Geochronology and geochemistry of Putnam-Nashoba metavolcanic and plutonic rocks, eastern Massachusetts: Constraints on the early Paleozoic evolution of eastern North America

Martin Acaster* Department of Earth Sciences, Heroy Laboratory, Syracuse University, M. E. Bickford† } Syracuse, New York 13244-1070

ABSTRACT Boston–Rhode Island outboard of the Laurentian margin during the approach and collision of Gondwana. An important event The Nashoba Block, the northern portion of the Putnam-Nashoba was the migmatization of the Fort Pond and Beaver Brook members of terrane, is a -bounded fragment of Late Proterozoic to early Pale- the Nashoba gneisses about 340 Ma. ozoic crust located in eastern Massachusetts. The Marlboro and Nashoba gneisses of the Putnam-Nashoba terrane are composed of a INTRODUCTION sequence of mafic, intermediate, and felsic volcanic, volcanogenic, and probable plutonic rocks. This sequence was intruded by plutons rang- The Putnam-Nashoba terrane, located in Massachusetts between the ing in composition from granite to gabbro. Major and trace element Boston–Rhode Island terrane (Goldstein, 1994) to the east and the Merri- data for the Marlboro and Nashoba gneisses and some of the granitic mack trough to the north-northwest (Fig. 1), is an enigmatic piece of the puz- plutons are consistent with formation in a calc-alkaline arc setting; zle that is the Appalachian orogen. A better understanding of the nature and some of the later granitic plutons are probably the result of crustal ana- timing of the events that led to the formation of the Putnam-Nashoba terrane, texis. The gabbroic plutons are slightly alkaline. and its accretion to Laurentia, will shed light on the Appalachian orogen as a U-Pb zircon age determinations for the Marlboro and Nashoba whole. The goal of this research was to constrain the timing of volcanism, gneisses indicate ages ranging from 584 ± 8 to 425 ± 2 Ma, but most are plutonism, and metamorphism in the northern part of the Putnam-Nashoba in the range 473 to 430 Ma. These ages indicate that significant arc vol- terrane; the approach was the U-Pb geochronology of selected units of the canism occurred during Late Ordovician and Silurian time in an ocean Marlboro and Nashoba gneisses and rocks that intrude them. basin separating Laurentia and Avalonia as Avalonia progressed to- A problem inherent in understanding the geology of orogenic belts, and ward its eventual collision with and accretion to Laurentia. the timing and nature of their formation, is the nature of the processes that A metamorphic and deformational event, interpreted to record the give rise to the orogens. Many orogens are not the result of simple, easily docking of the Putnam-Nashoba terrane (volcanic arc) with Laurentia datable, crustal plate collisions, but rather series of collisions of smaller mi- as the Boston–Rhode Island terrane (Avalonian fragment) impinged croplates, volcanic arcs, and seamounts along the leading edge of a conti- upon it, is constrained by a 425 ± 3 Ma monazite age for the Fish Brook nental plate. These events may or may not culminate in the closure of one gneiss and the ca. 390 Ma ages of the Straw Hollow diorite and Salem or more ocean basins through collision with another crustal plate. gabbro-diorite. These mildly alkaline mafic plutons, which intrude the The Appalachian orogen, including the Putnam-Nashoba terrane, has a Putnam-Nashoba terrane and the Boston–Rhode Island terrane, re- complex history, taking its shape from a number of orogenic events. None spectively, are members of a group of roughly contemporaneous intru- of these compressional events was a singular occurrence, but rather con- sions that yield ages from 430 ± 5 to 385 ± 10 Ma. The ages and chemi- sisted of a series of tectonothermal pulses of varying styles (e.g., Hatcher, cal similarity of these intrusions support the interpretation that the two 1989; Wintsch et al., 1992,1993; Rast and Skehan, 1993). To understand the terranes were proximal to each other by Early Silurian to Late Devon- tectonic history of an , it is necessary to understand how each ian time. The 360 ± 9 Ma syntectonic, peraluminous Andover Granite of the events that led to its creation affected its parts. In New England the and the 349 ± 4 Ma calc-alkaline phase of the Indian Head Hill granite Appalachian orogen is represented by a sequence of lithotectonic belts, indicates additional igneous activity in Early Mississippian time. from the Taconic in the west to the Avalonian in the east. Later metamorphism of rocks of the Putnam-Nashoba terrane was The Acadian , a diachronous event spanning Silurian and De- presumably due to changing pressure-temperature conditions during vonian time and having three proposed stages (Robinson et al., 1993), is oblique overthrusting and subsequent unroofing of the Putnam- perhaps the most complex of the orogenic events in which the lithotectonic Nashoba terrane during tectonic shuffling of the accreted Nashoba and fragments of New England were involved. The effects of this event are found throughout New England, northward into the Canadian Maritimes, *Current address: Roger N. Smith Associates, Inc., 800 N.W. Sixth Avenue, and in the Caledonides of . The inception of the orogenic event has Portland, Oregon 97209. been shown to be characterized by Silurian volcanism and related pluton- † Corresponding author; e-mail: [email protected]. ism in the Canadian Maritimes to the north of the Putnam-Nashoba terrane Data Repository item 9917 contains additional material related to this article.

GSA Bulletin, February 1999; v. 111; no. 2; p. 240–253; 17 figures; 2 tables.

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Figure 1. Generalized tectonic map of southeastern New England. CNF—Clinton-Newbury fault; BBF—Bloody Bluff fault; LCF—Lake Char fault; HHF—Honey Hill fault; WF—Willimantic fault. After Goldstein (1994).

(Dunning et al., 1990a, 1990b), and in Connecticut to the south (Sevigny southeastern New England reflects post-Acadian, Alleghanian, uplift and and Hanson, 1993). The results of our study indicate that this is also the case stacking in a westward-propagating sequence of thrust , followed by in the Putnam-Nashoba terrane, where arc volcanism began in Ordovician normal faulting during tectonic exhumation. time and continued into Silurian time. Rast and Skehan (1993) proposed that the Boston–Rhode Island terrane, GEOLOGIC SETTING the Putnam-Nashoba terrane, and the Merrimack trough were pieces of a once-larger Avalonian superterrane that was segmented during Cambrian Rocks of the Nashoba terrane include the Marlboro and Nashoba gneisses, rifting, then reassembled during successive stages of the Acadian orogeny. the Tadmuck Brook Schist, the Andover Granite, and the Sharpner’s Pond In this model, the oblique to head-on collision of the Merrimack trough with Diorite complex. The Marlboro and Nashoba gneisses are mostly Late Prot- Laurentia produced the main phase of the Acadian orogeny at about 380 erozoic to early Paleozoic, mafic and felsic rocks that have been thought to Ma. This was followed by the dextral emplacement of the Putnam-Nashoba have volcanic or volcanogenic sedimentary protoliths. The timing of struc- terrane and, finally, the Boston–Rhode Island terrane in Carboniferous to tural and metamorphic events has not been well constrained (see following). Early Permian time. Goldsmith (1991) believed that most of the deformation preceded intrusion Wintsch et al. (1992, 1993), in contrast, proposed that Avalonia under- of both the Silurian (430 ± 5 Ma; Zartman and Naylor, 1984) Sharpner’s thrust the Putnam-Nashoba terrane in Early Devonian time (ca. 390 Ma), Pond Diorite complex and the Ordovician to Silurian (ca. 400 Ma; Hepburn producing a series of hinterland-propagating thrust nappes. Continued un- et al., 1995; this work) older phase of the Andover Granite, and that peak derthrusting caused uplift and cooling of the hanging-wall Putnam-Nashoba metamorphism was essentially synchronous with the generation of the main rocks such that cooling ages decreased from east to west as the cover rocks phase of the Andover Granite. The subsequent structural and metamorphic were uplifted, denuded, and cooled between about 380 Ma in the east until history is complex and not well constrained. Munn (1987) interpreted fabrics about 250 Ma to the west in the Central Maine and Bronson Hill terranes. and mineral assemblages to indicate two progressive amphibolite facies This model indicates that the present distribution of lithotectonic terranes in metamorphic events (m1, m2) and one retrogressive greenschist facies event

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(m3). However, it is also possible that these features resulted from shearing alkalic granitic plutons of Ordovician to Devonian age. Late Proterozoic to and retrograde metamorphism during protracted uplift, cooling, and denuda- Mesozoic volcano-sedimentary basins are also present. One of these basins, tion following the initial high-grade metamorphism. For the Nashoba gneiss, the fault-bounded Newbury volcanic complex, is between the Putnam- Bober (1990) determined a temperature of about 600 ± 50 °C for m1 (no Nashoba terrane and the Boston–Rhode Island terrane. The Silurian to De- pressure determination) and temperature and pressure of about 600 ± 50 °C vonian (Shride, 1976) rocks of the Newbury volcanic complex are tilted, un- and 4.4 ± 0.5 kbar for m2. For the Marlboro gneiss, Bober determined a tem- metamorphosed, calc-alkaline continental-arc andesites and rhyolites. Hon perature of 580 ± 50 °C and a pressure of 5.6 ± 0.5 kbar for m1, whereas for et al. (1986) pointed out the similarity of the chemical composition of the m2 he obtained 520 ± 50 °C and 3.4 ± 0.5 kbar. volcanic rocks to the calc-alkaline intrusive rocks of the Nashoba terrane, The Putnam-Nashoba terrane has been interpreted as an upright homo- and suggested they may be a volcanic expression of them, preserved in a clinal sequence facing west (Bell and Alvord, 1976; Skehan and Abu down-dropped fault block. Metamorphism in the Boston–Rhode Island ter- Moustafa, 1976; Skehan and Murray, 1980), but it may be a zone of tight, rane adjacent to the Nashoba portion of the Putnam-Nashoba terrane is pre- bedding-parallel, axial-planar, upright folds which are not readily identified dominantly greenschist facies; however, farther south, Pennsylvanian rocks because of the strong deformation. Major folds were mapped in the west of the Narragansett basin are metamorphosed to amphibolite facies flank of the Putnam-Nashoba terrane by Hansen (1956) and are present in (Wintsch et al., 1993). Prograde metamorphism in Avalonian basement the Wachusett-Marlboro tunnel cross section of Skehan and Abu Moustafa rocks preceded emplacement of the ca. 600 Ma Dedham granodiorite (1976). These features may indicate a large-scale synformal structure in a (Zartman and Naylor, 1984; Hepburn et al., 1993; Thompson et al., 1996); downward-thinning, westward-dipping, wedge-shaped block between the thereafter, there was retrograde metamorphism. Boston–Rhode Island terrane and the Merrimack trough (Goldsmith, 1991). The Merrimack trough (Fig. 1) consists of the Precambrian(?) Rye For- Internal faults prevent recognition of repetition. mation, a high-grade mylonitized gneiss and schist thought to be strati- Geochronologic investigations have, until recently, been relatively un- graphically equivalent to the Nashoba gneisses of the Putnam-Nashoba ter- successful in resolving the timing of tectonic events in the Putnam-Nashoba rane (Lyons et al., 1982), and the Silurian or Ordovician to Precambrian terrane. Olszewski (1980) obtained a zircon U-Pb age of 730 ± 26 Ma for Merrimack Group, whose members include the Kittery, Eliot, and Berwick the Fish Brook gneiss, but that age was revised by Hepburn et al. (1995) to Formations. The Berwick Formation has been correlated with the Paxton +6 499 /-3 Ma, which they considered the age of the felsic volcanic protolith Formation (in Massachusetts) and the Hebron Formation (in Connecticut), of the gneiss. Duplicate analyses of monazite from the same sample yielded thus allowing extension of the Merrimack trough to its western contact with an age of 425 ± 3 Ma, which these authors believe to date a major meta- the Bronson Hill terrane across the Bonemill Brook fault (Rast and Skehan, morphism in Silurian time. Di Nitto et al. (1984) calculated an Nd model 1993; Wintsch et al., 1993).

age (TDM) greater than 450 Ma for a sample of the Marlboro gneiss, but di- The Merrimack Group is predominantly a sequence of metamorphosed rect dating of the volcanic rocks in the Marlboro gneiss has been unsuc- sedimentary rocks, including quartzites with local conglomerates, calcare- cessful. Zartman and Naylor (1984) reported Rb-Sr whole-rock ages rang- ous siltstones and sandstones, and turbidites with well-preserved graded ing from 408 ± 22 to 446 ± 32 Ma for the Andover Granite, the older phase bedding (Robinson and Goldsmith, 1991); metamorphic grade increases of which intrudes the Nashoba gneisses and the younger phase of which in- from greenschist to lower amphibolite facies in the east, westward into up- trudes the Marlboro gneisses. They also determined a zircon U-Pb age of per amphibolite facies, and finally into a zone of partial melting and a gra- 430 ± 5 Ma for the Sharpner’s Pond Diorite complex, a multiphase pluton dational contact into the Massabessic Gneiss Complex (Fagan, 1986). The that locally intrudes the Nashoba and Marlboro gneisses, as well as the older Massabessic Gneiss Complex is a body of metasedimentary and metavol- phase of the Andover Granite. Unfortunately, the dated outcrop of the canic rocks that is migmatized and intruded by granites of Precambrian age Sharpner’s Pond Diorite is in fault contact with the Nashoba gneisses. Hep- (623 ± 8 Ma) that were subjected to metamorphism and minor intrusion ca. burn et al. (1995) recently reported ages of 412 ± 2 Ma for an unfoliated per- 390 Ma (Aleinikoff et al., 1995). Older studies indicated possible Ordovi- aluminous phase of the Andover Granite, 349 ± 4 Ma for a part of the Indian cian and Permian ages for these orthogneisses (Besancon et al., 1977; Head Hill granite, and 395 ± 2 Ma for migmatite in the Nashoba terrane. Aleinikoff et al., 1979; Kelly et al., 1980; Olszewski et al., 1984; Bothner et Motions of bounding faults of the Putnam-Nashoba terrane (Fig. 1) were al., 1984; Zartman and Naylor, 1984). Goldsmith (1991) suggested that studied by Goldstein (1989, 1994), Nelson (1987), and Barosh (1984). rocks of the Massabessic Gneiss Complex are, like the Rye Formation, a Barosh suggested that motion on the Bloody Bluff and Clinton-Newbury fault-bounded block within the Merrimack trough (Bothner et al., 1994), fault systems was right lateral, west over east before Mesozoic time and left correlative with rocks of the Putnam-Nashoba terrane. lateral and normal during Mesozoic time. Goldstein (1989) believed that the The results of this study indicate that the Putnam-Nashoba terrane is an Or- Silurian-Devonian motions on the Bloody Bluff system were sinistral with dovician to Silurian volcanic arc that may have been situated on the leading a thrust component, and that Mesozoic reactivation was low-angle normal. edge of the Boston–Rhode Island terrane. The collision of the Putnam- Goldstein (1994) found that similar motions occurred on the Clinton-New- Nashoba terrane with Laurentia, and the onset of Acadian orogeny in south- bury fault and in the Wachusett zone. The Putnam-Nashoba ter- eastern New England, began in the Silurian Period. However, as will be seen rane is also extensively faulted internally. Two large fault systems, the Ass- later herein, a major problem regarding the timing of arc volcanism and the abet River and Spencer Brook systems, form apparent thrust contacts age of peak metamorphism in the Silurian remains unresolved. During the between the Marlboro and Nashoba gneisses and also between individual Devonian Period, orogenesis progressed westward into the Merrimack trough members of the Nashoba gneiss. There are a number of structurally based and the Kearsarge–central Maine synclinorium (Rast and Skehan, 1993). models for the internal stratigraphy of the Putnam-Nashoba terrane (Goldsmith, 1991). SAMPLING AND ANALYTICAL METHODS The Boston–Rhode Island terrane (Fig. 1), across the Bloody Bluff fault to the east of the Putnam-Nashoba terrane, is composed mostly of Late Prot- Sampling erozoic granitic rocks that intrude older metasedimentary and metavolcanic rocks having compositions characteristic of a convergent arc-margin setting The Marlboro and Nashoba gneisses are composed primarily of biotite- (Goldsmith, 1991). These rocks have been intruded by gabbroic to mostly hornblende quartzofeldspathic gneisses and amphibolites, the protoliths of

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which are considered to represent a sequence of volcanic rocks and vol- ples to 1050 °C. Total Fe was determined as Fe2O3. Analytical data are canogenic sedimentary rocks (e.g., Goldsmith, 1991). The ages and geo- given in Table 1. chemical signatures of these rocks should reveal the timing and tectonic set- Samples for geochronology were typically 100 kg or greater. Zircon and ting of the formation of the Putnam-Nashoba terrane. Consequently, we monazite were separated from powdered rock samples by standard methods collected eight samples from the Marlboro gneisses and six from the using a Wilfley table, heavy liquids, and magnetic separator, followed by Nashoba gneisses in an upward sequence that may be either structural or careful hand-picking to remove residual impurities. Isotopic analyses were depositional. Because a major goal of the research was zircon U-Pb done by standard methods of isotope dilution, modified from those of geochronology, all but three of these samples are of felsic lithologies. Sam- Krogh (1973), in the Isotope Geochemistry Laboratory at Syracuse Univer- ple locations are shown on Figures DR1 and DR2, which are in the Geo- sity between 1992 and 1994. Isotope ratio measurements were made on a logical Society of America Data Repository1. VG Sector 54 multicollector mass spectrometer equipped with an ion- Intrusive rocks, including the foliated two-mica phase of the Andover counting Daly multiplier system. Analytical blanks during these analyses Granite (Castle, 1964), a pegmatitic and a foliated phase of the Straw Hol- were between 40 and 300 pg for Pb, but most were less than 100 pg; U low diorite, and the unfoliated Indian Head Hill granite, were sampled on blanks were less than 10 pg. All analyses were corrected for blank, but these the basis of demonstrable crosscutting relationships that would allow con- corrections were, for the most part, negligible, because 0.5 to 1.0 mg multi- straints on the timing of metamorphism of their country rocks. The Nahant grain zircon and monazite samples were analyzed. Data were reduced and gabbro and a pegmatitic phase of the Salem gabbro-diorite, both from the ages calculated using the programs of Ludwig (1980, 1983). Natural con- adjacent Boston–Rhode Island terrane, were sampled to examine possible stants used are those of Steiger and Jaeger (1977). Analytical data are given contemporaneity with the Straw Hollow diorite and the Sharpner’s Pond in Table 2. Diorite (Zartman and Naylor, 1984) of the Putnam-Nashoba terrane. Con- temporaneous emplacement of similar magmatic rocks in the Putnam- GEOCHEMICAL AND GEOCHRONOLOGICAL DATA Nashoba terrane and the Boston–Rhode Island terrane would suggest that AND INTERPRETATION they had docked prior to the emplacement of the intrusions. Geochronology of Rocks with Probable Volcanic Protoliths Analytical Methods Marlboro Gneiss, Grafton Gneiss Member. The Grafton gneiss mem- Samples were prepared for chemical analysis by standard methods, in- ber is structurally the lowest member of the Marlboro gneiss we sampled. cluding jaw crusher and shatter box. Representative powdered rocks were The rock is a biotite quartzofeldspathic gneiss that was thought by Hepburn sent to the X-ray Fluorescence Centre, Department of Geology, St. Mary’s (1978) to be an intrusion. Goldsmith (1991), however, suggested that the University, Halifax, Nova Scotia, for determination of major and trace ele- protolith was probably a felsic volcanic rock. This suggestion is supported ment abundances. Loss on ignition (LOI) was determined by heating sam- by an observed alternating gradational transition into an overlying amphi- bolitic unit. Major and trace element analytical data are not available for this rock unit. Further discussion with J. Christopher Hepburn, Boston College 1GSA Data Repository item 9917, Figures DR1 and DR2, is available on re- (1998, personal commun.), has cast doubt on whether our sample was from quest from Documents Secretary, GSA, P.O. Box 9140, Boulder, CO 80301. E-mail: the Grafton gneiss or could have been of the Scituate granite gneiss. The im- [email protected]. Web: http://www.geosociety.org/pubs/ftpyrs.htm. plications of this uncertainty are discussed later in this paper.

TABLE 1. MAJOR AND TRACE ELEMENT ANALYTICAL DATA Metavolcanic Rocks Plutons Sample MG-1 MG-2 MSP1 MSP2A MSP28 NF-I IHH-1 SHD SHP SGP NG FG-1 SiO2 58.06 66.21 71.52 47.80 48.54 63.92 75.47 46.85 44.36 47.62 44.79 73.47 Al2O3 18.35 16.17 12.82 15.60 14.75 15.56 12.81 17.17 17.42 16.84 15.88 14.12 Fe2O3 6.17 4.78 5.40 10.21 10.54 7.27 1.81 9.45 12.77 13.05 12.01 1.76 MgO 3.79 1.62 1.63 8.47 8.28 2.99 0.13 6.13 4.69 3.64 5.21 0.39 CaO 6.36 3.23 2.17 9.87 10.77 1.92 0.88 8.35 9.12 7.61 9.50 1.36 Na2O 2.86 3.06 4.35 2.77 2.35 2.76 3.70 3.63 3.70 4.33 3.33 2.93 K2O 1.65 2.30 0.50 0.46 0.29 3.02 3.43 2.13 1.46 1.86 1.00 4.48 TiO2 0.88 0.57 0.56 1.70 1.80 0.94 0.18 1.93 2.02 2.26 3.92 0.20 MnO 0.11 0.05 0.08 0.20 0.21 0.11 0.02 0.10 0.11 0.21 0.16 0.03 P2O5 0.16 0.16 0.19 0.14 0.15 0.12 0.03 0.70 1.63 1.48 0.30 0.03 L.O.I. 1.00 1.30 0.20 1.30 1.00 0.70 0.20 2.20 1.50 1.10 2.50 0.70 Totals 99.39 99.45 99.42 98.52 98.68 99.31 98.66 98.64 98.78 100.00 98.60 99.47 Ba 308 350 142 44 32 468 458 496 381 643 344 410 Pb 65 108 6 13 5 148 65 6 13 148 20 160 Sr 440 290 629 355 418 165 440 629 355 165 618 133 Y 22 24 118 32 33 25 52 33 38 44 25 31 Zr 96 140 836 136 161 207 221 188 113 230 175 88 Nb 6 10 38 5 5 16 13 10 11 66 36 9 Th 10 14 10 10 10 10 20 10 10 10 10 10 Pb 20 23 23 13 10 22 18 11 12 13 14 41 Ga 20 17 27 19 18 20 17 21 22 28 20 15 Zn 76 63 93 66 77 95 15 48 55 120 122 31 Cu 18 8 18 84 67 9 19 15 15 50 51 5 Ni 23 5 9 84 76 56 5 15 5 10 71 5 V 163 91 41 292 273 140 17 313 56 279 292 19 Cr 50 15 23 160 376 114 17 5 5 5 120 14

Note: Oxide measurements given as weight percent; total Fe as Fe203. Trace element measurements given in parts per million by weight.

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TABLE 2. ISOTOPIC DATA FOR U-Pb ANALYSES Samples* Concentrations† Pb Isotopic compositions§ Radiogenic ratios# Ages (Ma)** Wt (mg) U (ppm) Pb (ppm) 206Pb 207Pb 208Pb 206Pb 207Pb 207Pb 206Pb 207U 207Pb 204U 206Pb 206Pb 238Pb 235U 206Pb 238U 235Pb 206Pb Marlboro Formation, Grafton Gneiss (GG-1) nm(2)CB 0.46 1027.10 73.40 1162.1 0.07048 0.16195 0.06685 0.54037 0.05863 417.1 ± 8 438.7 ± 9 553.4 ± 3 m(1)CB 0.45 1898.20 145.30 492.3 0.08659 0.20964 0.06676 0.54006 0.05867 416.6 ± 7 438.5 ± 7 555.0 ± 6 m(2)CP 0.24 644.10 53.30 1836.5 0.06684 0.18413 0.07713 0.62758 0.05901 479.0 ± 7 494.6 ± 7 567.5 ± 3 m(1)CA 0.29 156.00 12.60 648.5 0.07990 0.20358 0.07413 0.60237 0.05894 461.0 ± 7 478.7 ± 10 564.8 ± 26 Marlboro Formation, Sandy Pond Amphibolite Member (MSP-1) m(0) 0.35 315.80 28.60 861.2 0.08901 0.26002 0.07740 0.77442 0.07256 480.6 ± 8 582.3 ± 9 1001 ± 2 m(1) 0.42 175.50 15.80 368.4 0.09630 0.30115 0.07251 0.56992 0.05701 451.3 ± 7 458.0 ± 7 491.8 ± 3 m(2) 0.43 389.80 35.60 416.6 0.09128 0.34992 0.07136 0.55616 0.05653 444.3 ± 7 449.0 ± 7 473.2 ± 5 m(3) 0.27 373.20 35.80 254.7 0.11448 0.36451 0.07195 0.57209 0.05767 447.9 ± 7 459.4 ± 7 517.2 ± 9 Marlboro Formation, Millham Reservoir "Granulite" Member (MG-1 and MG-2) MG-1 m(4)CP 0.82 507.30 33.20 3350.1 0.05951 0.13640 0.06355 0.48384 0.05522 397.2 ± 7 400.7 ± 7 421.0 ± 1 m(5)CP 0.80 463.70 31.60 684.54 0.07622 0.19528 0.06095 0.46228 0.05501 381.4 ± 13 385.8 ± 14 412.4 ± 23 m(4)CCP 0.66 555.20 38.90 4168.4 0.05874 0.14382 0.06787 0.51758 0.05531 423.3 ± 11 423.5 ± 11 424.6 ± 2 MG-2 m(2) 0.22 573.90 37.10 803.0 0.07340 0.13366 0.06192 0.47238 0.05533 387.3 ± 6 392.8 ± 6 425.5 ± 4 m(3) 0.56 663.40 37.80 288.4 0.10539 0.21876 0.04718 0.35750 0.05496 297.2 ± 4 310.4 ± 5 410.5 ± 9 m(4) 0.25 1328.40 71.60 1242.0 0.06676 0.10614 0.05312 0.40352 0.05509 333.7 ± 5 344.2 ± 5 415.9 ± 4 Straw Hollow Diorite (SHD, diorite, and SHP, pegmatitic phase) SHD m(2) 0.37 501.40 35.90 1556.4 0.06438 0.27552 0.06164 0.46817 0.05508 385.6 ± 6 389.9 ± 6 415.6 ± 2 m(3) 0.35 487.10 33.20 823.1 0.07293 0.29437 0.05706 0.43506 0.05530 357.7 ± 5 366.8 ± 5 424.5 ± 2 m(4) 0.31 760.20 53.90 1165.0 0.06687 0.26880 0.06121 0.45927 0.05442 383.0 ± 6 383.8 ± 6 388.3 ± 4 SHP nm(1) 0.46 429.70 37.70 193.5 0.13014 0.38901 0.06109 0.46279 0.05494 382.3 ± 6 386.2 ± 7 409.7 ± 23 m(1) 0.37 259.10 18.80 747.6 0.07607 0.26882 0.06139 0.47987 0.05670 384.1 ± 6 398.0 ± 7 479.7 ± 17 Andover Granite (FG-1) m(1) 0.35 590.30 41.70 362.6 0.09454 0.27376 0.05802 0.43611 0.05452 363.6 ± 6 367.5 ± 6 392.5 ± 5 m(2) 0.36 1755.80 102.90 1449.7 0.06530 0.06870 0.05965 0.45511 0.05534 373.5 ± 6 380.9 ± 6 426.0 ± 2 m(3) 0.35 1934.70 113.90 1676.9 0.06511 0.05838 0.06054 0.47177 0.05652 378.9 ± 6 392.4 ± 6 473.0 ± 9 Indian Head Hill Granite (IHH-1) m(4)CCP 0.32 262.00 17.80 837.3 0.07223 0.32436 0.05590 0.42306 0.05489 350.6 ± 5 358.2 ± 5 407.7 ± 4 m(4)FCP 0.40 1560.3 97.30 677.3 0.09414 0.19365 0.05661 0.57130 0.07319 355.0 ± 5 458.9 ± 7 1019 ± 3 m(1) 0.28 320.10 20.10 566.1 0.07956 0.27252 0.05308 0.39410 0.05385 333.4 ± 5 337.4 ± 6 364.8 ± 6 m(2) 0.35 780.70 36.80 1786.3 0.06206 0.13699 0.04582 0.34610 0.05394 289.2 ± 4 302.2 ± 5 368.6 ± 16 m(3) 0.41 996.80 36.70 802.3 0.07592 0.18467 0.03405 0.27175 0.05788 215.9 ± 3 244.1 ± 4 525.2 ± 5 Salem Gabbro-Diorite (SGP, Pegmatitic Phase) nm(2) 0.45 362.20 24.30 791.7 0.07078 0.20377 0.06059 0.45445 0.05440 379.2 ± 6 380.4 ± 6 387.6 ± 11 m(3) 0.38 381.30 26.40 729.9 0.07320 0.20667 0.06201 0.46627 0.05453 387.8 ± 6 388.6 ± 6 393.3 ± 5 m(4) 0.46 228.20 15.70 672.8 0.07108 0.20042 0.06239 0.46857 0.05447 390.1 ± 6 390.2 ± 6 390.6 ± 12 Nahant Gabbro (NG-1) m(0) +200 0.42 434.10 18.80 384.1 0.10050 0.30751 0.03562 0.30945 0.06301 225.6 ± 5 273.8 ± 6 708.7 ± 15 m(0) -200 0.36 955.20 21.50 935.8 0.07213 0.28522 0.01920 0.15000 0.56650 122.5 ± 2 141.9 ± 2 477.9 ± 3 m(1) +200 0.32 282.20 24.70 1132.4 0.07007 0.27562 0.07562 0.59744 0.05730 469.9 ± 7 475.6 ± 7 503.2 ± 2 m(1) -200 0.40 240.2 19.10 1642.5 0.06572 0.25784 0.06998 0.54928 0.05693 436.1 ± 7 444.5 ± 7 488.6 ± 2 Nashoba Formation, Fort Pond Member (NF-1, melanosome, and NF-2, leucosome) NF-1 nm(2) 0.38 241.20 30.60 208.0 0.14337 0.28561 0.09830 1.12456 0.08297 604.5 ± 9 765.2 ± 13 1268 ± 14 m(2) 0.94 334.90 33.40 1746.1 0.08838 0.13181 0.09452 1.05023 0.08059 582.2 ± 9 729.0 ± 11 1211 ± 2 m(3) 0.48 370.00 36.00 1004.3 0.09168 0.13965 0.09139 0.98297 0.07801 563.7 ± 9 695.1 ± 10 1147 ± 2 m(6) 0.20 158.50 17.90 368.8 0.12102 0.23308 0.09658 1.10755 0.08317 594.3 ± 9 757.0 ± 11 1273 ± 3 mon-a 0.64 2987.60 508.90 1541.4 0.06298 2.51446 0.05441 0.40237 0.05364 341.5 ± 5 343.4 ± 5 355.8 ± 6 mon-b 0.41 2365.30 425.70 1410.3 0.06468 2.55820 0.05669 0.42546 0.05443 355.5 ± 6 360.0 ± 6 388.9 ± 2 mon-c 0.25 587.10 109.20 487.1 0.08500 2.49314 0.05832 0.44497 0.05534 365.4 ± 5 373.8 +6 426.0 ± 6 NF-2 m(1) 0.26 290.90 19.20 982.7 0.07111 0.29131 0.05693 0.44271 0.05640 356.9 ± 5 372.2 ± 6 468.1 ± 4 m(2) 0.36 361.00 23.60 386.6 0.09425 0.20734 0.05690 0.44672 0.05694 356.7 ± 5 375.0 ± 6 489.3 ± 3 m(4) 0.30 80.20 15.20 69.6 0.26699 1.56715 0.06124 0.50913 0.06030 383.2 ± 6 417.9 ± 7 614.2 ± 13 Nashoba Formation, Beaver Brook Member (BB-1) m(1) 0.21 1082.20 100.00 464.2 0.09991 0.16759 0.08224 0.78606 0.06932 509.5 ± 8 588.9 ± 9 908.2 ± 2 m(2) 0.14 1235.90 92.60 838.6 0.08189 0.12216 0.07154 0.63944 0.06483 445.4 ± 7 502.0 ± 8 768.8 ± 5 m(3) 0.15 1436.40 104.60 633.6 0.08532 0.12932 0.06842 0.59113 0.06266 426.6 ± 6 471.6 ± 7 696.9 ± 3 *nm = nonmagnetic; m = magnetic; numbers in parentheses indicate tilt on Frantz separator at 1.7 amperes; CB — coarse brown; CP — coarse prismatic; CA — coarse anhedral; CCP—coarse clear prisms; FCP—fine clear prisms. †Total U and Pb, corrected for analytical blank. §Measured ratios, not corrected for blank or mass discrimination. #Ratios corrected for 0.1% per amu mass discrimination, analytical blank and nonradiogenic Pb (Stacey and Kramers [1975] model). **Errors given at 2σ.

Our sample of the Grafton gneiss (GG-1) yielded clear, colorless, pris- age of 584 ± 8 Ma (Fig. 2), which is interpreted to be the primary crystal- matic zircons. After sieving to less than 150 µm, the zircons were purified lization age of the volcanic protolith of the Grafton gneiss. by handpicking so that all were free of cores and cracks. These were sepa- Marlboro Gneiss, Sandy Pond Amphibolite Member. We collected rated into four magnetic fractions and analyzed for U and Pb isotopic com- three samples of this predominantly amphibolitic member. Two samples position and concentration. The data (Table 2) yielded an upper intercept (MSP-2A, 2B) are amphibolites. Major element data (Table 1; Fig. 3) indi-

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304 Ma is the maximum age of the inherited component. However, this in- terpretation assumes that the zircon fractions have not undergone Pb loss since 445 Ma. An alternate interpretation is that the least-discordant frac- tion has no inheritance and that its 207Pb/206Pb age of 473 Ma (Table 2) is the maximum age of the rock and that the 1001 Ma 207Pb/206Pb age of the most discordant fraction is the minimum age of the inherited component. A conservative assessment of the data is that the crystallization age is be- tween 473 and 445 Ma. Marlboro Gneiss, Millham Reservoir “Granulite” Member. Two samples of this member were collected (MG-1, MG-2). Note that “gran- ulite” in this case is a local usage that describes texture and does not imply metamorphic grade. Major element data (Table 1; Fig. 3) indicate that the protolith of MG-1, a hornblende-cummingtonite plagioclase gneiss, is com- positionally andesite, whereas the protolith of MG-2, a biotite-plagioclase K-feldspar gneiss, is compositionally dacite. We separated three fractions of clear, light brown prismatic zircons from MG-1. Analytical data for U and Pb isotopic concentrations and composi- Figure 2. Concordia diagram for zircons from the Grafton gneiss. tions (Table 2; Fig. 6) indicate an upper intercept age of 425 ± 3 Ma. One fraction is concordant; we interpret the discordance of the other two frac- tions to result from normal Pb loss. We believe 425 ± 3 Ma to be the time of cate that their compositions are basaltic, whereas trace element data, plot- crystallization of the volcanic protolith. ted on the Ti-Zr-Y (Fig. 4) discrimination diagram of Pearce and Cann Sample MG-2 also yielded clear, light brown prismatic zircons. Three (1973), suggest that they are calc-alkaline basalts. fractions of these yielded U and Pb data (Table 2; Fig. 7) that are somewhat The third sample (MSP-1), a fine-grained felsic quartzofeldspathic more discordant than the data for MG-1. Regression yielded an upper inter- gneiss, has the major element composition of a low-K rhyolite (Table 1; cept age of 430 ± 7 Ma, within error in agreement with the age of MG-1.

Fig. 3). Trace element data, especially the relatively high Zr/TiO2 ratio, in- dicate that the gneiss is a metarhyolite. This sample yielded colorless, pris- Geochronology of Intrusive Rocks of the Putnam-Nashoba Terrane matic zircons with clearly visible inherited cores. We prepared four multi- grain magnetic separates for analysis. Data (Table 2; Fig. 5) for three Straw Hollow Diorite. We collected two samples from this body. Sam- fractions plot near concordia, whereas the fourth is displaced farther to the ple SHD is foliated hornblende diorite, whereas SHP is a slightly foliated right. If this array is interpreted as the result of mixing of zircon crystal- pegmatitic phase of the intrusion. Both samples have major and trace ele- lized during formation of the volcanic protolith with an older, inherited ment compositions (Table 1) that indicate somewhat alkaline affinities. component, regression of the data points yields a lower intercept age of 445 Each sample yielded clear, brown, anhedral zircons. These zircons were ± 4 Ma and an upper intercept age of 3194 ± 304 Ma. In this interpretation, analyzed for U and Pb concentrations and compositions (Table 2; Fig. 8). 445 ± 4 Ma is the crystallization age of the volcanic protolith and 3194 ± Of three fractions from sample SHD, one is nearly concordant with a

Figure 3. Total alkali vs. silica discrimination diagram after LeBas et al. (1986). Sample symbols: MSP-1—closed triangles; MSP-2A—open circles; MSP-2B—closed circles; MG-1—closed squares; MG-2—open squares; NF-1—open triangles.

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Figure 4. Ti-Zr-Y discrimination diagram after Pearce and Cann (1973). Sample symbols as in Figure 4. Fields: WPB—within-plate basalts; CAB—calc-alkaline basalts; LKT—low-K tholeiites; OFB— ocean-floor basalts.

Figure 6. Concordia diagram for zircons from a hornblende gneiss in the Milham Reservoir granulite member of the Marlboro Formation.

Figure 5. Concordia diagram for zircons from metarhyolite in Sandy Pond amphibolite member of the Marlboro Formation.

206Pb/238U age of 383 ± 6 Ma, a 207Pb/235U age of 384 ± 6 Ma, and a 207Pb/206Pb age of 388 ± 4 Ma. A second fraction plots to the right, indi- cating an inherited component, and a third is more discordant, indicating Pb loss. A regression excluding the point with inheritance yields an upper intercept age of 385 ± 4 Ma, but the lower intercept is negative, indicating some inheritance in the more discordant point. Of two fractions from sam- 206 238 ple SHP, one is concordant (but with a large uncertainty), with a Pb/ U Figure 7. Concordia diagram for zircons from biotite gneiss in the 207 235 207 206 age of 382 ± 6 Ma, a Pb/ U age of 386 ± 7 Ma, and a Pb/ Pb age of Milham Reservoir granulite member of the Marlboro Formation. 410 ± 23 Ma, whereas the second plots farther to the right, indicating an in- herited component. Given the nature of the data, regressions on either a Pb loss or a mixing- line model are dominated by the concordant point from sample SHD. Con-

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Figure 8. Concordia diagram for zircons from the foliated and peg- Figure 9. Concordia diagram for zircons from the foliated binary matitic hornblende diorite phases of the Straw Hollow diorite. granite phase of the Andover Granite.

sequently, we believe that 385 ± 10 Ma is a reasonable estimate of the age et al. (1995) reported an age of 349 ± 4 Ma for the Indian Head Hill granite; of the Straw Hollow diorite. we accept this as the crystallization age of this granite body. Foliated Two-Mica Phase of the Andover Granite. We collected two samples of this rock (FG-1, FG-2). Major and trace elemental data (Table 1) Intrusive Rocks of the Boston–Rhode Island Terrane indicate that both are rather typical peraluminous granites, with a significant sedimentary rock component in their source area. Salem Gabbro-Diorite. We collected a sample of the Salem Gabbro- Three fractions of euhedral zircon crystals from FG-1 were analyzed for Diorite of the Boston–Rhode Island terrane for comparison with the Straw U and Pb concentration and composition (Table 2; Fig. 9). Despite the lack Hollow diorite of the Putnam-Nashoba terrane. The two mafic plutons are of visible cores, the data array appears to be controlled by an inherited com- similar chemically (Table 1), both showing somewhat alkaline characteris- ponent, yielding an upper intercept age of 1490 ± 481 Ma and a lower in- tics. Similarity in age could indicate that mafic magmas were derived and tercept age of 359 ± 9 Ma. We interpret the lower intercept age as the mini- emplaced following the docking of the Putnam-Nashoba terrane and the mum age for this rock, whereas the 392 ± 5 Ma 207Pb/206Pb age of the Boston–Rhode Island terrane. least-discordant point is the maximum age. We note that Hepburn et al. We collected a sample of the pegmatitic phase of the Salem Gabbro- (1995) obtained a minimum age of 412 ± 2 Ma for an unfoliated phase of Diorite because of the likelihood that it would yield zircons. We analyzed the Andover Granite, indicating the complexity of this intrusive body. three unabraded fractions of brown, anhedral zircons for U and Pb isotopic Indian Head Hill Granite. One sample of the fine- to medium-grained, concentrations and composition (Table 2). The three fractions are concor- mostly unfoliated biotite granite phase of this pluton was collected. This dant within uncertainties (Fig. 11). When regressed through 0 ± 10 Ma, the phase intrudes an older, more dioritic, foliated phase of the pluton. The ma- three data points yield an upper intercept age of 392 ± 4 Ma, which we in- jor element chemical composition (Table 1) indicates that the analyzed phase terpret as the crystallization age. This age agrees within error with that of is a high-silica granite. Trace element data (Table 1), particularly concentra- the Straw Hollow diorite (385 ± 10 Ma) of the Putnam-Nashoba terrane. tions of Rb, Nb, and Y, are suggestive of a volcanic-arc granite if the data are The Straw Hollow diorite and the Salem Gabbro-Diorite are therefore co- plotted on the tectonic discrimination diagram of Pearce et al. (1984). eval. The similarity of their ages and chemical composition (Table 1) sug- Our sample yielded clear, euhedral, prismatic zircons. We analyzed five gest that they may have formed in a tectonic environment shared by the Put- fractions for U and Pb isotopic concentrations and composition (Table 2). nam-Nashoba terrane and the Boston–Rhode Island terrane. The data indicate variable Pb loss and inheritance as shown in Figure 10. Nahant Gabbro. A sample of the Nahant Gabbro, also from the Boston- Regression of fractions m1, m2, and m3, which appear to form a normal Pb- Rhode Island terrane, was collected for comparison with the Straw Hollow loss array, yields an upper intercept age of 346 ± 6 Ma, but the lower inter- diorite. Our sample NG-1 yielded a population of euhedral zircon prisms, in cept age of –388 ± 46 Ma clearly indicates that the fractions have an inher- strong contrast to both the Salem and the Straw Hollow rocks, which yielded ited component. Fractions m4 CCP and m4 FCP, which plot above and to anhedral zircon grains. We analyzed zircons from two magnetic fractions the right of the m1–m3 array, also clearly indicate an inherited component. from each of two size fractions (–160/+200 mesh and –200 mesh; Table 2). The least-discordant fraction (m1) has a 206Pb/238U age of 333 ± 5 Ma, a As shown in Figure 12, the two magnetic fractions from the –200 size fraction 207Pb/235U age of 337 ± 6 Ma, and a 207Pb/206Pb age of 365 ± 6 Ma. Thus, yield an upper intercept age of 489 ± 2 Ma with a small positive lower inter- on the basis of our data, 333 ± 5 Ma is probably the minimum age of the cept. The two magnetic fractions from the –160/+200 size fractions yield a rock, whereas 365 ± 6 Ma is likely the maximum age. We note that Hepburn similar upper intercept age of 493 ± 3 Ma, but have a lower intercept of –241

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Figure 10. Concordia diagram for zircons from the biotite granite Figure 12. Concordia diagram for zircons from the Nahant Gabbro. phase of the Indian Head granite.

Geochronology of Metamorphic Events

Nashoba Gneiss, Fort Pond Member. The Fort Pond Member is a bi- otite–K-feldspar gneiss that is characteristically strongly migmatized, con- sisting of ~30% quartzofeldspathic leucosome and 70% biotite-rich melanosome. We collected a sample of the melanosome (NF-1) and the leu- cosome (NF-2). Major and trace elemental data (Table 1; Fig. 3) are only available for the melanosome (NF-1). Sample NF-1 yielded opaque, brown, subhedral zircons. Four fractions of these zircons yielded U and Pb data (Table 2; Fig. 13) that define an ap- parent mixing line between an upper intercept age of 2269 ± 246 Ma and a lower intercept age of 450 ± 34 Ma. Because the extrapolation to the lower intercept is large, and because the extent of Pb-loss discordance of the zir- con fractions is not known, little information can be derived from these data except that the Fort Pond Member melanosome contains an inherited zircon component that is at least 1147 Ma, the 207Pb/206Pb age of the least-discor- dant fraction. We note that Hepburn et al. (1995) reported a monazite U-Pb age of 425 ± 3 Ma for the Fish Brook Gneiss, which is between the Marl- boro and Nashoba gneisses, and interpreted this as the age of m1 metamor- phism. Thus, the 450 ± 34 Ma age indicated by the lower intercept may be related to the formation of metamorphic zircon in this rock during an early metamorphism (the m1 of Munn, 1987). However, we that we could Figure 11. Concordia diagram for zircons from the pegmatitic phase not observe metamorphic overgrowths in the zircon fractions and the ana- of the Salem Gabbro-Diorite. lytical data can be interpreted in terms of mixing and Pb loss such that the lower intercept age may be meaningless. Sample NF-1 (melanosome) also yielded monazite. We analyzed three Ma, suggesting that the larger zircons have an inherited component. We take fractions of clear, yellow-orange monazite, obtaining the data (Table 2) plot- 489 ± 2 Ma, the result from the –200 mesh fractions, as the best interpretation ted in Figure 14. These data points form an array with an upper intercept age of the crystallization age of the Nahant Gabbro. Thus, although the Nahant of 939 ± 210 Ma and a lower intercept age of 337 ± 9 Ma. We interpret these Gabbro is chemically similar to both the 392 Ma Salem Gabbro-Diorite and results as indicating metamorphic growth, or recrystallization, of monazite the 384 Ma Straw Hollow diorite complex, its age indicates that it is a much ca. 337 Ma about older cores that may have formed ca. 425 Ma (207Pb/206Pb older body and cannot be related to either the Salem or the Straw Hollow. age of most discordant fraction; Table 2).

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Figure 13. Concordia diagram for zircons from the melanosome of Figure 15. Concordia diagram for zircons from the leucosome of the the migmatized Fort Pond Member of the Nashoba Formation. migmatized Fort Pond Member of the Nashoba Formation.

agreement between the monazite lower intercept age of the melanosome (NF-1) and the zircon lower intercept age of the leucosome, we interpret these data as indicating that high-grade metamorphism and migmatization of the Fort Pond member occurred ca. 336 Ma. This may be the m2 meta- morphism of Munn (1987). Nashoba Gneiss, Beaver Brook Member. We collected one sample (BB-1) of biotite-plagioclase–K-feldspar gneiss of the Beaver Brook Mem- ber. Major and trace elemental data are not available for this sample, which yielded clear, colorless, rounded to subrounded zircons that we interpret as detrital. As expected, these zircons yielded U and Pb data (Table 2) that in- dicate a significant inherited component. The three data points plotted in Figure 16 define an upper intercept age of 1449 ± 69 Ma and a lower inter- cept age of 345 ± 12 Ma. We interpret the lower intercept age as represent- ing the time of formation of metamorphic zircon as new crystals and rims about older, detrital zircons during high-grade metamorphism, noting the agreement, within error, with the age of high-grade metamorphism and migmatization determined from both samples of the Fort Pond Member.

SUMMARY

The Late Proterozoic age (584 ± 8 Ma) of our Grafton gneiss sample lends credence to the proposed Avalonian identity of the basement rocks Figure 14. Concordia diagram for monazite from the melanosome of of the Putnam-Nashoba terrane, because the formation of its protolith was the migmatized Fort Pond Member of the Nashoba Formation. broadly contemporaneous with major magmatism in other Avalonian ter- ranes. Units of the Grafton gneiss, presumably felsic metavolcanic rocks, appear to grade upward into the more mafic Sandy Pond member of the Sample NF-2, the leucosome, yielded clear, euhedral, prismatic zircons. Marlboro gneiss, indicating that 584 ± 8 Ma is the maximum age of the Three fractions yielded the data (Table 2) plotted in Figure 15. Although re- Marlboro gneiss. Alternatively, the Grafton gneiss may be in noncon- gression of these data is essentially based upon two points, we interpret it as formable contact with the lowermost member of the Marlboro gneiss. In a mixing line between zircons formed during crystallization of the leuco- this case the Grafton gneiss underlies the Marlboro Formation, but its age some and an older, inherited component, with a lower intercept age of 335 is still a maximum age for the Marlboro Formation. It has also been sug- ± 9 Ma and an upper intercept age of 1592 ± 236 Ma. Noting the excellent gested (J. Hepburn, 1993, personal commun.) that the Grafton gneiss is in

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a fault sliver of the Bloody Bluff fault zone and is a piece of the Boston–Rhode Island terrane to the east. As noted earlier, J. Hepburn (1998, personal commun.) has suggested from field observation that our sample may have been of the Scituate granite gneiss, which is clearly part of the Avalonian Boston–Rhode Island terrane. In either case, the age of our sam- ple provides no constraint on the maximum age of the Marlboro gneiss, and the 499 ± 6 Ma crystallization age of the Fish Brook Gneiss determined by Hepburn et al. (1995) constrains the upper age limit of the Marlboro gneiss and indicates that arc volcanism in the Putnam-Nashoba terrane began in Late Cambrian to Early Ordovician time. The 445–473 Ma crystallization age of the felsic volcanic protolith of the quartzofeldspathic gneiss layer in the Sandy Pond member of the Marlboro gneiss suggests that arc volcanism continued throughout the Ordovician Pe- riod. This rock is essentially coeval with the Amonoosuc volcanics (453 ± 2 Ma) and the Partridge formation (449 ± 3 Ma) of the Bronson Hill anticli- norium (Tucker and Robinson, 1990). The apparent basement in the Bron- son Hill, the 613 ± 3 Ma Dry Hill Gneiss, is, like the Grafton gneiss, roughly coeval with magmatism in Avalonian terranes. We interpret the ages of the biotite gneiss (430 ± 7 Ma) and the horn- blende gneiss (425 ± 2 Ma) of the Millham Reservoir “granulite” member of the Marlboro gneiss as the time of crystallization of their volcanic pro- toliths because (1) the rocks have textures and compositions indicating vol- Figure 16. Concordia diagram for zircons from the Beaver Brook canic origins, (2) the zircons have morphologies suggesting igneous origins, Member of the Nashoba Formation. and (3) the U-Pb systematics of the zircons indicate simple Pb-loss mecha- nisms for their discordance. Furthermore, ca. 425 Ma Silurian igneous ac- tivity and metamorphism have been documented elsewhere in the northern indicating Ar closure (~500 °C) at ca. 340 Ma. It is not clear how migmati- Appalachians and Europe (Dallmeyer, 1990; Dunning et al., 1990a, 1990b; zation occurred in the Fort Pond and Beaver Brook Members of the O’Brien et al., 1991). There is, however, a serious problem in interpretation, Nashoba Formation at this time when elsewhere rocks were cooling through because these rocks, and others in the Nashoba terrane, are intruded by the about 500 °C unless they were at a deeper crustal level. 430 ± 5 Ma Sharpner’s Pond Diorite complex (Zartman and Naylor, 1984), which is nonfoliated. Moreover, our ages for the Milham Reservoir “gran- ulite” member are essentially coeval with age of the earliest tectonothermal TECTONIC IMPLICATIONS event revealed by the 425 ± 3 Ma monazite age from the Fish Brook Gneiss (Hepburn et al., 1995), a metamorphism that presumably also affected the As noted earlier, Rast and Skehan (1993) proposed that the Boston- Milham Reservoir rocks. Thus, there is a timing problem; volcanic rocks Rhode Island terrane, the Putnam-Nashoba terrane, and the Merrimack that formed at the surface ca. 425 Ma must have undergone deformation and trough were pieces of a once-larger Avalonian superterrane that was seg- high-grade metamorphism very rapidly and then been intruded by the 430 mented during Cambrian rifting and reassembled during successive stages Ma Sharpner’s Pond Diorite. We do not have an answer to this dilemma, of the Acadian orogeny. The data of this study are not consistent with this which must be addressed by more detailed study of the metamorphic and tectonic model; they indicate that most of the rocks in the Putnam-Nashoba structural history of the Nashoba terrane. terrane were formed in a volcanic-arc environment during Ordovician and The Devonian crystallization ages of the 385 ± 10 Ma foliated Straw Hol- Silurian time. This is the most important conclusion of our study. low diorite, the 392 ± 4 Ma Salem Gabbro-Diorite, and the 402 ± 5 Ma Rb- Wintsch et al. (1992; 1993) proposed that the distribution of lithotectonic Sr age of a diorite that has been called an “older phase” of the Indian Head terranes in southeastern New England mostly reflects post-Acadian, Al- Hill pluton (Hill et al., 1984), indicate that igneous activity within the Put- leghanian, uplift and stacking in a westward-propagating sequence of thrust nam-Nashoba terrane and the Boston–Rhode Island terrane occurred con- nappes as the Avalon composite terrane (the Boston–Rhode Island terrane temporaneously at this time, allowing for the proposal that they had either ac- of this paper) was progressively underthrust from east to west. Uplift was creted to each other by about 395 ± 10 Ma or had always been parts of a followed by normal faulting during tectonic exhumation. Their interpreta- whole. The 359 ± 9 Ma age of the syndeformational, foliated two-mica gran- tion is supported by the fact that cooling ages, as determined from study of ite phase of the Andover pluton and the 349 ± 4 Ma age of the predominantly the U-Pb and 39Ar/40Ar systems in a number of minerals, generally decrease unfoliated, fine-grained, granitic phase of the Indian Head Hill pluton (Hep- from east to west, suggesting that rocks that cooled in late Paleozoic time burn et al., 1995), which intrudes the 402 ± 5 Ma diorite discussed previ- (ca. 280 Ma) were thrust eastward over Avalonian rocks that cooled earlier ously, indicate that magmatism continued into Early Carboniferous time. (ca. 400 Ma). Their thermochronologic data, derived from analysis of min- The migmatitic Fort Pond Member of the Nashoba Formation has a mon- erals from rocks in the Putnam belt portion of the Putnam-Nashoba terrane azite age of 337 ± 9 Ma and a zircon age of 335 ± 9 Ma. New zircon was and in the Willimantic , indicate cooling through monazite closure formed in the Beaver Brook Member at 345 ± 12 Ma. Thus, these rocks (~720 °C) at ca. 400 Ma with continued slow cooling to about 200 °C (K- reached pressure-temperature conditions at which partial melting could oc- feldspar closure) by Triassic time. Our data for the Nashoba belt of the Put- cur at about 340 Ma. We note that Hepburn et al. (1987) reported 40Ar/39Ar nam-Nashoba terrane do not preclude this general model. However, as cooling ages of 354 to 325 Ma for hornblendes from Marlboro Formation noted previously, our data indicate an event that caused the formation of amphibolites, indicating cooling through about 500 °C at this time; Wintsch migmatite in parts of the Nashoba Formation ca. 340 Ma. et al. (1992) presented 40Ar/39Ar data for hornblende from the Putnam belt Our data also suggest a similarity, on the basis of ages of basement and

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cover rocks, between the Putnam-Nashoba terrane and the Bronson Hill an- are shown in Figure 17. We interpret the Putnam-Nashoba terrane to repre- ticlinorium (Tucker and Robinson, 1990). In both the Bronson Hill anticli- sent a Late Cambrian to Silurian volcanic arc formed between Laurentia and norium and the Putnam-Nashoba terrane, Ordovician volcanic and sedi- a fragment of apparent Avalonian basement (represented by the mentary rocks were deposited with more-or-less contemporaneous Boston–Rhode Island terrane) during the approach of the Boston–Rhode Is- intrusion of plutons (older Andover in Putnam-Nashoba terrane; Swanzey, land terrane to Laurentia. The similarity of rock types and ages in the Bron- Pauchaug, Monson, and Fourmile gneisses in Bronson Hill anticlinorium). son Hill anticlinorium suggests that it was either a volcanic arc that formed Goldsmith (1991) suggested an Ordovician age for the Putnam-Nashoba closer to the margin of Laurentia, or, as suggested previously herein, a tec- terrane rocks and correlation with the Bronson Hill. The present geograph- tonically separated fragment of the Putnam-Nashoba terrane. In the former ical relationship of the Putnam-Nashoba terrane and Bronson Hill anticli- interpretation, the Merrimack trough was an interarc basin, between the Put- norium suggests that they may be reflective of similar tectonic environments nam-Nashoba terrane and the Bronson Hill anticlinorium, that received sed- on opposite sides of a closing interarc basin, now represented by the Merri- iment during formation and ultimate collision of the two volcanic arcs. In mack trough. If the Massabesic Gneiss Complex is the basement to the sed- the latter interpretation, the Merrimack trough may have been a basin that imentary rocks of the Merrimack trough, the Merrimack trough is an ensialic was caught between the two arc fragments during tectonic separation. The basin. If the Massabesic Gneiss Complex were tectonically underthrust, the ca. 425 Ma high-grade metamorphic event observed in the Putnam-Nashoba basin could have a cryptic oceanic basement. Alternatively, the Putnam- terrane is interpreted to represent the collision of the Boston–Rhode Island Nashoba terrane and the Bronson Hill anticlinorium may be parts of the same terrane with this arc (Putnam-Nashoba terrane) and the Laurentian margin. arc that were separated tectonically during oblique collision. The timing of this docking is further constrained by the ca. 390 Ma ages of Our interpretations of the tectonic history of the Putnam-Nashoba terrane the Straw Hollow diorite of the Putnam-Nashoba terrane and the Salem

370-340 Ma Collision with Gondwana resulting in further metamorphism in Nashoba Terrane rocks

Metamorphic front progressing westward; 400-370 Ma continued intrusion in Nashoba possibly related Gondwana to Clinton-Newbury volcanism

430-400 Ma Continued arc volcanism and plutonism; early metamorphism in Nashoba Arc Gondwana

460-430 Ma Boston Rhode Island Terrane Putnam/Nashoba arc volcanism Bronson Hill Arc volcanism

Merrimack Trough

490-460 Ma Boston Rhode Island Terrane

Laurentia Putnam/Nashoba Arc Bronson Hill Arc

Merrimack Trough

Figure 17. Diagrammatic representation of tectonic events in the formation of the Putnam-Nashoba terrane.

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Gabbro-Diorite of the Boston–Rhode Island terrane. The emplacement of Durham, University of New Hampshire, 108 p. Goldsmith, R., 1991, Stratigraphy of the Nashoba zone, eastern Massachusetts: An enigmatic ter- similar mafic bodies in each terrane at essentially the same time indicates a rane, in Hatch, N. L., Jr., ed., The bedrock geology of Massachusetts: U.S. Geological Sur- similar tectonic environment and supports the contention that they had vey Professional Paper 1366 E-J, p. F1–F22. docked prior to these intrusions. Goldstein, A. G., 1989, Tectonic significance of multiple motions on terrane bounding faults in the northern Appalachians: Geological Society of America Bulletin, v. 101, p. 927–938. During Devonian and Mississippian time, igneous activity continued Goldstein, A. G., 1994, A zone origin for the Alleghanian (Permian) multiple deformation within these accreted terranes as Gondwana approached the Laurentian in eastern Massachusetts: , v. 13, p. 62–72. margin, culminating with the observed ca. 335-360 Ma metamorphic event. Hansen, W. R., 1956, Geology and mineral resources of the Hudson and Maynard quadrangles, Massachusetts: U.S. Geological Survey Bulletin 1038, 104 p. This event was presumably caused by oblique overthrusting and subsequent Hatcher, R. D., Jr., 1989, Tectonic synthesis of the U.S. Appalachians, in Hatcher, R. D., Jr., unroofing of the Putnam-Nashoba terrane during tectonic shuffling of the Thomas, W. A., and Viele, G. W., eds., The Appalachian-Ouachita orogen in the United States: Boulder, Colorado, Geological Society of America, , accreted Nashoba and Boston–Rhode Island terranes outboard of the Lau- v. F-2, p. 511–536. rentian margin during the approach and collision of Gondwana. Hepburn, J. C., 1978, Preliminary reconnaissance bedrock geologic map of the Shrewsbury quad- rangle, Worcester County, Massachusetts: U.S. Geological Survey Open-File Report 78-951, 14p., 1 sheet. ACKNOWLEDGMENTS Hepburn, J. C., Hill, M., and Hon, R., 1987, The Avalonian and Nashoba terranes, eastern MA, U.S.A.: An overview: Maritime Sediments and Atlantic Geology, v. 23, p. 1–12. Hepburn, J. C., Hon, R., Dunning, G. R., Bailey, R. H., and Galli, K., 1993, The Avalon and This work was Acaster’s Master’s thesis at Syracuse University. The fi- Nashoba terranes (eastern margin of the Appalachian orogen in southeastern New England), nancial contribution of the university through teaching assistantships to in Cheney, J. T., and Hepburn, J. C. eds., Field trip guidebook for the northeastern United States: Amherst, Department of Geology and Geography, New England Intercollegiate Ge- Acaster and support of the Isotope Geochemistry Laboratory is gratefully ological Conference, Proceedings, 85th, v. 2, p. X1–X31. acknowledged. We thank Christopher Hepburn, Boston College, for helpful Hepburn, J. C., Dunning, G. R., and Hon, R., 1995, Geochronology and regional tectonic implica- discussion and guidance in the field. The manuscript was greatly improved tions of Silurian deformation in the Nashoba terrane, southeastern New England, U.S.A., in Hibbard, J. P., van Staal, C. R., and Cawood, P. A., eds., Current perspectives in the Appala- by the reviews of A. G. Goldstein and J. N. Aleinikoff. We also thank chian-Caledonian orogen: Geological Association of Canada Special Paper 41, p. 349–366. Hepburn, M. D. Thompson, and R. P. Wintsch for helpful comments. Hill, M. D., Hepburn, J. C., Collins, R. D., and Hon, R., 1984, Igneous rocks of the Nashoba Acaster thanks John Petruccione, Berkshire Environmental, Inc., and Paul Block, eastern Massachusetts, in Hanson, L. S., ed.; Geology of the Coastal Lowlands, Boston MA to Kennebunk ME: Salem, Massachusetts, Salem State College, New England Booms for software contributions, Robert Acaster and Susan Acaster for Intercollegiate Geological Conference, Proceedings, v. 76, p. 61–80. hardware donation, and Kim Hannula for literature search assistance. Hon, R., Hepburn, J. C., Bothner, W. A., Olszewski, W. J., Gaudette, H. E., Dennen, W. H., and Loftenius, C., 1986, Mid-Paleozoic calc-alkaline igneous rocks of the Nashoba Block and Acaster also thanks Marina Hyacinth for the Earth. Merrimack Trough, in Newberg, D. W., ed., Guidebook for field trips in southwestern Maine: Lewiston, Maine, Bates College, New England Intercollegiate Geological Confer- REFERENCES CITED ence, Proceedings, v. 78, p. 37–52. Kelly, W. J., Olszewski, W. J., Jr., and Gaudette, H. E., 1980, The Massabesic gneiss revisited; a Aleinikoff, J. N., Walter, M., and Fanning, C. 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