MASSIVE, RAPID MELTING OF THE CRUST AND THE GENERATION OF DISEQUILIBRIUM AND EQUILIBRIUM MELTING FACIES, MCMURDO DRY VALLEYS, ANTARCTICA

Rapid heating of igneous systems from cold storage to mobilized melt does not necessarily follow intuitive notions of melting. Due to the sluggish nature of chemical diffusion through felsic melts, it is possible that rapid melting may result in disequilibrium melting. Consequently, the liquid composition leaves the melting minimum composition in a path controlled by kinetics rather than equilibria. We believe this kinetically controlled phenomenon occurred in the McMurdo Dry Valleys of Antarctica, at the contact between the Basement Sill and Orestes Granite. Here, a relatively thick dolerite sill (~450m) induced a melting front into the overlying Orestes Granite, proceeding for ~100 years, and generating appreciable amounts of melt in a region 10’s of meters from the contact. Along the contact, the temperature increased nearly instantaneously and was well into the melting range of the Orestes Granite. With distance from the contact, heating rates decreased. The melt zone can be divided into two melting facies on the basis of melting style. Distal from the contact, the melt composition is near that of the minimum composition. This is due to relatively slow heating, a melt saturated with water, and a thin melt boundary between solids, such that transportation of reactants via diffusion could readily occur, the melt composition is near that of the minimum composition. However, proximal to the contact, where heating was rapid, the melt undersaturated in water, and diffusive distance of reactants relatively large, melt compositions significantly deviate from those expected from equilibrium models. Specifically, these disequilibrium melts are enriched in Or component, much more than models of equilibrium melting would suggest. Within the disequilibrium melting facies, disequilbrium textures appear, including the development of cellular textured plagioclase and the well-known rapakivi texture. This spatially preserved melt zone provides critical insights of rapid melting in the shallow crust.