the massabesic gneiss complex, new hampshire

Transcription

[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
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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
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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).
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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,
150
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.
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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.
Paragneiss to the right of the amphibolite, Permian aged pegmatites cut both orthoamphibolite and
paragneiss.
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Fig. 3. Bulk-rock Al2O3, MgO, Fe2O3 and TiO2 versus SiO2 diagrams. The amphibolites of the
Massabesic Gneiss complex define three compositional fields: the calc-silicate amphibolites (open squares)
plot at relatively high SiO2 concentrations (58-61 wt percent) compared to the orthoamphibolites. The
orthoamphibolites define two fields in Fe2O3 and TiO2 versus SiO2 space with the MORB amphibolites
(filled squares) being poorer in Fe2O3 and TiO2 at equivalent SiO2 contents compared to the continental rift
alkaline and tholeiite amphibolites (asterisks). The leucosomes (filled diamonds) and Permian, two-mica
granites (filled circles) contain less MgO, Fe2O3 and TiO2 and more Al2O3 compared to the paragneisses
(open diamonds).
On an AFM diagram (fig. 4A), the amphibolites that are relatively rich in SiO2 plot
in the calc-alkaline field. Additional data presented below indicate that these are
paramphibolites and not metabasites, hence their position in the AFM diagram has no
significance except that it shows this group is chemically distinct from the massive
amphibolites. One group of massive amphibolites plots as tholeiitic basalts in the AFM
diagram; the other group plots in the calc-alkaline field. In the FeO/MgO versus SiO2
diagram (fig. 4B), both sets of amphibolites plot in the tholeiitic field.
Paragneiss samples are plotted in (Na ⫹ Ca)/(Na ⫹ Ca ⫹ K) versus Si/(Si ⫹ Al)
(atomic proportions) in figure 5 which defines compositional fields for various
sedimentary rocks (Wintsch and Kvale, 1994). Most of the paragneiss samples plot as
graywackes, but one plots as a mudstone. This sample (MG-31) is highly sheared, and
metasomatic loss of plagioclase probably modified its initial composition.
Tectonic discrimination diagrams and trace element characteristics of orthoamphibolites.
—The Zr versus Ti diagram (fig. 6) shows the two massive amphibolite groups plot in
the low-K tholeiite and the ocean floor basalt fields and at greater Ti and Zr contents
along the extension of the ocean floor basalt field. In the Ti-Zr-Y diagram (fig. 7A), the
amphibolites plot in the field of ocean floor basalts and low-K tholeiites and as within
plate basalts. The Nb-Zr-Y diagram (fig. 7B) shows that several samples plot as N-type
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Table 1
Representative bulk-rock analyses, Massabesic Gneiss Complex, New Hampshire
MORB ⫽ mid oceanic ridge basalt; CRT ⫽ continental rift tholeiite; PA ⫽ paramphibolite; PG ⫽
paragneiss; OG ⫽ orthogneiss; LS ⫽ leucosome; G ⫽ granite.
MORBs with the remainder plotting in the within plate tholeiite and within plate
alkaline basalt fields.
Chondrite-normalized REE patterns of the massive amphibolites again show two
groups (fig. 8A). The amphibolites that plot as N-type MORBs in the Nb-Zr-Y diagram
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Fig. 4(A) AFM diagram for rocks of the Massabesic Gneiss complex. (B) FeO/MgO versus SiO2 diagram
(after Miyashiro, 1974).
have flat patterns ranging from 15 to 25 times chondrites. The patterns are LREE poor
which is consistent with MORB compositions (Bryan and others, 1976; Schilling and
others, 1983). With the exception of the positive Rb and K anomalies thought to be the
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Fig. 5. Atomic proportions of (Na ⫹ Ca)/(Na ⫹ Ca ⫹ K) versus Si/(Si ⫹ Al) for paragneiss samples
(after Wintsch and Kvale, 1994).
result of metasomatism during metamorphism of the basalts, these samples have
relatively flat patterns in the extended REE diagram (fig. 8B), again suggesting MORB
compositions (Sun and others, 1979). In figure 9, these amphiboles plot at (Ba/La)N
values of less than 2 at low (La/Sm)N which is characteristic of MORBs compared to
higher values of island arc tholeiites and calc-alkaline basalts (Kay, 1977; Sun and
others, 1979).
The other group of massive amphibolites has HREE and HFSE abundances
comparable to the MORB amphibolites, but these amphibolites are as rich as 100 to
500 times chondrites in LREE abundances (fig. 8A, B). This LREE enrichment, plus
the enrichment of other incompatible elements such as Rb, Ba, and K in the extended
REE diagram, shows these amphibolites to be similar to continental rift alkaline basalts
and tholeiites (BVST, 1981; Dupuy and Dostal, 1984; Bertrand, 1991) as suggested by
the tectonic discrimination diagrams. Another characteristic of continental rift magmas that distinguishes them from MORBs is the depletion of Nb and Ta in spiderdiagrams (fig. 8B; Thompson and others, 1983; Thompson and others, 1984; Dupuy and
Dostal, 1984; Bertrand, 1991). These negative anomalies are often erroneously por-
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Fig. 6. Ti versus Zr diagram. Data from the Middlesex Fells complex from Cardoza and others (1990)
and the Waterford Complex from Goldsmith (1987).
trayed as being exclusive to subduction-related magmas, but they may be present in
continental rift alkaline basalts and tholeiites as well as a result of crustal contamination (Cox and Hawkesworth, 1985; Dupuy and Dostal, 1984).
In summary, there are two groups of massive amphibolites in the Massabesic
Gneiss Complex, one group with MORB compositions, the other with continental rift
alkaline to tholeiitic compositions.
Trace element characteristics of the paramphibolites, paragneiss, and leucosomes.—Primitive mantle-normalized incompatible element abundances of the paragneiss and the
SiO2-rich amphibolites are shown in figure 10A. Compared to the orthoamphibolite
patterns, these patterns are rich in Rb, Th, and K, with negative Nb, Ta, P, and Ti
anomalies. The paragneiss is similar to the third group of SiO2-rich amphibolites that
plot in the calc-alkaline field of figure 4A.
Figure 10B shows chondrite-normalized REE patterns for paragneiss samples.
LREE are enriched to 130 times chondrites, HREE elemental abundances are relatively
flat at ⬃20 to 30 times chondrites. All the paragneiss samples display negative Eu
anomalies.
Leucosome samples are plotted in two tectonic discrimination diagrams (fig. 11;
Pearce and Cann, 1973; Pearce and others, 1984). The samples plot in the volcanic arc
and syn-collisional granite fields in the Nb versus Y diagram and in the volcanic arc
granitic field in the Rb versus Nb ⫹ Y diagram.
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Fig. 7(A) Ti-Zr-Y diagram (After Pearce and Cann, 1973) showing that the MORB amphibolites (filled
squares) plot in the ocean-floor and low-K tholeiite fields. The continental rift alkaline and tholeiite
amphibolites (asterisks) plot in the within-plate basalt field. (B) Nb-Zr-Y diagram showing that the MORB
amphibolites (filled squares) plot as N-type MORBs. The second group of orthoamphibolites (asterisks)
have within plate tholeiitic and alkaline affinities.
Discussion.—The presence of amphibolites with compositions of both MORB and
continental rift magmas in the Massabesic Gneiss Complex suggests the development
of a single magmatic series in Late Proterozoic magmatism. We suggest that the earlier
stages of continental rifting produced the transitional alkaline to continental rift
tholeiites whereas continual rifting led to further depletion of the underlying mantle,
eventually producing magmas of MORB compositions.
Fig. 8(A) Chondrite-normalized REE patterns for the Massabesic Gneiss Complex MORB amphibolites
and the continental rift alkaline to tholeiitic amphibolites (solid lines). The MORB amphibolites have flat
patterns with depleted LREE abundances (Bryan and others, 1976; Schilling and others, 1983). The
continental rift alkaline and tholeiite amphibolites are enriched in LREE, similar to continental rift magmas
of other localities (BVSP, 1981; Dupuy and Dostal, 1984) and to the Middlesex Fells amphibolites of
Massachusetts Avalon (gray lines, Cordoza and others, 1990). (B) Extended REE diagram (After Sun and
McDonough, 1989). The MORB amphibolites have flat patterns except for the positive K2O and Rb
anomalies (Sun and others, 1979) which appear to have been enriched by metasomatism during metamorphism. The Massabesic continental rift alkaline and tholeiite amphibolites are enriched in incompatible
elements and are similar to continental rift magmas of other localities (Dupuy and Dostal, 1984; Bertrand,
1991) and to the Middlesex Fells amphibolites of Massachusetts Avalon (gray lines, Cordoza and others,
1990).
M.J. Dorais, R.P. Wintsch, and H. Becker
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Fig. 9. Chondrite-normalized ratios of La/Sm and Ba/La showing fields for island arc and oceanic
basalts (After BSVP, 1981). Massabesic MORB orthoamphibolites plot at low (Ba/La)N values which is
consistent with the overall MORB signature of these samples.
Aleinikoff (1978) and Aleinikoff and others (1979) concluded that the dominant
rock type in the Massabesic Gneiss Complex is paragneiss. Our data support this
conclusion and suggest that the paragneiss is distinguished from the leucosomes by the
lower Al2O3 and higher MgO, Fe2O3, and TiO2 contents at equivalent SiO2 values (fig.
3). Minimum melts can dissolve only limited amounts of MgO, Fe2O3, and TiO2 (Miller
and others, 1985), hence the magmas that formed the orthogneisses and granites had
limits to the solubility of these elements.
The nature of the sedimentary protolith of the paragneiss can be inferred from
the REE abundances. The chondrite-normalized REE diagram (fig. 10B) includes
patterns of representative graywackes from several tectonic settings (Taylor and
McClennan, 1985). Graywackes from fore-arc settings have the lowest REE abundances, particularly the LREE. Graywackes shed from Andean-type continental arcs are
richer in REE, with LREE abundances ranging from 100 to 130 times chondrites.
Graywackes from passive margin settings that are rich in quartz overlap those from
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Fig. 10(A) Extended REE diagram for the paragneiss and paramphibolites of the Massabesic Gneiss
complex (Normalization constants after Sun and McDonough, 1989). (B) Chondrite-normalized REE
patterns for the paragneisses (solid lines) of the Massabesic Gneiss complex compared to patterns of
graywackes from fore-arc settings (gray dashed lines), Andean-type settings (gray dotted lines), and passive
margin settings (solid gray lines) (After Taylor and McClennan, 1985).
Andean-type settings but tend to be slightly more enriched with the LREE concentrations reaching 130 times chondrites. The Massabesic Gneiss Complex paragneiss has
LREE abundances that overlap both these LREE-rich graywackes. The relatively low
SiO2 contents of some of the paragneiss samples suggest that an Andean-type setting is
probably a more appropriate source region than a passive continental margin. Additionally, the relatively high Ni, Cr, and Sr contents of some of the paragneiss samples (table
2) suggest a more primitive volcanic component to the paragneiss that fits the
volcanoclastic origin suggested by Aleinikoff and coworkers (1979) and our data in
figure 5.
The conclusion that the dominant gneiss in the Massabesic Gneiss Complex is a
migmatized paragneiss is supported by plots of Massabesic Gneiss Complex rocks in
the ACF diagram (fig. 12). The leucosomes plot at high Al2O3 contents, separate from
the paragneiss that plots as typical clastic sediments (Orville, 1969). The massive
amphibolites plot within the labradorite-clinopyroxene-orthopyroxene and olivine
volume as do basalts. However, Orville (1969) demonstrated that many paramphibolites, being mixtures of clastic sediments and/or mudrocks with carbonates, plot in this
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Fig. 11. Nb versus Y and Rb versus Nb ⫹ Y diagrams (after Pearce and others, 1984). The leucosomes
plot in the volcanic arc field and the syn-collisional field in (A) and in the volcanic arc field in (B).
Representative trace element analyses, Massabesic Gneiss Complex, New Hampshire
Table 2
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MORB ⫽ mid oceanic ridge basalt; CRB ⫽ continental rift basalt; PA ⫽ paramphibolite; PG ⫽ paragneiss; OG ⫽ orthogneiss; LS ⫽ leucosome; G ⫽ granite; bd ⫽
below detection limits.
(continued)
Table 2
673
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Fig. 12. ACF diagram (symbols as in fig. 3). The orthoamphibolites (filled squares and asterisks) plot
within the Lab—CPX—OPX, Oliv volume. The paramphibolites (open squares) plot within the same
volume as the orthoamphibolites, but their compositions can be explained by the addition of dolomite to the
typical paragneiss as they plot along a line connecting the two endmembers.
same volume. The SiO2-rich amphibolites plot along a tie line between the paragneisses and dolomite. The similarity of the SiO2-rich amphibolites in trace element
abundances to known metasedimentary rocks, that is, the paragneiss samples (fig.
10A), suggests that the amphibolites also had metasediments as protoliths, hence these
amphibolites are paramphibolites whose major element compositions in the ACF
diagram (fig. 12) probably result from addition of dolomite to typical paragneiss.
The tectonic discrimination diagrams (fig. 11) suggests that the leucosomes
originated in volcanic arc settings. In actuality, the leucosomes could not have
originated in a volcanic arc setting; they clearly occur as migmatites produced in an
inferred syn-collisional environment. Their compositions were determined by the
chemical signature of the partially melted metasedimentary rocks and not tectonic
setting, and are another indication of graywacke source rocks. Similar interpretations
of tectonic discrimination diagrams are presented by Brown and others (1984), Clarke
(1992), and Forster and others (1997).
Thus all three rock types, the paramphibolites, paragneiss, and leucosomes, have
compositions that are compatible with derivation from graywackes in a continental
margin setting.
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neodymium isotopic compositions
Neodymium isotope data are given in table 3. Lacking a precise age, we use the
625 Ma age obtained from U-Pb data on zircon for subsequent calculations and initial
Nd isotopic compositions.
Figure 13A shows ⑀Nd versus time for the amphibolites and paragneisses. Both the
continental rift and MORB samples define a restricted range of ⑀Nd values (625 Ma)
from ⫹2.4 to ⫹4.0. The positive values indicate a mantle-derived (juvenile) magma
where the reservoir had been previously depleted such as what one would expect for
rift-related magmas. The amphibolites plot well above the range of values displayed by
Grenville rocks and are compatible with the Avalon-like crust at this time. Two
representative paragneiss samples (MG-1, MG-36) also plot within the Avalon field.
These samples are high-grade gneisses, being the dominant rock type of the Massabesic Gneiss Complex. In this diagram, one typical paragneiss (MG-28) is anomalous,
plotting in the Grenville field.
Figure 13B illustrates f Sm/Nd versus ⑀Nd for the continental rift samples, MORBs
and paragneiss where f Sm/Nd reflects the difference of Sm/Nd between sample and
CHUR (DePaolo and Wasserburg, 1976). Also plotted are the fields of Iapetus ocean
floor rocks and Avalonian rocks from Fryer and others (1997). At 625 Ma, the
Massabesic amphibolites plot within the Iapetus ocean floor rocks with the MORBs at
positive f Sm/Nd values as expected (fig. 13B). One sample (MG-10A) plots within the
Avalonian field, suggesting that this sample may have assimilated Avalonian crust. This
sample has an ⑀Nd (0) of ⫺4.0, also indicating assimilation of old crust. The Massabesic
paragneisses, including the anomalous sample MG-28 from the previous diagram, plot
within the Avalonian field as defined by Barr and Hegner (1992), Whalen and others
(1994), and Kerr and others (1995).
correlation of massabesic gneiss complex with avalon of se new england
Paragneiss and leucosomes.—The Avalon terrane of southeastern New England is a
composite terrane consisting of several domains that experienced different intensities
of Alleghanian metamorphism. The southwestern Hope Valley domain of O’Hara and
Gromet (1985) and a southeastern portion of the Esmond-Dedham domain each
experienced high grade Alleghanian metamorphism (Murry and others, 1990; Murry
and Dallmeyer, 1991), while the central Esmond-Dedham zone has escaped metamorphism since the late Precambrian (Skehan and Rast, 1990). No Acadian or Taconic
metamorphism has been identified. The Esmond-Dedham zone contains 600 to 650
Table 3
Nd isotopic data for Massabesic Gneiss Complex rocks
*Calculated using the parameters of Goldstein and others, 1984.
676
Fig. 13(A) Plot of ⑀Nd versus time for the Massabesic Gneiss complex orthoamphibolites and paragneiss.
The amphibolite samples plot in the Avalon field (Barr and Henger, 1992; Keppie and others, 1997; Fryer
and others, 1997; Pe-Piper and Piper, 1998). Two representative samples of paragneiss also plot in the Avalon
field whereas one paragneiss is anomalous, plotting in the Grenville field. (B) Plot of f Sm/Nd versus ⑀Nd for
Massabesic Gneiss Complex orthoamphibolite and paragneiss calculated at 625 Ma. The continental rift
samples plot in the negative f Sm/Nd and positive ⑀Nd field. The two MORB samples (MG-26, 7-13-95-1A) and
one continental rift tholeiite (MG-33) plot in the Iapetus ocean floor field (Fryer and others, 1997). One
continental rift sample (MG-10A) plots in the Avalon field (after Barr and Hegner, 1992), suggesting
possible assimilation of Avalonian material for this sample. All three paragneiss samples plot in the Avalon
field. DM ⫽ depleted mantle.
677
Ma plutons that range in composition from granite to diorite (Kovach and others,
1977; Zartman and Naylor, 1984; Thompson and others, 1996). Late Proterozioc mafic
volcanic rocks that erupted both prior and subsequent to the Late Proterozoic granitic
magmatism as well as Devonian anorogenic plutons are present in this zone. The ⬃620
Ma leucogneisses of the Hope Valley zone (Hermes and Zartman, 1985) experienced
Alleghanian metamorphism and have minor amounts of mafic rocks. Anorogenic
granites are absent.
The Massabesic Gneiss Complex has strong affinities with the Hope Valley portion
of Avalon composite terrane and also with the Pelham dome of Massachusetts. They all
share a common suite of ⬃620 Ma felsic orthogneisses that experienced Alleghanian
metamorphism, contain relatively minor amounts of mafic rocks, and share an absence
of anorogenic granites. Tucker and Robinson (1990) interpret the Pelham dome
paragneisses as immature feldspathic wackes with a quartz-rich continental source that
were deposited along a rifted continental margin (Rankin, 1994), a similar setting is
suggested by this study for the Massabesic Gneiss Complex. The ⬃625 Ma orthogneisses of the Massabesic Gneiss Complex are the same age as plutonic rocks of southeastern New England (Aleinikoff and others, 1995; Wintsch and Aleinikoff, 1987). Additionally, the Massabesic paragneiss has the same range of ⑀Nd (625) as other Avalonian
rocks (fig. 13B; Barr and Hegner, 1995) which is distinct from Grenvillian rocks at that
time.
Orthoamphibolites.—The question of how the Massabesic orthoamphibolites relate
to other Late Proterozoic amphibolites of the Avalonian terrane of southeastern New
England can be addressed by comparing the compositions of the Massabesic orthoamphibolites with amphibolites of the Middlesex Fells Formation of the Esmond-Dedham
zone of eastern Massachusetts and to amphibolites of the Waterford Complex of the
Hope Valley zone in Connecticut. The Middlesex Fells complex consists of a bimodal
association of felsic and mafic volcanic rocks occurring as roof pendants and large
blocks in the Dedham Granite north of Boston, Massachusetts (Cardoza and others,
1990). The mafic rocks have experienced low grade contact metamorphism by the
Dedham Granite. The Waterford complex (Goldsmith, 1987) consists primarily of
granodioritic rocks interpreted as a candidate for a caldera (Wintsch and others,
1990). Amphibolites are present throughout the complex in the upper part of the
section as dikes and flows.
For comparisons of the Massabesic Gneiss Complex amphibolites with those of
Avalon of southeastern New England to have any validity, the amphibolites must be the
same age. Although none were directly dated, a common age is implied. Massabesic
Gneiss Complex rocks are about 620 Ma which is our best estimate for the age of its
amphibolites. Southern Connecticut amphibolites occur as mafic enclaves in the dated
Waterford Complex and as interlayered volcanic rocks in the extrusive cap (Wintsch
and others, 1990). Thus these may be confidently identified as late Proterozoic. It has
been suggested that the Middlesex Fells rocks correlate with 700 to 800 Ma rocks of
Newfoundland (Strong and others, 1978; Strong, 1979; O’Brien and others, 1983).
This age difference between the Massabesic amphibolites and the Middlesex Fells
complex would invalidate any comparisons with the younger orthoamphibolites of the
Massabesic Gneiss Complex. However, a sill with Middlesex Fells compositions intruded the Cambridge Argillite which includes an ash bed which was dated at 642 Ma.
(Thompson, personal communication). This provides a maximum age for the Middlesex Fells amphibolite and permits an age correlation with the Waterford and the
Massabesic Gneiss Complexes. It is thus reasonable to explore regional variations in
basalt geochemistry.
Cardoza and others (1990) determined that the amphibolites of the Middlesex
Fells complex are alkaline to transitional basalts that have signatures of continental rift
678
environments. These rocks define a continuum of compositions at high Ti concentrations on the Zr-Ti diagram (fig. 6), overlapping the range of low-K tholeiites and ocean
floor basalt compositions shown by the Massabesic Gneiss Complex orthoamphibolites.
Figure 8A shows the chondrite-normalized REE patterns for the Middlesex Fells
amphibolites, the sill in the Cambridge Argillite and the amphibolites of the Massabesic Gneiss Complex. The alkaline basalts of the Middlesex Fells rocks are rich in LREE,
containing abundances up to 500 times chondrites. The sill plots at the REE-rich end
of the alkaline basalts. The transitional basalts defined by Cardoza and others (1990)
contain between 90 and 200 times chondritic abundances of the LREE, overlapping
with the Massabesic continental rift tholeiitic amphibolites. Figure 8B shows the
extended REE plots for these rocks. Both the sill and the amphibolites of the
Middlesex Fells complex are rich in incompatible elements. There is a range of
compositions between the alkaline basalts and the transitional basalts that plot at lower
incompatible element concentrations. The transitional basalts overlap the plots of the
continental rift tholeiites of the Massabesic Gneiss Complex.
Given that the Massabesic Gneiss Complex has strong correlations with Avalon
of southeastern New England, we interpret these similarities in amphibolite
compositions to suggest that the two suites of amphibolites may represent a
compositional continuum. A magmatic continuum would suggest that the continental rifting envisioned by Cardoza and coworkers proceeded to ocean basin formation as shown by the MORB compositions of the Massabesic Gneiss Complex
amphibolites. This scenario would suggest that the Esmond-Dedham zone of
Massachusetts contains alkaline magmas of the early rifting stages of the inland,
continental section of the rift which was followed by eruption of continental rift
basalts. The Massabesic Gneiss Complex would be the continental margin represented by the volcanoclastic sediments, continental rift tholeiites, and the initial
formation of adjacent ocean basin represented by the MORBs. Therefore, in spite
of the differences between the Esmond-Dedham and Hope Valley zones as defined
by O’Hara and Gromet (1985), a magmatic continuum from the Middlesex Fells to
the Massabesic Gneiss Complex amphibolites suggests that the zones were originally continuous.
Implications.—The suggestion that the Massabesic Gneiss Complex is representative of the oceanward margin of Avalon has a bearing on the nature of crustal materials
involved in the Acadian and Alleghanian orogenies. Because Avalon of southeastern
New England lacks evidence of involvement during the Acadian orogeny, one can infer
that the Acadian orogeny resulted from the collision of Laurentia and whatever rocks
were outboard of this portion of Avalon. If the Massabesic Gneiss Complex is the
trailing edge of that landmass, then we infer that its continental margin sediments and
adjacent oceanic crust collided with North America during the Acadian Orogeny and
could have produced metamorphic zircons at 390 Ma (Aleinikoff and others, 1995).
The MORB amphibolites of the Massabesic Gneiss Complex could therefore be the last
traces of the Avalonian side of the Iapetus ocean basin. The similar ⑀Nd (625 Ma) values
of the Massabesic orthoamphibolites to those of rocks interpreted to represent Iapetus
ocean floor by Fryer and others (1997) support this interpretation (fig. 13B).
conclusions
We interpret the Late Proterozoic Massabesic Gneiss Complex to correlate with
the Avalon terrane of southeastern New England and, based on neodymium isotopic
data, with the Avalon terrane of Canada. The Massabesic Gneiss Complex, the Pelham
dome, and the Hope Valley zone share similar ages, high-grade metamorphism and
cooling curves (Wintsch and others, 1992), strongly foliated gneisses in similar
lithologic packages, and have minor amounts of amphibolites of similar compositions.
679
The potential continuum from alkaline and continental rift tholeiitic magmas of the
Esmond-Dedham zone to continental rift tholeiites and MORB magmas of the Hope
Valley/Massabesic zone suggests that the two zones were continuous. The Acadian
Orogeny may have resulted from subduction of Iapetus ocean basin followed by
obduction of arc graywackes, the last traces of which are represented by the Massabesic
Gneiss Complex.
acknowledgments
We thank Mike Brown for his encouragement to proceed on the collaborative
project and Meg Thompson for the analysis of the Middlesex Fells sill. We are very
grateful to John Aleinikoff, Tom Armstrong, and Dave Stewart for reviews of an earlier
version of the manuscript and Dyk Eusden and Jo Laird for journal reviews. This
research was supported by NSF grant EAR-9418203 to Wintsch and EAR-9909410 to
Wintsch and Dorais.
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the massabesic gneiss complex, new hampshire

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