The rounded form of the pillows results from the rapid cooling of basaltic magma that erupts under water. Volcanoes that produce such basalts occur along mid-ocean ridges. The pillows at this location have been stretched during regional metamorphism.
Davidson describes the outcrop as follows:
“Stop 5-10. Pillowed basalts, Tudor formation, Grenville Supergroup
Mafic volcanic rocks along this stretch of highway lie on the east side of the ~1.27-Ga Elzevir tonalite batholith, which intrudes them. Pillows at this stop indicate the flows face away from the batholith. The metabasalts have tholeiitic chemistry; higher in the stratigraphic succession to the east lie andesitic and coarse volcaniclastic rocks that are clearly calc-alkaline....”
[A. Davidson, 1995, Tectonic History of the Grenville Province, Ontario
Field Trip Guidebook A5, Precambrian ‘95, Geological Survey of Canada, Open File 3142
http://ftp2.cits.rncan.gc.ca/pub/geott/ess_pubs/205/205286/of_3142.pdf ]
I recently found on the web a field trip guide prepared last year by graduate geology students at Ottawa University and Carleton University which includes four stops at volcanic rocks in the area south of Bon Echo Provincial Park and north of Kaladar. They describe the pillowed basalt outcrop as follows:
“Stop 2– Tudor Pillow Basalts - Highway 41 and Highway 506 intersection
This outcrop includes pillowed basalts belonging to the Tudor suite (erupted >1265 Ma),
which display various primary igneous textures such as pillow selvages, hyaloclastite and vesicles. The preservation of these textures allows us to determine which way the pillows were oriented upon eruption ... which way is it? This preservation of primary textures is rare within rocks of the Central Metasedimentary Belt, which has experienced multiple metamorphic events and several intense deformation events (i.e. the Elzevir, Shawinigan and Ottawan orogenic events)”
[ Seamus Magnus, Dave Lowe, Jamie Cutts, and Travis McCarron, 2013,
OCGC Field Trip – September 27th to 29th 2013
earthsci.carleton.ca/sites/default/files/Field%20Guide%202013.pdf ]
Magnus, Lowe, Cutts and McCarron give directions to three other nearby outcrops of volcanic rocks. Here is an abridged version of their description:
“Stop 1 – Mazinaw Lake Metavolcanics
North of Highway 41 and Mazinaw Road intersection
This outcrop contains intermediate to felsic volcaniclastic rocks of the Mazinaw Lake formation (1240-1250 Ma). ... , volcanic breccias and volcaniclastic rocks such as the tuffs observed in this outcrop. ...
Stop 3 – Metavolcanic and Metasedimentary Contact North of O’Donnell Road
Mafic metavolcanic rocks (west) and metapelitic rocks (east) are present on either side of
a small trodden path. This path represents an unconformable contact between the two
units. ...
Stop 4– Kashwakamak Hornblende Dacite
333199E, 4964264N – Ladyslipper Road, off of Myers Cave Road, south of lake
A calc-alkaline dacitic rock erupted at 1276 ±2 Ma (U -Pb zircon).... Interpreted as either a flow or dome, this represents the less dramatic form of intermediate volcanism.”
Given extra time, I would have visited those outcrops. The pillowed basalt, as noted above, records an eruption along a mid-ocean ridge. Dacite is believed to form when basaltic rocks are dragged down and melt along a subduction zone.
These rocks occur within the Mazinaw Terrane of the Central Metasedimentary Belt (“CMB”) of the Grenville Province of the Canadian Shield. Professor Nick Eyles of the University of Toronto commented in 2002 on the Central Metasedimentary Belt as follows:
“A number of different blocks (terranes) occur within the CMB (e.g., Bancroft, Elzevir, Mazinaw) and are separated by shear zones (large faults) that record the slippage between each terrane as they were pushed together. These terranes were separate land masses (basically micro-continents) with intervening seas which were swept together during the Grenville Orogeny.
Some geologists have suggested that modern-day Indonesia , with its subduction zones and limestone reef-fringed volcanic islands, provides a good model for the CMB. Rocks include pillow basalts, typical of oceanic crust, and pyroclastic debris deposited around explosive andesitic volcanoes at modern subduction zones. Mud and gravel that accumulated in the surrounding deep waters were deformed to schist and conglomerate. Other common metamorphic rocks are marbles formed from the alteration of limestone and quartzites derived from quartz rich sandstones. ...”
[Nick Eyles, 2002, Ontario Rocks - three billion years of environmental change, Fitzhenry & Whiteside, 339 pages, at page 105 ]
Professor Dugald Carmichael of Queen’s University at Kingston has commented on the rocks of the Central Metasedimentary Belt as follows:
“Starting about 1280 million years ago, ... volcanic eruptions created a chain of islands in the ocean. Like the present-day Antilles or the Philippines, these volcanic islands would have formed along the boundary between two converging tectonic plates. The islands grew larger as basaltic lava erupted from vents and fissures above and below sea level. Later the eruptions became violently explosive, spewing vast amounts of volcanic ash and cinders. In shallow intertidal lagoons around the islands, blue-green photosynthetic bacteria built small laminated mounds on a seafloor of precipitated calcium carbonate. Volcanic ash was eroded from the islands and washed into the lagoons, where later it consolidated into sandy limestone and limy shale. Deep underground beneath the volcanoes, between 1270 and 1240 million years ago, huge volumes of molten magma cooled and crystallized into granite and other types of igneous rock.
Meanwhile a continent, riding on a converging tectonic plate, was approaching the chain of islands. As the intervening ocean narrowed, submarine landslides broke loose from the leading edge of its continental shelf and slumped down the continental slope, mixing with mud to make a spectacular sedimentary breccia. When the continental shelf collided with the chain of volcanic islands, a great range of mountains was uplifted. Cobbles, pebbles, sand and clay were eroded from the mountains and deposited in sedimentary formations atop the eroded surface of what had been the chain of islands. Continuing tectonic convergence squeezed the sedimentary formations into tight folds, and all the older rocks were also squeezed. Deep underground, between 1170 and 1080 million years ago, high temperature and pressure changed the sedimentary and igneous rocks into metamorphic rocks, and once again granitic magma intruded, cooled and crystallized.’
[Bedrock in the Frontenacs, by Dugald Carmichael,
http://naturallyrichfrontenacs.com/bedrock.html ]
Food for thought when you are driving down Highway 7.
Christopher Brett
Perth, Ontario
