Dikes intruding tillite in the Precambrian of Ontario, Canada.

This outcrop has intrusions of dark-colored mafic dikes (now metamorphosed to greenstones) in glacial tillites. The glacial deposits are part of the famous Gowganda Formation, a ~2.3 billion year old deposit in Ontario, Canada representing a very ancient Ice Age. Other glacially-deposited sedimentary rocks of Paleoproterozoic age have been reported from Wyoming, Finland, India, southern Africa, and Australia. The glaciation was likely a widespread event, and may have persisted for a long time. This ice age is hypothesized to have been triggered by decreasing levels of methane gas (CH4) in the atmosphere as oxygen gas (O2) levels increased (the methane oxidized to carbon dioxide). This lessened the effects of greenhouse warming, and resulted in climate cooling.

The dikes are part of a tholeiitic gabbroic complex called the Nipissing Diabase. Specific intrusion types include dikes, sills, conical intrusions, and irregularly-shaped plutons. The major igneous minerals reported from the Nipissing Diabase are plagioclase feldspar and various types of pyroxene. Minor minerals include olivine, ilmenite, quartz, potassium feldspar, amphibole, magnetite, pyrite, and pyrrhotite (sulfides were observed in the Nipissing at this outcrop). Lithologies in the Nipissing actually range from gabbros to pyroxenites to granophyres. Regional metamorphism has reached greenschist to amphibolite grade, resulting in the presence of several common metamorphic minerals: actinolite, chlorite, epidote, amphibole, garnet, and biotite mica.

Minor peperitization has occurred here. Peperites are breccia-like, clastic-textured rocks formed when magma or lava encounters wet, unlithified sediments.

Stratigraphy of host rocks: Coleman Member, Gowganda Formation, lower Cobalt Group, Huronian Supergroup, Paleoproterozoic, ~2.3 Ga

Geology of dikes: Nippissing Diabase, Paleoproterozoic, 2.219 to 2.220 Ga

Locality: Percival Lake outcrop - roadcut on the eastern side of Route 144, opposite Percival Lake, north-northwest of the town of Cartier, Ontario, southeastern Canada (46° 45’ 47.38" North latitude, 81° 35’ 50.64" West longitude) (= stop 35 of Rousell et al. (2009) - A field guide to the geology of Sudbury, Ontario. Ontario Geological Survey Open File Report 6243, p. 165.)

Some info. from:

Blonde (1996) - Petrology and Metamorphism of Nipissing Diabase of May Township, Ontario. M.S. thesis. Carleton University, Ottawa, Ontario. 160 pp.

See info. at: