The following simplified geologic map shows the extent of the Lavant Gabbro Complex.
The towns of Lanark, Almonte, Carleton Place and Clyde Forks are shown on the map. The town of Perth is under the Legend.
The Lavant Gabbro Complex falls within the Sharbot Lake Terrane (sometimes called the Sharbot Lake domain) of the Central Metasedimentary Belt of the Grenville Province, Canadian Shield. The Sharbot Lake Terrane is comprised of marbles and metavolcanic rocks that have been intruded by gabbroic and granitic plutons. It is bounded on the east by the Maberly shear zone and on the west by the Robertson Lake shear zone (sometimes called the Roberton Lake mylonite zone). The metavolcanic rocks falling within the Sharbot Lake Terrane are not shown on the above map.
Much of the Lavant Gabbro Complex was mapped in detail in the 1980's by Liba Pauk, J. M. Wolff and Michael Easton of the Ontario Geological Survey, and outcrops of the complex can be easily found along a number of roads that cross the complex.
Fernando Carfu and Michael Easton in a paper published in 1989 described the complex as follows:
“The Lavant gabbroic complex is a composite intrusion, roughly 50 km long and up to 15 km wide. It consists of (i) a voluminous mafic suite, dominated by medium-grained gabbro to diorite, locally showing igneous layering and crosscutting relationships between several compositionally distinct phases; and (ii) a slightly younger granodiorite–monzogranite suite, which forms several small intrusive bodies and dikes cutting the gabbro. The intermediate to felsic phases occur mainly in the structurally higher part of the complex, and also intrude adjacent supracrustal rocks, including the marbles. Metamorphic grade of the complex varies from upper greenschist facies in the north to lower amphibolite facies in the south. Contact relationships are complex, and inclusions, roof pendants and slivers of all the country rocks are present. The gabbro body does not have an extensive contact aureole...”
They determined an age of 1224 ± 2 Ma for the gabbroic and associated monzogranitic rocks of the Lavant complex.
I’ve always been interested in layering in igneous rocks, and saw numerous examples of layering in anorthosite bodies in Labrador when I was employed as a summer student by the Geological Survey of Canada. I wondered whether any examples of layering in the Lavant Gabbro complex could be easily found.
In her Report 253 published in 1989 Liba Pauk of the Ontario Geological Survey mentioned:
“Primary rhythmic and graded layering of pyroxene cumulus occurs in a roadcut 1.2 km northwest of Black Creek Meadow (Photo 5). The layers strike 90 degrees and dip 30 degrees to the south. The primary rhythmic layering was also recorded ... some 1.5 km east of the map area at roadcuts of Lavant County Road 16.”
Her Photo 5 shows spectacular layering. The caption to her photo reads: Primary graded rhythmic layering in the Lavant Gabbro Complex. The only exposure found in the map area is located along Joes Lake Road, 1.2 km northwest of Black Creek Meadow.”
I did find her outcrop, but as it is now overgrown and covered with moss or lichen, the layering is no longer visible. For those looking for the outcrop, please note that the “Joes Lake Road” mentioned by Liba Pauk has been renamed Black Creek Road, and that the outcrop is on the east side of the road 1.3 km north of where Black Creek crosses the road. (More importantly, the current Joes Lake Road that is just south of Joes Lake is not the Joes Lake Road mentioned by Liba Pauk.)
In a paper published in 1994 Graham C. Wilson of the Ontario Geological Survey mentioned:
“The best layering observed in this area, and indeed in the course of the whole project, was observed in a satellite body on the east side of the main intrusive mass, west of Hopetown. The layers (Fig. 5c) are graded, and reflect modal variation between light feldspar and dark minerals (pyroxene or secondary amphiboles).”
Figure 5c appears at page 147 of Wilson’s report 5580. It shows spectacular layering. I did not find that outcrop, but did find two outcrops west of Hopetown on the north side of County Road 16 (a kilometer or so west of Highway 511) that show layering in the gabbroic rocks. Below are photographs of the two outcrops:
While the layering in the second outcrop is much better than the photograph shows, neither outcrop compares with the spectacular layering that I saw in Labrador.
In his Report 5693 published in 1988 Michael Easton of the Ontario Geological Survey also reported on primary igneous layering within the Lavant Gabbro Complex, which he mentioned consisted “of a variety of gabbro phases, ranging from gabbro to diorite to pyroxene gabbro to gabbroic anorthosite.” I have not yet found the time to look for his occurrences.
Christopher Brett
Perth, Ontario
References and Suggestions for Further Reading
Carfu, Fernando and Easton, R. Michael (1989), Sharbot Lake terrane and its relationship to Frontenac terrane, Central Metasedimentary Belt, Grenville Province: new insights from U-Pb geochronology, Can. J. Earth Sci. 34: 1239-1257
http://www.nrcresearchpress.com/doi/pdf/10.1139/e17-099
Easton, R. Michael (1988), Geology of the Darling Area Lanark and Renfrew Counties; Ontario Geological Survey, Open File Report 5693, 206 pages, at pages 60-66, Accompanied by Map 3113
http://www.geologyontario.mndmf.gov.on.ca/mndmfiles/pub/data/imaging/OFR5693/OFR5693.pdf
Easton, R. Michael and DeKamp, E.A. (1987), Darling Area, Lanark and Renfrew Counties; Report 34 at pages 220-228, in Ontario Geological Survey Miscellaneous Paper 137, Summary of Field Work and Other Activities 1987
Pauk, Liba (1989a), Geology of the Dalhousie Lake Area, Frontenac and Lanark Counties; Ontario Geological Survey Report 245, 57 pages, at pages 17-23, Accompanied by Map 2512
http://www.geologyontario.mndmf.gov.on.ca/mndmfiles/pub/data/imaging/R245/r245.pdf
Pauk, Liba (1989b), Geology of the Lavant Area, Lanark and Frontenac Counties; Ontario Geological Survey Report 253, 61 pages, at pages 26-29, Accompanied by Map 2515
http://www.geologyontario.mndmf.gov.on.ca/mndmfiles/pub/data/imaging/R253/R253.pdf
Wilson, G.C. (1994), Mafic-ultramafic intrusions, base-metal sulphides, and platinum group element potential of the Grenville province of southeastern Ontario: Ontario Geological Survey, Open File Report 5880, 196 pages at pages 125-128
http://www.geologyontario.mndmf.gov.on.ca/mndmfiles/pub/data/imaging/OFR5880/OFR5880.pdf
Wolff, J. M., 1985, Geology of the Sharbot Lake Area, Frontenac and Lanark Counties, Southeastern Ontario; Ontario Geological Survey Report 228, 70 pages at pages 29-37, accompanied by Map 2471
http://www.geologyontario.mndmf.gov.on.ca/mndmfiles/pub/data/imaging/R228/R228.pdf
