Retrograde net transfer reaction insurance
for pressure-temperature estimates
Matthew J. Kohn
Department of Geological Sciences, University of South Carolina, Columbia, South Carolina 29208, USA
Frank Spear
Department of Earth and Environmental Sciences, Rensselaer Polytechnic Institute, Troy, New York 12180, USA
ABSTRACT
Retrograde net transfer reactions significantly affect compositions of metamorphic minerals, yet are rarely considered when determining pressure-temperature (P-T) conditions. Two
natural amphibolite facies metapelites from the central Himalaya of Nepal exhibit extremely
common compositional patterns, including increases in Mn and Fe/(Fe + Mg) at the rims of garnets, which are the result of retrograde garnet dissolution and Fe-Mg exchange with biotite.
However, typical thermobarometric approaches for these rocks result in errors of hundreds of
degrees and 3–6 kbar compared with thermobarometry of nearby rocks and petrogenetic grids.
These large errors result because dissolution of high-Fe garnet has strongly affected the Fe/Mg
ratio of matrix biotite. X-ray maps help evaluate the extent and chemical effects of retrograde
reactions in these samples by identifying mineral regions that retain highest-T compositions, or,
through a new data-processing approach, by permitting correction of mineral compositions to
original high-T values. These approaches ensure against retrograde net transfer reactions and
should be applied routinely in thermobarometric studies—they ultimately yield P-T estimates
that are more petrologically reasonable, and permit rapid screening of samples for those least
affected by retrograde reactions. Reconsideration of thermobarometry in the central and eastern Himalaya indicates that retrograde net transfer reactions are extremely common. Therefore, previous thermobarometric studies based on garnet major element compositions from that
region should be reevaluated.
Keywords: geothermometry, geobarometry, metamorphism, Himalaya.
INTRODUCTION
Over the past 20 yr, thermobarometry has become a common petrologic tool. For example, in one international journal during the past decade,
more than 200 articles appeared that include an estimate of pressuretemperature (P-T) conditions based on mineral compositions and thermodynamically calibrated equilibria. Most workers apply standardized criteria
for selecting mineral compositions, especially for garnet, that assume that
reequilibration during cooling has occurred only via retrograde exchange
reactions (ReERs)—i.e., reactions that exchange Fe, Mg, or Mn among
minerals without significantly affecting modal abundances. However, most
compositional zoning systematics also indicate the importance of retrograde
net transfer reactions (ReNTRs)—reactions that cause net growth and dissolution of minerals. As demonstrated herein, P-T results for middle and
upper amphibolite facies rocks that have undergone ReNTR may be precise,
yet grossly incorrect by many kilobars and hundreds of degrees. Accurate
thermobarometry requires consideration of possible ReNTRs, as well as
design and application of chemical and textural tests to accommodate such
reactions. Data-processing techniques are described here that maximize the
likelihood of identifying peak equilibrium compositions and estimating
accurate P-T conditions, i.e., to ensure against ReNTRs. Metapelites with
garnet + biotite metapelites are emphasized, because these are most commonly used for thermobarometry.
REACTIONS
Exchange
Exchange reactions involve the exchange of two elements between
two minerals and form the basis for many thermometers, such as the Fe-Mg
exchange reaction between garnet and biotite (almandine + phlogopite =
annite + pyrope). Retrograde exchange reactions do not significantly change
mineral modes and cause divergence of mineral compositions (e.g., biotite
becomes more Mg rich while garnet becomes more Fe rich). If only retrograde exchange reactions occur, calculated temperatures will be below peak
temperatures.
Net Transfer
Net-transfer reactions involve the production and consumption of minerals. Kinetics may subordinate net-transfer reactions to exchange reactions
during cooling (e.g., Frost and Chacko, 1989), but unlike retrograde exchange
reactions, ReNTRs cause mineral compositions to shift in the same direction (e.g., biotite and garnet both become more Fe rich). This fact has been
discussed (e.g., Robinson, 1991; Spear, 1991; Spear and Florence, 1992;
Spear and Parrish, 1996) because if ReNTRs do occur, calculated temperatures may be higher than peak temperatures. However, no previous study
has shown how to quantitatively determine the extent of reaction or correct
compositions to retrieve peak P-T conditions. As described in the following,
these goals can be achieved via quantitative X-ray maps.
EXAMPLE 1: THE GARNET INNER RIM AND Mn KICK-UP
Mn contents and Fe/(Fe + Mg) profiles of most amphibolite facies garnets show a broad decrease from core toward the rim, with a sharp increase
at the rim (Fig. 1). Many workers assume that the Mn and Fe/(Fe + Mg)
trough (the inner rim) best represents the peak metamorphic composition,
and that matrix biotite either has not changed significantly or has become
more Mg rich via retrograde exchange reactions (Fig. 1); these workers then
pair the inner rim with matrix biotite for estimating peak P-T conditions.
Although these assumptions may be correct in some rocks, the Mn kick-up
Data Repository item 2000111 contains additional material related to this article.
Geology; December 2000; v. 28; no. 12; p. 1127–1130; 4 figures.
1127
is not commonly the result of simple diffusion: zoning in garnets around
isolated biotite inclusions shows only Fe-Mg zoning, not Mn. Rather, the Mn
kick-up is more likely due to garnet dissolution, i.e., net-transfer reaction.
This possibility is problematic because garnet has a much greater Fe/Mg
ratio than biotite, and garnet dissolution will enrich biotite and/or other
ferromagnesian minerals in Fe (Fig. 1). If near-peak garnet compositions
are preserved (e.g., in a core or near an undissolved rim) yet garnet dissolution is significant, then pairing of the trough composition with Fe-enriched
biotite may yield too high a temperature (Robinson, 1991; Spear and
Florence, 1992). If the Fe-enriched biotite exchanges with other biotite
grains, then although the compositional impact is lessened, the reaction is
also much harder to detect.
Sample DH-58 from the Darondi valley in central Nepal illustrates this
phenomenon (Fig. 2A and 2B). This sample is from a suite collected across
the Main Central thrust zone, as part of a larger P-T-t study (t = time) of inverted metamorphism. The marked Mn zoning pattern is interpreted as follows: (1) the garnet was once larger, and had a uniformly low Mn content
toward its rim, (2) the garnet dissolved during cooling and exhumation, returning Mn to the matrix, and (3) because biotite contains negligible Mn
compared to garnet, Mn increased on the rim of the garnet and backdiffused into the garnet. This scenario implies that the highest-temperature
part of the garnet remaining is reflected by the lowest Mn and Fe/(Fe + Mg)
values, but that biotite has become more Fe rich. The matrix biotite has a
fairly homogeneous Fe/(Fe + Mg) value of 0.52, although grains closest to
the garnet are ~0.01 higher. A common approach is to pair the lowest
Fe/(Fe + Mg) value of the garnet with the nearby matrix biotite, which
yields 17 ± 1.75 kbar and 825 ± 50 °C (Fig. 3; T is uniformly lower by only
30 °C if the most Mg-rich biotite is used). This result is surprising for several reasons: (1) the rock was collected close to the staurolite-in isograd,
which commonly occurs at ~600 °C (Spear and Cheney, 1989); (2) other
rocks nearby yield temperatures and pressures of 600–650 °C and 11 kbar;
and (3) the calculated P-T result corresponds to the granulite and eclogite
facies, and is close to muscovite-dehydration melting, yet there are no partial melts, granulites, or eclogites. However, recognizing the reaction that
D-Rim
A
Fe
(Fe + Mg)
Core
U-Rim
has taken place (garnet + muscovite = biotite + plagioclase), the amount of
Mn in the near-rim kick-up can be used as a semiquantitative measure of the
volume of garnet dissolved. This volume in turn can be used to determine
how much biotite was produced and hence how much the biotite composition has changed (see supplemental material for details).1
The garnet in Figure 2A has a volume of ~17 mm3. Based on the nearrim XSps profile, and assuming a central-cut through the garnet and quasiradial dissolution, the volume originally dissolved was at least 15 mm3 or
~45% of the original garnet. Similar analysis of another garnet from the same
rock indicates at least 40% dissolution. This large amount of dissolution is
1GSA Data Repository item 2000111, A step-by-step description of how X-ray
maps, compositions, and modal measurements may be used to correct mineral compositions for P-T calculations, is available on request from Documents Secretary,
GSA, P.O. Box 9140, Boulder, CO 80301-9140, [email protected], or at
www.geosociety.org/pubs/ft2000.htm.
DH-58
A
B
0.13
0.02
0.825
0.16
0.95
Fe
Fe + Mg
Mn
C
D
DH-60
0.85
Fe
Fe + Mg
0.047
0.88
0.10
Mn
E
Garnet
0.503
0.509
0.494
Troughs
0.489
Fe
(Fe + Mg)
XSps
Final Bt
Orig. Bt
Biotite
B
Orig. Bt
Final Bt
0.555
Figure 1. A: Schematic zoning profile across typical amphibolite-grade
garnet, illustrating general decrease in XSps (mole fraction spessartine)
and Fe/(Fe + Mg) from core toward rims, abrupt kick-up right at rim, and
resulting trough or inner rim. Dashed line shows original profile at peak
temperature. For dissolved rim (D-Rim), troughs occur in both XSps and
Fe/(Fe + Mg); for undissolved rim (U-Rim), trough occurs only in Fe/(Fe
+ Mg). Most published profiles are most similar to D-Rim profile. Ordinarily, trough composition is paired with matrix minerals for estimating
peak pressure-temperature (P-T ) conditions. Depending on original zoning profile and extent of dissolution and diffusional modification, Mn and
Fe/(Fe + Mg) trough locations may not coincide. Bt is biotite. B: Effect of
reactions on biotite Fe/(Fe + Mg). If garnet dissolves (D-Rim), biotite becomes more Fe rich and calculated temperatures may be too high; if garnet simply exchanges Fe and Mg, then biotite becomes more Mg rich
and calculated temperatures are too low. Vertical axes are not to scale.
1128
Figure 2. Images for Himalayan samples DH-58 and DH-60.
Values are for either XSps or Fe/(Fe + Mg). A–D: X-ray maps;
hot colors (red and yellow) are high values and cool colors
(blue and purple) are low values. These maps illustrate
troughs (or inner rims) and near-rim increases in Mn and
Fe/(Fe + Mg). X-ray maps for other garnets in same rocks
show virtually identical trends. Scale bars on Mn maps are
500 µm. E: Photomicrograph of DH-60 in plane-polarized
light. Large, high-relief grain toward left is garnet, dark
grains are biotite, and white grains are quartz, plagioclase,
and muscovite. Biotite Fe/(Fe + Mg) ratio is highest closest
to garnet and lowest farthest away.
GEOLOGY, December 2000
Standard
10
8
12
St + Bt
Stable
8
4
+P
DH-60
6
4
Ms
P (kbar)
l+
DH-60
DH-58
0
400
= Ms-absent (H) St + Bt
= Ms-bearing (H) Unstable
= MCT-zone (H)
DH-58
Rocks
near
DH-60
P (kbar)
16
ReNTRinsured
Qtz
Thermobarometry:
Rocks
n
DH-58ear
20
DISCUSSION AND IMPLICATIONS
Himalayan Metamorphism
A serious concern in Himalayan P-T research is that different studies of
similar areas yield radically different P-T results, or P-T conditions that
violate mineral stabilities (e.g., see Vannay and Grasemann, 1998; Fig. 4).
Qualitatively, the pressures and temperatures should be reconsidered in light
of ReNTRs. If ReNTRs are common in the Himalaya, then previous P-T
results are suspect; if ReNTRs are not evident, then some other explanation
for the P-T disparities must be invoked. To evaluate these possibilities, we
examined 14 published thermobarometric studies of the central and eastern
Himalaya, from central Nepal to Bhutan. Of these studies, only one shows
X-ray maps (6 samples; Davidson et al., 1997), six show core-rim or rim-rim
traverses across garnets analogous to the sketch in Figure 1 (23 samples:
Swapp and Hollister, 1991; Inger and Harris, 1992; Hodges et al., 1993;
Kaneko, 1995; Macfarlane, 1995; Vannay and Hodges, 1996), and three
report pairs of core and rim garnet compositions (17 samples: Hubbard, 1989;
Pognante and Benna, 1993; Neogi et al., 1998); the other four studies report
Kyanite Zone
(B&K)
le
600
700
800
900
T (°C)
Figure 3. Pressure-temperature (P-T ) results for samples. In DH-58,
pairing matrix biotite with composition of garnet with lowest Fe/(Fe +
Mg) ratio (i.e., trough) yields T of ~825 °C (Grt-Bt and Grt-Pl-Ms-Bt thermobarometer calibrations of Ferry and Spear, 1978; Berman, 1990;
Hoisch, 1990), ~200 °C higher than temperatures estimated from nearby
rocks. Accounting for effect of retrograde net transfer reactions (ReNTRs)
on biotite compositions yields petrologically more realistic minimum T
of ~575 °C. Because garnet rims were dissolved, peak-metamorphic
temperatures are not recoverable. In DH-60 pairing biotite nearest
garnet with garnet trough composition yields T of ~750 °C, ~100 °C
higher than estimated in associated rocks. Accounting for ReNTRs by
pairing matrix biotite far away from garnet with trough composition
yields more realistic T of ~625 °C.
GEOLOGY, December 2000
0
400
Migmatitic
Tibetan slab
(B&K)
500
Ms + Pl + Qtz
Unstable
ab
St
Q Ms + Pl + Qtz Melt Stable
+
Unstable
Pl
tz
2
500
ble
EXAMPLE 2: HETEROGENEOUS BIOTITE
Because retrograde exchange reactions cause calculated temperatures
to be lower than peak temperatures, some workers assume that the highest
temperature calculable in a sample is the best estimate of the peak. Other
workers, wishing to ensure the greatest likelihood of equilibration, pair
grains in closest proximity. Sample DH-60 (Fig. 2, C, D, and E), collected
~1 km structurally above DH-58 and at higher metamorphic grade, illustrates some pitfalls in these assumptions and also offers a different example
of how ReNTRs affect mineral compositions. Despite its nonequant habit,
the garnet Mn and Fe/(Fe + Mg) systematics in DH-60 are grossly similar to
those in DH-58, and indicate that the garnet has undergone ReNTR analogous to DH-58. However, biotite grains in DH-60 closest to the garnet have
Fe/(Fe + Mg) ratios of ~0.55, whereas those far away have ratios of 0.49
(Fig. 2E). Retrograde resorption of the garnet has increased the biotite
Fe/(Fe + Mg) ratio, but unlike the effect in DH-58, kinetics apparently limited the resulting increase to those grains nearest to the garnet.
Ignoring the ReNTR and pairing the biotite grains closest to the garnet
with the garnet with lowest Fe/(Fe + Mg) ratio yields ~750 °C and 13 kbar
(Fig. 3). This result is unacceptably high because there are no partial melts,
granulites, or eclogites in the section and because pressures and temperatures calculated from associated rocks with compositionally more homogeneous biotites are much lower (Fig. 3). However, if biotite far from the garnet is used, then temperatures are much more consistent with petrologic
observations and thermobarometry, yielding 625 °C and 10 kbar. It is unlikely that this biotite has changed composition significantly since the metamorphic peak, because there is modally very little garnet in DH-60 (1%).
Therefore, although we calculate that 13% of the garnet dissolved, average
biotite Fe/(Fe + Mg) was affected by ≤0.005 (or ~10 °C). However, if the
ReNTR were not recognized and its effects on biotite characterized chemically, spurious P-T conditions could have readily resulted.
Sta
supported by 0.25–1.0-mm-wide reaction zones around each garnet, defined
by a slightly higher biotite content than in the matrix, or by plagioclase with
a lower inclusion content than matrix grains; however, these textures are extremely subtle, and could readily be ascribed to small local differences in
bulk composition, or to strain effects, rather than the chemically documented
reaction. If XSps originally decreased toward the rims of the garnets (as is
likely), then even more garnet dissolved to produce the observed Mn increase, and the compositional and P-T corrections described next are minima. From the current modes of matrix biotite (27% ± 2%) and garnet
(7.35% ± 0.2%), the calculated amount of dissolved garnet (~6 modal percent), and the volume concentrations of Fe and Mg in biotite and garnet, the
original mode of biotite and its Fe/(Fe + Mg) ratio prior to garnet dissolution
can be estimated: these are ~19% and ~0.39. The revised biotite composition,
when paired with the garnet with minimum Fe/(Fe + Mg) ratio, yields an estimated T of ~575 °C, ~250 °C lower than the original estimate. The temperature estimate is still a maximum (because more of the garnet could have dissolved), but it is also much more consistent with petrologic considerations—it
should be higher than typical garnet-nucleation temperatures of 475–525 °C
for similar bulk compositions, but lower than the peak of metamorphism because the highest-temperature rim dissolved.
s
+
M
600
700
800
T (°C)
Figure 4. Pressure-temperatures (P-T ) results from Everest region,
based on Hubbard (1989; H) and Brunel and Kienast (1986; B & K); all
rocks are staurolite grade or higher (i.e., above staurolite [St] + biotite
[Bt]; Hubbard, 1989). Samples and areas that are black are reported P-T
conditions that are inconsistent with phase equilibria, and are therefore
interpreted by us as incorrect: four staurolite-zone samples, one sample
containing muscovite (Ms) + plagioclase (Pl) + quartz (qtz), and four
migmatitic rocks plot outside their assemblage stability fields. Most
Himalayan samples underwent retrograde exchange and net transfer
reactions, which we believe has biased P-T calculations.
1129
either a single composition (Brunel and Kienast, 1986) or no compositions
(Hodges et al., 1988a, 1988b, 1994). Of the 46 samples from which any zoning is inferable, 32 have clear Mn increases on garnet rims and another 4 have
textural evidence for retrograde resorption of garnet. Nine of the remaining
samples only have core-rim analyses, so near-rim increases could be present
but undocumented. Thus, at least 75% and possibly 90% of Himalayan samples
have undergone ReNTRs, and based on our analysis of DH-60 and DH-58,
their calculated peak P-T conditions are mislocated, possibly by hundreds of
degrees and several kilobars. Petrologic characterization is generally too poor
to revise P-T estimates. However, the occurrence of ReNTRs in the majority
of samples and the sensitivity of calculated P-T conditions to ReNTRs offer
both an explanation for some disparities and a caution against the quantitative
use or intercomparison of most thermobarometric results for the region.
Compositional Analysis
The examples and previous discussion lead to three generalizations
about chemical variations within metamorphic rocks, which guide our recommendations for data gathering and processing.
1. The peak-metamorphic compositions (i.e., the compositions corresponding to the maximum temperature) probably do not exist. Retrograde reactions always occur during cooling and tend to eradicate peak-metamorphic
compositions, and thus we should not assume that such compositions are retained. The alternative implicit assumption that extant compositions are our
best estimate of peak metamorphic compositions is clearly repudiated by
sample DH-58. The task of the thermobarometrist is to use mineral assemblage data, chemical variations, and textures to infer a reaction history, from
which better estimates of peak compositions and P-T conditions may be derived (Robinson, 1991; Spear and Florence, 1992).
2. All minerals are compositionally zoned, or different grains have distinct compositions. Insofar as at least one element has a slow diffusivity in
a given mineral, kinetics and/or fractional crystallization during prograde
metamorphism will always produce chemical zoning in growing minerals.
For reactant phases, dissolution and reprecipitation commonly result in
additional complex zoning patterns. Diffusional reequilibration can affect
all mineral compositions during cooling. Because many of these processes
are kinetically limited (e.g., diffusion), and because mineral distributions in
rocks are not usually homogeneous, different mineral grains should not
have identical compositions, as illustrated by biotite in DH-60.
3. Our ability to interpret natural compositional variability is far worse
than our ability to measure it. The variability in calculated temperatures in
a rock from measured compositions (e.g., Ferry, 1980; example 2 herein)
and the corrections to DH-58 are many times larger than the propagated
error for typical electron microprobe analyses (~±5 °C; Kohn and Spear,
1991). Thus, our ability to decide which mineral compositions were likely
in equilibrium is many times worse than the precision with which they are
typically measured. The greatest scientific gains will be made by analyzing
more points with lower precision via a technique such as quantitative X-ray
mapping, rather than by analyzing just a few spots or individual traverses
with high precision.
ACKNOWLEDGMENTS
Funded by National Science Foundation grants EAR-9903036 (to Spear) and
EAR-0073803 (to Kohn), and by the University of South Carolina. We thank Mark
Harrison and Rick Ryerson for providing the samples, and Cam Davidson and Darrell
Henry for detailed reviews.
REFERENCES CITED
Berman, R.G., 1990, Mixing properties of Ca-Mg-Fe-Mn garnets: American Mineralogist, v. 75, p. 328–344.
Brunel, M., and Kienast, J.-R., 1986, Etude petro-structurale des chevauchements
ductiles himalayens sur la transversale de l'Everest-Makalu (Nepal oriental):
Canadian Journal of Earth Sciences, v. 23, p. 1117–1137.
Davidson, C., Grujic, D.E., Hollister, L.S., and Schmid, S.M., 1997, Metamorphic reactions related to decompression and synkinematic intrusion of leucogranite,
high Himalayan crystallines, Bhutan: Journal of Metamorphic Geology, v. 15,
p. 593–612.
1130
Ferry, J.M., 1980, A comparative study of geothermometers and geobarometers in
pelitic schists from south-central Maine: American Mineralogist, v. 65,
p. 720–732.
Ferry, J.M., and Spear, F.S., 1978, Experimental calibration of the partitioning of Fe
and Mg between biotite and garnet: Contributions to Mineralogy and Petrology,
v. 66, p. 113–117.
Frost, B.R., and Chacko, T., 1989, The granulite uncertainty principle: Limitations on
thermobarometry in granulites: Journal of Geology, v. 97, p. 435–450.
Hodges, K.V., Hubbard, M.S., and Silverberg, D.S., 1988a, Metamorphic constraints
on the thermal evolution of the central Himalayan orogen: Royal Society of
London Philosophical Transactions, v. 326, p. 257–280.
Hodges, K.V., Le Fort, P., and Pecher, A., 1988b, Possible thermal buffering by
crustal anatexis in collisional orogens: Thermobarometric evidence from the
Nepalese Himalaya: Geology, v. 16, p. 707–710.
Hodges, K.V., Burchfiel, B.C., Royden, L.H., Chen, Z., and Liu, Y., 1993, The metamorphic signature of contemporaneous extension and shortening in the central
Himalayan orogen: Data from the Nyalam transect, southern Tibet: Journal of
Metamorphic Geology, v. 11, p. 721–737.
Hodges, K.V., Hames, W.E., Olszewski, W., Burchfiel, B.C., Royden, L.H., and
Chen, Z., 1994, Thermobarometric and 40Ar/39Ar geochronologic constraints
on Eohimalayan metamorphism in the Dinggye area, southern Tibet: Contributions to Mineralogy and Petrology, v. 117, p. 115–163.
Hoisch, T.D., 1990, Empirical calibration of six geobarometers for the mineral assemblage quartz + muscovite + biotite + plagioclase + garnet: Contributions to
Mineralogy and Petrology, v. 104, p. 225–234.
Hubbard, M.S., 1989, Thermobarometric constraints on the thermal history of the
Main Central thrust zone and Tibetan slab, eastern Nepal Himalaya: Journal of
Metamorphic Geology, v. 7, p. 19–30.
Inger, S., and Harris, N.B.W., 1992, Tectonothermal evolution of the High Himalayan
crystalline sequence, Langtang Valley, northern Nepal: Journal of Metamorphic
Geology, v. 10, p. 439–452.
Kaneko,Y., 1995, Thermal structure in the Annapurna region, central Nepal Himalaya:
Implication for the inverted metamorphism: Journal of Mineralogy, Petrology
and Economic Geology, v. 90, p. 143–154.
Kohn, M.J., and Spear, F.S., 1991, Error propagation for barometers: 2. Application
to rocks: American Mineralogist, v. 76, p. 138–147.
Macfarlane, A.M., 1995, An evaluation of the inverted metamorphic gradient at Langtang National Park, central Nepal Himalaya: Journal of Metamorphic Geology,
v. 13, p. 595–612.
Neogi, S., Dasgupta, S., and Fukuoka, M., 1998, High P-T polymetamorphism, dehydration melting, and generation of migmatites and granites in the Higher Himalayan crystalline complex, Sikkim, India: Journal of Petrology, v. 39, p. 61–99.
Pognante, U., and Benna, P., 1993, Metamorphic zonation, migmatization and
leucogranites along the Everest transect of eastern Nepal and Tibet: Record of
an exhumation history, in Treloar, P.J., and Searle, M.P., eds., Himalayan tectonics: Geological Society [London] Special Publication 74, p. 323–340.
Robinson, P., 1991, The eye of the petrographer, the mind of the petrologist: American Mineralogist, v. 76, p. 1781–1810.
Spear, F.S., 1991, On the interpretation of peak metamorphic temperatures in light of
garnet diffusion during cooling: Journal of Metamorphic Geology, v. 9,
p. 379–388.
Spear, F.S., and Cheney, J.T., 1989, A petrogenetic grid for pelitic schists in the system SiO2 - Al2O3 - FeO - MgO - K2O - H2O: Contributions to Mineralogy and
Petrology, v. 101, p. 149–164.
Spear, F.S., and Florence, F.P., 1992, Thermobarometry in granulites: Pitfalls and new
approaches: Journal of Precambrian Research, v. 55, p. 209–241.
Spear, F.S., and Parrish, R.R., 1996, Petrology and cooling rates of the Valhalla Complex, British Columbia, Canada: Journal of Petrology, v. 37, p. 733–765.
Swapp, S.M., and Hollister, L.S., 1991, Inverted metamorphism within the Tibetan
slab of Bhutan: Evidence for a tectonically transported heat-source: Canadian
Mineralogist, v. 29, p. 1019–1041.
Vannay, J.-C., and Grasemann, B., 1998, Inverted metamorphism in the High
Himalaya of Himachal Pradesh (NW India): Phase equilibria versus thermobarometry: Schweizerische Mineralogische und Petrographische Mitteilungen,
v. 78, p. 107–132.
Vannay, J.C., and Hodges, K.V., 1996, Tectonometamorphic evolution of the
Himalayan metamorphic core between the Annapurna and Dhaulagiri, central
Nepal: Journal of Metamorphic Geology, v. 14, p. 635–656.
Manuscript received March 13, 2000
Revised manuscript received September 1, 2000
Manuscript accepted September 20, 2000
Printed in U.S.A.
GEOLOGY, December 2000