material, and the quenching of basalt in contact with the country-rock inhibits local magma mixing. Basalt flux into the lower crust exceeding .01 m³/m²yr produces some regions with melt fractions greater than the critical melt fraction, although mixing is still largely inhibited by the quenched basalt rinds. Basalt flux greater than .1 m³/m²yr is required to induce a mixing scenario where local regions of crustal melting exceed the strength of the crust and can lead to mechanical collapse of the network of dikes.

Isolation of lower crustal melts from mantle basalts is consistent with both field examples and recent observations of elevated radiogenic osmium and low overall osmium from regions of the Cascade arc. Mixing of even small amounts of mantle basalt would dilute this signature.