KOMATIITE-PEBBLE CONGLOMERATE
This is the most amazing conglomerate I’ve ever
seen. This is the Timiskaming Conglomerate of Ontario,
Canada. It’s a polymict conglomerate, which means it consists of
pebbles of many different lithologies (rock types). Apart from the
striking colors and shapes, this polymict conglomerate is especially
spectacular for the presence of pebbles of rare rock types. The sample
shown below includes pebbles of the rare ultramafic extrusive igneous rock komatiite.
The komatiite pebbles are easily recognizable based on their spinifex
texture (long, needle-shaped crystals). There’s also has a
small clast of banded
iron formation.
The Timiskaming Conglomerate is a fluvial to alluvial
fan deposit. Its clasts are a wonderful variety of Archean rock types, including
intrusive igneous rocks (diorites, anorthosites), extrusive igneous rocks
(basalts, komatiites, dacites, rhyolites), sedimentary rocks (graywackes,
siltstones, cherts), and other clasts such as vein quartz and even pyrite
pebbles (Gray & Hutchinson, 2001).
The komatiite pebbles shown in the photos below are
consistent with eastern Ontario’s Pyke Hill Komatiite (2.703 to 2.715
billion years).
Unit & Age: Timiskaming Conglomerate, upper Neoarchean,
<2.674 billion years.
Locality:
Kirkland Lake, northeastern Timiskaming District, eastern Ontario, southeastern
Canada.
Polymict conglomerate (above & below; 6.0 cm across) having pebbles of
komatiite and banded iron formation (see labels below) from the Timiskaming
Conglomerate (Neoarchean, <2.674 billion years) at Kirkland Lake, Ontario,
Canada.
Polymict conglomerate (5.7 cm across) having komatiite pebble (lower right)
from the Timiskaming Conglomerate (Neoarchean, <2.674 billion years) at
Kirkland Lake, Ontario, Canada.
Reference Cited:
Gray, M.D. & R.W. Hutchinson. 2001.
New evidence for multiple periods of gold emplacement in the Porcupine Mining
District, Timmins area, Ontario, Canada. Economic Geology 96:
453-475.