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Geochemistry of Early Devonian calc-alkaline plutons in the Merrimack belt: implications for Mid-Paleozoic terrane relationships in the New England Appalachians.

A series of northeast-trending plutons extending from northeastern Massachusetts to southeastern Maine intruded the metasedimentary rocks of the Merrimack belt. The Early Devonian Dracut, Sweepstakes, Island Pond, Exeter, and Webhannet plutons are metaluminous and, with the exception of the granitic Webhannet pluton, are dominantly mafic to intermediate in composition. The plutons are calc-alkaline in character and have major, minor, and trace element compositions typical of magmas generated at destructive plate margins. These characteristics include mid- to high-K contents, enrichment of LILE, LREE, Ba, and Sr, and negative Nb and Ta anomalies.

Whole-rock chemical data indicate that the plutons of the Merrimack belt are similar in every measured geochemical parameter to ca. 400 Ma mafic to intermediate rocks of the New Hampshire Plutonic Suite. These similarities suggest that both groups of plutons were emplaced within the same magmatic arc and belong to the same magmatic suite. A magmatic suite common to both the Merrimack belt and Central Maine terrane suggests that the two lithotectonic zones were proximal to each other at ca. 400 Ma. Trace element differences between Merrimack belt and Sharpners Pond rocks suggest that the Putnam-Nashoba terrane represents a separate arc.

Located in the Central Maine terrane of New Hampshire, the Rochester pluton is geochemically distinct from the Siluro-Devonian plutons of the Merrimack belt and New Hampshire Plutonic Suite. The Rochester pluton has alkaline affinities but retains overall calc-alkaline features. High concentrations of incompatible elements (K, Ti, P, Ba, Rb, Zr) in the Rochester pluton are markedly similar to those observed in the 360 Ma Hardwick Tonalite of Massachusetts. The strong geochemical correlation between the Rochester and Hardwick plutons implies derivation from the same magmatic event and a common, Late Devonian origin. We suggest that these plutons may have originated in response to pull-apart rifting related to late-stage Acadian transpression.

Une serie de plutons orientee vers le nord-est s'etendant du nord-est du Massachusetts au sud-est du Maine, fait intrusion dans les roches metasendimentaires de la ceinture de Merrimack. Les plutons du Devonien inferieur Dracut, Sweepstakes, Island Pond, Exeter et Webhannet constituent des intrusions metalumineuses et, mis part le pluton granitique Webhannet, ils ont une composition en predominancc mafique a intermediaire. Les plutons ont une conformation calcoalcaline et leur composition du point de vue des principaux elements presents, de ceux presents en quantite restreinte et des elements traces est representative des magmas produits aux frontieres de plaques destructives. Leurs caracteristiques comportent notamment une teneur moyenne elevee en potassium, un enrichissement en LILE, en elements de terres rares legeres, en BA et en SR. de meme que des anomalies negatives de Nb et Ta.

Les donnees de Ia roche totale revelent que les plutons de la ceinture de Merrimack sont semblables, sous le rapport de chacun des parametres geochimeques mesures, aux magmas mafiques intermediaires d'il y a environ 400 Ma du cortege plutonique du New Hampshire. Ces similarites permettent de supposer que les deux groupes de plutons ont ete inseres l'interieur du meme arc magmatique et qu'ils appartiennent au meme cortege magmatique. L'existence d'un cortege magmatique commun Ia ceinture de Merrimack et au terrane de Central Maine permet de supposer que les deux zones lithotectoniques etaient proximales l'une de l'autre il y a environ 400 Ma. Les diffeences par rapport aux elements traces entre les magmas de la ceinture de Merrimack et ceux de Sharpners Pond laissent supposer que le terrane de Putnam-Nashoba represente un arc distinct.

Le pluton Rochester, situe dans le terrane de Central Maine du New Hampshire, est geochimiquement distinct des plutons siluro-devoniens de la ceinture de Merrimack et du cortege plutonique du New Hampshire. Le pluton Rochester presente des affinites alcalines tout en conservant ses caracteristiques calco-alcalines generales. Les concentrations elevees d'elements incompatibles (K, Ti, P, Ba, Rb, Zr) dans le pluton Rochester sont manifestement semblables a celles observees dans la tonalite d'il y a 360 Ma de Hardwick, au Massachusetts. La correlation geochimique prononcee entre les plutons Rochester et Hardwick suppose qu'ils decoulent du meme phenomene magmatique et qu'ils ont une origine commune remontant au Devonien superieur. Nous pensons que ces plutons tirent probablement leur origine d'une reaction une distension d'ecartement apparentee a une transpression acadienne tardive.

Traduit par la redaction

INTRODUCTION

Geochronological and geochemical studies in the last two decades have led to the recognition of several distinct lithotectonic zones along the eastern margin of the Appalachian Orogen in New England (Lyons et al. 1982; Zen 1983; Zartman 1988; Rankin 1994; Robinson et al. 1998). With this understanding has come controversy regarding the timing of their assembly and thus their relative contributions to the building of the orogen (Wintsch and Sutter 1986; Hatcher 1989; Armstrong et al. 1992; Thompson et al. 1992; Wintsch er al. 1992; Eusden and Lyons 1993; Rast and Skehan 1993; Wintsch et al. 1993). Unravelling the tectonic history of these lithotectonic terranes is complicated by the presence of multiple Paleozoic metamorphic and deformational events (Armstrong et al. 1992), the absence of fossil control, and generally poor exposure.

The Early to Middle Devonian Acadian orogeny is a tectonic event that has traditionally been attributed to the accretion of Avalonia to the North American craton (Osberg 1978; Dallmeyer et al. 1981; Williams and Hatcher 1983). Caught between the Putnam-Nashoba and Avalon composite terranes to the east and the Central Maine Terrane to the west (Fig. 1), the Merrimack belt is an enigmatic block in the orogen. Understanding its tectonic history is essential to understanding the assembly of the eastern terranes to central New England.

The presence of Late Silurian-Early Devonian calcalkaline intrusive rocks in both the Putnam-Nashoba and Central Maine terranes (Fig. 2) suggests that these zones were proximal to convergent plate margins during this time. Numerous hypotheses have been presented regarding the nature of these inferred subduction zones and their relationship to the tectonic assembly of the region. Several researchers (McKerrow and Ziegler 1971; Bradley 1983; Ludman et al. 1993) have suggested the presence of two Acadian subduction zones of opposite polarity on both margins of a closing intraplate basin to explain the distribution of arc-related extrusive and intrusive rocks in New England (Fig. 3a). Three potential models can explain the subsequent amalgamation and tectonic relationship of the Central Maine terrane, Merrimack belt, and Putnam-Nashoba terrane during the Late Silurian-Early Devonian. The first possibility is that the Merrimack belt sedimentary rocks constitute a distinct terrane that was accreted to the North Ame rican continent, with subduction of the lapetus Ocean generating magmas of the New Hampshire Plutonic Suite and the Merrimack belt after accretion. These magmas would have been contemporaneous, belonging to the same magmatic arc (Fig. 3b). A subvarient of this model is that the Merrimack belt was originally part of North America and was transposed to its present position by strike-slip faults after emplacement of the New Hampshire Plutonic Suite and Merrimack belt plutons.

A second possibility is that the Merrimack belt was formed outboard of North America and that the magmas of the Merrimack belt were generated by an eastward-dipping subduction zone beneath a previously assembled Merrimack belt, Putnam-Nashoba terrane, and Avalon terrane (Fig. 3c; Hepburn et al. 1995). In this scenario, the Merrimack belt plutons would be related to additional plutons farther east in the Putnam-Nashoba zone (Sharpners Pond) and Avalon terrane (Cape Ann). The third possibility is that the Merrimack belt and its associated plutons were formed independently of both North America and Putnam-Nashoba and are indeed a distinct terrane (Fig. 3d). The Early Devonian plutons of the intervening Merrimack belt, their tectonic affinity and relation to temporally equivalent intrusions in surrounding terranes, have been the subject of relatively little research.

In order to constrain which of these scenarios is most appropriate for the origin of the Merrimack belt and its plutons, five plutons in the Merrimack belt and one from the Central Maine terrane were selected for geochemical study. In this paper we summarize the whole-rock and mineral chemistry of these plutons. On the basis of these data we test the proposed models in Fig. 3 and address the following questions: (1) What was the tectonic setting in which the Devonian plutons of the Merrimack belt were generated and emplaced? (2) How do these magmas compare to Siluro-Devonian plutons produced at convergent plate margins in adjacent terranes? Did they originate in a common arc with either of the two neighbouring magmatic suites, or do they represent a geochemically distinct and thus tectonically unique event? If the Merrimack belt and either the Putnam-Nashoba or Central Maine terranes were juxtaposed prior to subduction-related magmatism, a close geochemical and temporal relationship between the plutons of the two lithotectonic zones would be expected.

GEOLOGICAL SETTING

Central Maine terrane

In central New Hampshire, metasedimentary rocks of the Central Maine terrane are predominantly metapelitic rocks comprising the Silurian Rangeley, Perry Mountain, Smalls Falls, and Madrid formations. These units are overlain by the Lower Devonian Littleton Formation and all are metamorphosed to amphibolite facies. The majority of the Silurian sediments were derived from source rocks to the west of a deep-water depositional basin that closed in the middle Devonian (Hatch et al. 1983; Moench and Pankiwskyi 1988, Osberg 1978; Dallmeyer et al. 1981; Williams and Hatcher 1983). The Littleton Formation consists of turbidites and minor amounts of volcanic rocks derived from an eastern source (Roy and Mencher 1976). All these formations experienced multiple episodes of deformation, which are presumed to be Acadian (Englund 1976; Nielson 1981; Lyons et al. 1982; Thompson et al. 1992; Eusden and Lyons 1993). Metamorphic grade is universally upper amphibolite facies, probably peaking in the early Devonian (Eusden and Ba rriero 1988).

Extensive granitoid magmas of the syn- to post-tectonic Devonian New Hampshire Plutonic Suite intruded the metasedimentary rocks of the Central Maine terrane. In New Hampshire, most of these plutons are felsic with only the Spaulding tonalite member of the New Hampshire Plutonic Suite containing relatively significant amounts of mafic to intermediate rocks. Mafic to intermediate plutons of the same age are more abundant in northeastern Vermont and western Maine (Moench and Pankiwskyi 1988; Nielson et al. 1989; Ayuso and Arth 1992; Dorais and Paige 2000), having intruded the Connecticut Valley belt, the Bronson Hill Anticlinorium, and the Central Maine terrane.

Merrimack belt

The Merrimack belt is a NNE-trending belt of metasedimentary rocks stretching from southern Connecticut to the Casco Bay area in southeastern Maine (Hussey and Bothner 1993). This sequence of Silurian(?) or older rocks (Aleinikoff et al. 1995) is composed of calcareous metasiltstone and argillaceous metasedimentary rocks of the Eliot, Berwick, and Kittery formations of the Merrimack Group in New Hampshire, the Oakdale Formation in Massuchusetts, and the Hebron gneiss in Connecticut. The intensity of metamorphism varies along and across strike. The metamorphic grade of the Merrimack belt increases to the west from garnet- to sillimanite-grade in Connecticut (Wintsch et al. 1993), from chlorite to andalusite in Massachusetts (Robinson 1981), and from chlorite to kyanite [+ or -] sillimanite in New Hampshire (Lyons et al. 1997; Kunk and Wintsch, unpublished data). In both Connecticut and New Hampshire, [Ar.sup.40]/[Ar.sup.39] thermochronologic data (Wintsch et al. 1992) show that the higher grade, western portio n of the belt was remetamorphosed in the Alleghenian, making the discrimination of overprinting fabrics difficult to interpret. A series of intrusions ranging in composition from gabbro to granite and generally becoming more felsic to the north intruded the tightly folded metasedimentary rocks Merrimack Group.

Putnam-Nashoba terrane

The Putnam-Nashoba terrane is composed of mafic metavolcanic and pelitic metasedimentary rocks (Goldsmith 1991) that are Late Cambrian to Early Ordovician in age (Hepburn et al. 1995). Post-metamorphic Late Silurian plutons of intermediate to granitic composition, including the 430[+ or -]5 Ma Sharpners Pond Diorite (Zartman and Naylor 1984), intruded the metasedimentary rocks of the Putnam-Nashoba terrane (Loftenius 1988).

Ductile faults and lithotectonic boundaries

Ductile faults on the eastern and western margin of the Merrimack belt separate it from packages of metasedimentary rocks with distinct thermal and metamorphic histories. The Norumbega fault system delineates the western boundary of the Merrimack belt and separates it from the Central Maine terrane. The eastern boundary between the Merrimack belt and the Putnam-Nashoba terrane is marked by the Clinton-Newbury fault and its extensions.

Tectonic/lithostratigraphic relationships among lithotectonic zones

Despite the thermal and metamorphic discontinuities observed across the Norumbega fault system (West et al. 1993), there is considerable debate regarding correlation of metasedimentary sequences of the Merrimack belt and Central Maine terrane and their potential stratigraphic and temporal relationship (Lyons et al. 1982; Bothner et al. 1984; Robinson et al. 1998; Bothner and Hussey 1999). The age of the Exeter pluton initially obtained by Gaudette et al. (1984) as 473[+ or -]37 Ma constrained the minimum age of the host metasedimentary rocks as Early Ordovician to Late Proterozoic. This age implied that the metasedimentary rocks of the Merrimack belt were older than the Silurian Central Maine terrane to the west and "exotic" to North America (Bothner et al. 1984; Olszewski and Gaudette 1988). Subsequent, more precise U-Pb dating of the Exeter pluton (406[+ or -]3 Ma, Bothner et al. 1993) has demonstrated that the minimum depositional age of the Merrimack belt metasedimentary rocks is Late Silurian and preclud es the designation of the Merrimack belt as a separate terrane on the basis of age alone.

Some workers have suggested that the Berwick Formation of the Merrimack belt is very similar to what was called the Vassalboro formation (now the Hutchins Corner Formation, Bothner and Hussey 1999) of the Central Maine terrane in Maine and the Paxton and Oakdale formations of Massachusetts (Billings 1956; Hussey 1962; Osberg 1980; Hussey et al. 1986). If these formations are indeed the same, then the suggestion that the Merrimack belt is a separate lithotectonic zone from the Central Maine terrane is negated (Robinson et al. 1998). Additionally, the Merrimack belt metasedimentary rocks contain detritial zircon as young as 420 Ma (Aleinikoff et al. 1995), indicating a Silurian or Early Devonian age of deposition, and thereby allowing correlation of the Central Maine terrane and Merrimack belt sequences (Bothner and Hussey 1999). In contrast, the Late Proterozoic to Early Paleozoic metasedimentary rocks of the PutnamNashoba terrane have a geologic history distinct from those of the Merrimack belt (Hepburn et al . 1995).

Within the Merrimack belt, a series of Early Devonian, mafic to intermediate intrusions extends from north-central Massachusetts to southeast Maine (Fig. 2). Previous studies (Billings 1956) grouped the mafic plutons of southeast New Hampshire as a distinct magmatic suite called the Hillsborough Plutonic Series. Based on similar ages, the Hillsborough Plutonic Series has subsequently been correlated with the New Hampshire Plutonic Suite (Lyons et al. 1997) of the Central Maine terrane (a U-Pb age of 406[+ or -]3 Ma for the Exeter Pluton; Bothner et al. 1993). Although the mafic and intermediate plutons of adjacent lithotectonic terranes have been the focus of several regional geochemical surveys (Zartman and Naylor 1984; Ayuso and Arth 1992; Hon et al. 1993; Hepburn et al. 1995; Dorais and Paige 2000), limited geochemical data exist for the plutons of the Merrimack belt. Acquisition, of petrochemical data for these rocks enables a more detailed interpretation of the tectonic setting in which they were emplace d which is important in reconstructing the geologic history of the region.

SAMPLING STRATEGY AND ANALYTICAL METHODS

The plutons of the Merrimack belt generally become more felsic from south to north. Each of the plutons shows varying degrees of compositional zoning and/or bimodal composition. Rocks representing the range of compositions for each pluton were selected in order to characterize this suite of rocks. Special emphasis was placed on mafic samples for several reasons. First, it is expected that the mafic rocks within the Merrimack belt have experienced relatively little crustal contamination and reflect most closely the geochemical characteristics of the original magmas and source regions. Additionally, comparison of the most primitive samples available yield the most information about the possible genetic relations of the magmas among the various lithotectonic zones and facilitate evaluation of the competing tectonic models presented in Fig. 3.

With this in mind, we selected samples displaying textures and whole-rock chemistry most representative of primitive liquids to compare with temporally and compositionally equivalent rocks in adjacent lithotectonic zones. Primitive, relatively uncontaminated mafic rocks are scarce within the New Hampshire Plutonic Suite. The best candidates for comparison are a series of mafic plutons located in northern Vermont, New Hampshire, and western Maine (Fig. 2b) spanning the Connecticut Valley, Bronson Hill, and Central Maine terranes. With the exception of the peraluminous Long Mountain pluton of northern New Hampshire, these plutons are metaluminous and have low [[delta].sup.18]O values (Dorais and Paige 2000.) indicative of relatively primitive melts. Likewise, the gabbro and diorite of the Sharpners Pond pluton represent similar intrusive rocks in the Putnam-Nashoba terrane. The 360[+ or -]1 Ma Hardwick pluton (Tucker and Robinson 1995), which represents a younger magmatic event within the Central Maine terrane, is used for geochemical comparison.

Major, minor, and selected trace element analyses were conducted using XRF techniques at the Nova Scotia Regional Geochemical Centre, St. Mary's University, Nova Scotia. Additional trace and rare earth element data were obtained by INAA performed at the Phoenix Memorial Laboratory at the University of Michigan. Mineral composition data were collected at Indiana University using a Cameca SX50 electron microprobe with 15 kV accelerating voltage and 20 nA beam current (10 nA, 10 [micro]m beam diameter for feldspar analyses).

PETROLOGY AND PETROGRAPHY

The sampled plutons of the Merrimack belt are composed dominantly of hornblende-biotite diorites and granodiorites. Mineral assemblages and textures are igneous, but display variable amounts of sub-solidus alteration. A more detailed characterization of the six individual plutons is included below.

Dracut Pluton

The Dracut Pluton is exposed over an area of 70 [km.sup.2] in north-central Massachusetts (Zen et al. 1983) in the southern portion of the Merrimack belt and intrudes the metasedimentary rocks of the Berwick Formation. Previous mapping and lithologic descriptions are given in Dennen (1943) and Hon et al., (1986). Rock compositions range from gabbro to tonalite, with fine- to medium-grained gabbro and diorite dominant. The pluton is heterogeneous on the outcrop scale, exhibiting considerable variability in grain size, phenocryst assemblage, and colour. The rocks are non-foliated, and are crosscut by volumetrically minor pegmatitic veins. Despite the presence of magmatic sulphide and local alteration minerals, many of the mafic rocks retain igneous textures and assemblages with minimal deuteric overprint.

Primary igneous minerals include euhedral laths of plagioclase, biotite, pyroxene, and amphibole. Orthopyroxene is more abundant than clinopyroxene and occurs as subhedral phenocrysts, some largely replaced by amphibole. Clinopyroxene (diopside) occurs as scarce, ragged cores within amphibole and rarely coexists with orthopyroxene. Dark brown subhedral phenocrysts of Ti-rich amphibole are ophitic to subophitic and commonly have biotite replacement rims.

Sweepstakes Pluton

The Sweepstakes Pluton, located in southeastern New Hampshire, intrudes the Eliot Formation of the Merrimack Group (Sundeen 1971) and is the smallest body sampled (~3.5 [km.sup.2]). The most abundant rock type is massive to moderately flow-banded diorite and quartz diorite. Several small bodies of coarse-grained norite cumulates occur in the centre of the body. With the exception of the northern portions of the Exeter pluton, the Sweepstakes shows the largest degree of sub-solidus alteration.

Minerals present include biotite, plagioclase, and light brown, secondary amphibole. Ilmenite is abundant and commonly rimmed by titanite overgrowths. Anhedral quartz is a minor, interstitial phase. Sericite commonly replaces plagioclase along cleavage planes. Other alteration products include epidote and minor chlorite.

Island Pond Pluton

Exposed approximately 6 kilometres to the north of the Sweepstakes Pluton, the Island Pond Pluton (~8 [km.sup.2]) crops out as a northeast-trending elliptical body (Sundeen 1971). Contact relationships are difficult to discern due to exceedingly poor exposure. The pluton exhibits a bimodal range of rock compositions with two volumetrically subordinate bodies of coarse-grained, massive diorite in the core of the intrusion surrounded by massive to weakly foliated granitic rocks.

The diorite contains normally zoned suhedral to anhedral plagioclase phenocrysts typically ranging from [An.sub.30] to [An.sub.25]. Pale to dark green amphibole is the dominant mafic silicate. Fe-Ti oxides are common along cleavage planes in hornblende. Lesser amounts of pale green to light yellow biotite are also present.

Exeter Pluton

The largest body (~130 [km.sup.2]) sampled is the northeast-trending Exeter Pluton, located in southeastern New Hampshire. Dominantly dioritic in composition, the pluton is zoned from gabbro in the southwest to granodiorite at its northern end. The rocks are medium- to coarse-grained and massive in texture. Mafic rocks in the southern portion of the pluton generally retain much of their original igneous character whereas deuteric alteration is most pervasive in the northern portion of the pluton.

The mafic rocks consist of hornblende, orthopyroxene [+ or -] clinopyroxene, biotite, plagioclase, and minor quartz. In some portions of the pluton, light to dark brown amphibole phenocrysts partially enclose normally zoned plagioclase laths in a subophitic texture. Other regions contain rocks with pale green phenocrysts of hornblende that are commonly intergrown with biotite. Ortho- and clinopyroxene occur as anhedral remnants partially replaced by amphibole. Alteration phases include actinolite and chlorite which are more abundant in the northern, more felsic portion of the pluton. Zircon and Fe-Ti oxides are common accessory minerals.

Webhannet Pluton

The northernmost and overall most felsic intrusion sampled from the Merrimack belt is the Webhannet Pluton which crops out over an are of ~150 [km.sup.2] 3 kilometres east of the New Hampshire-Maine border in southeastern Maine (Osberg et al. 1985). The coarse-grained pluton is concentrically zoned from granodiorite at its margin to K-feldspar megacrystic granite in the centre. Gaudette et al., (1982) reported a U-Pb zircon age of 403[+ or -]14 Ma for the Webhannet pluton.

Predominant minerals are potassium feldspar, quartz, and plagioclase. Perthitic microcline forms large (up to 12mm) phenocrysts. Biotite ranges from subhedral to interstitial. Subhedral to euhedral epidote crystals commonly have allanite cores and appear to be a primary magmatic phase. Apatite, zircon, and Fe-Ti oxides are common accessory minerals.

Rochester Pluton

The Rochester Pluton, located 15 kilometres northwest of the Exeter Pluton across the Norumbega fault system, is the only pluton of this study located in the Central Maine terrane. Rocks are medium- to coarse-grained diorite and exhibit moderate sub-solidus modification. With the exception of a poorly exposed fine-grained phase, the Rochester Pluton is similar in hand specimen to the Exeter Pluton.

Principle minerals are plagioclase and biotite. Plagioclase phenocrysts are subhedral to euhedral and show oscillatory zoning. Sericite partially replaces plagioclase in altered samples. Biotite is subhedral and contains abundant Fe-Ti oxide inclusions. Anhedral quartz occurs as a groundmass phase. Ilmenite is ubiquitous and commonly rimmed by secondary titanite. Common accessory minerals include zircon and abundant apatite.

MINERAL CHEMISTRY

Feldspar

Representative feldspar analyses are listed in Table 1. Plagioclase compositions for the mafic to intermediate rocks range from [An.sub.20] to [An.sub.73]. Compositions are more anorthitic in the southern plutons and become more albitic in the northern p]utons (Fig. 4). Phenocrysts are normally zoned with labradorite-andesine cores typically rimmed by oligoclase. Potassium feldspar ([Or.sub.91-95]) is found as a primary phenocryst only in the Webhannet pluton.

Pyroxene

Pyroxene analyses are given in Table 2. Pyroxene is relatively rare and occurs only in mafic samples of the Dracut and Exeter plutons. Orthopyroxene is dominant over clinopyroxene. Pyroxene grains in both plutons are slightly zoned with Mg- and Ca-rich cores and Fe-enriched rims. Pyroxene compositions are plotted in Fig. 5 and are typical of those generated in subduction-related magmas (Gill 1981).

Orthopyroxene compositions range from[Wo.sub.3][En.sub.72][Fs.sub.25] to [Wo.sub.2][En.sub.51][Fs.sub.47]. Clinopyroxene compositions cluster at [Wo.sub.46][En.sub.43][Fs.sub.11]. Magmatic temperatures of 895-910[degrees]C were obtained from coexisting pyroxenes using two separate geothermometers (Wood and Banno 1973; Wells 1978).

Amphibole

Amphibole analyses are listed in Table 3. Amphibole compositions are plotted in Fig. 6 and form two compositional trends. Primary Ti-rich amphiboles range from magnesio-hastingsite to magnesian hastingsitic hornblende (Leake 1978) and define a magmatic trend in which compositions become poorer in Al (total) and Ti at progressively higher values of (Fe+Mn)/(Fe+Mn+Mg). A second trend defining a positive slope toward amphiboles poorer in Ti and Al at lower Fe/(Fe+Mg) values and projecting away from the magmatic trend is formed by actinolitic hornblende, which suggests that it represents a secondary (sub-solidus) alteration trend. Likewise a positive correlation between Al (total) and Ti is demonstrated. The most Al- and Ti-enriched amphiboles occur in the Dracut pluton with compositions becoming poorer in both components to the north. Amphibole analyses for the Island Pond pluton show compositions richer in Fe and Al relative to magmatic amphibole from other plutons in the Merrimack belt.

Biotite

Biotite analyses are given in Table 4 plotted on Fig. 7. Biotite in the southern, more mafic plutons is richer in Ti and poorer in Al and

Fe compared to the northern plutons. Fe and Al content increases in the plutons in the northern portion of the belt. Biotite analyses from the Rochester pluton reveal higher Al and Ti contents than those in the Merrimack belt plutons.

BULK ROCK CHEMISTRY

Although the freshest available rocks were sampled, some samples show deuteric alteration. The effects of postmagmatic alteration in samples chosen for bulk rock analyses were assessed using petrographic and textural features where possible. The nature and extent of these effects varies within each pluton. Because some samples have been affected by post-emplacement alteration, mobile elements (K, Na, Sr, Rb) may be less accurate indicators of original igneous chemistry and have been used with caution. Accordingly, immobile elements such as Ti, Zr, Nb, Y, Ta, and rare earth elements were primarily used for the purpose of comparison. However, potential mobile element concentrations were considered when results are consistent on a suite scale.

Major Elements

The [SiO.sub.2] contents of the six sampled plutons range from 44 to 76% and generally increases from low values in plutons in the south to higher values in the northern plutons (Table 5). MgO [Fe.sub.2][O.sup.T.sub.3], CaO, [TiO.sub.2], and [P.sub.2][O.sub.5] decrease systematically with increasing [SiO.sub.2] content (Fig. 8a, b, c, g, h). Mg#'s [MgO/(MgO+[FeO.sup.T])] range from 29 to 76 and average 58. [Na.sub.2]O and [K.sub.2]O increase broadly with increasing [SiO.sub.2], and [Al.sub.2][O.sub.3] shows a slight decrease with increasing [SiO.sub.2].

The plutons exhibit calc-alkaline differentiation trends on an AFM plot (not shown). Most samples fall within the medto high-K calc-alkaline fields (Fig. 8f), except for the Rochester pluton, which plots completely within the shoshonitic field. The rocks are hypersthene- and diopsidenormative with or without normative quartz and none, including the Rochester pluton, contain normative nepheline. With the exception of the Webhannet pluton, the vast majority of the plutons have low aluminum saturation indexes [mol [Al.sub.3][O.sub.3]/([CaO+[Na.sub.2]O+[K.sub.2]O)] or A/CNK values (avg. = 0.85) and are metaluminous (Table 5).

Major element chemistry of the Rochester pluton contrasts with that of the Merrimack belt intrusions, especially in [TiO.sub.2] and [P.sub.2][O.sub.5] (Fig. 8g, h). Significant enrichment in these elements as well as [K.sub.2]O (Fig. 8f) denotes its alkaline affinity and unique chemistry relative to plutons of the Merrimack belt.

Trace elements

Trace elements are listed in Tables 5 and 6. Tectonic discrimination plots of Hf/3-Th-Ta (Fig. 9; after Wood et al. 1979) and Th/Yb vs. Ta/Yb (Fig. 10; after Pearce 1983) demonstrate that the magmas are subduction-related. Data for the Sharpners Pond pluton and New Hampshire Plutonic Suite are also plotted on these diagrams and indicate similar tectonic environments. Chondrite-normalized REE diagrams (after Ebihara and Anders 1982) demonstrate LREE-enriched patterns (Fig. 11). The Rochester pluton is distinctively enriched in LREE and MREE and this feature distinguishes it from plutons of the Merrimack belt. Extended element or spider diagrams exhibit patterns typical of those observed in subduction-related tectonic environments (Fig. 12; Pearce 1983). Relative to normal mid-ocean ridge basalts (NMORB), samples exhibit depletions in Nb, Hf, and Ta, enrichment in incompatible large ion lithophile elements (LILE) such as K, Ba, Th, U, Cs, and enrichment in LREE relative to heavy rare earth elements (HREE; La/Yb N = 23.7).

With the exception of the Rochester pluton, Zr and Ba versus [SiO.sub.2] generally form coherent trends (Fig. 13a, b) showing systematic increases with increasing [SiO.sub.2] and thereafter declining at [SiO.sub.2] contents > 70%. Samples from the Rochester pluton are anomalously enriched in Ba and Zr and plot within the same field as the Hardwick pluton of central Massachusetts (Shearer and Robinson 1988).

COMPARATIVE GEOCHEMISTRY

The major and minor element, whole-rock chemistry of the early Devonian calc-alkaline magmas of the Merrimack belt, New Hampshire Plutonic Suite, and Putnam-Nashoba terrane are broadly similar. Harker diagrams for most major elements and many trace elements display considerable overlap and, thus, do not distinguish the three magmatic suites from one another (not shown). Incompatible element diagrams however, show a strong correlation between the Rochester pluton and the Hardwick pluton of the Central Maine terrane (Fig. 13). [TiO.sub.2] and [P.sub.2][O.sub.5] compositions of the Rochester and Hardwick plutons are very similar and clearly discriminate these plutons from the others of this study (Fig. l3c, d).

Samples from the Merrimack belt and New Hampshire Plutonic Suite span the same [SiO.sub.2] range as the Sharpners Pond pluton but are enriched in Sr, Nb, Ba, Cs, Th, U, and LREE. In Fig. 14, the data define three distinct fields. Samples from the Merrimack belt and New Hampshire Plutonic Suite exhibit distinctly higher Sr content at a given Rb concentration than those of the Sharpners Pond. The Rochester and Hardwick plutons of the Central Maine terrane define a third grouping with the highest Sr content.

Plots of various trace element abundances and ratios for low silica samples (<60% [SiO.sub.2]) in Figs. 15 and 16 also group the Merrimack belt and New Hampshire Plutonic Suite magmas together in a distinct field separate from those of the Sharpners Pond pluton. Plutons of the Merrimack belt and New Hampshire Plutonic Suite exhibit high concentrations of Sr, Ba, and Ta relative to the Sharpners Pond pluton (Fig. 15). The plot of Th vs. Zr/Nb (Fig. 16a) distinguishes the plutons of the Merrimack belt and New Hampshire Plutonic Suite from the Sharpners Pond pluton and defines a third field for the Rochester and Hardwick Plutons. The separations observed in Fig. 16 are particularly noteworthy as Ce/Yb and Zr/Nb ratios are parameters least affected by postmagmatic alteration (Gill 1981). Plots of Ce, La/Yb, and Zr vs. MgO in Fig. 17 illustrate the similarities in trace element compositions in the most primitive samples of the Merrimack belt and New Hampshire Plutonic Suite and show that these trends differ from t hose of the Sharpners Pond magmas. Plutons of the Merrimack belt and New Hampshire Plutonic Suite have similar and elevated concentrations of Ce and La/Yb at the comparable MgO values relative to the Sharpners Pond pluton, whereas at high MgO concentrations, the Sharpners Pond samples are richer in Zr.

Chondrite-normalized REE patterns for each of the three magmatic suites further emphasize the similarities of the Merrimack belt and New Hampshire Plutonic Suite intrusions and distinguish them from the Sharpners Pond pluton (Fig. 11). Merrimack belt and New Hampshire Plutonic Suite samples show complete overlap and are characterized by higher LREE contents, steeper LREE/HREE slopes, and negligible to slightly negative Eu anomalies compared to the relatively flat LREE/HREE slopes of the Sharpners Pond pluton. N-MORB normalized extended element diagrams (Sun and McDonough 1989) for the three suites display similar shaped patterns indicative of a subduction-related arc environment (Fig. 12). The plutons of the Merrimack belt and New Hampshire Plutonic Suite are remarkably similar and enriched in LILE, LREE, and Ta relative to the Sharpners Pond Diorite. The Rochester Pluton shows significant enrichment in all of the aforementioned elements and is unlike all other plutons considered.

DISCUSSION

The recognition of suites, supersuites, and superunits of plutons within large magmatic and tectonic provinces on the basis of geochemical features (Larsen 1948; Cobbing et al. 1977; White et al. 1977; Bateman 1983) was a significant advancement in assessing igneous processes. A suite, supersuite, or superunit is a temporally and spatially related set of plutons that are fundamentally related and considered to be major assemblages of consanguineous rocks (Pitcher 1985; Atherton and Sanderson 1985). Wyborn et al. (1987), for example, defined the Boggy Plain Supersuite based upon its distinctly higher concentration of K, Ba, Sr, Rb, La, Ce, U, and Th relative to other I-type suites in the Lachlan Fold Belt. The application of the suite concept has also been beneficial in tectonic investigations where it has been used in the identification and correlation of discrete tectonic terranes (Chappell et al. 1988; Heatherington 1996; Kerr 1997; Barr and Kerr 1997). Similarly, a geochemical evaluation of magmas of the M errimack belt, New Hampshire Plutonic Suite, and Sharpners Pond pluton enables an assessment of the suite concept and an evaluation of the tectonic relationship of the lithotectonic zones that host these plutons.

Major, trace, and rare earth element data for the intrusions of the Merrimack belt and New Hampshire Plutonic Suite are remarkably similar. Based on the criteria of suite/supersuite/super-unit determination (e.g., Cobbing et a!. 1977), these geochemical similarities suggest that plutons of the Merrimack belt and New Hampshire Plutonic Suite belong to the same suite. This interpretation is consistent with that of Lyons et a!. (1997) who, rather than following Billings (1956) in classifying the Merrimack belt magmas as the distinct Hillsborough Plutonic Series, grouped the Devonian plutons of the Merrimack belt with the New Hampshire Plutonic Suite on the basis of age. The implication of the magmas belonging to a common suite is that the New Hampshire Plutonic Suite and Merrimack belt plutons formed within the same magmatic arc. If the New Hampshire Plutonic Suite and Merrimack belt plutons do indeed represent a single suite, this requires that the rocks these magmas intruded (i.e. the metasedimentary rocks of the Merrimack belt and Central Maine terrane) were adjacent to each other or continuous at the time of magmatism (~400 Ma, Fig. 4b), an interpretation also presented by Rankin (1994).

Although these data alone provide no information concerning the genetic relations of the metasedimentary rocks of the Central Maine terrane and Merrimack belt, other workers have considered the Merrimack belt and Central Maine terrane as equivalent on a lithostratigraphic basis (Billings 1956; Osberg 1980; Robinson et al. 1998; Bothner and Hussey 1999). Our data suggest that regardless of whether the Merrimack belt and the Central Maine terrane are indeed the same lithotectonic unit that was severed by the Norumbega fault system, or if they are different lithotectonic zones now juxtaposed across the faults, the two lithotectonic zones must have been contiguous during emplacement of the New Hampshire Plutonic Suite.

The Sharpners Pond pluton is similar in overall composition and tectonic setting to those of the New Hampshire Plutonic Suite and Merrimack belt, but has distinct trace element characteristics. Of particular significance is the contrast in trace element abundances observed in the most primitive samples from each suite. If magmas have different compositions in the earliest stages of their evolution (highest MgO contents, Fig. 17), it is likely they had unique parental magmas and are genetically distinct. Subtle differences in selected trace element abundances (Fig. 17), particularly in the most primitive samples, suggest that the Sharpners Pond pluton is genetically unrelated to the Merrimack belt and New Hampshire Plutonic Suite plutons.

Subduction of oceanic lithosphere was a prominent tectonic mechanism in the formation of the Appalachian orogen. The quantity, location, and polarity of subduction zones in the Late Silurian and Early Devonian however, remain highly contentious issues. The presence of two northeast-trending volcanic belts on either side of the Central Maine terrane and the Merrimack belt suggests the presence of one or more subduction zones, but the interpretation of these rocks remains equivocal. Bradley (1983) and Ludman et al. (1993) interpreted the Piscataquis volcanic belt of northern New England and southern Canada as arc volcanic rocks generated by northwest-dipping subduction beneath North America. Likewise, the Silurian Frontenac rift of Moench (1993) has been interpreted as a back-arc rifting related to northwestward subduction. Several workers (McKerrow and Ziegler 1971; Bradley 1983; Ludman et al. 1993) have suggested the Late Silurian-Early Devonian Coastal volcanic belt of eastern New England originated from eas tward dipping subduction beneath the western margin of impinging Avalon. Other researchers (McKerrow and Ziegler 1971; Bradley 1983; Ludman et al. 1993; Acaster and Bickford 1999) have speculated that the Merrimack belt ([+ or -] the Central Maine terrane) was a closing ensialic basin in the Late Silurian-Early Devonian with both the eastern and western margins subducting beneath Avalon and North America, respectively. Modern analogs of this tectonic setting however, such as the Mollocca Sea collision zone, are scarce (Hamilton 1978; Moore et at. 1981).

Across-arc variations in the chemistry of magmas formed in subduction zones are well established (i.e., Gill 1981; Tatsumi and Eggins 1995) and can be useful for deciphering subduction polarity in ancient arcs where other lines of evidence are absent. Systematic variations include an increase in the concentration of incompatible elements such as K, the LILE, and LREE with increasing distance above the subduction zone. On the basis of such relations, Hepburn et al. (1995) suggested that the west to east sequence of the Sharpners Pond pluton of Putnam-Nashoba and the Cape Anne Complex of the Avalon terrane are an expression of eastward subduction beneath these lithotectonic zones (Fig. 4c). They inferred that the Dracut and Exeter plutons in the Merrimack belt are the western most expressions of that subduction zone. However, the increase in LILE and LREE concentrations from Sharpners Pond pluton in the east to the Merrimack belt magmas to the west argues against this scenario (Figs. 11 and 12). If the Sharpner s Pond pluton and Cape Anne Complex do in fact represent an eastward dipping subduction zone, the enrichment in incompatible elements in the plutons of the Merrimack belt relative to the Sharpners Pond pluton indicate that the these two magmatic suites could not have formed within the same arc. The geochemical similarities between plutons of the Merrimack belt and the New Hampshire Plutonic Suite leads us to suggest that the Merrimack belt magmas resulted from the same westward-dipping subduction zone that produced the mafic to intermediate composition magmas of the New Hampshire Plutonic Suite (Bradley 1983; Ludman et at. 1993). Subsequent to the emplacement of the Merrimack belt plutons, the Merrimack belt may have been laterally transposed by Acadian and/or Alleganian dextral movement along the Norumbega fault system (Ludman et al: 1999; West 1999).

The Rochester Pluton is chemically distinct from all other sampled plutons and remarkably similar to the 360[+ or -]1 Ma Hardwick Tonalite located in the Central Maine terrane of north-central Massachusetts. This geochemical correlation suggests the possibility of a ~360 Ma age for the Rochester pluton. Furthermore, the alkaline affinity of the Rochester and Hardwick plutons may help constrain the polarity of subduction that gave rise to the magmas of the New Hampshire Plutonic Suite and Merrimack belt. The occurrence of alkaline magmas in back-arc, typically extensional, tectonic settings is well documented. Hole (1988) postulated that postsubduction alkaline magmas might retain a subduction zone geochemical signature up to 40 Ma subsequent to subduction. If the Rochester and Hardwick plutons represent back-arc magmatism relative to the mafic New Hampshire Plutonic Suite in Vermont, Maine, and New Hampshire, it would imply that subduction was initiated west of the Central Maine terrane and dipped to the southeast.

Two alternative interpretations of the Rochester and Hardwick plutons are possible: (1) these plutons may be the expression of back-arc maginatism associated with subduction unrelated to the New Hampshire Plutonic Suite; or (2) the Rochester and Hardwick plutons could be the result of extensional tectonics induced by the oblique collision of the Avalon with the eastern margin of North America. Given the lack of evidence for the former hypotheses and the presence of intense ductile deformation indicative of considerable tangential forces in this portion of the Central Maine terrane (Robinson et al. 1998), we favour a westward subduction model generating the mafic magmas of the New Hampshire Plutonic Suite and Merrimack belt with transpressive extension to account for the Rochester/Hardwick plutons. The geochemical signature of subduction such as bulk-rock negative Nb and Ta anomalies in the Rochester and Hardwick plutons were likely inherited from the underlying mantle which retained the metasomatic signature of Early Devonian subduction. The Keweenawan Rift represents an ancient example of such collision-induced rifting. Gordon and Hempton (1986) proposed that the Keweenawan Rift and associated magmatism developed in pull apart basins in response to continent-continent convergence of the Grenville Orogeny.

CONCLUSIONS

The Early Devonian plutons of the Merrimack belt represent a suite of calc-alkaline, subduction-related magmas. Similarities in age and composition of the Merrimack belt magmas and the New Hampshire Plutonic Suite suggest that both were emplaced within a common arc and define a single magmatic suite. This correlation also implies a North American affinity for the Merrimack belt, as it would be required to have been adjacent to the Central Maine terrane at the time of emplacement (Fig. 3b). Given that the Central Maine terrane and the Merrimack belt are currently separated by a major ductile shear zone, the notion that these two lithotectonic zones were along strike at the time of magmatic emplacement and subsequently juxtaposed to their current configuration is likely. The Sharpners Pond pluton has distinct trace element characteristics and defines a second subduction-related suite that was generated in a separate arc.

Marked geochemical similarities between the Rochester Pluton and Hardwick Tonalite suggest both a genetic relationship and a possible age of ~360 Ma for the former intrusion. The Rochester pluton has alkaline affinities while retaining calc-alkaline signatures. Although the Hardwick and perhaps Rochester plutons are 40 Ma younger than the dated mafic magmas of the New Hampshire Plutonic Suite and Merrimack belt, they may be expressions of back-arc rifting. If so, this indicates that the subducting lithosphere dipped eastward beneath the presently juxtaposed Central Maine terrane and Merrimack belt. Alternatively, our preferred interpretation is that these plutons formed in a pull-apart rifting environment resulting from oblique accretion of the Avalon terrane to the North American margin. The 360 Ma age of the Hardwick (and presumably the Rochester) pluton and high-grade metamorphism in north-central Massachusetts and southeast New Hampshire is enigmatic as it post-dates the Early Devonian onset of the Acadia n Orogeny by 40-50 million years. Thus, the setting of this protracted magmatic activity and its relation to previous Acadian magmatism requires additional investigation.

[FIGURE 6 OMITTED]

[FIGURE 7 OMITTED]

[FIGURE 8 OMITTED]

[FIGURE 11 OMITTED]

[FIGURE 12 OMITTED]

[FIGURE 13 OMITTED]

[FIGURE 14 OMITTED]

[FIGURE 15 OMITTED]

[FIGURE 16 OMITTED]

[FIGURE 17 OMITTED]
Table 1

Representative feldspar analyses, Merrimack Trough and Rochester plutons

 Plagioclase
Sample D-1-2a D-1-4a D-4-1a D-4-2a E-3-4ar

[SiO.sub.2] 55.77 57.34 53.59 50.70 53.93
[Al.sub.2][O.sub.3] 28.75 26.73 30.7 31.79 29.93
FeO 0.15 0.20 0.07 0.00 0.22
CaO 10.82 9.07 12.14 14.73 11.90
[Na.sub.2]O 5.29 6.36 4.57 3.30 4.75
[K.sub.2]O 0.31 0.14 0.03 0.00 0.05

Total 101.09 99.84 101.10 100.52 100.78

 % An 52 43 60 71 58

 Plagioclase
Sample E-3-4ac E-4-1a E-7-4a IP-5-1a IP-5-2a

[SiO.sub.2] 50.62 54.54 55.87 60.20 59.93
[Al.sub.2][O.sub.3] 32.2 28.9 27.7 25.12 25.24
FeO 0.05 0.02 0.15 0.09 0.08
CaO 15.2 10.88 10.13 6.00 6.43
[Na.sub.2]O 3.05 5.2 5.62 8.08 7.76
[K.sub.2]O 0.04 0.23 0.15 0.11 0.22

Total 101.16 99.77 99.62 99.6 99.66

 % An 73 53 50 29 31

 Plagioclase
Sample IP-5-3a IP-5-4a S-5-2a S-5-3ac

[SiO.sub.2] 60.61 61.80 56.87 56.00
[Al.sub.2][O.sub.3] 25.02 23.45 27.12 27.57
FEO 0.15 0.06 8.55 9.27
CaO 5.85 4.22 0.15 0.03
[Na.sub.2]O 8.14 8.93 6.72 6.28
[K.sub.2]O 0.11 0.08 0.06 0.16

Total 99.88 98.54 99.47 99.31

 % An 28 21 41 45
 Plagioclase
Sample S-5-3ar S-5-5a W-1-8a W-1-8a W-1-11a

[SiO.sub.2] 57.39 56.75 63.35 66.44 61.82
[Al.sub.2][O.sub.3] 27.16 27.56 23.39 20.56 24.15
FeO 0.11 0.01 0.04 0.05 0.07
CaO 8.06 8.68 3.62 0.75 4.61
[Na.sub.2]O 6.87 6.38 9.45 11.04 8.68
[K.sub.2]O 0.22 0.08 0.14 0.12 0.17

Total 99.81 99.46 99.99 98.96 99.5

 % An 38 43 17 4 22

 % Or

 Plagioclase
Sample W-1-10a R-4-1a R-4-5a R-5-4a R-5-4a

[SiO.sub.2] 63.19 59.88 56.02 55.90 58.19
[Al.sub.2][O.sub.3] 23.20 24.60 27.60 27.60 26.21
FeO 0.05 0.08 0.07 0.00 0.16
CaO 3.75 6.42 9.54 9.52 7.44
[Na.sub.2]O 9.24 7.76 6.00 5.96 7.02
[K.sub.2]O 0.14 0.17 0.14 0.09 0.07

Total 99.57 98.91 99.37 99.07 99.09

 % An 18 31 46 47 37

 % Or

 K-feldspar
Sample W-1-1a W-1-2a W-1-3a W-1-5a

[SiO.sub.2] 63.78 64.45 64.23 64.11
[Al.sub.2][O.sub.3] 19.01 18.72 18.77 18.94
FeO 0.02 0.06 0.04 0.02
CaO 0.00 0.00 0.00 0.00
[Na.sub.2]O 0.75 0.93 0.67 1.09
[K.sub.2]O 15.42 15.30 15.72 14.99

Total 98.98 99.46 99.43 99.15

 % An

 % Or 93 92 94 90
Table 2

Representative pyroxene analyses, Merrimack Trough plutons

 D-1-1a D-1-2a D-1-3a D-1-4a D-1-6a D-1-8a

[SiO.sub.2] 52.48 52.36 53.19 53.06 52.66 51.58
[TiO.sub.2] 0.26 0.19 0.32 0.16 0.23 0.60
[Al.sub.2][O.sub.3] 1.18 1.50 1.49 1.03 1.07 2.31
FeO 20.59 20.27 19.91 20.26 20.50 8.17
MnO 0.76 0.60 0.53 0.59 0.68 0.29
MgO 23.49 23.36 23.95 24.29 23.77 15.12
CaO 0.79 1.04 1.06 0.69 0.84 20.96
[Na.sub.2]O 0.14 0.09 0.11 0.10 0.10 0.48

Total 99.69 99.41 100.56 100.18 99.85 99.51

 D-1-8a D-1-9a D-4-1a D-4-2a D-4-3a E-3-2a

[SiO.sub.2] 52.42 52.93 53.52 52.62 52.65 51.17
[TiO.sub.2] 0.27 0.29 0.27 0.33 0.33 0.19
[Al.sub.2][O.sub.3] 1.39 1.34 0.97 1.43 2.03 0.76
FeO 8.24 8.47 18.61 7.83 17.37 28.17
MnO 0.29 0.30 0.48 0.33 0.44 0.80
MgO 14.76 14.89 24.81 15.33 25.38 17.66
CaO 22.09 21.48 1.16 21.80 1.43 1.38
[Na.sub.2]O 0.48 0.47 0.10 0.32 0.11 0.10

Total 99.94 100.17 99.92 99.99 99.74 100.23

 E-3-4a E-3-5a E-4-1a E-4-1a

[SiO.sub.2] 50.80 51.17 53.59 52.11
[TiO.sub.2] 0.21 0.15 0.09 0.35
[Al.sub.2][O.sub.3] 0.67 0.71 1.16 2.55
FeO 28.77 28.96 7.58 7.64
MnO 0.92 0.65 0.48 0.33
MgO 17.41 17.90 14.14 13.69
CaO 1.21 0.68 23.62 22.09
[Na.sub.2]O 0.12 0.06 0.39 0.51

Total 100.11 100.28 101.05 99.27
Table 3

Representative amphibole analyses, Merrimack Trough plutons

Sample D-4-1a D-4-1a D-4-3a D-4-3a D-4-4a D-4-4a

[SiO.sub.2] 42.32 43.08 42.37 42.37 42.96 43.60
[TiO.sub.2] 4.20 3.49 4.04 3.69 3.08 3.86
[Al.sub.2][O.sub.3] 11.82 11.22 11.25 11.29 11.46 11.59
MgO 13.16 13.66 13.63 13.50 13.62 13.70
CaO 11.64 11.71 11.24 11.50 11.33 11.70
MnO 0.13 0.12 0.20 0.14 0.18 0.11
FeO 11.79 10.90 11.11 10.95 11.34 10.80
[Na.sub.2]O 2.00 1.34 1.95 1.60 1.98 1.39
[K.sub.2]O 0.91 0.92 1.05 1.04 1.04 1.03
F 0.00 0.00 0.00 0.00 0.00 0.00
Cl 0.03 0.05 0.01 0.04 0.02 0.03

Total 98.00 96.49 96.85 96.08 97.01 97.81

Sample D-4-5a D-4-6a E-3-2a E-3-3a E-3-6a E-4-2a

[SiO.sub.2] 42.76 44.01 43.43 43.34 42.46 47.53
[TiO.sub.2] 3.81 3.79 2.32 2.38 3.12 0.98
[Al.sub.2][O.sub.3] 12.15 11.65 10.49 10.46 11.13 7.80
MgO 13.26 13.83 11.15 10.81 10.43 13.13
CaO 11.52 11.35 11.02 10.83 11.18 12.23
MnO 0.14 0.14 0.30 0.16 0.18 0.36
FeO 11.52 11.28 16.09 16.79 16.65 15.04
[Na.sub.2]O 1.85 1.93 1.46 1.46 1.59 0.98
[K.sub.2]O 1.03 0.97 0.99 0.96 0.96 0.75
F 0.00 0.00 0.00 0.00 0.00 0.00
Cl 0.06 0.06 0.02 0.05 0.04 0.11

Total 98.10 99.01 97.27 97.24 97.74 98.91
Sample E-4-3a E-4-4b E-7-1a E-7-3a IP-5-1a IP-5-2a

[SiO.sub.2] 46.03 45.68 45.48 45.36 41.46 41.25
[TiO.sub.2] 1.44 1.36 1.08 1.34 1.00 0.77
[Al.sub.2][O.sub.3] 8.90 8.83 7.61 8.41 11.28 11.35
MgO 12.29 12.05 10.96 10.51 8.00 8.00
CaO 11.91 12.18 10.97 11.07 11.30 11.20
MnO 0.37 0.27 0.57 0.34 0.88 0.85
FeO 15.57 15.81 18.11 18.57 21.43 21.32
[Na.sub.2]O 1.14 1.12 1.40 1.41 1.51 1.41
[K.sub.2]O 0.91 0.92 0.86 1.07 1.47 1.28
F 0.00 0.00 0.00 0.00 0.00 0.00
Cl 0.14 0.20 0.11 0.17 0.32 0.04

Total 98.70 98.42 97.15 98.25 98.65 97.47

Sample IP-5-3a IP-5-4a S-5-1a S-5-3a S-5-4a S-5-5a

[SiO.sub.2] 41.50 40.57 50.71 50.45 51.69 49.76
[TiO.sub.2] 0.84 0.95 0.27 0.25 0.25 0.41
[Al.sub.2][O.sub.3] 11.49 10.80 5.26 5.55 4.70 5.81
MgO 7.88 7.99 14.12 13.88 14.43 13.65
CaO 11.33 11.10 12.18 12.47 12.19 12.03
MnO 0.90 0.97 0.49 0.47 0.51 0.44
FeO 22.06 20.69 14.13 13.75 13.47 13.66
[Na.sub.2]O 1.48 1.48 0.58 0.50 0.47 0.64
[K.sub.2]O 1.38 1.30 0.38 0.39 0.31 0.45
F 0.00 0.00 0.00 0.00 0.00 0.00
Cl 0.00 0.01 0.03 0.00 0.05 0.01

Total 98.86 95.86 98.15 97.71 98.07 96.86
Table 4

Representative biotite analyses, Merrimack Trough and Rochester plutons

Sample D-1-1a D-1-2a D-1-2a D-1-4a E-4-2a E-4-6a

[SiO.sub.2] 36.56 36.40 36.92 36.52 35.90 36.60
[TiO.sub.2] 3.76 4.31 3.97 3.83 1.84 1.93
[Al.sub.2][O.sub.3] 14.32 14.09 14.09 14.33 15.92 16.01
MgO 15.38 14.96 14.54 15.09 12.71 13.49
CaO 0.00 0.00 0.02 0.02 0.01 0.00
MnO 0.08 0.08 0.12 0.15 0.25 0.19
FeO 15.08 15.10 15.62 15.15 18.30 18.01
[Na.sub.2]O 0.18 0.13 0.14 0.16 0.12 0.15
[K.sub.2]O 9.94 9.81 10.10 10.03 9.88 9.80
F 0.31 0.20 0.34 0.26 0.28 0.25
Cl 0.34 0.30 0.33 0.31 0.18 0.14

Total 95.95 95.38 96.19 95.85 95.39 96.57

Sample E-7-1a E-7-2a IP-5-1a IP-5-3a IP-5-5a IP-5-6a

[SiO.sub.2] 36.39 36.11 35.40 35.82 35.45 35.22
[TiO.sub.2] 2.97 3.92 2.04 2.45 2.08 2.29
[Al.sub.2][O.sub.3] 14.82 14.23 15.77 15.91 16.07 16.16
MgO 10.85 10.53 10.09 10.05 9.99 9.74
CaO 0.04 0.03 0.00 0.03 0.00 0.00
MnO 0.23 0.23 0.70 0.62 0.61 0.50
FeO 21.08 21.97 23.19 22.58 22.28 22.54
[Na.sub.2]O 0.12 0.15 0.15 0.14 0.17 0.16
[K.sub.2]O 9.80 9.78 9.50 9.62 9.57 9.58
F 0.23 0.24 0.29 0.31 0.31 0.27
Cl 0.20 0.18 0.04 0.02 0.01 0.03

Total 96.73 97.37 97.17 97.55 96.54 96.49
Sample S-5-1a S-5-2a S-5-3a S-5-5a W-1-1a W-1-3a

[SiO.sub.2] 36.24 36.89 36.18 36.55 35.52 35.23
[TiO.sub.2] 2.02 1.78 1.71 1.87 2.16 2.42
[Al.sub.2][O.sub.3] 16.15 16.60 16.83 16.59 15.64 16.30
MgO 11.84 12.32 12.17 12.19 9.05 7.63
CaO 0.00 0.06 0.00 0.00 0.03 0.01
MnO 0.23 0.30 0.29 0.29 0.53 0.38
FeO 18.49 18.68 18.30 18.59 22.97 24.70
[Na.sub.2]O 0.15 0.13 0.16 0.11 0.14 0.15
[K.sub.2]O 9.73 9.92 9.99 9.97 9.54 9.37
F 0.15 0.21 0.19 0.19 0.57 0.57
Cl 0.12 0.17 0.16 0.15 0.04 0.02

Total 95.12 97.06 95.98 96.50 96.19 96.78

Sample W-1-7a W-1-8a R-4-1a R-4-4a R-5-1a R-5-3a

[SiO.sub.2] 35.96 35.38 35.45 34.94 34.89 34.74
[TiO.sub.2] 2.13 2.59 3.14 3.38 3.03 3.09
[Al.sub.2][O.sub.3] 16.43 16.22 15.97 15.72 15.41 16.02
MgO 9.41 7.52 8.52 8.55 8.71 8.78
CaO 0.01 0.02 0.0l 0.00 0.04 0.09
MnO 0.49 0.46 0.55 0.59 0.63 0.56
FeO 20.81 24.07 22.85 23.52 23.74 23.86
[Na.sub.2]O 0.18 0.18 0.20 0.20 0.13 0.19
[K.sub.2]O 9.61 9.51 9.51 9.52 9.47 9.14
F 0.64 0.55 0.57 0.59 0.59 0.54
Cl 0.01 0.01 0.12 0.10 0.14 0.11

Total 95.68 96.50 96.89 97.11 96.78 97.12
Table 5

Major, minor, and trace element compositions of representative samples
from the Merrimack Trough and Rochester plutons

Pluton Dracut
Sample D-1 D-2 D-3 D-4 D-6

[SiO.sub.2] 47.90 46.69 47.12 49.75 47.83
[TiO.sub.2] 2.35 2.96 2.95 1.04 2.01
[Al.sub.2][O.sub.3] 18.29 18.10 18.06 16.70 17.25
[Fe.sub.2][O.sub.3] 12.07 11.45 11.11 8.83 10.86
MnO 0.20 0.19 0.18 0.17 0.18
MgO 6.14 5.25 5.25 10.06 8.14
CaO 10.73 10.28 10.54 10.87 11.30
[Na.sub.2]O 2.67 2.75 2.84 2.12 1.80
[K.sub.2]O 0.49 0.54 0.35 0.49 0.31
[P.sub.2][O.sub.5] 0.47 0.61 0.59 0.17 0.16
L.O.I. 0.48 1.66 2.16 1.52 1.22

Total 101.79 100.47 101.14 101.72 101.06

Trace elements *

Ba 0 221 73 280 235
Rb 24 28 25 31 25
Sr 559 582 610 647 650
Zr 88 82 85 74 73
Y 49 42 42 23 24
Nb 31 33 33 18 25
Sc 15 14 11 0 17
V 303 284 311 163 303
Cr 61 7 8 331 162
Co 49 45 42 46 55
Ni 10 7 1 44 19
Cu 26 33 26 25 45
Zn 99 100 96 71 77

Pluton Dracut Sweepstakes
Sample D-11 D-15 S-1 S-4 S-5

[SiO.sub.2] 49.31 74.37 54.43 55.38 61.86
[TiO.sub.2] 0.98 0.23 1.07 0.63 0.80
[Al.sub.2][O.sub.3] 17.41 15.24 18.06 16.88 17.28
[Fe.sub.2][O.sub.3] 8.29 1.62 7.29 7.26 5.19
MnO 0.16 0.02 0.15 0.22 0.10
MgO 9.19 0.68 6.03 7.56 3.56
CaO 11.78 3.26 7.45 7.98 5.49
[Na.sub.2]O 2.09 4.22 2.84 2.51 3.15
[K.sub.2]O 0.40 0.86 1.71 1.04 2.70
[P.sub.2][O.sub.5] 0.20 0.04 0.27 0.11 0.19
L.O.I. 1.58 1.15 1.74 1.67 0.78

Total 101.39 101.70 101.04 101.25 101.10

Trace elements *

Ba 0 434 46 222 661
Rb 30 68 102 57 114
Sr 694 469 398 584 376
Zr 78 120 70 105 124
Y 25 0 27 19 20
Nb 18 0 14 11 8
Sc 6 20 27 12 30
V 163 12 191 139 102
Cr 377 0 194 288 65
Co 39 0 26 28 12
Ni 32 1 24 20 21
Cu 24 0 21 15 11
Zn 68 30 85 81 71
Pluton
Sample S-6 S-11 S-12 S-14

[SiO.sub.2] 53.29 55.92 55.48 52.61
[TiO.sub.2] 0.50 1.26 1.40 0.36
[Al.sub.2][O.sub.3] 22.16 17.25 16.82 21.10
[Fe.sub.2][O.sub.3] 4.34 7.52 7.64 4.56
MnO 0.07 0.15 0.15 0.09
MgO 5.45 5.35 5.37 6.84
CaO 10.13 7.38 7.43 11.66
[Na.sub.2]O 3.31 2.94 2.86 2.99
[K.sub.2]O 0.54 1.94 1.92 0.20
[P.sub.2][O.sub.5] 0.08 0.23 0.24 0.03
L.O.I. 0.76 1.36 0.90 0.87
Total 100.64 101.30 100.21 101.32

Trace elements *

Ba 349 168 286 47
Rb 42 79 78 36
Sr 498 374 366 540
Zr 76 140 156 47
Y 5 31 31 4
Nb 3 17 17 3
Sc 0 5 11 0
V 73 159 173 94
Cr 267 144 174 41
Co 15 29 27 18
Ni 41 6 10 26
Cu 10 19 17 11
Zn 41 76 81 31

Pluton Island Pond
Sample IP-1 IP-2 IP-3 IP-5

[SiO.sub.2] 48.97 49.87 43.61 52.73
[TiO.sub.2] 2.18 2.25 3.54 1.87
[Al.sub.2][O.sub.3] 17.00 16.83 14.98 18.80
[Fe.sub.2][O.sub.3] 12.33 10.93 15.49 9.12
MnO 0.28 0.29 0.30 0.23
MgO 5.53 5.20 5.56 2.92
CaO 9.45 8.61 9.78 6.38
[Na.sub.2]O 3.14 3.63 2.96 4.57
[K.sub.2]O 0.89 1.24 1.73 2.63
[P.sub.2][O.sub.5] 0.62 1.03 2.09 0.77
L.O.I. 0.68 0.81 0.72 0.59
Total 101.07 100.69 100.76 100.61

Trace elements *

Ba 560 760 426 1817
Rb 69 65 53 63
Sr 873 851 889 990
Zr 111 190 112 381
Y 31 30 46 30
Nb 22 20 38 26
Sc 1 19 18 19
V 323 239 434 174
Cr 80 84 18 0
Co 58 47 76 29
Ni 67 32 8 0
Cu 46 31 38 20
Zn 122 125 147 145

Pluton Island Pond
Sample IP-6 IP-7

[SiO.sub.2] 70.22 71.26
[TiO.sub.2] 0.40 0.43
[Al.sub.2][O.sub.3] 16.56 16.45
[Fe.sub.2][O.sub.3] 2.38 2.27
MnO 0.04 0.04
MgO 0.86 0.89
CaO 2.38 2.23
[Na.sub.2]O 4.74 4.97
[K.sub.2]O 2.58 2.49
[P.sub.2][O.sub.5] 0.12 0.26
L.O.I. 0.49 0.58
Total 100.77 101.87

Trace elements *

Ba 543 394
Rb 155 164
Sr 530 388
Zr 227 206
Y 0 3
Nb 0 0
Sc 15 31
V 28 27
Cr 0 0
Co 0 0
Ni 0 0
Cu 0 0
Zn 58 75

* Obtained by XRF
Pluton Exeter
Sample E-1 E-2 E-3 E-4 E-5

[SiO.sub.2] 56.02 54.40 54.82 53.58 55.91
[TiO.sub.2] 0.97 1.09 0.98 1.14 1.17
[Al.sub.2][O.sub.3] 14.80 14.75 14.32 14.84 16.59
[Fe.sub.2][O.sub.3] 8.47 8.80 9.13 9.18 8.31
MnO 0.17 0.18 0.18 0.17 0.16
MgO 7.29 7.81 8.71 7.75 5.67
CaO 7.12 7.47 7.75 8.54 7.63
[Na.sub.2]O 2.87 2.69 2.60 2.30 2.90
[K.sub.2]O 2.70 2.57 2.25 2.03 1.66
[P.sub.2][O.sub.5] 0.27 0.30 0.27 0.29 0.26
L.O.I. 0.60 0.52 0.32 1.31 1.02

Total 101.29 100.57 101.33 101.14 101.28

Trace elements

Ba 302 634 562 598 538
Rb 25 31 26 29 68
Sr 448 485 479 463 474
Zr 85 74 73 79 120
Y 43 23 23 25 0
Nb 33 18 25 12 0
Sc 10 20 0 25 20
V 179 206 189 210 189
Cr 376 390 467 335 188
Co 34 38 40 41 33
Ni 137 134 166 117 55
Cu 23 24 42 23 0
Zn 90 90 88 86 92

Pluton Exeter Webhannet
Sample E-7 E-9 E-11 W-1 W-2

[SiO.sub.2] 60.39 57.61 56.92 69.86 67.26
[TiO.sub.2] 0.97 1.03 1.08 0.39 0.58
[Al.sub.2][O.sub.3] 15.68 15.84 15.57 15.30 16.63
[Fe.sub.2][O.sub.3] 6.79 7.57 7.34 2.21 3.43
MnO 0.13 0.15 0.14 0.05 0.07
MgO 3.64 5.10 5.90 0.60 0.97
CaO 5.63 6.81 4.94 1.72 1.92
[Na.sub.2]O 2.96 2.96 2.94 3.71 4.14
[K.sub.2]O 3.57 2.94 2.92 5.57 4.87
[P.sub.2][O.sub.5] 0.21 0.28 0.30 0.12 0.20
L.O.I. 0.50 0.56 2.63 0.80 0.65

Total 100.48 100.85 100.68 100.33 100.73

Trace elements

Ba 729 689 591 1262 1215
Rb 79 78 76 134 120
Sr 326 477 434 575 696
Zr 156 134 112 246 347
Y 33 32 32 5 16
Nb 19 21 20 0 5
Sc 27 5 19 23 34
V 141 178 153 22 53
Cr 101 182 245 0 0
Co 21 26 26 0 0
Ni 16 55 101 0 2
Cu 36 37 34 0 0
Zn 87 82 79 41 71

Pluton Webhannet
Sample W-3

[SiO.sub.2] 63.86
[TiO.sub.2] 0.99
[Al.sub.2][O.sub.3] 15.58
[Fe.sub.2][O.sub.3] 5.53
MnO 0.11
MgO 2.57
CaO 3.93
[Na.sub.2]O 3.45
[K.sub.2]O 3.31
[P.sub.2][O.sub.5] 0.34
L.O.I. 1.42

Total 101.09

Trace elements

Ba 878
Rb 102
Sr 416
Zr 281
Y 26
Nb 17
Sc 19
V 93
Cr 51
Co 17
Ni 31
Cu 28
Zn 91
Pluton Rochester
Sample W-4 W-5 W-6 R-1 R-2 R-3

[SiO.sub.2] 75.63 74.54 76.24 55.98 59.00 55.98
[TiO.sub.2] 0.19 0.25 0.13 2.82 2.00 2.51
[Al.sub.2][O.sub.3] 13.41 13.74 13.48 15.51 15.82 15.41
[Fe.sub.2][O.sub.3] 1.23 1.48 0.89 9.08 7.03 9.20
MnO 0.07 0.06 0.07 0.26 0.23 0.27
MgO 0.28 0.38 0.15 3.34 2.32 3.01
CaO 1.09 1.15 0.59 5.31 3.56 4.84
[Na.sub.2]O 3.89 3.84 4.08 2.58 3.05 2.91
[K.sub.2]O 4.43 4.79 4.64 3.35 5.38 3.62
[P.sub.2][O.sub.5] 0.05 0.07 0.05 1.43 1.12 1.31
L.O.I. 0.66 0.81 0.59 0.37 0.58 1.25
Total 100.93 101.11 100.90 100.03 100.10 100.30

Trace elements

Ba 169 91 62 1724 2653 1332
Rb 175 164 285 72 103 83
Sr 176 256 47 1242 944 853
Zr 120 150 82 478 413 481
Y 0 0 1 38 32 39
Nb 0 0 0 36 28 38
Sc 32 26 30 18 20 15
V 14 12 3 214 151 188
Cr 0 0 0 32 21 23
Co 0 0 0 27 24 34
Ni 0 0 0 27 15 13
Cu 0 0 0 28 15 21
Zn 36 37 25 145 111 168

Pluton
Sample R-4 R-5 Br (1) BR (2)

[SiO.sub.2] 57.98 57.81 38.20 37.88
[TiO.sub.2] 2.25 2.26 2.60 2.61
[Al.sub.2][O.sub.3] 15.19 15.34 10.20 9.97
[Fe.sub.2][O.sub.3] 8.37 8.37 12.88 12.74
MnO 0.24 0.24 0.20 0.20
MgO 2.68 2.62 13.28 13.18
CaO 4.55 4.46 13.80 13.43
[Na.sub.2]O 2.85 2.96 3.05 3.01
[K.sub.2]O 4.05 3.96 1.40 1.36
[P.sub.2][O.sub.5] 1.15 1.15 1.04 1.03
L.O.I. 0.92 0.19 3.00 3.00
Total 100.22 99.36 99.65 98.41

Trace elements

Ba 1720 1277 1050 1156
Rb 90 90 47 42
Sr 803 824 1320 1317
Zr 440 395 250 225
Y 37 37 30 39
Nb 37 37 98 94
Sc 3 35 25 0
V 169 164 235 291
Cr 21 22 380 360
Co 28 29 52 69
Ni 10 17 260 272
Cu 22 22 72 71
Zn 144 144 160 160

(1)Values for standard as reported in Govindaraju 1989

(2)Values for internal standard analysis
Table 6

Rare earth and selected trace element data obtained by INAA

Sample D-1 D-4 D-15 E-1 E-3 E-7 IP-2 IP-3 IP-7

La 17.89 19.51 29.88 25.49 59.42 37.14 50.73 68.36 32.41
Ce 53.09 43.86 53.25 53.39 111.57 78.45 111.52 151.12 63.16
Nd 39.90 25.16 21.49 23.37 49.27 35.90 59.43 53.91 30.30
Sm 11.34 6.97 3.18 5.97 5.33 7.51 9.96 14.16 4.23
Eu 2.80 2.11 1.33 1.46 1.41 1.69 2.63 3.31 0.94
Tb 1.77 0.93 0.33 0.71 0.69 1.10 1.13 1.57 0.54
Dy 11.63 2.54 2.09 4.96 4.95 6.57 4.00 7.36 3.86
Yb 3.81 2.50 0.93 2.71 2.57 3.21 1.88 2.41 1.25
Lu 0.55 0.48 0.18 0.43 0.39 0.48 0.30 0.38 0.20
Cs 1.34 4.69 0.71 3.02 3.16 3.36 1.45 12.09 7.59
Hf 2.33 2.69 2.63 5.00 3.90 11.23 2.40 3.20 5.12
Ta 0.89 0.40 0.90 0.67 0.56 1.12 1.15 1.22 0.99
U 0.73 1.08 6.55 2.35 2.25 2.05 0.37 1.83 6.15
Th 0.41 0.72 8.94 7.40 7.23 9.44 3.49 3.08 8.49
La/Yb 4.70 7.80 32.16 9.41 23.12 11.57 26.98 28.37 25.93

Sample R-1

La 142.33
Ce 303.74
Nd 157.93
Sm 22.49
Eu 4.91
Tb 1.83
Dy 8.14
Yb 2.63
Lu 0.33
Cs 1.25
Hf 15.29
Ta 1.78
U 1.62
Th 12.92
La/Yb 54.12
Sample R-2 R-4 S-1 S-6 S-12 W-1 W-3 W-5

La 121.23 139.13 33.81 7.48 33.67 95.56 69.17 53.58
Ce 265.89 283.44 64.40 15.20 65.22 167.47 139.30 95.83
Nd 114.24 118.31 31.34 12.76 41.76 53.43 72.41 25.19
Sm 20.37 21.06 4.48 1.71 6.59 8.47 10.77 4.37
Eu 4.30 4.24 0.97 1.00 1.56 1.96 1.93 0.83
Tb 1.61 1.69 0.71 0.17 0.98 0.64 0.99 0.45
Dy 6.89 8.02 4.85 1.92 5.04 3.73 6.04 1.79
Yb 2.16 2.81 1.09 0.80 2.56 1.55 2.70 1.18
Lu 0.29 0.36 0.21 0.13 0.41 0.26 0.37 0.22
Cs 2.10 3.34 7.66 1.69 5.48 1.92 1.74 1.39
Hf 13.02 15.69 5.24 1.38 5.38 6.86 8.68 4.13
Ta 1.63 1.81 1.06 0.25 0.71 0.87 1.35 1.22
U 1.93 2.05 7.15 0.61 2.47 4.39 2.00 3.63
Th 12.55 17.51 8.64 1.13 7.49 16.02 15.84 30.84
La/Yb 56.13 49.51 31.02 9.34 13.15 61.65 25.62 45.41

Sample 688 (1) 688 (2)

La 5.00 5.03
Ce 13.10 13.03
Nd <35.00 21.10
Sm 2.20 2.43
Eu 0.85 0.90
Tb 0.36 0.39
Dy 3.20 3.36
Yb 2.10 1.90
Lu 0.33 0.36
Cs <1.50 0.52
Hf 1.46 1.50
Ta 0.28 0.34
U 0.32 0.83
Th 0.28 0.38
La/Yb 2.38 2.65

(1)Values for standard NIST 688 as reported in Bower 1992

(2)Values for analysis of internal standard NIST 688


ACKNOWLEDGEMENTS

We thank Jim Brophy for an earlier review of the paper and David West and David Gibson for helpful journal reviews. We also thank David McMullin for assistance with the illustrations. This study was supported by NSF grants EAR-9418203 to Wintsch and EAR-9909410 to Wintsch and Dorais.

Date Received: February 4, 2000

Date Accepted: February 3, 2001

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Brandon G. Watts (1), *

Michael J. Dorais (2), (ss)

Robert P. Wintsch (3)

(1.) Department of Geological Sciences, Indiana University, Bloomington, IN 47405, U.S.A.

(2.) Department of Geology, Brigham Young University Provo, UT 84602, U.S.A. <dorais@byu. edu>

(3.) Department of Geological Sciences, Indiana University Bloomington, IN 47405, U.S.A. <wintsch@indiana.edu>

* Current address: Earth Tech Inc. 1420 King Street, Suite 600, Alexandria, VA 22314, U S.A. <Brandon_Watts@earthtech. com>

(ss) Corresponding author
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Publication:Atlantic Geology
Geographic Code:1U100
Date:Jul 1, 2000
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