The Sebago pluton, located in southwestern Maine, is a two-mica granite that intruded the Silurian and Devonian metasedimentary rocks of the Central Maine Terrane during the Early Permian (Osberg et al. 1985; Tomascak et al. 1996). The Sebago pluton has an irregular oval shape with a long axis striking northwest-southeast and is approximately 90 km in length with a maximum width of ~35 km (fig. 1). Previous gravity studies have shown the pluton to be a 1 km thick sheet (Hodge et al. 1982), dipping shallowly to the northeast at about 3 degrees (Carnese 1983).

The Sebago pluton is part of the New Hampshire Plutonic Suite, a suite of syn- and post-kinematic granitic rocks from the Silurian to the Permian that extends from southwestern Maine northwesterly into New Hampshire. Rb-Sr isotopic studies conducted by Hayward and Gaudette (1984) dated the Sebago pluton as Carboniferous. Another study by Aleinikoff (1984) using U-Pb zircon methods estimated the crystallization age of the Sebago pluton to be around 325 Ma. More recently, however, Tomascak et al. (1996) found the pluton to be Permian, intruding around 292 ± 3 Ma.

The Sebago pluton is composed of a pink to white moderately foliated biotite-muscovite granite (Hussey 1989) that Wise (1995) further describes as a peraluminous, fertile granite enriched in rare alkalis, Al2O3, SiO2, Ga, Sn, B, F, and P. Creasy (1979) and Wise (1995) have demonstrated that the Sebago pluton has two spatially distinct phases. The outer phase is a texturally heterogeneous granite consisting of coarse pegmatites and multiple metasedimentary xenoliths from the adjacent country rock, while the inner phase is a more texturally homogeneous medium- to fine-grained granite containing very few xenoliths. Creasy (1979) hypothesized that the heterogeneous phase records the anatectic origin of this granite and marks the regional contact between the granites of the Sebago pluton and the surrounding country rock.

Hodge et al. (1982) examined the relationship between the shape of granitic plutons and their method of emplacement in southern Maine and New Hampshire and found that most of the plutons in southern Maine, including the Sebago pluton, are fairly thin horizontal sheets (1 to 10 km thick) that formed in the middle to lower crust.

Carnese (1983) conducted detailed gravity studies across a number of granitic plutons in western Maine and found the plutons in northern Maine to have a characteristic inverted tear-drop shape, while in southern Maine the plutons became progressively flatter with dips of ~3 degrees to the northeast. Carnese (1983) also determined that most of the plutons in this region extend below their northern surficial contact with the surrounding country rock.

Considering the Sebago pluton to be a sheet-like body and using the subsurface geometry determined by Carnese (1983), with an approximate surface width of 45 km, De Yoreo et al. (1989) calculated the thickness of the Sebago pluton to be about 2 km. This calculation is in fairly close agreement with the estimated thickness of 1 km given by Hodge et al. (1982). Using this model, De Yoreo et al. (1989) further claimed that the Sebago pluton is positioned with its top lying along the northern contact with the country rocks and the bottom of the pluton lying along the southern boundary. This geometry has been used to explain the Barrovian metamorphism reported by Thomson and Guidotti (1989) on the southern margin on the Sebago pluton.

Geoscience Services of Salem, Inc. (1986) compiled a regional gravity anomaly map for southwestern Maine using gravity data collected during seventeen separate gravity surveys over the last forty years. In this study, Geoscience Services of Salem, Inc. (1986) found the gravitational effect of the Sebago pluton to be fairly small, creating maximum anomalies of only 4 to 6 mGals. Based on the magnitude of these anomalies, Geoscience Services of Salem, Inc. (1986) concluded that the pluton is quite thin, reaching a maximum thickness of approximately 600 meters near the center, with the southern portion reaching a thickness of only 300 meters. It is important to note, however, that Geoscience Services of Salem, Inc. (1986) did not attribute any of the regional gravity trend as an effect of the Sebago pluton itself. Conversely, Hodge et al. (1982) concluded that the regional field was a combination of a deep-subsurface gravity field and the gravity effect of the Sebago pluton. Using this method, Hodge et al. (1982) calculated a greater thickness for the Sebago pluton, with a maximum thickness exceeding 1 km.

Geoscience Services of Salem, Inc. (1986) also found that along the northern contact with the country rocks, the Sebago pluton extends 10 km to 15 km north below the surface. This result is in agreement with the work of Carnese (1983), who also found the pluton to project below the surface north of its mapped surficial boundary. The southern boundary of the pluton, as mapped by gravity, however, did not correlate well with the mapped surficial contacts (Geoscience Services of Salem, Inc. 1986).

The gravity data for this study was collected at 216 stations located in the region defined by 43û45'N x 44û00'N and 70û15'W x 70û45'W (fig. 2a). All of the gravity measurements were taken using a Lacoste and Romberg Model G Land Gravity Meter. The necessary elevation control for these measurements was obtained using benchmarks set by the U.S. Geological Survey, U.S. Coastal and Geodetic Survey, the Maine Department of Transportation, and the water level of Sebago Lake. The data were then corrected for latitudinal and elevation effects, instrument and tidal drift, regional gravity trends, and finally linked to the International Gravity Standardization Net 1971 station PORTLAND B, located in Portland, Maine. Due to the low relief throughout the study area, no terrain corrections were applied to the gravity data.

From the corrected gravity data, the Bouguer gravity anomaly, the regional gravity field, the residual gravity anomaly, and the horizontal gravity gradient were calculated for the region. The gridding technique used was minimum curvature with a grid spacing of 1 grid point per square kilometer. Finally using a 3-D iterative mathematical inverse technique developed by Cordell et al. (1992), the contact between the pluton and the underlying metasedimentary rocks was calculated from the residual anomaly map.

In order to properly analyze gravity anomaly maps, it is necessary to know the density contrast between the separate geologic units. This was determined based on 20 randomly selected samples of the Sebago granite and 10 samples of the metasedimentary rocks. The average density calculated for the Sebago pluton was 2.59 g/cm3 with a standard deviation of 0.05 g/cm3, while average density for the metasedimentary rocks was found to be 2.75 g/cm3 with a standard deviation of 0.08 g/cm3. From these average densities, the density contrast between the pluton and the metasedimentary rocks was calculated to be -0.16 g/cm3, in close agreement with the density contrast of -0.17 g/cm3 found by Hodge et al. (1982) and Geoscience Services of Salem, Inc. (1986).

The Bouguer gravity anomaly map generated in this study is defined by a decrease of 32 mGals from the southeastern corner of the study area to the northwestern corner, with a range of -4 mGals to -36 mGals (fig. 2a). The Bouguer gravity anomaly represents the total gravity variation caused by the subsurface geology in the region. Consequently, local gravity variations are often obscured by regional gravity trends in a Bouguer gravity anomaly map. However, the Bouguer anomaly map does show some distinct correlations between the gravity field and the local bedrock geology. As expected, based on the negative density contrast between the pluton and the metasedimentary rocks, more negative Bouguer anomalies are seen associated with exposures of the Sebago granite and less negative anomalies correspond to the metasedimentary rocks. Furthermore, in the southeastern quadrant of the study area sharp changes in the Bouguer anomaly are seen corresponding to the contact between the pluton and the country rock. In particular, the position and orientation of the ridge-shaped gravity high located at 70° 25’W x 43° 48’N correlates well with the exposed metasedimentary rocks in this region.

However, although the Bouguer anomalies correspond well with the geologic contacts in the southeastern quadrant, the same degree of agreement is not found in the southwestern quadrant of the study area. In order to determine if this disagreement is an effect of the regional gravity field obscuring the geologic contact, the residual gravity anomaly is calculated across the study area.

The regional gravity field found across the study area was fairly strong, dipping to the northwest at a rate of 0.54 mGals/km. Kane and Bromery (1968) attributed this regional field to the White Mountain plutonic-volcanic complex in New Hampshire. Kane et al. (1972) hypothesized that the regional gravity field in southern Maine is caused by one of three sources: 1) a lower density contrast between the pluton and metasedimentary rocks than previously calculated, 2) a deep subsurface source, or 3) thin sheet-like granitic rocks that are unexposed at the surface. Based on the density data from this study and Hodge et al. (1982), the first explanation seems unlikely. Moench and Zartman (1976) argue for the third option, emphasizing that the Sebago pluton is known to dip below the surface north of its northern contact with the country rock. No one, however, has yet produced definitive evidence to distinguish between the second and third possibilities (Hodge et al. 1982).

The residual gravity anomaly map created in this study shows a range of anomalies from 3 mGals to -8 mGals (fig. 2b). Because the regional gravity field has been subtracted, the only effects that should be expressed in the residual anomaly map are those caused by the local geology. Similarly to the Bouguer anomaly map, the correlation between the residual anomalies and the mapped bedrock geology in the southeastern quadrant of the study area is fairly good with negative anomalies associated with exposures of the Sebago granite and positive anomalies correlating to the metasedimentary rocks (fig. 2b). In particular, the same distinctive northeasterly trending ridge-shaped gravity high, located around 70û25'W x 43û48'N, can be seen corresponding to the metasedimentary rocks in this region. Moreover, in the central portion of the study area, negative residual anomalies reaching ~ -7 mGals are associated with the mapped Sebago granite. However, in the southwestern quadrant of the study area, the residual anomalies remain strongly negative south of the mapped surficial contact between the pluton and the country rock, and it seems probable that in this region, the pluton projects southward below the cover of the metasedimentary rocks.

Another abnormality in the relationship between the residual gravity anomaly and the bedrock geology occurs in the northeastern corner of the study area. Here, positive residual anomalies of ~1 mGal are seen associated with the mapped Sebago granite. This supports the conclusion drawn by Hodge et al. (1982) and Geoscience Services of Salem, Inc. (1986) that the gravity signature of the Sebago pluton is very small. Moreover, the heterogeneous nature of the granite in this area may cause slight variations in the density contrast that could account for these positive anomalies.

The horizontal gravity gradient represents the rate of change in the residual gravity anomaly. Consequently, areas in which a lithologic contact is located between two geologic units of distinct densities should show high horizontal gravity gradients. Conversely, low gradients will be associated with areas where one geologic unit of uniform thickness is present, or two units of low density contrast.

Figure 2c shows the horizontal gravity gradient across the study area. As expected, the gravity gradient is fairly low throughout most of the northern and central portions of the study area, which are mapped exclusively as Sebago granite. Moreover, in the southeastern quadrant, high gravity gradients are seen associated with the contact between the pluton and country rock.

However, in the southwestern quadrant of the study area, no increase in the horizontal gradient is found corresponding to the contact between the pluton and the metasedimentary rocks. This further supports the hypothesis that the pluton does not terminate at this contact, but rather continues to the southwest beneath the surface.

The model of the subsurface contact between the Sebago pluton and the underlying metasedimentary rocks plus a cross-sectional view of the pluton are shown in figure 3. The model shows the thickest portions of the pluton (~1.8 km) to occur at the bottom of a bowl-shape located near the southwestern contact. Moreover, the model shows the pluton thinning toward the northern and eastern portions of the study area, where the average thickness of the pluton is less than 0.5 km. Thus, in contrast to the original model of the Sebago pluton as a thin sheet (Hodge et al. 1982) dipping to the northeast at ~3 degrees (Carnese 1983), this study suggests that the pluton extends into the subsurface below its geologic contact with the metasedimentary rocks in the southwestern portion of the study area.

The Cordell et al. (1992) modeling method is limited by the fact that the resulting subsurface contact is only as accurate as the residual anomaly from which the model was derived. Thus, if the residual anomaly is slightly too large or too small, this error will be expressed in the depth of the subsurface contact. Moreover, the Cordell et al. (1992) modeling routine will only show the presence of the pluton in areas in which the residual anomaly is negative. Therefore, if the residual anomaly is positive in certain regions where the pluton is known to exist from field observations, the model cannot adjust its results accordingly. This is particularly problematic with plutons that have very weak gravity signatures, such as the Sebago pluton. A final limitation of the Cordell et al. (1992) modeling method is that a single density contrast must be used, presenting problems if there are density variations across the pluton.

The model generated in this study clearly contradicts the previously proposed northeasterly dipping sheet-like geometry for the Sebago pluton. Three main conclusions concerning the subsurface shape of the Sebago pluton can be drawn from the gravity data collected in this study: 1) the southern contact of the pluton dips shallowly to the south at about 2 degrees (fig. 3) with the adjacent metasedimentary rocks located above the pluton, 2) the overall shape of the entire pluton is one of an arched sheet dipping shallowly to the north below its northern contact and shallowly to the south below its southern contact with a very thin (0.5 km thick), nearly horizontal interior (fig. 4), and 3) the thickness of the pluton is highly variable, ranging from <0.5 km to ~1.8 km across the study area.

The regional implications of this study are numerous. Using the sheet-like, northeasterly dipping slab model, Thomson and Guidotti (1989) proposed that the last, peak metamorphic event, which is spatially related to the Sebago pluton, is Carboniferous in age. In their model, the metasedimentary rocks near the southern contact of the pluton were underneath the pluton and are characterized by Barrovian metamorphism, while those along the west, north, and eastern contacts are characterized by upper-sillimanite zone metamorphism, and abundant migmatites and pegmatites. However, this study suggests that both the northern and southern margins of the pluton dip shallowly beneath the cover of the metasedimentary rocks. Therefore, if the Sebago pluton formed during a single magmatic episode, the relationship of the metamorphic zones and pluton shape proposed by Thomson and Guidotti (1989) must be modified, as must the thermal modeling of De Yoreo et al. (1989).

In addition to these geometric inconsistencies with respect to the metamorphism, Tomascak et al. (1996) pointed out that the geochronologic constraints on the timing of peak metamorphism surrounding the Sebago pluton are inconsistent with their new 293 ± 2 Ma age for the batholith. Possibly given the range in ages (from Carboniferous to Permian) now reported for the pluton (Aleinikoff 1984; Tomascak et al. 1996), the metamorphic zones could have been caused by multiple phases of "Sebago magmatism" over an extended period of time during the Carboniferous and Permian and the batholith is itself composite, as suggested by Gibson and Lux (1996).

To resolve these conflicting geophysical and geologic interpretations, we suggest that the portion of the granite extending south of the southern Sebago contact is an older granite that is contiguous, but not coeval with the classic Sebago sheet-like pluton. The younger sheet-like pluton would have been above the unmigmatized metasedimentary rocks, which contain few if any pegmatites, and is now eroded away. In this model the northeasterly dipping sheet-like geometry for the Sebago pluton could be preserved and the Barrovian metamorphic signature in the south would be a product of contact metamorphism associated with the sheet (Thomson and Guidotti 1989), while the older contiguous granite underneath these metasedimentary rocks is cut by the sheet and unrelated to the Barrovian metamorphism.

To test this model, geologic mapping needs to be done along the contact, and especially within the pluton, to define the cross cutting relationships between the multiple phases of two-mica granite magmatism. Additionally, the geochronologic control needs to be increased by dating numerous portions and phases of what must be a composite Sebago batholith.

The results of this study show: 1) the southern contact of the Sebago pluton dips shallowly to the south at ~2 degrees with the metasedimentary rocks located above the pluton, 2) the present, post-erosional shape of the pluton (or plutons) is an arched sheet, and 3) the thickness of the pluton is highly variable. These results question the mode of emplacement, timing of intrusion(s), and petrogenesis of the Sebago pluton. In particular, the contradictory Carboniferous and Permian ages given for the pluton by Aleinikoff (1984) and Tomascak et al. (1996), respectively, suggest the Sebago pluton is in fact a composite batholith that is composed of a younger granite, which forms the classic Sebago sheet-like pluton, and a contiguous older granite extending south of the southern Sebago contact.

We thank the Howard Hughes Medical Institute for the funding for this project, Tim LeSiege at the Maine Department of Transportation, and Bob Moose and Curt Crow for providing the benchmark information necessary to conduct this study. Finally we thank Wallace Bothner, Charles Guidotti, and John Creasy for their insightful comments and suggestions relating this project to the geologic history of the area.

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Carnese, M.J. 1983. Gravity Studies of Intrusive Rocks in West Central Maine. M.S. thesis, University of New Hampshire, Durham, NH.

Cordell, L., Phillips, J.D., and Godson, R.H. 1992. U.S. Geological Survey Potential-Field geophysical software Version 2.0. U. S. Geological Survey, Open-File 92-18.

Creasy, J.W. 1979. Geologic Report to Accompany the Preliminary Geologic Map and Structure Sections of the Poland 15’ Quadrangle, Maine. Maine Geological Survey, Open-File 79-15.

De Yoreo, J.J., Lux, D.R., and Guidotti, C.V. 1989. A Thermal Model for Carboniferous Metamorphism near the Sebago Batholith in Western Maine. In Studies in Maine Geology, vol. 3. Edited by R.D. Tucker and R.G. Marvinney. Maine Geological Survey, pp. 19-35.

Geoscience Services of Salem, Inc. 1986. Gravity and its Geological Interpretation: The Sebago Pluton and Vicinity, Southwestern Maine. Maine Geological Survey, Open-File No. 86-15.

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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, 104: 185-195.

Wise, M.A. 1995. Petrology of the Sebago Batholith and Related Pegmatites. In 1995 NEIGC Guidebook to Field Trips in Southern Maine and Adjacent New Hampshire. Edited by A.M. Hussey, II and R.A. Johnston. Maine Geological Survey, pp. 229-242.

Figure 1

Figure 2 (a & b)

Figure 2 (c)

Figure 3

Figure 4