U-Pb age of the Sebago batholith, southwestern Maine: Metamorphic and tectonic implications

JOHN N ALEINIKOFF ) ROBERT H MOENCH J U'S' GeoloSical Survey, Box 25046, Federal Center, M.S. 963, Denver, Colorado 80225 JOHN B. LYONS Department of Earth Sciences, Dartmouth College, Hanover, New Hampshire 03755

ABSTRACT cuts the white. He suggested a quarry exposure in northern New England that other plutons, for the site of sample Me/PM 1-81 (pink phase). including the Sebago, were also . This Two phases (pink and white ) of Archie W. Berry, Jr., mapped the eastern part of study was initiated to determine the age of intru- the Sebago batholith of southwestern Maine the batholith in the Norway (15') quadrangle. sion of the Sebago batholith and to determine have been dated by the U-Pb zircon method. According to his observations (1982, oral com- whether the two recognized phases of granite Identical upper concordia intercepts of both mun.), the pink color appears to be due to alter- within the batholith are coeval. The results have rocks indicate an intrusive age of 325 ± 3 m.y. ation, and therefore two varieties may not be led us to reinterpret metamorphic and tectonic for the batholith. The lower intercept of the present. In Figure 1, the batholith is shown as a features of this area of New England. pink-phase sample, 114 ± 13 m.y., is inferred uniform body because these color variations to represent episodic lead loss due to the cannot be mapped separately. Sample Me/No PREVIOUS GEOCHRONOLOGY intrusion of the nearby Pleasant 1-82 (white phase) was collected at the eastern Table 1 lists numerous isotopic age determi- Mountain stock. The lower intercept of the margin of the batholith. nations that were made on rocks located near white-phase sample, 18 ± 21 m.y., suggests The southwestern portion of Maine has been the Sebago batholith. Four small stocks (Hallo- only modern dilatancy lead loss. Monazites studied by many New England geologists inter- well, Togus, Three Mile Pond, and Haitian d; see have ages of 272 m.y. (pink) and 282 m.y. ested in regional geology, metamorphism, tec- Fig. 1) have Devonian crystallization ages. (white) which are thought to be cooling ages. tonics, and geochronology (for example Pan- Gaudette and others (1982) dated the Lyman Rb-Sr whole-rock data have low initial kiwskyj and others, 1976; Moench and Zartman, pluton, located ~20 km due south of the Sebago ""Sr/^Sr ratios of 0.7031 (pink) and 0.7053 1976; Holdaway and others, 1982). Devonian batholith, at 322 ± 12 m.y., and Hay ward and (white). These data, in conjunction with pub- ages have been obtained from several plutonic Gaudette (1984) recently determined a Carbon- 40 39 lished Ar/ Ar, Rb-Sr, K-Ar, and fission- bodies in the vicinity (Dallmeyer and Van Bree- iferous Rb-Sr age for the Sebago. Analyses of track ages, suggest that little or no uplift men, 1981; Dallmeyer and others, 1982), and 40Ar/39Ar ratios in micas and hornblende by occurred in this part of New England until the the assumption was made by most investigators Dallmeyer (1979), Dallmeyer and Van Bree- and that the uplift rate from 275 m.y. to 225 m.y. was ~3 times as rapid as was the rate for 225 m.y. to the present. The Carbonif- TABLE 1. RADIOMETRIC AGES FROM ROCKS LOCATED NEAR THE SEBAGO BATHOLITH erous age of the Sebago batholith suggests Body Mineral Technique Age (m.y.) Reference that currently accepted metamorphic and tec- no. tonic interpretations for southwestern Maine Hallowell Whole-rock Rb-Sr 387 ± 11 1 and for east-central New Hampshire require Togus Whole-rock Rb-Sr 394 t 8 1 Three Mile Pond Whole-rock Rb-Sr 381 ± 14 I revision. Songo Zircon U-Pb 381 ± 4 9 Lyman Whole-rock Rb-Sr 322 ± 12 3 Pleasant Mtn. Biotite K-Ar 112 ± 3 4 Hallowell Biotite Ar-Ar 275 ± 5 1 INTRODUCTION Hallowell Muscovite Ar-Ar 285 ± 5 1 Togus Biotite Ar-Ar 268 ± 5 1 Togus Muscovite Ar-Ar 298 Î 5 1 Three Mile Pond Biotite Ar-Ar 300 t 5 1 The Sebago batholith, exposed in southwest- Three Mile Pond Muscovite Ar-Ar 301 ± 5 1 Three Mile Pond Hornblende Ar-Ar 351 t 5 1 ern Maine and east-central New Hampshire, is Hartland Biotite Ar-Ar 363 ± 5 2 2 the largest plutonic body in New England Hartland Hornblende Ar-Ar 362 ± 5 Meta. terrain Hornblende Ar-Ar 333 5 2 (-2,700 km ). The batholith is approximately Sebago Biotite Ar-Ar 227 5 Hallowell Biotite K-Ar 266 Ï 8 6 kidney-shaped and elongated in a west-north- Hartland Biotite K-Ar 363 ± 11 6 North Jay Biotite K-Ar 253 ± 8 6 west-east-southeast direction, with a maximum Hallowell Apatite FT 219 ± 22 7 North Jay Apatite FT 150 ± 15 7 length of ~80 km and a maximum width of—35 White Mtn. bath. Zircon FT 163 ± 14 8 km (Fig. 1). During reconnaissance mapping in White Mtn. bath. Apatite FT 94 ± 8 8

the western part of the batholith, Norman L. 1. Dallmeyer and Van Breemen (1981). 6. Zartman and others (1970). Hatch (1981, oral commun.) recognized two 2. Dallmeyer and others (1982). 7. Naeser and Brookins (1975). 3. Gaudette and others (1982). 8. Doherty and Lyons (1980). varieties of granite, pink and white. According 4. Foland and Paul (1977). 9. Lux and Aleinikoff ( 1985) 5. Dallmeyer (1979). to his observations, the pink variety consistently

Geological Society of America Bulletin, v. 96, p. 990-996,4 figs., 4 tables, August 1985.

990

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mapping). In contrast to the predominantly sub- vertical structure of central Maine, dips of gneiss- ic layering or stratification are gentle in the northwest-trending belts, probably subparallel to the subhorizontal contacts of the Sebago batho- lith. Gravity studies indicate that the Sebago batholith is, in fact, a subhorizontal sheet-like body having a calculated thickness of < 1 km (Hodge and others, 1982). The two samples dated in this study are both two-mica in which biotite exceeds mus- covite by a ratio of ~ 3:1 and in which there is minor, but pervasive, sericitization of plagioclase and chloritization of biotite. Sample Me/PM 1- MAP. LOCATION 81, the pink, and possibly younger, granite, is medium to coarse grained, allotriomorphic and subporphyritic with microcline-perthite pheno- crysts (36%), plagioclase (An12; 21%), quartz (33%), biotite (7%), muscovite, possibly late (2%), and accessory Fe-Ti oxides, zircon, mona- zite, and apatite. The white granite (Me/No 1- 82) has essentially the same mineralogy; the rock is equigranular, and plagioclase is slightly more calcic (An16). Both rocks have been weakly deformed subsequent to crystallization, Figure :I . Location of New as is shown by tapered twinning in plagioclase Hampshire Plutonic Suite bodies and by undulatory extinction in quartz. Chemi- and metam orphic zones in south- cal analyses of eight samples of the Sebago gran- central Mai ne (modified from Gui- ite (Table 2) indicate very little difference in dotti, 1983). Metamorphic zones are composition. GS (greensichist) , E (epidote-am- Zircons from both dated samples are subdi- phibolite), J VA (low-rank amphibo- vided into two populations (Fig. 2), nonmag- lite), AB (i nedium-rank amphibo- netic and magnetic. The nonmagnetic fractions lite), and J lC (high-rank amphib- contain zircons that have a range of color (clear olite). Loca tions of samples in this to light brown), length-to-width (L/W) ratio study are shown. Plutons men- (1-6:1), shape (prismatic euhedral to semi- tioned in t he text and table are rounded), and presence of inclusions (none to Sebago (S) , Songo (So), Hartland very numerous). Fine euhedral zoning and (HT), Hall Dwell (H), Togus (T), darker cores are occasionally present. In con- 50 MILES Three Mile Pond (TMP), North Jay trast, most zircons in the magnetic fractions are I I I (NJ), Lyman (L), and Pleasant opaque (dark brown to black), subhedral, and 0 50 KILOMETERS Mountain stock (PM). range in L/W from -2-4:1.

men (1981), and Dallmeyer and others (1982) geologists indicate that the body is composed of GEOCHRONOLOGY delineate a range of ages becoming younger to rather homogeneous pink and white two-mica the southwest across the northeast termination granite. Inclusions of metasedimentary rocks are Approximately 50 kg each of the pink and of the belt of Permian K-Ar mica ages (Faul and locally abundant throughout the body and are white phases were collected for zircon U-Th-Pb others, 1963; Zartman and others, 1970). The particularly abundant in a belt 4-10 km wide geochronology, and a whole-rock split of each Sebago batholith is near the northeast end of this along the eastern side of the pluton. Pegmatites was obtained for Rb-Sr analysis. The pink phase belt of disturbed K-Ar ages. Fission-track ages are common. The well-known gem pegmatites (sample Me/PM 1-81) was collected from a (Table 1) on apatite and zircon from rocks lo- at Paris and Norway, Maine, are at the north- large quarry exposing homogeneous granite in cated to the east and west of the batholith also central border of the pluton. the interior of the batholith ~4 km northeast of follow the trend of becoming younger toward Although the long dimension of the batholith the cross-cutting Pleasant Mountain stock. The the southwest. lies athwart the predominantly northeast-trend- white phase (sample Me/No 1-82) was col- ing structural grain of New England, this grain lected from a small quarry near the northeast GEOLOGIC SETTING AND bends abruptly to the northwest in a belt as margin of the batholith. The site of the white ROCK DESCRIPTION much as 25 km wide along the northeast and sample is within the wide marginal zone of the southwest sides of the batholith (Guidotti, 1965; batholith containing abundant metasedimentary The Sebago batholith has not been studied in Creasy, 1979; Gilman, 1977; Hussey and Pan- layers and wisps, and the rock of the quarry is detail, but reconnaissance mapping by several kiwskyj, 1975; A. W. Berry, Jr., 1982, unpub. massive, equigranular, and shows no obvious ev-

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TABLE 2. MAJOR-ELEMENT ANALYSES OF PINK AND WHITE PHASES OF THE SEBAGO BATHOLITH intercept ages of 324 ± 4 and 114 ± 13 m.y. (95% confidence). The upper intercept is inter- Oxide Nor-1* Nor-2* Nor—3A* Nor-4B* Me/No 1-82* Nor-3B* Nor-4B* Me/PM 1-8 it (white) (white) (white) (white) (white) (pink) (pink) (pink) preted as being the age of crystallization and thus indicates a Carboniferous age of intrusion. Si02 71.8 72.2 71.7 70.8 72.6 71.2 72.1 71.5 The lower intercept is interpreted as being the TiOj 0.13 0.17 0.26 0.28 0.27 0.31 0.21 0.20 A1203 15.4 15.1 14.6 15.4 14.1 14.8 14.5 15.0 age of episodic lead loss. This sample was col- Fe203 0.80 0.89 1.61 1.87 1.88 1.66 1.48 1.95 MgO 0.21 0.27 0.40 0.41 0.35 0.34 0.35 0.45 lected ~4 km east of the Pleasant Mountain CaO 1.01 1.13 1.23 1.27 0.72 1.10 1.15 1.12 stock, a White Mountain Plutonic-Volcanic Na20 3.17 3.15 2.91 3.08 2.98 3.01 3.12 3.87 K2O 6.52 6.03 5.84 5.39 5.80 5.94 5.53 5.40 Suite augite syenite dated by Foland and Faul P2O5 0.11 0.10 0.19 0.29 0.10 0.15 0.17 0.09 MnO <0.02 <0.02 <0.02 <0.02 <0.02 <0.02 <0.02 0.02 (1977) at 112 ± 3 m.y. It seems likely that the LOI 0.89 0.77 0.88 0.76 0.40 0.69 0.68 0.47 heat supplied by an apophysis of the intrusion of Total 100.00 99.91 99.69 99.58 99.21 99.23 99.34 100.07 the stock caused lead loss in zircons from the *XRF analysis by A. J. Band, K. Stewart, nearby pink phase of the Sebago batholith. J. Taggart. U.S. Geological Survey, Denver, Colorado. +XRF analysis by X-ray Assay The (-150) Mag fraction was excluded from Laboratories, Ltd., Ontario, Canada. the best-fit calculation because it drastically in- creased the uncertainties in both intercept ages. Two possible causes for its location off the dis- idence of contamination by country rocks. In stants for uranium and thorium and 235U/238U cordia are: (1) modern dilatancy lead loss (Gol- bulk composition (Table 2), however, this sam- are those values recommended by Steiger and dich and Mudrey, 1975) after the 114-m.y. ple differs from other samples of Sebago granite Jäger (1977). lead-loss episode; (2) inheritance from older (both pink and white) in having slightly greater U-Th-Pb isotopic data for zircon and mona- rocks. The (+150) Mag fraction contains twice Si02 and slightly less A1203, CaO, and Na20. zite from both phases of the Sebago batholith as much uranium as the (-150) Mag fraction This sample was intentionally collected to de- are listed in Table 3 and plotted in Figure 3. and should be more susceptible to leaching of termine if proximity to xenolithic contamination Clear, nonmagnetic zircons from both phases radiogenic lead (dilatancy lead loss) as the rock would affect the U-Pb isotopic systematics. contain between 1,400 and 2,100 ppm U, and is exhumed to the surface. The Pb/U ratios in Zircon and monazite were extracted, using dark, magnetic zircons contain between -5,000 the coarse fraction do not indicate disturbance of conventional mineral separation techniques, in- and 11,000 ppm U. The uranium concentration the U-Pb systematica after 114 m.y. ago, and cluding Wilfley Table, heavy liquids and mag- in the clear, nonmagnetic zircons is higher than thus the second interpretation is favored. Prior netic separator; then they were split into size, is typical for granitic zircons (Gebauer and Grü- to episodic lead loss 114 m.y. ago, the

color, and magnetic susceptibility fractions rang- nenfelder, 1979); the concentrations in the mag- 207Pb/206Pb age of the (_150) Mag fraction ing from 2 to 18 mg. Analytical procedures for netic zircons are quite high, and 11,000 ppm U thus, was probably slightly older than was the measurement of lead, uranium, and thorium iso- in the Me/PM 1-81 (+150) Mag fraction is ex- age of the granite, due to a small amount of topes are given in Aleinikoff and others (1979). tremely unusual. inheritance of radiogenic lead in the finer- Common lead in zircon and monazite was cor- When plotted on a concordia diagram (Fig. grained zircons. Subsequent episodic lead loss rected using the Stacey and Kramers (1975) 3), four of five fractions from the pink phase moved that data point on a trajectory toward value for 325-m.y.-old lead (see Table 3). Total (Me/PM 1-81) form an excellent linear array. A 112 m.y., along a somewhat different path than laboratory blank lead monitored throughout the best-fit line calculated through those 4 data was followed by the other data points that did course of the study was <1.5 ng. Decay con- points [Me/PM 1-81 (-150) Mag excluded] has not contain inheritance.

A1 * , * • ¥ Hi » u t • 9 r-%

A B

Figure 2. Photomicrographs of zircons from the white and pink phases of the Sebago batholith. Average length of grains shown is -200 pin. A. Me/No t-82 (+150)NM. Most grains are clear, and some show euhedral zoning. B. Me/PM 1-81 (+I50)Mag. Most grains are dark gray to opaque.

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Monazite from Me/PM 1-81 has concordant 0.054, Pb/U and Pb/Th ages at 272 (±2) m.y. Because monazite is known to be more easily reset than is zircon (although probably at a temperature of

>600 °C, Gebauer and Grunenfelder, 1979) and 0.050 because of the excellent linearity of the zircon data, the 272-m.y. date is thought not to be the crystallization age. The monazite data indicate that the batholith last cooled to a temperature 0.046 less than about 600 °C -50 m.y. after Carbonif- D 00 CO erous time of emplacement and crystallization. rN -D As suggested by Holdaway and others (1982) Ci- to and Cheney and Guidotti (1979), on the basis of o CM metamorphic assemblages in surrounding rocks, 0.042 the two-mica granite probably intruded crust at a pressure of ~3.5 kbar (corresponding to a depth of ~ 11.5 km) and remained at that depth into Permian time. 0.1 The 3 size fractions of nonmagnetic zircon from sample Me/No 1-82 (white phase) form a linear array with concordia intercept ages of 325 ± 2 m.y. and 18 ± 21 m.y. The upper intercept 0.034 coincides with the pink-phase zircon data and 0. indicates that the two phases are the same age. 207pb/235|J The lower intercept age indicates modern lead loss, perhaps due to dilatancy (Goldich and Figure 3. Concordia plot of Pb/U isotopic data from zircon and monazite of the pink and Mudrey, 1975) when the rock was exhumed at white phases of the Sebago batholith. Dots are zircons from Me/No 1-82; circles are zircons the surface. These zircons were not affected by from Me/PM 1-81; squares are monazites from both samples (see Table 3 for data). the intrusion of the Pleasant Mountain stock (~20 km to the southwest). Conversely, zircons in the pink-phase sample did not suffer modem (5,600 ppm U) than did the coexisting nonmag- the monazite from the pink phase. The slight lead loss, possibly because the heat from the netic zircons, similar to the uranium concentra- reverse discordance (207Pb/206Pb <207Pb/ Pleasant Mountain intrusion caused annealing of tions in the dark magnetic zircons in the pink 235U <206Pb/238U; Table 3 and Fig. 3) is prob- the crystal lattices. Following the 110-m.y. re- phase. The 207Pb/206Pb age of 349 m.y. from ably due to minor uranium loss. There are two heating, there probably was insufficient radia- the Me/No 1-82 (+150)DK zircons probably possible causes for this 10-m.y. difference in tion damage to permit modern lead loss due to indicates a minor amount of inheritance. Mi- monazite ages. (1) The white-phase sample was dilation. We conclude that the most reasonable croscopic examination reveals cores in some of collected at the northeast margin of the batho- explanation for different U-Pb systematics of the the dark zircons, suggesting inclusion of xeno- lith; thus, this part of the rock cooled faster than two samples is their respective locations within crystic material, possibly derived from residual did the interior. (2) A regional uplift and cooling the Sebago batholith. zircon in the anatectic melt or assimilation of the gradient existed in this part of New England One size fraction of dark magnetic zircon wall rocks during intrusion. where uplift occurred more rapidly to the north- from the white phase was hand-picked and ana- Monazite in the white phase has a 207Pb/ east. We cannot choose a preferred cause lyzed. It contained considerably more uranium 206Pb age of 282 (±2) m.y., 10 m.y. older than because both seem viable. The white-phase

TABLE 3. U-Th-Pb ISOTOPIC DATA FOR ZIRCON AND MONAZITE FROM GRANITES OF THE SEBAGO BATHOLITH

Sample Fraction Concentrations Atomic % Ages (m.y.) 204 206 207 208 name U Th Pb Pb Pb Pb Pb ^Pb ^Pb 207^ 208pb

238u 235y 206pb 232^

(+150)NM 1,396 62.2 0.010 90.6 4.91 4.43 295 297 313 (-150+2001NM 1,522 73.2 0.015 90.4 5.00 4.61 316 317 322 (-200)NM 1,563 70.7 0.023 89.8 5.07 5.11 296 298 313 (+150)Mag 11,028 380.8 0.046 91.5 5.42 3.02 231 235 280 (-150)Mag 4,999 208.6 0.061 84.4 5.32 10.18 255 260 305 (+150)NM-Mon.* 9,097 68,653 1,193 0.007 28.6 1.58 69.83 272 272 272

(+150JNM 1,506 59.5 0.085 86.5 5.81 7.62 247 254 318 (~200+250)NM 1,518 75.9 0.104 86.3 6.09 7.52 309 310 323 (~325)NM 2,116 88.7 0.093 87.2 5.97 6.69 263 269 324 (+150)DK 5,619 234.3 0.119 84.6 6.27 8.97 252 262 349 (+150)NM-Mon* 10,172 52,049 1,110 0.024 37.5 2.29 60.21 294 293 282

Note: common lead correction of204Pb: 206Pb: 207Pb: 208Pb = 1:18.2:15.6:38 (Stacey and Kramers, 1975). •Monazite; all other samples are zircon.

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sample cooled through the closure temperature TABLE 4. Rb-Sr WHOLE-ROCK DATA FROM GRANITES OF THE SEBAGO BATHOLITH GRANITE before the middle part of the batholith as a result of normal cooling from the margins inward dur- Sample Rb (ppm) Sr (ppm) 87Rb/86Sr 87Sr/86Sr IR» ing crystallization. As shown by Zartman and Me/PM 1-81 284.5 116.0 7.1206 0.73605 0.7031 others (1970) using K-Ar dates, however, rocks Me/No 1-82 316.2 112.7 8.1547 0.74298 0.7053 to the east of the Sebago have ages older than do •Initial 87Sr/86Sr calculated from the data using an intrusive age of 325 m.y. rocks to the west, suggesting to these authors that the belt of young ages was primarily caused by differential uplift. Thus, cooling and/or uplift may account for the differences in the monazite granite at Milford, New Hampshire (275 m.y.; ever, include the following. (1) There may be a ages. Aleinikoff and others, 1979), the Narragansett 50-m.y. gap between the 275 m.y. and 325 m.y. Rb-Sr analyses of whole-rock splits from Pier Granite in southern Rhode Island (275 magmatism in New England. (2) All muscovite- both phases are listed in Table 4. Because the m.y.; Kocis and others, 1978), and the Lyman bearing plutons discussed by Sinha and Zietz age of the Sebago batholith was precisely de- pluton (322 m.y.; Gaudette and others, 1982). (1982) have 87Sr/86Sr initial ratios >0.708. The termined by the U-Pb isotopic systematics, the In addition, lower intercept ages on zircon from tectonic, geologic, and geochemical origins of purpose of the limited Rb-Sr data was to the Fishbrook Gneiss and the Westboro Forma- plutons in New England thus may not be appli- calculate initial 87Sr/86Sr ratios for comparison tion in Massachusetts are 289 m.y. and 278 cable to models for the southern Appalachians. with other New England granites. Sebago gran- m.y., respectively (Olszewski, 1980). The range Hercynian activity has also been well docu- ite initial 87Sr/86Sr ratios are 0.7031 (pink in ages (275 to 325 m.y.) is younger than most mented in France (Michard-Vitrac and others, phase) and 0.7053 (white phase), and Hayward of the post-tectonic plutonic rocks of New Eng- 1980), Great Britain (DeSouza, 1982), and and Gaudette (1984) similarly determined an land. In the southern Appalachians, Fullagar Central Europe (VanBreeman and others, initial 87Sr/86Sr ratio of 0.7051 for an unspeci- and Butler (1979) showed that many plutonic 1983). fied sample of Sebago granite. These results are bodies were intruded during the late Paleozoic in agreement with the interpretation of inheri- orogeny, 265 to 325 m.y. ago. Sinha and Zietz II. Metamorphism tance in the dark zircons from the white phase (1982) proposed a Hercynian magmatic arc in (Table 3). The presence of muscovite in the Se- the southern Appalachians which is marked by The Sebago batholith lies at the northeast 87 86 bago granite suggests a metasedimentary anatec- an eastern belt of low Sr and initial Sr/ Sr termination of the high-grade sillimanite terrane tic origin, but the Rb-Sr data indicate that the and relatively high K2O and Si02, and a west- of Thompson and Norton (1968) and between a source rock for the granite either must have been ern belt of lower K2O and Si02 and higher Sr high-rank amphibolite (sillimanite + K-fs) ter- only slightly older or had a low Rb/Sr ratio. and initial 87Sr/86Sr. If a similar magmatic arc 87 86 rane on the north and a medium-rank amphibo- The initial Sr/ Sr of the Sebago granite is existed in Maine, the Sebago batholith may be a lite (sillimanite + muscovite) terrane on the similar to the initial ratio of 0.7044 obtained by part of the eastern belt; the locations of plutons south (Fig. 1; C. V. Guidotti, 1983). Holdaway Gaudette and others (1982) from the 322-m.y.- that could be assigned to a western belt have yet and others (1982) identified four metamorphic old Lyman pluton. They described the Lyman as to be determined. Apparent differences, how- phases (Ml to M4) in this region of Maine, consisting of pink biotite granite and gray two- mica granite. These similarities between the Lyman and Sebago plutons suggest a compara- ble origin. T T" 'WR zircon tU/Pb^ (Rb-Sr) DISCUSSION 600 monazite (Pb/Pb)

I. Hercynian (Late Paleozoic) Magmatism in hornblende

New England o (Ar/Ar)

A Hercynian age for the Sebago batholith in- £400 dicates that the extent of Hercynian magmatism < in New England is larger than previously cc muscovite (Ar/Ar) LU realized. Other Hercynian granites include the Q.

Doherty and Figure 4. Cooling and uplift curves for intrusive bodies of Lyons (1980) south-central Maine, modified from Dallmeyer and Van Bree- men (1981). Data for all rocks except Sebago zircon and monazite are listed in Table 1. Curves labeled "Devonian plutons" were derived by Dallmeyer and Van Breemen (1981) from data from the Hallowell, Togus, Three Mile Pond, and 200 400 North Jay plutons. AGE (M.Y.I

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presumably all Acadian. They noted that the batholith located just west of the Sebago 2. Zircons in the pink phase may have been K-feldspar-sillimanite isograd (M3) approxi- batholith. Closure temperature estimates are reset by intrusion of the Pleasant Mountain mately parallels the north boundary of the Se- from Harrison and others (1979), except for stock -112 m.y. ago. Zircons in the white phase bago batholith at distances between 5 and 30 monazite (Gebauer and Grunenfelder, 1979) lost lead only recently, probably due to dilation. km from the contact and that "the leveling of the and zircon (assumed to be -650 °C on the basis Dark magnetic zircons in both phases contain highest-grade temperatures southwestward to- of minimum melt temperature of two-mica inherited radiogenic lead. In the pink phase, in- ward the Sebago batholith, a muscovite-bearing granites, Evans, 1965). The Sebago batholith, heritance occurs only in the finer zircons, pluton, does support a close cause and effect intruded at 325 m.y. at a depth of 10-12 km, whereas in the white phase, the coarse zircons relation between metamorphism and plutonism" cooled slowly until 275 m.y. ago. From 275 to also contain inheritance. (p. 583). On the other hand, the Lyman pluton 225 m.y., the batholith cooled from at least 600 3. Metamorphic isograds in the vicinity of (322 ±12 m.y.) to the south cuts across several °C to -300 °C, indicating a rapid regional cool- the batholith approximately parallel its shape metamorphic zones with no apparent effect on ing rate of 6 °C/m.y. From Mesozoic time to the and may be at least partly Hercynian in age. the isograds (Fig. 1). Moench and Zartman present, Sebago rocks cooled at a uniform rate 4. Cooling and uplift of the Sebago batholith (1976) also recognized four Devonian meta- of-1.3 °C/m.y., whereas the Devonian plutons appear to have followed a complicated path in- morphic events in this region, although they dis- to the northeast cooled at -0.4 °C/m.y. (data volving little or no uplift in the middle Carbon- agree with Holdaway and others (1982) about from Dallmeyer and Van Breemen, 1981). iferous through the middle Permian, rapid the exact timing of these events. Using the chro- While the entire region was uniformly uplifted cooling from 275 to 225 m.y. ago, and slow nology of Moench and Zartman (1976), a fifth and rapidly cooled from -275-225 m.y., the cooling since the middle of the . A re- metamorphic event could be applied to the in- uplift rate of rocks to the southwest was greater gional uplift gradient was created, beginning trusion of the Sebago batholith. We conclude than those to the northeast during the past 225 -225 m.y. ago. Different rates of cooling may that the regional metamorphic history is even m.y. This resulted in the present-day exposure of be related to changes in the tectonic regime. more complex than are the Acadian M1-M4 different levels of the crust, as shown previously scenarios envisioned by Holdaway and others by field and gravity studies (Moench and others, ACKNOWLEDGMENTS (1982) and Moench and Zartman (1976). It 1982; Carnese and others, 1982). thus seems quite likely that significant Carbonif- The scenario proposed by Dallmeyer and We thank Edward F. Duke (Dartmouth Col- erous regional metamorphism may be widely Van Breemen (1981) is a two-stage model in- lege) and Archie W. Berry, Jr., (University of superimposed on Acadian metamorphism. Fur- volving relatively rapid cooling from -385 to Maine, Farmington) for sample collection; thermore, on the assumption that regional met- 225 m.y. followed by relatively slow cooling David A. Plume and Randy T. Ogg for mineral amorphism and plutonism are closely related, until the present. We favor the slightly more separations; and Lynn B. Fischer, Carol A. all of the major granitic plutons of northern New complicated model, for the Sebago rocks, of Berkenbaugh, Kiyoto Futa, and Karin Barovich England will have to be dated accurately be- maintenance of established crustal levels for chemical extractions and isotopic analyses. fore we can fully understand metamorphic throughout the late Paleozoic until -275 m.y., Berry, who has mapped the Norway quadrangle, chronology. rapid cooling for 50 m.y., followed by slow selected the site of sample Me/No 1-82. cooling and uplift to the present. The regional Norman L. Hatch suggested the site of sample III. Cooling and Uplift uplift gradient across this part of New England Me/PM 1-81. We thank Charles W. Naeser, was established since only 225 m.y. ago, sup- H. Roberta Dixon, Robert A. Ayuso, and Dallmeyer and Van Breemen (1981, p. 71) porting the interpretation of Zartman and others Joseph G. Arth for very careful reviews. concluded that "at least across its northeastern (1970) that the Permian K-Ar mica ages were terminus, the New England Permian mica belt caused by uplift and erosion, not a discrete Per- does not reflect widespread late Paleozoic met- mian event. It should be emphasized, however, that previously unrecognized, significant plu- amorphism [i.e. Permian] but, rather that the REFERENCES CITED area was continually maintained at elevated tonio activity occurred in the Carboniferous in Aleinikoff, J. N., Zartman, R. E., and Lyons, J. 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Earth Sciences, v. 17, p. 1407-1416. MANUSCRIPT ACCEPTED JANUARY 11, 1985

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