Morphology and chemistry of early precambrian metabasalt flows, Utik Lake, Manitoba

Abstract

A 400m thick, sequence of Early Precambrian, tholeiitic metabasalt flows consisting of interlayered massive and pillowed units was mapped in detail at Utik Lake, Manitoba. Metamorphic grade is low to middle greenschist facies but most flows have preserved primary textures. A typical flow is composed of randomly oriented plagioclase laths (An55) in a matrix of actinolite and minor iron-titanium oxide grains. The flows are aphyric, except for sparse pseudomorphosed pyroxene grains near the base of the thickest massive flow, and plagioclase microporphyritic textures in the lower parts of many massive flows. The central parts of other massive flows locally contain plagioclase laths with belt-buckle and swallow-tail morphologies suggestive of rapid cooling from a crystal-poor residual liquid in the flow. No chemical variations were observed across flows or between flows... Some massive flows have strongly altered flow-top breccias. The alteration involved calcium addition, and is characterised by the presence of diopside, minor epidote and clinozoisite in interfragmental areas and replacing original material. This alteration, and other alteration present as fracture fillings and clots in some pillowed flows, may represent reaction with ocean water. Most massive and pillowed flows extend across the entire area, a distance of 500m. Other flows are less extensive and terminate within the map area. In one observed flow termination, a massive flow abruptly changed into an interconnected pillowed flow unit. In some massive flows with brecciated tops, the breccia is absent near the inferred termination of the flow.