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Ichthyoglypts and other igneous animals, Topsham
Last month's Geologic Site of the Month - Bradbury Mountain, Pownal - made a passing reference to graphic granite - a variety of granite with a peculiar mineral texture - namely, the "...intergrowth of subhedral skeletal quartz prisms in an alkali feldspar or sodic plagioclase host, which occurs in granite pegmatites and rarely as megacrysts in granites" (Barker, 1970).
A picture is worth a thousand words, so...
...here is a picture of a slab of graphic granite from the Hussey Quarry in Bowdoinham, Maine.
The unusual shapes of the dark quartz crystals prompted earlier workers to propose some striking names for the rock, including "runite" (from the similarity of the quartz crystals to Norse runes), "Hebraic granite," and, my favorite, "ichthyoglypts" because of the fancied resemblance of the quartz rods to tailless fish (Fersmann, 1928, as cited in Whalstrom, 1939). The likeness to crude writing is the origin of the most commonly used term - graphic granite.
This locality also has the advantage of being adjacent to the Cathance River Nature Preserve. If your interest in the pegmatites wanes, you can wander onto one of several hiking trails that crisscross the preserve.
The following ten pictures were taken adjacent to the Purington and Consolidated #1 quarry pits and along the Heath Trail. The pits themselves are now filled with water, but blasting has left many large boulders with excellent exposures of fresh pegmatite, many showing graphic textures.
Arguments over the origin of graphic granite mirrored closely the debate on the origin of granite - a metasomatic/replacement origin versus a magmatic origin (see Whalstrom, 1939; Spencer, 1945; Barker, 1970; and Fenn, 1986).
The optical continuity of the enclosing alkali feldspar, the lack of a consistent crystallographic orientation between the quartz and feldspar host, and the cross-cutting nature of late quartz veins in some feldspar crystals provided support for a replacement hypothesis (Whalstrom, 1939).
The similarity of the texture of graphic granite to eutectic structures in metallic systems suggested that graphic granite may have crystallized from a eutectic or cotectic composition - a liquid composition where two or more solid phases are in equilibrium with the liquid and crystallizing simultaneously. This is supported by the relatively narrow range the quartz/feldspar ratio of graphic granite - in the range of 24-percent to 32-percent quartz. A fixed ratio of crystallizing phases is characteristic of a cotectic or eutectic composition. Bulk compositions of some graphic granites do not lie on the quartz-feldspar cotectic at reasonable temperatures and pressures, however.
The general consensus at the present time is that graphic granite does form by the simultaneous crystallization of alkali feldspar and quartz, but not necessarily at equilibrium conditions. Experimental studies (Fenn, 1986; Lentz and Fowler, 1992) suggest that graphic granite textures form under non-equilibrium conditions in rapidly cooling silicate liquids saturated or nearly saturated with a volatile (water-rich?) phase. The unusual texture is the result of oscillation between silica saturated (quartz crystallizing) and silica under-saturated (feldspar crystallizing) conditions.
In the Topsham area, graphic granite is characteristic of the youngest suite of pegmatites, dated from 268 to 275 million years old - of Permian age (Tomascak and others, 1996). Older pegmatites in the region, of Middle to Late Devonian age, do not have the graphic texture. The different pegmatites are shown on the geologic map of the Bowdoinham quadrangle (West and Cubley, 2006), and summarized in the Bath 100K bedrock geology report by Hussey and Berry (2002).
Barker, D. S., 1970, Composition of granophyre, myrmekite and graphic granite: Geological Society of America, Bulletin, v. 81, p. 3339-3350.
Fenn, P. M., 1986, On the origin of graphic granite: American Mineralogist, v. 7l, p. 325-330 (pdf format - 626Kb).
Hussey, A. M., II, and Berry, H. N., IV, 2002, Bedrock Geology of the Bath 1:100,000 Map Sheet, Coastal Maine: Maine Geological Survey, Bulletin 42, 50 p. (online version of B-42)
Lentz, D. R. and Fowler, A. D., 1992, A dynamic model for graphic quartz-feldspar intergrowths in granitic pegmatites in the southwestern Grenville Province: Canadian Mineralogist, v. 30, p. 571-585.
Tomascak, P. B., Krogstad, E. J., and Walker, R. J., 1996, U-Pb monazite geochronology of granitic rocks from Maine; implications for late Paleozoic tectonics in the Northern Appalachians: Journal of Geology, v. 104, no. 2, p. 185-195.
Spencer, E., 1945, Myrmekite in graphic granite and in vein perthite: Mineralogical Magazine, v. 27, p. 79-98.
Wahlstrom, E. E., 1939, Graphic granite: American Mineralogist, v. 24, p. 681-698.
West, D. P., Jr., and Cubley, J. F., 2006, Bedrock Geology of the Bowdoinham Quadrangle, Maine: Maine Geological Survey, Open-File Map 06-54, 1:24,000 (online version of 06-54).
Web site by Marc C. Loiselle, Maine Geological Survey
Originally published on the web as the September 2008 Site of the Month.
Last updated on April 12, 2012
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