Monday 17 July 2017

The Pike Lake Pluton - A Layered Syenite Intrusion in Lanark County

The Pike Lake Pluton lies about 12 kilometres (8 miles) south of Perth between Pike Lake and Black Lake.  It falls mainly within North Burgess Township of Lanark County (now Burgess Ward of Tay Valley Township), and extends westward into North Crosby Township of Leeds County.

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

Wednesday 12 July 2017

Outcrops of Orbicular Granite in Lanark County, Ontario

“Orbicular granite (also known as orbicular rock or orbiculite) is an uncommon plutonic rock type which is usually granitic in composition. These rocks have a unique appearance due to orbicules - concentrically layered, spheroidal structures, probably formed through nucleation around a grain in a cooling magma chamber. Almost one third of known orbicular rock occurrences are from Finland. The occurrences are usually very small.”
https://en.wikipedia.org/wiki/Orbicular_granite

There is a tremendous amount written on orbicular rocks.  Kennan and  Lorenc (2008) provide a discussion of the early discoveries of orbicular rocks.  They mention that “In 1802, Leopold von Buch described some outcrops of orbicular granite in the Karkonosze granite, Lower Silesia, Poland. ...  The Silesian discovery predates that of the well-known orbicular diorite (Napoleonite) in outcrop on Corsica and, thus, may be the first ever record of this distinctive rock type in its geological context.”

David Levenson (1989) provides the following summary: “Orbicular rocks are nonsedimentary rocks that contain concentric shells of different texture and/or mineralogy about a central core. Occurrences have been described from every continent, and orbicules have been found in ultramafic to felsic, igneous, and metamorphic rocks. Host rocks are fine- to coarse-grained, and include dykes, sills, stocks, batholiths, lavas, gneisses, schists, and migmatites. Orbicular facies are generally local, less than several hundred meters in greatest dimension. ... Orbicules range from <3 cm to >30 cm in diameter. Shells number from one to more than twenty, and may be spaced irregularly or in geometrical progression.”

Numerous theories have been advanced to explain the formation of orbicular rocks.  Sage (1987), who noted that they occur in magmatic, metamorphic and migmatic terrains and have various compositions, provides a list of eight proposals from Levenson (1966) and mentions a ninth advanced by Elliston (1984).  The nine theories are: liquid immiscibility; fluctuations of a melt about a eutectic or eutectoid due to variations in pressure and temperature; reactions between magmas and xenoliths; xenoliths moving through parts of magma of different compositions; crystallization of concentric envelopes of contrasting composition in a highly viscous magma that retards viscous diffusion; excessive crystallization of one component around a nucleus, followed by excessive crystallization of a second component, as a result of  alternating supersaturation;  rhythmic supersaturation and crystallization about centres in a magma in a manner analogous to Leisegang ring formation;  layers built up on a previously formed nucleus in a silicate melt, with diffusion of material, and crystallization later than the formation of the orbicular structure.   More recent theories involve nucleation in small pockets of H2O-rich superheated silicate melt, with the orbicule shell structures crystallizing and growing from supercooled boundary layers within the  magma  (Owen, 1991; Ort, 1992; Grosse et al, 2010; Smillie and Turnbull,  2014).

Lanark's Outcrops


What is not widely known is that Lanark County is home to at least three occurrences of orbicular granite, which are mentioned in the notes to the following map and described in the following doctoral thesis.

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.    
http://www.collectionscanada.gc.ca/obj/thesescanada/vol2/QMM/TC-QMM-124004.pdf
http://digitool.library.mcgill.ca/thesisfile124004.pdf

Surprisingly, the outcrops of orbicular granite are not mentioned in the following paper:

Dugas, Jean, 1950,
Perth map-area Lanark and Leeds Counties Ontario; Geological Survey of Canada, Paper no. 50-29, 1950; 22 pages (1 sheet), https://doi.org/10.4095/101399

In the descriptive notes to Map 1089A,  Jean Dugas mentions:

“Small bodies of granite are exposed in various parts of the map-area.  Among these, and too small to be mapped separately, are exposures of an ‘orbicular’ granite, composed of granite ovoids and basic rims.  These can be best observed on lot 20, con. 1, North Crosby township.”

I have to admit I’ve known about that reference for over five years and have not yet made the time to visit the occurrence, largely because the two options are (a) to ford a stream and walk across a swamp, or (b) launch my canoe at the far end of Pike Lake and paddle to the outcrop.

Jean Dugas (1952) discusses the occurrences of orbicular granite at pages 67-73 of his doctoral thesis. He mentions three occurrences:
- on lot 20, concession I of North Crosby township, on the southwest end of Pike Lake;
- on lot 16, concession VIII of North Burgess township (to the northeast of Pike Lake);
- on lot 18, concession III of South Sherbrooke township, north of Christie Lake.   
Outcrops of granite on those lots and concessions in North Crosby township and North Burgess township are shown on Map 1089A.   On the following extract from Map 1089A I've shown the three locations by florescent pink boxes.




At page 68 of his thesis Jean Dugas includes four photographs of the outcrops: three photographs of the outcrop  on lot 20, concession I of North Crosby township; one of the outcrop on lot 16, concession VIII of North Burgess township; but none of the outcrop on lot 18, concession III of South Sherbrooke township.  At page 70 he includes photographs of thin sections from the outcrop in North Crosby township.
   
Jean Dugas (1952)  mentions (at pages 67, 69; 72-73) that the outcrop near the southwest end of Pike Lake “can be observed very plainly. A scarp on the east side shows a good exposure of rock. The band may reach 20 feet in width. It seems to be in the form of a lens: at a distance of 1000 feet from the best exposure, it is only about two feet wide. It is composed of rounded pieces of granite, in places longitudinally arranged along the strike and dip and surrounded by a thin, very basic rim of amphibolite composition. The size of the granite pieces ranges from one inch to more than one foot and the rim is one half to one inch wide. The shape of the granite and the interstitial material approaches that of pillow structure (cf. photo no. 13, p. 68). The granite is composed of pink microcline and quartz with a few pyroxene crystals penetrating from the basic rim (cf. photo no. 17, p. 70). The rim itself is composed of augite and oligoclase for the most part.
...
The study of the Perth orbicular granite is particularly difficult as there is no recognizable matrix and no non-orbicular granite in adjacent areas from which the rock may have derived. However, it is remarkable that the center of orbicules is free from ferric minerals and there is sharp increase of these towards the rim. No microcline is observed in the rim and the composition is esboitic (oligoclasic). There is also evidence of replacement between sodic feldspar and microcline (cf. photo no. 18, p. 70).”

In his thesis Jean Dugas also describes a leopard pegmatite from Lot 12, concession V of North Burgess township and includes a photograph of the outcrop at page 79.   He mentions “At the Silver Queen mine, a pegmatite shows locally a faint sinuous system of black stringers composed of ferromagnesian minerals.  This type of pegmatite has been called ‘leopard pegmatite’ (c.f'. photo no. 2l, p. 79), and is probably of orbicular nature.”

The best references to orbicular rocks that I have been able to find are the following French language GEOWIKI articles: 

Les roches orbiculaires
http://www.geowiki.fr/index.php?title=Les_roches_orbiculaires

Les types de roches orbiculaires
http://www.geowiki.fr/index.php?title=Les_types_de_roches_orbiculaires

Liste des occurrences dans le monde
http://www.geowiki.fr/index.php?title=Liste_des_occurrences_dans_le_monde    

Interestingly that list of worldwide occurrences of orbicular rocks does not include the outcrops in Lanark County.

Also worth a look are the following videos on the Granito Orbicular of Chile: 
https://www.youtube.com/watch?v=j_yx3sTz3co    5:08
https://www.youtube.com/watch?v=HkfIiGNl41A    1:33
https://www.youtube.com/watch?v=yaMEfVZMvPc      1:02

Christopher Brett
Perth, Ontario


References and A Reading List on Orbicular Rocks:

Adams, Frank. D., 1897
 Nodular granite from Pine Lake, Ontario;  Geological  Society  of  America
Bulletin,  vol. ix, pp. 163-172; reprinted as McGill University, Papers from the Department of Geology, No. 8. ,  https://archive.org/details/cihm_02281       

Affholter, Kathleen A. and Lambert,  E. E., 1982
Newly described occurrences of orbicular rock in Precambrian granite, Sandia and Zuni Mountains, New Mexico; in  Wells, S. G.; Grambling, J. A.; Callender, J. F.; [eds.] New Mexico Geological Society Guidebook, 33rd Field Conference, Albuquerque Country II, 1982, pages 225-232
https://nmgs.nmt.edu/publications/guidebooks/downloads/33/33_p0225_p0232.pdf

Blake, William P., 1904
Origin of Orbicular and Concretionary Structures, Transactions of the American Institute of Mining Engineers, vol. XXXVI,  677- 682
library.aimehq.org ...bulletin of the AIME 1905 1-6-047.pdf

Bräunlich, Matthias
Orbiculite - eine Auswahl:
http://www.kristallin.de/orbiculite/kugelgesteine2.htm#Anker1
Orbicular Rocks
http://www.kristallin.de/orbiculite/orbicular_rocks1.htm

Elliston, John N. 1984,
Orbicules: An indication of the crystallisation of hydrosilicates, I;  Earth-Science Reviews, Volume 20, Issue 4, August 1984, Pages 265-344
https://doi.org/10.1016/0012-8252(84)90021-7
   
Enz, Robert D., Albert M. Kudo, and Douglas G. Brookins; 1979
Igneous origin of the orbicular rocks of the Sandia Mountains, New Mexico, Geological Society of America Bulletin, February, 1979, v. 90, p. 138-140
https://doi.org/10.1130/GSAB-P2-90-349

Grosse, Pablo; Toselli, Alejandro J.; and  Rossi, Juana N., 2010
Petrology and geochemistry of the orbicular granitoid of Sierra de Velasco (NW Argentina) and implications for the origin of orbicular rocks; Geological Magazine, vol. 147,  no. 3, 451-468
DOI: 10.1017/S0016756809990707
   
Hitchcock, Edward; Edward Hitchcock, jr., Albert D. Hager, Charles H. Hitchcock. 1861
Report on the Geology of Vermont: descriptive, theoretical, economical, and scenographical.  Vol. 2, pp. 563, 564, 721, (the nodular granite of Craftsbury; the Craftsbury pudding granite)
https://babel.hathitrust.org/cgi/pt?id=mdp.39015086763169;view=1up;seq=20

Hudec, P. P.,  1964
Geology of the Big Trout Lake Area District of Kenora (Patricia Portion); Ontario Department of Mines, Geological Report No. 23 “A rather peculiar orbicular volcanic rock has been observed ... [which] consists of very fine-grained siliceous and chloritic concentric features, with fine quartz  in the centre, a ring of fine chlorite around it, surrounded by a ring of chlorite of different composition.   The large rings  (1/2  to l  inch   in   diam.) have small orbicular features within them.   The rock is dark, hard, and brittle.”

Kemp, J. F., 1894
An orbicular Granite from Quonochontogue, Beach, Rhode Island, Transactions of the New York Academy of Sciences, Volumes 13-14, pages 140 - 144
https://books.google.ca/books?id=AcoAAAAAYAAJ

Kennan, Pádhraig S. And Lorenc, Marek W.,  2008
Orbicular granite near Jelenia Góra in southwestern Poland: the first outcrops; 
Mineralogia, Volume 39, Issue 3-4, pp. 79-85
Open Access: DOI: https://doi.org/10.2478/v10002-008-0006-4

Kessler, H.S. and Hamilton, W.E., 1904      
The Orbicular Gabbro of Dehesa, California; American Geologist, Vol. Xxxiv, No. 3, 123-140 with five plates 
https://archive.org/stream/panamericangeolo341904desm#page/n165/mode/2up   

Lahti, S. (Editor), 2006
Orbicular Rocks in Finland 2005, with contributions by P. Raivio and I. Laitakari.
Espoo (Geological Survey of Finland) ISBN: 951-690-911-6. , 177 pages

Lawson, Andrew C., 1904
The Orbicular Gabbro of Dehesa, San Diego County, California;   Volume 3, Issue 17 of University of California publications: Bulletin of the Department of Geology; pp 383-396
https://archive.org/stream/bulletinofde319021904univ#page/n549/mode/2up

Levenson,  David  J., 1966,
Orbicular  rocks:  A  review:  Geological  Society  of  America Bulletin, v. 77, p. 409- 426
http://dx.doi.org/10.1130/0016-7606(1966)77[409:ORAR]2.0.CO;2
   
Levenson,  David J. (1989)
Orbicular Rocks; in Petrology, Encyclopedia of Earth Science, pp 415-416, Springer
link.springer.com/10.1007/0-387-30845-8_168
DOI   10.1007/0-387-30845-8_168

Ort, Michael H., 1992
Orbicular volcanic rocks of Cerro Panizos: Their origin and implications for orb formation;
Geological Society of America Bulletin, August 1992, vol. 104 no. 8 1048-1058
DOI: 10.1130/0016-7606(1992)104<1048:OVROCP>2.3.CO;2

Owen,  J. Victor, 1991
Significance of epidote in orbicular diorite from the Grenville Front zone, eastern Labrador; Mineralogical Magazine, June 1991, Vol. 55, pp. 173-18I
http://www.minersoc.org/pages/Archive-MM/Volume_55/55-379-173.pdf

Sage, R. P., 1987
Geology of Carbonatite-Alkalic Rock Complexes in Ontario: the Prairie Lake Carbonatite Complex, District of Thunder Bay; Ontario Geological Survey Study 46, 91 pages at 20-22
http://www.geologyontario.mndmf.gov.on.ca/mndmfiles/pub/data/imaging/S046/S046.pdf

Smillie, Robert W. and Turnbull, Rose E,  2014
Field and petrographical insight into the formation of orbicular granitoids from the Bonney Pluton, southern Victoria Land, Antarctica; Geological Magazine,   Volume 151, Issue 3,  May 2014, pp. 534-549           DOI: https://doi.org/10.1017/S0016756813000484

Watson, Thomas Leonard, 1904
Orbicular Gabbro-Diorite from Davie County, North Carolina; The Journal of Geology, Volume 12, pp 294 - 303    https://archive.org/details/jstor-30055824

Zihkel, Ferdinand, 1894
Lehrbuch der Petrographie, 2d ed., vol. 2, pages 50, 51-
https://archive.org/stream/lehrbuchderpetr01zirkgoog#page/n59/mode/2up