Geological Survey Studies in Maine : Volume 4 1989

Interpretation of the Regional Significance of the Chain Lakes Massif, Maine based upon Preliminary Isotopic Studies

Michael M. Cheatham* William J. Olszewski Henri E. Gaudette Institute for the Study of Earth, Oceans and Space Science and Engineering Research Building University of New Hampshire Durham, New Hampshire 03824

*Present address: Department ofGeological Sciences Snee Hall Cornell University Ithaca, 14853

ABSTRACT

The Chain Lakes massif of northwestern Maine and adjacent Quebec contrasts sharply in lithology, metamor­ phism, deformation, and age with surrounding rocks, and has been considered a suspect terrane in the northern Appalachians. The massif is composed of aquagene metavolcanic and metasedimentary rocks, massive granofels and poorly stratified gneisses containing clasts of volcanic, plutonic, and sedimentary rocks which show evidence of previous deformation and metamorphism. Rocks of the Chain Lakes massif have been metamorphosed to upper amphibolite-granulite fades, although a later retrograde event (epidote-pumpellyite fades) is also recognized. Mild thermal effects and cataclasis resulted from overthrusting of the Boil ophiolite in the southern portion of the massif. The massif has generally been assigned a Precambrian age. Although previous age work on the massif is ambiguous, stratigraphic constraints and isotopic results from surrounding rocks constrain the age of the massif only to the pre-Late Ordovician. A total of 40 whole rock samples have been petrographically examined, 27 of which were analyzed by Rb-Sr and Sm-Nd isotopic techniques in order to determine the number, grades, and ages of the various metamorphic events affecting the massif, the age of the protolith material, and to constrain the age of deposition of the material in the massif. Petrographically, two distinct mineral assemblages characterize the rocks of the Chain Lakes massif. The first consists of quartz-plagioclase-sillimanite-biotite-muscovite, and relict K-feldspar (largely altered to white mica and epidote). The relict K-feldspar suggests second sillimanite grade associated with the upper amphibolite­ granulite fades metamorphism. The second assemblage consists of extensive growth of chlorite, muscovite, epidote, and calcite, associated with retrograde metamorphism. The Rb-Sr whole rock results are scattered, but yield an errorchron age of 684 ± 76 Ma. This is interpreted to represent the high-grade metamorphic event and resetting of the Rb-Sr whole rock systems at that time. Rb-Sr mineral analyses of three samples yield ages of 378, 394, and 419 Ma, interpreted as the result of resetting during a retrograde metamorphic event, possibly related to Acadian metamorphism and intrusion. Sm-Nd whole rock results are too limited to provide an isochron, but calculated crustal residence ages relative to depleted mantle range from

125 M. M. Cheatham and others

1050 to 1796 Ma. These crustal residence ages are in agreement with the Sr evolution of the massif (Sr crustal residence age of 1457 Ma) and zircon U-Pb results (1534 Ma). The sedimentary rocks of the Chain Lakes massif were probably deposited between "'684 Ma and "'1500 Ma. The timing of the metamorphic events and the crustal residence ages for the massif are similar to other areas in New England, but no definitive correlations can be made at present. The isotopic signatures of the massif are similar to high-grade terrains of comparable age from other areas around the world. The petrographic and isotopic results reinforce the unique nature of the Chain Lakes massif and point out the importance of the massif in understanding the tectonic evolution of the northern Appalachian orogen.

INTRODUCTION

The Chain Lakes massif of northwestern Maine and ad­ application of Rb-Sr and Sm-Nd whole rock isotopic methods jacent Quebec (Fig. I) is composed dominantly of metamorphic and Rb-Sr mineral isotopic analyses to rocks of the massif. rocks. Lying within the Gander Superterrane of Williams and These analyses were carried out in order to obtain information Hatcher ( 1983), the Chain Lakes massif contrasts strikingly in about the age and origin of the Chain Lakes massif, the timing lithology, metamorphic grade, and structural style with the sur­ of thermal events affecting the rocks of the massif, and to provide rounding Paleozoic rocks and is itself considered a "suspect information on the tectonic evolution of the massif in a regional terrane" (Williams and Hatcher, 1983; Zen, 1983a). context. The age, origin, and relationships of the Chain Lakes massif to the surrounding rocks are uncertain, and except for small GEOLOGY OF THE CHAIN LAKES MASSIF enclaves of Chain Lakes-like rocks in Quebec, no similar rocks are known in New England. In this regard, the massif is unusual, Lithology and Stratigraphy and an understanding of its geology, age, and evolution should reveal information pertaining to the early tectonic development The upper I km of the Chain Lakes massif is composed of of this part of the northern Appalachians. This paper reports the aquagene metavolcanic and metasedimentary rocks. The

INDEX MAP EXPLANATION - •7" 0 5 10 ~I1~1 ...... 1 i.__.__ I -~I Mile s 10

A rea o l map at righl

so 100 I I I Miles 710

LOCATION MAP

OF THE

CHAIN LAKE S MASSIF, £bm Boil Mountain oph1ohte NOR THWESTERN MAINE AND THE le D Yc1 Chain Lakes masSJ I G ENE RALIZED GEOLOGY >--..._ Thrust 'l-- Fautt OF SU RROUNDING ROCKS i ·-Deer Pond fault z.. Thrasher Peaks fault 70°15' 3--Squlflgun fautt

Figure J _ Location map of the Chain Lakes massif in northwestern Maine and the generalized geology of the surrounding rocks (from Moench, 1984).

126 Regional significance of the Chain Lakes massif

remainder of the exposed massif consists dominantly of massive GN granofels to poorly stratified gneisses (Boudette et al., 1984). The term granofels, which has been applied to most of the rocks 19ir of the lower sequence of the Chain Lakes massif, is used to ~~ indicate high-grade metamorphic rocks where layering is cryptic or absent. Those units described as gneisses show a foliation 8 interpreted to be parallel to compositional layering (Boudette, 1970; Biederman, 1984). \ The granofels and gneisses consist of rocks with pebble- to \ boulder-sized clasts of vein quartz, mafic and felsic plutonic and A volcanic rocks, and sedimentary rocks which show evidence of 7 \ previous deformation and metamorphism (Boudette, 1970; /~ ,//I Boudette and Boone, 1982). The clasts are supported by a matrix / \ of quartz, plagioclase, biotite, chlorite, muscovite, and sil­ ,,,./ limanite (Boudette, 1970; Albee and Baudette, 1972; Boudette / I and Boone, 1982; Biederman, 1984; Boudette et al., 1984). Boudette et al. (1984) divided the lower sequence of the Chain Lakes massif into eight geographic/lithologic units (Fig. 2). These are: (I) semipelitic gneiss at Twin Bridges; (2) layered granofels and gneiss at Bag Pond Mountain; (3) granofels and gneiss at Sarampus Falls; (4) massive granofels at Bugeye Pond; (5) polycyclic breccia at McKenney Pond; (6) spotted granofels LEGEND at Kibby Mountain; (7) polymictic breccia at Kibby Mountain; and (8) diorite. The stratigraphic relationships between these units are un­ 1. Semipelitic gn eiss at Twin Bridges certain, although the diorite intrudes the spotted granofels at 2. Granofel s and gneiss at Bag Pond Moun tain 3. Gran?lels and gneiss at Sarampus Falls Kibby Mountain (6). Other than this relationship, no 4. Massive granofels at Bugeye Pond 5 Polycyclic breccia at McKenney Pond stratigraphic sequence is implied by the above list. 6 Spotted granofels at Kibby Mountain 7 Polymictic brecc1a al Kibby Mountain The metamorphic history of the massif is complex and still B Oiorite uncertain. The Chain Lakes massif has been metamorphosed to upper amphibolite-granulite facies (Boudette and Boone, 1982; These units are based on geographic vana11ons in lithology: fil! slrat~graphy is 1mpl1f;-d Beiderman, 1984 ), but Boudette ( 1982) also recognized a retrograde metamorphic event of at least epidote-pumpellyite facies in both the Chain Lakes massif and the surrounding pre-Silurian rocks. He suggested an Ordovician age for this Figure 2. Geologic map (from Boudette et al., 1984) of the Chain Lakes retrograde metamorphism. Additionally, local fault-related massif showing the geographic variations in lithology. No stratigraphy is impled by these units. retrograde contact metamorphism due to the emplacement of the Boil Mountain ophiolite has affected the rocks of the massif. This zone is recognized by an increase in the size and amount of muscovite in the matrix of the rocks as the contact with the massif from the cyclically bedded, well-graded, light gray to ophiolite complex is approached. The Devonian stratigraphic dark gray turbidites (Boucot, 1961; Albee and Boudette, 1972) section around the Chain Lakes massif shows the effects of an of the Seboomook Group (Baudette et al., 1984; Baudette and Acadian prograde metamorphism to chlorite-biotite grade Boone, 1984; Pollock, 1987). The Seboomook Group contains (Albee and Boudette, 1972; Biederman, 1984) prior to the in­ an early Devonian, Gedinnian-Siegenian age fossil assemblage trusion of the undeformed and unmetamorphosed Lexington (Boucot, 1961; Albee and Baudette, 1972). batholith at 399 Ma (Gaudette and Boone, 1985). Although the The Attean pluton (Boudette, 1982; Lyons et al., 1983; Acadian metamorphism appears to have had minimal effects on Aleinikoff and Lyons, 1984), a hornblende-bearing quartz mon­ the mineralogy of the rocks of the massif, its effects on the Rb-Sr zonite, intrudes the massif in the east. The eastern flank of the mineral ages are evident (see below). Attean pluton is unconformably overlain by the rocks of the Silurian and Devonian Moose River sequence (Boucot, 1961 ; Structure and Relationships to Surrounding Rocks Albee and Boudette, 1972; Baudette et al., 1984). This Siluro­ Devonian sequence unconformably overlies the Chain Lakes The Chain Lakes massif is bounded on the north, northwest, massif (Boudette, 1970; Boudette et al., 1984) south of the Attean and west by the Thrasher Peaks fault (Fig. 1) which separates the pluton (Fig. I).

127 M. M. Cheatham and others

To the southeast, south, and southwest the Chain Lakes eastern portion of the massif, recognized a similar lithology massif is in contact with the Boil Mountain ophiolite and the Jim which they referred to as the "granofels unit'', and assigned it to Pond Fonnation (Boudette, 1982). The Boil Mountain ophiolite the pre-Silurian. Thin section study of this unit showed it to is composed of layered harzburgite/lherzolite and pyroxenite, consist of quartz, feldspar (altered to white mica and epidiorite/gabbro and clinopyroxenite, and calcic plagiogranite clinozoisite), biotite, muscovite, chlorite, and more rarely sil­ (Boudette, 1982). The Jim Pond Formation consists of pillow limanite and garnet. Abundant polymictic lithic fragments were basalts at the base, overlain by vent facies felsic volcanic rocks, also noted. iron fonnation, and metaclastic rocks including melange and Ages of the surrounding rocks constrain the age of the rocks quartzwacke olistostrome (Boudette, 1982). The contact of the of the Chain Lakes massif. Zircons from the Attean pluton yield Chain Lakes massif with the ophiolite complex is interpreted as an upper intercept age of 443 ± 4 Ma (Lyons et al., 1986). a tectonic surface, probably a thrust fault along which the Zartman et al. (1970) report a K/Ar age of370 Ma for the Chain ophiolite complex has been ramped up on the massif (Boudette, of Ponds pluton. Lux ( 1983) obtained a 40Ar;3 9 Ar hornblende 1982). Mild thennal metamorphism and cataclasis is associated age of 371 Ma for the Spider Lake pluton. Zircons from the with this contact (Boudette, 1982; and discussed above). The calcic plagiogranite of the Boil Mountain ophiolite gave an age present contact between the Chain Lakes massif and the Boil of 520 ± 12 Ma (Eisenberg, 1981). U-Pb analyses of zircons Mountain ophiolite dips steeply to the south. The Jim Pond from the Jim Pond Formation give an age of 500 ± IO Ma Formation overlies the Boil Mountain ophiolite and is also thrust (Aleinikoff and Moench, 1985). The available isotopic results over the massif along its southwesternmost margin. thus support a middle Ordovician or older age for the Chain In the west and southwest, the massif is intruded by the Lakes massif. Chain of Ponds pluton, dominantly composed of with The rocks of the Chain Lakes massif were originally as­ hornblende-bearing granodiorite, and the Seven Ponds pluton, a signed to the Precambrian by Ells (l 887). Boudette (1970, granite. On the northwest, the Chain Lakes massif is in fault 1982), Naylor et al. ( 1973), St-Julien and Hubert (1975), Lyons contact with the Spider Lake pluton along the Thrasher Peaks et al. (1982), Zen (1983a), Williams and Hatcher( 1983), Bieder­ fault (Fig. I). A small body of granodiorite known as the Skinner man (1984), Moench (1984), and Osberg et al. (1985) have all pluton intrudes completely within the massif (Figs. I, 2). upheld this age assignment based on the limited isotopic studies Preliminary interpretations of seismic reflection profiling of the rocks of the Chain Lakes massif by Naylor et al. (l 973) (Stewart et al., 1985, 1986) from the Chain Lakes region showed and Biederman (1984). The Naylor et al. study used U-Pb that the massif is apparently floored by a southeasterly dipping analyses of zircons from the matrix of the granofels. They decollement. They interpret this as a southeast over northwest obtained an upper intercept age of 1534 ± 59 Ma and a lower thrust of either Acadian or Taconic age. The rocks of the Chain intercept of 153 ± 126 Ma (these ages and errors are recalculated Lakes massif can be interpreted as allochthonous, and most by the present authors from the original data; Naylor, pers. probably derived from the southeast. comm., 1985). The upper intercept age was interpreted by Besides the parts of the massif proper which occur across Boudette and Boone ( 1982) to represent the age of prograde the international border in Quebec, small areas of Chain Lakes­ metamorphism. However, these ages are based on only two like rocks occur to the northwest in Quebec (Williams and zircon fractions which are approximately 50% discordant and St-Julien, 1982; Cousineau and St-Julien, 1986). very close to one another. The true errors for these ages are unknown; the errors quoted above are for an assumed 1% ex­ Previous Work on the Age and Origin ofthe perimental error, and thus no estimate of scatter can be made. In Chain Lakes Massif addition, as we discuss below there is evidence that these zircons contain a large fraction of grains of detrital origin which may Early geologic mapping in the Chain lakes massif was done represent different sources with a wide range of ages. by Marleau (l 957) in Canada, and by Albee (l 961) and Green Biederman (l 984) did not propose an age for the high-grade and Guidotti ( 1968) who assigned rocks of the massif to the metamorphism in the Chain Lakes massif, but did suggest that Ordovician, through correlation with the Ammonoosuc vol­ the rocks of the massif passed through a temperature of 500°C canics. Harwood ( 1970, 1973), working in the westernmost part in the very late Precambrian to early Cambrian. This age assign­ of the massif, also considered the rocks of the Chain Lakes ment is based on the 40 Ar/39Ar release spectra obtained from two massif to be Ordovician in age, but assigned the rocks to the samples of hornblende from the Bugeye Pond member. Magalloway Member of the Dixville Formation. West of the The origin of the Chain Lakes massif is even more uncertain Seven Ponds pluton, Harwood (1973) described a coarse-grained than its age. Boudette and Boone ( 1982) have suggested that the biotite gneiss containing rounded masses of clear to milky white Chain Lakes massif represents one of the following features: a granular quartz in a matrix of quartz, plagioclase, biotite, mus­ tillite, a tectonic melange, an olistostrome, an explosive volcanic covite, chlorite, sillimanite, and magnetite. This gneiss was also assemblage, a plutonic roof complex, an infracrustal rift zone, or mapped as roof pendants within the Seven Ponds pluton (Har­ a metasuevite (impact origin) complex. The question of its wood, 1973). Albee and Boudette ( 1972), while working in the origin remains open.

128 Regional significance of the Chain Lakes massif

ANALYTICAL METHODS The isotopic compositions of Rb, Sr, Sm, and Nd were determined on a Nier-type, 9-inch radius, 60° sector, solid source A total of 40 samples, each approximately 15-20 kg in mass spectrometer in the Isotope Geochemistry Laboratory at weight, were collected from various units within the massif. the University of New Hampshire. Rb, Sr, and Sm were Every effort was made to collect the freshest possible material. analyzed as metals, Nd as an oxide. Sr analyses were corrected However, megascopic examination of samples in the laboratory for fractionation to 86s r;88sr = 0.1194 (Steiger and Jager, 1977). 146 144 resulted in selection of 27 samples from the McKenney Pond, Nd ratios were normalized to a Nd/ Nd ratio of 0.72413 Bugeye Pond, and Sarampus Falls members for isotopic (Wasserburg et al. , 1981). Analytical errors from powder repli- 81. .86 8 7 .86 . analysis. Rb/Sr and Sm/Nd whole rock and Rb/Sr mineral cates are Rbt Sr = l.0%; Srt Sr = 0.07%; coefficient of 147 144 143 14 analyses were carried out in conjunction with petrographic ex­ correlation=0.011; Sm/ Nd=0.2%; Nd/ 4Nd =0.02%; amination of the rocks. Modal abundances of the minerals in coefficient of correlation= 0.01. All errors quoted are± 1.0 O'm . each sample were determined by visual estimation in thin sec­ Isochrons are calculated using the equations of York ( 1969). tion. Decay constants are those recommended by Steiger and Jager 147 12 Each whole rock sample was prepared for isotopic analysis (1977) for Rb and U; the A. value used for Sm was 6.54 x 10- 1 by standard techniques. Minerals were separated from the whole y( (DePaolo and Wasserburg, 1976). Replicate analyses of rock samples by standard heavy liquid and magnetic separation Eimer and Amend standard Sr carbonate during the course of the procedures. Sample dissolution and chemical separation of Rb study gave a value of 0.70793 ± 0.00005 ( 1.0 O'm; N = 22) for and Sr from the whole rock and mineral samples was done using 87sr/8°sr. Analyses of standard whole rock BCR-1 for Nd HF-HCl04 acid and cation exchange procedures. Sm and Nd fielded a value of 0.511863 ± 0.000013 ( 1.0 crm; N = 8) for 43 14 were separated on anion exchange resin using nitric acid­ Nd/ 4Nd, which agrees well with the accepted value of methanol mixtures (Faris and Warton, 1962). 0.511847 (Wasserburg et al., 1981 ).

TABLE I. MINERAL MODES

Sample Q+ F(An) B M c A s E 0 MCKENNEY POND 397 31 32(24) 3 10 9 tr 8 2 3 398zl 27 31(21 ) II 13 10 tr 5 tr 2 399 29 36(25) 9 10 II tr 3 tr I 400 28 25(19) 8 10 7 tr 17 2 2 401 30 26(20) 8 15 9 tr 4 3 I 404 35 28(26) 5 15 10 tr 3 I 3 405 32 33(24) 3 12 13 I 2 2 l SARAMPUS FALLS 416zl* 30 30(17) 18 12 tr tr 417 40 38( 18) 5 15 tr tr tr I 418 35 31(22) II 15 3 I 2 419 30 28(24) 10 20 8 tr 2 420 32 28(23) 12 15 8 tr 2 421 34 24(19) 10 16 14 tr tr 422 38 27(25) 8 8 12 tr 2 423 34 34(18) 6 10 12 2 424 40 15(25) 10 23 10 tr tr tr 425 40 15(24) 15 27 tr tr 2 tr 426 38 27(18) tr 10 23 tr tr I 427 46 33(20) 12 8 tr tr tr I 436 46 30(24) 3 10 5 tr 2 tr 2 BUGEYE POND 428 40 30(19) tr 12 15 tr 429 35 20(22) tr 25 18 tr tr 430 30 20(23) tr 10 37 tr tr tr I 431 40 37(19) 4 10 6 tr tr tr 2 432 35 34(20) 6 14 10 tr tr I 433 39 30(18) 6 10 9 tr 3 2 434 43 30(19) 15 5 3 tr 2 tr 435 47 35(24) 7 4 3 tr 2 I

+Q - Quartz; F(An) - Feldspar. dominantly plagioclase but may include some potassium feldspar, value in parentheses is the anorthite content of the plagioclase; B - Biotite; M - Muscovite; C - Chlorite; A - Apatite; S - Sillimanite; E - Epidote; 0 - Opaque minerals, including ilmenite, magnetite, sulfides, etc. Short dashes indicate minerals not detected in thin section. All samples contain trace amounts of zircon. *This sample includes 3% staurolite and 5% andalusite. This sample has not been analysed isotopically.

129 M. M. Cheatham and others

RESULTS TABLE 2. Rb-Sr WHOLE ROCK ANALYSES

87 Petrography Sample Rb{ppm) Sr(ppm) 87Rb/!6Sr Sr/!6Sr

SARAMPUS FALLS Petrographic and isotopic analyses were carried out on the 417 100 245 1.185 0.7269 Sarampus Falls ( 13 samples, one sample 4 I 6z I was not analyzed 418 92 241 1.103 0.7269 isotopically), Bugeye Pond (8 samples), and McKenney Pond (7 419 10 1 245 1.192 0.7263 420 90 290 0.899 0.7248 samples) members. Modal abundances of major minerals for 421 94 224 1.2 12 0.7304 each sample are provided in Table I. Boudette et al. (1984) have 422 123 187 1.909 0.7334 indicated a mappable difference between the Sarampus Falls and 423 91 183 1.438 0.7325 147 Bugeye Pond members. Our examination of the 21 samples of 424 89 1.755 0.7327 425 114 180 1.825 0.7306 these two members in both hand specimen and thin section could 426 81 290 0.811 0.7236 find no consistent difference between the two members, and for 427 90 286 0.910 0.7239 this reason our petrographic descriptions for these two members 436 103 232 1.291 0.7266 BUGEYE POND are treated together. 428 86 176 1.4!0 0.7281 Megascopically, the Sarampus Falls and Bugeye Pond 429 84 218 1.115 0.7275 samples are medium to dark gray, medium grained, quartz­ 430 60 276 0.622 0.72 14 1.070 p lagioc lase-bioti te-m uscov ite-chlorite gneisses and/or 431 80 216 0.7260 432 91 215 1.212 0.7267 granofels, with minor clasts of quartz + feldspar and rare clasts 433 90 282 0.928 0.7245 of metapelite. Foliation, when present, is most apparent on the 434 93 199 1.35 1 0.7303 outcrop scale, but is also discernible in some hand specimens and 435 76 202 1.088 0.7297 MCKENNEY POND thin sections. 397 50 13 1 1.099 0.7248 Thin section examination shows the following ranges in 398zl 67 139 1.41 8 0.7335 mineral compositions: 30-47% quartz, 5-35% plagioclase, 0- 399 53 179 0.853 0.7227 400 89 158 1.636 0.7306 18% biotile (showing some alteration), 4-27% muscovite, 0- 401 72 166 1.259 0.7297 35% iron-rich chlorite, 0-2% sillimanite, and varying trace 404 81 145 1.615 0.7288 amounts of apatite, ilmenite, magnetite, carbonate, epidote, tour­ 405 47 190 0.710 0.7199 maline, and zircon. We have tentatively identified potassium feldspar (altered to white mica and epidote) in a few samples, but no analyses or staining have been done to corroborate this. 07400

Potassium feldspar has also been identified by Biedermann CHAIN LAKES MASSIF ( 1984) within a portion of the Sarampus Falls member. Rb·Sr Whole R()Ct( (aNda ta) Q The samples from the McKenney Pond member are all very • similar in their textural relationships and mineral assemblages. 0.7300 e Sarampus Falls Megascopically, the rocks are medium lo dark gray, medium en~ O Bugeye Pond grained, q uartz-p lagioc lase-bioti te-chlorite-m uscovite-si 1- Q McKenney Pond .."'-;: limanite granofels. The samples analyzed are generally massive ....en and lack foliation. .. 0.7200 Thin section examination provides the following ranges in Age m 684 +/- 76 Ma mineral composition: 25-35% quartz, 10-21 % plagioclase, 3- Initial Ratio= 0.7156 +/· 0.0014 MSWD = 12.4 11 % partially altered biotite, 7-13% chlorite, 10-15% muscovite, 2-17% fibrous sillimanite, and variable trace amounts of car­ bonate, apatite, epidote, ilmenite, magnetite, zircon, and pyrite. 0.7100 0.00 0.50 1.00 1 50 2.00 Clasts are present and are typically composed of coarse-grained e1Rb/e6sr quartz and feldspar. Less common are clasts of metapelite Figure 3. Rb-Sr whole rock diagram for all of the analyses of the Chain consisting of fibrous sillimanite, biotite, chlorite, and muscovite. Lakes massif rocks carried out for this study. Errors are+ 1.0 crm. Albee and Boudette ( 1972) reported clasts with hornblende in this member, although none were found in the thin sections we examined. data yield a regression line (errorchron) with an age of 684 ± 76 Rb/Sr Whole Rock Results Ma and initial ratio of0.7 156 ±0.0014 (MSWD = 12.39). If the three members are treated separately, the resulting three er­ The Rb-Sr isotopic results are listed in Table 2. A plot of all rorchrons (Fig. 4) vary in age, but overlap at 1.0 cr. The separate of the results for the three members is given in Figure 3. These regression results are: McKenney Pond: 849 ± 202 Ma, initial

130 Regional significance of the Chain lakes massif

0.7400 ------ratio= 0.7123 ± 0.0036, MSWD = 23.50; Bugeye Pond: 707 ± 172 Ma, initial ratio= 0.7157 ± 0.0028, MSWD = 9.83; Saram­ CHAIN LAKES MASSIF pus Falls: 611±96 Ma, initial ratio= 0.7169 ± 0.0018, MSWD McKenney Pond Samples • = 10.10. 0.7300 Rb-Sr Whole Rock There does not appear to be any significant difference in Rb ci5 and/or Sr contents of the three members (Table 2), or the relative

0.00 0.50 1.50 2.00 and gneiss represent a mixture of a number of rock types repre­ sented as clasts, each with a prior isotopic history; (2) the generally high grade of the early metamorphism of the massif which has most likely reset the Rb-Sr isotopic systematics on a whole rock scale; and (3) the variable effects of the Acadian 0.7400 --.....--..-- .....------..----- metamorphism. 1n spite of these difficulties, we feel that the regression line "age" represents a significant event in the CHAIN LAKES MASSIF geologic evolution of the Chain Lakes massif. We interpret the Bugeye Pond Samples regression line "age" as representing the approximate time of the 0.7300 Rb-Sr Whole Rock first prograde metamorphic event that effected the massif. Our reasons for this interpretation are given in detail below.

Rb/Sr Mineral Analyses 0.7200 Age= 707 +I- 172 Ma Initial Ratio= 0.7157 +/- 0.0028 Two samples of the Sarampus Falls member (UNH 420 and MSWD=9.8 UNH 422) and one sample of the McKenney Pond member (UNH 40 I) were chosen for mineral separation and mineral 0.7100 ______.... ______..... __ ..._ __ Rb-Sr isotopic analyses. The individual mineral separates in­ 0.00 0.50 1.50 2.00 cluded a plagioclase fraction, muscovite fraction, and biotite fraction, and a heavy mineral fraction consisting mostly of opaque minerals (mainly magnetite, ilmenite, and pyrite), zir­ con, apatite, and biotite. The results from these mineral analyses are listed in Table 0.7400 ------3 and plotted in Figure 5. These mineral isochrons include the whole rock point for each sample. The isochron for UNH 40 I CHAIN LAKES MASSIF does not include the heavy mineral or the biotite fraction, both Sarampus Falls Samples • of which were highly altered. 0.7300 Rb-Sr Whole Rock UNH 401 (McKenney Pond) gave an isochron (plagioclase­ ~ .,(/) whole rock-muscovite) with an age of 378 ± 61 Ma, initial ratio "';:: = 0.7228 ± 0.0010, MSWD = 0.06. UNH 420 (Sarampus Falls) (/) .,.... yields an isochron with an age of 419 ± 14 Ma, initial ratio= 0. 7200 Aoe = 611 +/- 96 Ma 0.7198 ± 0.0004, MSWD = 0.50; UNH 422 (Sarampus Falls) Initial Ratio= 0.7169 +/- 0.0018 gives an isochron with an age of 394 ± 11 Ma, initial ratio = MSWD= 10.1 0.7234 ± 0.0006, MSWD = 1.28. The ages for the three mineral isochrons agree within 1.0 a of each other. We interpret these 0.7100 ______..... __ ages as representing a metamorphic event which reset the Rb-Sr 0.00 0.50 1.50 2.00 isotopic systematics of the minerals. Ages ranging from 370 to 415 Ma are typical of ages for the Acadian orogeny in eastern New England. In northwestern Maine, near the Chain Lakes Figure 4. Rb-Sr whole rock diagrams for the individual lithologic units massif, the Acadian orogeny is constrained by the ages of the of the Chain Lakes massif. Errors are+ 1.0 Om. Attean quartz monzonite ( 443 Ma, Lyons et al., 1986), which has

131 M. M. Cheatham and others

TABLE 3. Rb-Sr MINERAL ANALYSES 0.7400 ..-----T"'""-...... --....-----...---...---

Fraction 87 6 87 Sample Rbf sr sr;86Sr CHAIN LAKES MASSIF 401 Feldspar 0.670 0.7264 UNH401 , McKenney Pond Rb-Sr !.tneral 5-patates Muscovite 1.478 0.7307 0 .7300

~ 420 Feldspar 0.549 0.7231 (/) Muscovite 3.702 0.7419 "'"'-;: Biotite 2.689 0.7356 ....(/) Heavy* 1.616 0.7299 (I) 422 Feldspar 1.218 0.7307 0.7200 Age• 378 +I· 61 Ma Muscovite 6.475 0.7599 lnttial Ratio • 0. 7228 +/· 0.0010 Biotite 5.028 0.7512 MSWD•0.06 Heavy* 3.717 0.7447

*Heavy fraction is the suite of minerals that sinks in methylene iodide, no attempt was made to separate this fraction into separate pure mineral fractions. 0.7100 0.00 0.50 1.00 1.50 2.00 87 86 Rbt Sr

been affected by Acadian metamorphism, and the Lexington 0.7450 batholith (399 Ma, Gaudette and Boone, 1985), which is unaf­ fected by Acadian metamorphism. Plutons intruding the massif have dates as low as 370 Ma (see above). We interpret the CHAIN LAKES MASSIF mineral ages as representing resetting due to a combination of UNH420, Sarampus Falls Rb-St Mneral Separates Acadian metamorphism and heating from the plutonic rocks that 0.7350

~ intrude the Chain Lakes massif. These events have undoubtedly (/) CD contributed to the scatter seen in the combined whole rock "'-;: regression line. ....Cf) '° 0.7250 Sm/Nd Whole Rock Results Age. 419 +/- 14 Ma lnttial Ratio. 0.7198 +I· 0.0004 MSWD•0.50 Sm-Nd isotopic analyses have been carried out on seven

samples from the McKenney Pond member. The isotopic results 0.7150 are listed in Table 4. Although an isochron could not be drawn 0.00 1.00 2.00 3.00 4.00 87 86 for these results because of the limited spread in 147sm1 144Nd Rbt Sr ratios, crustal residence ages (Tcr) have been determined for each sample (Table 5). Crustal residence ages represent a hypotheti­ cal age at which the source of the material that makes up the o.noo massif was derived from a bulk earth source (CHUR, OePaolo

and Wasserburg, 1976) or a depleted mantle (OM) source. The 0.7600 CHAIN LAKES MASSIF rocks of the Chain Lakes massif represent a mixture of sources UNH422, Sarampus Falls and as a result, the crustal residence ages will represent a mjxing Rb-Sr Mineral Separates age rather than the age at which a single and distinct source was Cf) 0.7500 CD derived from the mantle. Such mixture ages may have little to '°-;: do with the geologic history or evolution of a rock unit (Arndt ....(/) (I) 0.7400 and Goldstein, 1987). However, other results (discussed below) suggest that the Chain Lakes crustal residence ages may provide Age. 394+/- 11 Ma lnttial Ratio• 0.7234 +I· 0.0006 information about the sources of material for the massif. The Tcr 0.7300 values are not affected by metamorphism, even high-grade MSWD• 1.28 metamorphism, as long as the Chain Lakes massif as a whole has 0.7200 remained a closed system. Our reasons for assuming closed 0.00 2.00 4.00 6.00 8.00 87 86 system behavior for the Chain Lakes massif are discussed below. Rbt Sr The Tcr values for these seven samples range from 763 Ma to 1788 Ma with respect to CHUR, with an average of 1382 Ma. Figure 5. Rb-Sr mineral isochron diagrams for three samples of the With respect to OM, the values range from l 050 Ma to 1796 Ma Chain Lakes massif. F-feldspar fraction; WR-whole rock; H-heavy with an average of 1496 Ma. The implications of these Tcrvalues mineral fraction; B-biotite fraction; M-muscovite fraction. Errors are+ are discussed in more detail below. 1.0crm.

132 Regional significance ofthe Chain Lakes massif

DISCUSSION (1) The metamorphic grade was high enough to reset the Rb-Sr isotopic systems on the whole rock scale. The fine­ The mineral assemblages and textures of the Sarampus grained matrix probably is more easily reset than the coarse­ Falls, Bugeye Pond, and McKenney Pond members suggest that grained clasts, and this may contribute to the scatter in the at least two, and possibly three, metamorphic events have af­ isochron. fected the Chain Lakes massif. The first of these was a high­ (2) This interpretation does not conflict with the other grade metamorphism that reached at least the first sillimanite available geologic and geochronologic data presented above. isograd, and possibly the second sillimanite isograd (Biederman, (3) This "age" is very similar to other ages (both metamor­ 1984 ). Muscovite grew in all three members at the time of the phic and intrusive) in eastern New England, as discussed below. high-grade metamorphism as shown by the inclusion of biotite A period of high-grade metamorphism with a similar timing in large muscovite grains. The rocks of the massif show high­ is seen in areas of New England and Maritime Canada. Gaudette grade textural equilibrium in the development of granoblastic et al. (I 985) report a Rb-Sr whole rock age of 686 ± 30 Ma for and granoblastic-polygonal textures. The weak fol iation seen in the biotite gneiss of the Kelly's Mountain Complex of Cape many of the rocks probably dates from this first high-grade event, Breton Island. Olszewski and Gaudette (in prep.) and Gaudette since the foliation is generally defined by the high-grade et al. (1983) report a Rb-Sr whole rock age of 692 ± 46 Ma for minerals. the amphibolite grade Folly River Schist (part of the Bass River We interpret the 684 ± 76 Ma "age" as reflecting the reset­ Complex) of the Cobequid Highlands in Nova Scotia. Granitic ting, either completely or partially, of the Rb-Sr systematics gneisses that intrude the Bass River Complex yield Rb-Sr whole during this high-grade event. This "age" then gives at least an rock ages of 647 ± 22 Ma and 630 ± 18 Ma. A major period of approximate age for the high-grade metamorphism. intrusion and metamorphism occurs in the Green Head Group There are a number of reasons for believing that the regres­ and associated rocks near Saint John, New Brunswick between sion line age and the high-grade metamorphism are related: 700 and 850 Ma (Olszewski and Gaudette, 1982) based on Rb-Sr whole rock and zircon U-Pb analyses. Cormier ( 1969) obtained a Rb-Sr whole rock age of 750 ± 80 Ma for the volcanic rocks of the Coldbrook Group which unconformably overlie the Green TABLE 4. WHOLE ROCK Sm/Nd RESULTS Head Group. In Maine, Stewart and Wones ( 1974) report an age of 750 ± 150 Ma by Rb-Sr whole rock analyses of fol iated Sample 147Sm/144Nd 143Nd/144Nd medium- to high-grade rocks on Seven Hundred Acre Island in Penobscot Bay. Stewart and Lux ( 1988) report a Rb-Sr whole 397 0.1040 0.51103 398z l 0.1016 0.51098 rock age of 620 ± 20 Ma for a coarse muscovite pegmatite that 399 0.1092 0.51098 intrudes these same rocks on Seven Hundred Acre Island. The 400 0.0911 0.51091 same pegmatite yields a K-Ar age of 594 ± 18 Ma (Stewart and 401 0.1030 0.5 1090 Lux, I 988). Argon release spectra from hornblendes separated 404 0.1076 0.51140 405 0.1058 0.51078 from amphibolite and gamet-amphibolite on Seven Hundred Acre Island yield apparent ages of 670 and 650 Ma (Stewart and Lux, 1988). In south-central New Hampshire, Kelly et al. (1980) obtained a Rb-Sr whole rock age of 68 1 ± 16 Ma for a syntectonic intrusive in the Massabesic Gneiss Complex. A similar age was TABLE 5. Nd CRUSTAL RESIDENCE AGES AND £Nd AND Esr found by zircon U-Pb (68 I± 147 Ma). A zircon U-Pb age of730 VALUES FOR MCKENNEY POND SAMPLES ± 26 Ma was obtained by Olszewski (1980) for the Fishbrook

1 1 2 Gneiss of northeastern Massachusetts. Sample Tc,(CHUR) Tcr(DM) £Nd £5/ Most of these rocks have been included in the Avalonian 397 1347 1471 -15.96 288.1 terrane (Williams and Hatcher, 1983; Zen, 1983a; Zartman, 398z l 1388 1499 -16.94 411.6 1988) of eastern New England. The similarity in timing of 399 1508 1593 -16.94 258.3 400 1346 1459 -18.3 1 370.5 metamorphism between the Chain Lakes massif and these other 401 1532 1607 -18.50 357.7 units suggests three possibilities: that the timing of metamor­ 404 763 1051 - 8.73 344.9 phism is coincidental and that the Chain Lakes massif and 405 1788 1796 -20.85 2 18.6 Avalonia are two separate terranes that have never interacted; 141 44 1Crustal Residences Ages are in 106 years; CHUR parameters: ( Srnl Nd)o that they are two separate terranes which interacted in the late =0 .1967, ( 143Nd / 144.Nd' )o =0.5 11 847 (Wasserburg et al.. 198!1,; Depleted Precambrian to produce similar timing of metamorphism; or that 14 7 143 Mantle (DM) parameters: ( Srnt' ~d)o =0 .2300. ( Nd;' Nd)o = the Chain Lakes massif represents Avalonian terrane. 0.512245. 2 £ values are relative to CHUR at the present time. CHUR parameters for Sr: This last possibility we feel can be dismissed because of 87 87 Rbf 6Sr = 0.0837, Sr/6Sr = 0.7045. numerous differences between the massif and typical Avalonian

133 M. M. Cheatham and others terrane (e.g., southeastern Massachusetts). For example, there .---...... ----.---r----.----r------...---.------. 0. 740 are no rocks like the Chain Lakes massif in the typical Avalonian terrane of southeastern New England (or anywhere else in New Sr Evolution of the England for that matter), the grade of metamorphism of the rocks Chain Lakes massif 0.730 of the massif is significantly higher than that of most of the Avalonian terrane, and the Paleozoic geologic histories of the Intercepts: CHUR: 1417 +/· 250 Ma ..,"' Chain Lakes massif and the Avalonian terrane are different (e.g., (fl ~ the Ordovician alkalic plutonism seen in southeastern Mas­ DM: 1457 +I· 275 Ma 0.720 -"' sachusetts is absent in the massif; the Cambrian is represented ~ by shallow water marine facies sediments in Massachusetts, not 684 Ma 0.7156 ophiolite as in the Chain Lakes massif; Acadian metamoprhism 0.710 is very weak in the Avalon terrane of southeastern Mas­ sachusetts). CHUA In addition, the late Precambrian event in the Avalon terrane DM ....__...... __ __.__ ...... _ _ _.___ ....._ _ _.___ ....__ .... 0.700 of southeastern Massachusetts is dominated by the intrusion of 2000 1500 1000 500 0 large calc-alkalic plutons, metamorphism is weak, and deforma­ Age(Ma) tion is dominantly brittle shear and cataclasis (Zen, 1983b). In Figure 6. Sr evolution diagram for the Chain Lakes massif. The Sr contrast, the rocks of the Chain Lakes massif underwent high­ 87 6 evolution is based on extrapolation of the present average Srf Sr grade metamorphism in the late Precambrian as we suggest, it 1 value for all of the samples analyzed (0.7275) back through the initial lacks major late Precambrian calc-alkalic plutonism, and defor- ratio at 684 Ma (circle with cross) to the intersections with a chondritic mation is dominantly ductile. ' undifferentiated reservoir (CHUR) and typical depleted mantle (DM). We feel that these are strong arguments against directly correlating the Chain Lakes massif with the Avalonian terrane. The similarity in timing may be fortuitous, but our tenative interpretation would be that it represents interaction between the can also calculate a crustal residence age from the average Rb-Sr Chain Lakes massif and the Avalonian terranes in the late isotopic results. Precambrian. Figure 6 shows the Sr evolution for the Chain Lakes massif Our correlation of the high-grade metamorphism with the as a whole. Using the average 87 Sr/86sr value for the samples 684 Ma regression line "age" sets a minimum age for the forma­ at present (0.7275), a chord has been constructed through that tion of the Chain Lakes massif. A maximum age can be deter­ value and the value of the initial ratio at 684 Ma (0. 7156). The mined by the Sm-Nd provenance or crustal residence ages. The intersection with a source with CHUR Sr parameters (Mc­ Tcr value relative to OM of 1496 Ma would represent the Culloch and Chappell, 1982) is 1417 Ma; the intersection with maximum age for the Chain Lakes massif. We realize that this a depleted mantle source is 1457 Ma. The errors on these ages Tcr value almost certainly represents a mixture of different are 250 and 275 Ma respectively. The value for the depleted sources with different crustal residence ages. However, the mantle intersection age (SrTcr) is very close to the value of 1496 massif must be younger than the youngest source rock, and the Ma for the Tcr relative to depleted mantle obtained from the Nd youngest source rock can not be older than the average crustal data. residence age (or else the crustal residence age would be older). Both of these Tcr values are similar to the age of 1534 Ma Thus, the Sm-Nd crustal residence age sets a maximum age for from the zircon results of Naylor et al. (1973) discussed above. the massif. Additional support for the significance of this crustal These zircons were taken from the matrix of the Sarampus Falls residence age comes from Rb-Sr and U-Pb results as well. member. Examination of the zircon population from sample Average crustal residence ages derived from Sm-Nd and CLM-1 analyzed by Naylor et al. ( 1973), shows that the zircons Rb-Sr results assume that the Chain Lakes massif as a whole has are well rounded to subhedral, frosted and pitted grains. We remained closed to Sm, Nd, Rb, and Sr transport. Isotopic interpret these characteristics as suggesting a detrital origin for systems may be reset on both the mineral and whole rock scale, the zircons from the matrix material. but if the massif as a unit remains closed, the isotopic systems The age of 1534 Ma for the zircon from the Sarampus Falls will retain information about the average crustal residence ages member of the Chain Lakes massif represents a mixture of for the sources. The closed system behavior of the Chain Lakes zircons from different sources. However, its similarity to the Tcr on the scale of the massif is suggested by the lack of evidence of values from the Sr and Nd results suggest that the massif may partial melting or the removal by anatexis of a component of the represent a well-mixed combination of sources. This crustal massif. There is no evidence of long distance transport of Sr or residence age of circa 1500 Ma may provide an upper limit for Rb within the massif represented, for example, by extensive the age of the Chain Lakes massif. The isotopic results may pegmatite or aplite dikes, epidote veins, etc., except near in­ constrain the age of the Chain Lakes massif between 684 Ma and trusions. Thus if the massif has remained a closed system, we 1500 Ma.

134 Regional significance ofthe Chain Lakes massif

Certainly crustal residence ages must be approached (4) The rocks of the Chain Lakes massif were metamor­ cautiously (Arndt and Goldstein, 1987), but the similar crustal phosed to high grade at about 684 Ma, resetting the whole rock residence ages from three independent techniques suggest an Rb-Sr systems. average source age for the massif of -1500 Ma. (5) Subsequent low-grade metamorphic effects from the Similar old, mixed source ages have been found for other Acadian orogeny and the intrusion of plutons reset the Rb-Sr rocks in New England and Maritime Canada. Olszewski and mineral ages at about 400 Ma. Gaudette ( 1982) found detrital zircons with ages of 1641 Ma in (6) Although the Chain Lakes massif has many similarities high-grade rocks of the Brookville Gneiss in New Brunswick. in isotopic ages to other areas in New England and Maritime Olszewski ( 1980) found detrital zircons ranging in age from Canada, there are significant differences in rock types, geologic 1500 Ma to 2000 Ma in rocks of the Avalon terrane and the history, and metamorphism. The Chain Lakes massif cannot be Nashoba block of southeastern Massachusetts. Old zircon com­ reliably correlated with other rocks in the northern Appalachians, ponents (>I 000 Ma) are found in many of the binary in although it has probably interacted with other terranes since the New England (Harrison et al., 1987). However, the significance late Precambrian. The massif appears to be a unique terrane in of these similarities in age is at present unknown. New England. Although the age of the Chain Lakes massif can be tenative­ Our isotopic results cannot provide unequivocal informa­ ly constrained between 684 Ma and 1500 Ma, its exact age of tion about the origin of the Chain Lakes massif. In fact, none of formation (presumably deposition) cannot be determined based the possible modes of origin outlined by Boudette and Boone on the available data. The lowest Tcr age we obtained is 763 Ma ( 1982) can be discounted based on this study. However, Archean by Sm/Nd for sample 404. Thus it is possible that portions of the or early Proterozoic ages of formation for the Chain Lakes massif Chain Lakes massif are as young as - 760 Ma. The material as suggested by Boudette and Boone ( 1982) seem to be contrary making up the Chain Lakes massif could have undergone a to the available isotopic data. number of periods of erosion, transportation, deposition, Our results also support the idea that the Chain Lakes massif lithification, and metamorphism between 1500 Ma and the high­ is not basement of Grenville affinity (Ayuso et al., 1987; grade metamorphism at -684 Ma. Boudette and Boone, 1976; Naylor, 1975, 1976; Osberg, 1978; These age constraints on the Chain Lakes massif are consis­ Stewart et al., 1985, 1986; Williams and Hatcher, 1983; Zen, tent with its present day £Nd and Esr values when compared to l 983a), at least in the sense that it is directly related to Grenville other Precambrian high-grade terrains. Figure 7 is an £Nd - Esr rocks where high-grade metamorphism occurred at 900 to 1100 diagram for the samples from the McKenney Pond member, based upon the data listed in Table 5. Data from key geologic provinces are also provided for comparison. The results for the Chain Lakes massif show high positive £sr values and moderate­ 20 ly negative £Nd values. Both are consistent with other high­ grade gneissic terrains such as the Grenville (Area Din Fig. 7; BULK EARTH McCulloch and Wasserburg, 1978), 1500 Ma granulites (Area E, Ben Othman et al., 1984), and Canadian Shield gneisses older than 2500 Ma (Area F, McCulloch and Wasserburg, 1978). Thus uz an old source age for the Chain Lakes massif is reasonable when w compared to other areas. --- ·20 CONCLUSIONS

Our major conclusions from this study are: ( l) The materials of the Chain Lakes massif were deposited .400.....~...._~...... ~...... 1..~--'...._~._~...._~ ...... ~...... 1.~~..____. -60 0 120 360 480 some ti me between -1500 Ma and -684 Ma based on Rb-Sr and Sm-Nd whole rock, and zircon U-Pb analyses. (2) The sources for the sedimentary material of the Chain Figure 7. ENd-£5,diagram comparing results for the Chain Lakes massif Lakes massif were derived from continental crust whose ultimate with other rocks of varying age and location. The values for the Chain sources in the upper mantle average about 1500 Ma in age. The Lakes samples are listed in Table 5. The£ rock units from other areas sedimentary material that made up the massif was probably a that are plotted on the diagram are: A: MORB (O'Nions et al., 1979); B: Granitoid rocks older than IOOO Ma (Allegre and Ben-Othman, mixture of sources of different ages. 1980); C: Granitoid rocks younger than 400 Ma (Allegre and Ben-Oth­ (3) The isotopic signatures of the rocks of the Chain Lakes man, 1980); D: Grenville gneisses (McCulloch and Wasserburg, 1978); massif are consistent with derivation from an older mixture of E: 1500 Ma granulites (Ben-Othman et al., 1984 ); F: Canadian Shield sources and are similar to high-grade areas of similar age from gneisses older than 2500 Ma (McCulloch and Wasserburg, 1978); G: other parts of the world. Liberian charnockites, 3300 Ma (Ben-Othman et al., 1984).

135 M. M. Cheatham and others

Ma. However, Grenville rocks may have been one of the sources tant that tectonic models attempt to establish and evaluate what for the sediments of the massif. the existence and extent of this unique terrane means to the Recent work by Aleinikoff and Moench (1985, 1987), Bar­ evolution of the northern Appalachian orogen. reiro and Aleinikoff ( 1985), and Ayuso et al. ( 1987) suggests that Precambrian rocks with isotopic signatures similar to the Chain ACKNOWLEDGMENTS Lakes massif may underlie the Kearsarge-central Maine synclinorium of New England (Lyons et al., 1982). Whether This study was part of the senior author's M.S. thesis these basements have any relationship to the Chain Lakes massif submitted to the Department of Earth Sciences, University of is unknown. Other occurrences of rocks lithologically similar to New Hampshire. This project and investigation of the evolution the Chain Lakes massif are found only in Quebec within the of the Eastern Margin Precambrian Complexes have been sup­ melange of the St. Daniel and Brompton Formations (Williams ported by the National Science Foundation (Grants EAR-81- and St-Julien, 1982; Cousineau and St-Julien, 1986). These 21423 and EAR-79-27093) to Henri E. Gaudette and William J. occurrences may be rafts of Chain Lakes rock rather than ex­ Olszewski. Dr. Richard S. Naylor graciously provided us with posure of a Chain Lakes basement. zircon sample CLM-1 from his earlier study. Thanks go to Gene Rocks of the Chain Lakes massif are unique in the northern Boudette for his continuing interest and insights, assistance in Appalachians. How widespread their occurrences are is present­ the field sampling program, and his willingness to read and ly unknown. The nature of the basement beneath the Kearsarge­ comment upon many versions of the manuscript. We thank Dave central Maine synclinorium is also essentially unknown. Stewart for reading and commenting upon an early version of Geophysical evidence (Stewart et al., 1985) is consistent with a the paper, and John Aleinikoff, Chuck Guidotti, Marc Loiselle, greater extent of the Chain Lakes massif to the southeast than the and Bob Ayuso for careful and thoughtful reviews of the surface expression in northwestern Maine suggests. It is impor- manuscript.

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136 Regional significance of the Chain Lakes massif

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