Retrograde metamorphism of eclogite in the southern Appalachian Mountains, U.S.A. A case involving seamount subduction?
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J. metamorphic Geol., 2001, 19, 433±443
Retrograde metamorphism of eclogite in the southern
Appalachian Mountains, U.S.A.±A case involving seamount
subduction?
R. N. ABBOTT, 1 AND J . P. GR E E NW O OD 2
1
Department of Geology, Appalachian State University, Boone, North Carolina 28608, USA,
(E-mail: abbottrn@appstate.edu)
2
Institute of Geophysics and Planetary Physics, University of California at Los Angeles, Los Angeles, California 90095, USA
ABSTRA CT This work supports a growing body of evidence that the Ashe Metamorphic Suite (AMS) of the eastern
Blue Ridge province in North Carolina has an ensimatic origin and is part of a subduction-related
accretionary meÂlange, marking the Taconic suture between the North American craton and the Inner
Piedmont. In a palinspastic reconstruction, the thrust fault at the base of the AMS appears to have
intercepted the greatest depths (i.e. highest-P metamorphic rocks) beneath parts of the AMS now exposed
adjacent to the Grandfather Mountain window. The greatest volume of ma®c rock is found in these same
areas. We suggest that the nascent, subduction-related, basal thrust fault was de¯ected downward by an
obstacle in the form of an isolated, ma®c volcanic edi®ce on the oceanic crust±a sea mount.
Pelitic and ma®c rocks dominate the AMS. North of the Grandfather Mountain window, retrograded
eclogite occurs in the amphibolite near the base of the AMS. Textures and mineralogy indicate that an
original eclogite assemblage was subjected to the following sequence of parageneses:
(a) Eclogite(I) facies: omphacite+garnet+quartz,
(b) Eclogite(II) facies: omphacite+garnet+epidote+quartz,
(c) Symplectic (diopside+plagioclase)+garnet+epidote+quartz,
(d) Amphibolite facies: (diopside+plagioclase)+garnet+epidote+hornblende+quartz,
(e) Amphibolite facies: plagioclase+garnet+epidote+hornblende+quartz.
P±T conditions, estimated from geothermobarometry applied to relevant mineral compositions, are
c.720 uC and c.16 kbar for (b) eclogite(II) facies; c.655 uC and c.8.5 kbar for (e) amphibolite facies.
Key words: Appalachian Mountains; Blue Ridge province; eclogite; retrograde metamorphism; seamount
subduction.
metamorphism during the Taconic orogeny (Goldberg
I NTRODUC TION
& Dallmeyer, 1997). The AMS is exposed north-east
The objectives of this paper are: (1) to review the and south-west of the Grandfather Mountain window
metamorphic petrology of pelitic and ma®c rocks in the (Rankin et al., 1973; Goldberg et al., 1989), where it
Ashe Metamorphic Suite (AMS) north-east of the forms a principal Neoproterozoic to Early Paleaozoic
Grandfather Mountain window (Fig. 1); (2) to discuss (?) sequence of sedimentary and volcanic rocks in the
new constraints on metamorphic conditions (P,T) southern Blue Ridge Belt. The AMS structurally
and regional metamorphic gradients in pressure and overlies Mesoproterozoic basement and is structurally
temperature; (3) to describe retrograded eclogites in overlain by the ABMS (Rankin et al., 1973; Fullagar &
the AMS; (4) to re-evaluate the P±T path of the AMS; Odom, 1973; Fullagar & Bartholomew, 1983; Abbott &
and (5) to discuss a novel origin for part of the unit, Raymond, 1984). The Ashe and Alligator Back
as a subducted seamount. Metamorphic Suites are con®ned to the Spruce Pine
thrust sheet of Goldberg et al. (1989). The thrust sheet
structurally overlies a deeper, basement-cover sequence
of lower metamorphic grade (Bryant & Reed, 1970).
G E N E R A L G E O L OGY
Major rock types in the AMS include pelitic schist,
The AMS and the Alligator Back Metamorphic Suite quartzofeldspathic schist and gneiss and hornblende
(ABMS) and their inferred lithostratigraphic equiva- schist and gneiss. The hornblende schist and gneiss are
lents in the Blue Ridge Belt of the southern Appa- interpreted to be metabasalt (e.g. Rankin, 1970; Misra
lachian orogen (Fig. 1), were metamorphosed to the & Conte, 1991). Minor components of the suite include
amphibolite facies of regional Barrovian facies series eclogite (Abbott & Raymond, 1997; Willard & Adams,
# Blackwell Science Inc., 0263-4929/01/$15.00 433
Journal of Metamorphic Geology, Volume 19, Number 4, 2001434 R. N. ABBOTT & J. P. GREENWOOD
Fig. 1. Metamorphic zones in the eastern Blue Ridge (modi®ed from Drake et al., 1989). Bold lines are major thrust faults
separating the Blue Ridge belt from the Valley and Ridge province (V & R) to the north-west and the Inner Piedmont (IP) to the
south-east. Thrust faults within the Blue Ridge belt separate the western Blue Ridge (®ne stippling) from the eastern Blue Ridge
and also form the borders of the Grandfather Mountain window (GMW). The Ashe and Alligator Back Metamorphic Suite are
the principal constituents of the eastern Blue Ridge Belt. Three pseudo-invariant points are indicated by ®lled circles, where the
garnet, staurolite and kyanite zones meet. Direction of increasing metamorphic pressure and direction of increasing metamorphic
temperature in the Eastern Blue Ridge Belt are indicated at each of the pseudo-invariant points. Inferred regions of high pressure
metamorphism are shown in coarse stippling. Eclogite locales are indicated by X's. Eclogite south-west of Grandfather Mountain
window has been described by Willard & Adams (1994). Retrograded eclogite immediately north of Grandfather Mountain
window is described in this report.
1994; Adams et al., 1995) and ultrama®c rocks. The & Raymond, 1984; Miller et al., 1997; Goldberg &
ultrama®c rocks are variously interpreted to be residual Dallmeyer, 1997). Incomplete retrograde recrystalliza-
mantle blocks, ophiolite fragments, or intrusions (e.g. tion is widely developed.
Rankin et al., 1973; Abbott & Raymond, 1984;
McSween & Hatcher, 1985; Wang & Glover, 1991).
The ABMS is dominated by pelitic rocks, but also P EL I TI C R O CK S
contains hornblende schists and rare ultrama®c rocks
North of the Grandfather Mountain window, pelitic
(Rankin et al., 1973; Conley, 1987).
rocks in the AMS are mainly muscovite schists. The
Raymond et al. (1989) argued that the AMS and at
essential minerals are muscovite, quartz and plagio-
least parts of the ABMS are a subduction-related
clase. Where quartz and plagioclase dominate, the
accretionary meÂlange. The hypothesis is supported by
rocks are properly called quartzofeldspathic schists or
block-in-matrix structures involving blocks of ultra- gneisses. Depending on the grade of metamorphism
ma®c rocks, amphibolite and eclogite in a matrix of the and bulk composition, other minerals include various
pelitic schist and gneiss (Raymond et al., 1989; Adams combinations of biotite, garnet, chlorite, staurolite and
et al., 1995) and more recently by high pressure, kyanite. Accessory minerals include Fe-oxides, apatite
eclogite facies metamorphism locally near the base of and tourmaline. The distribution of mineral assem-
the AMS (this study; Willard & Adams, 1994; Adams blages is consistent with a Barrovian metamorphic
et al., 1995). Trace element and REE geochemistry of facies series.
amphibolite in the AMS (Misra & Conte, 1991) Regional (Fig. 1) and detailed metamorphic maps
support an ensimatic origin for the basalt protolith of the central Blue Ridge Belt (Hadley & Nelson,
of the amphibolite. 1971; Espenshade et al., 1975; Brown, 1985; Abbott &
The allochthonous Neoproterozoic rocks, especially Raymond, 1984; McSween et al., 1989; Abbott et al.,
the AMS and ABMS, have been affected by at least 1991; Butler, 1991; Abbott & Raymond, 1997) reveal
three metamorphic and deformational events (Butler, three occurrences of an AFM (A=Al2O3±Na2O±
1972, 1973; Abbott & Raymond, 1984; Adams et al., K2O±CaO, F=FeO, M=MgO; Thompson, 1957)
1995). The major prograde event has generally been pseudo-invariant point (intersection of isograds).
assigned to the Ordovician Taconic orogeny (Abbott Pseudo-invariant points of the type described hereRETROG RAD E EC LOGITE IN TH E SOU TH ERN A PPA L AC HIAN S 43 5
have not been recognized elsewhere, although Labotka
(1981) has predicted their existence in theory and
thermodynamic calculations (Spear et al., 1995)
indicate reasonable crustal conditions (P, T) for their
existence.
Each of the pseudo-invariant points is located at
the intersection of the three mapped isograds. The
three isograds that de®ne the point of intersection,
correspond to the AFM reactions
Grt+Chl=Bt+St, [Ky]
St=Bt+Ky+Grt, [Chl]
and Grt+Chl=Bt+Ky [St].
Mineral abbreviations follow Kretz (1983). Mapping
of reaction isograds suggests invariance (with respect
to P and T) only within the limits of resolution per-
mitted by the spatial distribution of relevant, observed
mineral assemblages. Spear et al. (1995) have shown
that conditions at such an intersection of reactions
depend, among other factors, on the CaO and MnO
content of the rock. Indeed, while the point of
intersection of the reaction isograds cannot actually º
be invariant, the point of intersection has character-
Fig. 2. Schematic phase relationships in pelitic rocks (®ne
istics of invariance in the context of a limited range of lines) and ma®c rocks (bold line) in AMS and ABMS. Slopes
bulk compositions in pelitic rocks. That the intersec- of AFM reactions (pelitic rocks) are consistent with
tions can be identi®ed at all on the basis of the calculations by Spear et al. (1995) for conditions at pseudo-
distribution of mineral assemblages suggests that the invariant point of P=7.5 kbar, T=640 uC, XMn(Grt)=0.25.
The shaded region is the staurolite zone of a typical Barrovian
CaO and MnO contents, among other bulk composi- metamorphic facies series.
tional factors, are more-or-less the same in relevant
rock types in the AMS and ABMS.
Theoretically, two other reactions, of the Spruce Pine thrust sheet, close to the Grand-
father Mountain window. Signi®cantly, the locations of
St=Grt+Ky+Chl, [Bt] eclogites south-west of the Grandfather Mountain
and St+Chl=Bt+Ky, [Grt] window (Willard & Adams, 1994) and north-east of the
Grandfather Mountain window (Abbott & Raymond,
are involved at each intersection, but corresponding 1997) support this interpretation.
isograds apparently were not produced, probably 2 These areas with evidence for the highest pressures of
because of inappropriate bulk compositions. As noted metamorphism do not coincide with the areas that
above, P±T conditions for this kind of intersection experienced the highest temperatures. This follows
remain poorly constrained, for reasons just noted from inferred gradients in pressure and temperature.
(Spear et al., 1995). Based on determinations near one 3 The geothermal gradient, at the time of metamorph-
of the pseudo-invariant points in the AMS (McSween ism, varied systematically from place to place laterally
et al., 1989) north-east of the Grandfather Mountain across the volume of crust affected. In the highest-
window, near Boone, North Carolina, P±T are pressure regions (close to Grandfather Mountain
estimated to be c.7.5 kbar and 600±650 uC. Relation- window), the temperatures were comparatively low;
ships around the point of intersection are shown thus, the geothermal gradient was comparatively steep
schematically in Fig. 2. The P±T slopes of the various (i.e. a cooler geotherm). To the north-east and to the
reactions are consistent with calculations by Spear et al. south-west, away from the Grandfather Mountain
(1995). Signi®cantly, the slope (dP/dT) of reaction [Ky] window, comparable temperatures correspond to lower
is positive, but very steep to nearly vertical, while the pressures. Thus the geothermal gradient was compara-
slope of reaction [Chl] is negative. In Fig. 1, the arrows tively shallower (i.e. a hotter geotherm).
show the direction of increasing metamorphic P and T
at each of the pseudo-invariant points. If the relation-
M A FI C R O C K S
ships are even qualitatively correct, important conclu-
sions naturally follow: Hornblende schist and gneiss are by far the dominant
1 Inferred metamorphic gradients in pressure (Fig. 1) ma®c component of the AMS and ABMS. The essen-
indicate that the highest pressure of metamorphism in tial minerals are hornblende, quartz and plagioclase
the AMS and ABMS should be recorded near the base and varieties of amphibolite are distinguished by436 R. N. ABBOTT & J. P. GREENWOOD
various combinations of garnet, biotite, epidote-zoisite pseudo-invariant point in such a way that the P±T
and magnetite. Locally, extreme variants, dominated slope (dP/dT) of the reaction is constrained to be
by epidote (epidotites) or garnet (garnetites), occur as negative (Fig. 2).
lenses (dm-scale) and thin (mm- to cm-scale) layers, Grt±Hbl and Grt±Bt geothermometry is consistent
parallel to the foliation. Accessory minerals include with Grt±Bt geothermometry for the pelitic rocks
titanite, apatite, ilmenite, zircon and iron sulphides. (McSween et al., 1989). Temperatures range from 530
Late replacement minerals include white mica, calcite, to 620uC in the low-grade zone (Hbl±Bt±Ep) and from
chlorite and ferric oxides. 610 to 730 uC in the high-grade zone (Hbl±Grt). The
Abbott & Raymond (1984) identi®ed two meta- map distribution of estimated temperatures (McSween
morphic zones in the ma®c rocks of the AMS±ABMS et al., 1989) is consistent with the general direction of
north-east of the Grandfather Mountain window the increasing emperative, inferred from relationships
(Fig. 3). In terms of CFM minerals (C=CaO+K2O+ in the pelitic rocks.
Na2O-Al2O3, F=FeO-Fe2O3, M=MgO; Abbott, Retrograded eclogite is known from only one area
1982), the zones are separated by a reaction isograd, north-east of the Grandfather Mountain window.
There are three sites (Fig. 3, precise locations given
Bt+Ep=Grt+Hbl. in caption), which are close to each other and in nearly
Quartz, plagioclase and Fe±oxide are involved in the the same structural position, close to the base of the
reaction. The lefthand side corresponds to the low- AMS at its westernmost edge. It is not possible to know
grade, where the common CFM mineral assemblage if the three sites are parts of a contiguous area of
is Hbl+Bt+Ep (coexisting with quartz, plagioclase, retrograded eclogite, because the intervening areas are
Fe±oxide). Garnet and hornblende are not compatible. covered by thick soil. The locations coincide, however,
On the high-grade side, diagnostic CFM assemblages with the area of highest metamorphic pressures in
are Grt+Hbl, Grt+Hbl+Ep and Grt+Hbl+Bt. the AMS, as inferred from the general direction of
The isograd passes practically through the AFM increasing pressure in the pelitic rocks.
The retrograded eclogites occur as thin (cm-scale),
granoblastic layers in otherwise typical amphibolite.
The essential minerals are symplectic intergrowths of
diopside and plagioclase (representing former ompha-
cite), generally euhedral to subhedral garnet (RETROG RAD E EC LOGITE IN TH E SOU TH ERN A PPA L AC HIAN S 43 7
garnet, grains of symplectic diopside±plagioclase, or
both. By itself or with lesser amounts of other
minerals±mainly garnet and symplectic diopside+plag-
ioclase±the epidote forms aggregates that show a well-
developed, equigranular mosaic texture. A second
type of epidote (with quartz) occurs between garnet
and symplectic diopside-plagioclase in some samples
as crude coronas, suggesting that it resulted from an
early retrograde reaction involving garnet and
omphacite. Locally, euhedral garnet is embedded in
symplectite (diopside+plagioclase) with only minor
quartz, no epidote and no hornblende, suggesting an
original metamorphic assemblage of omphacite and
garnet.
Hornblende seems not to have any special site for Fig. 4. Eclogite tetrahedron, de®ned by components, jd =
NaAlSi2O6, di±hd = Ca(Mg,Fe)Si2O6, ts = CaAl2SiO6 and
nucleation. Grains of hornblende occur along every Qtz = 3 SiO2. The quartz component is taken as three units
kind of grain boundary involving combinations of of SiO2, so that all compositions in the tetrahedron are
garnet, epidote and symplectite. Hornblende also normalized to six oxygen atoms. In this way, all compositions
developed at boundaries between grains of symplectic represent approximately the same volume; hence, modal
relationships are preserved (approximately). The bulk
diopside+plagioclase cutting across the vermicular composition of typical eclogite is in the shaded plane, e.g.
texture of the symplectite. composition marked X.
Based on textural relationships, the following
sequence of assemblages (excluding accessory minerals)
is envisioned, starting with the earliest, highest-grade ts±component. Presumably, production of epidote
assemblage, from omphacite by the ®rst reaction is less important
(a) Eclogite(I) facies: Omp(I)+Grt+Qtz, than production of epidote from garnet by the second
(b) Eclogite(II) facies: Omp(II)+Grt+Ep+Qtz, reaction. The reader will note that the component of
(c) Symplectic (Di+Pl)+Grt+Ep+Qtz, where garnet in the second reaction is chemically equivalent to
(Di+Pl) is symplectite, combined chemical components of Omp(I) in the ®rst
(d) Amphibolite facies: (Di+Pl)+Grt+Ep+Hbl+ reaction. The ®rst reaction has the effect of changing
Qtz and the composition of the omphacite toward the (di±hd)±
(e) Amphibolite facies: Pl+Grt+Ep+Hbl+Qtz. jd join, directly away from epidote. In the context of
Reactions relating assemblage (a) to (b) and Fig. 4, the reaction may be written simply,
assemblage (b) to (c) can be represented conveniently Omp(I)+H2O+O2=Omp(II)+Ep.
in the compositional space de®ned by principal
The bulk composition of the rock is in the triangle Qtz±
components of omphacite (Omp) plus quartz, that is
Grt(Ep)±Omp(II). Hence, the reaction producing the
jd=NaAlSi2O6, ts=CaAl2SiO6, di-hd=Ca(Mg,Fe)-
symplectite necessarily takes the form,
Si2O6 and Qtz=3 SiO2 (Fig. 4). Assuming the rock
system was open with respect to H2O and O2, epidote Omp(II or I)+Grt(or Ep)+Qtz=Di+Pl,
plots in the same place as garnet in this simpli®ed
de®ning the compatibility tetrahedron Di±Pl±Qtz±
representation. The appearance of epidote is thought to
Grt(Ep) (assemblage c), within which the bulk
be controlled mainly by the availability of O2 and H2O,
composition must reside. Admittedly, the amount of
according to one or both of the following reactions garnet (or epidote) involved in the reaction may be
involving components of an original, tschermakitic small. The absence of epidote from the paragenesis of
omphacite, Omp(I), components of garnet, or both. the eclogite south-west of the Grandfather Mountain
CaFeSi2O6+CaAl2SiO6+0.5 H2O+0.25 O2 window (Willard & Adams, 1994) may be due simply to
hd and ts in Omp(I) lower fugacity of O2.
=Ca2FeAl2Si3O12(OH)
Ep
EC LOGI TE FA CIES CONDI TI ONS
Ca2FeAl2Si3O12+0.5 H2O+0.25O2
in Grt Chemical analyses of the minerals in assemblage (c),
(Di+Pl)+Grt+Ep+Qtz are presented in Table 1,
=Ca2FeAl2Si3O12(OH)
where the clinopyroxene and plagioclase are compo-
Ep
nents of symplectite. Mineral compositions (Table 1 &
The ts-content of most omphacite (Deer et al., 1992; Table 4) were determined by one of us (JPG) at the
Cameron & Papike, 1980) is low, typically much less University of Tennessee, Knoxville, using a Cameca
than 10%. Omphacites analysed by Willard & Adams SX050 electron microprobe, appropriate natural and
(1994) from eclogite in the AMS contains less than 9% synthetic standards and PAP (ZAF) matrix correction438 R. N. ABBOTT & J. P. GREENWOOD
Table 1. Chemical Analyses of Minerals in Retrograded Eclogite.
No. of analyses Grt core Grt rim Di Pl Ep
Wt. % oxides* (standard deviation): 9 8 5 5 9
SiO2 37.7(0.1) 37.7(0.2) 51.0(0.2) 58.1(0.3) 37.8(0.4)
TiO2 0.15(0.04) 0.06(0.03) 0.18(0.05) ± 0.15(0.04)
Al2O3 20.83(0.14) 20.77(0.21) 2.47(0.17) 25.78(0.27) 25.17(1.14)
Cr2O3 0.04(0.03) 0.04(0.05) 0.04(0.02) ± 0.08(0.08)
FeO* 24.85(0.68) 25.99(0.53) 11.28(0.43) 0.26(0.10) 9.74(1.27)
MnO 0.93(0.30) 0.52(0.06) 0.03(0.02) ± 0.04(0.03)
CaO 13.07(0.59) 12.13(0.78) 22.91(0.09) 8.13(0.28) 23.76(0.26)
MgO 2.51(0.34) 2.68(0.19) 10.95(0.25) ± 0.04(0.01)
Na2O 0.03(0.01) ± 0.78(0.02) 7.03(0.13) ±
TOTAL 100.11 99.89 99.64 99.30 96.78
Constraints used in
calculating chemical formula:
Oxygen atoms p.f.u. 12 12 6 8 12
Cations p.f.u: 8 8 X+Y+Z=4 no constraint no constraint
Tetrahedral coordination:
Si 2.96(0.00) 2.98(0.01) 1.926(0.007) 2.62(0.02) 2.94(0.01)
Al ± ± 0.074(0.007) 1.37(0.01) ±
Fe3+** 0.04 0.02 ± nd nd
Fe+2 ± ± ± 0.01(0.00) ±
SUM 3.00 3.00 Z=2.00 4.00 2.94
Octahedral coordination:
Al 1.93(0.01) 1.93(0.01) 0.036(0.002) ± 2.31(0.08)
Ti 0.01(0.00) ± 0.005(0.002) ± 0.01(0.00)
Cr ± ± 0.001(0.001) ± 0.01(0.00)
Fe+3** 0.06 0.07 0.083(0.015) ± nd
Fe+2 ± ± 0.273(0.015) ± 0.63(0.09)
Mn ± ± 0.001(0.001) ± ±
Ca ± ± 0.927(0.004) ± ±
Mg ± ± 0.616(0.013) ± ±
Na ± ± 0.057(0.001) ± ±
SUM 2.00 2.00 X+Y=2.00 ± 2.96
Dodecahedral coordination in garnet:
Fe+3** 0.03 0.02 ± ± ±
Fe+2 1.50(0.04) 1.63(0.08) ± ± ±
Mn 0.07(0.02) 0.03(0.00) ± ± ±
Ca 1.10(0.05) 1.02(0.06) ± ± ±
Mg 0.30(0.04) 0.30(0.04) ± ± ±
SUM 3.00 3.00 ± ± ±
Large cations in plagioclase and epidote:
Ca ± ± ± 0.39(0.01) 1.98(0.01)
Na ± ± ± 0.61(0.01) ±
SUM ± ± ± 0.99 1.98
alm 0.500 0.543 hd 0.273 an 0.39
sps 0.023 0.010 di 0.616 or ±
grs 0.367 0.340 ts 0.076 ab 0.61
pyr 0.100 0.100 jd 0.057
*All Fe reported as FeO. `±' below detection. K2O, below detection in all averages.
**Fe3+ estimated, based on constraints used in calculating chemical formula (see above).
procedures. An accelerating potential of 15 kV, with a reasonable range. The densities for the diopside and
20 nA beam and 20 s counting times were used. plagioclase were calculated from values for the end
In this section we estimate the P±T conditions for the member species. Converting all of the An in plagioclase
eclogite facies assemblages by applying geothermoba- to ts in the omphacite (tsmax models) results in ts±
rometers for the equilibria garnet±omphacite (Pattison contents that are more than twice the amount, noted
& Newton, 1989) and plagioclase±quartz±omphacite above, typically found in omphacite. A second pair of
(Gasparik & Lindsley, 1980). To this end, the com- models, however unrealistic, was calculated assuming
position of the original omphacite is ®rst estimated. that the original omphacite contained no ts (ts0 model).
The composition of the original omphacite was In these model compositions all of the ts±equivalent in
calculated from the average composition of the diop- the plagioclase and an equal amount of di+hd in the
side and plagioclase in the symplectite and a simple diopside would come from garnet, not omphacite. A
visual estimate of the modal proportions of diopside third pair of models was extrapolated from the tsmax
and plagioclase in the symplectite (Table 2). The ®ne and ts0 models, for an intermediate amount of ts=0.07.
grain size of the plagioclase and diopside in the These are the ts0.07 models, which along with the ts0
symplectite for practical purposes precluded estimation models were subsequently used in the geothermo-
by point count. Two proportions were used in our barometry. We assume, on this basis, that the original
calculations, 40/60 (40% plagioclase/60% diopside) and jadite content of the omphacite was between 0.27
50/50 (50% plagioclase/50% diopside), to re¯ect a and 0.42.RETROG RAD E EC LOGITE IN TH E SOU TH ERN A PPA L AC HIAN S 43 9
Table 2. Estimated Original Omphacite Composition.
Average plagioclase and diopside in symplectite:
Plagioclase, ab0.61 an0.39 density=2.692 g/cm3 G.F.W. (AZ4O8)=269.4 g
Diopside, di0.616 hd0.273 ts0.076 jd0.057 density=3.312 g/cm3 G.F.W. (XYZ2O6)=194.8 g
Model 40/60 50/50
Pl Di Pl Di
Volume 40.0 cm3 60.0 cm3 50.0 cm3 50.0 cm3
Moles 0.3997 1.0205 0.4997 0.8499
Molar equivalents of pyroxene components:
ts 0.1559 0.0770 0.1948 0.0642
jd 0.2438 0.0581 0.3048 0.0486
di 0.6283 0.5233
hd 0.2782 0.2317
Omphacite (combined molar pyroxene components from plagioclase and diopside):
ts 0.2329 0.2590
jd 0.3019 0.3534
di 0.6283 0.5233
hd 0.2782 0.2317
SUM 1.4413 1.3674
Omphacite, tsmax model (Mole fractions of components in original omphacite):
ts 0.1616 0.1894
jd 0.2095 0.2584
di 0.4359 0.3827
hd 0.1930 0.1694
Omphacite, ts0 model (ts = 0 in omphacite. All ts-equivalent in plagioclase
plus an equal amount of di+hd in diopside came from garnet, not omphacite):
ts 0.00 0.00
jd 0.31 0.42
di+hd 0.69 0.58
Omphacite, ts0.07 model (linear extrapolation between tsmax and ts0 models):
ts 0.07 0.07
jd 0.27 0.36
di+hd 0.66 0.57
Summary of omphacite models used in calculating P±T conditions:
1. (40/60) ts0 jd31
2. (40/60) ts0.07 jd27
3. (50/50) ts0 jd42
4. (50/50) ts0.07 jd36
Table 3. Geothermometry and Geobarometry, Eclogite Facies.
omphacite. These results are identi®ed as Pl-Qtz-
Omp(...) 1k, 2k, 3k and 4k in Table 3. Combining results
Equilibrium Conditions for the appropriate equilibria yields P±T conditions
1 Grt(core)-Omp(40/60,ts0, jd31) P=x115.7+0.18182(T)
ranging from 640 uC at 13.5 kbar (omphacite of jd27,
2 Grt(core)-Omp(40/60,ts0.07, jd27) P=x103.4+0.18182(T) ts0.07) to 840 uC at 18.9 kbar (omphacite of jd42, ts0).
3 Grt(core)-Omp(50/50,ts0, jd42) P=x133.7+0.18182(T) The average of the results, which we consider to be the
4 Grt(core)-Omp(50/50,ts0.07, jd36) P=x114.6+0.18182(T)
1' Pl-Qtz-Omp(40/60,ts0, jd31) P=5.60+0.0135(T) best estimate, is approximately 720 uC and 15.7 kbar.
2' Pl-Qtz-Omp(40/60,ts0.07, jd27) P=6.25+0.0113(T) This would correspond to an equilibrium omphacite of
3' Pl-Qtz-Omp(50/50,ts0, jd42)
4' Pl-Qtz-Omp(50/50,ts0.07, jd36)
P=4.20+0.0175(T)
P=5.25+0.0150(T)
jd34 and ts0.035. These conditions are comparable to
Equilibrium conditions for Grt(core)-Omp-Pl-Qtz T, uC P, kbar those estimated for the eclogites elsewhere in the
1+1' Omp(40/60,ts0, jd31) 720 15.3 AMS south-west of the Grandfather Mountain window
2+2k Omp(40/60,ts0.07, jd27) 640 13.5
3+3k Omp(50/50,ts0, jd42) 840 18.9
(Willard & Adams, 1994; Adams et al., 1995).
4+4k Omp(50/50,ts0.07, jd36) 720 16.0
Equilibria 1-4 Pattison & Newton (1989), Equilibira 1'-4'Gasparik & Lindsley (1980).
R E T R O G R A D E A M P H I B O L I T E F A C IE S
C ON DI TIO NS
Results of the geothermobarometry are presented in
Table 3. The P±T relationships were calculated for the Chemical analyses of the minerals in assemblage (e),
garnet-omphacite equilibrium, using the formulation Pl+Grt+Ep+Hbl+Qtz, obtained by electron micro-
by Pattison & Newton (1989). The garnet core probe, are presented in Table 4 . In this section, we
composition was paired with each of the ts0.07 and ts0 estimate the P±T conditions for the eclogite facies
omphacite models. The results are identi®ed as assemblages by applying various geothermobarom-
Grt(core)±Omp(...) equilibria 1, 2, 3 and 4 in Table 3. eters. First, we discuss the origin of hornblende during
Calculations using the garnet rim composition indi- the retrograde metamorphism of the symplectite
cated reequilibration to amphibolite facies conditions. assemblage (c).
P±T relationships were also calculated for the plagio- Assemblage (c), Di+Pl+Grt+Ep+Qtz, the
clase-omphacite-quartz equilibrium, using the formu- immediate predecessor to the hornblende-bearing
lation by Gasparik & Lindsley (1980). The resulting P± assemblage, is amenable to a CFM representation
T relationships pertain to minimum pressure for the (Abbott, 1982), wherein the retrograde appearance of440 R. N. ABBOTT & J. P. GREENWOOD
Table 4. Chemical Analyses of Minerals in Amphibolite
Assemblage.
No. of analyses Grt core Grt rim Hbl Pl
Wt. % oxides* (standard deviation): 1 1 7 1
SiO2 37.8 37.9 44.6(1.0) 59.2
TiO2 0.11 0.04 0.84(0.15) 0.02
Al2O3 21.28 21.18 10.54(0.81) 25.36
Cr2O3 ± 0.06 0.02(0.02) ±
FeO* 24.11 26.21 15.58(0.14) 0.35
MnO 0.65 0.80 0.06(0.02) ±
CaO 12.65 10.11 11.57(0.06) 7.17
MgO 3.35 3.79 11.64(0.44) ±
K2 O ± ± 0.06(0.01) ±
Na2O ± 0.03 1.91(0.11) 7.56
TOTAL 99.95 100.12 96.82 99.66
Fig. 5. CFM diagrams (C = CaO+Na2O+K2O-Al2O3, Constraints used in calculating chemical formula:
F = FeO-Fe2O3, M = MgO, Abbott, 1982), projected from Oxygen atoms p.f.u.: 12 12 no constraint 8
quartz and plagioclase, illustrating schematically the retrograde Cations p.f.u.: 8 8 X+Y+Z=15 no constraint
conversion of eclogite to amphibolite. A hypothetical bulk Tetrahedral coordination:
composition is indicated by the ®lled circle, expressed at high Si 2.96 2.94 6.66(0.10) 2.65
Al ± ± 1.34(0.10) 1.34
grade (a) by symplectic (Di+Pl)+Grt +Ep+Qtz (see Fig. 4).
Fe(3+)** 0.04 0.06 ± nd
At lower P±T conditions (b) in the presence of H2O, the same Fe(2+) ± ± ± 0.01
bulk composition is expressed by Hbl+Grt+Ep+Pl +Qtz. SUM 3.00 3.00 Z= 8.00 4.00
Octahedral coordination:
Al 1.96 1.93 0.51(0.06) ±
hornblende can be understood qualitatively. The Ti 0.01 ± 0.09(0.02) ±
relevant relationships are shown in Fig. 5. Chemical Cr
Fe(3+)**
±
0.03
±
0.07
±
nd
±
±
analyses for garnet (rim), diopside and epidote from the Fe(2+) ± ± 1.94(0.03) ±
retrograde eclogite assemblage (Fig. 5a, Table 1) and Mn ± ± 0.01(0.00) ±
Ca ± ± 1.85(0.01) ±
chemical analyses for garnet (rim) and hornblende Mg ± ± 2.59(0.08) ±
from the amphibolite assemblage (Fig. 5b, Table 4) SUM 2.00 2.00 X+Y= 7.00 ±
Dodecahedral coordination in garnet:
suggest that the reaction for the ®rst appearance of Fe(3+)** 0.03 0.06 ± ±
hornblende takes the form, Fe(2+) 1.48 1.51 ± ±
Mn 0.04 0.05 ± ±
Di+Grt (+Pl+H2O+O2)=Hbl+Ep. Ca 1.06 0.94 ± ±
Mg 0.39 0.44 ± ±
The positions of two-phase regions Grt±Di in Fig. 5(a) SUM 3.00 3.00 ± ±
and Hbl±Ep in Fig. 5(b) indicate that garnet is a minor Large cations in hornblende and plagioclase:
Ca ± ± ± 0.34
participant in the reaction. For practical purposes the K ± ± 0.01(0.00) ±
reaction may be simpli®ed to Na ± ± 0.55(0.03) 0.66
SUM ± ± 0.56 1.00
Di (+Pl+H2O+O2)=Hbl+Ep. alm 0.493 0.503 an 0.344
sps 0.013 0.017 or 0.000
H2O and O2 necessarily appear on the left side of the grs 0.353
pyr 0.130
0.313
0.147
ab 0.657
reaction. The appearance and growth of hornblende is
thus limited by the availability of these volatile species. *All Fe reported as FeO. `±' below detection.
**Fe(3+) estimated, based on constraints used in calculating chemical formula (see above).
Because epidote is part of the retrograde eclogite Note: No Fe(3+) was estimated for hornblende. Chemical formula for hornblende was
assemblage (Fig. 5a), the fugacity of O2 was already calculated according to Graham & Powell (1984).
high enough to stabilize this mineral before the
appearance of hornblende. Presumably the availability Table 5. Geothermometry and Geobarometry, Amphibolite
of H2O was more important than the fugacity of O2 Facies.
with regard to the appearance and growth of Equilibria Reference T, uC P(655 uC), kbar
hornblende. Conversely, the diopside-plagioclase sym-
plectite assemblage would be preserved where the Grt(rim)-Hbl
Grt(rim)-Hbl-Pl (Mg)
Graham & Powell (1984)
Kohn & Spear (1990)
655 ±
7.9
availability of H2O was limited. Grt(rim)-Hbl-Pl (Fe) Kohn & Spear (1990) 8.5
In the eclogites south-west of the Grandfather Grt(rim)-Di-Pl-Qtz Newton & Perkins (1982) 8.6
Mountain window, described by Willard & Adams
(1994), hornblende starts to form in a different way,
earlier in the paragenesis, before the breakdown of composition (Table 4) are reported here, because
omphacite to symplectic Di+Pl. Willard & Adams calculated temperatures using the one garnet core
suggested the reaction, composition were considered unrealistically high. The
temperature was estimated to be 655 uC, based on
3 Grt+7 Omp+2 H2O=2 Hbl+4 Pl.
Fe-Mg exchange between garnet (rim) and hornblende,
Results of the geothermobarometry are presented in according to the formulation by Graham & Powell
Table 5. Only results involving the average garnet rim (1984). This temperature was used in subsequentRETROG RAD E EC LOGITE IN TH E SOU TH ERN A PPA L AC HIAN S 44 1
estimations for the pressure, using the Mg end-
member and Fe end-member formulations for the
equilibrium involving garnet (rim), hornblende and
plagioclase, according Kohn & Spear (1990). The
estimated pressure is 7.9 and 8.5 kbar respectively for
the Fe and Mg end-member equilibria. The pressure
was estimated to be 8.6 kbar (at 655 uC) for the garnet
rim±diopside±plagioclase±quartz equlibrium in the
symplectite assemblage (Table 1), using the formula-
tion of Newton & Perkins (1982). The estimated
conditions are consistent with previous estimations
(McSween et al., 1989) for pelitic and ma®c assem-
blages from the same area.
DI SCUSSI ON
A hypothetical retrograde P±T path is given in Fig. 6.
Two points on the path are (1) the estimated minimum
P±T conditions for the original omphacite-bearing
eclogite facies assemblage and (2) the estimated
equilibrium conditions for the amphibolite facies Fig. 6. P±T diagram illustrating schematically the retrograde
assemblage. The stippled region encloses P±T condi- path of eclogites in the AMS. Phase relationships from Fig. 2
tions estimated by McSween et al. (1989) for amphi- are positioned such that point i is consistent with conditions
bolites and pelitic schists in the AMS. estimated by McSween et al. (1989). Slopes of reactions are
consistent with calculations by Spear et al. (1995). Al2SiO5
The distribution of retrograded eclogite in the AMS polymorphic transformations, involving And, Ky and Sil, are
is consistent with the metamorphic pressure gradients from Holdaway (1971). Stippled region encloses range of P±T
preserved in the pelitic rocks. Parts of the AMS that conditions in AMS, as estimated by McSween et al. (1989).
experienced the greatest uplift and hence originated at
the greatest depths, are adjacent to the Grandfather
Mountain window. In a palinspastic reconstruction,
this means that the basal thrust fault intercepted the
greatest depths beneath parts of the AMS now exposed
adjacent to the Grandfather Mountain window. Early
during subduction (Abbott & Raymond, 1984; Willard
& Adams, 1994; Adams et al., 1995), along what would
become the basal thrust fault of the AMS, downward
de¯ection of the surface of the fault would explain the
interception of the highest-P parts of the AMS.
Downward de¯ection of the nascent fault suggests
that the affected parts of the AMS were different from
other parts of the eastern Blue Ridge (i.e. parts further
away from the general vicinity of the Grandfather
Mountain window) with regard to physical properties
of the crust at the time. Perhaps the most signi®cant,
distinguishing feature of the AMS-ABMS adjacent to
the Grandfather Mountain window is the great volume
of ma®c rocks (Rankin et al., 1972). A cross-strike
section through the AMS just north of the Grandfather
Mountain window consists of up to 75% essentially
uninterrupted amphibolite. Rankin (1970) indicated
only that the original thickness of the AMS `must...be
measured in miles.' Presumably, at least in the AMS Fig. 7. Model for aborted subduction of a seamount.
adjacent to the Grandfather Mountain window, the Diagrams illustrate four stages in the process, from the initial
thickness of the amphibolite component of the AMS stage (top) to the ®nal stage (bottom). Filled circle indicates
would be measured in `miles'. We offer the possibility the position of the protolith for eclogite in the AMS. Numbers
that the nascent thrust fault was de¯ected downward 1±6 mark positions in the North American plate (left) plate. In
the second stage, subduction is displaced from the east side
quite simply by an obstacle, in the form of what was (right) to the west side (left) of the seamount and de¯ected
originally an isolated ma®c volcanic edi®ce on oceanic downward beneath the seamount. The seamount is accreted to
crust±a seamount. The idea is illustrated in Fig. 7. the Piedmont terrane (right).442 R. N. ABBOTT & J. P. GREENWOOD
Trace element and REE geochemistry of AMS±ABMS In: Paleozoic Structure, Metamorphism, and Tectonics of
amphibolites (Misra & Conte, 1991) are generally the Blue Ridge of Western North Carolina: Carolina
Geological Society Field Trip Guidebook (eds Stewart, K. G.,
consistent with this interpretation. Misra & Conte Adams, M. G. & Trupe, C. H.), 87±101.
(1991) describe three compositional groups of basalts. Abbott, R. N., Raymond, L. A. & McSween, H. Y., 1991.
The paleotectonic setting for group I is back-arc or Taconic invariant points in the central Blue Ridge Belt and
plume; for group II, spreading centre (N-MORB); and their structural/tectonic implications. Geological Society of
for group III, transitional between spreading centre America Abstracts with Program, 23, 1.
Adams, M. G., Stewart, K. G., Trupe, C. H. & Willard, R. A.,
and plume (T-MORB). 1995. Tectonic signi®cance of high-pressure metamorphic
rocks and dextral strike-slip faulting along the Taconic
suture. In: Current Perspectives in the Appalachian±
C O N C L U S IO N S Caledonian Orogen (eds Hibbard, J. P., van Staal, C. R. &
Cawood, P. A.). Geological Association Canada, pp. 21±42.
Eclogite, retrograded or otherwise, is scarce in the Brown, P. M., 1985. Geologic Map of North Carolina.
Appalachian orogen (Adams et al., 1995). In the Department of Natural Resources and Community
southern Appalachian Blue Ridge, the evidence that Development, Division of Land Resources, North Carolina.
such rocks provide for early high-P conditions Bryant, B. & Reed, J. C., 1970. Geology of the Grandfather
(c.16 kbar, c.720 uC, in the present case) is important, Mountain window and vicinity, North Carolina and
Tennessee. U.S. Geological Survey Professional Paper, 615.
if not crucial, for the interpretation of the AMS and Butler, J. R., 1972. Age of Paleozoic regional metamorphism
ABMS as a subduction-related metamorphic complex. in the Carolinas, Georgia, and Tennessee, southern
Other characteristics of the AMS and ABMS (Ray- Appalachians. American Journal of Science, 272, 319±333.
mond et al., 1989; Adams et al., 1995) support the view Butler, J. R., 1973. Paleozoic deformation and metamorphism in
part of the Blue Ridge thrust sheet, North Carolina. American
that large parts of the metamorphic complex were an Journal of Science, 273-A, 72±88.
accretionary meÂlange and that the AMS and ABMS Butler, J. R., 1991. Metamorphism. In: The Geology of the
together mark the suture between the ancient North Carolinas (eds Wright, J. W. & Zullo, V. A.), pp 127±141.
American craton and the Piedmont terrane (Raymond University of Tennessee, Knoxville.
et al., 1989; Adams et al., 1995). Subduction was most Cameron, M. & Papike, J. J., 1980. Crystal chemistry of silicate
pyroxenes. Mineralogical Society of America, Reviews in
likely toward the east (Fig. 7), such that the meÂlange Mineralogy, 7, 5±116.
accumulated along the western edge of Piedmont Conley, J. F., 1987. Geology of the Piedmont of Virginia±
terrane (Raymond et al., 1989). The geochemistry of Interpretations and problems. Contributions to Virginia
amphibolite in the AMS (Misra & Conte, 1991) de®nes Geology±II. Virginia Division of Mineral Resources
three compositional groups. Collectively, the groups Publication, 7, 115±149.
Deer, W. A., Howie, R. A. & Zussman, J., 1992. An Introduction
argue strongly for two dominant environments for the to the Rock-Forming Minerals, 2nd edn. John Wiley & Sons,
protolithic basalt, ocean ridge (N-MORB) and ocean New York.
plume (T-MORB, sea mount). Such materials were the Drake, A. A., Sinha, A. K., Laird, J. & Guy, R. E., 1989. The
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Society of America, Boulder, Co.
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Thus, the AMS adjacent to the Grandfather Mountain B., 1975. Geologic Map of the east half of the Winston-Salem
window may represent the deformed, sheared, partially quadrangle, North Carolina-Virginia. U.S. Geological Survey,
dismembered, but still largely contiguous, parts of a Miscelaneous Geologic Investigations, Map Ix709B.
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Raleyh, NC.
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Raymond, 1997). The paper has bene®tted greatly from Precambrian gneisses in the Blue Ridge province of north-
the comments of reviewers H. McSween and G. Ernst western North Carolina and adjacent parts of Virginia
and Editor M. Brown. and Tennessee. Geological Society of America Bulletin, 84,
3065±3080.
Gasparik, T. & Lindsley, D. H., 1980. Phase equilibria at high
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