Archean Ultramafic Flows in Munro Township, Ontario

A unique exposure within the Abitibi orogenic belt of northeastern Ontario provides an exceptionally clear view of early Precambrian (Archean) ultramafic volcanic rocks. Approximately 60 ultramafic flow units are exposed over a stratigraphic thickness of 125 m. Individual flow units range in thickness from 0.5 to 15 m, average about 3 m, and display remarkable strike lengths and internal structure, which suggest extreme fluidity.

The upper margin of the flow units (0.1 to 1.0 m) is thicker than the lower margin (about 1 cm), and the upper margin is highly fractured, thereby providing an easily recognizable top determinator. Many flow units contain a zone of spinifex, ranging in width from zero to 5 m, which underlies and is gradational with the flow top. Spinifex is a textural term to describe an array of criss-crossing sheafs or booklets characterized by numerous closely spaced and parallel blade- or plate-like crystals of olivine. The texture is generally accepted as having formed by rapid cooling of a crystal-free ultramafic liquid in situ. The variation in size of bladed olivine crystals within a spinifex zone is always from coarse (up to 1 m long) at the bottom to fine at the top, thereby providing a primary top determination. In flow units not containing a spinifex zone the chilled and fractured flow top grades downward into a zone of moderately foliated peridotite which generally constitutes the remaining part of a flow unit.

Chemically, the flow units resemble similar ultramafic rocks described from northeastern Ontario, western Australia, and the Barberton area of South Africa; one unique feature is the exceptionally high alumina content of the clinopyroxene