KOMATIITE-PEBBLE CONGLOMERATE
This is the most amazing conglomerate IÕve ever
seen. This is the Timiskaming Conglomerate of Ontario,
Canada. ItÕs 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). ThereÕs 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 OntarioÕs 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.