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Carboniferous U-Pb Age of the Sebago Batholith, Southwestern Maine: Metamorphic and Tectonic Implications

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Carboniferous U-Pb Age of the Sebago Batholith, Southwestern Maine: Metamorphic and Tectonic Implications Carboniferous 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 Devonian. This Two phases (pink and white granite) 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 Cretaceous 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- Permian 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 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/96/8/990/3445241/i0016-7606-96-8-990.pdf by guest on 29 September 2021 CARBONIFEROUS U-Pb AGE, SEBAGO BATHOLITH, MAINE 991 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 granites 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.
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