Metamorphism and Melting at an Exposed Example of the Conrad Discontinuity, Kapuskasing Uplift, Canada

Abstract

The nature of the mid-crustal velocity discontinuity, identified in seismic refraction experiments in widely-spaced probes of the continents, has been debated for many years; recent reflection profiles have added new constraints. In central Ontario, Canada, the thrustbounded Kapuskasing uplift exposes a continuous oblique cross-section of Archean crust to a depth of some 25 km, including a Conrad-like discontinuity, allowing direct observation of the nature and origin of this complex feature. Lithologically, it represents a gradational change from higher level, homogeneous tonalitic gneiss in the amphibolite facies (Wawa gneiss terrane) to a deeper-level, layered heterogeneous sequence in the upper amphibolite and granulite facies (Kapuskasing structural zone). Accompanying lithological changes are increases in density of the order of 0.1 g.cm^-3 and aggregate P-wave velocity of approximately 0.35 km.sec^-1. Migmatitic structures in the high-grade Kapuskasing rocks suggest that partial melting during metamorphism was responsible for production of tonalité, which could have coalesced, risen and ponded above the discontinuity in the Wawa terrane. Other effects that contributed to the gross crustal density stratification include the pre-metamorphic emplacement of more mafic intrusions and anorthosites at depth.