Quantitative estimates of emplacement level of postmetamorphic mafic dykes and subsequent erosion magnitude in the southern Kapuskasing uplift

Abstract

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 interstitial amphibole and quartz. Dyke crystallization conditions can be estimated using equilibria involving amphibole by assuming that igneous mineral rims reached equilibrium with the interstitial phases.TWQ software using 33 amphibole-bearing equilibria yields temperatures ranging from 552 to 811 °C, with an average of 680 °C. The average of two calibrated pressure-sensitive equilibria (2 diopside + 2 quartz + tschermakite = tremolite + 2 anorthite and diopside + 5 quartz + pargasite = tremolite + anorthite + albite) ranges from 0.32 to 0.70 GPa for Matachewan dykes and from 0.30 to 0.57 GPa for Kapuskasing dykes. Because dyke pressures are lower than metamorphic pressures in the amphibolite- and granulite-facies Archean country rocks, the values represent feasible emplacement pressures.The regional patterns of erosion level derived from dyke pressures have similarities to those based on other lines of evidence. Across the Kapuskasing uplift, easterly deepening structural levels are indicated by increasing metamorphic pressure (~0.4–1.1 GPa). The 0.30–0.70 GPa dyke pressures reflect similar geometry and constrain the magnitude and timing of uplift. Modest amounts of uplift (0–8 km) prior to emplacement of Matachewan dykes may have been related to faulting in the Kapuskasing structural zone. A similar magnitude (4–7 km) may have occurred in the interval between dyking events, but the main uplift (10–17 km) of high-grade rocks of the Kapuskasing structural zone took place after emplacement of Kapuskasing dykes.