Spinifex and swirling olivines in a komatiite lava lake, Munro Township, Canada

Abstract

Spinifex veins, and veins filled with swirling, tabular olivine grains, intrude the upper part of a 120 m thick lava lake in Munro Township, Ontario, Canada. These veins are 20–200 cm wide, tens of metres long and are oriented roughly parallel to the upper surface of the lava lake. The spinifex texture differs from that found in simple layered komatiite flows: rather than increasing downwards, the size of skeletal olivine grains is greatest in the centres of the veins, or varies irregularly. Upper contacts meander and veinlets penetrate overlying rock; lower contacts are transitional. The other type of vein contains 50–70% of ∼5 mm long hopper olivine grains whose orientation varies widely to produce a swirling, contorted fabric. Both types of vein are believed to have formed when lava from the centre of the lake flowed into fractures in the solidified crust. The lava lake has a dunitic lower portion and an olivine porphyry upper portion. The composition of chill samples, the compositions of olivine phenocrysts, and variations in major element abundances throughout the unit are used to show that the lava originally was komatiite with about 22% MgO. The lava lake is overlain by mafic scorias and breccias with moderate vesicularity and welding structures suggesting eruption in shallow water.