Petrochemistry and Tectonic Origin of the Ammonoosuc Volcanics, New Hampshire—Vermont

Petrochemistry and tectonic origin of the Ammonoosuc Volcanics, New Hampshire—Vermont

JOHN N. ALEINIKOFF Department of Earth Sciences, Dartmouth College, Hanover, New Hampshire 03755

ABSTRACT The volcanic rocks are mapped over a fairly large area in western New Hampshire, generally occurring in the form of bedded am- The Middle Ordovician Ammonoosuc Volcanics of the Branson phibolite or greenstone and locally as pillows. Billings (1937) dis- Hill anticlinorium in the upper Connecticut River valley, New tinguished several facies within the Ammonoosuc Volcanics, in- Hampshire—Vermont, is composed of amphibolite, greenstone, cluding soda-rhyolite, volcanic conglomerate, greenstone, and and felsic schists. Major-element analyses of rocks believed to have meta-andesite. Rocks of basaltic composition constitute 50% of the been basaltic flows, such as pillowed greenstones and dense ara- formation. Lyons (1955) mapped the Post Pond Volcanic member phibolites, reveal that they are tholeiitic in composition. The felsic as chlorite and hornblende schists that were probably originally schists do not have igneous compositions, therefore indicating con- basalt flows but later were reworked with admixtures of sedimen- tamination by sedimentary detritus. Regional metamorphism ap- tary detritus. In this study, only pillowed metabasalts or thick, pears to have been isochemical. However, sea-floor alteration prior dense amphibolites that suggested flows were sampled. Rocks with to regional metamorphism probably depleted the basalts in MgO well-preserved pillow structure, most with a dark rind representing and slightly enriched them in Si02 and P205. On the basis of the original glassy chill at the pillow margin, were collected at three trace-element discrimination diagrams (Ti-Zr, Ti-Zr-Y, Ti-Zr-Sr), localities in the Hanover (Lyons, 1955) and Mt. Cube (Hadley, two distinct basaltic populations exist, suggesting an abyssal 1942) quadrangles of Vermont—New Hampshire (Fig. 1). Exact lo- oceanic affinity for one group and an island-arc affinity for the other. It is proposed that the opening of the proto—Atlantic Ocean (Iapetus) in early Paleozoic time and its subsequent closure during the Taconic orogeny would explain the interfingering of Cambrian to Middle Ordovician abyssal tholeiite, island-arc tholeiite, and eugeosynclincal metasedimentary rocks in the northern Appa- lachians.

INTRODUCTION

The Ammonoosuc Volcanics is a series of metamorphosed flows and pyroclastic and volcaniclastic sedimentary rocks cropping out in a north-northeast—trending belt along the Bronson Hill anticlinorium in eastern Vermont, western New Hampshire, and central Massachusetts. This formation extends northward into western Maine (Harwood, 1966; Boudette, 1970) and southward into central Connecticut, where it is locally known as the Middletown Formation (Rodgers, 1970, p. 102). In almost all respects, the volcanic rocks typify metamorphosed eugeosynclinal flows and tuffs. The purpose of this study has been to deduce, using major- and minor-element chemistry, the paleogeographic setting of the volcanic rocks with reference to plate tectonic theory. Similar work on lower Paleozoic lavas has been conducted by Kean and Strong (1975) in central Newfoundland, by Bloxam and Lewis (1972) in Great Britain, and by Fumes and Faerseth (1975) in Norway. The plethora of synonymous terminology in the literature (for example, "abyssal," "oceanic," "ocean ridge," "ocean rift") greatly compli- cates any attempt to synthesize data in accordance with the previ- ous research of various authors. In most cases, the terms "abyssal" and "island-arc" have been employed, although original terminology has been preserved in figures extracted from appropriate publi- cations. Although originally mapped as two separate formations —the Post Pond Volcanic Member of the Orfordville Formation

and the Ammonoosuc Volcanics, respectively (see Lyons, 1955; KILOMETERS Hadley, 1942) — recent detailed stratigraphic correlations by Thompson and others (1968) have shown that the two units are Figure 1. Index map for location of pillow metabasalts and amphibolites equivalent. discussed in text.

Geological Society of America Bulletin, v. 88, p. 1546-1552, 7 figs., 4 tables, November 1977, Doc. no. 71102.

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TABLE 1. LOCATION OF SAMPLES metamorphism" (alteration that occurs at depth in rifting axial zones — that, is, oceanic ridges) and "regional metamorphism" Sample Geographic location Metamorphic (alteration that occurs at converging plate margins in orogenic name zone zones — that is, active continental margins). Miy ashiro (1972) suggested that most of the oceanic crust that Plainfield Beaver Brook on Rt. 12, 1.6 km Garnet north of Plainfield, N.H.; and biotite underlies the thin, surface magnetized zone (0.5 to 2 km thick) is lat 43°36'08" N, metamorphosed. Greenschist-facies metamorphism is caused by long 72°19'32" W high heat flow whose source is either mantle upwelling or simple Plainfield Bryant Brook, 2.4 km east of Biotite igneous intrusion. Oceanic metabasalts and metagabbros differ Plainfield, N.H.; lat 43°32'30" N, from regionally metamorphosed amphibolite in that thermal long 72°20'04" W metamorphism at the ocean ridge produces granulose-textured Norwich 1.6 km northwest of Norwich, Vt.; Biotite rocks with little or no foliation. Shido and others (1971) suggested lat 43°43'56" N, long 72°18'35" W that most chemical inhomogeneities found in metamorphosed pil- Lyme Hewes Brook, 3.2 km southwest of Garnet lows are probably a function of initial alteration at the time of Lyme, N.H.; lat 43°46'58" N, eruption, that is, on the sea floor, although regional metamorphism long 72°10'55" W may also influence chemical reactions. Hart and others (1974), in a Lyme Quarry 8.8 km southwest of Lyme, Garnet compilation of several studies, showed that sea-floor alteration of N.H.; lat 43°44'20" N, basalts causes a generally consistent pattern of enrichment in FeO long 72°15'35" W and K20 and depletion in Si02, CaO, and MgO. Scott and Hajash (1976) described the different effects of low-temperature and high- temperature alteration. K20 is enriched, and MgO, CaO, and Si02 cations are given in Table 1. Agglomerate with mafic and felsic are depleted from pillow interiors by low-temperature, sea-water clasts and rocks called "soda-rhyolite flows" by Billings (1937) interaction. High-temperature, hydrothermal alteration enriches were sampled in the Littleton-Moosilauke area (see Fig. 1). The the pillow interiors in MgO and depletes them in CaO, K20, and metabasalts can be divided into two groups. Plainfield and Nor- Si02. In Table 3, a comparison is made between average abyssal wich pillows (Hanover quadrangle) contain megacrysts of corroded tholeiitic compositions determined by Hyndman (1972) and Engel plagioclase, whereas the Lyme samples (Mt. Cube quadrangle) lack and others (1965) and representative Ammonoosuc pillow any large grains. The mineralogy of the Ammonoosuc Volcanics metabasalts from the three localities in central New England. varies with composition and metamorphic grade. Basaltic rocks Chemical compositions of the Ammonoosuc Volcanics compare range from greenschist to amphibolite, composed of quartz, quite favorably with those averages for abyssal tholeiites listed in plagioclase, calcite, chlorite, hornblende, and clinozoisite. Felsic Table 3, except for MgO (which is very low) and Si02 and P205 volcanic rocks are quartz, plagioclase, and sericite schists. (which are slightly high). CaO in both the average abyssal tholeiite and the Ammonoosuc Volcanics is variable. Whatever processes METHODS OF CHEMICAL ANALYSIS may have changed the chemistry of present-day volcanic rocks presumably would also have operated on Ordovician submarine Analyses of Si02, Ti02, A1203, FeO (total Fe as FeO), MnO, extrusions. The rather close chemical similarities between the Am- MgO, CaO, Na20, and K20 in the samples were made by atomic monoosuc Volcanics and present-day abyssal tholeiites suggests absorption spectrophotometry. A colorimetric method was used to that for most elements, regional metamorphism has not caused ex- + analyze P205, while H20 and H20~ were determined following tensive chemical change in the Ordovician lavas. the method of Shapiro and Brannock (1955). Loss on ignition was There are at least three possible causes for the low MgO concen- measured by weighing the sample before and after heating at trations in the Ammonoosuc Volcanics: (1) fractional crystalliza- 1000°C. The percentage error in the analyses of the various oxides tion of magma, (2) submarine alteration, and (3) geochemical var- (as determined by replicate analyses of known standards) was with- iation during regional metamorphism. Figure 2, after Kay and in 2.5% of the amount reported; Na20 (5%) is an exception. others (1970), is a plot of FeO versus MgO for basalts from the Selected trace-element (Ni, Cr, Zr, Y, Sr, and Rb) concentrations were determined by x-ray fluorescence. Values for U.S. Geologi- cal Survey Standard BCR-1 run as an unknown fell within the accepted range, ensuring precision in the analyses. Table 2 lists the analyses of mafic volcanic rocks. Felsic volcanic rocks (metatuffs and agglomerates) were also analyzed, but their bulk chemistry and norms indicate that they are volcaniclastic, and, therefore, were not amenable to resolution of the question of tectonic provenance by the following methods. In order to discern a specific primary igneous origin, the chemical determinations of metamorphic rocks must be corrected to represent the original volcanic compositions. Following the method of Irvine and Baragar (1971), all compositions have been recalculated without water and other volatiles, and Fe203 has been made equal to Ti02 + 1.5%. Results were then normalized to 100%. All binary and ternary plots presented below use recalculated (volatile-free) data.

METAMORPHISM

At least two different types of metamorphism must be considered Wt. % MgO in order to decide what geochemical variations, if any, have oc- Figure 2. FeO-MgO (total Fe as FeO), modified from Kay and others curred since extrusion of the lavas. In the terminology of Miy ashiro (1970). Circles = Plainfield and Norwich pillow metabasalts; squares = (1972), the two most important processes are "ocean-floor Lyme pillow metabasalts and amphibolites.

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TABLE 2. CHEMICAL ANALYSES OF PILLOWED GREENSTONES AND AMPHIBOLITES

H-l H-2 H-3 H-4 H-5 H-7 M-8 M-10

Si02(%) 51.49 52.06 53.31 51.54 52.93 50.29 56.94 51.13 Ti02 2.01 2.29 2.36 1.42 1.64 2.08 0.51 1.08 AI2O3 14.60 15.19 14.79 17.74 17.09 15.31 15.40 15.15 FeO * 10.90 11.38 11.49 9.50 8.63 8.99 8.78 10.28 MnO 0.20 0.18 0.21 0.16 0.18 0.15 0.14 0.17 MgO 4.94 4.31 4.53 5.56 5.02 3.95 4.32 5.00 CaO 11.69 8.57 9.49 9.96 10.23 12.05 8.12 10.82 NazO 2.93 3.26 3.55 3.79 3.51 3.87 2.51 3.13 K2O 0.23 0.20 0.35 0.14 0.62 0.20 0.14 0.24 P2O5 0.40 0.16 0.35 0.25 0.22 0.35 0.20 0.42 + H2O 1.54 2.25 1.42 2.47 2.28 0.89 2.10 0.95 H2O- 0.13 0.17 0.13 0.13 0.13 0.14 0.20 0.16 + LOI 0.64 0.49 0.59 0.78 0.15 3.14 0.66 1.94 Total 101.70 100.51 102.57 103.44 102.63 101.41 100.02 100.47

Ni (ppm) 56 62 47 70 69 52 64 126 Cr 43 44 38 60 49 41 58 84 Zr 137 126 171 120 128 170 48 50 Y 41 50 46 30 33 40 15 24 Sr 157 92 127 199 259 207 139 117 Rb <3 <3 11.5 <3 8 3 <3 <3

Note: Column headings are sample numbers. * Total iron as FeO. Loss on ignition.

Mid-Atlantic Ridge (lat 45°N and 30°N), Gorda Ridge, and Juan TECTONIC SETTING OF AMMONOOSUC de Fuca Ridge, with analyses of the Ammonoosuc Volcanics METABASALTS superimposed. If low MgO in the Ammonoosuc pillow lavas is due to fractional crystallization of olivine and subsequent enrichment Major Elements of iron, the analyses presumably might fall on or near the Juan de Fuca Ridge (iron-enriched) trend. However, since all points plot on Because the original tectonic setting of these basalts is the prob- the low-MgO side of this trend, it could be concluded that frac- lem under consideration, all possible tectonic and petrologie prov- tional crystallization is not a likely mechanism for the low-MgO inces must be considered as potential source areas. All graphs values of the Ammonoosuc rocks. Unmetamorphosed basalts with presented in this paper to indicate a petrologie province or process very low MgO concentrations (approximately 4.5%) have been re- are based, as noted above, on "corrected" chemical analyses — that ported from both oceanic and continental margin areas (see is, analyses for which the values for volatiles have been subtracted McBirney and Williams, 1970; Anderson and others, 1975), but and the analyses recalculated to 100%. Although, as shown above, these "ferrobasalts" are very highly enriched in both FeO (13% to the Ammonoosuc rocks have probably been postdepositionally 14%; total Fe as FeO) and Ti02 (3.5% to 4%), owing to fractional altered, most elements do not seem to have been affected. Thus, an crystallization at low pressure and low oxygen fugacity. Ti02 was igneous origin can be discerned if the proper (unchanged) elements considered by Chayes and Velde (1965) to be an excellent dis- are used as discriminators. criminator for distinguishing various types of basalts, because it is Using the standard MacDonald and Katsura (1961) plot (Fig. 3), relatively immobile. The Ti02 contents of 1.0% to 2.5% in the all rocks of the Ammonoosuc Volcanics except one amphibolite fall Ammonoosuc Volcanics thus indicate that it is improbable that in the "tholeiitic" field. Unfortunately, tholeiitic basalts exist in fractional crystallization of olivine has occurred to such an extent large quantities not only in ocean-rift environments (abyssal that the formation of a ferrobasalt with very low MgO has resulted. tholeiites) but also in island-arc sequences (Jakes and Gill, 1970; It is therefore concluded that the chemical compositions of the Miyashiro, 1974) and as continental flood basalts. Ammonoosuc pillows have been altered either on the sea floor, or Using Hyndman's (1972) tabulated average analyses for 144 during regional metamorphism. On the basis of existing data and continental tholeiites and 161 abyssal tholeiites (Table 3), rocks of literature, low-temperature sea-floor alteration appears to be more the Ammonoosuc Volcanics (with Na20/K20 approximately 10 probable. Frey and others (1974) and Ridley and others (1974) and A1203 ranging from about 15% to 18% by weight) appear to found both altered and fresh pillows in Deep Sea Drilling Project be abyssal in nature. The slight possibility exists, however, that (DSDP) cores. In the altered pillows, Ridley and others (1974) felt Na20 and (or) K20 have been mobilized, and thus the present ratio confident that low MgO (4.7%) is due to submarine alteration. is a metamorphic overprint. However, Hart and others (1974) have Therefore, prior to regional metamorphism, the Ammonoosuc Vol- shown that pillow basalts altered by interaction with sea water are canics was probably altered to some extent by loss of MgO and always enriched in K20. Thus, the very low values of K20 in the perhaps addition of Si02, P205, and H20. All other constitutents Ammonoosuc Volcanics indicate either no enrichment on the sea seem to have remained fairly immobile, or at least constant. It is floor or that there has been metamorphic depletion of an altered unclear why only MgO would decrease. abyssal basalt.

TABLE 2. (Continued)

M-U M-12 M-13a M-13b M-14 M-15 M-17 M-3

49.65 50.96 49.55 48.54 52.24 50.52 46.50 47.93 0.90 0.93 1.02 0.95 2.32 2.26 0.93 1.34 15.53 17.78 17.69 18.45 16.73 15.52 14.77 18.20 10.43 9.72 9.28 9.26 10.63 10.32 7.86 11.16 0.16 0.14 0.11 0.13 0.19 0.19 0.14 0.12 4.76 4.98 4.80 4.86 5.16 5.15 4.72 4.55 11.52 10.57 12.96 13.07 10.35 11.55 15.07 8.80 2.57 3.16 2.82 2.83 2.95 2.26 3.17 2.54 0.23 0.16 0.31 0.27 0.11 0.52 0.18 2.14 0.21 0.13 0.29 0.28 0.22 0.16 0.13 0.45 0.55 0.81 0.52 0.55 0.89 0.59 0.82 0.51 0.10 0.13 0.10 0.09 0.13 0.13 0.14 0.19 2.75 1.23 2.58 2.24 1.01 1.08 6.19 2.55

99.36 100.70 102.03 101.52 102.93 100.25 100.62 100.48

133 47 55 51 72 61 50 107 81 52 50 55 43 47 46 66 47 52 51 57 156 150 47 60 28 21 13 14 40 41 19 25 150 155 134 205 194 163 181 66 <3 <3 <3 4 <3 10 <3 52

With the discovery of large piles of tholeiitic lavas in island-arc Trace Elements sequences (for example, JakeS and Gill, 1970), methods for distinguishing lavas erupted at mid-oceanic rift zones from those devel- There has been a great deal of work recently on trace-element oped in island arcs and ocean islands have only recently been de- and rare-earth-element trends in basalts (for example, Kay and vised. Ridley and others (1974) have constructed a binary plot of others, 1970; Tanaka and Sugisaki, 1973; Frey and others, 1974). Ti02-P205 with fields for abyssal tholeiite, ocean-island tholeiite, Using the Pearce and Cann (1973) discrimination diagrams of and alkali basalt (Fig. 4). Many points for the Ammonoosuc Vol- Ti-Zr-Y and Ti-Zr-Sr for analyses of the Ammonoosuc Volcanics, canics plot outside of Ridley's fields, possibly because of addition of these pillow lavas appear to be most closely related to basalt of P205 by submarine alteration (see Table 3). If P205 in the Am- abyssal tholeiite affinity (see Figs. 5, 6). However, both figures monoosuc rocks were in the range of 0.15 to 0.20 (data from Shido show that some of the pillows show a closer relationship to island- and others, 1974, for fresh abyssal pillow basalts), all points would arc basalts. As an indication that this ambiguity of basalt prove- fall within the abyssal tholeiite field, thus eliminating the possibility nance is real, all plots based on major- or minor-element composi- of Hawaiian-type ocean-island tholeiitic volcanic rocks. Similarly, tion (Figs. 2 to 7) definitely show two distinct populations. The it seems unlikely that the Ammonoosuc metabasalts could be cor- boundaries of each basalt class in the discrimination diagrams are relative with Columbia River—type continental tholeiites because based on world-wide averages of modern basalts, which may or of their low K20 and high Na20/K20 and their occurrence with may not be exactly applicable to the lower Paleozoic Ammonoosuc eugoesynclinal sedimentary rocks. Discrimination between abyssal Volcanics. Figure 7 shows the distribution of Ammonoosuc pillows and island-arc tholeiites is discussed below. on a modified Pearce and Cann (1973) plot of Ti-Zr. Also plotted

TABLE 3. COMPARISON OF AVERAGE COMPOSITION OF THOLEIITES AND INTERIORS OF PILLOWED AMMONOOSUC VOLCANICS FROM NEW HAMPSHIRE-VERMONT

Continental Abyssal Abyssal Ammonoosuc Volvanics tholeiite'' tholeiite1 tholeiite+ Norwich Plainfield Lyme

Si02 (wt %) 50.7 49.3 49.34 52.71 51.37 49.91 TiO, 2.0 1.8 1.49 2.33 2.12 1.03 AI2O3 14.4 15.2 17.04 14.62 15.64 17.82 Fe203 3.2 2.4 3.82 3.66 2.54 FeO 9.8 8.0 8.61§ 8.24 6.19 7.27 MnO 0.2 0.17 0.17 0.21 0.15 0.11 MgO 6.2 8.3 7.19 4.48 4.04 4.83 CaO 9.4 10.8 11.72 9.38 12.31 13.05 NazO 2.6 2.6 2.73 2.93 3.95 2.84 K2O 1.0 0.24 0.16 0.23 0.20 0.31 P2O5 0.21 0.16 0.40 0.36 0.29

Na20/K20 2.6 10.83 17.06 12.74 19.75 9.16

1 Data from Hyndman (1972). f Data from Engel and others (1965). § Fe O as total iron.

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1.0

0.6 -

a."

44 46 48 50 52 54

Wt. % Ti02 Wt. % SiO, Figure 4. P205-Ti02 after Ridley and others (1974). Circles = Plain- Figure 3. Modified MacDonald-Katsura silica-alkali plot. Boundary field and Norwich pillow metabasalts; squares = Lyme pillow meta- line from MacDonald and Katsura (1961). Circles = Plainfield and Nor- basalts and amphibolites. wich pillow metabasalts; squares = Lyme pillow metabasalts and amphibolites; stars = metabasalts from Cady (1969) that show abyssal affinity; diamonds = metabasalts from Cady (1969) that show island-arc the premetamorphic rock was olivine basalt. Those metabasalts affinity. with the lowest Si02, TiC^, Zr, and Y and which lack plagioclase megacrysts (that is, Lyme rocks) are probably the least differ- are lower Paleozoic (upper Precambrian to Devonian) lavas of the entiated. Higher concentrations of Ti02, Si02, Zr, and Y, coupled Appalachian-Caledonian chain from Great Britain (Bloxam and with the presence of plagioclase megacrysts in Plainfield and Nor- Lewis, 1972) and Norway (Furnes and Faerseth, 1975). Some wich metabasalts, characterize the more differentiated lavas. How- Ammonoosuc basalts plot in an area where all three petrogenic ever, another possibility based on the trace-element data would put basalt series overlap. Thus, their specific provenance appears to be the Lyme rocks, low in Zr and Ti, into the island-arc setting, uncertain. It may be significant that some Norwegian and British whereas the metabasalts richer in those elements could have lower Paleozoic lavas correspond closely to the New England pil- evolved in a mid-ocean rift environment. The present stratigraphic low basalts. relations of the New England rocks are not clear, owing to faulting Table 4, modified from Jakes and Gill (1970) and Pearce and and highly complex folding during the Taconic and Acadian Cann (1973), compares average selected trace elements for the orogenies; thus, it is not possible to distinguish conclusively be- three main petrogenetic basalt series with trace elements of the tween these two different origins of the metabasalts. Ammonoosuc Volcanics. Zr,Y, Ni, and perhaps Rb and Sr show Bird and Dewey (1970) proposed a Taconic subduction zone for affinities for the abyssal tholeiite series. Cr, however, is about five the Appalachians, with the concomitant formation of an Ordovi- times lower than the average concentrations in abyssal tholeiites, cian island arc on which there were eruptions of felsic Am- corresponding more closely to the island-arc series; this discrep- monoosuc Volcanics. It is conceivable, but not yet proven by any ancy remains unresolved. The Rb (Table 4) and bulk chemical field data known to me, that the interfingering of abyssal tholeiite analyses definitely appear to rule out a calc-alkalic or high A1203 with clastic and volcaniclastic material took place along this basalt trend (see Hyndman, 1972, p. 120-122). Taconic subduction zone. Any tectonic model devised for the Appalachian-Caldeonian chain must explain the widespread occur- SUMMARY OF CHEMISTRY AND PETROLOGY rence of lower Paleozoic and Precambrian tholeiitic basalt, not only in the areas mentioned in this paper, but in many localities Average compositions of abyssal tholeiites (Engel and others, throughout New England. Cambrian(P) pillow basalts, part of an 1965; Shido and others, 1974; Hyndman, 1972) in general com- obducted(P) ophiolite sequence, occur on strike in western Maine pare favorably to compositions of Ammonoosuc Volcanics. The (Boudette, 1970). Cady (1969) listed analyses for 14 Cambrian and only major-element discrepancy involves MgO, where Am- Ordovician amphibolites and greenstones of oceanic tholeiitic monoosuc Volcanics average about 2.5% by weight less than affinity occurring in several localities in central and western Ver- average abyssal tholeiites; the minor-element discrepancy is Cr, mont, as well as five analyses of island-arc affinity from the same which is approximately five times less abundant than in abyssal areas (see Fig. 3). Pieratti (1976) determined that a volcanic tholeiites. (Eocambrian?) suite in western Vermont shows similar affinity to Use of "incompatible" elements such as Ti, Zr, Ni, and Y sup- oceanic rift basalt. It is therefore important to note that, at numer- ports the conclusion drawn from major-element analyses. This ous localities in the northern Appalachians during early Paleozoic conclusion is reinforced by the occurrence of similar rocks from and possibly latest Precambrian time, abyssal tholeiitic lavas were Great Britain and Norway erupted during early Paleozoic time. being erupted. These tholeiites may thus represent oceanic rift The distribution of trace elements in the Ammonoosuc Volcanics lavas related to the opening of the proto—Atlantic Ocean during suggests a petrologic sequence. High Ni concentration implies that latest Precambrian and early Paleozoic time. Closure of the

Ti/100 Ti/100

2 r L y ^ v u u v y y * i g r//2 2 ' " " " " si * x * * Y -3 r Figure 6. Ti-Zr-Sr after Pearce and Cann (1973). Abyssal basalts plot in Figure 5. Ti-Zr-Y after Pearce and Cann (1973). "Within-plate" basalts region A, island-arc basalts in B, and calc-alkalic basalts in C. Circles = plot in region A, island-arc basalts in B and C, abyssal basalts in C, and Plainfield and Norwich pillow metabasalts; squares = Lyme pillow calc-alkalic basalts in C and D. Circles = Plainfield and Norwich pillow metabasalts and amphibolites. metabasalts; squares = Lyme pillow metabasalts and amphibolites.

proto-Atlantic (Iapetus) would account for the present juxtaposi- tion of oceanic lavas with continental margin or arc-trench sedimentary rocks.

CONCLUSIONS

The Ammonoosuc Volcanics pillow metabasalts are abyssal tholeiites and island-arc tholeiites. Felsic metavolcanic rocks can- not be classified in a petrogenetic scheme, because their chemical compositions indicate contamination by admixtures of detritus. Pillows collected from three localities along the Bronson Hill anticlinorium represent varying degrees of differentiation of the primary magma, but none of the basalts can be termed "primitive" because of the high Ti02 content. Sea-floor alteration has probably depleted the metabasalts in MgO. Other changes possibly attributable to submarine alteration include enrichment in H20 and P205, and possibly Si02. The presence of palimpsest selvage and pillow structure indicates that metamorphism has not caused large-scale geochemical varia- Zr (ppm) tion. This is substantiated by the fact that the composition of the Figure 7. Ti-Zr modified from Pearce and Cann (1973). Abyssal basalts Ammonoosuc Volcanics compares quite favorably to compositions plot in regions A and B, island-arc basalts in B and C, and calc-alkalic of present-day average abyssal tholeiites. basalts in B and D. Circles = Plainfield and Norwich pillow metabasalts; The abyssal tholeiite suite of the Ammonoosuc Volcanics is very squares = Lyme pillow metabasalts and amphibolites; stars = lower similar to lower Paleozoic and possibly Eocambrian metabasalts in Paleozoic pillow lavas and greenstone from Norway (Furnes and Faerseth, western and central Vermont. A tensional (rifting) environment re- 1975) and Great Britain (Bloxam and Lewis, 1972). lated to the opening of the proto-Atlantic Ocean has been postu- lated as the petrogenetic origin. Island-arc tholeiites in the Am- TABLE 4. COMPARISON OF SELECTED TRACE ELEMENTS FROM THREE PETROGENETIC BASALT SERIES monoosuc Volcanics probably erupted during subsequent closing WITH AMMONOOSUC VOLCANICS of the proto-Atlantic. Element Calc- Island- Abyssal Ammonoosuc Volcanics ACKNOWLEDGMENTS alkalic arc tholeiite Lyme Plainfield- (ppm) (ppm) (ppm) (ppm) Norwich I thank John B. Lyons for his invaluable guidance throughout the (ppm) entire project, from initial discussions and collections of samples to careful criticism of the manuscript. Douglas Rumble guided me to Zr 105 50 95 52 145 several choice localities in the Mt. Cube quadrangle. Robert C. Y 20 20 30 20 40 Reynolds and Richard W. Birnie contributed valuable suggestions Sr 380 100-200 70-150 143 175 pertaining to the chemical analyses. I also thank William S. Condit Rb 30 3-10 0.2-5.0 2 5 and Gerald G. Carlson for critically reading the manuscript. The Cr 56 0-50 200-400 62 46 work was supported in part by a Sigma Xi Grant-in-Aid, by a Ni 18 0-30 30-200 79 61

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Dartmouth College Fellowship, and by National Science Founda- Kay, R., Hubbard, N. J., and Gast, P. W., 1970, Chemical characteristics tion Grant DES72-01525. The manuscript was reviewed by Mar- and origin of ocean ridge volcanic rocks: Jour. Geophys. Research, land P. Billings and Peter Robinson, who offered many valuable v. 75, p. 1585-1613. suggestions. However, the conclusions expressed are solely mine. Kean, B. F., and Strong, D. F., 1975, Geochemical evolution of an Ordovi- cian island arc of the central Newfoundland Appalachians: Am. Jour. Sci., v. 275, p. 97-118. REFERENCES CITED Lyons, J. B., 1955, Geology of the Hanover quadrangle, New Hampshire-Vermont: Geoi. Soc. America Bull., v. 66, p. 105-145. Anderson, R. N., Clague, D. A., Klitgord, K. D., Marshall, M., and MacDonald, G. A., and Katsura, T., 1961, Chemical composition of Nishimori, R. K., 1975, Magnetic and petrologie variations along the Hawaiian lavas: Jour. Petrology, v. 5, p. 82-133. 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