metamorphic, thermal, and tectonic evolution of...
TRANSCRIPT
JOURNAL OF PETROLOGY VOLUME 43 NUMBER 11 PAGES 2097–2120 2002
Metamorphic, Thermal, and TectonicEvolution of Central New England
FRANK S. SPEAR1∗, M. J. KOHN2, JOHN T. CHENEY3 ANDF. FLORENCE4
1DEPARTMENT OF EARTH AND ENVIRONMENTAL SCIENCES, RENSSELAER POLYTECHNIC INSTITUTE,
TROY, NY 12180, USA2DEPARTMENT OF GEOLOGICAL SCIENCES, UNIVERSITY OF SOUTH CAROLINA, COLUMBIA, SC 29208, USA3DEPARTMENT OF GEOLOGY, AMHERST COLLEGE, AMHERST, MA 01002, USA4SCIENCE DIVISION, JEFFERSON COMMUNITY COLLEGE, WATERTOWN, NY 13601, USA
RECEIVED APRIL 19, 2001; REVISED TYPESCRIPT ACCEPTED MAY 8, 2002
Mountain, Skitchewaug and Big Staurolite nappes. Reactivation ofA new, detailed tectonic model is presented for the Acadian orogenicthis fabric during thrusting is recorded in some rocks of the Bigbelt of central New England (Vermont and New Hampshire) thatStaurolite nappe by rotated garnets that grew during near-isothermalaccounts for a wide range of petrological and structural observations.loading. Only the sillimanite isograd crosses the Fall Mountain–Three belts are considered: the Eastern Vermont, Merrimack, andSkitchewaug nappe boundary. Metamorphic breaks across the Skit-intervening Bronson Hill belts. Specific observations in easternchewaug–Big Staurolite nappe boundary, at the base of the BigVermont that are accounted for in the model include the following.Staurolite nappe, and at the margin of the Keene and Alstead domesP–T paths are clockwise with maximum pressures near the Athens,require post-metamorphic thrusting when P–T conditions were inChester, and Strafford domes of 8–11 kbar, but with maximumthe greenschist facies. These observations can be explained by apressures decreasing to 3–5 kbar at the boundary with the Bronsonrelatively simple model involving in-sequence thrusting from east toHill belt. Differential exhumation of the Vermont domes relative towest commencing in central New Hampshire at 400–410 Ma.the rocks in easternmost Vermont is required by the recorded differencesPreservation of the low-grade belt along the Vermont–New Hamp-in maximum pressure (5–6 kbar; 15–20 km) and the present-dayshire border requires that crustal thickening in Vermont was notgeographical separation (7–10 km). Specific observations in Newcaused by emplacement of New Hampshire nappes onto easternHampshire that are explained include the following. P–T paths inVermont and that the nappes of western New Hampshire had timethe Merrimack belt are counter-clockwise with maximum pressuresto cool before final juxtaposition against the low-grade belt. Coolingof 4–5 kbar and are related to high regional heat flow and heatages constrain this final juxtaposition to have occurred in thetransfer by early Acadian plutons. P–T paths in the Bronson HillCarboniferous, suggesting that the Acadian was a prolonged eventbelt are intimately associated with structural position. An earlyspanning as much as 100 Myr.contact metamorphism is evidenced in the Skitchewaug and Fall
Mountain nappes near contacts with the early Acadian Bethlehemgneiss (>400–410 Ma). Peak metamorphic temperature risesupwards in the nappe sequence (an inverted metamorphic sequence) KEY WORDS: New England; Vermont; New Hampshire; Acadian; invertedwhereas peak pressures decrease. Near-simultaneous intrusion of the metamorphism; P–T pathsBethlehem gneiss and Kinsman quartz monzonite is required toaccount for the low-P, high-T metamorphism observed in theChesham Pond and Fall Mountain nappes. The dominant schist-
INTRODUCTIONosity, which is related to isoclinal folding, postdates early contactmetamorphism in the Fall Mountain and Skitchewaug nappes, and The Acadian of central New England (Fig. 1) consists of
three belts: the eastern Vermont, Bronson Hill, and thepre-dates peak metamorphism and isothermal loading in the Fall
∗Corresponding author. E-mail: [email protected] Oxford University Press 2002
JOURNAL OF PETROLOGY VOLUME 43 NUMBER 11 NOVEMBER 2002
Fig. 1. Map of New England showing the trends of the major tectonicbelts discussed in this paper: the eastern Vermont, Bronson Hill, andMerrimack belts. The eastern Vermont belt (dark gray shading) ischaracterized by dominantly clockwise P–T paths whereas the Mer-rimack belt (light gray shading) is characterized by dominantly counter- Fig. 2. Generalized geological map (same area as Fig. 3) showingclockwise paths. The intervening Bronson Hill belt is characterized by metamorphic isograds. In Vermont, kyanite-grade rocks are exposedcomplex P–T paths that are intimately related to structural position. in the domes. In New Hampshire, high-grade rocks are exposed in theBox shows location of Figs 2 and 3. Black dot along the Bronson Hill structurally highest nappes to the east. A distinct metamorphic lowbelt shows location of samples from Littleton, NH (8874, 8835, 8809 (chlorite–biotite grade) with closely spaced isograds exists at the bound-and 8848; Fig. 9a). ary between the eastern Vermont and Bronson Hill belts.
Merrimack belts. The traditional view (e.g. White &On closer inspection, the metamorphic histories of theJahns, 1950; Doll et al., 1961; Rosenfeld, 1968; Thompson
three belts contrast markedly (Figs 1 and 2). Rocks from& Norton, 1968; Thompson et al., 1968; Naylor, 1969,eastern Vermont were metamorphosed to the staurolite–1971; Robinson & Hall, 1980; Hatch et al., 1983; Rob-kyanite zone (Fig. 2) and experienced dominantly clock-inson et al., 1991) of the tectonic evolution of this regionwise P–T paths (Fig. 1; Armstrong et al., 1992; Menardis one in which two sedimentary basins, the Connecticut& Spear, 1994; Armstrong & Tracy, 2000) reachingValley trough of eastern Vermont and the Merrimackpressures of 8–11 kbar on the flanks of the Chester andtrough of central New Hampshire, coexisted beginningAthens domes (Kohn & Spear, 1990; Ratcliffe et al., 1992;in the Silurian and extending through the late EarlyKohn & Valley, 1994; Menard & Spear, 1994). InDevonian (Emsian). The Bronson Hill terrane, situatedcontrast, the Merrimack belt is characterized by low-between the two basins, is believed to be the remnant ofpressure, high-temperature metamorphism (Fig. 2) andthe Taconian arc responsible for the Ordovician Taconicdominantly counter-clockwise P–T paths (Fig. 1 inset).orogeny (e.g. Tucker & Robinson, 1990). Both basinsMetamorphic parageneses in the Bronson Hill belt arewere deformed and metamorphosed during the Middleintimately related to a series of nappes in which rocks ofDevonian Acadian orogeny, which is believed to havehigher metamorphic grade are found in higher structuralbeen caused by collision of the Avalon terrane from thelevels. This inverted metamorphic sequence in the Bron-east. Large-scale, west-directed recumbent folding andson Hill belt has been recognized for several decadesthrusting are characteristic of all three terranes, and(Chapman, 1953; Thompson et al., 1968), and recentthe pattern of isograds (Fig. 2) suggests a continuous
metamorphic gradient across central New England. findings (e.g. Spear, 1992, 1993; Kohn et al., 1997) reveal
2098
SPEAR et al. TECTONIC MODEL OF CENTRAL NEW ENGLAND
that juxtaposition of higher-grade rocks upon lower-grade rocks could not have occurred during the peakof metamorphism, but must have occurred followingsubstantial cooling of the high-grade rocks (Kohn et al.,1997). Differences in the P–T evolution of the threeAcadian belts continue through their cooling histories asevidenced by thermochronology studies (e.g. Harrison etal., 1989; Spear & Harrison, 1989).
The purpose of this paper is to present a synthesis ofthe metamorphic history of rocks from central NewEngland and to show how the metamorphism relatesto the tectonic assembly of the terrane. Key to theinterpretation of the tectonic assembly of this region isthe relationship between the P–T evolution and the fabricdevelopment in each structural level. The P–T historieshave been deduced from the metamorphic re-crystallization using a variety of methods includingthermobarometry, garnet zoning analysis (e.g. Spear &Selverstone, 1983), pseudomorph textures, and com-parison of inferred reactions with petrogenetic grids. Thetectonic fabrics are related to major deformation eventsthat involve isoclinal folding and transport. Therefore,the relative timing of a specific part of a P–T path to afabric with a known tectonic significance reveals thedepth and thermal conditions of the crust when thedeformation occurred. For example, several nappes inwestern New Hampshire experienced P–T paths thatinclude a segment of isothermal or near-isothermal load-ing. The loading is interpreted to have occurred in
Fig. 3. Generalized geological map of central New England includingresponse to emplacement of higher-level nappes. There-parts of New Hampshire and Vermont. The boundary between thefore, the relationship between the dominant fabric in the eastern Vermont and Bronson Hill belts is shown as a heavy dotted
lower nappe and the metamorphic recrystallization that line labeled CYL (Chicken Yard line) and ML (Monroe line). Symbolsshow location of samples for which P–T information is provided. CD,records the change in pressure reveals when the lowerChester Dome; AthD, Athens dome; SD, Strafford dome; PD, Pomfretnappe was deformed (the fabric-producing event) relativedome; BG, Bethlehem gneiss; KQM, Kinsman quartz monzonite; AD,
to the emplacement of the higher-level nappes (the load- Alstead dome; KD, Keene dome; MD, Mascoma dome; EVB, Easterning event). Vermont belt; BHB, Bronson Hill belt; MB, Merrimack belt; OB,
Orfordville belt, FM, Fall Mountain. A–A′ shows location of cross-The paper focuses on a transect at the approximatesections in Figs 15 and 16. Box shows location of Fig. 5. Light dashedlatitude of Fall Mountain, New Hampshire (see Fig. 3) line shows Vermont–New Hampshire state border.
starting with the lowest Acadian structural levels of east-ern Vermont and working eastwards and structurallyupwards. Along each part of the transect, the P–T history, EASTERN VERMONT P–Tfabric development, and their relationship will be stressed.
EVOLUTIONA summary of mineral assemblages, compositions, andpeak P–T conditions for all samples referenced in this The eastern Vermont belt includes the Connecticut Val-
ley synclinorium, a sequence of Silurian to Devonianstudy is presented in Table 1. Particular emphasis isplaced on the evolution of western New Hampshire (the metasedimentary and metaigneous rocks, and a sequence
of Cambrian to Ordovician rocks that flank a seriesBronson Hill terrane), because it forms the transitionzone between the fundamentally distinct eastern and of major north–south-trending domes (Athens, Chester,
Strafford), in which are exposed higher-grade and, in thewestern belts. Important variations also occur along striketo the north in western New Hampshire, and these will case of the Athens and Chester domes, Proterozoic
age rocks (e.g. White & Jahns, 1950; Doll et al., 1961;be summarized where appropriate. Finally, the thermal,baric, and structural evolution of the region based on Rosenfeld, 1968) (Fig. 3).
Two major periods of deformation affected the rocksthese data will be integrated into a synthesis that con-strains the timing of the juxtaposition of the distinct of the eastern Vermont belt during the Acadian, an
earlier nappe stage and a later dome stage (e.g. Rosenfeld,tectonic slices.
2099
JOURNAL OF PETROLOGY VOLUME 43 NUMBER 11 NOVEMBER 2002
Tab
le1
:A
ssem
blag
es,
com
posi
tion
san
dP–
Tco
nditio
nsof
sam
ples
used
toco
nstr
ain
tect
onic
evol
utio
nof
cent
ral
New
Eng
land
Are
aan
dS
amp
leTy
pe
Ass
emb
lag
eP
rpA
lmS
ps
Grs
Plg
Bt
Ch
lS
tH
bl
TP
Fig
.Fi
g.
Ref
eren
ce
zon
eo
rFM
FMFM
FMlo
c.P
&T
nap
pe
Easte
rnV
erm
on
tb
elt
Str
affo
rdd
om
e,ea
stfl
ank
Grt
TM
549
Co
reG
rt+
Ch
l+B
t+M
s+P
lg+
Qtz
0·06
20·
536
0·31
80·
084
0·21
–0·2
4∗
∗—
——
—2
4M
enar
d&
Sp
ear
(199
4)
Rim
Grt+
Ch
l+B
t+M
s+P
lg+
Qtz
0·05
70·
568
0·25
40·
120
0·32
0·48
10·
454
——
475
4500
24
Men
ard
&S
pea
r(1
994)
Str
affo
rdd
om
e
Grt
K87
-18
Rim
Grt+
Bt+
Hb
l+P
l+Q
tz0·
098
0·64
00·
054
0·20
90·
140·
450
——
0·51
157
510
500
24
Ko
hn
&S
pea
r(1
990)
Grt
V80
cR
imG
rt+
Ch
l+M
s+P
arag+
Hb
l+P
l+Q
tz0·
101
0·63
60·
106
0·15
60·
18—
0·43
0—
0·59
459
093
002
4La
ird
&A
lbee
(198
1)
Grt
V80
dR
imG
rt+
Ch
l+H
bl+
Pl+
Qtz
0·12
20·
580
0·15
30·
144
0·23
—0·
383
—0·
515
590
7900
24
Lair
d&
Alb
ee(1
981)
Ath
ens
do
me
Grt
V10
6dR
imG
rt+
Bt+
Ch
l+M
s+H
bl+
Pl+
Qtz
0·11
90·
696
0·06
30·
122
0·22
0·48
80·
442
—0·
581
590
8300
24
Lair
d&
Alb
ee(1
981)
Grt
V11
3bR
imG
rt+
Bt+
Ch
l+H
bl+
Pl+
Qtz
0·09
80·
615
0·11
10·
176
0·23
0·46
10·
404
—0·
552
560
8800
24
Lair
d&
Alb
ee(1
981)
Grt
V11
8dR
imG
rt+
Bt+
Ch
l+M
s+H
bl+
Pl+
Qtz
0·11
60·
602
0·15
00·
131
0·22
0·44
30·
362
—0·
532
560
7800
24
Lair
d&
Alb
ee(1
981)
Grt
V11
8eR
imG
rt+
Bt+
Ch
l+H
bl+
Pl+
Qtz
0·11
00·
574
0·13
20·
184
0·33
0·50
10·
445
—0·
645
680
8000
24
Lair
d&
Alb
ee(1
981)
Grt
V11
9cR
imG
rt+
Bt+
Ch
l+H
bl+
Pl+
Qtz
0·11
90·
580·
164
0·13
70·
260·
390
0·38
9—
0·54
857
576
002
4La
ird
&A
lbee
(198
1)
Sax
ton
sR
iver
Grt
N-1
Rim
Grt+
Bt+
Ch
l+M
s+P
arag+
Hb
l+0·
081
0·67
40·
020
0·22
50·
190·
490
0·48
0—
0·58
056
095
002
4K
oh
n&
Sp
ear
(199
0)
Plg+
Qtz
Grt
N-2
Rim
Grt+
Bt+
Ch
l+M
s+P
arag+
Hb
l+0·
072
0·71
90·
014
0·19
50·
180·
550
0·48
5—
0·56
755
090
002
4K
oh
n&
Sp
ear
(199
0)
Plg+
Qtz
Str
affo
rdd
om
e
St–
Ky
TM
445
Co
reG
rt+
Bt+
Ch
l+M
s+P
lg+
Ep+
Qtz
0·04
50·
520
0·18
00·
255
0·47
∗∗
——
——
24
Men
ard
&S
pea
r(1
993,
1994
)
Nea
rri
mG
rt+
Bt+
Ch
l+M
s+P
lg+
Ep+
Qtz
0·07
00·
560
0·12
00·
250
0·20
∗∗
——
475
5800
24
Men
ard
&S
pea
r(1
993,
1994
)
Rim
Grt+
St+
Bt+
Ms+
Plg+
Qtz
0·12
00·
760
0·06
00·
060
0·24
0·43
0—
n.a
.—
530
6800
24
Men
ard
&S
pea
r(1
993,
1994
)
St–
Ky
TM
534
Co
reG
rt+
Bt+
Ch
l+M
s+P
lg+
Ep+
Qtz
0·05
10·
571
0·15
20·
226
0·20
∗∗
——
504
7375
24
Men
ard
&S
pea
r(1
993,
1994
)
Nea
rri
mG
rt+
Bt+
Ch
l+M
s+P
lg+
Ep+
Qtz
0·07
30·
638
0·04
20·
247
0·47
∗∗
——
——
24
Men
ard
&S
pea
r(1
993,
1994
)
Rim
Grt+
Bt+
Ms+
Plg+
Qtz
0·09
50·
740
0·04
50·
120
0·28
0·44
80·
407
——
540
7000
24
Men
ard
&S
pea
r(1
993,
1994
)
St–
Ky
TM
650
Rim
Grt+
Ch
l+H
bl+
Pl+
Qtz+
Ep
0·07
70·
662
0·01
50·
246
0·30
—0·
523
—0·
585
——
24
Men
ard
&S
pea
r(1
993,
1994
)
St–
Ky
TM
825a
Co
reG
rt+
Bt+
Ch
l+M
s+P
lg+
Ep+
Qtz
0·04
60·
652
0·07
40·
228
0·15
0·59
00·
554
——
511
7787
24
Men
ard
&S
pea
r(1
993,
1994
)
(zo
ne
E)
Nea
rri
mG
rt+
Bt+
Ch
l+M
s+P
lg+
Ep+
Qtz
0·07
20·
642
0·03
10·
255∗
∗∗
——
——
24
Men
ard
&S
pea
r(1
993,
1994
)
(B)
Rim
(A)
Grt+
Bt+
Ms+
Plg+
Qtz
0·06
00·
750
0·04
00·
150
0·15
0·50
00·
457
——
530
9000
24
Men
ard
&S
pea
r(1
993,
1994
)
St–
Ky
TM
590a
Co
reG
rt+
Ch
l?+
Bt+
Ep+
Ms+
Pl+
Qtz
0·04
60·
501
0·20
00·
250
0·45
∗∗
——
——
24
Men
ard
&S
pea
r(1
994)
Rim
Grt+
St+
Ky+
Bt+
Ms+
Pl+
Qtz
0·08
40·
716
0·09
90·
101
0·14
0·41
2—
n.a
.—
550
8500
24
Men
ard
&S
pea
r(1
994)
2100
SPEAR et al. TECTONIC MODEL OF CENTRAL NEW ENGLAND
Are
aan
dS
amp
leTy
pe
Ass
emb
lag
eP
rpA
lmS
ps
Grs
Plg
Bt
Ch
lS
tH
bl
TP
Fig
.Fi
g.
Ref
eren
ce
zon
eo
rFM
FMFM
FMlo
c.P
&T
nap
pe
Ch
este
rd
om
e
St–
Ky
V&
HS
eere
fere
nce
for
det
ails
of
min
eral
chem
istr
y2
4Va
nce
&H
olla
nd
(199
3)
Ath
ens
do
me
St–
Ky
V15
2R
imG
rt+
Bt+
Ch
l+H
bl+
Pl+
Qtz
0·12
40·
639
0·08
90·
149
0·24
0·46
90·
412
—0·
499
600
7500
24
Lair
d&
Alb
ee(1
981)
St–
Ky
V15
4bR
imG
rt+
Bt+
Ch
l+H
bl+
Pl+
Qtz
0·09
40·
645
0·09
20·
169
0·20
0·42
50·
390
—0·
543
540
8700
24
Lair
d&
Alb
ee(1
981)
St–
Ky
15A
Rim
Grt+
Bt+
Ch
l+S
t+M
s+P
arag+
Plg+
0·13
80·
707
0·00
20·
153
0·04
/0·1
50·
425
0·38
2—
590
—2
4K
oh
n&
Valle
y(1
994)
Qtz
St–
Ky
15B
Rim
Grt+
Bt+
Ch
l+M
s+P
arag+
Plg+
Qtz
0·11
80·
739
0·01
10·
132
0·21
0·48
40·
456
——
600
8500
24
Ko
hn
&Va
lley
(199
4)
St–
Ky
1SR
imG
rt+
Ch
l+M
s+P
arag+
Plg+
Qtz
0·14
50·
688
0·00
20·
165
0·05
/0·1
5—
0·39
0—
——
—2
4K
oh
n&
Valle
y(1
994)
St–
Ky
1N2
Rim
Grt+
Bt+
Ms+
Plg+
Qtz+
Ep
0·08
80·
618
0·12
10·
172
0·20
0·47
6—
——
560
8450
24
Ko
hn
&Va
lley
(199
4)
St–
Ky
1N3
Rim
Grt+
Bt+
Plg+
Qtz+
Ep
0·09
50·
585
0·14
20·
178
0·22
0·45
9—
——
570
>785
02
4K
oh
n&
Valle
y(1
994)
St–
Ky
1N7
Rim
Grt+
Bt+
Hb
l+P
lg+
Qtz+
Ep
0·09
50·
509
0·18
70·
208
0·27
0·44
1—
—0·
476
590
8250
24
Ko
hn
&Va
lley
(199
4)
Bro
nso
nH
ill
belt
Bel
low
sFa
lls,
VT
Grt
zon
eB
F-53
Co
reG
rt+
Bt+
Ch
l+M
s+P
lg+
Qtz
0·06
20·
783
0·14
40·
010
0·31
∗∗
——
56
Th
isst
ud
y
Rim
Grt+
Bt+
Ch
l+M
s+P
lg+
Qtz
0·06
10·
775
0·14
30·
020
0·14
0·64
0—
—49
5–51
63·
2–3·
85
6T
his
stu
dy
Grt
zon
e93
-19
Co
reG
rt+
Bt+
Ch
l+M
s+P
lg+
Qtz
0·05
70·
779
0·12
20·
041∗
∗∗
——
56
Th
isst
ud
y
Rim
Grt+
Bt+
Ch
l+M
s+P
lg+
Qtz
0·06
80·
821
0·08
00·
031
0·02+
0·15
0·64
70·
635
——
490–
535
3·2–
4·3
56
Th
isst
ud
y
Litt
leto
n,
NH
Big
St.
8874
Grt
core
Grt+
Bt+
Ch
l+M
s+P
lg+
Qtz
0·09
00·
675
0·20
00·
034
0·14
∗∗
——
——
19a
Flo
ren
ceet
al.
(199
3)
nap
pe
Mat
rix
Grt+
Bt+
Ch
l+M
s+P
lg+
Qtz
0·10
60·
725
0·13
80·
030
0·11
0·53
40·
473
——
575
5·6
19a
Flo
ren
ceet
al.
(199
3)
Big
St.
8835
bG
rtco
reG
rt+
Bt+
Ch
l+M
s+P
lg+
Qtz
0·05
20·
732
0·09
60·
120
0·42
∗∗
——
——
19a
Flo
ren
ceet
al.
(199
3)
nap
pe
Mat
rix
Grt+
Bt+
Ch
l+M
s+P
lg+
Qtz
0·08
50·
822
0·01
00·
083
0·29
0·61
80·
574
——
565
5·2
19a
Flo
ren
ceet
al.
(199
3)
Big
St.
8809
Grt
core
Grt+
Bt+
Ch
l+M
s+P
lg+
Qtz
0·07
50·
704
0·17
40·
047
0·25
∗0·
496
——
——
19a
Flo
ren
ceet
al.
(199
3)
nap
pe
Mat
rix
Grt+
Bt+
Ch
l+M
s+P
lg+
Qtz
0·05
40·
404
0·03
20·
021
0·18
0·52
6—
——
540
4·8
19a
Flo
ren
ceet
al.
(199
3)
Big
St.
8848
Grt
core
Grt+
Bt+
Ch
l+M
s+P
lg+
Qtz
0·10
70·
792
0·10
10
0·22
∗∗
——
539
3·6
19a
Flo
ren
ceet
al.
(199
3)
nap
pe
Mat
rix/
Grt+
Bt+
St+
Ms+
Plg+
Qtz
0·08
90·
870·
007
0·03
40·
130·
608
0·49
20·
865
—56
55·
51
9aFl
ore
nce
etal
.(1
993)
rim
2101
JOURNAL OF PETROLOGY VOLUME 43 NUMBER 11 NOVEMBER 2002
Tab
le1
:co
ntin
ued
Are
aan
dS
amp
leTy
pe
Ass
emb
lag
eP
rpA
lmS
ps
Grs
Plg
Bt
Ch
lS
tH
bl
TP
Fig
.Fi
g.
Ref
eren
ce
zon
eo
rFM
FMFM
FMlo
c.P
&T
nap
pe
Mas
com
a–O
rfo
rdvi
lle
Big
St.
nap
pe
D84
-1c
Grt
core
Grt+
Bt+
Ch
l+M
s+P
lg+
Qtz
0·06
10·
693
0·17
70·
069
0·23
∗∗
——
——
29b
Ko
hn
etal
.(1
997)
Rim
Grt+
Bt+
Ch
l+M
s+P
lg+
Qtz
0·06
50·
735
0·13
90·
061
0·14
0·60
60·
582
——
530
5·1
29b
Ko
hn
etal
.(1
997)
Big
St.
nap
pe
77-1
2yC
ore
Grt+
Bt+
Ch
l+H
bl+
Plg+
Qtz
0·12
60·
655
0·11
00·
109
0·40
∗∗
——
——
29b
Ko
hn
etal
.(1
997)
Rim
Grt+
Bt+
Ch
l+H
bl+
Plg+
Qtz
0·15
80·
681
0·05
80·
103
0·27
0·43
40·
402
—0·
541
609
6·8
29b
Ko
hn
etal
.(1
997)
Big
St.
nap
pe
K87
-110
aG
rtco
reG
rt+
Bt+
Ch
l+M
s+P
lg+
Qtz
0·04
20·
466
0·41
30·
079∗
∗∗
——
——
29b
Ko
hn
etal
.(1
997)
rim
Grt+
Bt+
Ch
l+M
s+P
lg+
Qtz
0·06
80·
626
0·22
50·
081
0·21
0·46
60·
450
——
450
4·2
29b
Ko
hn
etal
.(1
997)
Big
St.
nap
pe
MC
9bG
rtco
reG
rt+
Bt+
Ch
l+M
s+P
lg+
Qtz+
Ep
0·05
20·
485
0·26
90·
194
0·74
∗∗
——
——
29b
Ko
hn
etal
.(1
997)
Rim
Grt+
Bt+
Ch
l+M
s+P
lg+
Qtz+
Ep
0·05
70·
487
0·25
80·
198
0·47
0·50
00·
476
——
504
4·7
29b
Ko
hn
etal
.(1
997)
Big
St.
nap
pe
K87
-82g
Grt
core
Grt+
Bt+
Ch
l+M
s+P
lg+
Qtz
0·04
50·
461
0·36
10·
133
0·37
∗∗
——
——
29b
Ko
hn
etal
.(1
997)
Rim
Grt+
Bt+
Ch
l+M
s+P
lg+
Qtz
0·06
50·
621
0·17
10·
142
0·21
0·47
90·
467
——
475
6·1
29b
Ko
hn
etal
.(1
997)
Bel
low
sFa
lls
Big
St.
nap
pe
BF-
18c
Grt
core
Grt+
Bt+
Ch
l+M
s+P
lg+
Qtz
0·07
50·
752
0·13
00·
043
0·22
∗0·
507
——
430–
480
3·2–
3·5
59c
Sp
ear
etal
.(1
990)
Mat
rix
Grt+
Bt+
St+
Ms+
Plg+
Qtz
0·10
00·
841
0·02
60·
033
0·12
0·54
0—
0·84
0—
495
5·6
59c
Sp
ear
etal
.(1
990)
Big
St.
nap
pe
BF-
52a
Grt
core
Grt+
Bt+
Ch
l+M
s+P
lg+
Qtz+
Ep
?0·
020
0·56
80·
190
0·22
10·
25∗
0·57
2—
——
—5
7-p
ho
toT
his
stu
dy
Mat
rix
Grt+
Bt+
St+
Ms+
Plg+
Qtz
0·08
60·
870
0·00
80·
036
0·02
0·59
4—
0·84
0—
580–
620
—5
7-p
ho
toT
his
stu
dy
Big
St.
nap
pe
BF-
22G
rtco
reG
rt+
Bt+
Ch
l+M
s+P
lg+
Qtz
0·09
70·
785
0·06
40·
054∗
∗∗
——
——
57-
ph
oto
Th
isst
ud
y
Mat
rix
Grt+
Bt+
St+
Ms+
Plg+
Qtz
0·09
30·
805
0·06
40·
038
0·08
0·56
2—
0·85
5—
——
57-
ph
oto
Th
isst
ud
y
Ski
tch
ewau
gB
F-64
Co
nta
ctG
rt+
St+
Bt+
Ms+
Plg+
Qtz
n.a
.n
.a.
n.a
.n
.a.
n.a
.n
.a.
——
——
510
-ph
oto
Th
isst
ud
y
nap
pe
Mat
rix
Grt+
Bt+
Ch
l+M
s+P
lg+
Qtz
n.a
.n
.a.
n.a
.n
.a.
n.a
.n
.a.
n.a
.—
——
—5
10-p
ho
toT
his
stu
dy
InG
rt+
Bt+
Ch
l+M
s+S
t(re
lic)
n.a
.n
.a.
n.a
.n
.a.
n.a
.n
.a.
n.a
.n
.a.
——
—5
10-p
ho
toT
his
stu
dy
pse
ud
o-
mo
rph
2102
SPEAR et al. TECTONIC MODEL OF CENTRAL NEW ENGLAND
Are
aan
dS
amp
leTy
pe
Ass
emb
lag
eP
rpA
lmS
ps
Grs
Plg
Bt
Ch
lS
tH
bl
TP
Fig
.Fi
g.
Ref
eren
ce
zon
eo
rFM
FMFM
FMlo
c.P
&T
nap
pe
Ski
tch
ewau
gB
F-12
bC
on
tact
Grt+
An
d+
Bt+
Ms+
Plg+
Qtz
0·07
40·
825
0·05
80·
043∗
∗—
——
——
510
-ph
oto
Th
isst
ud
y
nap
pe
Po
st-
Grt+
Bt+
Ch
l+M
s+P
lg+
Qtz
0·07
10·
839
0·05
50·
033
n.a
.0·
602
0·56
5—
—48
0–52
5—
510
-ph
oto
Th
isst
ud
y
thru
st
Ski
tch
ewau
gB
F-86
bC
on
tact
Grt+
Bt+
An
d+
Ms+
Plg+
Qtz
0·12
60·
730
0·10
00·
044
0·28
∗—
——
——
511
Sp
ear
etal
.(1
990)
nap
pe
Mat
rix
Grt+
Bt+
Ch
l+M
s+P
lg+
Qtz
0·11
20·
738
0·08
70·
063
0·16
0·49
00·
436
——
500
55
11S
pea
ret
al.
(199
0)
InG
rt+
Ky+
St+
Ch
l+M
s+Q
tzn
.a.
n.a
.n
.a.
n.a
.n
.a.
——
0·82
0—
——
5—
Sp
ear
etal
.(1
990)
pse
ud
o-
mo
rph
Fall
Mtn
BF-
14G
rt-1
Grt+
Ch
l+M
s+B
t+P
lg+
Qtz
All
five
gar
net
gro
wth
zon
esar
en
ot
∗—
—50
02
512
Sp
ear
etal
.(1
990)
;
nap
pe
Ko
hn
etal
.(1
997)
klip
pe
K92
-12
Grt
-2G
rt+
Als+
Bt+
Ms+
Plg+
Qtz
pre
sen
tin
asi
ng
lecr
ysta
l.—
——
625
2–4
512
Sp
ear
etal
.(1
990)
;
Ko
hn
etal
.(1
997)
BF-
9G
rt-3
Grt+
Als+
Bt+
Pl+
Qtz+
LS
eere
fere
nce
sfo
rd
etai
lso
fco
mp
osi
tio
ns.
——
—72
04·
55
12S
pea
ret
al.
(199
0);
Ko
hn
etal
.(1
997)
Grt
-4G
rt+
Als+
Bt+
Ms+
Plg+
Qtz
——
—65
0–57
54·
55
12S
pea
ret
al.
(199
0);
Ko
hn
etal
.(1
997)
Grt
-5G
rt+
Ch
l+M
s+B
t+P
lg+
Qtz
——
—47
54·
55
12S
pea
ret
al.
(199
0);
Ko
hn
etal
.(1
997)
Gils
um
Fall
Mtn
BF-
78C
on
tact
Grt
?+A
nd+
Bt+
Ms+
Pl+
Qtz
∗∗
∗∗
∗∗
——
——
—5
12S
pea
ret
al.
(199
5)
nap
pe
roo
tzo
ne
Grt
core
/G
rt+
Sil+
Bt+
Ms+
Pl+
Qtz+
L0·
114
0·69
00·
165
0·03
10·
300·
542
——
—62
03
512
Sp
ear
etal
.(1
995)
mat
rix
Grt
rim
Grt+
Sil+
Bt+
Ms+
Pl+
Qtz+
L0·
084
0·76
10·
120
0·03
50·
210·
579
——
—53
53·
85
12S
pea
ret
al.
(199
5)
Fall
Mtn
89-2
2bG
rtco
reG
rt+
Ch
l+B
t+M
s+P
l+Q
tz0·
092
0·79
70·
075
0·03
60·
260·
612
∗—
—60
04
512
Sp
ear
etal
.(1
995)
nap
pe
roo
tzo
ne
Grt
rim
/G
rt+
Sil+
Bt+
Ms+
Pl+
Qtz
0·08
60·
844
0·02
70·
043
0·20
0·59
3—
——
550–
535
4·2
512
Sp
ear
etal
.(1
995)
mat
rix
Ch
esh
amLM
-1P
eak
Grt+
Crd+
Bt+
Sil+
Qtz+
Plg+
Kfs+
L0·
178
0·75
20·
037
0·03
40·
350·
502
——
—70
03·
2–3·
85
13S
pea
r(1
992,
1993
)
Po
nd
nap
pe
∗Ass
um
edp
rese
nt
inea
rly
par
agen
esis
bu
tn
ot
avai
lab
lefo
ran
alys
is.
n.a
.,n
ot
anal
yzed
;—
,ab
sen
tfr
om
asse
mb
lag
e;FM=
Fe/(
Fe+
Mg
).
2103
JOURNAL OF PETROLOGY VOLUME 43 NUMBER 11 NOVEMBER 2002
Fig. 4. P–T diagrams summarizing peak metamorphic conditions and P–T paths for samples from the eastern Vermont belt (see Fig. 3 forsample locations). (a) Samples from Silurian and Devonian metasediments (Kohn & Spear, 1990; Menard & Spear, 1994). Two shaded boxesand arrows are all from sample TM825a. (b) Samples from pre-Silurian metasediments (Laird & Albee, 1981; Kohn & Spear, 1990; Kohn &Valley, 1994). Curved arrow in (b) shows P–T path from Vance & Holland (1993). Box and straight arrows in (b) show peak P–T conditionsand P–T paths from Kohn & Valley (1994). Al2SiO5 triple point (Holdaway, 1971) and melting reactions (Huang & Wyllie, 1973, 1974; Vielzeuf& Clemens, 1992) shown for reference.
1968). The dominant fabric in the Silurian–Devonian Vermont belt were produced in the Acadian. In contrastto the relatively high pressures recorded in the vicinityrocks is related to WSW-directed transport and nappe
emplacement (White & Jahns, 1950; Rosenfeld, 1968; of the domes, the peak P–T conditions at the garnetisograd east of the domes are 450–500°C and 4–5 kbarMenard & Spear, 1994) and early garnet growth in
the domes and vicinity is synchronous with this fabric (Menard & Spear, 1994) (Fig. 4: sample TM549).Published P–T paths for Silurian–Devonian rocks of(Rosenfeld, 1968; Woodland, 1977; Menard & Spear,
1994; Armstrong et al., 1997). Additionally, Rosenfeld eastern Vermont are dominantly clockwise (e.g. Fig.4a; see also Menard & Spear, 1994), although near-(1968) described late top-to-the-east shearing, recorded
in rotated garnets and thus synchronous with late garnet isothermal loading is also evident over parts of somepaths. P–T paths of pre-Silurian rocks (Fig. 4b) displaygrowth. In contrast, garnet growth east of the domes
near the garnet isograd entirely postdates the dominant heating with loading (e.g. Kohn & Valley, 1994), andheating with unloading (e.g. Vance & Holland, 1993).fabric (Menard & Spear, 1994; sample TM549). Late
chlorite overgrowths on the fabric and replacement of However, the significance of the core P–T conditions ofthe Gassetts schist examined by Vance & Holland (1993)garnet are common across the region.
Peak metamorphic conditions during the Acadian oro- is not clear, because of the possibility that this mayrepresent pre-Acadian recrystallization (e.g. Rosenfeld,geny reached staurolite–kyanite grade in the deepest
exposed Silurian–Devonian rocks of the domes and es- 1968; Karabinos, 1984). Of particular interest is theabsence of sillimanite in regionally metamorphosed rockstimated P–T conditions are in the range of 500–650°C
and 8–11 kbar (Kohn & Spear, 1990; Menard & Spear, from this part of Vermont, indicating that the P–T pathsdid not enter the sillimanite field.1994) (Fig. 4a). Similar peak P–T conditions are cal-
culated from pre-Silurian rocks using garnet rim + The differences in P–T conditions between the domesand the garnet isograd to the east reveal significantmatrix compositions (Fig. 4b) (Kohn & Spear, 1990;
Armstrong et al., 1992; Vance & Holland, 1993; Kohn differential uplift across strike. For example, the peakpressure of sample TM825a from the Strafford dome& Valley, 1994; Armstrong & Tracy, 2000). The similarity
of peak P–T conditions, an Acadian Sm/Nd age for (Fig. 4a) is 10 ± 1 kbar whereas the peak pressure ofsample TM549 from the garnet isograd (Fig. 4a) is onlygarnet rim growth from Gassetts, Vermont (378 Ma;
Vance & Holland, 1993) and late Acadian hornblende 5 ± 1 kbar (Menard & Spear, 1994). The difference inpeak pressures of >5 kbar implies some 18 km ofcooling ages (355–379; Laird et al., 1984; Spear & Har-
rison, 1989) indicate that all of the peak metamorphic structural relief between these samples. The present-dayseparation of these samples is of the order of only 7mineral assemblages observed throughout the eastern
2104
SPEAR et al. TECTONIC MODEL OF CENTRAL NEW ENGLAND
km, requiring >10 km of differential uplift, presumably evolution of the belt. Large-scale, west-vergent recumbentfolds have long been recognized in the area (e.g. Thomp-during emplacement of the domes.
The temperatures recorded at the time each rock son et al., 1968; Thompson & Rosenfeld, 1979; Robinsonet al., 1991), but recent mapping and the association ofexperienced its peak pressure (525 ± 25°C and 480 ±
20°C, respectively) suggest a relatively steep geotherm, distinct metamorphic parageneses with structural levelsuggests that thrust faults are also common in the region.if it may be assumed that these P–T conditions occurred
at the same time. In both rocks, the garnet core overgrows Figure 5 shows our structural interpretation in a part ofsouthwestern New Hampshire. The Fall Mountain andor is synchronous with the nappe-stage fabric, suggesting
that the timing of the peak pressure conditions recorded Chesham Pond thrust faults are shown essentially asby the garnet cores may have been similar. If so, the mapped by Thompson et al. (1968) and Chamberlainimplied geothermal gradient was>2·5°C/km (45°C/18 (1986), respectively. The Skitchewaug nappe outliers inkm). Such a steep instantaneous geotherm is exactly the western map region of Fig. 5 are shown to bewhat is predicted from one-dimensional crustal thickening consistent, with minor modifications, with recent map-models during the early stages of post-thickening re- ping by Armstrong et al. (1997). The root zone of thelaxation (e.g. England & Thompson, 1984). Comparison Skitchewaug thrust fault is shown to be consistent withof the P–T conditions recorded at the peak temperatures stratigraphic mapping and the distribution of pseudo-(7·5 ± 1·5 kbar, 580°C for sample TM825a and again morphs (discussed below). The existence of a structural5 ± 1 kbar, 480°C for sample TM549) indicate a break in the position of the Skitchewaug thrust fault wasseparation of only >10 km at this time and an in- first inferred by Spear (1992, 1993) and was subsequentlystantaneous geothermal gradient of>10°C/km (100°C/ verified by geological mapping (e.g. Armstrong et al.,10 km). The change from an early geothermal gradient 1997). A thrust fault has also been inferred around theof 2·5°C/km to 10°C/km is best explained as the re- borders of the Alstead and Keene domes based onlaxation of a geotherm by thermal conduction that had metamorphic disparity between the cover sequence andbeen perturbed by crustal thickening (e.g. England & the underlying rocks (Kohn & Spear, 1999).Thompson, 1984). Finally, a major decollement has been inferred in the
western part of the region just to the east of the ChickenYard line–Monroe line, here called the Western NewHampshire Boundary Thrust (WNHBT). This thrust
MERRIMACK AND BRONSON HILL fault floors a sequence of garnet- and staurolite-gradeBELTS P–T EVOLUTION rocks that stretches from Massachusetts to northern New
Hampshire, which will here be referred to colloquiallyThe Merrimack belt includes the Merrimack (Centralas the Big Staurolite nappe. The existence of the WNHBTMaine) synclinorium, a series of Silurian to Devonianis inferred from the sharp metamorphic discontinuitymetasedimentary rocks that were extensively intruded bybetween rocks of the Big Staurolite nappe and the under-early Acadian plutons (the Kinsman quartz monzonitelying garnet- and chlorite-zone rocks (see, e.g. Figs 15–17,and Bethlehem gneiss) as well as younger plutons. De-below), which requires considerable post-metamorphicformation consisted of west-directed folding, thrust fault-displacement. It is a testament to the difficulty of purelying and nappe formation (e.g. Thompson et al., 1968;stratigraphic mapping in this terrane that this fault hasThompson, 1985; Chamberlain, 1986; Robinson et al.,not previously been identified, because the metamorphic1991). Between the Merrimack and eastern Vermontparageneses absolutely require its existence. Re-belts lies the Bronson Hill belt. The oldest rocks of theconnaissance field mapping by Spear and Cheney hasBronson Hill belt are the metaigneous suites of theidentified a shear zone of several meters width with west-Ordovician Oliverian magma series, which crop out invergent kinematic indicators in the position of this fault,the cores of a series of gneiss domes. Mantling the domesbut further mapping is required to trace its extent.are metavolcanic and metasedimentary rocks with agesAdditionally, mapping by Armstrong (1995; see alsoranging from late Ordovician to Devonian. These rocksArmstrong et al., 1997) has identified a major shear zonehave also been deformed by isoclinal folding and a seriesnear Bellows Falls, VT, in approximately the requiredof west-directed thrust faults that place higher-grademetamorphic position (the Westminster West Shearmetamorphic rocks upon lower-grade rocks forming anZone), and the WNHBT may also be related to thisinverted metamorphic sequence (Chapman, 1953;structure.Thompson et al., 1968; Spear, 1992, 1993).
In southwestern New Hampshire in the vicinity of FallMetamorphic parageneses in the Bronson Hill belt ofMountain (Figs 3 and 5), there is evidence for threewestern New Hampshire are intimately associated withtectonic foliations. Bedding (S0) and a bedding-parallelstructural level (Chapman, 1953; Thompson et al., 1968;foliation (S1) are both overprinted and commonly erad-Spear et al., 1990, 1995; Spear, 1992, 1993; this study),
which has led to a reinterpretation of the structural icated by a dominant penetrative foliation or crenulation
2105
JOURNAL OF PETROLOGY VOLUME 43 NUMBER 11 NOVEMBER 2002
Fig. 5. Map of a part of western New Hampshire and adjacent Vermont (see Fig. 3 for location) showing metamorphic isograds (dashed lines),inferred structural levels (patterns), and thrust faults (barbed lines). Grt, garnet isograd; St, staurolite isograd; Sil, sillimanite isograd; Mig,migmatite isograd; Crd, garnet + cordierite isograd. Locations of thrust faults are based on a combination of stratigraphy and distribution ofmetamorphic parageneses. Symbols show location of key mineral assemblages.
cleavage (S2). This foliation is interpreted to be associated Each of the nappes has experienced a different P–Thistory. Most importantly, the part of the P–T pathswith early west-vergent isoclinal folding (see Thompsonalong which the dominant fabric (S2 in southwestern Newet al., 1968; Robinson et al., 1991) and locally has beenHampshire and S1 in west–central and northern Newreactivated during later thrusting, although it is importantHampshire) is developed differs between nappes, and thisto note that S2 may not be of the same age or origin inrelationship can be used to infer the depth and thermalrocks of all structural levels. Towards the WNHBT, aconditions of the crust during deformation. The pertinentthird tectonic fabric (S3) is common. The S3 fabric rangesobservations will be presented below, beginning within intensity from kink bands to penetrative shear fabricthe lowest (westernmost) structural level and workingthat completely disrupts the S2 fabric. Greenschist-faciesstructurally upwards (eastward).alteration is common near the WNHBT and the intensity
of the alteration correlates with the intensity of the S3
deformation, suggesting that fluids responsible for thisalteration gained access along shear zones. In contrast,
Low-grade beltonly two tectonic fabrics have been observed in west–central and northwestern New Hampshire [e.g. in the A zone of chlorite–biotite-grade rocks containing thevicinity of the Orfordville belt and Mascoma dome (Fig. assemblage quartz + chlorite ± biotite + muscovite3) and in the Littleton area (Fig. 1)]. In addition to + albite + K-feldspar occurs at the boundary betweenbedding (S0), a strong foliation (S1) is axial planar to the Vermont and New Hampshire sequences (theisoclinal folds, and again is thought to be the result of Chicken Yard line–Monroe line; Figs 2, 3 and 5). Thewest-directed fold and thrust nappes (Rumble, 1969; Chicken Yard line has been variously described as anSpear & Rumble, 1986; Kohn et al., 1992). A cross- unconformity, a normal stratigraphic succession, and acutting cleavage (S2) is locally present and thought to be fault (Trzcienski et al., 1992; Thompson et al., 1997).
Mylonites up to several meters thick occur at the Chickenthe result of post-nappe (D2) upright folding.
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Yard line with strain features such as mica fish and pyrite Orfordville beltcubes with asymmetric tails. Quartz deformation was In the northern half of the map area of Fig. 3, a sequenceductile whereas feldspar deformation was brittle, sug- of rocks collectively referred to as the Orfordville beltgesting that mylonitization occurred at temperatures be- crops out. Originally mapped as a separate formationtween 300 and 400°C, consistent with the metamorphic believed to be the oldest rocks in New Hampshire (Had-grade. The presence of greenschist-facies recrystallization ley, 1942), they are now correlated with other Ordovician,in the mylonitic rocks near the Chicken Yard line suggests Silurian and Devonian rocks of the Bronson Hill sequencethat at least some of the shearing took place under these (Thompson et al., 1968).conditions. The P–T evolution of rocks within the Orfordville belt
At the latitude of Fall Mountain, isograds near the has been described by Spear & Rumble (1986), Kohn etal. (1992) and Florence et al. (1993). P–T paths areChicken Yard line are closely spaced; locally the garnettypically clockwise with maximum pressures of 6–7 kbarzone occurs only a few tens of meters east of the Chickenand maximum temperatures of 475–575°C, dependingYard line (Figs 2 and 5). Garnet-bearing assemblageson the metamorphic grade (see Kohn et al., 1992, fig. 7;(localities BF-53 and 93-19) contain garnet + biotite +Florence et al., 1993, fig. 15). The metamorphic gradientchlorite+ muscovite+ plagioclase+ quartz± graph-within the belt is normal, with higher-grade rocks exposedite. Biotite is porphyroblastic and pre-dates much of thein deeper structural levels. If the chlorite–biotite-gradedeformation inasmuch as it is deformed into fish-likerocks to the west of the Ammonoosuc fault are rep-structures. Garnet is small (0·5 mm diameter) and gen-resentative of high structural levels of the Orfordvilleerally makes up only a few modal percent of the as-belt rocks, then the range of metamorphic temperaturessemblage. The dominant foliation in rocks of the garnetimplied by staurolite–kyanite-grade rocks in the deepzone (S2) is a penetrative crenulation cleavage. Figure 6astructural levels (>575°C) and chlorite–biotite-gradeillustrates small pressure shadows of the S2 fabric de-rocks in the high structural levels (>450°C) must haveveloped around a low-grade garnet from sample BF-53,been of the order of 125°C. At 25°C/km this suggests 5suggesting that garnet growth in this sample pre-datedkm of structural throw across the Ammonoosuc fault.S2. Figure 6 also shows X-ray maps of the chemical
Garnet growth in schists and felsic metavolcanics ofzoning in this garnet. Mg, Fe, Mn and Ca are very nearlythe Orfordville belt was, at least in part, synchronousunzoned [Xprp = 0·06; Xalm = 0·76; Xsps = 0·14; Xgrs =with development of the nappe fabric whereas staurolite0·04; Fe/(Fe+Mg)= 0·93] because of limited progressand kyanite typically overgrow this fabric [samples 77-on the garnet-producing reaction chlorite + quartz =15A, 79-149D, 68-422V in fig. 4 of Spear & Rumblegarnet+ H2O. Plagioclase is zoned from approximately(1986)]. Chemical zoning profiles from the syntectonicAn31–33 in the core to An2–7 on the rim. The P–T conditionsparts of these garnets typically show decreasing Mn,of garnet crystallization for two samples from the garnetantithetically increasing Fe, and little change in Ca orzone (BF-53 and 93-19) are estimated to be 475–530°C,Fe/(Fe + Mg). Plagioclase inclusions typically become3–4 kbar based on garnet–biotite geothermometrymore albitic from core to rim. The decrease in XAn atand garnet–plagioclase–muscovite–biotite–quartz geo-nearly constant XGrs and Fe/(Fe + Mg) results in cal-barometry (Fig. 6f ). The lack of significant reactionculated P–T paths of nearly isothermal loading for theprogress involving garnet in these samples precludessyntectonic portions of the garnets. These P–T pathsdetermination of the P–T path.suggest that metamorphic recrystallization occurred in
The garnet isograd roughly parallels the Chicken Yard response to loading from higher-level nappes, and theline to the north until it enters the Orfordville belt, where rotated garnets from which the P–T paths are derivedit displays a distinct northward bulge (Fig. 2). This is suggest that there was reactivation of the dominant fabricbelieved to be largely due to the distribution of bulk in these rocks during this higher-level nappe em-compositions suitable for the formation of garnet: in placement.the Orfordville belt the isograd is drawn based on theoccurrence of garnet in felsic metavolcanic rocks. There-fore, the garnet isograd in the southern part of the
Big Staurolite nappeOrfordville belt should not be taken as indicative ofconstancy of peak metamorphic temperature. Along the To the east of the garnet zone at the latitude of Fallwestern part of the Orfordville belt, the garnet and Mountain, across the WNHBT, lies what is here calledstaurolite–kyanite zones are juxtaposed against chlorite– the Big Staurolite nappe. This nappe can be traced frombiotite-grade rocks along the Ammonoosuc fault. In- central Massachusetts to northern New Hampshire andterpreted originally as a west-side-up thrust fault (Billings, is informally named after the characteristic metamorphic1937; Hadley, 1942), it is now believed to be a normal paragenesis of late (post-D2), large staurolite por-
phyroblasts, which are common in many rocks of thefault (west-side-down) (e.g. Thompson et al., 1968).
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Fig. 6. Sample BF-53 (garnet zone: Grt + Chl + Bt + Ms + Pl + Qtz). (a) Photomicrograph (plane-polarized light). Box in (a) showslocation of (e). (b)–(d) X-ray maps showing zoning in garnet. It should be noted that there is very little zoning as a result of limited reactionprogress. (e) X-ray map showing plagioclase zoning. (f ) P–T diagram for samples BF-53 and 93-19 (also garnet zone) showing peak metamorphicconditions for garnet-zone samples (this study).
nappe. Rocks of the Big Staurolite nappe are equivalent Along strike to the north of Fall Mountain (Fig. 3), themetamorphic grade decreases to garnet and, locally,to rocks of the Hardscrabble, Garnet Hill, and Salmon
Hole Brook synclines in the Orfordville and Littleton biotite grade (locations MC-9 and D84-1; Fig. 3), andincreases again to the staurolite zone (again with largeareas of New Hampshire, respectively.
Much of the Big Staurolite nappe is in the staurolite staurolite crystals) near Littleton, New Hampshire (Fig.1). From this along-strike variation, it is surmised that azone and is characterized by distinctively large staurolite
crystals (1–5 cm) that typically overgrow the dominant near-vertical metamorphic gradient exists within thisnappe. Furthermore, near Littleton, the Ammonoosuc(S2) foliation in the rocks (Fig. 7). In some samples, a
well-developed crenulation cleavage is preserved within fault has juxtaposed low-grade equivalents (chlorite–biotite zone) of the large staurolite rocks of the Salmonstaurolite crystals (Fig. 7b) whereas in other samples, S2
is penetrative and staurolite overgrows a straight foliation Hole Brook syncline against the Walker Mountain syn-cline (Billings, 1937, 1992; Moench, 1989, 1992). If these(Fig. 7a). Fabrics within and surrounding garnet crystals
suggest that garnet growth in different samples was pre-, rocks are indeed equivalent, the range in temperatureswithin the Big Staurolite nappe in the Littleton regionsyn-, or post-tectonic. For example, garnet crystals in
both samples in Fig. 7 are pre-tectonic whereas a sample must have been greater than 100°C (i.e. 575°C at thebase and <475°C at the top).illustrated by Kohn et al. (1992, sample D84-1C, fig. 8)
is syn-tectonic. Metamorphic parageneses have been described forrocks of the Big Staurolite nappe by Spear et al. (1990;Peak metamorphic conditions in the Big Staurolite
nappe near Fall Mountain are in the staurolite zone. near Fall Mountain, NH: sample BF-18c), Florence et al.
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Fig. 7. Photomicrographs showing texture of large (late) staurolitesamples within the Big Staurolite nappe. (a) BF-52a. Staurolite hasovergrown S2 (nappe stage) fabric. Inclusion-free staurolite is presentin cores and may represent early (contact) staurolite. Margins ofstaurolite are altered to chlorite. (b) Sample BF-22. Staurolite overgrowsS2 (nappe stage) crenulations. Numerous small (early, contact) garnetsare present inside staurolite and in matrix.
Fig. 8. X-ray maps showing garnet zoning in Big Staurolite nappesample BF-52a. (a) Fe/(Fe + Mg); (b) spessartine; (c) grossular; (d) XAn,Pl.(1993; near Littleton, NH: samples 8835b, 8848, 9047c,Matrix is Qtz + Ms + Bt + Pl. Zoning is indicative of growth
8874, 8809, K8826, LT2a), and Kohn et al. (1992; zoning by the reaction chlorite + quartz ± epidote = garnet +H2O. Numbers are mole fractions of indicated components.near Hanover, NH: samples MC-9b, D84-1c, K87-110A,
K87-82G, and 77-12Y). Sample BF-52a is typical ofthese, and shows garnet growth zonation (Fig. 8) withhigh Mn cores and bell-shaped zoning profiles (XSps = changes in grossular content of garnet reveals increases0·18–0·02). Ca decreases strongly in sample BF-52a in pressure during growth (Fig. 9). Significantly, along(XGrs= 0·24–0·05) suggesting epidote was present in the the entire strike of the Big Staurolite nappe from theassemblage during initial garnet growth, but garnets from Massachusetts border to Littleton, NH (over 150 km),other samples have lower Ca contents and are relatively P–T paths consistently show nearly isothermal loadingunzoned in Ca [e.g. sample BF-18c; figs 11 and 12 of of 1–4 kbar (3–15 km) during garnet growth (Fig. 9),Spear et al. (1990)]. Fe/(Fe+Mg) decreases only slightly which is interpreted to have been caused by the em-from core to rim (from 0·96 to 0·93). The small decrease placement of higher-level nappes (the Skitchewaug, Fallin Fe/(Fe+ Mg) in a garnet growing in the assemblage Mountain, and Chesham Pond nappes). Furthermore,garnet+ chlorite+ biotite+ muscovite+ plagioclase many garnets from the Big Staurolite nappe that grew+ quartz indicates only a small increase in temperature during isothermal loading are syn-tectonic, suggesting
local reactivation of foliation during nappe emplacement.during growth. Correlation of plagioclase zoning with
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Fig. 9. Summary of P–T evolution of samples from the Big Staurolitenappe. (a) Littleton, NH, area (Florence et al., 1993; see Fig. 1 forlocation); (b) Mascoma–Orfordville area (Kohn et al., 1992; see Fig. 3 Fig. 10. Photomicrographs of muscovite pseudomorphs after staurolitefor location); (c) Bellows Falls area (Spear et al., 1990; see Fig. 5 for (a: sample BF-64) and andalusite (b: sample BF-12) in the Skitchewauglocation). All P–T paths show loading in response to emplacement of nappe (see Fig. 5 for locations). Late chlorite crosscuts fabric in theoverlying Skitchewaug, Fall Mountain and Chesham Pond nappes. It matrix (several are indicated in each photo). Pseudomorph reaction isshould be noted that samples in (a) are 150 km north of the sample in staurolite or andalusite + biotite = garnet + chlorite + muscovite.(c) indicating consistent metamorphic response to loading along strike. It should be noted that the S2 (nappe stage) fabric wraps around butThe three models in (c) invoke different assumptions about the as- does not deform pseudomorph muscovite.semblage present during garnet growth: I, Grt + St + Bt + Chl; II,Grt + St + Bt; III, Grt + Chl + Bt.
staurolite can still be found (Fig. 10a). In still otherRocks in which garnet apparently postdates the dominant samples, a second generation of minerals has grownfabric are interpreted as having not experienced sig- within the mica pseudomorph including the mineralsnificant reactivation of foliation whereas rocks in which staurolite, kyanite and fibrolitic sillimanite [e.g. fig. 9 ofgarnets apparently pre-date the dominant fabric must Spear et al. (1990)]. Another characteristic feature of manyhave experienced considerable flattening of the foliation samples is the development of chlorite porphyroblasts infollowing loading and garnet growth. the matrix that cut across and include the S2 foliation.
Andalusite pseudomorphs (Fig. 10b) are restricted tothe upper levels of the nappe whereas staurolite pseudo-
Skitchewaug nappe morphs (Fig. 10a) are found in the lower part. NearFall Mountain (e.g. sample BF-12, Fig. 5), andalusiteRocks of the Skitchewaug nappe (Fig. 5) have a distinctivepseudomorphs occur within a few meters of the Bellowsparagenesis. Common in the schists of this nappe areFalls pluton, which lies structurally above the nappe. Thepseudomorphs comprising predominantly white micasearly andalusite and staurolite porphyroblasts that arethat are after either andalusite or staurolite (Fig. 10; seeunique to the Skitchewaug nappe are interpreted asalso Spear et al., 1990, Fig. 9). In most samples, the
pseudomorph reaction is complete but in others relict contact metamorphic minerals and it is inferred that
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SPEAR et al. TECTONIC MODEL OF CENTRAL NEW ENGLAND
an inverted metamorphic and thermal gradient existedduring this early contact metamorphic event.
The white mica that forms the pseudomorphs is notdeformed, indicating that the pseudomorph reaction oc-curred following development of the dominant fabric, S2.Furthermore, S2 appears to be wrapped around thepseudomorphs (e.g. Fig. 10a), suggesting that the por-phyroblasts were present at the time of S2 development.Therefore, the parageneses of these samples suggest thesequence (1) early porphyroblast formation, (2) de-velopment of S2 (the nappe stage fabric related to isoclinalfolding), and (3) development of the pseudomorphs andmatrix chlorite.
A P–T path for rocks from this structural level wascalculated by Spear et al. (1990) from zoning in garnetand plagioclase from a sample containing pseudomorphsafter andalusite [sample BF-86B; figs 9b, 14, 15 and 16of Spear et al. (1990)—it should be noted that the samplelocation is misplaced in fig. 1 of that paper and shouldbe closer to sample BF-89]. Garnet from this sample iszoned with slightly decreasing Mn and increasing Ca.
Fig. 11. P–T evolution of pseudomorph-bearing, Skitchewaug nappeFe, Mg and Fe/(Fe+Mg) are relatively unzoned except samples. (a) Sample BF-86b (from Spear et al., 1990; see Fig. 5 forat the rim where Fe/(Fe + Mg) increases from 0·85 to location). The three P–T paths (Traverses 1A, 2A, and 2B) are from
three core–rim traverses on a single garnet. (b) Generalized P–T paths0·89. Plagioclase zoning and inclusions within garnetfrom Skitchewaug nappe samples with reactions and AFM diagramsindicate An32 in equilibrium with garnet core and An22 superimposed. Low-temperature path is staurolite pseudomorph re-
in equilibrium with garnet rim. The preferred model for action from (a) (see also Fig. 10a); high-temperature path is andalusitepseudomorph reaction (see Fig. 10b).the evolution of this sample is garnet core growth with
the early assemblage garnet + biotite + andalusite +muscovite+ plagioclase+ quartz and later growth with
be noted also that the absence of deformation in thethe assemblage garnet+ biotite+ chlorite+muscovite pseudomorphs themselves suggests little reactivation of+ plagioclase + quartz. The P–T path [Fig. 11; seeS2 during loading.
also fig. 16 of Spear et al. (1990)] shows a period of nearlyisobaric heating through the andalusite field (contactmetamorphism from the Bellows Falls pluton) followed
Fall Mountain nappeby 2–3 kbar (>7–10 km) of loading. The sharp changefrom isobaric heating to isothermal loading occurs at the Rocks from the Fall Mountain klippe have been theassumed assemblage change. As with the lower-level subject of intensive study (Spear et al., 1990; Spear &nappes, the increase in pressure recorded in the P–T Kohn, 1996; Kohn et al., 1997) and rocks from the rootpaths of the Skitchewaug nappe is interpreted to have zone have been considered by Spear et al. (1995) (Fig.occurred in response to the emplacement of higher-level 12). Rocks from both places have P–T paths that involvenappes (the Fall Mountain and Chesham Pond nappes). an episode of nearly isobaric heating in the andalusiteThe pseudomorphing reaction suggested by Spear (1992, field followed by loading of >2 kbar. Rocks from the1993) is the retrograde progress of the typical prograde Fall Mountain klippe underwent dehydration meltingreactions garnet + chlorite + muscovite = staurolite (Spear & Kohn, 1996; Kohn et al., 1997) as did some+ biotite+ H2O or garnet+ chlorite+ muscovite= rocks from the root zone.andalusite + biotite + H2O. Because the garnet from The early, low-pressure metamorphism experiencedwhich the P–T path was calculated was produced by by rocks of the Fall Mountain nappe is evidenced by earlythese reactions, the pseudomorph reactions must have andalusite porphyroblasts that have been pseudomorphedalso proceeded during loading. These reactions both have by sillimanite [e.g. fig. 2a of Spear et al. (1990)]. Andalusitepositive P–T slopes (Fig. 11b). Accordingly, an increase pseudomorphs are most abundant in the vicinity ofin pressure will stabilize the low-temperature assemblage, the Bethlehem gneiss (Bellows Falls pluton), and areprovided sufficient H2O is added to the rock. The source interpreted as a contact metamorphic assemblage. Theof the fluids necessary to drive these reactions to the left andalusite pseudomorphs are generally oriented in ran-is not known, but the thrust faults associated with the dom planar arrays but sometimes form a mineral lineation
and are locally folded by F2 (nappe stage) folds.loading are likely conduits for fluid migration. It should
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Fig. 12. P–T diagram showing summary of P–T paths from the FallMountain nappe (see Fig. 5 for sample locations). Black path: P–T Fig. 13. P–T diagram showing P–T path of Chesham Pond nappe
rocks. Parallelogram shows near-peak P–T conditions calculated frompath of Fall Mountain klippe (Spear et al., 1990; Kohn et al., 1997).Grey path: P–T path of Fall Mountain root zone (Spear et al., 1995). thermobarometry. Muscovite breakdown reactions and garnet + cor-
dierite stability field shown for reference (from Spear et al., 1999).Muscovite breakdown and melting reactions shown for reference. Fivegenerations of garnet growth (Grt1–Grt5) are documented in the FallMountain klippe (Kohn et al., 1997).
grade reaches the cordierite + garnet zone and mig-matites are typical. K-feldspar is common in these rocks,Five generations of garnet growth have been docu-suggesting that partial melting occurred following mus-mented in rocks of the Fall Mountain klippe (Kohn etcovite breakdown to Al2SiO5 + K-feldspar, which re-al., 1997) and two to three generations in rocks of thequires a prograde P–T path below>4 kbar (Spear et al.,root zone (Spear et al., 1995). The early generation of1999). Peak P–T conditions are 725°C, 3–4 kbar andgarnet (Grt1) pre-dates the S2 fabric whereas the latestthe path is slightly counter-clockwise (Fig. 13). Sig-generations (Grt4 and Grt5) clearly postdate it [e.g. fig.nificantly, the entire P–T evolution occurred at low2c of Spear et al. (1990) and fig. 2 of Spear et al. (1995)].pressure, and there is no evidence of a loading event asThe absence of obvious inclusion relationships makesis seen in the Fall Mountain nappe P–T paths.it difficult to ascertain the relationship between fabric
Leucosomes in migmatites from the Chesham Ponddevelopment and intermediate-generation garnets (Grt2nappe are locally concordant with the S2 fabric, andand Grt3). Some of the leucosomes from the migmatitessome leucosomes are sigmoidal. However, the bulk ofare mildly deformed, displaying sigmoid shapes, but mostthe leucosomes are undeformed, and micas produced onare undeformed, indicating that development of S2 wascrystallization of the leucosomes are completely un-over before the peak temperature was achieved. Latedeformed. These observations suggest that deformationmuscovites produced during crystallization of leucosomeswas over by the peak of metamorphism.crosscut the S2 fabric and are completely undeformed.
Peak temperatures experienced by rocks of theThe loading experienced by rocks of the Fall MountainChesham Pond and Fall Mountain nappes are similar,nappe is interpreted to have occurred in response toalthough the pressure at the peak temperature was loweremplacement of the higher-level Chesham Pond nappe,in the structurally higher Chesham Pond nappe. At-and the change in pressure recorded by rocks of the Falltainment of a peak temperature of 720–750°C at aMountain nappe (�P = 2·5 kbar) suggests that thepressure of 3 or 5 kbar requires input of heat, and theChesham Pond nappe was >8 km thick. Additionally,likely candidates are the Bethlehem gneiss and Kinsmaninasmuch as the loading was followed by isobaric heatingquartz monzonite. A single pluton at a temperature ofby as much as 100°C, it is inferred that the Chesham900°C intruding a country rock at 500°C can achieve aPond nappe was hot (locally over 750°C) when it wasmaximum temperature in the contact aureole ofemplaced.>700°C, but the temperature of the contact aureoledrops off to >600°C in a few hundred meters from thecontact. Although this type of aureole is consistent with
Chesham Pond nappe what is observed in the Skitchewaug nappe beneath theBellows Falls pluton, it is too steep a gradient to beThe highest structural level in central New England is
the Chesham Pond nappe. Parageneses of rocks from consistent with the regional low-pressure, high-tem-perature metamorphism seen in the Fall Mountain andthis structural level have been described by Chamberlain
(1986), Spear (1992) and Spear et al. (1999). Metamorphic Chesham Pond nappes. A possible explanation is that
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the high-grade metamorphism in these nappes was pro-duced by overlapping thermal aureoles from both theBethlehem gneiss and Kinsman quartz monzonite. Over-lapping thermal aureoles require that both plutons intrudewithin a few tens of thousands of years of each other,because the time constant for thermal decay of thesekilometer-thick, sheet-like plutons is only of the order of30 kyr. More-or-less simultaneous intrusion is within theprecision of the ages of crystallization of these plutons[i.e. the Bethlehem gneiss has been dated at 395–410Ma (Aleinikoff & Moench, 1987; Moench, 1989; Moench& Aleinikoff, 1991; Kohn et al., 1992) and the Kinsmanquartz monzonite has been dated at 402–413 Ma (Lyons& Livingston, 1977; Barreiro & Aleinikoff, 1985)]. Ascenario that fits the available petrological data calls forthe Bethlehem gneiss to intrude first, producing contactaureoles above and below. The Chesham Pond nappeis then emplaced with simultaneous intrusion of the
Fig. 14. P–T diagram summarizing the P–T conditions of intrusionKinsman quartz monzonite, causing the increase in pres-of the Oliverian magma series of the Alstead and Keene domes (fromsure seen in the Fall Mountain nappe followed by raisedKohn & Spear, 1999). Stippled boxes are garnet core (magmatic) P–T
temperature that produced anatexis. Although field maps conditions; gray boxes are garnet rim (retrograde) P–T conditions;are insufficiently precise to permit verification because unfilled boxes show P–T conditions of the immediate cover rocks to
the domes (AD, Alstead dome; KD, Keene dome). Following coolingof poor outcrop exposure, it is likely that the Kinsmanfrom magmatic temperature, there is no evidence that the dome rocksquartz monzonite intruded along the Chesham Pond were ever heated again to conditions of the immediate cover rocks.
thrust, and presumably helped lubricate the fault surface.The Fall Mountain thrust is coincident with the Beth-lehem gneiss in places and it is similarly likely that thrust Retrograde metamorphismmovement was aided by a partially molten intrusion. High-temperature ‘retrograde’ metamorphism is de-However, at least some of the movement on the Fall veloped in some lithologies, and is especially prevalentMountain thrust must have occurred following cooling in the Chesham Pond and Fall Mountain nappes. Rocksof the Fall Mountain nappe and Bethlehem gneiss to of both nappes locally experienced partial melting and>550°C because rocks of the underlying Skitchewaug the water released from the crystallization of leucosomesnappe reveal no temperature rise following loading by promoted hydration reactions during cooling. For ex-the Fall Mountain nappe, which would have occurred ample, in both the Fall Mountain klippe and the Cheshamhad the nappe been near its thermal peak of 725°C. Pond nappe, retrograde progress of the reaction sil-
limanite + biotite = garnet + K-feldspar + melt ispervasive and in the Fall Mountain klippe, retrograde
Alstead and Keene domes progress of muscovite + quartz = sillimanite + K-feldspar + melt has produced abundant late muscovite.The Bronson Hill anticlinorium contains a series of domes
that are cored by gneisses of the Ordovician Oliverian In rocks that contain the subsolidus assemblage sillimanite+ biotite+ garnet+ muscovite+ quartz, cooling hasmagma series (the Alstead, Keene and Mascoma domes
of Figs 3 and 5). The cover sequence to the domes (i.e. produced late garnet and muscovite by the water-absentreaction sillimanite + biotite = garnet + muscovitethe Big Staurolite nappe; Fig. 5) has generally been
interpreted as being para-autochthonous to the dome (i.e. garnet generation G4 of Fig. 12; Spear et al., 1990,1995, 1999; Kohn et al., 1997).rocks. However, Kohn & Spear (1999) presented evidence
that the Oliverian magmas intruded at pressures of 8–10 Greenschist-facies retrograde metamorphism is de-veloped locally throughout western New Hampshire. Inkbar and that the metamorphic grade (greenschist facies)
of the dome rocks themselves never reached the same some localities it is pervasive (i.e. most rocks in somelocalities contain late chlorite alteration). For example,grade (amphibolite facies) as the cover sequence during
Acadian metamorphism (Fig. 14). These observations Kohn et al. (1997) described from the Fall Mountainklippe a fifth generation of garnet growth in rocks,require both a structural break between the dome rocks
and the overlying metavolcanic and metasedimentary generally in association with chlorite by the reactionsillimanite + biotite + H2O = garnet + chlorite +cover sequence and that the two rock suites were not
juxtaposed until the cover rocks had cooled somewhat muscovite (i.e. Grt5, Fig. 12). Widespread chloritizationis also evident in rocks at the base of the Big Staurolitefrom their peak temperature.
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nappe. The intensity of chloritization increases as the emplacement of the overlying nappes. The absence ofWestern New Hampshire Boundary Thrust is ap- such a pressure increase in the Chesham Pond nappeproached to the extent that rocks at the base of the suggests that it is the highest structural level.unit are nearly completely converted to greenschist-facies Figure 17 summarizes the peak P–T conditions in aassemblages (although pseudomorphs after staurolite re- schematic diagram in which the different structural levelsmain). Greenschist-facies assemblages (chlorite + albite are stacked vertically. It should be noted that this rep-+ epidote after plagioclase + hornblende) are also resentation is not meant to imply that the New Hampshiredeveloped in shear zones at the margin of the Alstead sequence rocks were ever as far west as the Chester domedome (Kohn & Spear, 1999). in Vermont. Clearly apparent is the fall in temperature
In summary, high-grade retrogression was produced and pressure going upward from the Vermont domespervasively from melt crystallization and then locally towards the Chicken Yard line (a normal metamorphicwhere the appropriate water-absent assemblage could sequence), and the discontinuities in pressure and tem-proceed to react. From this observation it does not appear perature across thrust surfaces through the inverted meta-that retrograde fluids permeated the nappe sequence morphic sequence in the New Hampshire rocks. It isduring initial cooling [see also Kohn et al. (1997) for important to emphasize that Fig. 17 does not representisotopic evidence of closed-system behavior at high tem- a crustal thermal structure at any point in time, but is,perature]. Greenschist-facies retrogression, however, is rather, a representation of the maximum P and T con-widespread, has affected all nappes, and is most intense ditions achieved in each structural level. Indeed, thein and near faults and shear zones. Furthermore, Kohn marked P–T discontinuity at the Chicken Yard lineet al. (1997) presented stable isotope evidence that this virtually requires that the New Hampshire nappes werelate retrograde hydration occurred at Ζ475°C. From cool before being emplaced into their present position.these observations, it is concluded that the final assemblyof the nappe pile probably occurred at the time ofgreenschist-facies alteration and that fluid causing the
Tectonic summaryretrogression probably gained access along the faults andThe combined petrological and textural observationsshear zones.presented above provide constraints on the tectonic as-sembly of the central New England metamorphic beltduring the Acadian orogeny. The earliest metamorphismSUMMARY AND INTERPRETATIONobserved is the contact metamorphic aureoles associated
OF P–T EVOLUTION ACROSS with the Bethlehem gneiss (Bellows Falls pluton), whichproduced andalusite-bearing assemblages both above andCENTRAL NEW ENGLANDbelow the pluton in the Fall Mountain and SkitchewaugFigures 15–17 summarize the maximum P–T conditionsnappes, respectively (Fig. 18a). Intrusion of the Kinsmanand P–T paths experienced by rocks across strike inquartz monzonite at a structurally higher level must havecentral New England. Maximum pressures range fromoccurred only shortly thereafter, along with thrusting ofnearly 10 kbar in the eastern Vermont domes to 6 kbarthe Chesham Pond nappe over the Fall Mountain nappein central New Hampshire (Figs 15 and 17). Maximum(Fig. 18b). Continued westward thrusting emplaced thetemperatures in eastern Vermont are only of the orderFall Mountain and Chesham Pond nappes on the Skit-of 600°C whereas maximum temperatures in centralchewaug nappe (Fig. 18c).New Hampshire approach 750°C. The pattern of iso-
It is significant that the early contact metamorphicgrads shows strong correlation with structural positionassemblage of the Fall Mountain nappe is similar to that(Fig. 15d). Isograds in eastern Vermont are symmetricalof the underlying Skitchewaug nappe (andalusite +about the domes, with grade increasing with structuralgarnet + biotite), but then the Fall Mountain nappedepth. In western New Hampshire, isograds depictingexperienced a considerable rise in temperature followingearly metamorphic history are coincident with inferredloading whereas the Skitchewaug nappe experiencedthrust faults. Only the sillimanite isograd crosses structuralonly minor heating or isobaric cooling. The simplestboundaries (see Figs 2 and 5).explanation is that the nappes were not vertically jux-P–T paths also show marked differences across striketaposed at the time of the peak of metamorphism in the(Fig. 16). Vermont P–T paths are clockwise whereasFall Mountain nappe. For this reason, the emplacementthose in western New Hampshire are dominantly counter-of the Fall Mountain nappe on the Skitchewaug nappeclockwise. It is particularly significant that the loweris inferred to have occurred following cooling of the high-nappes in the New Hampshire sequences all involve angrade assemblages in the upper nappes. The amount ofepisode of increasing pressure in their P–T paths, whereastime required for the cooling need not be great, inasmuchthe Chesham Pond nappe does not. The pressure in-
creases are interpreted to have occurred during the as the plutons are not thick and crystallization probably
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Fig. 15. Summary of peak metamorphic P–T conditions and cross-sections across central New England (line A–A′ in Fig. 3). (a) Peak pressures.(b) Peak temperatures. (c) Geological cross-section. (d) Cross-section with isograds superimposed. The gray area in (d) shows the structuralposition of the pseudomorph-bearing samples of the Skitchewaug nappe (e.g. Fig. 10). Question mark in (a) denotes uncertainty in Acadian peakpressure in the Oliverian dome rocks.
occurred within a few hundred thousand years of the the Littleton Formation (e.g. Hadley, 1942; Rumble,1969), which contains Lower Emsian fossils (Boucot &time of intrusion.
Ages of the Bethlehem gneiss and Kinsman quartz Arndt, 1960; Boucot & Rumble, 1980). The LowerEmsian has recently been dated at 408± 2 Ma (Tucker etmonzonite fall in the range 393–413 Ma (Lyons &
Livingston, 1977; Barreiro & Aleinikoff, 1985; Aleinikoff al., 1998), providing a tight upper age limit for plutonism.Metamorphic monazite and zircon ages of 408–392 Ma& Moench, 1987; Kohn et al., 1992) with considerable
overlap between published ages. These plutons intrude have been reported from central New Hampshire and
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Fig. 16. Summary of P–T paths superimposed on cross-section A–A′ (Fig. 3). The following should be noted: (1) the decrease in maximumpressure heading eastward from the Vermont domes to the Vermont garnet zone; (2) the increase in maximum pressure from the New Hampshiregarnet zone to the Big Staurolite nappe; (3) the general decrease in maximum pressure and increase in maximum temperature going eastwardup the nappe sequence in New Hampshire from the Skitchewaug to the Chesham Pond nappe. Abbreviations are as in Figs 3 and 5.
Fig. 17. Schematic diagram showing maximum temperatures and pressures achieved by rocks in different structural levels across strike fromthe Chester Dome, VT, to the Chesham Pond nappe, NH. The temperatures and pressures shown are not representative of the thermal andbaric structure at any time because the petrological evidence suggests that the New Hampshire nappes were cool before being emplaced to thewest. The inverted metamorphic sequence going upward through the New Hampshire nappes should be noted.
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The structural evolution shown in Fig. 18 is that of anin-sequence thrust stack. Total shortening is difficult toestimate, but the current exposure of the nappes can beused to provide a minimum estimate. The distance acrossstrike from the Chicken Yard line to the root zone ofthe Fall Mountain nappe near Gilsum, NH, is >20 km(Fig. 5). Assuming that before erosion the nappes werestacked vertically in the vicinity of the Chicken Yardline, a minimum shortening of 75% is implied (�l/l =60 km/80 km).
Onset of metamorphism in the Silurian–Devonianrocks of eastern Vermont is not well constrained. AnSm–Nd age of 378 Ma on the rim of a garnet from thepre-Silurian Gassetts schist on the west flank of theChester dome (Vance & Holland, 1993) suggests that therocks in the core of the domes were undergoing progrademetamorphism at this time, and it is reasonable to assumethat the cover rocks were as well. However, ages of
Fig. 18. Sequence of schematic cross-sections showing relative positions monazite from Silurian–Devonian rocks of 354 Ma (Wingof major tectonic units in western New Hampshire in the initial phase
et al., 1999; Ferry, 2000) suggest that metamorphism inof the Acadian orogeny. CPN, Chesham Pond nappe; FMN, Fallthe cover sequence may have been much younger thanMountain nappe; SKN, Skitchewaug nappe; BSN, Big Staurolite nappe;
KQM, Kinsman quartz monzonite; BG, Bethlehem gneiss. (a) Situation was previously thought.at>400–410 Ma. Onset of intrusion of the Bethlehem pluton into the The 8–11 kbar peak pressures in the vicinity of theMerrimack trough sediments. Dashed line shows positions of the future
domes in eastern Vermont require significant crustalChesham Pond thrust. (b) Intrusion of the Kinsman pluton and stackingof the Chesham Pond nappe onto Fall Mountain nappe. Dashed line thickening, and the question naturally arises: ‘What wasshows position of future Fall Mountain thrust within the Bethlehem emplaced on eastern Vermont to cause the crustal thick-gneiss. (c) Stacking of Fall Mountain and Chesham Pond nappes onto
ening?’ One possibility is that the New Hampshire nappeSkitchewaug nappe. It should be noted that the FMN and CPN mustsequence was thrust directly on eastern Vermont to causehave cooled to >550°C before emplacement onto the SKN. Dashed
line shows position of future Skitchewaug Mountain thrust. (d) Stacking the Barrovian metamorphism, a model favored by Spearof higher nappes onto Big Staurolite nappe. Minimum shortening (1992, 1993). However, this scenario is no longer favoredimplied by this stacking is 75% (�l/l = 60 km/80 km).
for several reasons. First, the peak pressures in garnet-zone rocks now exposed along the Chicken Yard andMonroe lines are only 3–4 kbar whereas the peak pres-
Maine (Barreiro et al., 1988; Eusden & Barreiro, 1988; sures of rocks in the overlying Big Staurolite nappe areSmith & Barreiro, 1990; Zeitler et al., 1990). These ages 5–6 kbar. If the garnet-zone metamorphism was causedare from rocks in structural levels above the Fall Mountain by emplacement of the New Hampshire nappe sequence,nappe (e.g. the Chesham Pond nappe) and are generally this could be accommodated only if 3–10 km of erosionconsistent with the model presented here that the meta- of the New Hampshire nappes had occurred before theirmorphic peak in these high-level nappes was caused by emplacement onto the garnet-zone rocks. Second, theheat from the syn-tectonic plutons. 28–35 km of burial implied by the 8–11 kbar pressures
Emplacement of the Skitchewaug and higher nappes experienced by rocks of the Vermont domes is sig-on the Big Staurolite nappe is the next event recorded nificantly greater than the thickness of the New Hamp-in the rocks of western New Hampshire (Fig. 18d). It is shire nappe sequence, especially if the thickness is limitednot clear whether this happened immediately following to 10–15 km based on the 3–4 kbar pressures of thethe emplacement of the Fall Mountain nappe, or after a garnet-zone rocks in the low-grade belt. Third, the ap-hiatus in deformation. Before the isothermal loading parent absence of high-grade metamorphism in the Oli-experienced by rocks of the Big Staurolite nappe, the verian gneisses of the Alstead and Keene domes (i.e.P–T conditions were in the lower to middle garnet zone Kohn & Spear, 1999) requires that the rocks of the Big(Fig. 9), and there is some evidence that the geothermal Staurolite and higher nappes were placed on top of thesegradient may have been slightly elevated, at least locally, dome rocks after they cooled below >500°C. Fourth,at this time. For example, tails of some P–T paths in Fig. the pervasive greenschist-facies alteration at the base of9 are in the andalusite field and Florence et al. (1993) the Big Staurolite nappe and elsewhere throughout thehave described early andalusite followed by garnet + New Hampshire nappe sequence suggests significant fluidstaurolite + kyanite + biotite assemblages from the infiltration at temperatures of 400–450°C, presumably
along structural breaks. The simplest explanation is thatSalmon Hole Brook Syncline in the Littleton area.
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monazites from Vermont (Wing et al., 1999; Ferry 2000).This evidence suggests that the assembly of the centralNew England metamorphic belt was a protracted processthat extended through the Devonian into the Car-boniferous. Additional ages on the timing of the meta-morphism in Vermont, the different structural levels ofNew Hampshire, and the greenschist-facies alterationshould help constrain further the timing of this assembly.
Fig. 19. Tectonic scheme depicting the proposed model for the relativepositions of the New Hampshire nappe sequence, the Vermontsequence, the intervening low-grade metamorphic belt and the BronsonHill arc at the time of burial of the Vermont belt. CONCLUSIONS
A combination of P–T studies and fabric analysis providesa framework within which to interpret the sequence ofthis infiltration occurred when these faults were activetectonic assembly in central New England. The modeland the nappe sequence had cooled to greenschist-faciespresented here is broadly consistent with prior geologicalconditions.mapping based on stratigraphic correlations, althoughOur revised scenario invokes burial of rocks of thethere are numerous examples where the thrusts proposedVermont sequence beneath rocks that now form the low-here (e.g. Fig. 5) require reinterpretation of earlier map-grade belt, producing a more-or-less normal meta-ping (e.g. Thompson et al., 1968; Thompson & Rosenfeld,morphic gradient (Fig. 19). It is proposed that this might1979; Armstrong et al., 1997). Indeed, some of the resultshave been the Bronson Hill arc, with the New Hampshireof this study are not at all obvious based solely onnappes eastward of the arc (Fig. 19). Major recumbentstratigraphic mapping. Clearly, considerable additionalfold nappes are mapped in eastern Vermont, and pre-work needs to be done, especially in constraining thesumably formed during this partial subduction, con-timing of different events. Testing of this model can betributing to crustal thickening. Additional burial mustbest achieved from detailed analysis of the timing of peakhave occurred along thrust faults or distributed shearmetamorphism across strike from petrologically well-zones within the eastern Vermont belt. Substantial re-characterized structural levels. Ultimately, any satis-flectors are observed beneath western New Hampshirefactory model will have to reconcile all of these data.in the COCORP seismic line (Ando et al., 1984), and it
is possible that one or more of these reflectors representsthe proposed thrust faults, although direct evidence islacking.
ACKNOWLEDGEMENTSFollowing burial to >30 km depth, eastern Vermontexperienced >7 km of differential uplift of the dome The results and interpretations presented in this paperrocks relative to the lower-pressure rocks toward the have benefited from many years of discussions withChicken Yard–Monroe lines. Finally, the New Hamp- numerous individuals. The authors are particularly in-shire nappe sequence was thrust westward along the debted to the insights of J. B. Thompson, Jr, D. Rumble,Western New Hampshire Boundary Thrust fault ac- III, and P. Robinson, whose work has influenced thecompanied by greenschist-facies alteration. It is also thinking of all of the authors. Insightful and informativeproposed that ramp anticlines associated with related reviews were provided by T. Dempster and S. Harley.thrust faults produced the Alstead and Keene domes. This work was supported by NSF grants EAR-8916417,
These hypotheses are testable with thermochronology. EAR-9220094, and EAR-0073747 (to F.S.S.).For example, the timing of this late shearing is not known,but the greenschist-facies metamorphism that is producedin the shear zones, at the base of the Big Staurolitenappe, and, to a lesser degree, throughout the New REFERENCESHampshire nappe sequence, suggests a temperature of Aleinikoff, J. N. & Moench, R. H. (1987). U–Pb geochronology and
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Spear, F. S. & Rumble, D., III (1986). Pressure, temperature and Vielzeuf, D. & Clemens, J. D. (1992). The fluid-absent melting ofstructural evolution of the Orfordville Belt, west–central New Hamp- phlogopite + quartz: experiments and models. American Mineralogistshire. Journal of Petrology 27, 1071–1093. 77, 1206–1222.
Spear, F. S. & Selverstone, J. (1983). Quantitative P–T paths from White, W. S. & Jahns, R. H. (1950). Structure of central and east–centralzoned minerals: theory and tectonic applications. Contributions to Vermont. Journal of Geology 58, 179–220.Mineralogy and Petrology 83, 348–357. Wing, B. A., Ferry, J. M. & Harrison, T. M. (1999). The age of
Spear, F. S., Hickmott, D. D. & Selverstone, J. (1990). Metamorphic andalusite and kyanite isograds in New England from Th–Pb ionconsequences of thrust emplacement, Fall Mountain, New Hamp- microprobe dating of monazite. Geological Society of America, Abstractsshire. Geological Society of America Bulletin 102, 1344–1360. with Programs 31, A-40.
Spear, F. S., Kohn, M. J. & Paetzold, S. (1995). Petrology of the Woodland, B. G. (1977). Structural analysis of the Silurian–Devonianregional sillimanite zone, west–central New Hampshire, U.S.A. with rocks of the Royalton area, Vermont. Geological Society of Americaimplications for the development of inverted isograds. American Bulletin 88, 1111–1123.Mineralogist 80, 361–376. Zeitler, P. K., Barreiro, B., Chamberlain, C. P. & Rumble, D. (1990).
Spear, F. S., Kohn, M. J. & Cheney, J. T. (1999). P–T paths from Ion-microprobe dating of zircon from quartz–graphite veins at theBristol, New Hampshire, metamorphic hot spot. Geology 18, 626–629.anatectic pelites. Contributions to Mineralogy and Petrology 134, 17–32.
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