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Home > Explore! > Bedrock Geology > Field Localities > Geology at Mt. Pisgah

Migmatite in New Gloucester

The Site

In 2005, a section of Maine Route 26 north of Turnpike Exit 63 (Gray) was rebuilt and relocated. One section of new road in New Goucester, about 1.3 miles north of the Maine Wildlife Park, required significant blasting of bedrock to reduce the road steepness. The resulting roadside rock cut exposures are the subject of this web site. If you choose to visit on your own, please use extreme caution, because traffic moves fast on this new road. Do not park vehicles on Route 26. Instead, use the DOT Park and Ride Lot near the beginning of Sabbathday Road. The UTM coordinates for this parking lot are 0391688, 4866759 (NAD 27, Zone 19).

Glimpse into the Earth

These rocks are in the eastern part of a large region of granite shown on the 1985 Bedrock Geologic Map of Maine as the Sebago batholith (Figure 1). As the 1985 map suggests, the eastern part of the batholith is complicated, and contains a mixture of granite and metamorphic rocks (Figure 2). Granite is an igneous rock. It forms when other rocks are heated to sufficient temperature in the earth to partially melt; the melted rock then collects and flows into larger bodies of melt which then solidify into granite. Not all igneous rocks are granite, and not all granites are alike, but all igneous rocks form by solidification from molten rock. The dark colored metamorphic rocks are what remain of the pre-existing rocks which were heated and partially melted. Rocks in which igneous and metamorphic rocks are found together are called migmatite.

bedrock geology map of Maine
Figure 1		 location map
Figure 2		 roadside outcrop
Figure 3		 roadcut exposure
Figure 4

Many roadside exposures in this area, such as the one in Figure 3, appear to consist mostly or entirely of light gray to white granite. The vertical cuts beside the road, however, expose rocks beneath the surface, which show that there are many varieties of rock (Figure 4).

Migmatite

The proportion of light-colored granite to dark-colored metamorphic rock, and the shapes of the various types of rock are variable in the exposures, both along the rock face from one end to the other, and from bottom to top. A layered structure is dominant, with the layers generally horizontal, though careful tracing of individual layers shows that they may curve gradually up and down, and their thicknesses change from bulging to pinched. In some places, the granite layers are thick (Figure 5); in other places thick layers of metamorphic rock dominate (Figure 6). In detail, the granite cuts across the metamorphic layers to form an interconnected network (Figure 7). In extreme cases, granite has engulfed the metamorphic rocks so that only disconnected wisps remain (Figure 8). These sorts of relationships would be difficult to see by looking down on the tops of bedrock outcrops normally found in woods and fields.

thick layers of granite
Figure 5		 thick metamorphic layers
Figure 6		 granite cutting across metamorphic layers
Figure 7		 granite engulfing metamorphic rock
Figure 8

Interaction between Granite and Metamorphic Rock

The metamorphic rocks have an internal fabric that was probably produced by heat and pressure which affected them as solid rocks, before the rock melted. Similar fabric is found in metamorphic rocks well outside the granite. Therefore, some of the mineral alignment and folding in the metamorphic rocks is probably older than the granite. On the other hand, there are structural features produced by heat and pressure which affect both the granite and the metamorphic rocks together. These features indicate that the granite was not emplaced quietly and passively, but in a dynamic environment. Igneous and metamorphic rocks are deformed by the same folds (Figure 9, Figure 10), and beautifully scalloped edges show that some of the metamorphic rocks were soft and malleable due to heat and pressure at the time that the molten granite was flowing around them (Figure 11).

folds
Figure 9		 another fold
Figure 10		 scalloped edges
Figure 11

Granite Types

There are several different varieties of granite at this locality. The most obvious variable is the size of the mineral grains that comprise the rock. Rocks with large mineral grains are said to be coarse-grained (Figure 12, Figure 13), and those with small mineral grains are said to be fine-grained (Figure 14). The predominant minerals of all true granites are feldspar and quartz. In the rocks here, the feldspar is milky white and the quartz is clear to translucent gray. Black mica (biotite) is almost always present, but in small amounts. Red garnet has been found in a few of the rocks here (Figure 13). Two granites of different grain size may have similar mineral compositions, and two granites of similar grain size may have different mineral compositions.

coarse-grained granite
Figure 12		 another coarse-grained granite
Figure 13		 fine-grained granite
Figure 14

Interaction among Granite Types

According to the fundamental rule of cross-cutting relationships, an igneous rock which cuts across another rock is younger than the other rock. At this field site, coarse-grained and fine-grained granites are found in many different relationships, suggesting either a complicated process or a complicated history of granite formation, or both. In some places, the fine-grained granite clearly cuts across coarse-grained granite (Figure 15). In some places, veins of coarse-grained granite cut through fine-grained granite (Figure 16). In many places, especially in the coarse-grained granite, there are granites of different grain sizes mixed together in patches that merge with each other without distinct boundaries (Figure 17). Some thick and fairly straight layers of very coarse-grained granite cut through fine-grained granite (Figure 18), although in this particular case, their boundary is not clean, and suggests that the two rocks were molten at the same time (Figure 19). And in some places, thin layers of fine-grained granite cut cleanly across all the other granite types, indicating that it is the youngest rock, at least in this particular place (Figure 20).

fine-grained granite cutting coarse-grained granite
Figure 15		 coarse-grained granite cutting fine-grained granite
Figure 16		 patches of mixed grain size
Figure 17		 very coarse granite cutting fine-grained granite
Figure 18		 boundary not clean between grain sizes
Figure 19		 fine-grained granite is youngest
Figure 20

Taken together, these relationships suggest that many of the granite types were probably molten and actively moving through the rock at the same time, but that some are clearly younger than others. Much of the volatility in the melting environment may be ascribed to the metamorphic rocks, which yield fluids when heated. Mixing of fluids rich in water or carbon dioxide with granitic magma can significantly and suddenly influence the grain size and mobility of the magma.

Younger Diabase Dikes

In addition to the migmatites described above, these bedrock exposures contain several thin, nearly vertical dikes of black rock, which cut straight through everything else (Figure 21). They are fine-grained igneous rocks called diabase, or basalt. They formed when magma from deep in the earth was forced upward through fractures. In contrast to the granites, which intruded when the metamorphic rocks were hot, the diabase dikes intruded through cold, brittle rock and did not interact with the wall rocks. This is illustrated by the straightness of the vertical walls, and the consistent dike thickness. Some dikes do have small jogs or offsets (Figure 22), but these occur around imperfections in the host rock, where it did not break open cleanly. Close inspection of the sides of the diabase dikes shows that the contacts with the host rock are sharp and brittle (Figure 23).

diabase dike
Figure 21		 jog in dike
Figure 22		 sharp contact
Figure 23

Importance

These extensive roadside exposures allow for detailed study of the character and relationships between the metamorphic rocks and the granitic melt, and also among the several varieties of granite. These studies lead to an understanding of geologic processes during the time of melting and intrusion. A series of geology students at the State University of New York College at Buffalo, under the direction of Prof. Gary Solar, and at the State University of New York at Oswego, under the direction of Prof. Paul Tomascak, have been studying these exposures in detail, both through careful descriptive field studies and a variety of laboratory studies. Some of this work has been presented at professional meetings by LaFleur and others (2008), Nyitrai and others (2009), and Bohlen and others (2010). These studies are gradually telling the story of how granite formed at this place. Similar meticulous studies have been done at other sites in the region (Hayes and Solar, 2006; Gulino and others, 2001; Kalczynski and Solar, 2008; Thalhamer and Solar, 2010).

This site is also important in understanding the nature of the Sebago batholith, a dominant feature of southern Maine geology, and the largest plutonic complex in Maine. Since the time of the 1985 State bedrock map, several workers have shown that the batholith is more complicated than had been appreciated previously. Quadrangle mapping, especially east of Sebago Lake, delineated several varieties of granite (Creasy, 1996; Creasy and Robinson, 1997). The regional map was redrawn to emphasize the area of migmatite east of the granite (Berry and Hussey, 1998). Several chemical and isotopic studies demonstrated a variety of melt sources and ages among the granites and migmatites (Tomascak and others, 1996a, 1996b; Cirmo and others, 2006; McAdam and others, 2009). As these lines of research evolve, regional interpretations of the history and significance of the Sebago batholith are continually reassesed (Tomascak and others, 1996a, 1996b; Creasy and others, 1998; Guidotti and others, 1998; Solar and Tomascak, 2001, 2002; Tomascak and Solar, 2001; Tomascak and others, 2008).

References cited

Berry, H.N., IV, and Hussey, A.M., II (editors), 1998, Bedrock geology of the Portland 1:100,000 quadrangle, Maine and New Hampshire: Maine Geological Survey Open-File Map 98-1.

Bohlen, T., Solar, G.S., and Tomascak, P.B., 2010, The eastern contact zone of the Sebago pluton, SW Maine: Implications for timing of emplacement: Geological Society of America, Abstracts with Programs, v. 42, p. 160.

Cirmo, A.C., Tomascak, P.B., and Solar, G.S., 2006, Geochemistry of undeformed granites of the Sebago migmatite domain, southern Maine: Geological Society of America, Abstracts with Programs, v. 38, p. 343.

Creasy, J.W., 1996, Preliminary report: Bedrock geology of the Naples and Raymond quadrangles: Maine Geological Survey, Open-File Report 96-4, 2 maps (Naples, and Raymond ) scale 1:24,000, 9 p. report.

Creasy, J.W., and Robinson, A.C., 1997, Bedrock geology of the Gray 7.5-minute quadrangle, Cumberland County, Maine: Maine Geological Survey, Open-File Report 97-3, map (scale 1:24,000), 9 p. report.

Creasy, J.W., Engelman, M.R., Robinson, A.C., and Shyka, P.M., 1998, Geology of the Sebago batholith, southwestern Maine: Geological Society of America, Abstracts with Programs, v. 30, p. 12.

Guidotti, C.V., Berry, H.N., IV, Thomson, J.A., Cheney, J.T., and Hames, W.E., 1998, The geologic envelope enclosing the Sebago batholith: Geological Society of America, Abstracts with Programs, v. 30, p. 22-23.

Gulino, C., Solar, G.S., and Tomascak, P.B., 2007, Structural, textural and petrographic variations in rocks on Bruce Hill, eastern Sebago migmatite domain, southern Maine: Geological Society of America, Abstracts with Programs, v. 39, p. 77.

Hayes, S.G., and Solar, G.S., 2006, Migmatite-granite relations at the centimeter- to meter-scale: Geological Society of America, Abstracts with Programs, v. 38, p. 415.

Kalczynski, M.J., and Solar, G.S., 2008, Structural and mineralogical variations associated with the southwestern contact of the Sebago pluton with the Sebago migmatite domain, SW Maine: Geological Society of America, Abstracts with Programs, v. 40, p. 27.

LaFleur, L.L., Solar, G.S., and Tomascak, P.B., 2008, Documentation of a pluton's contact zone in the middle crust: Details and implications of a new exposure of the eastern contact of the Sebago pluton and the eastern Sebago migmatite domain, southern Maine: Geological Society of America, Abstracts with Programs, v. 40, p. 26.

McAdam, S.M., Tomascak, P.B., and Solar, G.S., 2009, Elemental geochemistry of migmatites within the Sebago Migmatite Domain, southwestern Maine: Geological Society of America, Abstracts with Programs, v. 41, p. 80.

Nyitrai, K.A., Solar, G.S., and Tomascak, P.B., 2009, Mineral and textural variations in granites of the Sebago pluton at its eastern contact zone, southern Maine: Geological Society of America, Abstracts with Programs, v. 41, p. 31.

Solar, G.S., and Tomascak, P.B., 2001, Is there a relation between transpressive deformation and pluton emplacement in southern Maine?: Geological Society of America, Abstracts with Programs, v. 33, p. A-6.

Solar, G.S., and Tomascak, P.B., 2002, Transpression and granite magmatism in southern Maine: Integrated structural and geochemical studies: Geological Society of America, Abstracts with Programs, v. 34, p. A-77.

Thalhamer, E.J., and Solar, G.S., 2010, Mineralogical and textural variations in the migmatite-granite complex near the western contact of the Sebago pluton, SW Maine: Geological Society of America, Abstracts with Programs, v. 42, p. 159.

Tomascak, P.B., and Solar, G.S., 2001, Integrated structural and geochemical studies of granite magmatism, Maine Appalachians: EOS, v. 82 (47), abstract V52B-07, p. F1409.

Tomascak, P.B., Krogstad, E.J., and Walker, R.J., 1996a, U-Pb monazite geochronology of granitic rocks from Maine: Implications for late Paleozoic tectonics in the Northern Appalachians: Journal of Geology, v. 104, p. 185-195.

Tomascak, P.B., Krogstad, E.J., and Walker, R.J., 1996b, Nature of the crust in Maine, USA: Evidence from the Sebago batholith: Contributions to Mineralogy and Petrology, v. 125, p. 45-59.

Tomascak, P.B., Grade, M.S., and Solar, G.S., 2008, Isotopic heterogeneity and potential variable sources of granitic rocks of the Sebago migmatite domain, southern Maine: Geological Society of America, Abstracts with Programs, v. 40, p. 26.

Text and photos by Henry N. Berry, IV

Originally published on the web as the July 2010 Site of the Month.

Last updated on August 18, 2010