Stratigraphy and Petrology of the Mulcahy Lake Layered Gabbro: An Archean Intrusion in the Wabigoon Subprovince, Ontario

The Mulcahy Lake gabbro is an Archean layered intrusion of tholeiitic bulk composition located in the Wabigoon subprovince. The intrusion is 6 km thick at the thickest and is exposed over an area of 63 km2. It intrudes basaltic to siliceous volcanics of the Crow Lake-Savant Lake greenstone belt and is intruded by the Atikwa batholith. Zircon U-Pb data indicate crystallization at 27322+1·0/–0·9 m·y. Principal phases are plagioclase, orthopyroxene, augite (and pigeonite in iron-rich rocks), olivine, hornblende and magnetite. Olivine is confined to several horizons. Apatite and then zircon are prominent accessory phases at advanced stages of fractionation. Plagioclase, pyroxenes and olivine are cumulate phases. Hornblende is invariably an intercumulus phase. Magnetite is ubiquitous throughout the intrusion, generally as a cumulate phase, and forms centimeter thick layers in fractionated rocks. Fractionation followed a tholeiitic trend with iron enrichment in the liquid. The intrusion is divided into lower, mixed, middle, upper and marginal zones. The lower and middle zones are 2·0 and 2·5 km thick respectively. The upper zone is approximately 1 km thick, and the marginal zone is measured in hundreds of meters. A 200 m thick mixed zone is interposed between the lower and middle zones. The base of the lower zone consists of ultramafic units containing olivine of Fo82. The top of the zone has olivine of Fo28. Fractionation of the lower zone, from the floor up, was interrupted by the introduction of pristine liquid which mixed with more dense and cooler residual liquid in the chamber to form the mixed zone. Further introduction of several minor pulses of liquid constructed the lower part of the middle zone. The upper part of the middle zone was constructed from a major pulse of liquid plus several minor pulses each of which is represented by reversals in cryptic layering. The upper zone consists of ultramafic to iron-rich gabbro cumulates formed by cooling through the roof plus horizons formed by influx of pristine liquid. Marginal zone rocks represent cooling through the walls of the intrusion. Rhythmic layering is well developed in lower and middle zone cumulates. Petrofabric data show that orthopyroxene has a lineation in the plane of layering and parallel to structures suggestive of flow. Plagioclase laths also have a preferred orientation in many cumulates and in unlayered gabbros as well. Flow, possibly laminar, of liquid-crystal material is suggested and m