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 sug- gests 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 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/96/12/1493/3419322/i0016-7606-96-12-1493.pdf by guest on 01 October 2021 71° Northern part IT m- - Southern part 72° "Berlin 43° — -0^ 1 N. H. MAINE Ammonoosuc VT. fault _N. H. MASS- "MASS! W P •m MASS. 73" 42» CONN. m tu 0 10 20 30 40 50 KM ~r JL I J M 10 30 Ml LONG ¡SLAND SOUND 41° 73° 72° A Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/96/12/1493/3419322/i0016-7606-96-12-1493.pdf by guest on 01 October 2021 EXPLANATION 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). Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/96/12/1493/3419322/i0016-7606-96-12-1493.pdf by guest on 01 October 2021 1496 G. W. LEO 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