QUANTITATIVE ESTIMATES OF EMPLACEMENT LEVEL OF POSTMETAMORPHIC MAFIC DYKES AND SUBSEQUENT EROSION MAGNITUDE IN THE SOUTHERN KAPUSKASING UPLIFT

The Early Proterozoic Matachewan (2.454 Ga) and Kapuskasing (2.04 Ga) dyke swarms of the central Superior Province have similar petrographic features, including zoned igneous plagioclase and clinopyroxene and i nterstitial amphibole and quartz. Dyke crystallization conditions can be estimated using equilibria involving amphibole by assuming that ign eous mineral rims reached equilibrium with the interstitial phases. TW Q software using 33 amphibole-bearing equilibria yields temperatures r anging from 552 to 811 degrees C, with an average of 680 degrees C. Th e average of two calibrated pressure-sensitive equilibria (2 diopside + 2 quartz + tschermakite = tremolite + 2 anorthite and diopside + 5 q uartz + pargasite = tremolite + anorthite + albite) ranges from 0.32 t o 0.70 GPa for Matachewan dykes and from 0.30 to 0.57 GPa for Kapuskas ing dykes. Because dyke pressures are lower than metamorphic pressures in the amphibolite- and granulite-facies Archean country rocks, the v alues represent feasible emplacement pressures. The regional patterns of erosion level derived from dyke pressures have similarities to thos e based on other lines of evidence. Across the Kapuskasing uplift, eas terly deepening structural levels are indicated by increasing metamorp hic pressure (similar to 0.4-1.1 GPa). The 0.30-0.70 GPa dyke pressure s reflect similar geometry and constrain the magnitude and timing of u plift. Modest amounts of uplift (0-8 km) prior to emplacement of Matac hewan dykes may have been related to faulting in the Kapuskasing struc tural zone. A similar magnitude (4-7 km) may have occurred in the inte rval between dyking events, but the main uplift (10-17 km) of high-gra de rocks of the Kapuskasing structural zone took place after emplaceme nt of Kapuskasing dykes.