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«[American Journal of Science, Vol. 301, September, 2001, P. 657– 682] THE MASSABESIC GNEISS COMPLEX, NEW HAMPSHIRE: A STUDY OF A PORTION OF THE ...»

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[American Journal of Science, Vol. 301, September, 2001, P. 657– 682]

THE MASSABESIC GNEISS COMPLEX, NEW HAMPSHIRE: A STUDY OF

A PORTION OF THE AVALON TERRANE

MICHAEL J. DORAIS*, ROBERT P. WINTSCH**, and HARRY BECKER***

ABSTRACT. Geochemical data from the 625 Ma Massabesic Gneiss Complex of

southern New Hampshire show strong affinities for other Avalonian rocks of southern

New England and suggest continental rifting in the Late Proterozoic. Migmatized paragneiss, the dominant rock type in the complex, has major and trace element compositions that are compatible with graywackes from continental arcs. The paragneiss also has strong lithologic, metamorphic, and isotopic similarities to the rocks of the Hope Valley zone of Connecticut Avalon, suggesting a possible Hope Valley— Massabesic correlation. At 625 Ma, the paragneiss Nd values are similar to Avalonian crust in other locations of the orogen.

Two types of amphibolite are present in minor amounts in the paragneiss of the Massabesic Gneiss Complex. The first type is a paramphibolite and consists of calc-silicate layers in the Massabesic paragneiss, the second type is metaigneous. Major and trace element abundances reveal that the protoliths of the orthoamphibolites range from continental rift alkaline basalts and tholeiites to N-type MORBs. Orthoamphibolite Nd (625 Ma) values range from 2.4 to 4 as expected of rift-related magmas derived from partial melting of a depleted mantle source and have the same values as Iapetus ocean floor rocks of similar age. Orthoamphibolite major and trace element geochemical characteristics overlap those of the Middlesex Fells amphibolites of the Esmond-Dedham zone of eastern Massachusetts Avalon, which range from alkaline to transitional basalts erupted in a continental rift setting. The compositions of orthoamphibolites define a potential magmatic continuum produced by batch partial melting of the mantle initiated during continental rifting and proceeded to ocean basin formation.

The inferred continuity of mafic magmatism from the Esmond-Dedham (Middlesex Fells Formation) to the Massabesic Gneiss Complex (and Hope Valley zone) suggests that these zones are not distinct lithotectonic zones but are parts of a single landmass. Massachusetts Avalon (Esmond-Dedham) represents the continental section of Avalon where the alkaline to transitional magmas of the early rifting stages are preserved. According to our tectonic reconstruction, the Massabesic Gneiss Complex is the oceanward, continental margin represented by volcanoclastic sediments with the MORBs representing the initiation of ocean basin development. The leading edge of this landmass, of which the Massabesic Gneiss Complex is the only observable remnant, collided with Laurentia during the Acadian Orogeny. The inboard, thicker, more continental trailing-edge, that is, platform Avalon (Esmond-Dedham) collided later during the Alleghanian Orogeny.

introduction Considerable progress has been made in understanding the geologic history of the Appalachian region in New England, primarily aided by new geochronological data that have revealed the complexities of New England geology (Zartman, 1988;

Rankin, 1994; Robinson and others, 1998; and references therein). New England is now thought to consist of several distinct terranes or composite terranes including the Rowe-Hawley, Connecticut Valley, Bronson Hill, Central Maine, Merrimack, PutnumNashoba, and Avalon lithotectonic zones that have different ages and/or metamorphic histories (fig. 1). From west to east, they were affected increasingly by the Taconic, Acadian, and Alleghanian orogenies respectively (Rankin, 1994).

*Department of Geology, Brigham Young University, Provo, Utah 84602 **Department of Geological Sciences, Indiana University, Bloomington, Indiana 47405 ***Department of Geology, University of Maryland, College Park, Maryland 20742 658 M.J. Dorais, R.P. Wintsch, and H. Becker—The Massabesic Gneiss Complex, Fig. 1. Generalized geologic map of New England (after Zartman, 1988) showing the lithotectonic zones and the location of the Massabesic Gneiss Complex. Other exposures of Avalon terrane rocks occur in the Willimantic and Pelham domes and the largest exposure is in southeastern New England. The southeastern New England Avalon contains at least two domains, the Hope Valley and the Esmond-Dedham.

Rocks of the Avalon composite terrane are exposed in southeastern New England in Rhode Island and in large areas of southeastern Connecticut and eastern Massachusetts (fig. 1). Avalon is thought to represent a fragment of North Africa/Amazonia that accreted to North America and remained part of North America after Mesozoic rifting of Pangea and the opening of the Atlantic Ocean (Schenk, 1971; Rast and others, 1976;

Williams, 1978; Williams and Hatcher, 1982; O’Brien and others, 1983; Nance and Murphy, 1994). The terrane is teconically important because the collision of this continental block with the margin of Laurentia is thought to have caused either the early Devonian Acadian or the Late Paleozoic Alleghanian orogenies (Osberg, 1978;

Dallmeyer and others, 1981; Williams and Hatcher, 1983; Wintsch and others, 1992).

However, in spite of increasing amounts of geochemical and geochronological data, the specific role of Avalon in these orogenies is still unclear.





Rocks with Avalonian affinities underlie several of the above mentioned allochthonous terranes, extending under the cover rocks as far inland as central Massachusetts, New Hampshire, and Maine (Zartman, 1988; Stewart and others, 1993; Tomascak and others, 1996). The western-most surficial exposures of Avalon in New England are thought to be the Willimantic and Pelham domes in Connecticut and Massachusetts and the Massabesic Gneiss Complex in New Hampshire (Wintsch, 1979; Aleinikoff and others, 1979; Zartman and Naylor, 1984; Hodgkins, 1985; Wintsch and others, 1990).

The correlation of these inliers with Avalon is based on several features. They share a common lithologic assemblage including metaigneous rocks with Late Proterozoic ages as defined by U-Pb zircon crystallization ages. This age is well established in southeast Avalon and the Willimantic dome at about 620 Ma (Wintsch and Aleinikoff, 1987; Zartman and others, 1988; Wayne and others, 1992). In the Pelham dome, Tucker and Robinson (1990) also determined an age of 615 Ma. Aleinikoff and New Hampshire: A study of a portion of the Avalon Terrane others (1995) reevaluated earlier studies of the orthogneiss of the Massabesic Gneiss Complex (Besancon and others, 1977; Aleinikoff and others, 1979) using the ion microprobe and determined an age of 625 Ma. Thus all the orthogneisses have a remarkably common age of 620 Ma.

The “type” New England Avalon and the three inlying domes also share a common late Paleozoic moderate to high grade metamorphism. Evidence for this event comes from U-Pb crystallization and overgrowth ages of metamorphic zircon, monazite, and sphene and from 40Ar/39Ar cooling ages of amphiboles and micas. A prograde event is well documented by the metamorphism of Pennsylvanian sediments and by Ar cooling ages (Dallmeyer and Takasu, 1992). This metamorphism persists west to the Honey Hill—Lake Char fault system in eastern Connecticut and Massachusetts where hornblende cooling ages range from 275 to 255 (Wintsch and others, 1992). Sphene and hornblende ages of 305 Ma (Getty and Gromet, 1992) and 280 Ma (Wintsch and others, 1992) in the Willimantic dome agree well with sphene and hornblende ages of 292 Ma (Tucker and Robinson, 1990) and 287 Ma (Spear and Harrison, 1989) in the Pelham dome. In the Massabesic Gneiss Complex, a metamorphic event of a similar age is defined by monazite (289 Ma, Aleinikoff and others, 1979; 282 Ma, Eusden and Barreiro, 1988), sphene (276-263 Ma, Eusden and Barreiro, 1988), and hornblende (260-250 Ma, West, 1993; Lux and West, 1993). Only the Massabesic Gneiss Complex has any evidence of pre-Alleghanian metamorphism. If a 390 Ma zircon in an amphibolite (Aleinikoff and others, 1995) proves to be metamorphic, then the Massabesic Gneiss Complex may have a more complicated (Acadian) metamorphic history than its sibling Avalonian outliers. Based on the geochronological evidence, there is no question that all these bodies shared a late Proterozoic igneous event as well as a Late Paleozoic metamorphic event in the earliest Permian.

In this study, we examined the whole-rock geochemical and Nd isotopic characteristics of paragneiss, leucosomes, and amphibolites of the Massabesic Gneiss Complex

with the intent to:

1. Constrain the provenance and tectonic setting of the paragneiss;

2. Discriminate between ortho- and paramphibolites to constrain the tectonic setting of the orthoamphibolites;

3. Determine the Nd isotopic compositions of the various Massabesic Gneiss rock types;

4. Compare all these data with those available for Avalonian rocks of southeastern New England in order to draw a larger-scale picture of the petrogenesis and tectonic history of the Avalon terrane than can be reconstructed from any one zone.

regional geology and geology of the massabesic gneiss complex The Massabesic Gneiss Complex consists primarily of Late Proterozoic and Permian sillimanite-zone migmatites (Aleinikoff, 1978; Aleinikoff and others, 1979;

Lyons and others, 1982) that extend in a northeast-southwest trend across the southern portion of New Hampshire (fig. 1). Emerson (1917), Sriramadas (1966), Carnein (1976), and Aleinikoff (1978) documented the variability of the complex, which was given the name Massabesic Gneiss after the exposures surrounding Massabesic Lake southeast of Manchester, New Hampshire. To the southeast, the complex is bounded by the Merrimack belt considered to be in gradational contact with the Massabesic Gneiss Complex as a migmatized equivalent of the Berwick Formation (Bothner and others, 1984; Fagan, 1985). More recently, Goldsmith (1991), Pouliot (1994), Larson (1999), and Larson and others (1999) interpreted the contact to be a ductile fault. To the northwest, the complex is separated from the rocks of the Central Maine terrane by a blastomylonite (Armstrong and others, 1999a, b).

660 M.J. Dorais, R.P. Wintsch, and H. Becker—The Massabesic Gneiss Complex, The Massabesic Gneiss is a complex of migmatitic gneisses of variable texture and structure. Based on zircon morphology and bulk-rock compositions, Aleinikoff (1978) and Aleinikoff and others (1979) concluded that the dominant rock type in the complex is paragneiss. In most outcrops, the gneissosity of the paragneiss is defined by a preferred orientation of biotite in alternating layers of biotite-rich and biotite-poor, quartz-plagioclase-K-feldspar gneiss with 0.5 to 1 cm grain size. This banding is variably migmatitic with Mehnert’s (1971) schlieren type the most dominant. The biotite-rich folia locally contain rare sillimanite, garnet, and muscovite. Leucosome occurs as stringers to small pods within the biotite-rich folia (fig. 2A). Some locations contain pods of leucosome that are sufficiently large for the outcrops to have been called orthogneiss (Aleinikoff, 1978).

Within the paragneiss are relatively rare, small bands and lenses of amphibolite with a foliation parallel to that of the host paragneiss (Bothner and others, 1984;

Larson, 1999; Larson and others, 1998). Larson and coworkers interpreted these amphibolites to be calc-silicates, consisting of amphibole, plagioclase, epidote, clinopyroxene, quartz, diopside, garnet. Other amphibolites are more massive, occurring as blocks or boudins with dimensions of several meters (fig. 2B). Contacts between these massive amphibolites and paragneiss have been sheared, leaving the relative premetamorphic age relations difficult to determine.

Cutting both paragneiss and leucosome are undeformed two-mica granites and pegmatites. One of these, the Damon Pond granite at Milford, New Hampshire, is Alleghanian (Aleinikoff and others, 1979) and probably represents partial melts from deeper in the Massabesic Gneiss Complex. The associated pegmatites probably are also Permian in age because of their lack of metamorphic fabric.

analytical methods Bulk-rock major and selected trace element analyses were conducted by XRF techniques at Michigan State University. Analyses of additional trace elements were obtained by INAA at the Phoenix Memorial Laboratory at the University of Michigan.

Nd isotopic compositions and Nd and Sm concentrations were measured using isotope dilution—thermal ionization mass spectroscopy techniques at the Isotope Geochemistry Laboratory, University of Maryland. After adding a mixed REE (149Sm, Nd) spike, the samples were dissolved in HF-HNO3 at 210°C in screw-top Teflon beakers in Parr bombs for two days. After separation of the REE fraction on a primary column (AG50W-X8), Nd and Sm were separated on AG50W-X4 resin using 0.2 M methylactic acid. Blanks were 500 pg for Nd and less than 100 pg for Sm, and blank corrections are insignificant or small. Isotopic ratios were measured on a SECTOR 54 mass spectrometer with multiple collectors operating in the dynamic mode. Measurements of the La Jolla Nd standard over the analysis period yielded 143Nd/144Nd 0.511847 10 (2s, fractionation corrected to 146Nd/144Nd 0.7219). All 143Nd/144Nd ratios are corrected to a value of 0.511860 for the La Jolla Nd standard. Nd values were calculated using 143Nd/144Nd 0.512638 and 147Sm/144Nd 0.1966 for the presentday bulk silicate earth (Jacobsen and Wasserburg, 1980).

bulk-rock compositions Major elements.—Figure 3 illustrates selected major element compositions (table 1) of rocks of the Massabesic Gneiss Complex. Amphibolite compositions define three fields: massive amphibolites that appear to be boudinaged dikes define two fields that contain approx 48 wt percent SiO2 but have different TiO2 and Fe2O3 contents. Small bands and lenses of amphibolites define a third field at greater SiO2 contents that range between 58 and 61 wt percent. With only one sample as an exception, the paragneiss plots at lower Al2O3 and higher MgO, Fe2O3, and TiO2 contents at equivalent SiO2 values compared to the leucosome and granite.

New Hampshire: A study of a portion of the Avalon Terrane Fig. 2(A) Photograph of typical Massabesic migmatized paragneiss. (B) Photograph of a continental rift tholeiitic amphibolite exposed in road cuts at exit 8 on interstate 93 in Manchester, New Hampshire.



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