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Abstract
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Introduction
The use of thermobarometry to determine the extent of metamorphism has been used in various
manners for the past hundred years. The original studies of Barrow and Buchanan based
approximate temperatures and pressures of metamorphism upon the presence or absence of
certain index minerals. The recognition that mineral composition could be used to further
determine the extent of metamorphism was made clear through the varying types of a given
mineral (e.g. garnets, pyroxenes, amphiboles) depending on the tectonic history of a province.
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Geologic History
The crystalline rocks of New England have been deformed during three major orogenies: the
Taconian (mid-Ordovician), the Acadian (early Devonian) and the Alleghanian (PennsylvanianPermian) (Rankin, 1994). The polymetamorphic history of the various terranes has been the
object of study for decades and the overprinting relationships among the various orogenies are
still not well understood. Sutter et al. (1985) utilized 40Ar/39Ar and K-Ar data from the
Berkshires of western Massachusetts (Fig. 1) to define the boundary between rocks
metamorphosed primarily in the Taconian orogeny and rocks overprinted by the Acadian
orogeny. Similar research for the Bronson Hill terrane was conducted by Boyd et al. (1993) to
discriminate between rocks metamorphosed principally in the Acadian orogeny and those later
remetamorphosed by the Alleghanian orogeny in the Permian.
3.1
Taconian Orogeny
The Taconian orogeny occurred during the Ordovician from ~470 to 440 Ma (Rankin, 1994).
The Champlain thrust emplaces the lower Cambrian Dunham Dolostone over the deformed midOrdovician Iberville Shale (Stanley, 1987). The thrust is unconformably overlain by lower
Devonian rocks in the Hudson Valley (Bosworth et al., 1988; Thompson et al., 1993), which
defines the Taconic orogeny as Ordovician in age. Tucker and Robinson (1990) have dated a
series of subduction-related volcanics and plutons in the Bronson Hill terrane as having ages
between ~470 and 435 Ma. Ordovician and Silurian K-Ar and 40Ar/39Ar cooling ages of biotite
and hornblende in the Berkshires are due to heating during the Taconian orogeny (Sutter et al.,
1985).
3.2
Acadian Orogeny
The Acadian orogeny is Devonian and ranges from ~400 to 380 Ma in age (Rankin, 1994). The
Catskill Delta is a clastic wedge of Devonian sediments that was deposited as a result of the
Acadian orogeny in New England (Faill, 1985). Plutons across New England crystalized and
were deformed during the Acadian (Bradley et al., 1998). Late Devonian and Mississippian KAr and 40Ar/39Ar cooling ages of hornblende and micas reflect cooling from the Acadian orogeny
(Clark and Kulp, 1968; Sutter et al., 1985; Harrison et al., 1989; Spear et al., 1989; Tucker and
Robinson, 1991; Moecher et al., 1997).
3.3
Alleghanian Orogeny
The Alleghanin orogeny transpired between ~320 and 270 Ma during the Pennsylvanian and
Permian (Rankin, 1994). In Pennsylvania, folded Carboniferous sediments are unconformably
overlain by undeformed Triassic sediments, defining the Alleghanian orogeny. Similar
constraints are also found in New England, where the Pennsylvanian sediments of the
Narragansett Basin have been metamorphosed to staurolite grade, but have late Permian mica
cooling ages (Mahler and Mosher, 1994). The increasing number of isotopic ages for crystalline
rocks in New England are indicative of a wide-spread Permian cooling event (Clark and Kulp,
1968; Zartman, 1988; Harrison et al., 1989; Spear et al., 1989; Wintsch et al., 1993; Boyd et al.,
1995).
3.4
Extent of the Orogenies
K-Ar and Ar40/Ar39 research into muscovite, biotite, and amphibole cooling ages has been used
to delineate the extent of the three orogenies within New England. This process is based on the
fact that the argon closure temperature for muscovite is ~350E C, biotite ~275E C, and
amphibole ~500E C. For muscovite, Zartman et al. (1970) define ages >350 Ma as being
Taconian, 350-260 Ma as Acadian, and 260-200 Ma as Alleghanian in age.
3.4.1 Acadian versus Taconian
The line between rocks solely metamorphosed in the Taconian Orogeny versus rocks
metamorphosed in the Acadian Orogeny was drawn by Zartman et al. (1970) using K-Ar ages for
muscovite. The boundary of the Acadian overprint in western New England was further refined
by Sutter et al. (1985) based on both K-Ar and 40Ar/39Ar cooling ages for muscovite, biotite, and
amphibole (Fig. 1).
3.4.2 Alleghanian versus Acadian
Zartman et al. (1970) differentiated between rocks with solely Acadian cooling ages and rocks
with an Alleghanian overprint in the muscovite ages. The western age of the Alleghanian
overprint approximately coincident with the boundary between the Bronson Hill Terrane and the
Connecticut Valley Synclinorium. In southern New England, Alleghanian muscovite cooling
ages extend from the Bronson Hill to the western edge of the Avalon terrane. The bulk of the
Avalon terrane, however, has ages >260 Ma. Rocks within the Putnam-Nashoba Terrane
recorded Acadian cooling ages in the northern portion, but several Alleghanian muscovite ages
are located within the southern section of the terrane.
Several muscovite ages in the northern
portion of New Hamphshire and the mid-section of Maine are Alleghanian in age, but a large
area of both states had not been analyzed as of 1970.
Boyd (1995) analyzed hornblende and muscovite for 40Ar/39Ar cooling ages along a traverse of
the Bronson Hill Terrane in conjunction with data from previous studies (Brookins, 1970;
Brookins and Armstrong, 1980; Wintsch and Sutter, 1986; Harrison et al.,1989; Spear and
Harrison, 1989; and Tucker and Robinson, 1990). Boyd concluded that the muscovite ages are
all approximately equal at 250 Ma for the length of the terrane (fig. 1 and 2), but the amphibole
cooling ages are latitude dependent. An age gradient from north to south existed with the
amphibole ages, with the older cooling ages in the northern section of the terrane (fig. 2). Boyd
proposed that the loading of the Bronson Hill Terrane was unequal, with a greater amount of
burial in the southern portion of the terrane (fig. 3). Wintsch et al. (2001) used cooling path
models to demonstrate that the rocks in the southern portion of the Bronson Hill Terrane were
exhumed at a greater rate than those in the northern section of the terrane, in order for all of the
muscovite cooling ages to be equivalent (fig. 4).
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Geologic Setting
The Bronson Hill is a north-south trending terrane that is continuous from Long Island Sound in
southern Connecticut to the Maine-Quebec border. Traditionally considered the island arc that
formed over an east-dipping subduction zone during the Taconian Orogeny, the terrane was
accreted during the Ordovician to Laurentia and later metamorphosed in the Acadian and
Alleghanian orogenies (Bradley, 1983; Harrison et al., 1989).
The rocks collected for this study contain amphibolite from the Bronson Hill terrane along a
traverse from southern Connecticut through Massachusetts. The amphibolite was sampled from
the Partridge Formation and the Ammonoosuc Volcanics, both of which are Ordovician in age
(Zartman and Leo, 1985; Tucker and Robinson, 1990). The two formations represent volcanics
extruded onto the island arc. Specimens were located a maximum of 18 km from each other on
a north-south traverse of the terrane, with at least one sample per 15' quadrangle (fig. 5 and
appendix 1). These amphibolites were chosen for the assemblage amphibole + plagioclase "
garnet to be analyzed for petrologic and thermobarometric calculations using Holland and
Bludny=s (1994) amphibole-plagioclase thermometer, the Kohn and Spear (1989) garnethornblende-plagioclase-quartz thermometer, and the Dale et al. (2000) hornblende-garnetplagioclase thermobarometer.
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Methods
Specimens used for this study were located a maximum of 18 km from each other on a northsouth traverse of the Bronson Hill Terrane, with at least one sample per 15' quadrangle (fig. 5 and
appendix 1). These amphibolites were chosen for the assemblage amphibole + plagioclase "
garnet to be analyzed for petrologic and thermobarometric calculations using Holland and
Bludny=s (1994) amphibole-plagioclase thermometer, the Kohn and Spear (1989) garnethornblende-plagioclase-quartz thermometer, and the Dale et al. (2000) hornblende-garnetplagioclase thermobarometer.
Rock samples were cut perpendicular to the main foliation whenever identifiable and billets were
also aligned parallel to mineral lineations where visible. The thin sections were singly polished
for analysis on the Cameca 500 SX Microprobe at Indiana University, as well as microstructural
analysis with the optical microscope.
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Plagioclase
6.1
Petrography
Within the thin sections studied, plagioclase is anhedral and round to oval in shape. When oval,
the long direction of the plagioclase is aligned with the dominant foliation of the sample.
Plagioclase and quartz are commonly intergrown and embay one another.
6.1.1 Optical Zonation
Visibly zoned metamorphic plagioclase was observed in a number of the thin sections. Optical
zonation of metamorphic plagioclase has been discussed by Nord et al. (1978), Crawford (1966),
Stoddard (1985) and Passchier and Trouw (1998). Plagioclase zoning is visible due to the
difference in extinction angles between various portions of the grain (Nord et al., 1978; Stoddard,
1985). The boundary between different zones may be either gradual or sharp depending on the
amount of equilibration the grain has undergone (Passchier and Trouw, 1998). The samples
examined ranged from several distinct zones to having only indistinct cores and rims (fig. 6).
Of the samples analyzed, the presence of optically zoned plagioclase ranged from no optical
zonation visible to 80% of the plagioclase crystals within a given thin section visibly zoned (fig.
7). In the Massachusetts section of the traverse, plagioclase zoning is rare. When elongate, the
rims tend to align with the dominant foliation and rim-core or rim-mantle boundaries are diffuse,
though core-mantle boundaries are sharp in mid-Massachusetts. In Connecticut, zoned
plagioclase accounts for 50 to 80% of the total plagioclase grains. The elongate rims continue to
be commonly aligned with the dominant foliation. Though in central Connecticut, the
boundaries between the cores and mantles are sharp, the mantle-rim boundaries are diffuse. In
the remaining portions of the state, the boundaries between cores and rims are diffuse, with no
distinguishable mantles.
6.1.2 Twinning
Within the thin sections studied, several types of feldspar twinning were observed. Passchier
and Trouw (1998) differentiate between growth twinning (e.g. Carlsbad and Albite) and
deformation twins (also called mechanical twins), which form to accommodate strain. The latter
twins tend to be concentrated along the rim of individual grains and taper inwards towards the
center of the feldspar. Deformation twins are most likely to form at metamorphic temperatures
under 400E C (Passchier and Trouw, 1998).
One sample contained very rare Carlsbad growth twins (99ERG07c), but deformation twins were
the most common found in the thin sections analyzed. The percentage of deformation twins for
each sample was estimated and forms a pattern along strike of the traverse. Deformation twins
are common in the Massachusetts samples and in the southernmost Connecticut rocks, but are
rare throughout most of Connecticut (Fig. 8).
6.1.3 Petrographic Interpretation
Passchier and Trouw (1998) describe the deformational behavior of plagioclase as being
dependent on the temperature at which the deformation occurred and if the grains were heated to
a higher temperature after the deformation.
At low temperatures (<400E C), plagioclase will be likely to deform via deformation twinning
and undulose extinction. Deformation twinning can occur at higher temperatures, but is less
abundant. Added heat to the system may provide sufficient energy for the plagioclase to
Aheal@ due to dislocation climb and recrystallization. These processes can begin in feldspars at
~400-500E C. Deformation twins that are present within a thin section normally indicate that
the rock was deformed at temperatures less than 400E C and the sample was not consequently
reheated to greater than 400E C.
The zoning of plagioclase indicates that though the rock was at a temperature great enough for
plagioclase to grow, the rock did not reach a sufficient temperature for the grains to equilibrate
and become homogenous. The growth zonation can occur due to several different circumstances
as outlined by Spear (1995): a change in P-T conditions, a change in mineral assemblage, the
fractionation of material into the core of the mineral (like in igneous fractional crystallization), or
the change of the bulk composition of the rock due to infiltration and metasomatism. The
circumstances that cause the variation in conditions can occur during continuous or
discontinuous crystal growth. The cation diffusion in plagioclase has been documented by
Grove et al. (1984) as being very slow, so that plagioclase cannot easily reequilibrate except by
dissolution and reprecipitation.
From north to south along the Bronson Hill traverse zoning and deformation twinning are
dominant at different periods. Of the samples analyzed, those in Massachusetts and the
southern-most section of Connecticut had a large percentage of plagioclase grains with
deformation twins. Zoning of the plagioclase was prevailing feature in most of Connecticut.
The rocks in Massachusetts were deformed at low temperatures but were not consequently heated
to temperatures greater than 400E C. In Connecticut, the low percentages of twins indicate that
the rocks were also deformed. The deformation twins may have formed at higher temperatures
or the samples were deformed and then heated to >400E C, which healed most of the twins. The
high percentage of twins in southern Connecticut may be due to a high amount of differential
stress at temperatures >400E C that occurred during the rapid uplift of the southern-most section
of the terrane as suggested by Wintsch et al. (2001).
6.2
Microprobe Analysis
Of the samples collected and cut for thin sections, ?? thin sections were analyzed with the
electron microprobe at Indiana University for plagioclase (fig. 9). Plagioclase was analyzed
using a beam size of 10 kV and a current of 10 nA on the Cameca 500 SX microprobe.
6.2.1 Anorthite value vs Latitude
The anorthite content of plagioclase ranges systematically from wide range in the northern
sections of the travers to a low variability in northern Connecticut before rising once again in
southern Connecticut (Fig. 10). The plagioclase composition varies along the length of the
Bronson Hill Terrane from ~An15 to An90. Anorthite content has a mean value of An46. The An
values within individual plagioclase have a difference between 0.1 and 25.1, with a mean value
of 5.7. Within a thin section, the An value differs from between a change of only 1% to just
over 50%.
6.2.2 Plagioclase Stability
Carpenter (1994) defined regions of stability for specific anorthite values that includes several
gaps on a binary phase diagram (Fig. 11). At temperatures lower than 400E C, plagioclase solid
solution is very limited and except for extremely albitic or anorthitic compositions, two crystals
are stable with one another. Greater than 400E C, solid solution in plagioclase becomes more
prevalent, increasing in range with composition.
6.2.3 Implications of Microprobe Plagioclase Studies
The systematic variance of plagioclase compositions from north to south in the Bronson Hill
terrane may be directly linked to the stable areas of plagioclase solid solution as defined by
Carpenter (1994). Rocks in the northern section of Massachusetts were heated to lower
temperatures than those in southern Connecticut and Massachusetts. The possible anorthite
solid solution would have been limited at low temperatures and extend over a greater range at
higher temperatures. This temperature variation is also evidenced by the presence of migmatites
in the southernmost section of Connecticut, pegmatites in the southern to central portion of
Connecticut and in the Massahusetts section of the Bronson Hill terrane, and quartz veins in midto northern Connecticut (fig. 12).
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Amphibole
7.1
Petrography
Two types of amphibole are located within the thin sections examined: the dominant form is an
aluminous amphibole ranging from edenite to ferro-pargasite hornblende and ferro-tschermak
hornblende to tschermak in composition and the second is tremolite. The aluminous amphibole
dominates within the samples studied, but coexists with tremolite is within the thin sections
99ERG17 and 00ERG12 (fig. 5).
7.1.1 Aluminous Amphibole
The aluminum-rich amphiboles are aligned with the dominant foliation in all of the thin sections
studied. The aluminous amphibole defines the foliation, except in samples where the mode of
aluminum is low. Deflection of the amphibole foliation occurs in some of the garnet bearing
samples around the porphyblasts.
Aluminous amphibole grains are anhedral and elongate. Opaque inclusions are common within
the amphiboles, but equant quartz inclusions are rare. In a few samples, the amphibole contain
inclusions of sphene (numbers?).
7.1.2 Tremolie
When tremolite is present within the thin section, it is aligned parallel to the dominant aluminous
amphibole foliation. The tremolite can occur both as isolated crystals or as beards elongate in
the direction of foliation on aluminous amphibole grains. Inclusions of aluminous amphibole
are common within the tremolite and the aluminous amphibole is often truncated by the
tremolite. The tremolite can increase the aspect ratio of the amphiboles by from 3:1 to 9:1.
7.1.3 Implications of Tremolite and Aluminous Amphibole Relationships
Tremolite forms at low pressures and temperatures during greenschist facies metamorphism.
The aluminous amphibole present, in contrast, formed during amphibolite facies metamorphism
documenting a change from high-grade to low-grade metamorphism (fig. 13). The alignment of
the tremolite with the dominant aluminous amphibole foliation is evidence that the differential
stress in both cases must have been in the same direction.
7.2
Microprobe Analyses of Amphibole
The amphibole was analyzed using a beam size of 10 kV and a current of 10 nA on the Cameca
500 SX microprobe. Only aluminous amphiboles were analyzed and their compositions are
shown in figure 9.
7.2.1 Amphibole Compositions
The analyzed amphiboles plot on an approximately straight line on both the Mg / Mg + Fe vs Si
diagram for Na + K > 0.5 cations and Na + K < 0.5 cations (fig. 14 and 15). The amphiboles on
the diagram with less that 0.5 cations of sodium and potassium plot in the edenite, edenitic
hornblende, and ferro-pargasitic hornblende fields. What about the amphs in that field that has
no name? What are they called? Are they even amphs? On the > 0.5 cation diagram,
amphiboles are located within the ferro-tschermak hornblende, ferro-tschermak, and tschermak
fields. The majority of the amphiboles have less than 0.5 cations of sodium and potassium. A
few of the analyses produced amphiboles with less than 6 cations of silica and are probably due
to experimental error.
7.2.2 Igneous versus Metamorphic stability of aluminous amphiboles
Leake (1971) defined the extent of igneous amphibole stabilities based on the amount of sodium,
potassium, calcium, and silica within a given amphibole (fig. 16).
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Amphibole-Plagioclase Thermometry
8.1
Requirements for Thermometry
Holland and Blundy (1994) developed two thermometers for the amphibole-plagioclase system.
Thermometer A is only applicable for temperatures between 400 and 900E C. The amphiboles
must have NaA > 0.02 pfu, Alvi < 1.8 pfu, and Si from 6.0-7.7 pfu. Plagioclase is required to
have an An number of less then 90. Thermometer B is stable from 500-900E C. Plagiolcase
must be between An10 and An90. Amphiboles are required to have XNaM4 > 0.03, Alvi < 1.8 pfu,
and Si from 6.0-7.7 pfu.
8.1.1 Quartz Saturation
The two thermometers of Holland and Blundy (1994) have several constraints placed upon them,
but the main distinguishing factor is the presence or absence of quartz. The edenite-tremolite
reaction requires silica saturation, while the edenite-richerite calculation is to be applied to silica
undersaturated rocks. In silica saturated rocks the two thermometers should be in agreement
with one another. When aSiO2 < 1.0, the edenite-tremolite calculation will be consistently higher
than the edenite-richerite calculation. When the converse is observed, Holland and Blundy
(1993) surmise either the ferric iron has been incorrectly recalculated or the hornblende and
plagioclase do not represent an equilibrium assemblage.
8.2
Experimental Results
Rim-rim compositions of amphibole and plagioclase were entered into the Holland and Blundy
(1993) thermometers. Several types of results were returned: 1. the quartz-saturated
thermometer was consistently higher then the quartz-undersaturated thermometer; 2. the quartz-
saturated and undersaturated thermometers crossed at some pressure between 0 and 15 kbar; or 3.
the quartz-saturated thermometer was consistently lower then the quartz-undersaturated
thermometer (Figure 17, Table 1).
8.3
Discussion of Thermometry Results
The higher temperatures resultant from the silica-undersaturated thermometer indicate
disequilibrium, since the ferric iron was corrected for amphibole correctly. Conventual wisdom
assumes that the rims of individual grains in contact with each other should be in local
equilibrium. This logic, however, does not hold true for this traverse of amphibolites.
Conversations with Spear (2001) suggested that using a plagioclase of An50 might produce
results where either the two thermometers would produce identical results or the silica-saturated
thermometer would have higher values. Calculations based on this premise also returned results
in which the silica-undersaturated temperatures were higher than the silica-saturated ones.
Wintsch and Yi (in press) argued that the amphibole within the Glastonberry Gneiss of the
Bronson Hill Terrane had retained its igneous composition in the presence of two subsequent
metamorphic events. The metamorphic zoning patterns of the plagioclase within the various
thin sections is a strong indication that at least the feldspars underwent growth during the
Acadian and/or Alleghanian orogenies. As an argument against this theory, however, the
amphibole is always aligned with the dominant foliation and is normally the foliation forming
mineral indicating it did participate during the metamorphic events.
Amphibole can contain both Fe2+ and Fe3+, which can not be distinguished from one another with
the electron microprobe. Several methods exist to estimate the amount of Fe2+ vs Fe3+ are
summarized by Robinson et al. (1982). The various methods result in statistically different
amounts of Fe3+ for the same analysis in most cases. Holland and Blundy (1993) specify their
own recalculation for Fe3+, but this is also a >best= estimate. Though Holland and Blundy
(1993) point to not recalculating the ferric/ferrous iron ratio according to their method as a
possible reason for the quartz-undersaturated temperature to be higher than the quartz-saturated
temperature, it is also possible that the error lies in the assumptions being made by the Holland
and Blundy (1993) recalculation.
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Discussion
9.1
Agreement with Thermal Modeling of Wintsch et al. (2001)
9.2
warnings as to using the H&B plag-amph thermometer