The Pike Lake Pluton is comprised of syenite and has been dated at 1178 ± 3 Ma by Davidson and van Breemen (2000), who placed it in the Frontenac terrane of the Central Metasedimentary belt of the Grenville Province.
The pluton is shown on the following geological map and described in Jean Dugas' doctoral thesis, where it is identified as the Black Lake body.
Wilson, Morley E. and Dugas, Jean, 1961,
Map 1089A, Geology, Perth, Lanark and Leeds Counties, Ontario, Geological Survey of Canada; Geology by Morley E. Wilson, 1930 and Jean Dugas, 1949; Descriptive notes by Jean Dugas.
https://doi.org/10.4095/107951
Dugas, Jean, 1952,
Geology of the Perth map area, Lanark and Leeds Counties, Ontario; Ph. D., McGill, 189 pages, four maps. Map 1089A replicates a map that is part of the thesis.
http://www.collectionscanada.gc.ca/obj/thesescanada/vol2/QMM/TC-QMM-124004.pdf
http://digitool.library.mcgill.ca/thesisfile124004.pdf
Below is an extract from Map 1089A showing the Pike Lake pluton.
The Bedding (Igneous Layering)
Worth noting on Map 1089A is that bedding (igneous layering) is shown for at least 22 locations within the boundaries of the pluton, and that at 14 locations the map shows foliation. Further, in the text of his thesis Dugas (1952) remarked on the good bedding within the syenite body and included a spectacular photo of bedding in the syenite body (his photo number 54 at page 159 of his thesis). Dugas places the outcrop shown in photo number 54 in Lot 24, Concession VI of North Burgess township. (Locating that outcrop is problematic as his Black Lake syenite body (the Pike Lake pluton) does not appear to outcrop in Lot 24 of Concession VI.)
One interesting feature within the Pike Lake Pluton, that is shown on the above extract from Map 1089A, is a thin lens of pyroxenite syenite along the Eastern margin (close to Black Lake) that is parallel to the bedding and extends over a length of at least one and a half miles (2.4 kilometers). The lens appears to be about 1/16 of a mile (100 meters) thick. It would be interesting to determine whether this is layering within the syenite body, or a separate intrusion.
The Syenite
Jean Dugas (1952) identified four main syenite bodies within his map area: the McGowan-Lanark body, the Bathurst body, the Crosby body and the Black Lake body. What recent authors call the Pike Lake pluton, Jean Dugas called the Black Lake body.
Jean Dugas summarized the common characteristics of the four main syenite bodies as follows (pages 52-53):
“1. The rock is commonly pink though it may grade to grey or brown where magnetite is most abundant. The brown coloring is probably caused by staining of the feldspars by iron hydroxide.
2. The rock varies in grain size from 1 mm. to 5 mm. and may be porphyritic.
3. The predominant mafic minerals are biotite and hornblende (near hastingsite in composition), pyroxene being only minor in amount.
4. The magnetite-ilmenite content is high, though not uniformly distributed. All syenite bodies give strong magnetic anomalies, though sections are poorly magnetic.
5. Accessory minerals are mainly apatite and sphene, the latter commonly forming a rim around magnetite crystals. Sphene is probably secondary after ilmenite (cf'. photo no. 71 p. 54).
6. The feldspars are microcline and oligoclase (Ab. 85).
7. The foliation is prominent near the margins of the body.
8. Migmatitic complex of' granite and syenite is common along the margins of these larger syenite masses. “
He also comments specifically on the Black Lake Body:
At pages 51-52,
"Another lens-shaped body (Black Lake body) lies between Pike Lake and Black Lake. This body has a maximum width of slightly more than a mile and extends for more than four miles."
At pages 59 - 60
“Black Lake body
This is one of the most interesting of the syenite bodies. The rock at the center of the body, though variable in grain size and color, is typically brownish, medium grained and massive. Away from the center of the body, the syenite has similar composition but is very well banded (ct. photo no. 54 p. 159). Further from the center the rocks are syenite-granite migmatite and on the outer edges are garnet gneiss. Good exposures of the latter occur on the shore of Pike Lake. Reference to this body is made in subsequent chapters."
At pages 152, 153
“Magnetic Data
Glancing at the geological map and the air survey magnetic contour intervals, there can be no doubt that all major magnetic anomalies are caused by syenite or quartz syenite bodies. ... [T]he Black Lake body is probably the best example of a sharp magnetic anomaly, though readings do not exceed 1,700 gammas. Contacts could have been traced fairly accurately from the magnetic contours alone.”
At pages 158, 161
“No absolute evidence of the magmatic origin of the syenites, granites and diorites can be given. The beat example of these features is within the Black Lake syenite body ( cf. figure p. 160). The center of this body is massive and around the massive rocks, without sharp contact, very well bedded rocks (cf. photo no. 54, p. 159) have obviously an igneous syenite composition. The rock then grades to a migmatitic complex of syenite-granite and, on the shore of Pike Lake, to a typical biotite-garnet gneiss.”
OGS Re-mapping the Perth Sheet
In my blog posting from last December I mentioned that Dr. Easton of the Ontario Geological Survey is currently remapping the Perth Sheet. Within the Perth map area Dr. Easton (2015, 2016a) has divided the Frontenac terrane into 3 subdomains: the western, central and eastern subdomains. From the text of his reports it appears that he places the Pike Lake Pluton within his western subdomain, commenting (2015, at page 18-6): “Frontenac suite intrusions in the western subdomain consist of pre- to syntectonic (e.g., Pike Lake pluton, 1178±4 Ma), syntectonic (e.g., Bennett Bay pluton, 1164±2 Ma) and posttectonic (e.g., North Cosby pluton, 1157±3 Ma) intrusions (all ages from Davidson and van Breemen 2000). This relative age range is also observed in the smaller Frontenac suite intrusions. Older intrusions, especially the smaller bodies, appear to be more quartz rich (quartz syenite, quartz syenite) than the younger intrusions (syenite, monzonite).” He also notes that “The western subdomain consists of both felsic and mafic Frontenac suite intrusions that are rimmed by migmatitic, compositionally varied, quartzofeldspathic gneisses that are commonly cut by smaller irregular bodies and sills of Frontenac suite intrusive rocks. It is not clear if these smaller intrusive bodies are simply sheet-like injections of magma emplaced during deformation, or if they represent cupolas or roof-pendants of a larger intrusive body at depth.” His final report should be worth reading.
Christopher Brett
Perth
References and Suggested Reading
Davidson, A. and van Breemen, O., 2000
Age and extent of the Frontenac plutonic suite in the Central metasedimentary belt, Grenville Province, southeastern Ontario; Geological Survey of Canada, Current Research 2000-F4; Radiogenic Age and Isotopic Studies: Report 13; 15 pages;
https://books.google.ca/books?isbn=0660182300
publications.gc.ca/collections/Collection-R/GSC-CGC/M44-2000/M44-2000-F4E.pdf
Easton, R. M., 2015
Project Unit 15-014. Precambrian and Paleozoic Geology of the Perth Area, Grenville Province; in Summary of Field Work and Other Activities, 2015. Ontario Geological Survey, OFR 6313
at pages 18-1 to 18- 13
http://www.mndm.gov.on.ca/en/news/mines-and-minerals/summary-field-work-and-other-activities-2015
Easton, R.M., 2016a.
Precambrian and Paleozoic geology of the Perth area, Grenville Province; in Summary of Field Work and Other Activities, 2016, Ontario Geological Survey, Open File Report 6323, p.17-1 to 17-13.
Easton, R.M., 2016b.
Metasomatism, syenite magmatism and rare earth element and related metallic mineralization in Bancroft and Frontenac terranes: A preliminary deposit model; in Summary of Field Work and Other Activities, 2016, Ontario Geological Survey, Open File Report 6323, p.18-1 to 18-9.
Both of Dr. Easton's 2016 reports can be downloaded from:
http://www.mndm.gov.on.ca/en/news/mines-and-minerals/summary-field-work-and-other-activities-2016
