Journal of Petrology 2004 45(7):1481-1512; doi:10.1093/petrology/egh023
Journal of Petrology 45(7) © Oxford University Press 2004; all rights reserved
Orthopyroxene–Corundum in Mg–Al-rich Granulites from the Oygarden Islands, East Antarctica
N. M. KELLY1,* and
S. L. HARLEY2
1 DIVISION OF GEOLOGY AND GEOPHYSICS, SCHOOL OF GEOSCIENCES, F05, UNIVERSITY OF SYDNEY, SYDNEY, N.S.W. 2006, AUSTRALIA
2 SCHOOL OF GEOSCIENCES, UNIVERSITY OF EDINBURGH, GRANT INSTITUTE, KINGS BUILDINGS, EDINBURGH EH9 3JW, UK
RECEIVED
MARCH 4, 2003;
ACCEPTED
FEBRUARY 2, 2004
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ABSTRACT
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High-Mg–Al, silica-undersaturated metapelites from the
Oygarden Group of islands, East Antarctica, preserve clear evidence
for the stable coexistence of the assemblage orthopyroxene +
corundum in natural rocks. The quartz-absent metapelite occurs
as pods and isolated layers within a high-strain zone related
to deformation during the
c. 0·93 Ga Rayner Structural
Episode. Assemblages that include orthopyroxene, corundum, sapphirine,
sillimanite, cordierite, garnet and kornerupine are developed
across a pre-existing compositional zoning, leading to contrasting
mineral Fe–Mg ratios. The assemblage orthopyroxene–corundum
is shown to exist in only a very restricted range of bulk compositions
and
P–
T histories. Simplified qualitative FMAS grids have
been constructed for kornerupine-absent and -present systems,
illustrating MAS terminations and divariant equilibria that
help to describe the mineral assemblage and reaction history.
Reaction textures that include coronas of sapphirine and sillimanite
separating orthopyroxene and corundum, and symplectites of orthopyroxene
+ sapphirine ± cordierite/plagioclase, orthopyroxene
+ sillimanite ± cordierite/plagioclase and orthopyroxene
+ sapphirine + sillimanite embaying garnet, imply a clockwise
P–
T–
t evolution. Conditions of
P > 9–10
kbar and
T 
800–850°C were attained prior to an initial
phase of decompression that was accompanied by heating of up
to
100°C. Peak temperatures of
T 850–900°C were
achieved at
P 9 kbar followed by near-isothermal decompression
to pressures of
P 5 kbar. This clockwise isothermal decompression
path contrasts markedly with anticlockwise isobaric cooling
paths recorded elsewhere in the Rayner Complex, and reflects
a second phase of orogenesis within the Rayner Structural Episode.
KEY WORDS: decompression; kornerupine; orthopyroxene–corundum; reaction textures; sapphirine; symplectite
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INTRODUCTION
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Aluminous magnesian gneisses are commonly reported from high-grade
metamorphic terranes. Rocks with such compositions are useful
for the reconstruction of pressure–temperature–time
(
P–
T–
t) paths as the mineral assemblages they contain
commonly preserve spectacular reaction textures that document
segments of
P–
T–
t paths (Droop & Bucher-Nurminen,
1984
; Windley
et al., 1984
; Waters, 1986
; Droop, 1989
). Because
of their unusual composition relative to country rocks, these
gneisses are also commonly subjected to partial alteration of
their margins, thereby creating variations in bulk composition
across individual layers. As the position of a particular multi-variant
equilibrium in
P–
T space generally depends upon the compositions
of the phases involved, an assemblage with given phase compositions
may pass through a reaction earlier or later than the same assemblage
containing phases with different compositions (e.g. Hensen,
1971
). Hence, segments of a
P–
T path can be ascertained
by comparing rocks with subtle differences in their bulk compositions,
especially variations in Fe–Mg ratios, dependent only
on their spatial position with respect to layer boundaries.
Silica-undersaturated metapelites with highly magnesian compositions have been reported from a number of localities in East Antarctica, including MacRobertson Land (Fig. 1; Sheraton et al., 1982; Dunkley et al., 1999), the Napier Complex (Motoyoshi et al., 1995), the Vestfold Hills (Harley, 1993), and the Rauer Group (Harley, 1998a). This paper describes a new locality, in the Oygarden Group of islands, East Antarctica, where silica-undersaturated metapelitic gneiss preserves assemblages that include orthopyroxene, corundum, sapphirine, kornerupine and garnet, developed within distinct zones across inherited compositional domains. Contrasting mineral compositions and subtly different reaction textures are preserved in the compositionally different zones that are inferred to have formed as a result of a prior metasomatic event. Coexisting orthopyroxene and corundum, which are predicted by theoretical petrogenetic grids to form a stable assemblage under restricted P–T conditions (e.g. Hensen, 1987), but have been loosely inferred to exist in nature (e.g. Windley et al., 1984; Bertrand et al., 1992; Goscombe, 1992; Kihle & Bucher-Nurminen, 1992), or isolated within extensively developed reaction textures (Ouzegane et al., 2003), are shown more clearly here to have reached textural equilibrium at near-peak P–T conditions. These rocks illustrate the importance of subtle variations in bulk composition on resulting reaction products and assemblages, and the preservation of reaction textures and phases. Garnet, sapphirine, orthopyroxene–corundum, and kornerupine-bearing parageneses provide ‘snapshots’ of a clockwise P–T–t evolution that is interpreted to have resulted from prograde thickening of the crust in Kemp Land during the Rayner Structural Episode and decompression from high pressures and temperatures. This P–T path contrasts markedly with the dominantly anticlockwise P–T–t paths dominated by apparent isobaric cooling, reported elsewhere in the Rayner Complex (Clarke et al., 1989; Fitzsimons & Thost, 1992; Thost & Hensen, 1992; Hand et al., 1994; Nichols, 1995; Boger & White, 2003).
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Fig. 1. (a) Location of the Oygarden Islands in Kemp Land, East Antarctica. The map indicates the extent of the Rayner Complex, Rayner tectonic reworking of Archaean crust in Kemp Land, and Neoproterozoic–Cambrian tectonic reworking of Proterozoic crust in Prydz Bay and west of Enderby Land. (b) Sketch map of the outcrop hosting the Si-undersaturated metapelite pods, illustrating the relationship of pods with respect to S4 layering. (c)–(f) detailed sketches of Pods 1, 2, 4 and 10, illustrating the relationship between compositional zones, internal layering and foliation to external S4 layering. Shaded boxes with numbers indicate the locations of XRF sample sites, with numbers referring to analyses listed in Supplementary Table 1.
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GEOLOGICAL SETTING
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The Oygarden Group of islands forms some of the most westerly
exposures of the Proterozoic Rayner Complex in Kemp Land (
Fig. 1a),
which is interpreted to represent Archaean crust that was
reworked by the Rayner Structural Episode during the earliest
Neoproterozoic (Sheraton & Black, 1983
; Clarke, 1987
; Sheraton
et al., 1987
; Grew
et al., 1988
; Kelly
et al., 2002
). Lying
near the western extremity of this zone of reworking (Sheraton
& Black, 1983
; Sheraton
et al., 1987
), the Oygarden Group
is considered to have formed part of the Napier Complex from
at least
c. 2·5 Ga, and was tectonically reworked at
c. 0·93 Ga (Kelly
et al., 2002
). The Napier Complex,
an Archaean ultrahigh-temperature metamorphic terrane, experienced
multiple episodes of deformation and metamorphism between
c.
2·98 Ga and
c. 2·45 Ga (Sheraton
et al., 1987
;
Harley & Black, 1997
).
The Rayner Complex is a composite granulite-facies metamorphic terrane that comprises both reworked Archaean and Proterozoic crust. Orogeny between 1·0 and 0·9 Ga occurred as a result of convergence between an Indo-Napier craton (comprising the Napier Complex and peninsular India) and part of what now constitutes East Antarctica, during the Rayner Structural Episode (RSE: Sandiford & Wilson, 1984; Sheraton et al., 1987). The RSE is characterized by two main phases of deformation that are separated in time by the intrusion of voluminous charnockite in MacRobertson Land and the northern Prince Charles Mountains at 0·98–0·96 Ga (Young & Black, 1991; Kinny et al., 1997; Young et al., 1997; Zhao et al., 1997). The location of the Oygarden Group, which records only the second phase of this deformation (Kelly et al., 2000, 2002), makes this area critical to the understanding of the evolution of this region.
The Oygarden Group of islands was affected by at least five episodes of deformation (D1–D5) over a 1·8 Ga period during the Archaean and Proterozoic. The structures in the islands are dominated by those that formed during the RSE: D3 and D4. At least two deformation events or sets of structures are recognized to pre-date D3/D4. An S1 gneissosity, defined in part by a migmatitic layering in layered felsic orthogneiss, is identified with confidence only where cut by a homogeneous felsic orthogneiss that intruded during D1 at c. 2·75 Ga (Kelly et al., in preparation). Layered composite orthogneiss, the most abundant lithology, is composed of layers of felsic, intermediate and mafic gneiss that alternate on a centimetre to metre scale. Subordinate lithologies include mafic dykes, calc-silicate gneiss, layered quartz-rich metasediments, quartz-bearing metapelitic gneiss, and silica-undersaturated metapelitic gneiss. D2 resulted in transposition of S1 and the homogeneous felsic orthogneiss into S2 at c. 2·45 Ga (Kelly et al., in preparation).
The RSE D3 deformation is characterized by the effects of east-directed thrusting that occurred at medium- to high-P granulite-facies conditions at c. 0·93 Ga (Kelly et al., 2002). S3 assemblages in mafic granulite, including garnet, orthopyroxene, clinopyroxene, plagioclase and quartz, suggest that D3 occurred at P 9–10 kbar and T 800–850°C (Kelly et al., 2000). D4 resulted in a 2–3 km wide extensional shear zone that recrystallized rocks in the south of the island group. D4 occurred at granulite-facies conditions, similar to those that accompanied D3 (Kelly et al., 2000). The last event recognized, D5, is characterized by mylonites and ultramylonites that formed at amphibolite-facies conditions and cut structures that formed during the RSE. Mylonites show a broadly northward sense of transport.
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OUTCROP DESCRIPTION AND ASSEMBLAGES
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High Mg–Al silica-undersaturated metapelite occurs as
more than 15 distinguishable pods and lenses in an outcrop approximately
20 m
x 10 m in size within a
D4 high-strain zone (
Figs 1b and
2a). The pods form part of a layered sequence that includes
garnet-rich felsic and intermediate gneisses, quartz-rich gneiss,
and also lenses of quartz-bearing pelitic and semi-pelitic gneiss.
S4 layering envelopes the pods of silica-undersaturated metapelite,
but
S4 is not penetratively developed within the pods. Most
pods preserve similar assemblage relationships, summarized using
four pods (1, 2, 4 and 10) as examples that span the common
compositional and textural variations observed (
Fig. 1c–f;
Table 1). Three main composition/assemblage types occur within
the pods, with other subordinate compositions and assemblages
occurring as layers bounding or enveloping the pods. These main
types are: (1) garnet-poor, where garnet occurs only as fine-grained
and volumetrically insignificant inclusions in sapphirine or
sillimanite; (2) garnet-rich, where garnet is abundant in outer
zones of pods; (3) kornerupine-bearing, which may or may not
contain abundant garnet. Nearly all pods preserve an assemblage
zoning pattern characterized by anhydrous cores dominated by
coarse-grained orthopyroxene and sapphirine with a transition
zone into 10–15 cm wide, phlogopite-rich schistose rinds.
The anhydrous cores commonly preserve a foliation (
S3) that
is oblique to the enveloping
S4 layering and foliation developed
in schistose rinds (e.g.
Fig. 1c). The orientation of
S3 is
not consistent between pods, which is interpreted to reflect
variable rotation of pods during
D4 deformation.
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Fig. 2. (a) Outcrop photograph of Opx–Spr–Crn-bearing metapelite (Pod 1), enveloped by S4 layering. (Geological hammer for scale is 40 cm in length.) (b) Back-scattered electron (BSE) image of garnet inclusions in S3 sapphirine (sample OG522A, Pod 1; scale bar represents 100 µm). (c) BSE image of relic corundum grains within an extensive growth of sapphirine and sillimanite (scale bar represents 500 µm). (d) BSE image of a sapphirine–sillimanite intergrowth in orthopyroxene from the transition zone in Pod 1 (sample OG522A). The sapphirine is interpreted to have completely replaced corundum, and sillimanite is beginning to dominate the texture (scale bar represents 500 µm). (e) Plane-polarized light photomicrograph of S3 kornerupine, sapphirine, phlogopite and orthopyroxene (sample OG566; width of field of view is 4 mm). (f) Plane-polarized light photomicrograph of an orthopyroxene–kornerupine intergrowth embaying a garnet porphyroblast (sample OG566; width of field of view is 4 mm).
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Garnet-poor (quartz-absent) assemblages
The core assemblages in garnet-poor pods (Pods 1 and 4;
Table 1)
are composed of
S3 sapphirine, orthopyroxene and corundum,
with or without minor sillimanite, cordierite and phlogopite.
Rutile also occurs as part of
S3, and is the only Fe–Ti
oxide present in any zone. Garnet is low in abundance and occurs
only as fine-grained inclusions (less than 0·5 mm;
Fig. 2b)
in elongate sapphirine grains (Pod 1), and is absent from
the core of Pod 4. Sapphirine commonly occurs as elongate grains
and clusters of grains up to 2 cm in length, and may also form
elongate ‘cuneiform’ grains in orthopyroxene. In
Pod 4, orthopyroxene may occur as large (up to 5–6 mm),
elongate grains with lobate and eroded grain boundaries, and
abundant fine-grained rutile inclusions. Sapphirine may form
large xenoblasts that contain inclusions of rounded orthopyroxene.
Corundum is scattered in this pod and is not confined to the
core region. Where corundum occurs it is always separated from
orthopyroxene by a thin (up to 0·3 mm), but complete,
corona of sapphirine (
Fig. 3a). Importantly, the grain size
of sapphirine in the coronas is appreciably smaller than that
of the orthopyroxene and corundum grains that they separate.
Sillimanite may rarely form an outer, continuous or discontinuous
rim between sapphirine and orthopyroxene (similar to
Fig. 3b),
and can appear to embay these minerals. Cordierite is rare,
forming narrow rims between sillimanite and orthopyroxene and
inclusions in orthopyroxene. Rare phlogopite occurs as inclusions
in, or along grain boundaries of orthopyroxene.
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Fig. 3. Grey-scale X-ray intensity maps (512 x 512 pixels) for selected textures illustrating variation in elemental abundance. All maps except (a) have been recalculated to cations per 24 oxygens; (a) is a raw count intensity map. (a) Al map from Pod 1, edge of core (sample OG524A): narrow corona of sapphirine separating corundum from orthopyroxene (analysis step-size 4 µm). (b) Al map from Pod 1, transition zone (sample OG524A): corona of sapphirine and sillimanite between corundum and orthopyroxene (top left of map), and coarse sapphirine developed around corundum (bottom of map; analysis step-size 4 µm). (c) Al map from Pod 1, schistose rind (sample OG524B): intergrowth of sapphirine and cordierite in a corona between corundum and orthopyroxene. Sapphirine also occurs as elongate grains (analysis step-size 4 µm). (d) Si map from Pod 10, schistose rind (sample OG581): garnet surrounded by an intergrowth of orthopyroxene, sillimanite and plagioclase (analysis step-size 3 µm). (e) Si map from Pod 4, sillimanite-rich gneiss (sample OG577): symplectite of orthopyroxene, sillimanite and sapphirine, with garnet inclusions in sillimanite (analysis step-size 2 µm). (f) Al map from Pod 10, sillimanite–sapphirine gneiss (sample OG582A): garnet surrounded by a symplectite of orthopyroxene, sapphirine and plagioclase (analysis step-size 4 µm).
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A two-stage transition zone occurs between the core and schistose
rind. The first stage involves a marked increase in sillimanite
(
Fig. 2c and d), which interfingers with orthopyroxene and sapphirine,
and a minor increase in phlogopite content (
Fig. 4). In some
local domains orthopyroxene and sapphirine form rounded inclusions
in larger sillimanite grains. The second stage is characterized
by a marked increase in phlogopite and cordierite. Corundum
is present throughout the transition zone, and is commonly separated
from orthopyroxene by double coronas of sapphirine and sillimanite
(
Fig. 3b). A schistose rind up to 15 cm in width occurs on all
pods (
Fig. 1c–f), and in garnet-poor types is characterized
by a smaller grain size and lower abundance of rutile compared
with the core. Sillimanite-rich domains occur; however, cordierite-rich
domains are predominant. Sapphirine and orthopyroxene both appear
to be in overall textural equilibrium with other phases, but
locally may be partially embayed by sillimanite or cordierite.
Rarely, sapphirine may contain inclusions of phlogopite, orthopyroxene,
corundum and garnet. Corundum is surrounded by coronas of sapphirine
and either sillimanite or cordierite (
Fig. 3c). Proximal to
the margin of the schistose rind, phlogopite and sapphirine
are foliated parallel to the pod margin and to
S4 (
Fig. 1b and c).
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Fig. 4. Variation in mineral modal percentages across two selected pods (1 and 4), illustrating the rimward decreases in modal abundances of orthopyroxene, sapphirine and rutile, and increases in sillimanite, cordierite and biotite. (Figure not to scale; see Fig. 1b for true distances.)
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Garnet-rich (quartz-absent) assemblages
These assemblages (summarized here using Pod 10 as an example;
Fig. 1f;
Table 1) show similar assemblage and zoning patterns
to other pods but are comparatively garnet-rich, contain plagioclase
and have no corundum. Core assemblages have higher abundances
of cordierite, sillimanite and phlogopite compared with other
pods, which is interpreted to reflect more intense metasomatic
alteration prior to the development of the assemblages. The
core assemblages may preserve cordierite-poor domains that contain
sub-idioblastic, tabular sapphirine grains that appear needle-like
in cross-section. Cordierite-rich domains that surround cordierite-poor
areas contain large poikiloblastic sapphirine (>2 mm) and
sillimanite intergrown with, or containing inclusions of, orthopyroxene,
cordierite and garnet. Cordierite occurs as rims on and interfingering
with orthopyroxene, sapphirine and sillimanite. An outer core,
with higher abundance of phlogopite and sillimanite, may envelop
the main core, and commonly contains interstitial patches of
plagioclase. The transition zone differs from other pods in
its preservation of garnet porphyroblasts (up to 1 mm) and higher
modal abundance of cordierite. Intergrowths of tabular sapphirine
with orthopyroxene in this zone may be partially replaced by
coarse-grained cordierite, which may also embay coarse-grained
orthopyroxene, sapphirine and sillimanite. Garnet is present
as inclusions in sapphirine, orthopyroxene and cordierite.
The schistose rind is relatively garnet-rich compared with other pods. However, garnet-poor compositional sub-domains do occur that are similar to schistose rinds in garnet-poor rocks. Garnet-rich domains contain coarse-grained garnet, orthopyroxene and sillimanite, with or without sapphirine. In one sample (OG581) a quartz vein (approximately 2 mm in width) cuts the quartz-absent rock, armoured from the latter by a 0·25–0·5 mm wide moat of orthopyroxene. A zone of coarse-grained plagioclase up to 3 mm in width separates this moat from coarse-grained garnet in the quartz-absent host, and is commonly rimmed by cordierite. Rare symplectites of orthopyroxene and cordierite may also occur adjacent to garnet grains. Approximately >5 mm from the quartz vein, garnet is surrounded and embayed by symplectites of orthopyroxene + sillimanite, with or without plagioclase and cordierite (Fig. 3d). In sub-domains 2–3 cm away from the large garnet grains described above, the assemblage is composed of a coarse-grained intergrowth of sillimanite and orthopyroxene, and medium-grained garnet.
Kornerupine-bearing (quartz-absent) assemblages
Kornerupine has been identified in a single pod (Pod 2) within the suite (Fig. 1d). This pod preserves core and transition assemblages similar to those in garnet-poor rocks. However, the schistose rind has higher garnet abundance compared with garnet-poor assemblages described above. All zones are kornerupine-bearing. Kornerupine occurs in the core assemblage as elongate grains, generally <1 mm in length, that define S3 (Fig. 2e). Kornerupine may contain inclusions of garnet and corundum, be overgrown by orthopyroxene, and occur as inclusions in S3 sapphirine. Corundum inclusions are separated from kornerupine and orthopyroxene by coronas of sapphirine. Garnet occurs as fine-grained inclusions in sapphirine, whereas phlogopite may contain inclusions of sapphirine. In the schistose rind sapphirine, sillimanite, phlogopite and kornerupine surround coarse-grained (up to 2 mm) garnet, which may be embayed by kornerupine and orthopyroxene intergrowths (Fig. 2f). Garnet may also occur as coarse-grained inclusions in sapphirine, sillimanite and orthopyroxene. Sapphirine may contain inclusions of kornerupine. Adjacent to the core, the rind assemblage is aligned parallel to S3, but closer to the pod margin, sapphirine, sillimanite, phlogopite and kornerupine are parallel to S4.
Other quartz-absent assemblages
Present in the outcrop are compositionally distinct layers that may occur adjacent to the schistose rind on a single side of a pod (e.g. Pods 4 and 10), on both sides (Pod 1) or completely surrounding the pod (Pod 2; Fig. 1). In Pod 1 (Fig. 1c), the straight boundary between the schistose rind and a sillimanite-rich gneiss is parallel to S3 preserved in the core of the pod, and oblique to the enveloping S4. It is possible that these compositional layers reflect a primary compositional layering or are the result of metasomatic alteration of a more homogeneous protolith prior to D4. Sillimanite-rich gneiss (Pods 1 and 4) contains an assemblage composed of randomly oriented, coarse-grained orthopyroxene, sillimanite, phlogopite, cordierite and plagioclase, with minor garnet. Patchy phlogopite and sillimanite may define S4 close to the pod margin, but away from the pod margin sillimanite may occur as radiating aggregates. Sillimanite may be separated from orthopyroxene by medium- to coarse-grained rims of cordierite, which contain inclusions of phlogopite, sillimanite and orthopyroxene. In Pod 4 the sillimanite-rich gneiss preserves spectacular ‘cuneiform’ intergrowths of sillimanite and sapphirine in orthopyroxene (Fig. 3e). Garnet occurs as inclusions in sapphirine and sillimanite. Sillimanite–sapphirine gneiss, occurring as a distinct layer or outer rind (Pods 1, 2, 10; Table 1), is composed of coarse-grained sillimanite, sapphirine, orthopyroxene, phlogopite and garnet. In Pod 10, sillimanite-absent domains occur where large garnet porphyroblasts (2–4 mm) are embayed by symplectites of orthopyroxene and random, ‘spinifex-like’ arrays of tabular sapphirine, with or without plagioclase (Fig. 3f). Smaller garnet grains may also occur with coarse-grained orthopyroxene, sapphirine and plagioclase, and as inclusions in elongate sapphirine grains. Phlogopite is rare.
Quartz-bearing metapelite
Quartz-bearing metapelitic gneiss associated with quartz-absent rocks commonly preserves similar assemblages and reaction textures. The rocks predominantly preserve a coarse-grained S4 assemblage of orthopyroxene, garnet and sillimanite, with medium-grained biotite, plagioclase, quartz and rutile, with or without K-feldspar. In more leucocratic samples, S4 is defined by alternating garnet–orthopyroxene-rich and quartzo-feldspathic domains. In some samples coarse-grained plagioclase (2–3 mm) occurs in leucosomes and contains fine-grained vermicular inclusions of quartz and K-feldspar. In leucocratic domains, cordierite occurs as large grains, but also forms part of symplectites with orthopyroxene that separate garnet and biotite, or biotite and quartz.
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MINERAL CHEMISTRY OF QUARTZ-ABSENT METAPELITE
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Electron microprobe point analyses and X-ray intensity maps
were collected using the Cameca Camebax SX-50 electron microprobe
housed at the Electron Microscope Unit, University of New South
Wales, operating with an accelerating voltage of 15 kV, beam
current of 20 nA, beam width of 1–5 µm and PAP data
reduction software supplied by the manufacturer and described
by Pouchou & Pichoir (1984)
. X-ray intensity maps were collected
with a 1–3 µm beam size and count times of 300 ms
at each point. X-ray intensity data (
Figs 3 and
6) were reprocessed
to display weight percent oxide values and molecular proportions
following the method described by Clarke
et al. (2001)
, which
uses the computer software Matlab incorporating a matrix correction
algorithm based on the empirical
-factor approach of Bence &
Albee (1968)
. Additional samples were analysed at the Department
of Geology and Geophysics, University of Edinburgh, using a
Cameca Camebax Microbeam electron microprobe operating at an
accelerating voltage of 20 kV and beam current of 25 nA. No
systematic differences were detected in compositions between
minerals analysed in Edinburgh and Sydney. Microprobe analyses
representing the range of mineral compositions are presented
in
Table 2. Back-scattered electron images (
Fig. 2) were collected
at the University of Edinburgh, using a Phillips XL30 scanning
electron microscope operating at an accelerating voltage of
20 kV. In addition to analysis by electron microprobe, kornerupine
was analysed by secondary ion mass spectrometry (SIMS) using
the Cameca IMS-4f at the Department of Geology and Geophysics,
University of Edinburgh, following methods outlined in detail
by Hinton (1995)
. A primary
16O
– beam at
8 nA was accelerated
through 10 kV and positive secondary ions drawn at a 75 V offset.
1H,
7Li,
9Be and
11B, and a suite of other major and trace elements
(
Table 3), were analysed as isotopic ratios referenced against
28Si, and ppm concentrations of elements calculated by reference
to ion yields determined using the NIST (SRM610) glass standard.
Across each pod the
XMg values [= Mg/(Mg + Fe)] preserved by
all ferromagnesian minerals gradually decrease from core domains
to the schistose rinds (
Fig. 5;
Table 1), but are variable in
sillimanite-rich gneiss and sillimanite–sapphirine gneiss.
This decrease in mineral
XMg correlates with an increase in
phlogopite abundance in the rinds. Relative
XMg values [Mg/(Mg
+ Fe
tot)] between minerals occur in the same order in nearly
all pods and zones, with cordierite > sapphirine > phlogopite
> orthopyroxene > garnet. When considering Fe
3+,
of sapphirine may in some cases be equal to that
of cordierite, and in rare circumstances phlogopite may be slightly
more magnesian than sapphirine. Where present, kornerupine
XMg may be less than sapphirine (Pod 2 core zone) or intermediate
between sapphirine and cordierite values. However, when compositions
are recalculated to consider Fe
3+, kornerupine may become more
magnesian than sapphirine (Pod 2 core zone) or marginally less
magnesian (Pod 2 schistose rind).
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Fig. 5. Mineral XMg distribution diagram for Pods 1, 2, 4 and 10. Average mineral XMg compositions are plotted against the average orthopyroxene XMg particular to the sample. The consistent relationship between mineral XMg and orthopyroxene XMg across each of the four pods illustrated should be noted. Dashed curves are Mg distribution lines that describe the XMg of mineral x for a given orthopyroxene XMg. Error bars indicate the standard deviation of average values at 1 level.
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Sapphirine
Compositional zoning of sapphirine is characterized by a systematic
decrease in
XMg [= Mg/(Mg + Fe
tot)] in grains with proximity
to the edge of pods. Pod 1 core compositions have
XMg 0·93–0·94,
and decrease to
XMg 0·90–0·92 in the schistose
rinds (
Table 1). Pod 2 sapphirine has slightly lower
XMg values
(0·91–0·89). Sapphirine from the sillimanite-rich
and sillimanite–sapphirine gneiss also commonly has lower
XMg compared with the schistose rinds. Sapphirine in all pods
and zones has between 4·5 and 4·3 Al cations (per
10 oxygens) and shows a rimward increase in Al and decrease
in Si. Sapphirine rims adjacent to corundum or sillimanite are
commonly more enriched in Al when compared with rims adjacent
to orthopyroxene, garnet or biotite. Compositions approach a
stoichiometric ratio of 7:9:3 (Mg:Al:Si; where Al = 4·5,
Si = 0·75 cations p.f.u.), with cores (higher Si) trending
approximately towards 33% of a 2:2:1 composition (Al = 4, Si
= 1 cations p.f.u.;
Fig. 7a). Analysed compositions lie off
the ideal Tschermak's substitution line (AlAlSi
–1Mg
–1)
indicating a proportion of Fe
3+ in total Fe. Fe
3+ concentration,
calculated by assuming 14 cations per 20 oxygens and using the
structural formula M
7(M)O
2[T
6O
18] (after Moore, 1969
), where
Fe
3+ = Al
VI – Al
IV [Al
VI = 6 – (Si + Ti); Al
IV =
Al
tot – Al
VI], ranges between 0·05 and 0·18
cations p.f.u. (average 0·10 ± 0·3). These
values show no consistent trend within or between pods, or in
relation to the Tschermak's substitution. Fe
3+/Fe
total ranges
between 0·14 and 0·48, but averages 0·35
± 0·04. By taking Fe
3+ into account
XMg values
increase by between 0·02 and 0·05 units (
Tables 1 and
2a).
Orthopyroxene
Individual grains are rarely zoned with respect to XMg and values within domains are relatively uniform. However, rare grains preserve weak increases in XMg at rims adjacent to garnet and sapphirine. Orthopyroxene XMg compositions typically decrease from core domains (0·89–0·91) to schistose rinds (0·84–0·87; Pod 10 0·81). Sillimanite-rich gneiss and sapphirine–sillimanite gneiss orthopyroxenes have XMg 0·84–0·86, which is similar to, or lower than the schistose rinds, with the exception of Pod 10 (Table 1). Individual orthopyroxene porphyroblasts show a consistent rimward decrease in Al2O3 from a broad core plateau that is high in Al2O3 (up to 9·5 wt %) to narrow (<35 µm), comparatively Al2O3-poor rims (>3 wt %; Fig. 6a). Zoning in Al2O3 is most marked when orthopyroxene is adjacent to cordierite or plagioclase, and in some cases significant rimward depletions in Al2O3 are observed in orthopyroxene adjacent to sapphirine (Fig. 6b) and phlogopite. Fe3+ has been calculated for orthopyroxene from charge balance based on four cations and six oxygens; contents are minor (0·030 ± 0·02 cations p.f.u.) and show no systematic variation or zoning features. Fe3+ in Fetotal averages 0·11 ± 0·08, increasing XMg by generally less than 0·03 units, but is higher in some cases (Tables 1 and 2a).
Biotite
All biotite is close to phlogopite in composition [classified
after Deer
et al. (1992, p. 284)
]. Consistent with other ferromagnesian
minerals,
XMg of phlogopite decreases systematically from core
zones (0·93–0·90) to schistose rinds (0·85–0·88),
whereas phlogopites from sillimanite-rich and sapphirine–sillimanite
gneiss are similar in composition or slightly less magnesian
(
XMg 0·85–0·89). Total F in phlogopite
is commonly between 1·2 and 1·5 wt %, with no
systematic variation between zones or pods. As F rarely accounts
for more than 2% of the total percentage of elements in the
phase, up to 6 wt % of total oxides is unaccounted for in the
analysis. This is inferred to be H
2O or Cl in the hydroxyl site,
suggestive of a relatively hydrous phlogopite.
Garnet
General grain compositions are variable within and between pods, although predominantly following the Mg zoning patterns already described for other minerals. The most magnesian garnet occurs as inclusions in sapphirine in the core zone of Pod 10 [XPyp = 0·74–0·77, where XPyp = Mg/(Mg + Fetot + Ca + Mn)] and Pod 1 (XPyp = 0·67–0·70). In general, XGrs [Ca/(Mg + Fetot + Ca + Mn)] = 0·02–0·03. Most garnets display a systematic decrease in XPyp between the core and schistose rind (Table 1), with minor increases in XGrs and XAlm [Fetot/(Mg + Fetot + Ca + Mn)]. Sillimanite-rich and sillimanite–sapphirine gneiss typically have garnet compositions similar to, or slightly more magnesian than schistose rind garnet. Garnet porphyroblasts, which are preserved only in Pods 2 and 10, commonly preserve a rimward decrease in XPyp, with a broad core compositional plateau and narrow zoned rim (e.g. Fig. 7b). The core composition of some garnet grains in the schistose rinds may be as high as XPyp = 0·67 (Pod 2), similar to the composition of garnet inclusions in sapphirine from core domains. The composition of the rims of these grains is equivalent to that of smaller garnet porphyroblasts and inclusions in sapphirine in the schistose rind domains.
Kornerupine
Electron microprobe analyses indicate XMg 0·89–0·91 (where Fe2+ = Fetot) and a (Mg,Fe)O:Al2O3:SiO2 ratio that is close to 11:10:11, such that kornerupine lies to the Si-poor side of the Crn–Opx tie-line in MAS (Fig. 7c). However, considering the Fe3+ content in Fetotal [after Grew et al. (1999): Fe3+ in kornerupine is inferred to be equal to that estimated for associated sapphirine], approximately 35% in the Oygarden samples, increases XMg to 0·93–0·94. These values are similar to XMg values previously recorded for kornerupine (Droop, 1989; Goscombe, 1992; Vry, 1994; Carson et al., 1995; Friend, 1995; Grew, 1996). In addition to a change in XMg, removing Fe3+ from Fetotal shifts the position of kornerupine to being effectively collinear with orthopyroxene–corundum in MAS. Total Al cations p.f.u. (for 21·5 oxygens) are approximately 6·9 and AlIV [=5 – (Si + B)] is calculated to lie between 1·06 and 1·16. The Oygarden kornerupines are ‘boron-poor’ examples (after Friend, 1995), with nine SIMS analyses yielding an average of 0·94 wt % B2O3, 0·48 wt % F, 0·85 wt % H2O, 0·05 wt % Na2O, 91 ppm Be and 314 ppm Cl (see Table 3 for full analyses).
Other phases
In cordierite (where present), XMg varies from cores (0·96–0·97) to schistose rinds (0·94–0·95), consistent with the XMg trends recorded by the other minerals (Table 1). Cordierite in sillimanite-rich gneiss commonly has a similar composition to that in the schistose rind (XMg = 0·94–0·95). Plagioclase occurs in a number of zones in Pod 10, and only in sillimanite-rich gneiss in Pod 1, and has a variable composition with XAn [=Ca/[Ca + Na + K)] between 0·30 and 0·55. Calcium contents are higher in samples from Pod 10 (average XAn 0·50) and individual grains commonly preserve a rimward increase in XAn. Zoning is especially pronounced where plagioclase is adjacent to garnet. Sillimanite is near pure Al2SiO5, with Fe being the only impurity (<0·3 wt % Fe2O3, measured as FeO). Fe values show no systematic variation when in association with sapphirine. Corundum contains less than 0·15 wt % Fe2O3 and less than 0·3 wt % Cr2O3.
|
MINERAL CHEMISTRY OF QUARTZ-PRESENT METAPELITE
|
|---|
Garnet from quartz-bearing metapelitic gneiss is a pyrope–almandine
mix (
XAlm = 0·35–0·46, and
XPyp = 0·51–0·62)
with low grossular contents (
XGrs = 0·02–0·03).
Large garnet grains (>2 mm in diameter) show a subtle rimward
increase in Fe and a decrease in Mg content, commonly by approximately
XAlm = 0·03. Orthopyroxene has
XMg = 0·75–0·83
[Mg/(Mg + Fe
tot)] with alumina contents varying between 0·37
and 0·11 cations per six oxygens, and shows a consistent
rimward decrease in porphyroblastic orthopyroxene of up to 0·17
cations.
XMg of cordierite ranges between 0·90 and 0·94
and rarely preserves zoning. However, some symplectic cordierite
has higher Mg contents than grains adjacent to biotite or garnet.
Plagioclase is oligoclase in composition, with
XAn = 0·18–0·32
and
XAb = 0·80–0·66. Where plagioclase occurs
as exsolution lamellae in perthite, it has the same composition
as larger matrix grains. Alkali feldspar is sanidine to orthoclase
in composition, with
XOr = 0·73–0·92 and
XAb = 0·27–0·08. Rare analyses taken from
exsolution lamellae in K-feldspar have higher albite components
(
XAb 
0·87). Biotite compositions range from phlogopite
to eastonite, with
XMg of between 0·79 and 0·91.
Sillimanite generally has less than 0·5 wt % Fe
2O
3 (analysed
as FeO).
|
BULK-ROCK GEOCHEMISTRY
|
|---|
Bulk-rock composition data were obtained using standard X-ray
fluorescence (XRF) techniques at the University of New South
Wales and are available in Supplementary Data
Table 1, which
may be downloaded from the
Journal of Petrology website at
http://www.petrology.oupjournals.org/.
The components MgO–FeO–SiO
2–Al
2O
3 make up
97–98% of core samples and 94–97% of schistose rind
samples, with the remainder being mostly K
2O. Compositional
relationships are therefore easily compared with analysed mineral
compositions using a MAS ternary diagram (
Fig. 8). The whole-rock
analyses confirm that the rocks are undersaturated with respect
to silica (38–47%), are highly magnesian (
XMg = 0·85–0·93),
and have core compositions characteristically low in CaO and
Na
2O. Data indicate systematic K enrichment in the transition
zones and schistose rinds of the pods, an increase that is reflected
in the increased modal abundance of phlogopite. In addition,
most pods also show a minor, although systematic, enrichment
in Fe and Si in the rinds. No major elements exhibit consistent
depletion patterns; however, an apparent decrease in Mg was
observed in data from some pods. Ti is commonly depleted in
transition zones and enriched in the schistose rind correlating
with a decrease in rutile abundance and grain size with increasing
distance from the core. This may also indicate mobility of Ti
from the core into the schistose rind where it has been incorporated
in phlogopite. The presence of plagioclase with a significant
albite component (
XAb = 0·7 in Pod 1 outer zones;
XAb = 0·45–0·69 in Pod 10;
Table 1) suggests
that a proportion of Na was present. There are no consistent
zoning patterns with respect to Na, although some pods do have
a rimward increase (Pods 1 and 10), therefore Na metasomatism
accompanying silica cannot be confirmed.
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|
Fig. 8. (a) (MgO + FeO)–Al2O3–SiO2 ternary diagram showing the positions of analysed minerals. This diagram does not consider Fe–Mg relations between phases. (b)–(e) enlarged portions of (a), showing the positions of bulk compositions as determined by XRF for Pods 1, 4, 10, and 12. Abbreviations to the right of each sample number: C, core zone; Cb, core zone from Pod 10B; Tr, transition zone; R, schistose rind; Rb, schistose rind from Pod 10B; CR, country rock; SG, sillimanite-rich gneiss; SsG, sillimanite–sapphirine gneiss. (f) FMAS compatibility diagram for Pod 1, showing phase (symbol to the right of sample number) and selected bulk-rock compositions (symbol to the left of sample number) projected from orthopyroxene. Each bulk composition and mineral has been projected from the orthopyroxene composition particular to that sample.
|
|
On a MAS compatibility diagram, most bulk-rock compositions
plot within the orthopyroxene–sillimanite–sapphirine
tie-triangle and commonly on the silica-poor side of the orthopyroxene–corundum
tie-line (
Fig. 8). The compositions plot between garnet and
phlogopite, and close to orthopyroxene, which correlates with
the high proportion of orthopyroxene within core assemblages
and abundance of phlogopite in rind assemblages. Exceptions
to this include the quartz-bearing country rock from Pods 1
and 12, and samples OG577 (Pod 4) and OG581 (Pod 10). Sample
OG577 is from sillimanite-rich gneiss that envelopes Pod 4 and
its composition plots close to the orthopyroxene–sillimanite
tie-line. Sample OG581 (schistose rind) plots above the orthopyroxene–sillimanite
tie-line within the stability field orthopyroxene–sillimanite–cordierite,
which corresponds to the observed textures and the absence of
sapphirine in the assemblage.
|
THERMOBAROMETRY
|
|---|
The majority of quartz-absent assemblages described here are
not appropriate for conventional thermobarometry. As a result,
P–
T estimates were made from garnet–orthopyroxene-bearing
S3 and
S4 assemblages from quartz-bearing metapelite that formed
layers within the same outcrop as the silica-undersaturated
metapelite. Average
P and
T calculations using the coarse-grained
assemblage garnet, orthopyroxene, plagioclase, sillimanite,
biotite, quartz and cordierite (sample OG562) using THERMOCALC
v.3.0 [Powell & Holland (1988)
, based on the internally
consistent dataset of Holland & Powell (1998)
], gave an
average
P = 9·9 ± 0·3 kbar (for
T = 850°C;
aH2O = 1·0; 1
) and an average
T = 844 ± 27°C
(for
P = 10 kbar; 1
). Using
aH2O = 0·5 reduced pressures
by 1 kbar but raised temperatures by 50–60°C, whereas
using
aH2O = 0·1 reduced pressures by only 0·5
kbar and raised temperatures by
140°C. Using the ‘average
P–
T’ function on the same assemblage gave
P = 10·1
± 0·6 kbar and
T = 875 ± 65°C (1
).
Excluding cordierite from this assemblage gave slightly lower
pressure and similar temperature estimates: average
P–
T of
P = 8·7 ± 1·3 kbar and
T = 876 ±
66°C (1
). A second sample (OG589) gave similar, although
poorly constrained, results for the
S4 assemblage garnet, orthopyroxene,
plagioclase, biotite, cordierite and quartz, with an average
P–
T estimate of
P = 8·4 ± 2·1 kbar
and
T = 819 ± 156°C (1
).
Temperature estimates for suitable quartz-absent samples where garnet and orthopyroxene are in textural contact and interpreted to be in chemical equilibrium, or inferred to have originally been in textural contact, have also been obtained using garnet–orthopyroxene equilibria (Fe–Mg: Ganguly et al., 1996; Mg–Al: Harley & Green, 1982; Fe–Al: Aranovitz & Berman, 1997). A 10 kbar pressure estimate, obtained above, was used. Adjacent rim compositions of garnet and orthopyroxene produced estimates of T 810–910°C (for P = 10 kbar; Fe–Mg calibration) and T 850–960°C (for P = 10 kbar; Mg–Al calibration). The Fe–Al calibration was consistently lower than the Fe–Mg and Mg–Al calibrations and gave estimates of T 740–845°C (for P = 10 kbar). To investigate the effects of post-peak re-equilibration of Fe–Mg between orthopyroxene and garnet, the approach of Fitzsimons & Harley (1994) was used to ‘back-calculate’ the peak composition of garnet–orthopyroxene pairs. This technique produced internally consistent (convergent) estimates generally in the range T 910–960°C. Results obtained using garnet–orthopyroxene thermobarometers alone are only indicative of the general metamorphic conditions, and do not provide tight constraints on the P–T evolution. However, these results are consistent with estimates from THERMOCALC and those obtained using conventional garnet–orthopyroxene–plagioclase–quartz thermobarometry (see Supplementary Data Tables 2 and 3, which may be downloaded from the Journal of Petrology website). In all, the application of thermobarometry suggests peak or near-peak P–T conditions of P 9–10 kbar and T 850–950°C.
|
REACTION HISTORY AND PHASE RELATIONS
|
|---|
Reaction textures observed in the orthopyroxene–sapphirine
granulites occur in all compositional zones. This implies that
the reaction textures post-date the development of the compositional
zones and hence were not a direct result of interaction with
a metasomatic fluid. Instead, the textures are interpreted to
have been formed as a consequence of changes in ambient pressure–temperature
conditions. For the purposes of describing the phase relations,
the minerals orthopyroxene–corundum–sapphirine–sillimanite–cordierite–garnet–spinel
have been investigated in the simplified FeO–MgO–Al
2O
3–SiO
2 system (FMAS). By comparing the assemblages and reaction textures
observed with existing petrogenetic grids (Waters, 1986
; Hensen,
1987
), a qualitative partial
P–
T grid involving FMAS univariant
reactions has been constructed (
Fig. 9), and relevant FMAS divariant
equilibria within this system have been modelled in terms of
their MAS end-member analogues. The new grid observes known
Vr contraints and restricts sapphirine-bearing assemblages to
high temperatures by its terminal reaction relations. In addition,
the grid restricts orthopyroxene–corundum to low temperatures
relative to sapphirine, cordierite to low pressures and garnet
to high pressures. The new reaction grid differs from previously
published grids (e.g. Windley
et al., 1984
; Droop, 1989
; Goscombe,
1992
) in ignoring gedrite and spinel, and in being focused on
the high-pressure reactions involving assemblages including
orthopyroxene and corundum. In contrast to the grids of Droop
(1989)
and Goscombe (1992)
, the grid presents FMAS univariant
reactions linked to MAS terminations and divariant equilibria.
The new grid incorporates and satisfies the topology of the
[Krn Spl Qtz] univariant reaction (Hensen & Green, 1973
;
Hensen, 1986
, 1987
) and the kornerupine-present experimental
results of Wegge & Schreyer (1994)
, and is consistent with
observations made for similar rocks from the Limpopo Belt (Windley
et al., 1984
; Droop, 1989
).
Kornerupine-absent equilibria
Early Opx–Crn and pre- to syn-S3 assemblages
The
S3 minerals sapphirine, orthopyroxene and sillimanite each
contain relics of Mg-Grt. Although from the textures it is difficult
to uniquely deduce reactions leading to the early formation
of this garnet, its decrease in modal abundance suggests the
traversal of the divariant FMAS [Crn Spl Crd] reaction
 | (1a) |
with decreasing pressure and rising or falling
temperature. This is, therefore, suggestive of a clockwise
P–
T path, although the precise d
P/d
T of this path segment cannot
be constrained. In some instances corundum is produced at the
apparent expense of garnet, leading to the [Spl, Crd, Sil] assemblage
orthopyroxene, sapphirine and corundum, with minor garnet. This
can be explained by the end-member MAS analogue
 | (1b) |
Orthopyroxene and corundum can also be produced
from sapphirine and cordierite on an up-pressure path in highly
magnesian bulk compositions. However, there is no textural or
mineralogical evidence for an early sapphirine + cordierite
assemblage, or sapphirine + cordierite + orthopyroxene in garnet-absent
cases.
Reaction (1b) lies at high pressure in Fig. 9 and has a slope that is constrained using the thermodynamic dataset of Holland & Powell (1998), and is consistent with the same reaction as calculated by Ouzegane et al. (2003). This Mg end-member reaction lies at P = 16·25 ± 0·25 kbar (1) for T = 850°C and XMgTs = 0·12 (Fig. 10; Gasparik & Newton, 1984). However, the reaction is displaced to lower pressures with increasing Fe content in garnet. For example, using the highest recorded magnesium content for relic garnet from a core zone (XPyp = 0·77), with the corresponding orthopyroxene for that zone (XMg = 0·88), displaces this curve to P 11·7 kbar for T = 850°C (calculated using THERMOCALC, v.3.1, Powell & Holland, 1988). This pressure may be considered an upper limit to conditions reached during D3, and agrees with textural evidence for the absence of kyanite either as inclusions in garnet or pseudomorphed by sillimanite. The position of the kyanite–sillimanite boundary between 800 and 850°C is P = 10·2–11·3 ± 0·11 kbar (1, Fig. 10; calculated using THERMOCALC, v.3.1, Powell & Holland, 1988). These pressure estimates are also consistent with estimates made using S3 assemblages from mafic granulite (P 9–10 kbar; Kelly et al., 2000
TOP
ABSTRACT
INTRODUCTION
) vs higher-Si cores (
). (b) Compositional traverse across a garnet porphyroblast from the rind of Pod 2. Garnets typically show a decrease in XPyp mirrored by an increase in XGrs and XAlm + XSps adjacent to rims. The garnet is approximately 0·5 mm in diameter. (c) (MgO + FeO)–SiO2–Al2O3 ternary plot for Pod 2 (Krn-bearing); it should be noted that kornerupine in this plot is uncorrected for Fe3+ in total Fe, and therefore plots just below the Opx–Crn tie-line. (d) (MgO + FeO)–SiO2–Al2O3 ternary plot showing chemographic relationships between phases from Pod 1 (Krn-absent). (e) Partial ternary diagram illustrating the position of the minerals Crd, Sil, Crn, Krn, Spr and Grt projected from Opx onto the SiO2–FeAl2O4–MgAl2O4 (quartz–hercynite–spinel) plane, using mineral compositions corrected for Fe3+. The minor deviation of Krn to the magnesian side of the Opx–Spr–Crd plane should be noted.