OROGENESIS, PARTIAL MELTING OF CRUST, AND STUDIES OF MIGMATITES

The core of orogens is typically composed of high-grade metamorphic rocks, containing evidence of melting reactions, and migmatites that represent the solidified remnants of partially molten rocks. Recent geophysical work in Tibet and the Altiplano has uncovered a seismically-slow and low-resistivity lower crust, interpreted in terms of a partially molten layer, confirming the geologic observations. Therefore, partial melting is a key orogenic process. In order to constrain the role of melting during orogenesis, field studies of migmatites ought to concentrate on the geochronologic, metamorphic, and structural records. An excellent example of a migmatite terrane is exposed in the Shuswap metamorphic core complex in the Canadian Cordillera. Thermochronology demonstrated fast cooling of the migmatites from ~800°C to ~300°C between 55 Ma and 49 Ma. Metamorphic petrology has corroborated these results and shown a decompression of ~5 kbar preserved in the metamorphic textures. This decompression must have been rapid; it corresponds to gravitational collapse of the thickened crust by ductile flow accompanied by fast cooling and exhumation. The structural record of migmatites is more problematic because contorted structures that are typical of migmatites on the mesoscale are difficult to unravel. The anisotropy of magnetic susceptibility (AMS), used commonly in granite studies, is emerging as a new structural tool to determine flow directions in migmatites. A pilot study conducted in the Archean Morton Gneiss, Minnesota River Valley, demonstrates the power of AMS. Rock cores spaced ~10 cm, ~10 m, and ~1 km show extremely good consistency in the directions of K1, K2, K3, the principal axes of the magnetic susceptibility ellipsoid, irrespective of the apparent structural complexity observed at a given outcrop. The K1-K2 plane is close to the regional macroscopic layering in the migmatite, and the K1 direction is constant over the sampled region. Furthermore, the combination of AMS (both low and high-field studies) and image analysis of appropriately cut rock faces, has the potential to resolve the kinematics of viscous flow in migmatites. The systematic application of AMS to migmatitic terranes will constrain the role partial melting plays during orogenesis.