Trondhjemite and Metamorphosed Quartz Keratophyre Tuff of the Ammonoosuc Volcanics (Ordovician), Western New Hampshire and Adjacent Vermont and Massachusetts

Home , Ammonoosuc Volcanics, Keratophyre

Trondhjemite and metamorphosed quartz keratophyre tuff of the Ammonoosuc Volcanics (Ordovician), western New Hampshire and adjacent Vermont and Massachusetts

GERHARD W. LEO U.S. Geological Survey, National Center 928, Reston, Virginia 22092

ABSTRACT INTRODUCTION

The Ammonoosuc Volcanics and equivalent rocks of Ordovician The leucocratic, but potassium-poor, nature of trondhjemites and age are exposed in the Oliverian domes along the Bronson Hill anti- tonalites, as well as their significant role in crustal evolution since the early clinorium (BHA) between northern New Hampshire and southern Archaean (Barker and others, 1981), has generated a great deal of interest Connecticut. In western New Hampshire and adjacent Vermont and in this rock type. Since the early Proterozoic, tonalites, including trond- Massachusetts, the Ammonoosuc lithology consists of a lower, mainly hjemites, have constituted a minor, but significant, component of mag- mafic unit of hornblende-plagioclase amphibolite, and an upper, matic arcs and convergent continental-oceanic margins where they are mainly felsic, metamorphosed quartz keratophyre tuff. These litholo- found in association with K-poor mafic volcanic rocks (see Barker, gies are locally interlayered, and both are intruded by sills, dikes, and 1979a, and Barker and others, 1981, for examples). plugs of trondhjemite. Trondhjemite also constitutes the interior The purpose of this paper is to report the occurrence and discuss the gneissic "core" of several small domes or plutons. The trondhjemite significance of trondhjemite and chemically similar metamorphosed quartz 1 is highly siliceous (Si02 = 73%-81%), low in A1203 (11.3%-13.5%), keratophyre tuff within the Middle Ordovician Ammonoosuc Volcanics generally contains <1% K20, and thus resembles some trondhjemites in in western New Hampshire. These rocks form part of the Bronson Hill island-arc or continental-margin settings. Chemical trends of both anticlinorium, a major structure in the New England Appalachians (Fig. 1) trondhjemite and Ammonoosuc Volcanics (felsic and mafic) are essen- which was the locus of intense magmatic activity during the Ordovician tially calc-alkaline. Taconic orogeny. The anticlinorium appears to be the remnant of a mag- Variations in both major and trace elements of trondhjemites in matic arc formed during convergence of the ancestral Atlantic (Iapetus) several of the domes suggest several somewhat different sources along ocean (Kay, 1951; Thompson and others, 1968; Osberg, 1978; Robinson the BHA. Overall, however, the major- and minor-element chemistry and Hall, 1980, see especially the interpretive cross sections; Hall and of the trondhjemites is closely similar to that of the Ammonoosuc Robinson, 1982; Lyons and others, 1982). The occurrence and origin of quartz keratophyre tuff. These rocks could have been produced either these trondhjemitic rocks is regionally significant, as similar lithologies are by partial melting or by fractional crystallization of basaltic source found throughout the Appalachian chain (Higgins, 1972; Whitney and rocks. The partial-melting model is preferred because of the largely others, 1978; Southwick, 1979; Payne and Strong, 1979; Malpas, 1979; bimodal basalt-quartz keratophyre Ammonoosuc assemblage in which Pavlides, 1981; Leo and others, 1984). In many places, however, including andesitic and other intermediate compositions are virtually lacking. the Bronson Hill anticlinorium, it has not been possible to relate quartz The relatively thin Ammonoosuc section appears to preclude genera- keratophyre and keratophypre tuffs to a magmatic source. The spatially tion of trondhjemite at the presently exposed base of an island arc, as related and chemically almost identical trondhjemite provides at least a has been postulated for very similar trondhjemite-amphibolite assemb- partial answer as to the origin of the felsic Ammonoosuc tuffs. Its recogni- lages (Twillingate trondhjemite, Little Port Complex) in Newfound- tion, furthermore, clarifies the former mistaken assumption that the land. Instead, generation of the felsic Ammonoosuc rocks more likely trondhjemite, especially where it constitutes larger masses, is comagmatic occurred at deeper levels along a subduction zone dipping eastward with Oliverian granitic gneiss (for example, Chapman, 1942; Kruger, under the BHA, as postulated in current plate-tectonic models. The 1946). close juxtaposition in space and time of sialic crust and Ammonoosuc Volcanics may explain the calc-alkaline trends of the latter and suggests a paleotectonic environment of convergent oceanic-continental plate margins, possibly with significant crustal shortening across the 'Inasmuch as all of the rocks discussed herein are regionally metamorphosed, the word "metamorphosed" to describe the quartz keratophyre is henceforth omit- arc. ted but implied.

Additional material for this article (sample numbers, descriptions, and locations for Tables 1 and 2) is available free of charge. Request Supplementary Data 85-31 from the GSA Documents Secretary.

Geological Society of America Bulletin, v. 96, p. 1493-1507, 14 figs., 3 tables, December 1985.

1493

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Northern part IT

m- - Southern part 72° "Berlin 43° — -0^ 1 N. H. MAINE

Ammonoosuc

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MASS. 73" 42» CONN.

m tu 0 10 20 30 40 50 KM ~r JL I J M 10 30 Ml

LONG ¡SLAND SOUND

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Layered rocks Intrusive rocks including volcanic rocks but including orthogneisses excluding granitic gneisses

+ + ^ ± ±. Cretaceous to Portland Formation and related White Mountain Plutonic- Triassic volcanic rocks of the Volcanic Suite Connecticut Valley Unconformity

Devonian to Silurian and Devonian rocks Felsic to intermediate plutonic Silurian undivided rocks, mostly gneissic

Unconformity

* < >

Ammonoosuc Volcanics and Gneisses in cores of Oliverian Highlandcroft Plutonic Suite Partridge Formation and related domes. (See (New Hampshire) (Middletown Gneiss and figure captions for names Collins Hill Formation, re- corresponding to letter spectively, in southern Con- symbols) necticut); also includes Or- fordville Formation (southwest New Hampshire) and Brim- field Group (Middle Ordovi- cian or older; Massachusetts Ordovician and Connecticut)

HI Albee Formation (New Hampshire)

Other Ordovician rocks, undivided (southwest New Hampshire) Unconformity

Proterozoic Z Metasedlmentary and Gneiss in cores of domes metavolcanic rocks — Contact — Fault B

Figure 1. A. Geologic setting of the Bronson Hill anticlinorium, emphasizing the Oliverian domes and Ammonoosuc Volcanics. Adapted from Robinson and Hall (1980, Fig. 1) and Billings (1955). Letter symbols for Oliverian and other domes (place names where appropriate) as follows (generally north to south): J, Jefferson; Ld, Landaff; M, Mascoma; L, Lebanon; trondhjemite intruding Ammonoosuc near White River Junction, Vermont; trondhjemite-tonalite northeast of Plainfield, New Hampshire; C, Croydon; U, Unity; A, Alstead; V, Vernon; W, Warwick; P, Pelham; Mo, Monson; G, Glastonbury. Other units in Explanation (Fig. IB). Names of domes or plutons not referred to in text, as well as the geology east and west of the Ammonoosuc Volcanics, are mainly omitted. Heavy dashed lines enclose hypothetical areas of oceanic crust (see text).

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REGIONAL RELATIONSHIPS DESCRIPTION OF LITHOLOGIES

The Ammonoosuc Volcanics of Ordovician age are distributed along Ammonoosuc Volcanics the Bronson Hill anticlinorium from northern New Hampshire to Long Island Sound (Fij|. 1). The Bronson Hill anticlinorium is marked by a The Ammonoosuc section adjacent to the Oliverian plutons iri west- series of gneiss domes (Oliverian domes) and elongate anticlines. It shows ern New Hampshire consists, predominantly, of two lithologies: quartz- the following pre -Mesozoic stratigraphic succession: (1) gneisses and re- plagioclase granofels and hornblende-plagioclase amphibolite.2 Northwest lated rocks in the interiors of domes, (2) metamorphosed Middle Ordovi- of the Ammonoosuc fault, volcanic features such as pillow lavas, volcanic cian sedimentary and volcanic rocks (including Ammonoosuc Volcanics), breccias, agglomerates and conglomerates, and tuffs showing graded bed- and (3) an unconf ormably overlying sequence of metamorphosed Silurian ding and cross-bedding are well preserved (Billings, 1937). No ignimbrites and Lower Devonian sedimentary and volcanic rocks. The metamorphic have been identified, although their presence cannot be ruled out. On the grade is middle to upper amphibolite facies in most places southeast of the southeast side of the Ammonoosuc fault, where the metamorphic grade is Ammonoosuc fault (see Fig. 1), and greenschist facies northwest of the higher, primary textural volcanic features have been largely obliterated, fault. but pyroclastic accumulations, often well graded and having clasts up to Billings's (1937) pioneering studies of several Oliverian domes 30 cm across, are common and well preserved in parts of the Ammonoo- showed that the major deformation is post-Early Devonian, inasmuch as suc section. Where a succession within the Ammonoosuc sequence has the thick Lower E>evonian sequence has been involved in the deformation. been identified, the lower part is predominantly mafic, and the uppsr part, More recent investigations (Thompson and others, 1968; Robinson and predominantly felsic (for example, Robinson, 1963; Naylor, 1969; Hall, 1980) have ¡shown that the doming followed the development, in the Schumacher, 1980a, 1983). Rocks of intermediate composition are rela- Early Devonian, of giant nappes that extend at least from northern tively scarce, and there is no significant andesitic component. The ::atio of Massachusetts to west-central New Hampshire. Except for the three small mafic to felsic Ammonoosuc, although nowhere accurately determined, is trondhjemite-cored domes discussed in this paper, Oliverian gneisses are estimated, approximately, at 5:1 to 5:2, locally as much as 5:3. Recent calc-alkaline granite plutons locally grading to granodiorite and tonalite. mapping and isotopic dating (Moench and Aleinikoff, 1985; Aleinikoff Oliverian plutons, in some places, intrude Ammonoosuc Volcanics; else- and Moench, 1985) have resulted in further structural and lithologic sub- where, the natuie of the contact is undetermined (Zartman and Leo, division of the Ammonoosuc in northwestern New Hampshire and 1985). Trondhjemite was not found to grade to, nor to be cut by, Oliverian southwestern Maine. gneiss; rather, it is invariably associated with Ammonoosuc Volcanics. Mafic Ammonoosuc southeast of the Ammonoosuc fault (Fig. 1) is Where Ammonoosuc with associated trondhjemite is in close spatial jux- mostly hornblende-plagioclase-(biotite) amphibolite. Rocks containing taposition with Oliverian gneiss (for example, in the Berlin area, the Ca-poor amphiboles (cummingtonite, anthophyllite, and aluminous ged- Landaff pluton, and at the northern end of the Croydon dome; see Fig. 1), rite), with or without accompanying hornblende, constitute a subordinate, trondhjemite car. be difficult to distinguish from fine-grained Oliverian but distinctive, Ammonoosuc lithology (compare Robinson and Jaffe, granitic gneiss. 1969; Schumacher, 1980b, 1983). These rocks span the compcsitional Recent U-Pb isotopic age determinations on zircons from granitic range from mafic to felsic and thus constitute a limited departure from the gneiss plutons of 6 domes (Zartman and Leo, 1985) indicated a composite predominantly bimodal assemblage. They are conspicuously low in CaO, age of 444 ± 8 m .y., in good agreement with the earlier determinations of slightly low in MgO and K20, and somewhat enriched in Na20 and Naylor (1969). Field relations indicate that the Ammonoosuc is at least FeO*,3 compared to nongedritic samples (Leo, unpub. data). These differ- somewhat older than the Oliverian gneisses, and recent 40Ar/39Ar age ences correspond partly, but not entirely, to those in similar reeks de- spectrum dating suggests a difference of 10-20 m.y. (Kunk and Sutter, scribed by Schumacher (1983), who concluded that low-temperature 1984; Zartman and Leo, 1985). On the basis of available evidence, there is hydrothermal alteration and/or weathering could account for the distinc- no indication either of significant anatexis of the Oliverian plutons or of tive chemistry of these rocks. significant disturbance of their U-Th-Pb isotope systematics that is related Felsic Ammonoosuc granofels is typically white to very pale gray, to the Acadian orogeny. fine-grained, and equigranular, although graded sequences with clasts up The Ammonoosuc Volcanics that everywhere mantle the Oliverian to 10 cm long are known. The granofels consists of quartz, sodio plagio- gneissic plutons consist mainly of a bimodal assemblage of amphibolite clase and subordinate biotite, locally accompanied by cummingtonite, (formerly tholeiitic basalt and mafic tuff) and siliceous, K-poor granofels anthophyllite, garnet, magnetite, and/or epidote. These rocks are :hinly to (formerly quartz keratophyre and related tuff) (Leo, 1980). On the War- massively layered, typically interbedded with amphibolites, and are inter- wick dome and elsewhere in northern Massachusetts, both the lower and preted as being metamorphosed quartz keratophyre tuffs. In several upper Ammonoosuc assemblages are thicker and more varied, and the domes, the Ammonoosuc sequence is underlain by massively bedded, upper part includes rhyolitic rocks with higher, although erratic, K2O K-poor felsic granofels generally similar to the fine-grained variety of felsic contents in the range of 1.5% to 5.5% (Schumacher, 1983). The lithology Ammonoosuc itself (stratified core gneiss of Naylor, 1969; see Table 2 and gross chemistry of the Ammonoosuc Volcanics show some similarities below, nos. 24-26). These rocks appear to be unrelated to the to oceanic island arcs, such as the Fiji arc, and, to a lesser extent, the nonstratified, granitic gneisses of the domes, and so they are henceforth Tonga-Kermadec and Marianas arcs. As will be shown, however, the sum referred to as "sub-Ammonoosuc quartz keratophyre." of chemical and lithologic features of the Ammonoosuc has no close analogue in any modern arc. It is concluded that the Ammonoosuc is most reasonably interpreted as being a continental-margin arc, having been 2 developed by (1) partial melting of a basaltic source; (2) subsequent erup- A more diversified Ammonoosuc lithology that includes calcareous and tion/ emplacement of the felsic phase in disproportionate amounts relative aluminosilicate-bearing rocks in the lower part of the section and metarhyolites containing as much as 5.5% K20 in the upper part has been described ir. northern to its source; and (3) possibly, limited contamination by sialic crust which Massachusetts and southwestern New Hampshire (Schumacher, 1983). constituted the source of the Oliverian plutons. 3FeO* = total iron as FeO.

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Figure 3. Trondhjemite sill, ~1 m thick, intruding layered mafic and felsic Ammonoosuc (under hammer). Differential erosion of the two units is locally the principal criterion for distinguishing trond- Figure 2. Slightly crosscutting sill of trondhjemite in felsic Am- hjemite from layered felsic Ammonoosuc granofels. First Avenue near monoosuc, Berlin, New Hampshire. Hill Street, Berlin, New Hampshire.

Figure 4. Post Pond volcanics (= Ammonoosuc Volcanics) in- jected by trondhjemite (white); combination constitutes gneiss at White River Junction of Lyons (1955). Cut along 1-91, ~2 km north of White River Junction. Extremely shattered aspect is probably related to post-Acadian movement on the Ammonoosuc fault -0.5 km to the east. The lensoid, boudinaged appearance of both trondhjemite and amphibolite matrix indicates contemporaneous and mutually intrusive emplacement in a plastic, incompletely crystallized condition. Hammer (right center) gives scale.

Trondhjemite Figure 5. Angular trondhjemite blocks in matrix of amphibolite; Sills, dikes, and irregular masses of white to pale gray, even-textured basalt here was the intrusive phase. Route 1-91, 2 km north of White trondhjemite consisting of quartz + sodic plagioclase + biotite ± muscovite River Junction. ± epidote ± chlorite ± hornblende cut Ammonoosuc Volcanics in various localities along the Bronson Hill anticlinorium. Especially good exposures occur in the Berlin, New Hampshire, area (Figs. 2 and 3) near West South of the Lebanon-White River Junction area, trondhjemite Lebanon, New Hampshire, and near White River Junction, Vermont. At forms several continuous exposures 3-20 km long and 1-4 km wide. the latter locality, in cuts on 1-91, dikes of trondhjemite and Ammonoosuc These include a subelliptical trondhjemitic to tonalitic plug 10 km south- amphibolite show clear evidence of mutual intrusion (Figs. 4 and S) and east of Lebanon, New Hampshire (gneiss east of Plainfield, New confirm the contemporaneity of the intrusive and eruptive Ammonoosuc Hampshire, as termed by Lyons, 1955, p. 121); the northern end of the phases. Croydon dome, where trondhjemite intrudes mafic Ammonoosuc in a

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zone tens of metre!! wide (Fig. 6); and all of the gneissic cores in the Unity, Alstead, and Vernon domes (Fig. 1). The belt of larger trondhjemite exposures appears to be devoid of Oliverian granite gneiss. This granite- free belt, extending -100 km south from White River Junction to the Vernon dome, evidently reflects the present erosion surface on oceanic crust (see dashed lines, Fig. 1).

CHEMISTRY

Analytical Techniques

Major- and trace-element compositions of trondhjemites are listed in Table 1. Data for selected Ammonoosuc Volcanics and underlying quartz keratophyre are listed in Table 2. Accompanying triangular diagrams were generated on a volatile-free basis by a computer program: Graphic Norma- tive Analysis Program (GNAP) of the U.S. Geological Survey (Bowen, 1971; Stuckless and Van Trump, 1979; T. L. Wright, 1983, personal

commun.). Major-element oxides, for most samples, and water and C02 for all samples were determined by rapid-rock analysis methods described Figure 6. Shadowy tabular inclusions of Ammonoosuc amphibo- by Shapiro (1975). Some major-element analyses were obtained by X-ray lite in trondhjemitic gneiss, northeastern part of Croydon dome spectroscopy, as were Rb, Sr, Ba, Nb, Zr, and Y. Rapid-rock analyses were (Fig. 1). Roadcut on west side of 1-89 ~1 km west of North Grantham done by F. Brown, N. Skinner, Z. Brown, and H. Smith; X-ray spectro- (Mascoma, New Hampshire, IS' quadrangle). Height of cut ~8 m.

TABLE 1. MAJOR AND TRACE-ELEMENT CHEMISTRY OF TRONDHJEMITE ASSOCIATED WITH AMMONOOSUC VOLCANICS, WESTERN NEW HAMPSHIRE AND ADJACENT VERMONT AND MASSACHUSETTS

Analysis Chondrite normalizing vjlues^ West Lebanon- Landaft White Plain Warwick pluton River Junction field Croydon dome Unity dome dome

Major elements (percent)

73.2 76.7 77.8 80.6 78.8 77.1 75.5 73.3 75.8 78.3 79.0 75.4 78.7 73.1 74.7 75.1 74.8 77.0 Ti02 0J3 0.24 0.10 0.16 0.07 0.23 0.23 0.21 0.30 0.18 0.10 0.21 0.21 0.21 0.22 0.22 0.22 0.21 Al,2 0 3 12.! 12.3 11.9 11.4 11.7 11.6 11.6 13.2 12.5 11.3 11.8 12.6 11.3 13.5 13.8 13.3 12.9 Fe203 l.V 0.47 0.88 0.00 0.92 2.1 0.90 1.5 1.85 0.79 0.49 1.57 0.67 1.1 1.7 1.2 0.68 FeO O.i« 0.88 0.88 0.44 0.54 1.2 2.2 2.1 1.60 1.00 1.10 1.10 0.76 2.2 1.9 0.8 0.92 MnO 0.15 0.05 0.06 0.02 0.14 0.03 0.04 0.02 0.05 0.03 0.05 0.08 0.0 14 0.12 0.01 1 0.07 MgO 0.113 1.2 0.49 0.00 0.21 0.54 1.3 0.99 0.50 0.14 0.55 0.43 0.65 0.98 0.96 0.35 5 0.30 CaO l/' 1.6 0.98 0.66 1.1 1.2 0.85 3.1 2.77 1.72 0.34 3.12 1.40 3.6 3.5 2.9 2.19 5.05 3.7 Na20 5.\ 4.6 5.6 6.21 5.6 5.7 5.5 3.1 3.83 4.45 3.53 5.2 4.2 3.8 4.53 KjO 1.. 1.2 0.31 0.26 0.82 0.20 0.03 1.2 1.22 1.28 0.98 1.26 0.25 0.35 1.0 1.4 1.01 H,0+ 0.!W 0.58 0.56 0.22 0.58 0.57 1.2 1.4 0.28 0.33 0.46 0.35 0.53 0.57 0.19 0.39 3 0.37 0.49 0.16 0.00 0.08 0.05 0.07 0.04 0.04 0.10 0.05 0.03 0.01 0.01 0.08 0.14 0.13 2 0.07 0.03 0.08 0.D8 0.09 0.07 0.04 0.03 0.05 0.05 0.05 0.02 0.03 0.01 0.06 0.21 0.09 0.01 0.05 5 0.04 C02 0.01 0.03 0.01 0.0 0.06 0.23 0.24 0.86 0.02 0.06 0.01 0.01 0.03 0.02 0.01 100 Total 99 100 100 100 101 101 100 101 101 100 100 100 100 101 100 100 FeO' 0.75 0.52 0.77 1.0 0.87 0.85 0.70 0.78 0.87 0.92 0.74 0.85 0.68 0.77 0.84 0.84 FeO* + MgO Trace elements (ppm)

Rb 32 53 7 <2 20 3 <2 29 22 16 22 41 2 9 34 46 21 98 Sr 174 146 51 38 90 98 103 120 119 97 39 101 112 159 121 88 205 96 Ba 763 225 107 70 404 69 30 326 340 676 270 290 26 99 236 282 95 216 Th 3.7 8.0 ND 11.6 10.9 3.5 ND ND 3.3 5.6 4.7 3.3 ND 3.6 ND 4.0 4.7 6.0 Cr 1 ND 3 ND ND 2 2 2 1 3 ND 2 Zr 174 135 ND 240 66 121 148 62 100 180 150 100 ND 56 80 110 Hf 51 3.3 ND 7.3 2.6 4.1 ND ND 2.8 5.0 4.2 3.0 ND 2.5 ND 2.6 2.5 3.1 Nb ND ND ND 10 <5 <5 <5 ND 10 10 ND ND ND ND ND ND 16 Ta 0.3 0.42 ND 0.63 0.76 0.56 ND ND 0.22 0.37 0.31 0.20 ND 0.34 ND Y ND ND ND 70 26 40 45 23 ND 70 43 ND ND ND 18 ND ND 40 La ND 26.4 ND ND 26.6 ND 15.4 10.0 ND ND 11.8 ND 15.2 9.9 12.6 18.8 ND ND 0.315 Ce ND 46.5 ND ND 48.7 ND 31.8 19.2 ND ND 27.2 ND 32.1 20.5 23.6 37.2 ND ND 0.813 Nd ND 20. ND ND 22. ND 19. ND ND 14.6 ND 17. 20. ND ND 0.597 Sm ND 4.3 ND ND 4.7 ND 4.83 2.6 ND ND 4.00 ND 4.1 2.79 2.8 4.20 ND ND 0.192 Eu ND 0.73 ND ND 0.421 ND 1.03 0.50 ND ND 0.392 ND 0.52 0.57 0.69 0.71 ND ND 0.0722 Gd ND 4.2 ND ND 4.3 ND 5.3 2.8 ND ND 4.4 ND 4.0 3.1 3.3 ND ND 0.259 Tb ND 0.66 ND ND 0.64 ND 0.90 0.45 ND ND 0.86 ND 0.78 0.63 0.54 0.66 ND ND 0.047 Tm NE 0.48 ND ND 0.33 ND 0.64 0.32 ND ND 0.54 ND 0.53 0.42 0.37 0.41 ND ND 0.030 Yb NE' 3.20 ND ND 2.74 ND 4.81 2.23 ND ND 5.22 ND 3.8 2.91 2.89 3.20 ND ND 0.208 Lu NE' 0.539 ND ND 0.404 ND 0.721 0.393 ND ND 0.768 ND 0.57 0.483 0.470 0.53 ND ND 0.0323 K/Rb 28:: 188 368 1079 340 553 124 344 460 664 370 255 1038 323 244 253 269 85.6 K/Ba ,I:LO 44.2 24.1 30.8 16.8 24.1 8.30 30.6 29.8 15.7 30.1 36.1 79.8 29.3 35.2 41.2 59.4 38.8 Rb/Sr .18 0.36 0.14 0.05 0.22 0.03 0.02 0.24 0.18 .17 0.56 0.41 .02 0.06 .28 0.52 0.10 1.02 Eu/Eu" 0.52 0.28 0.63 0.30 0.29 0.47 0.60 0.57 (Ce/Yb)„ 3.72 4.54 1.69 2.20 1.33 2.16 1.80 2.09 2.97

tvalues for Tb and Tm from Haskin and others, 1968; remainder from Sun and Hanson, 1975. Note: ND, not determined; • -, determination inconclusive or inapplicable. FeO* = total iron as FeO. cn = chondrite-normalized. Locations and sample descriptions in appendix.

TABLE 2. MAJOR- AND TRACE-ELEMENT CHEMISTRY OF SELECTED SAMPLES OF AMMONOOSUC AND SUB-AMMONOOSUC QUARTZ KERATOPHYRE AND AMMONOOSUC AMPHIBOLITE

Chondrite normalizing Anal. no. 19 20 21 22 23 24 25 26 27 28 29 30 31 values^

Sub-Ammonoosuc Ammonoosuc quartz keratophyre quartz keratophyre Ammonoosuc amphibolite

Major elements (percent)

Si02 75.2 78.4 78.8 77.0 75.0 75.4 77.3 77.2 48.8 52.9 49.3 48.6 49.1 Ti02 0.25 0.12 0.16 0.21 0.16 0.29 0.12 0.17 1.8 0.87 1.1 2.0 0.87 AI2O3 11.5 11.9 10.6 12.5 10.5 12.9 11.6 12.8 16.0 17.3 16.7 15.5 16.3 Fej03 2.0 0.46 0.86 0.88 0.54 1.4 1.3 0.97 1.9 1.7 3.4 4.0 FeO 1.5 1.2 0.88 0.96 2.5 0.92 1.0 0.72 6.8 7.9 8.2 7.1 MnO 0.01 0.06 0.08 0.06 0.02 0.04 0.05 0.03 0.17 0.08 0.15 0.25 0.13 MgO 0.79 0.53 1.3 0.65 2.2 0.42 0.76 0.21 6.6 8.0 7.2 7.7 CaO 1.6 0.54 0.72 1.6 2.3 2.8 1.1 2.7 11.0 5.9 8.8 11.4 8.6 NTJO 5.5 5.00 3.6 5.2 4.1 4.4 5.3 4.3 3.9 3.4 2.0 4.4 KJO 0.12 0.99 1.6 0.31 0.05 0.55 0.10 0.19 1.9 0.86 0.53 0.03

H2O 0.45 0.48 0.88 0.52 0.54 0.40 0.60 0.34 0.66 1.7 0.97 0.48 0.64 HJO- 0.11 0.03 0.06 0.06 0.05 0.09 0.05 0.07 0.05 0.09 0.05 0.00 0.01

P205 0.05 0.03 0.05 0.07 0.07 0.12 0.05 0.04 0.23 0.15 0.17 0.36 0.13 co2 0.01 0.01 0.01 0.08 0.01 0.00 0.02 0.08 0.01 0.01 0.00 0.01 0.01 Total 99 100 100 100 99 100 99 100 99 100 99 100 99 FeO* 0.81 0.75 0.56 0.73 0.33 0.84 0.46 0.89 0.58 0.56 0.54 0.61 0.58 FeO* + MgO Tract elements (ppm)

Rb 4 39 32 11 3 12 <2 6 52 34 42 9 <2 Sr 81 43 64 93 214 263 98 181 222 221 206 174 241 Ba 48 359 794 245 86 281 28 232 83 282 592 89 38 Th 4.5 8.1 5.9 4.0 5.5 4.4 3.4 3.8 .8 2.2 1.4 .4 1.1 Cr 1 1.0 2.0 1.0 241 90 257 212 — Zr 180 5.5 133 59 71 — 38 Hf 4.4 4.3 3.7 3.2 5.2 3.6 6.5 3.7 3.3 1.3 2.0 3.6 1.2 Nb ND 12 ND ND ND ND ND ND <5 <5 <5 ND <5 Ta .65 .58 .47 .21 .50 .26 .23 .31 .44 .29 .09 .21 .06 Y 60 34 13 21 — 16 U 16.6 28.9 ND 13.8 18.7 11.6 10.5 ND ND ND ND ND ND 0.315 Ce 35.9 56.8 ND 25.6 40.5 22.1 22.5 ND 23.4 21.1 18.7 22.0 14.2 0.813 Nd 22.1 26.8 ND 14.6 26.3 16. 18. ND 17.9 11.1 12.2 18.7 8.42 0.597 Sm 5.65 6.20 ND 3.13 6.67 3.7 5.3 ND 5.10 2.55 3.26 5.69 2.31 0.192 Eu 1.22 0.758 ND 0.573 1.016 0.78 1.31 ND 1.78 0.835 1.10 2.04 0.744 0.0722 Gd 7.3 7.5 ND 3.8 7.8 4.3 7.6 ND 2.60 3.71 7.31 2.62 0.259 Tb 1.05 1.08 ND 0.52 1.20 0.77 1.56 ND ND ND ND ND ND 0.047 Dy ND ND ND ND ND ND ND ND 6.95 2.61 3.92 7.80 2.63 0.325 Er ND ND ND ND ND ND ND ND 4.19 1.59 2.41 4.67 1.55 0.213 Tm 0.63 0.49 ND 0.494 0.61 0.56 1.16 ND ND ND ND ND ND 0.030 Yb 4.75 4.06 ND 2.33 5.27 4.1 9.46 ND 3.82 1.49 2.24 4.15 1.40 0.208 Lu 0.755 0.661 ND 0.402 0.771 0.61 1.29 ND ND ND ND ND ND 0.0323 K/Rb 250 210 416 234 138 380 416 262 175 509 170 489 125 K/Ba 20.8 22.8 16.7 10.5 4.8 16.2 29.6 6.80 110 55.9 12.1 49.4 6.5 Rb/Sr 0.05 0.91 0.50 0.12 0.01 0.46 0.02 0.03 0.22 0.14 0.20 0.05 0.01 Eu/Eu* 0.59 0.34 0.51 0.41 0.60 0.64 0.99 0.98 0.98 0.93 (Ce/Yb)^ 1.93 3.58 2.81 1.97 1.38 0.61 1.57 3.62 2.14 1.36 2.59

^Values for Tb and Tm from Haskin and others, 1968; remainder from Sun and Hanson, 1975. Sole: data are in ppm. Comments in Table 1 apply to this table also.

TABLE 3. COMPARISON BETWEEN AMMONOOSUC TRONDHJEMITE scopy analyses were done by H. J. Rose, Jr., P. Hearn, J. Lindsay, B. AND TRONDHJEMITE AS DEFINED BY BARKER (1979b) McCall, S. Wango, and R. Johnson, all of the U.S. Geological Survey. Th,

Cr, Hf, Ta, and rare-earth elements (REE) were determined (but not for 19 Ammonoosuc-related Low-alumina trondhjemites trondhjemite (Barker, 1979b) amphibolites; Table 2, no. 27-31) by instrumental neutron activation by (percent) (percent) C. A. Palmer, G. A. Wandless, P. A. Baedecker, and L. J. Schwarz of the

U.S. Geological Survey (epithermal INA for mafic samples; Baedecker Si02 73.1-80.6 >68, usually <75 AI2O3 11.3-13.5 < 14 (S> 75% Si02 and others, 1977). REE for amphibolite samples were determined by FeO' + MgO 0.44-4.44, avg 2.7 <3.4 FeO*/MgO 2.1-13.3, avg 5.6 2-3 isotope-dilution mass spectrometry by L. Peter Gromet (Brown CaO 0.34-3.6, avg 1.9 1.5-3.0 University). Na20 3.1-6.2, avg 4.7 4.0-5.5 K2O 0.03-1.4, avg 0.77 -2.5, typically <2

Analytical Results dum between 0.5% and 3.5% in about half of the analyses), and alkalic Major Elements. The trondhjemites have variable, but generally rocks are lacking (Leo, unpub. data). This is a generally accepted criterion

high, Si02 (73.1% to 80.6%), low A1203 (11.3% to 13.5%), and in several of an orogenic setting (Martin and Piwinskii, 1972). other respects correspond to low-Al trondhjemite as redefined by Barker Harker diagrams for six major constituents of the trondhjemites, no-

(1979b) (Table 3). Indeed, the Si02 content of these rocks is somewhat tably Na20, CaO, FeO*, and KzO (Fig. 7) show significant scatter, which higher than in most published trondhjemite analyses. Overall, they resem- appears to preclude a single magma source.4 A degree of linearity, perhaps ble several Precambrian and early Paleozoic trondhjemites along the Ap- indicative of local differentiation, is apparent for several groups of samples, palachians and elsewhere that have been interpreted as representing island-arc environments, and they also bear some resemblance to trond- 4Thirty-two analyses of the allegedly homogeneous Twillingate trondhjemite hjemites in some modern island arcs, as discussed below. Alumina satura- in north-central Newfoundland (Payne and Strong, 1979) show comparable scatter tion varies between metaluminous and peraluminous (normative corun- in the same constituents.

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FeO + O.9 Fe203 1 1 1' 1 1 1 1 1 1 1 1

AI2O3

• • • A A A • °A •

• • * A

I i 1 1 1 I 1 1 1 1 1

FeO* A • • • »0 A — A -

A 1 1 1 1 1 I 1 1 1 1 1

MgO • • - V - •A OA • «0 1 1 1 T 1 IA 1 A 1 1

• Na20 + K20 MgO CaO - A A° - •A A Figure 8. AMF diagram for Ammonoosuc trondhjemite undiffer- A entiated (•). Fields for felsic and mafic Ammonoosuc Voicanics (Leo, • • A • • unpub. data) shown for reference (numbers of analyses in - • • * A parentheses). 1 1 1 1 1 1 1 ? L.'VL 1 • • • the spectrum, an indication of metasomatic alteration according i:o Hughes Na20 • «6 (1972). It is probably significant that nearly all of the quartz keratophyre A •A • • • points plot outside the "spectrum," and about one-third of the trondhjem- A * O ites and one-half of the amphibolites plot within it (supposedly indicative A - - of primary compositions). If Hughes's criteria are accepted, this implies a pervasive and preferential loss of alkalies, notably K, from the tuffaceous Ammonoosuc, and their preferential retention in trondhjemites (compare K2O A* • intrusive and extrusive keratophyres from the Virgin Islands; Fig;. 10) and • A j> A amphibolites. The situation is complicated in the following ways. (1) The 1 1 t 1 • 1 P •i 1*1 ! 70 75 80 Ammonoosuc suite, at least southeast of the Ammonoosuc fault, is all

Si02 WEIGHT PERCENT metamorphosed at amphibolite-facies grade, thus any former spOitic alteration is not petrographically evident; and (2) there appears to be no Figure 7. Harker diagrams for trondhjem- consistent basis on which to rationalize differences between altered and ites associated with Ammonoosuc Voicanics. • "primary" trondhjemites (they are petrographically similar). Ths Hughes = Berlin, New Hampshire, area; • = West plot, moreover, does not accommodate compositional fields for primary Lebanon, New Hampshire-White River Junc- trondhjemites and high-Si dacites (Hughes, 1972, Fig. 4), because it as- tion, Vermont; O = Unity dome; • = Alstead sumes, a priori, that most such rocks are altered, which is not necessarily dome; • = remaining samples (see Table 1). true. It should be noted that, inasmuch as the alteration considered here FeO* = total iron as FeO. involves only K and Na, over-all bulk compositions of nominally primary and nominally metasomatised rocks may not differ greatly from each as from the Alstead dome and, to a lesser extent, for the White River other. Junction and Berlin samples. There are, however, no consistent patterns Trace Elements Other than Rare Earths. Rb, Sr, and Ba show for the region as a whole, which suggests a lack of homogeneity of source varying and unsystematic abundances, although concentrations of both Rb materials in different areas, notably in CaO. This is further shown by and Sr are relatively low (average for 19 samples, 25 ppm and 109 ppm, trace-element abundances (see Figs. 11 and 12 below). respectively). Elemental ratios (Table 1) are accordingly unsystematic, and An AMF diagram (Fig. 8) confirms the general similarity between plots of abundances show much scatter. The scatter on the Rb-Sr diagram trondhjemites and Ammonoosuc quartz keratophyre. The overall Ammo- (Fig. 11) compares with that of Ammonoosuc Voicanics as a whole, and noosuc trend (Leo, unpub. data) is calc-alkaline, lacking the conspicuous with Rb and Sr abundances in suites of younger trondhjemite!; and It- iron enrichment seen in some tholeiitic suites. A normative An-Ab-Or poor dacites, as well as with abundances in the Twillingate trondhjemite diagram (Fig. 9) confirms the similar chemistry of all felsic Ammonoosuc (Payne and Strong, 1979) and Little Port Complex (Malpas, 1979; the of the region (trondhjemite and quartz keratophyre) and also emphasizes latter are hereafter referred to collectively as "Newfoundland trondhjem- the compositional gap between Ammonoosuc Voicanics and the Oliverian ites"). The scatter in the large-ion lithophile (LIL) elements is likely to be granitic gneiss. related to post-intrusion alteration and/or metamorphism (compare A measure of the mobility of alkalies in these rocks can be gained Fig. 10), or even to late-magmatic processes. from Figure 10, which shows the "igneous spectrum" of Hughes (1972). Relatively immobile elements such as Th, Zr, Hf, and Ta (Fig. 12) More than half of the plotted Ammonoosuc points fall off the left side of show more consistent abundances, as well as moderately linear variation

Figure 9. Ab-An-Or normative diagram with the classification of O'Connor (1965), modified by Barker (1979b). A: Ammonoosuc-related trondhjemite (•). Shown for comparison are fields of felsic and mafic Ammonoosuc Volcanics and Oliverian granitic plutons (Leo, unpub. data), as well as Twillingate trondhjemite (Payne and Strong, 1979) and Little Port Complex (Malpas, 1979). B: Fields for trondhjemite (shown by •) and low-K dacite from Fiji (GUI and Stork, 1979), and for low-K dacite from Tonga-Kermadec (Bryan, 1979), shaded; B = Saipan dacite (Meijer, 1983; Barker and others, 1976).

Figure 10. KzO + Na20 versus K20/(K20 + Na20) x 100. Curves define the "igneous spectrum" of Hughes (1972). • = Ammonoosuc trondhjemites; O = Ammonoosuc quartz keratophyre; • = Ammo- noosuc amphibolites (data from Table 2 and Leo, unpub. data); • , a , and • are, respectively, averages of (moderately altered) mafic, intermediate, and silicic volcanic rocks and associated plutonic rocks from Fiji (Gill, 1970; Gill and Stork, 1979); B = Saipan dacite (Barker and others, 1976); x = intrusive keratophyres, ® = extrusive keratophyres, Virgin Islands (Donnelly, 1966, cited by Hughes, 1972). See text for discussion.

both major and rare-earth elements (Figs. 8 and 14). The single trondhjemite sample falling out of the convergent-plate mrgins field cannot be reasonably accounted for. Four samples of mafic Ammonoosuc straddle the boundary between the fields of primitive arc tholeiites and N-type MORB. Three of the samples, two pillow basalts and one feeder dike, were collected in the Plainfield-Norwich area (Fig. 1) (Aleinikoff, 1977), and some of Aleini- kofPs (1977) samples from this area were regarded by him, on the basis of other minor- and trace-element characteristics, to be of abyssal origin. A x 100 more detailed evaluatin of the possible significance of these samples is (K20 + Na20) beyond the scope of this paper. Rare-Earth Elements. Rare-earth-element (REE) patterns for se-

when plotted against Si02; however, little linear variation is evident within lected samples of trondhjemite, Ammonoosuc quartz keratophyre, sub- subgroups as in Figure 7. On a Ta-Th-Hf/3 plot (Fig. 13), designed to Ammonoosuc quartz keratophyre (Naylor, 1969), and mafic Ammonoo- discriminate between tectonic settings for both felsic and mafic rocks suc are shown in Figure 14. The REE data on rocks other than (Wood and others, 1979; Wood, 1980), most data points cluster in the trondhjemite are given to provide a basis for comparison and discussion. field of convergent plate margins with calc-alkaline affinity. To the extent All of the patterns show moderate enrichment in LREE. Such enrichment that the diagram is valid, this agrees with the calc-alkaline trends shown by is characteristic of calc-alkaline island-arc volcanics, but it is also observed

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70 75 80 1 1 1 1 1 1 1 1 1 1 1 1 Th (ppm)

A • - • A A • O 1 A 1 1 1 1 1 1 1 1 1 1 1 Zr (ppm)

• A • • O • - • OA

»A •

1 1 1 1 1 1 1 1 1 1 1 1

— Hf (ppm) • A • 4 0 AAAO A • A • T , 1 1 1 1 1 1 1 1 1 1 Ta (ppm) •

A • • 0 A t O A

0 1I I1 1I 1I I1 1I I 1I 1I L1- 1 I 70 75 80

S¡02 (WEIGHT PERCENT)

Figure 12. Plot of Th, Zr, Hf, and Ta against Si02, Figure 11.. Plot of Rb against Sr. • = Ammonoosuc trondhjem- Ammonoosuc trondhjemite. Symbols as in Figure 7. ite. Fields shown for comparison include calc-alkaline Oliverian gneisses (Leo, unpub. data), Ammonoosuc quartz keratophyre (Table 2), Newfoundland trondhjemites, Tonga, and Fiji. B = Saipan dacite. Hf/3

in some tholeiitic rocks (for example, see Basaltic Volcanism Study Pro- ject, p. 202, Fig. 1.2.7.10; Masuda and others, 1975), orogenic andesites (Gill, 1981, Fig. 5.12b), and trondhjemites (Phelps, 1978, Fig. 4D) with medium to high K contents. Corresponding rocks with low K contents, by contrast, have; REE patterns that are typically flat or have depleted LREE, and also have lower over-all REE abundances (Basaltic Volcanism Study Project, p. 202, Fig. 1.2.7.9; Gill, 1981, Fig. 5.12a; Masuda and others, 1975; Phelps. 1978; Barker and others, 1978). The Ammonoosuc Volcan- ics combine generally low K abundances (Tables 1 and 2) with a calc- alkaline trend, as confirmed by other criteria discussed herein. Trondhjemite. The patterns for trondhjemite (Fig. 14A) show moder- 5 ate REE fractionation (Ce/Ybcn = 1.33-4.54), with Ce abundances of approximately 25 to 60 x chondrites; moderate to pronounced negative Eu anomalies (Eu/Eu* = 0.28 to 0.63); and relatively flat HREE. These patterns are generally comparable to those of I0W-AI2O3 trondhjemites (Barker and others, 1976) and thus are in accord with the major-element

Figure 13. Plot of Th-Ta-Hf/3. After Wood (1980). See text for 5 cn = chondrite-normalized; Eu* = (Sm + Gd)cn/2. discussion.

100 100 r- Ammonoosuc quartz keratophyre - Sub-Ammonoosuc quartz keratophyre

•o c o .c O ü u O QC

La Ce Sm Eu Gd Tb Tm Yb Lu La Ce Sm Eu Gd Tb Tm Yb Lu

100

Figure 14. Rare-earth element (REE) plots of trondhjemite and layered Ammonoosuc Volcanics. A and B, trondhjemites; symbols as in Figure 7; WRJ = White River Junction (VT). C, Ammonoosuc quartz keratophyre; D, sub-Ammonoosuc quartz keratophyre; E, Ammonoosuc amphibolites. Numbers on patterns correspond to analysis numbers in Tables 1 and 2.

chemistry of these rocks (Tables 1, 3). The shape of the trondhjemite trondhjemites. LREE fractionation (Ce/Ybc = 1.93-3.58) is slightly less patterns, with the exception of sample 11, generally suggests a residue than in the trondhjemite patterns. The similarity between the quartz kera- containing plagioclase and pyroxene. tophyre and trondhjemite patterns indicates a minimum of differentiation, Ammonoosuc Quartz Keratophyre. Patterns for 4 samples (Fig. 14C) or of winnowing of heavy accessory minerals during the process of erup- are generally similar to trondhjemite patterns (shaded field) but have tion and deposition of the tuff, and constitutes a major basis for regarding slightly higher abundances of LREE (Ceo, = 30-70 x chondrites) and the quartz keratophyre composition as being close to primary. slightly less pronounced Eu anomalies (Eu/Eu*,;,, = 0.34 - 0.59) than the Sub-Ammonoosuc Quartz Keratophyre Tuff. Patterns for two sam-

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pies (Fig. 14D) differ unexpectedly from each other. No. 25 is comparable production of both high-Al203 and low-Al203 trondhjemite (Barker and to most of the patterns discussed so far (Ce/Yb^, = 1.38, Eu/Eu* = 0.60), Arth, 1976); however, granophyric, relatively low-K rocks formed by whereas no. 24 shows significantly and progressively higher HREE abun- fractional crystallization are typically much more iron-enriched than is the

dances relative to no. 25. The negative slope of the pattern (Ce/Ybcn = Ammonoosuc trondhjemite, and they also contain ~ 10% less Si02 (Pres- 0.61), together with the relative depletion of Ce, suggests a more marked nall, 1979). Furthermore, trondhjemites derived by fractional crystalliza- effect of a clinopyroxene residue than is seen in the other silicic rocks. tion are likely to be part of a differentiation suite comprising hornb'endite Ammonoosuc Amphibolite. Patterns of five Ammonoosuc amphibo- and hornblende-bearing gabbro, diorite, and tonalite (Barker and Arth, lites (Fig. 14E) show moderately fractionated LREE and HREE 1976). By contrast, trondhjemites derived by partial melting of basaltic (Ce/Ybcn = 1.36 i:o 3.62) and negligible Eu anomalies (Eu/Eu* = 0.93 to source rocks ordinarily constitute bimodal suites with mafic rocks, inter- 0.99). Compositions of these amphibolites are tholeiitic (slightly to mediate compositions being scarce or absent. Given the essentially bimo- strongly hypersthene-normative; Leo, unpub. data). The REE patterns dal character of the Ammonoosuc Volcanics, the partial-melting resemble some published patterns for island-arc tholeiites as discussed hypothesis thus seems more applicable in the present instance. above, but they are generally more similar to patterns of calc-alkaline, Several laboratory studies confirm the feasibility of producing trond- island-arc basalts (JakeS and Gill, 1970; Arth, 1981). The Ammonoosuc hjemites by partial melting of basalt. Holloway and Burnham (1972) and patterns, however, are distinct from those of ocean-ridge basalts, which Helz (1976) produced trondhjemitic liquid by partial melting (~65o-18% have lower over-ill REE abundances and typically show moderately to glass) of olivine basalt in the temperature range of 700-875 °C at P^ = 5 AN strongly depleted light REE (Frey and others, 1974; Kay and Hubbard, kbar and under various conditions of PH2O> D Kushiro and others

1978; Sun and others, 1979). (1972) produced a K20-poor dacitic melt (20% glass) from spinel lherzo- lite at much higher P-T (26 kbar and 1190 °C). Discussion The relatively high proportion of felsic Ammonoosuc (estimated range, 20%-40% of the total section) is excessive compared to the proportion The various hypotheses regarding the origin of trondhjemites have of dacitic glass, in the approximate range of 6%-20% (typically < 10%), been recently discussed at length (Barker, 1979a). A good summary of the produced by experimental melting. This discrepancy indicates that asso- principal modes of origin is given by Payne and Strong (1979, ciated felsic and mafic rocks at the present erosion level are not at, or near, p. 504-507). They include (1) anatexis of K-poor sediments such as their source; rather, the siliceous melt phase appears to have been preferen- graywackes; (2) Na-metasomatism of pre-existing granitic rocks, usually tially separated and to have migrated upward before being erupted or by sea water; (3) fractional crystallization of basaltic magma; and (4) emplaced, in association with a relatively smaller volume of basalt that partial melting of eclogite or amphibolite. could have been generated at a higher level. Comparable volumetric dis- The first of these alternatives implies the presence of epiclastic- parities are found elsewhere along the Appalachians (see references cited volcaniclastic rocks underlying the Ammonoosuc that could have pro- in Introduction, also Seiders, 1978), as well as in Fiji, the most nearly vided a source for anatectic generation of trondhjemite. Although such an analogous modern island arc, where dacites and trondhjemites of Miocene origin has been proposed for the trondhjemitic northern part of the Glas- age constitute about one-half of the basement (Gill and Stork, 1979). tonbury Gneiss in Massachusetts and Connecticut (Leo and others, 1984), The alternate possibility must be considered: that the felsic Ammo- it probably is not relevant to the trondhjemite of western New Hampshire. noosuc may have had a different, perhaps granitic, crustal source but lost Northwest of the Oliverian domes, the Ammonoosuc Volcanics are under- most of its potassium during anatexis and/or subsequent metamcrphism. lain by the Ordovician Albee Formation, which consists of quartzite and The latter alternative, however, appears unlikely for two reasons. First, the slate northwest of the Ammonoosuc fault, and equivalent rocks of higher Oliverian plutons, temporally and spatially associated with Ammonoosuc metamorphic grade (quartzite and semipelitic to pelitic schists) on the Volcanics and presumably formed by anatexis of sialic crust, have normal

southeast side of the fault (Billings, 1937, p. 472-475). Quartzofeldspathic calc-alkaline K20 contents (avg -4.5%; Leo, unpub. data). Second, Fo- rocks likely to be affected by anatexis at geologically reasonable tempera- land and Loiselle (1981) postulated an eclogitic source to produce syenite

tures are scarce in the Albee; in any case, the regional metamorphic grade (range of K20 = 5.7% to 7.9%) in the central part of the Jefferson dome is well below that associated with anatectic melting. The same is true of the (Fig. 1). If Foland and Loiselle are right, it is unlikely that rocks "stratified core gneiss" (Naylor, 1969) that underlies Ammonoosuc Vol- conspicuously low in potassium due to its selective removal and migration canics in a few of the domes, which retains primary structures, granular would have formed in the same temporal and spatial environment in textures, and is fir from an anatectic condition. which potassium was being selectively incorporated in nearby syenites. The second alternative, Na-metasomatism of pre-existing granite, can The evidence for some loss of K20, preferentially, from quartz keratophyre be ruled out for the Ammonoosuc trondhjemite because (1) trondhjemite (Fig. 10) does not invalidate the above argument. usually differs markedly from Oliverian granite in its style of emplacement; An attempt was made (by J. G. Arth, U.S. Geological Survey) to test and (2) with few exceptions, granite and trondhjemite are texturally, com- the partial-melting model for trace-element behavior on severe.1 trond- positionally, and spatially distinct and show no evidence of mutual intru- hjemites from the Alstead dome (Table 1, analyses 13-15), on the assump- sion or gradation, even where trondhjemite constitutes the plutonic interior tion that associated amphibolite (Table 2, analysis 31) is representative of of a dome. the parent rock at depth. The results of the modeling were somewhat inconclusive. Two cases were considered: (1) the parent rock is amphibo- Partial Melting versus Fractional Crystallization lite melting to dacitic liquid with a residue of hornblende and plagioclase and (2) the parent is basalt (based on the norm of the same amphibolite) The remaining mechanisms here considered for trondhjemite genesis melting to dacitic liquid with a residue of plagioclase and clinopyroxene. are fractional crystallization of a basaltic liquid and partial melting of The first model resulted in patterns (not shown) which were strongly basaltic source rocks. Either mechanism can hypothetically lead to the depleted in heavy REE and which do not correspond to the observed

trondhjemite patterns (Fig. 14B). The presence of a significant hornblende anticlinorium falls within the Dunnage or Gander zones of Williams residue therefore can be ruled out. The second model resulted in REE (1978) and is structurally on strike with the Twillingate area. The BHA abundances that were mostly higher than observed ones, requiring exces- has long been regarded as an island arc (Hitchcock, 1883; Kay, 1951; sive melting (>20%) to produce REE abundances approximating those of Berry, 1968) over an east-dipping subduction zone (Osberg, 1978; Robin- the Alstead dome trondhjemite. Nevertheless, the discrepancy between son and Hall, 1980) active in Cambrian-Ordovician time during the clos- model and observed REE abundances was less in the second case than in ing of Iapetus. An accumulating body of chemical data on the the first. In view of the uncertainties in the choice of modeling parameters, Ammonoosuc Volcanics and Oliverian gneissic plutons (Leo, this paper including (1) the lack of specific knowledge of the relation between parent and unpub. data; Schumacher, 1983) is providing a basis for the island rock and melting product and (2) the variations in abundances of REE and arc-convergent-plate margin model briefly described earlier. The point of LIL elements in different samples quite likely related to posteruption meta- major concern here is a conjectural tectonic framework which accounts for somatism and metamorphism, it is safe to conclude only that the parent the depth required at which the felsic Ammonoosuc phase could be gener- rock for the felsic Ammonoosuc phases was not amphibolite but, instead, ated, whether by partial melting or by fractional crystallization. The max- was probably anhydrous basalt containing pyroxene ± olivine and little, or imum thickness of the Ammonoosuc in western New Hampshire has been no, hornblende. Such basalt presumably constituted a large proportion of estimated at < 1 km (Billings, 1937, p. 480); this is too thin, by at least an the primary Ammonoosuc pile and was subsequently metamorphosed to order of magnitude, to account for posteruptive anatexis near its base. In amphibolite during the Acadian orogeny. this regard, the analogy with the Twillingate trondhjemite generated at an A conjectural sequence of Ammonoosuc volcanism, taking the estimated depth of 12 km (~ 4 kbar) is inapplicable. Genesis of the Am- known data into account, is as follows. monoosuc assemblage more probably occurred along the subduction zone 1. (Middle Ordovician) Local and explosive? eruptions of postulated in current plate-tectonic models of the region, or in the overly- sub-Ammonoosuc quartz keratophyre tuff ("stratified core ing mantle wedge. gneiss") derived either by fractional crystallization or by partial An island-arc origin for the Ammonoosuc Volcanics, however, is melting of an anhydrous basaltic source. subject to certain restraints imposed by the unusual lithology and geo- 2. Eruption of basalt, accompanied by emplacement of dikes and chemistry of these rocks: (1) the bimodal assemblage, with a dispropor- plugs of trondhjemite derived from the same mafic source. tionately high percentage of felsic rocks, discussed above; (2) the 3. A gradual transition to felsic volcanism resulting in deposition of combination of low K with relative enrichment in incompatible elements interlayered basaltic tuffs and flows and quartz keratophyre tuff (Ba, La, Zr, Th, and, to a lesser extent, Ta; see Tables 1 and 2); (3) the and associated coarser pyroclastic rocks (Ammonoosuc moderately fractionated REE patterns (compare Gill and Stork, 1979, Volcanics). Fig. 5); and (4) the generally calc-alkaline trends. This combination of 4. (Early to Middle Devonian) Regional deformation and metamor- features has no close analogue in modern island arcs, whether tholeiitic phism related to the Acadian orogeny which, among other effects, (oceanic) or calc-alkaline (continental margin). Perhaps the closest ana- converted primary Ammonoosuc basalt to amphibolite. The logue is the Tertiary volcanic-plutonic assemblage of Fiji (see Fig. 9B) in hydrous character of the Acadian metamorphism throughout which the relative proportions of felsic and mafic rocks are comparable to much of the Bronson Hill anticlinorium is apparent in the large the Ammonoosuc Volcanics; however, the chemistry of the Fijian rocks is and seemingly random variations in LIL elements in all of the more consistently tholeiitic than that of the Ammonoosuc (Gill and Stork, Ammonoosuc (Tables 1 and 2). 1979). Moreover, the SiC>2 content of even the most siliceous rocks does The Newfoundland trondhjemites of Late Cambrian age show some not exceed -76%, whereas about half of the Ammonoosuc trondhjemite features very similar to those of the Ammonoosuc trondhjemite: (1) and quartz keratophyre samples have SiC>2 contents between 76% and 80% contemporaneous and syntectonic intrusion of metabasalt dikes and (Tables 1 and 2). As suggested earlier, this may be due to differential trondhjemite; (2) bulk compositions of trondhjemites with markedly high, alteration of the Ammonoosuc. Rocks of the Tonga-Kermadec arc (Bryan although variable, SiC>2 and low AI2O3; and (3) generally similar and others, 1972; Ewart and Bryan, 1972; Ewart and others, 1973; Bryan, trace-element abundances (REE data not presented), although the abun- 1979) have been invoked as a comparison to the Ammonoosuc (Schu- dances vary less in the Newfoundland rocks than in the Ammonoosuc macher, 1983), but, in fact, the dacites of this bimodal assemblage show assemblage. Malpas (1979, p. 471-472) and Payne and Strong (1979, p. only limited compositional overlap with the felsic Ammonoosuc rocks 491), moreover, described the development of veinlets and networks of (Fig. 9B), are less siliceous (60%-69% Si02) and are higher in Fe and Ca trondhjemite from amphibolite. Amphibolitization of primary tholeiite than is the Ammonoosuc; they also differ in other respects. The Tonga- with concurrent production of trondhjemite by partial melting is attributed Kermadec suite as a whole corresponds to island-arc tholeiite. Another to Taconic regional metamorphism. Payne and Strong (1979) estimated limited analogue to the Ammonoosuc is the Marianas arc, particularly the depth of anatexis to be -12 km. In the absence of REE data, no Saipan, where andesite is associated with highly siliceous dacite (Schmidt, confirming relationship between assumed source and melt can be estab- 1957; Barker and others, 1976; Meijer, 1983), but these rocks also show lished by modeling. the typical island-arc tholeiite characteristics. The dacite, moreover, may not be related to the andesite in any simple way (Meijer, 1983). Evidence of anatexis comparable to that in the Newfoundland localities was not found in the Ammonoosuc assemblage, quite probably be- The foregoing discussion shows that the Ammonoosuc is not compa- cause the exposed level of erosion is not deep enough. The similarities, in tible with Ringwood's (1974) early phase of island-arc development other respects, to Newfoundland trondhjemites suggest a similar pedo- resulting in the dominantly basaltic island-arc tholeiitic suite with low genesis in all three areas. Payne and Strong (1979) regarded the volcanics abundances of incompatible elements and unfractionated REE patterns. of central Newfoundland as an island-arc sequence built upon oceanic The generally calc-alkaline character of the Ammonoosuc is in better crust, and the Twillingate trondhjemite pluton as representing the base of accord with more evolved arcs showing calc-alkaline chemistry (Ring- the volcanic pile close to the transition to the crust. The Bronson Hill wood, 1974), such as the Indonesian or Andean continental-margin arcs.

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The major difficulty with such analogies is that the latter arcs are domi- account for the generation and rise of the Ammonoosuc Volcanics assem- nated by andesite, which is a minor type in the Ammonoosuc assemblage; blage in the Oliverian arc. also, they are net K-poor. Quite clearly, neither of Ringwood's (1974) two The distribution of island-arc-related oceanic crust along the BHA stages of arc development can account for the mixed characteristics of the appears to be irregular, as pointed out earlier; it is exposed at the present Ammonoosuc Volcanics, which seem to reflect conditions intermediate erosional surface along the belt of trondhjemite-cored domes (Fig. 1). between tholeiitic and calc-alkaline arcs. Elsewhere along the axis of the BHA, sialic crust, which evidently consti- The patterns of the silicic Ammonoosuc rocks suggest a pyroxene tuted the protolith for the Oliverian plutons, overlies, or is juxtaposed with, residue, and so it is fair to postulate that they were generated at a depth oceanic crust. Recent dating of six Oliverian plutons (Zartman and Leo, exceeding 60 km. This corresponds to 20 kbar, below which, pyroxene is a 1985) indicates a primary Ordovician age, lacking evidence for older, stable phase (Aith and Barker, 1976). Depths of this magnitude are com- inherited ages. West of the Oliverian domes, the Dry Hill Gneiss of the patible with a subduction zone. Given the described ambiguities of the Pelham dome (Fig. 1) has yielded a U-Pb zircon age of 565 m.y. (Naylor Ammonoosuc assemblage and the known field relationships, a reasonable and others, 1973), and the Ordovician Highlandcroft plutonic series sug- conjecture is that the Ammonoosuc does reflect a relatively early stage of gests the presence of inherited zircons with an age of as much as 1,500 m.y. island-arc magmatism with initial production of iron-poor tholeiitic basalt (Lyons and others, in press). East of the BHA, Avalonian (-600 m.y.) and associated K-poor felsic rocks, as outlined above. The calc-alkaline ages have been documented as far as the Fitchburg pluton (Lyons and trend may have been produced by limited contamination of basaltic others, 1982; J. B. Lyons, 1983, personal commun.). A boundary between magma by sialic crust east of the axis of the Bronson Hill anticlinorium, Avalonian and Ordovician sialic crust must lie somewhere east of the partial melting of which subsequently produced the Oliverian granite plu- BHA. tons. In effect, the early oceanic arc evolved into a continental-margin arc, acquiring some features of each. Such a model cannot be quantitatively tested with the available data because of a lack of reliable primary compo- NOTE ADDED IN PROOF sitions of a basaltic parent (this lack also inhibits the modeling of trondhjemite genesis). In any case, the close juxtaposition in time and space of the Preliminary U-Pb isotopic ages of zircon (R. E. Zartman, 1985, Oliverian gneiss«» and somewhat older mantling volcanics makes at least personal commun.) obtained from trondhjemite along New Hampshire some chemical interaction between them appear plausible. The relative Route 16 9 km north of the center of Berlin (sample no. OL55A) permit narrowness of the Ammonoosuc-Oliverian belt, moreover, suggests a sig- the interpretation that this rock is essentially coeval with the Middle to nificant degree of crustal shortening across the arc in the course of the Upper Ordovician gneisses of the Oliverian domes (Zartman and Leo, Taconic plate collision. 1985). Minor discrepant behavior in the data arising from a small compo- The speculative scenario outlined above strongly suggests a congres- nent of inherited zircon, however, would also allow for a somewhat sional tectonic regime. In this context, it is worth noting the well- younger, possibly Silurian, age of the trondhjemite and, by implication, of established association of bimodal (and, typically, alkalic) plutonic-vol- the associated Ammonoosuc Volcanics. The disparity between these new canic suites with anorogenic (tensional) environments, whereas orogenic data and the somewhat older estimated Ammonoosuc age based on the (compressional) environments are associated with calc-alkaline suites work of Kunk and Sutter (1984, discussed above) cannot be eval oated at showing a spectrum of compositions (Martin and Piwinskii, 1972). This present. Clearly, further work will be required to clarify the age(s) of these distinction, for example, constitutes the major basis for the interpretation rocks. of the Silurian-Devonian Maine coastal volcanic belt as having formed under conditions of crustal tension (Gates and Moench, 1981). Due to the ACKNOWLEDGMENTS above-described ambiguities of the Ammonoosuc assemblage (bimodal suite, calc-alkaline chemistry), this question cannot be answered in any Thanks are due L. Peter Gromet for REE analyses of amphibolites clearcut way on the basis of chemistry and lithology, except to note that and Joseph G. Arth for help with trace-element modeling. The paper has alkalic rocks are completely lacking from the Ammonoosuc, and even benefited from reviews by Arth, Richard Goldsmith, John B. Lyons, and normal rhyolites are negligible, at least in western New Hampshire. There one anonymous reviewer. I am indebted to David Gottfried for stimulating is, moreover, little field evidence for extensional tectonics except sporadic discussions. northeast-trending amphibolite dikes of pre-Acadian age, which, however, are not obviously related to the Ammonoosuc Volcanics and may well be REFERENCES CITED

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