Contrasting Archean ultramafic igneous bodies in Dundonald and Clergue Townships, Ontario

Abstract

A differentiated sill of ultramafic and gabbroic rocks in Dundonald and Clergue Townships (25 miles northeast of Timmins, Ontario) may be classed as a "stratiform intrusion", whereas several peridotite-pyroxenite lenses in the pillow lavas overlying the sill have characteristics that have not been reported in previously described ultramafic bodies.The sill was intruded as a nearly horizontal, conformable sheet, 4 × 2 miles in extent. It was differentiated as crystals settled from the cooling magma to form a 1000-ft thick basal layer of peridotite overlain by 500 ft of augite pyroxenite, 700 ft of gabbro, and 200 ft of granophyric gabbro. After solidification it was folded and faulted. The sequence of cumulus minerals and the cryptic layering throughout much of the sill are consistent with fractional crystallization models based on analogous experimental data.The lenses are generally stratibound, but are locally transgressive to the volcanic country rocks. The lenses range in surface extent from [Formula: see text] mile to 200 ft × 10 ft. Compositional zoning is parallel to the contacts; pyroxene increases in amount towards both contacts, in some places to the exclusion of olivine. The pyroxene-rich marginal parts and pyroxene-rich bands within the lenses are characterized by skeletal olivine, chromite crystals, and bundles of acicular pyroxene crystals, which indicate crystallization from a supercooled liquid. This conclusion is supported by the discovery of one pyroxene-rich sample that contains a calcic clinobronzite (Wo7.2En79.3Fs13.5) mantled by a sub-calcic augite with a higher percentage of iron than the core. The lenses are thought to have been injected as a mixture of olivine crystals and pyroxene-rich liquid into the volcanic sequence when this was still accumulating. Olivine became concentrated at the center of each lens as a result of flow. Rapid chilling of the marginal liquid occurred at the time of injection, giving rise to the skeletal textures.