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
Very interesting! This is the first time I've heard of orbicular granite in Ontario. I'm from Manitoba. I recently found an 18Kg boulder of orbicular granite-diorite in a local gravel pit. Thank you for this great collection of information! Todd
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