A REVIEW OF MIGMATITE DYNAMICS: POSSIBLE IMPLICATIONS FOR ARCHEAN AND PROTEROZOIC CRATONIC REGIONS

Cratonization is an essential process in the growth of continental crust. Many cratons are characterized by migmatite terranes either at exposure level or in the near subsurface. These rocks were once partially molten and as such, were able to accommodate significant lateral and vertical flow owing to their relative low viscosity. Phanerozoic migmatites are commonly exhumed across active and recent orogens and their study informs the conditions of their formation (temperature-depth) and their history of emplacement (geometry, rate). This review summarizes structural, metamorphic, and geochronologic data from Phanerozoic migmatites, tests the physics of migmatite flow using numerical experiments, and offers insight into how migmatites have shaped the continental crust through time.

Phanerozoic migmatites typically contain partially molten metasedimentary, metavolcanic, and older basement rocks. During flow these rocks are mixed in complex ways and are intensely deformed. Migmatites that are exposed in domes cooled rapidly following significant isothermal decompression, such that their metamorphic signature of crystallization (felsic composition) is typically low-pressure (LP) and high-temperature (HT). Recent work has demonstrated that refractory inclusions in these migmatites, such as mafic pods or boudins, were metamorphosed to HP granulites or even eclogite indicating that the migmatites entrained these rocks from near-Moho depth. According to field-based observations and numerical modeling, the low-viscosity, near-Moho rocks are preferentially exhumed to the near surface in domes or large metamorphic complexes, such that the crust is partially overturned. The immediate effect of such dramatic flow and heat transfer is stabilization of the crust (cratonization). The prediction is that migmatites exhumed to the near-surface in a single tectonic event are likely to contain a record of HP metamorphism. This record is preserved only in the most refractory mafic or pelitic inclusions; a systematic study of these inclusions in Archean and Proterozoic migmatite terranes might yield important information on the building and stabilization of cratons.