Metamorphism of the Canadian Shield, Ontario, Canada. I. The Superior Province

Abstract

This paper is an outgrowth of the compilation of metamorphic information for the Canadian Shield in Ontario, and complements the recently compiled Metamorphic Map of the Canadian Shield. The paper contains a summary of the Archean metamorphic history of the Superior Province in Ontario on a subprovince basis, with an emphasis on the limitations of the existing data and the reasoning applied in extrapolating metamorphic boundaries. Little of the early metamorphic history of the Superior Province (pre-2715 Ma) is preserved, but there is local evidence for events at 2870-2850, 2810, and 2730 Ma within some of the older blocks of crust. The present distribution of metamorphic grade and age of metamorphism largely reflects pan-Superior events in the interval 2710-2640 Ma that occurred subsequent to coalescence of a system of island arcs, back-arcs, oceanic plateaus and microcontinents between 2720 and 2690 Ma. The distribution of metamorphic events and facies is the result of three interrelated patterns. 1) There is a relationship between subprovince type, metamorphic grade and age, with granite-greenstone subprovinces generally preserving older greenschist-to lower-amphibolite-facies events, metasedimentary subprovinces preserving younger middle-amphibolite-to granulite-facies events, and high-grade gneiss subprovinces preserving the youngest events. 2) Discrete metamorphic episodes between 2720 and 2640 Ma are associated with periods of major plutonism. 3) There is a pattern of increasing complexity of metamorphic history with increasing metamorphic grade. The present distribution of metamorphic facies in the Superior Province also was influenced by uplift, tilting, and erosion during the Paleoproterozoic. The timing of lode-gold and rare-element-pegmatite mineralization within the Superior Province corresponds closely with metamorphic evolution; it is consistent with models whereby gold-bearing fluids and pegmatite-forming melts develop, in part, as a result of granulite-facies metamorphism of the lower crust.

Shallow impact: Isotopic insights into crustal contributions to the Sudbury impact melt sheet

“…As such it is likely that the gneisses evolved with variable U/Pb ratios prior to ca. 2.6 Ga partial U loss during granulite facies metamorphism (see Easton, 2000). It is not possible to resolve the absolute Pb isotope evolution of these gneisses, although it is evident that at 1850 Ma they would have had Pb isotope ratios similar to the least-radiogenic feldspar data.…”

Post-orogenic thermal evolution of newborn Archean continents

“…Using several review studies (Card, 1990;Moser et al, 1996;Easton, 2000;Percival et al, 2006Percival et al, , 2012Heaman et al, 2011;Biczok et al, 2012, and references therein) we have produced a simplified geological map and a schematic chronology of craton formation in the Superior Province ( Figs. 1 and 2).…”

“…The crustal thickness must be larger than today's value of 40 km because of erosion. At the regional scale, the average metamorphic grade corresponds to pressures of 0.3 GPa (Easton, 2000), corresponding to a paleodepth of about 10 km. We shall therefore show results for 50 km-thick crust with a uniform crustal heat production of 1.5 μW m −3 , corresponding to the present-day value corrected for radioactive decay.…”

Insights on Archean continental geodynamics from gravity modelling of granite–greenstone terranes

“…Large batholiths are round to elliptical in shape and can form domal structures, as in the older terranes, but amphibolite to granulite grade metamorphic country rocks are absent (outside of contact metamorphic aureoles), arguing against the crustal diapirism model. For instance, the supracrustal rocks in the very large Abitibi granite-greenstone belt are everywhere characterized by greenschist to sub-greenschist grade metamorphism, with the exception of narrow contact metamorphic aureoles that are present around some plutons (Easton, 2000). The structural style of the younger belts, especially their continuous linear nature, is more suggestive of accretion at plate margins by plate tectonic processes than of crustal diapirism (Choukroune et al, 1997).…”

“…In the Abitibi Subprovince, the regional metamorphic grade is greenschist to subgreenschist (Easton, 2000). The structures that most strongly influence the regional-scale map pattern are upright, shallowly plunging, doubly-plunging anticlines and synclines with wavelengths of tens of kilometers, and east-west striking shear zones that are continuous for tens to hundreds of kilometers.…”