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Lucerne Granite, Lucerne-in-Maine
The rock is a very coarse-grained grayish-white granite. Its distinctive feature is the abundance of large feldspar grains, an inch or more across, which give a spotted or mottled appearance visible from the road (Figure 3). A closer look (Figure 4) shows that there are many sizes of feldspar grains in the rock, producing a seriate rock texture (having a series of sizes). Not all granites have this texture, some having grains of approximately equal size (equigranular), and others having grains of two predominant sizes (porphyritic).
The arrangement of minerals and the way they are grown together is a consequence of the way the rock formed. Granite is an igneous rock that forms by slow solidification from molten rock. As minerals first grow in the cooling magma, they tend to form regular crystal shapes. But when the minerals grow large enough to touch each other, their outer edges take on whatever irregular shapes are necessary to fill in the remaining spaces. Also, certain minerals (such as feldspar) tend to grow in a smaller number of larger grains, while other minerals (such as quartz) tend to grow in a larger number of smaller grains.
One particular texture found in the Lucerne Granite has one feldspar growing around the outside of another feldspar, partially or completely enclosing the inner, older feldspar. The younger, enclosing feldspar is always plagioclase; but the older, inner feldspar may be either alkali feldspar or plagioclase. The photo in Figure 5 shows that both may occur near each other in the same rock. Only some of the feldspars in the rock have this sort of texture. This texture of one feldspar surrounding another, is sometimes called "rapakivi texture" from descriptions of certain granites in Finland. Rapakivi texture is found in several of the granites on the Maine coast.
In detailed studies of other granites in coastal Maine, workers have demonstrated that intrusion of mafic magma into a body of partially molten granite could produce both mafic enclaves and plagioclase rims around feldspars in granite (Wiebe and others, 2004; Hogan and others, 2007; Lux and others, 2007; for example). Several observations suggest that a similar process may have affected the Lucerne Granite, although the details have not been worked out (Flanagan-Brown, 1999).
More about the Lucerne Pluton
The Lucerne pluton is one of the larger granite bodies in Maine. A comprehensive field study was reported by David Wones (1980), and a map of the pluton has been published by the U.S. Geological Survey (Wones and Ayuso, 1993). Its age is Devonian, approximately 380 million years old (Zartman and Gallego, 1979; Hogan and others, 2001).
Microscopic views of granite, show that the mineral grains may have different sizes and shapes, and are interlocked like pieces in a jigsaw puzzle.
Flanagan-Brown, Riley, 1999, Microgranular mafic enclaves as evidence for mingling and mixing of a mafic magma with the Lucerne Granite of central Maine: Geological Society of America, 1999 annual meeting, Abstracts with Programs, v. 31, no. 7, p. 268.
Hogan, J. P., Seaman, Sheila, and Williams, Michael, 2001, Geochemical and age mapping of monazite from the Lucerne Granite, Maine: Geological Society of America, Northeastern Section, 36th annual meeting, Abstracts with Programs, v. 33, no. 1, p. 68.
Hogan, J. P., Lux, D. R., Gibson, David, Hooks, Benjamin, and O'Donnell, S. P., 2007, Plagioclase mantled alkali feldspars: A record of thermal perturbations in felsic magma chambers: Geological Society of America, Abstracts with Programs, v. 39, no. 1, p. 64.
Lux, D. R., Hooks, Benjamin, Gibson, David, and Hogan, J. P., 2007, Magma interactions in the Deer Isle granite complex, Maine; field and textural evidence: The Canadian Mineralogist, v. 45, p. 131-146.
Wiebe, R. A., Manon, M. R., Hawkins, D. P., and McDonough, W. F., 2004, Late-stage mafic injection and thermal rejuvenation of the Vinalhaven Granite, coastal Maine: Journal of Petrology, v. 45, p. 2133-2153.
Wones, D. R., 1980, Contributions of crystallography, mineralogy, and petrology to the geology of the Lucerne pluton, Hancock County, Maine: American Mineralogist, v. 65, nos. 5-6, p. 411-437.
Wones, D. R., and Ayuso, R. A., 1993, Geologic map of the Lucerne Granite, Hancock and Penobscot Counties, Maine: U.S. Geological Survey, Miscellaneous Investigations Series, I-2360, map, scale 1:125,000.
Zartman, R. E., and Gallego, M. D., 1979, USGS(D)-BUB-8 (Sample 139), USGS(D)-BUB-9 (Sample 140), USGS(D)-BUB-10 (Sample 141), and USGS(D)-ORA-301 (Sample 142): in Marvin, R. F., and Dobson, S. W., (editors), Radiometric ages: Compilation B, U. S. Geological Survey: Isochron West, no. 26, p. 18-19.
Text and photos by Henry N. Berry, IV
Originally published on the web as the August 2010 Site of the Month.
Last updated on April 12, 2012
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