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 anti- clinorium in eastern Vermont, western New Hampshire, and cen- tral Massachusetts. This formation extends northward into western Maine (Harwood, 1966; Boudette, 1970) and southward into cen- tral Connecticut, where it is locally known as the Middletown Formation (Rodgers, 1970, p. 102). In almost all respects, the vol- canic 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 vol- canic 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 terminol- ogy 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. 1546 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/88/11/1546/3429055/i0016-7606-88-11-1546.pdf by guest on 25 September 2021 PETROCHEMISTRY AND TECTONIC ORIGIN OF AMMONOOSUC VOLCANICS 1547 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 ac- cepted 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,