JOURNAL OF PETROLOGY
VOLUME 43
NUMBER 2
PAGES 271–290
2002
High f O2 Metasomatism During Whiteschist
Metamorphism, Zambezi Belt, Northern
Zimbabwe
S. P. JOHNSON∗ AND G. J. H. OLIVER
CRUSTAL GEODYNAMICS GROUP, SCHOOL OF GEOGRAPHY AND GEOSCIENCES, UNIVERSITY OF ST. ANDREWS,
ST. ANDREWS KY16 9AL, UK
RECEIVED SEPTEMBER 4, 2000; REVISED TYPESCRIPT ACCEPTED AUGUST 10, 2001
The Kadunguri Whiteschists are a group of talc- and kyanitebearing lithologies that occur in the Chewore Inliers from the
Zambezi Belt of northern Zimbabwe. They crop out on the southern
margin of the Chewore Ophiolite Terrane, a Mesoproterozoic ophiolite
and island arc, as a 5 km × 1·5 km, southeasterly dipping, semicontinuous block, and contain the second known natural occurrence
of yoderite. Major element analyses define the whiteschists within
the relatively simple MFASH system. Major and trace element
analyses indicate that the whiteschists originate from the metasomatic
alteration of alkalic ocean-island-type metabasalts similar to those
in the underlying Ophiolite Terrane. Synmetamorphic or metasomatic
mineral parageneses indicate peak P–T conditions of between 13
and 15 kbar at 550–600°C, and the highly oxidizing nature of
all reactions indicates the presence of a high fO2 metasomatic fluid.
The peak P–T conditions require that this synmetamorphic, exotic
metasomatic fluid was available at depths near 55 km. The age of
high-pressure metamorphism is constrained within the Pan African
tectonothermal cycle at 550–520 Ma. Tectonometamorphism in
the Zambezi Belt is related to a period of extensive crustal thickening
possibly related to amalgamation of Gondwanaland.
The transcontinental, Neoproterozoic to Cambrian, Pan
African Belt is a composite orogen that stretches across
southern Africa and separates the Congo Craton from
the Kalahari Craton. It includes the Damara Belt of
Namibia, the Lufilian Arc of Zambia and the Democratic
Republic of Congo, and the Zambezi Belt of southern
Zambia and northern Zimbabwe. It interacts complexly
at its eastern margin with the north–south-trending East
African Orogen of northern Mozambique and East
Africa.
The Zambezi Belt is a complex polyorogen recording
major tectonothermal events at >890–880 Ma and
520–550 Ma, both of which predominantly rework older
crustal components (Mesoproterozoic to Archaean) with
the addition of minor juvenile material (Hanson et al.,
1988, 1993, 1994; Wilson et al., 1993; Armstrong et al.,
1999). Within this belt and within basement inliers of
the Lufilian Arc, are a string (150 km long, 50–100 km
wide) of high-pressure, kyanite-bearing, meta-gabbroic
eclogites and whiteschist fragments (Fig. 1) (Vrana &
Barr, 1972; Vrana et al., 1975; Cosi et al., 1992; Johnson
& Oliver, 1998; Dirks & Sithole, 1999; John et al., 1999,
2000). Until recently these high-pressure, moderate-temperature fragments have been poorly studied but some
are now the focus of detailed investigations (e.g. Johnson
& Oliver, 1998; Oliver et al., 1998; Dirks & Sithole, 1999;
John et al., 1999, 2000; Johnson, 2000a, 2000b, 2001).
Results reveal that the mafic eclogites have mid-ocean
ridge basalt (MORB)-type signatures relating to the subduction and subsequent exhumation of oceanic-type crust
to and from depths greater than 50 km ( John et al., 1999,
2000). Such widespread high-pressure metamorphism
indicates that at some point in the extended history of the
Zambezi Belt this region was subject to tectonothermal
regimes similar to those of subduction zones or perhaps
∗Corresponding author. Present address: Tectonic Special Research
Centre, Department of Geology and Geophysics, University of Western
Australia, Nedlands, W.A. 6907, Australia. Telephone: 00-61-08-93807849. Fax: 00-61-08-9380-7848. E-mail: [email protected]
 Oxford University Press 2002
Congo Craton; high pressure; Kalahari Craton; metasomatism; whiteschist; Zambezi Belt
KEY WORDS:
INTRODUCTION
JOURNAL OF PETROLOGY
VOLUME 43
NUMBER 2
FEBRUARY 2002
Fig. 1. Tectonic map of central, southern Africa illustrating the location of the Chewore Inliers (CI) and main localities of eclogite and whiteschist
bodies. LK, Lake Kariba; MG, Makuti Group; RG, Rushinga Group. Box outlines the position of Fig. 2.
‘A’ type high-pressure (HP)–ultra-high-pressure-metamorphic (UHPM) belts (Maruyama et al., 1996). The
current interpretation of the Zambezi Belt tectonothermal
cycles, by Hanson et al. (1993, 1994), Vinyu et al. (1997)
and Dirks et al. (1998), cannot account for the production
of these high-pressure fragments, as, in their view, the
main tectonometamorphism is related to a high-temperature–low-pressure extensional event at 890–880 Ma
followed by minor thrust-related burial at 520–550 Ma,
with or without the closure of a narrow Red Sea-type
basin. Clearly there is need to elucidate the nature and
timing of this high-pressure event, not only to further
understand the evolution of the Zambezi Belt but also
for its wider implications on the timing and collision
between the Congo and Kalahari Cratons and its bearing
on the amalgamation of Gondwanaland.
Whiteschists
The term whiteschist was first used by Schreyer (1974) to
describe the stable, high-pressure, equilibrium assemblage
of talc and kyanite that occurs in the MASH (MgO, Al2O3,
SiO2 and H2O) system. However, the term has now been
expanded to include those assemblages that derive from
272
Fig. 2. Simplified geological map of the Chewore Inliers after Oliver
et al. (1998). Box outlines the position of Fig. 3.
JOHNSON AND OLIVER
WHITESCHIST METAMORPHISM
the high-pressure reaction between talc and kyanite in the
M(F)ASH system to form such minerals as orthoamphibole,
cordierite, sapphirine, chlorite, kornerupine, garnet,
yoderite, haematite ± phengite, ± rutile.
A longstanding problem surrounding these lithologies
is the nature of the protolith, as primary (sedimentary or
igneous) M(F)ASH rocks are extremely rare in nature
and thus all explanations require metasomatism. A feature
of this alteration process is that any multivalent ions (Fe
or Mn) predominantly occur in the highest oxidation
state, i.e. Fe3+, Mn3+, indicating the presence of a hydrothermal or metasomatic fluid with excess O2. Such O2rich fluids (a fluid with a high oxygen fugacity or f O2)
might normally be expected to occur in hydrothermal
systems near the surface of the Earth; however, the highpressure nature of whiteschist metamorphism indicates
that such an exotic fluid must occur at depths of
50 km or more. This paper describes the exceptional
metasomatic and tectonometamorphic evolution of a
high-pressure whiteschist occurrence in northern
Zimbabwe.
THE CHEWORE INLIERS
The Chewore Inliers are a group of isolated, basement
horsts within the Zambezi Valley of northern Zimbabwe
(Figs 1 and 2). Their location is crucial to understanding
the development of this complex tectonic region, as they
lie at the triple junction between the Zambezi Belt in
northern Zimbabwe, the re-tectonized Irumide Belt in
southern Zambia and the East African Orogen in Mozambique. Primary investigation of the Inliers was conducted by the Geological Survey of Zimbabwe in the
early 1990s and the results (including geochronological
and metamorphic studies) were published by Both
(1991, 1992) and Goscombe et al. (1994, 1996, 1997,
1998, 2000) (Fig. 2). Further work by Johnson &
Oliver (1998, 2000), Oliver et al. (1998) and Johnson
(1999) has subdivided the most southerly terrane, the
Zambezi Terrane, into what has been identified as a
Mesoproterozoic, dismembered ophiolite (the Chewore
Ophiolite) and an island-arc sequence (the Kaourera
Arc). These were accreted onto the Congo continental
margin before or during the Neoproterozoic Pan
African Orogeny.
The Kadunguri Whiteschists crop out on the southeastern margin of the Ophiolite Terrane (Fig. 2) and
form a semi-continuous block of whiteschist some 5 km
× 1·5 km in size. Aeromagnetic maps at 1:50 000 scale
(Anonymous, 1992) reveal that they continue under the
Karoo sedimentary cover, giving a total area of 10 km
× 3 km ( Johnson & Oliver, 1998). The best exposed
and most lithologically diverse section is centred around
Kadunguri Hill (GR[ST 9619 3498]) (Fig. 3).
STRUCTURAL SETTING
Structural development of the Kadunguri
Whiteschists
The earliest structure observed within the Kadunguri
Whiteschists is a coarse-grained, random fabric (S1) composed of radially arranged high-pressure minerals such
as gedrite or talc and kyanite. However, a penetrative
tectonic fabric [both foliation (S2) and lineation (L2)]
consisting of aligned high-pressure minerals (talc and
kyanite) is developed within a zone of 50–500 m width
that borders the Kadunguri Thrust (Fig. 3) and that
separates the Kadunguri Whiteschists from the underlying Ophiolite Terrane. The S2 foliation is defined by
the alignment of talc blades and small (1 mm) tabular
kyanite crystals. The L2 lineation is variably developed
in all the talc-bearing lithologies within this zone and is
defined by the alignment of the long axes of the tabular
kyanite laths, elongate rutile crystals and minor aggregations of quartz. The orientations of these fabrics
are illustrated in Fig. 3b. All mineral phases appear to
have grown syntectonically within this fabric and do not
display any textures that might be interpreted as being
modified by a later tectonic event. This suggests that the
high-pressure metamorphism (including the random S1
fabrics) and this deformation were synchronous.
Correlation with the Ophiolite Terrane
The structure of the Ophiolite Terrane has been described in detail by Goscombe et al. (1994, 1998, 2000)
and Johnson (1999). Johnson (1999) has divided the
Ophiolite Terrane into smaller structural domains, each
of which displays a similar structural evolution but differs
in the orientation of these structures as a result of gross,
large-scale heterogeneities during shear-dominated deformation. The structural evolution of the Ophiolite
Terrane is dominated by a single progressive event. An
initial S1 gneissic layering is isoclinally folded into rootless
shear folds with the development of a predominant S2–L2
tectonic fabric. Shear sense indicators show a SW over
NE tectonic transport direction. This tectonic fabric has
subsequently been reoriented into upright open folds
with the development of intermittent, poorly constrained,
crenulation cleavages. A SW–NE to south–north shortening direction is inferred. Late-stage crenulation cleavages, with poorly constrained orientations, are also
present on either side of the Kadunguri Thrust (Fig. 3).
The orientation of the main S2–L2 fabrics passes without
loss of orientation or integrity from the Kadunguri
Whiteschists across the Kadunguri Thrust into the Ophiolite Terrane (Fig. 3a and b). It is therefore concluded
that the dominant fabrics within both were developed
during the same shear-dominated tectonic event.
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FEBRUARY 2002
Fig. 3. Detailed geological map of the Kadunguri Hill region. Circled stars indicate the localities of samples collected for whole-rock X-ray
fluorescence (XRF) major and trace element analyses (see Table 2). Inset are equal-area stereographic projections illustrating the orientation of
structural fabrics within (a) the adjacent domain of the Chewore Ophiolite and (b) the Kadunguri Whiteschists. In both projections, crosses
represent poles to foliation and dots represent mineral elongation lineations.
Whiteschists
Foliated
LITHOLOGIES AND PETROLOGY
The Kadunguri Whiteschists have been subdivided into
five distinct mineralogical and textural units. The five
units form a predominantly southeasterly dipping, layered
sequence (Fig. 3).
This sub-unit occurs at the base of the whiteschist
sequence and is in tectonic contact with the underlying
meta-basalts of the Ophiolite Terrane. This rock is mostly
274
JOHNSON AND OLIVER
WHITESCHIST METAMORPHISM
formed of quartz (40%), talc (25%), kyanite (20%), haematite (7%), rutile (5%) and dravite (3%). Talc, kyanite,
rutile and ribbon quartz form a persistent foliation
whereas the kyanite long axes define a impersistent
lineation (Fig. 4a). Dravite and haematite are generally
coarser than the fabric that wraps around these phases.
Textural equilibrium is interpreted for all phases except
for occasional random chlorite blades that replace talc
and kyanite. In some cases the tabular kyanite laths
are fully replaced and pseudomorphed by the random
chlorite blades. This replacement suggests a retrograde,
chlorite-producing reaction, involving both talc and kyanite as reactants.
This lithology is fundamentally different from that of
the other two main whiteschist units in that it does not
contain primary talc and that the main phase (gedrite)
is randomly oriented. Although the contact between this
unit and the underlying Unfoliated Whiteschist has yet
to be identified in outcrop, it is interpreted that it is
relatively sharp, as the boundary can be traced to a
distance of <10 m.
Yoderite Whiteschist
The Yoderite Whiteschist crops out as a 12 m × 3 m
pod between the Foliated and Unfoliated Whiteschists
(Fig. 3) and contains the second known natural occurrence of the high-pressure magnesium–aluminium
silicate mineral yoderite [Mg2Al6Si4O18(OH)2]. The rock
is formed from coarse-grained (up to 4 cm), randomly
oriented chlorite (40%), kyanite (30%), haematite (10%),
dravite (10%), talc (5%) and yoderite (5%). All phases
are interpreted to be in textural equilibrium (Fig. 4d). A
description of the composition of the mineral yoderite
and a comparison with the type locality of this mineral
(Mautia Hill, Tanzania; McKie, 1959) has been given
by Johnson & Oliver (1998). The pod is surrounded
by a semi-continuous band of coarse-grained quartz,
haematite and kyanite of 1–5 m thickness (described in
the next paragraph). Field evidence suggests that the pod
is a discrete unit, that the boundary between the two is
sharp and straight, and that there is no mineralogical or
textural grading between the two ( Johnson & Oliver,
1998).
Unfoliated
This sub-unit is located structurally above and is mineralogically identical to the Foliated Whiteschists. All
phases are randomly oriented and are interpreted to be
in textural equilibrium, except for the minor, late chlorite
that replaces talc and kyanite.
Orthoamphibole Whiteschists
This unit occurs at the structural top of the exposed
sequence. It is composed of gedrite (35%), kyanite (30%),
dravite (15%), quartz (10%), haematite (9%) and rutile
(1%). All phases are randomly oriented and are interpreted to be in textural equilibrium (Fig. 4b) except
for the occasional, minor replacement of gedrite by talc
(Fig. 4c). Acicular aggregates of gedrite (up to 2 cm in
length) with interlocking stubby, skeletal kyanite crystals
and an interstitial fine-grained matrix of quartz, kyanite,
haematite and dravite give the rock a massive appearance
(Fig. 5a). However, at some localities, this massive unit
is interbanded with millimetre- to centimetre-scale, equigranular and fine-grained (up to 0·5 mm) haematite,
kyanite and quartz bands. This banding can in places
make up 60% of the total rock volume and is observed
to change sharply, both texturally and mineralogically,
into the massive unit over a distance of 1–2 mm.
Some Orthoamphibole Whiteschist specimens display
minor disequilibrium textures where talc is observed to
replace gedrite (Fig. 4c). In the tabular gedrite sections,
the talc replaces the margins (usually the {001} form) as
a mass of very fine-grained (<0·1 mm) talc laths that are
aligned parallel to the {100} form. In basal sections, talc
replaces gedrite along, and is aligned parallel to, the
{210} and {21̄0} cleavage planes. Oriented talc laths
mimic the basal amphibole cleavage in the gedrites that
have been totally pseudomorphed. These textures are
interpreted to represent a prograde reaction with the
production of both talc and kyanite (see Metamorphic
Development).
Quartz–haematite–kyanite band
This unit crops out as an intermittent vertical sheet of
1–5 m thickness that coincides roughly with the decrease
in strain between the Foliated and Unfoliated Whiteschists and is the rock that envelops the Yoderite Whiteschist (see Johnson & Oliver, 1998, fig. 3). This rock is
composite in nature, with the bulk of the lithology being
formed from fine-grained (<0·5 mm), equigranular quartz
(45%) and haematite (45%) with minor, randomly oriented kyanite (10%). This fine-grained portion is crosscut by predominantly parallel-sided, coarse-grained (up
to 2 cm), semi-continuous bands of randomly oriented
haematite (55%) and quartz (45%) or haematite (50%),
quartz (40%) and kyanite (10%) (Fig. 5b). This banding
is parallel to the margins of the vertically oriented body;
however, at MR [9662 3494] the coarse haematite, quartz
and kyanite banding takes the form of an anastamosing
network (Fig. 5c), suggesting that this is not a primary
igneous or sedimentary feature but of metasomatic origin.
275
Fig. 4. (a) Plane-polarized photomicrograph of the Foliated Whiteschist. The lithology comprises 0·1–0·25 mm aligned talc (Tc) and 0·25–0·5 mm kyanite (Ky) laths, 1–2 mm granular
quartz (Q ) and elongate–irregular haematite (H). Dravite (not shown in photomicrograph) occurs as >1 mm, semi-spherical porphyroblasts. Field of view is 5 mm. (b) Plane-polarized
photomicrograph of the Orthoamphibole Whiteschist. The acicular, radially arranged gedrite crystals (G) and interstitial kyanite (Ky) and quartz (Q ) should be noted. Field of view is 5 mm.
(c) Plane-polarized photomicrograph illustrating the replacement of gedrite (G) with talc (Tc) in the Orthoamphibole Whiteschist. The field of view displays a single gedrite crystal (basal
section) with a quartz crystal in the top left of the picture. The gedrite has been replaced by talc along the cleavage planes (best illustrated in the top right of the picture). This is interpreted
as a prograde reaction [reaction (4) in main text] between gedrite and quartz to produce talc, kyanite and haematite. Field of view is 5 mm. (d) Plane-polarized photomicrograph of the
Yoderite Whiteschist. C, chlorite; K, kyanite; Y, yoderite. Field of view is 3·5 mm.
JOURNAL OF PETROLOGY
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Fig. 5. (a) Field view of the Orthoamphibole Whiteschist (MR[9579 3348]). Pencil is 6 cm in length. Typical occurrence of the Orthoamphibole Whiteschist displaying acicular, radially
arranged rosettes of gedrite with an interstitial fine-grained matrix of quartz, kyanite and haematite. At other localities, this unit contains centimetre-scale bands of fine-grained quartz and
haematite (see main text). (b) Field view of the 1 m thick quartz–haematite–kyanite band (MR[9634 3510]). Pencil is 10 cm in length. This unit is composed of a very fine-grained matrix
of quartz and haematite (± talc), which is cut by parallel bands, up to 1 cm thick, of coarse-grained quartz, haematite and kyanite. These smaller-scale bands are parallel to the margins
of the main body. (c) Field view of the quartz–haematite–kyanite band at MR[9662 3494]. At this locality, the quartz–haematite–kyanite band contains a sub-parallel, anastamosing network
of quartz, haematite and kyanite veins. Field of view is 75 cm. (d) Field view of a coarse-grained, cross-cutting quartz and haematite (H) sheet. Pen is 15 cm in length. Quartz crystals form
up to 90% of the rock and contain tabular, randomly oriented haematite crystals up to 4 cm long.
JOHNSON AND OLIVER
WHITESCHIST METAMORPHISM
277
JOURNAL OF PETROLOGY
VOLUME 43
Coarse-grained, cross-cutting veins
These veins, of >1 m thickness, are restricted in outcrop
but can be found cross-cutting all the units described
above. They are of granoblastic quartz (90%) with dravite
(10%), or haematite (10%), or haematite (8%) and kyanite
(2%). All are very coarse grained with individual quartz
grains up to 1 cm in diameter and euhedral dravite, or
haematite, or haematite and kyanite crystals up to 6 cm
in length (Fig. 5d). At one outcrop (MR [9654 3531])
exceptionally large radially arranged green kyanite crystals 50–60 cm in length occur within a coarse-grained
(up to 3 cm) granoblastic quartz matrix. These veins
are presumed to be an expression of synmetamorphic
pegmatite formation.
Mineral chemistry
Mineral analyses were conducted at St. Andrews University using a JEOL JCXA-733 Superprobe with sequential wavelength-dispersive spectrometry detectors.
The accelerating voltage was 15 kV, and the probe current 20 nA, with a 1–2 m beam diameter. Representative
mineral analyses are given in Table 1.
Talc. Talc in the Foliated, Unfoliated and Yoderite
Whiteschists contains minor Al3+ [0·18–0·19 per formula
unit (p.f.u.)] and has the formula [Mg2·9Al0·1]
[Si3·9Al0·1O10](OH)2. In the Orthoamphibole Whiteschist some Al3+ and minor Na+ substitute into the
structure giving a formula of [Mg2·6Al0·24Na0·1]
[Si3·9Al0·1O10](OH)2.
Kyanite. Kyanite shows minor substitution of Fe3+ for
Al3+ giving an average formula of [Al1·98Fe0·2][SiO5].
Gedrite. According to the nomenclature of Leake (1978),
the orthoamphibole is classified as magnesio-gedrite
Na0·4(Mg5·8Fe0·2)(Al1·0[(Si6·7Al1·3)O22]OH)2 having an Mg
to Fe ratio [XMg = Mg/(Mg + Fe)] of 0·96.
Chlorite. Within the Yoderite Whiteschist the matrix chlorite is sheridanite [Mg4·5Al1·3Fe0·05][(Si2·8Al1·2)O10(OH)8]
whereas chlorite inclusions are penninitic [Mg2·7Al1·8
Fe1·0][(Si3·2Al0·8)O10(OH)8] in composition [see Johnson &
Oliver (1998) for further details]. Chlorite within the
Foliated and Unfoliated Whiteschist is sheridanite
[Mg4Al1·4Fe0·5][(Si2·9Al1·1)O10(OH)8]. The sheridanite and
penninite have XMg ratios of 0·98 and 0·75, respectively.
Dravite. Boron has not been analysed but by assuming
the standard dravite formula it has the formula
(Na0·8)(Mg2·4Fe0·6)(Al6)[Si6O18](BO3)3(OH)4 and an XMg
ratio of 0·8. Na+ is low, possibly as a result of evaporation
under the probe beam.
NUMBER 2
FEBRUARY 2002
Yoderite. The occurrence and composition of yoderite has
been described in detail by Johnson & Oliver (1998). It
has an average composition of Mg2Al5·7Fe0·3Si4O18(OH)2
with an XMg ratio of 0·86.
BULK-ROCK CHEMISTRY AND
METASOMATISM
The aim of this section is to determine whether the
Kadunguri Whiteschists are related via metasomatism to
the Chewore Ophiolite lithologies and if so, to determine
the nature and relative degree of metasomatism. As
there are apparently no known primary M(F)ASH rocks,
whiteschists must develop from the metasomatic alteration of a parent lithology. Considering that the Kadunguri Whiteschists have a relatively large outcrop area
(30 km2) compared with most other whiteschist occurrences (up to a few 100 m2), Johnson & Oliver (1998)
suggested that the Kadunguri protolith must have been
relatively uniform and probably igneous in nature. As
the whiteschists are in contact with voluminous metabasalts and meta-island-arc lithologies of the Ophiolite
Terrane and, only 90 km to the west, Vrana & Barr
(1972) interpreted similar whiteschists as metasomatized
meta-basalts, it was tentatively proposed that the Kadunguri Whiteschists had a similar origin ( Johnson &
Oliver, 1998).
Major and trace elements were analysed on glass
discs and pressed powder pellets, respectively, and were
analysed using a Phillips PW 1450/20 X-ray fluorescence
spectrometer with a side-window rhodium tube at the
University of St. Andrews.
Results
Table 2 lists the results of the whole-rock major and
trace element analyses for the main whiteschist lithologies.
The whiteschists are constrained within the MFASH
system (with all Fe as Fe2O3) but with variable proportions
of TiO2 (up to 1·78 wt % in rutile) and B2O3 (>1 wt %
in tourmaline). Figure 6 is a modalized triangular plot,
illustrating the variation in the main, major element
components (SiO2–MgO–Al2O3). It is evident that the
Foliated, Unfoliated and Orthoamphibole Whiteschists
lie along a similar, straight-line trend towards progressive
MgO enrichment whereas the Yoderite Whiteschist deviates from this trend, plotting further from the SiO2
axis, indicating enrichment in both MgO and Al2O3.
Figure 7 shows a series of trace element ratio diagrams
plotting those trace elements usually considered immobile
during metamorphism and hydrothermal alteration
(Humphris & Thompson, 1977; Brekke et al., 1984;
Brouxel et al., 1989). In all diagrams there is a similar
278
0·04
CaO
279
0·18
0·00
0·02
0·00
2·90
0·00
0·00
0·00
7·02
Al
Fe2+
Fe3+
Mn
Mg
Ca
Na
K
Total
oxygens
11
0·00
Ti
No. of
3·89
Si
Cation proportion
94·443
30·45
MgO
Total
0·00
MnO
0·00
0·33
Fe2O3
K2O
0·00
FeO
0·22
2·45
Al2O3
Na2O
0·00
0·09
TiO2
10
4·82
0·00
0·00
0·00
0·00
0·00
0·07
0·00
3·99
0·00
2·01
100·914
0·00
0·00
0·03
0·01
0·00
1·58
0·00
62·09
37·20
60·87
SiO2
wt %
24·5
15·63
0·00
0·78
0·01
2·35
0·01
0·58
0·00
5·86
0·05
5·99
87·333
0·00
2·54
0·08
9·97
0·04
4·86
0·00
31·50
0·42
37·93
14
9·86
0·00
0·00
0·00
3·97
0·00
0·00
0·51
2·49
0·00
2·91
89·373
0·02
0·00
0·02
28·76
0·02
0·00
6·60
22·70
0·00
31·30
Chlorite
23
15·28
0·00
0·36
0·01
5·77
0·00
0·15
0·00
2·30
0·01
6·67
97·121
0·02
1·40
0·06
29·13
0·01
1·53
0·00
14·69
0·14
50·15
8
12·06
0·00
0·00
0·00
0·02
0·00
0·19
0·00
7·80
0·00
4·04
100·080
0·00
0·00
0·00
0·13
0·02
2·06
0·00
60·76
0·00
37·12
Kyanite
11
6·97
0·00
0·10
0·00
2·60
0·00
0·03
0·00
0·34
0·01
3·89
93·364
0·01
0·76
0·04
29·03
0·01
0·52
0·00
4·32
0·21
58·51
Talc
Gedrite
Dravite
Talc
Kyanite
Orthoamphibole Whiteschists
Foliated and Unfoliated Whiteschists
11
6·99
0·00
0·02
0·00
2·86
0·00
0·01
0·00
0·19
0·00
3·91
90·390
0·02
0·18
0·01
28·80
0·00
0·21
0·00
2·46
0·01
58·71
Talc
10
6·01
0·00
0·00
0·00
0·00
0·00
0·03
0·00
4·00
0·00
1·99
100·090
0·00
0·00
0·00
0·00
0·00
0·59
0·00
62·33
0·00
37·17
Kyanite
Yoderite Whiteschist
Table 1: Electron microprobe data for the mineral phases within the Kadunguri Whiteschists
14
9·94
0·00
0·00
0·00
4·56
0·00
0·00
0·05
2·52
0·00
2·81
86·650
0·01
0·01
0·02
32·84
0·02
0·00
0·64
22·99
0·00
14
9·94
0·00
0·00
0·02
2·72
0·00
0·00
0·98
2·58
0·00
3·21
88·230
0·02
0·02
0·16
19·10
0·00
0·05
12·13
22·90
0·00
33·63
Inclusions
Matrix
30·12
Chlorite
Chlorite
24·5
12·01
0·01
0·68
0·05
2·43
0·00
0·42
0·00
6·05
0·04
5·95
85·810
0·06
2·18
0·28
10·20
0·00
3·49
0·00
32·13
0·29
37·19
Dravite
19
19·00
0·00
0·00
0·00
2·00
0·00
0·28
0·00
5·73
0·00
4·00
98·170
0·01
0·01
0·01
12·43
0·02
3·43
0·00
45·12
0·03
37·11
Yoderite
JOHNSON AND OLIVER
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JOURNAL OF PETROLOGY
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FEBRUARY 2002
Table 2: Whole-rock, major and trace element analyses for the Kadunguri Whiteschists, Chewore Ophiolite and
Kaourera arc lithologies
Ophiolite Terrane lithologies
Kadunguri Whiteschists
Marginal-
HFSE-
HFSE-
HFSE-
Serpentinized
Orthoamphibole Unfoliated
Foliated
Yoderite
basin∗
depleted
enriched
enriched
ultramafic
Whiteschist
Whiteschist
Whiteschist
A-OIB§
rock¶
low-K island OIB‡
Whiteschist
arc†
SiO2
48·60
49·73
48·06
49·50
42·24
57·16
56·37
51·95
TiO2
0·78
0·68
3·12
2·24
0·27
1·78
1·38
1·15
35·46
0·27
Al2O3
14·66
15·20
11·93
17·63
2·7
15·96
16·07
15·50
23·80
Fe2O3
11·33
10·78
19·18
10·94
21·89
8·66
7·59
2·29
12·97
MnO
0·22
0·21
0·27
0·15
0·15
0·02
0·03
0·00
0·02
MgO
10·50
8·64
4·66
3·89
21·11
13·88
16·24
18·99
20·35
CaO
9·35
9·70
8·72
8·05
8·12
0·06
0·02
0·11
0·18
Na2O
2·99
3·39
1·43
3·78
0·02
0·27
0·40
0·10
0·33
K2O
0·32
0·23
0·70
1·13
0·02
0·04
0·01
0·02
0·00
P2O3
0·09
0·05
0·38
0·31
0·02
0·08
0·06
0·07
0·05
LOI
2·00
2·10
1·30
2·70
3·50
2·80
1·80
10·60
6·80
100·84
100·28
99·73
100·30
100·04
100·73
99·95
100·78
100·23
WRXMg
0·48
0·44
0·26
0·49
Nb
1
2
12
33
2
17
16
9
26
Y
16
15
44
28
2
21
29
12
2
Ce
5
6
34
72
0
5
22
6
4
Sc
42
38
44
24
25
20
27
9
29
V
64
Total
0·17
0·61
0·68
0·89
0·61
245
167
551
226
157
19
25
29
La
2
2
9
29
0
1
9
2
1
Cr
700
199
95
222
5000
174
132
92
105
Ni
270
237
49
79
722
50
128
54
69
Geochemical data from Johnson (1999).
∗Sample SJ 203, a marginal basin-type meta-basalt from the Chewore Ophiolite.
†Sample SJ 286, a low-K tholeiitic, island-arc meta-basalt from the Kaourera island arc.
‡Sample SJ 72, a tholeiitic ocean-island meta-basalt from the Kaourera island arc.
§Sample SJ 132, an alkalic ocean-island, seamount-type meta-basalt from the Kaourera island arc.
¶Sample SJ 213c, serpentinized ultramafic rock from the Chewore Ophiolite.
LOI, lost on ignition.
linear trend indicating progressive removal of trace elements, presumably from a common protolith. It should
be noted that the trace elements Zr, Nb and Y, which
are usually considered immobile, are apparently highly
mobile in these rocks. The mobilization of these elements
attests to the extreme nature of the metasomatic process
and/or the exotic composition of the metasomatic fluid.
Parental composition
The geochemistry of the Chewore Ophiolite lithologies
is diverse, and they contain marginal basin meta-basalts,
serpentinized ultramafic rocks, high field strength element
(HFSE)-depleted meta-basalts (low-K island-arc), islandarc meta-andesites and meta-dacites, HFSE-enriched
[ocean-island basalt (OIB)] tholeiitic meta-basalts and
HFSE-enriched alkalic meta-basalts [for detailed analyses, see Johnson & Oliver (2000)]. Representative wholerock major and trace element anlayses are given in Table
2 and the compositional fields for the Chewore lithologies
are overlain on Figs 6 and 7.
As these whiteschists display a progressive, linear trend
of metasomatic trace element removal, it is not unreasonable to assume that the composition of the parental
lithology lies along this line at the higher trace element
ratio end. In all major and trace element ratio diagrams
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Fig. 6. Modalized SiO2, MgO, Al2O3 triangular compatibility diagram (in the presence of excess H2O, FeO and Fe2O3). The diagram shows
the positions of the coexisting phases and whole-rock major element analyses for the various Kadunguri Whiteschists, Chewore Ophiolite and
Kaourera Arc lithologies. Key to whiteschists: Φ, Orthoamphibole Whiteschist; Χ, Random Whiteschist; Β, Foliated Whiteschist; Ε, Yoderite
Whiteschist. Key to compositional fields: diagonal stripes, Kaourera Arc lithologies; fine stipple, tholeiitic ocean-island meta-basalts; unshaded
field, marginal basin meta-basalts of the Chewore Ophiolite; vertical stripes, serpentinized ultramafic; light grey field, alkalic ocean-island metabasalts. Key to abbreviations: dr, dravite; g, gedrite; ky, kyanite; pen, penninite; qtz, quartz; sher, sheridanite; tc, talc; y, yoderite. (For discussion,
refer to text.)
(e.g. Figs 6 and 7) only the alkalic ocean-island metabasalts consistently display a good fit with this
metasomatic trend and thus they are assumed to be the
parent lithology.
Metasomatic alteration
Another way of examining the process of metasomatic
alteration and fluid–rock interactions is by using the
composition–volume approach. If the composition of the
protolith is known then the concentrations of the elements
(either weight percent or moles of elements) in the altered
rock can be compared directly with those within the
parent or least-altered equivalent. The result is an estimate of the relative gains and/or losses of elements
resulting from the metasomatic process. This method
was proposed by Gresens (1967) and a graphical solution,
termed an isocon diagram, devised by Grant (1986). A
useful application of this method is the determination of
the relative oxygen concentrations during metasomatism.
Such a calculation is essential for these whiteschist lithologies, as all multivalent ions occur in the highest
oxidation state, suggesting a metasomatic fluid saturated
in O2. Figure 8a–d shows a series of isocon diagrams
calculated in moles of element concentrations by the
computer program ‘Gresens 92’ (Potdevin, 1993) assuming that the parental lithology has a composition
identical to the alkali ocean-island meta-basalt of the
Chewore Ophiolite (Table 2). The densities of each rock
unit were also used to calculate the relative mass–volume
changes. Element concentrations that have undergone
no change from the parent to altered rock will fall on a
line intersecting the origin. This line is termed an ‘isocon’
and can be related to the equation
Ca = (Mo/Ma) Co
where Ca and Co are the final and original concentrations
and Ma and Mo are the final and original masses of
the rock (Grant, 1986). In this case a line passing
through Al [as it is considered to be the least mobile;
Gresens (1967)] and intersecting the origin is taken to
be the isocon. For the Yoderite Whiteschist the isocon
is only estimated, as it is evident that even Al is mobile
in this rock. Elements that plot above the isocon
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Fig. 7. Various trace element ratio diagrams for the Kadunguri Whiteschists, Chewore Ophiolite and Kaourera Arc lithologies, showing that
whiteschists lie on a trend originating from a parental alkalic ocean-island meta-basalt. Symbols are the same as in Fig. 6.
represent a gain in concentration compared with the
parent, whereas elements that plot below the line
represent losses in concentration and are thus removed
from the system by the metasomatic fluid. Figure 8a–d
shows the following:
(1) the mobile major elements K, Na, Ca and Mn are
completely removed by the fluid phase;
(2) all trace elements, even those normally considered
immobile during metamorphism and hydrothermal alteration, are in fact highly mobile during this metasomatic
event;
(3) although there is an initial increase in Si concentration, there follows a progressive decrease and removal of Si into the fluid phase;
(4) there is a progressive decrease in Fe3+ concentration
in the three main whiteschist lithologies, but an increase
of Fe3+ within the Yoderite Whiteschist;
(5) there is a stable concentration of excess O2 in all
lithologies;
(6) there is a significant increase in Al concentration
in the Yoderite Whiteschist;
(7) there is a massive and progressive increase in Mg
concentration in the whiteschists compared with the
meta-basalt.
Evidence for removal or mobilization of these major
elements by the metasomatic fluid can also be directly
observed in the field. For example, the Si, Fe, B and Na
removed by the fluid is recrystallized as coarse, crosscutting veins such as the quartz–dravite or quartz–
haematite veins. Fe and Si (including minor Al) is also
recrystallized around the Yoderite Whiteschist as a
quartz–haematite–kyanite band of 1–5 m thickness and
as coarse anastamosing quartz–haematite veins within
this unit (see Lithologies and Petrology).
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Fig. 8. (a)–(d) are a series of isocon diagrams [based on Gresens (1967) and Grant (1986) and calculated using the computer program ‘Gresens
92’ by Potdevin (1993)], calculated in moles of elements, to show the progressive, metasomatic alteration of a typical A-OIB meta-basalt of the
Ophiolite Terrane to Kadunguri Whiteschist compositions. In (a) the isocon is defined by the best-fit line to the elements, Ti, Fe3+, Al, 0·3Nb,
0·2Zr and 0·5Sc. In (b) and (c) the isocon is defined by that of constant aluminium whereas in (d) the position of the isocon is determined
roughly from a position that lies below 0·3O2 and above 0·5Si. (For detailed discussion, refer to main text.)
METAMORPHIC DEVELOPMENT
Metamorphic reactions in the M(F)ASH system have
been studied experimentally by many workers (Yoder,
1952; Schreyer & Yoder, 1968; Schreyer & Seifert, 1969a,
1969b; Schreyer, 1977, 1988; Massonne, 1989, 1995;
Fockenberg & Schreyer, 1991, 1993, 1994; Fischer et al.,
1999). There are no direct exchange thermometers or
net transfer reactions between mineral phases within the
MFASH system and thus estimations of P–T conditions
are based on the recognition of key metamorphic reactions and equilibrium assemblages compared with those
derived experimentally.
TiO2 is a 1–2 wt % trace element occurring in all
Kadunguri lithologies as rutile. As it occurs only within
one phase it is unlikely that it would change any estimation
of P–T of the main lithologies; however, its occurrence
as a metamorphic phase needs to be documented. It is
likely that Ti originated in ilmenite within the basalt and
was metamorphosed to sphene at low grade and then to
rutile at whiteschist grade with the removal of all Ca2+
into the metasomatic fluid in a reaction similar to the
following:
ilmenite + H2O[or Si(OH)4] = sphene + Fe2O3 =
rutile + Ca2 + .
(1)
Foliated and Unfoliated Whiteschists
The stability of the equilibrium assemblage, talc +
kyanite + quartz, at high pressure, has been the focus
of experimental studies (Yoder, 1952; Schreyer, 1977,
1988; Schreyer & Seifert, 1969a, 1969b; Massonne, 1989,
1995). These results indicate that talc and kyanite are
produced by the high-pressure (>7–20 kbar), moderatetemperature (>500–600°C) reaction between chlorite
and quartz. Even though metasomatism has been extensive (see above) we propose to identify appropriate
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Fig. 9. Petrogenetic grid for the MASH and MFASH systems illustrating the upper and lower stability of talc + kyanite (after Schreyer, 1977,
1988; Massone, 1989, 1995). Numbered isopleths refer to the Al3+ concentration (p.f.u.) in talc, which have been experimentally derived by
Massone (1995). car, carpholite; chl, chlorite; en, enstatite; g, gedrite; ky, kyanite; qtz, quartz; tc, talc.
reactions using the compositions of the active minerals
as analysed by microprobe.
The bulk whole-rock, major element chemistry of the
Kadunguri Whiteschists lies in the two-phase field of
chlorite (sheridanite) and quartz (Fig. 6). It is assumed
that the following reaction took place in a similar P–T
frame to that derived experimentally by Schreyer (1977)
and Massone (1989) (Fig. 9):
chlorite +
quartz +
100 Mg3·97A12·5Fe0·5Si2·9O10(OH)8 +355·5 SiO2 +
oxygen =
talc +
14 O2 = 137 Mg2·9A10·19Si3·9O10(OH)2 +
kyanite +
haematite + water
111·5 A12SiO5 + 25 Fe2O3 + 263 H2O.
(2)
[ged]
Talc blades in both whiteschists contain significant
concentrations of Al3+ (>2·5 wt % and 0·184 p.f.u.)
(Table 1). Experimental results of Massone (1995) have
illustrated the systematic P–T-dependent substitution of
Al3+ into the talc structure for lithologies in the MASH
system. As neither the reaction products nor primary
reactant phases of reaction (2) contain Fe(2+ or 3+), this
reaction can also be modelled in the simple MASH
system. The isopleths for Al3+ p.f.u. in talc are shown in
Fig. 9 and it is evident that talc with an Al3+ concentration
of 0·184 p.f.u. (Table 1) indicates formation at >10 kbar
and 600–750°C. The upper thermal limit for this assemblage in the MASH system is 800°C (with the production of enstatite) and in the MFASH system it is
>850°C (7–20 kbar) with the production of gedrite and
quartz. At lower pressures (<7 kbar) this assemblage is
bracketed at much lower temperatures (650–750°C) by
the formation of cordierite (Schreyer, 1977). Because
both whiteschists contain a texturally equilibrated assemblage of talc and kyanite and show no evidence for
the breakdown to gedrite, enstatite or cordierite, it is
postulated that temperatures never exceeded 800–850°C.
It is concluded that this stable assemblage indicates
metamorphic conditions between 10 and 20 kbar at temperatures above 500–600°C and below 850°C.
Orthoamphibole Whiteschists
The presence of orthoamphiboles in MFASH lithologies
has commonly been ascribed to the high-temperature
(>850°C), high-pressure (7–20 kbar) breakdown of talc
and kyanite to produce gedrite and quartz (Schreyer,
1977; Munz, 1990; Fischer et al., 1999) (Fig. 9).
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However, most orthoamphibole-bearing MFASH lithologies have considerably greater whole-rock XFe,
SiO2 wt % and hence lower XMg values than that of
these whiteschists (Table 2 and Fig. 6). It is possible
that orthoamphibole is developed in preference to talc
in a reaction similar to (2), at similar P–T, in these
lower XMg lithologies. Such a relationship was also
postulated by Robinson (1982) and Munz (1990). The
Orthoamphibole Whiteschist has the lowest XMg wholerock value (Table 2), 0·85 in comparison with 0·9 and
0·97 for the Unfoliated and Foliated Whiteschists,
respectively. Considering this much lower whole-rock
XMg value, the primary chlorite (in the two-phase field
of quartz and chlorite) must have been richer in Fe,
with a composition near that of pennenite–clinochlore.
A chlorite with this composition is used to derive the
following reaction:
[tlc]
chlorite +
100 Mg3·4A12·5Fe0·75Si3O10(OH)8 +
quartz + sodium + oxygen =
150 SiO2 +23 Na+ + 15 O2 =
gedrite +
58·6 Na0·4(Mg5·8Fe0·2)(A11·0[(Si6·7A11·3)O22]OH)2+
kyanite + haematite + water
56·6 A12SiO5 +31·6 Fe2O3 + 341 H2O.
(3)
Figure 10 (XMg concentration vs T°C) illustrates the
Schreinmakers analyses for the six phases (chlorite,
quartz, kyanite, gedrite, talc, water) in the MFASH
system, similar to that proposed by Massone (1989). From
this diagram it is evident that there is only one unique
XMg whole-rock composition where gedrite can be stable
with chlorite and the other four phases, namely the
invariant point (IP[1]). In lithologies with a lower XMg
than that at the invariant point, chlorite and quartz react
producing gedrite, whereas those with higher XMg values
will develop talc. The sharp lithological contact (traceable
to within 10 m) between the Orthoamphibole Whiteschist
and the underlying Unfoliated Whiteschist may equate
to invariant point [1] (see Lithologies and Petrology).
The minor disequilibrium textures present within some
of the Orthoamphibole Whiteschists are interpreted as a
prograde reaction (Fig. 10), with the production of both
talc and kyanite via the following reaction:
[chl]
The lack of new kyanite growth as porphyroblasts suggests
that this phase may be growing in structural continuity
around old kyanite inclusions formed via reaction (3),
within the original gedrite porphyroblast. This is demonstrated in that all gedrite crystals undergoing reaction
(4) contain significantly larger (up to 0·5 mm larger)
inclusions of kyanite than those that are in stable or
metastable equilibrium. As the effect of the XMg composition on reactions (3) and (4) has not been experimentally studied, it is unclear how these reactions
relate in P–T space to those outlined by Massone (1989).
However, it is unlikely that they would be significantly
different, even with the presence of minor quantities of
Na (0·1 p.f.u) within the gedrite structure. The concentration of Al3+ within these talc crystals is relatively
high compared with that of the other whiteschist lithologies (0·340 p.f.u. compared with 0·184 p.f.u.). This is
presumably due to talc production being related to the
breakdown of Al-rich gedrite in the more complex
MFASH system.
Recent experimental data for the upper stability limit
of gedrite containing up to 1 wt % impurities give
800–850°C at pressures between 9 and 15 kbar (Fischer
et al., 1999). The higher pressure (15 kbar) limit results
in the production of aluminium silicate, enstatite and
melt whereas the lower pressure (9 kbar) limit results
in cordierite-bearing assemblages (Fischer et al., 1999).
Although the Schreinmakers analyses (Fig. 10) examine
only the lower-temperature portion of the gedrite stability
field, there is no reason why cordierite or enstatite should
not become involved in the breakdown of this mineral
assemblage, especially as the talc- and quartz- or kyaniteabsent reactions have been replaced by reactions involving lower-pressure mineral phases. Therefore it is
assumed that the upper stability limit for these gedritebearing assemblages is no different from that derived
experimentally by Fischer et al. (1999). As there is a lack
of enstatite, cordierite or melt production, it can be
assumed that this assemblage did not exceed temperatures
of >800–850°C at pressures between 9 and 15 kbar.
Yoderite Whiteschist
Johnson & Oliver (1998) have documented the metamorphic and PT evolution of the Yoderite Whiteschist
in detail and indicated that the quartz-free yoderite
assemblage formed via the reaction
gedrite +
10 Mg5·8A12·3Fe0·2Na0·3Si6·7O22(OH)2 +
quartz +
water + oxygen =
27·7 SiO2 +12·3 H2O + O2 =
talc +
kyanite +
22·3 Mg2·6A10·34Na0·1Si3·9O10O10(OH)2 +7·7 A12SiO5 +
haematite
1 Fe2O3.
(4)
talc +
sheridanite
50 Mg3Si4O10(OH)2 + 120 Mg4·5A12·5Fe0·05Si2·8O10(OH)8
+ kyanite + haematite + oxygen =
5O2 =
+850 A12SiO5+Fe2O3+
yoderite +
water
350 Mg2A16Fe0·3Si4O18(OH)2 + 183 H2O
(5)
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Fig. 10. Schreinmakers analyses for the five main phases (chlorite, quartz, kyanite, gedrite and talc) of the Orthoamphibole Whiteschist in the
MFASH system. It should be noted that the y-axis of the diagram represents the XMg whole-rock concentration of the whiteschist lithology and
not pressure. chl, chlorite; g, gedrite; hem, haematite; ky, kyanite; qtz, quartz; tc, talc.
the P–T range of which is constrained between 590 and
650°C at 13–21 kbar (Fig. 11) (Schreyer & Yoder, 1968;
Schreyer, 1977; Massonne, 1989; Fockenburg & Schreyer, 1991, 1993, 1994). The lack of mineral phases such
as pyrope, staurolite, enstatite, cordierite and kornerupine
indicates that metamorphic conditions did exceed the
yoderite stability field and hence temperatures did not
exceed 800°C at pressures between 7 and 20 kbar.
Summary of metamorphism
It is evident from the evaluation of the key metamorphic
assemblages that these whiteschists developed during a
metamorphic episode with peak P–T conditions between
13 and 15 kbar at temperatures >500°C but never
>800°C. All whiteschist reactions require input O2
and hence are all characterized by highly oxidizing
conditions.
DISCUSSION
Metasomatism, metamorphism and the
origin of the metasomatic fluid
Intuitively, as a result of the presence of excess O2 in the
metasomatic fluid and the highly oxidizing nature of all
the whiteschist reactions, it might seem more likely that
metasomatism occurred near the Earth’s surface, possibly
during sea-floor hydrothermal alteration rather than at
55 km depth. However, the evidence that high-pressure
metamorphism and metasomatism were intrinsically
linked is as follows:
(1) the quartz–haematite–kyanite band that surrounds
the yoderite-bearing lithology and other cross-cutting
veins are clearly metasomatic in origin. The presence of
primary haematite and Fe3+ in all Fe-bearing highpressure mineral phases indicates that the metasomatic
fluid must have had a high f O2.
(2) All Kadunguri Whiteschist lithologies contain
haematite and are dominated by highly oxidizing
reactions, thus requiring a fluid with high f O2 concentration.
(3) The presence of kyanite (rather than andalusite or
sillimanite) within the quartz–haematite–kyanite band
and other cross-cutting veins suggests that these crystallized from the metasomatic fluid at moderate to high
pressure, presumably similar to that of the whiteschistgrade (high-pressure, moderate-temperature) metamorphism.
(4) The orientation of the quartz–haematite–kyanite
band, parallel to the strain gradient, suggests that it was
also synchronous with deformation.
The presence of high f O2, Mg-rich fluids at great
depths (>55 km) has yet (as far as known to the authors)
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JOHNSON AND OLIVER
WHITESCHIST METAMORPHISM
might provide a source for the high f O2 in the deep
crust. However, the tectonic origin of this exotic fluid is
highly enigmatic and as such will not be speculated upon
further.
Age of tectonometamorphism
Fig. 11. Petrogenetic grid for the MFASH system showing the PT
stability of yoderite (from Fockenberg & Schreyer, 1994). The shaded
arrow is the estimated P–T path for the Yoderite Whiteschist [this
paper and Johnson & Oliver (1998)]. chl, chlorite; co, corundum; cord,
cordierite; en, enstatite; korn, kornerupine; ky, kyanite; py, pyrope; qtz,
quartz; st, staurolite; tc, talc; y, yoderite.
to be documented. The scarcity of whiteschist occurrences
in the geological record attests to the rarity of this exotic
fluid and hence the whiteschist-forming process. Of the
whiteschist occurrences that are known, the main examples, which include the Kokchetav Massif (Maruyama
& Parkinson, 2000; Parkinson, 2000), the Pamirs and
Hindu Kush (Grew et al., 1990; Hubbard et al., 1999;
Searle et al., 2001) and the Dora Maira Massif (Sharp et
al., 1993), are all associated with the deep subduction of
continental lithosphere. The production of such an exotic
high f O2 fluid may be the result of the deep, rapid
subduction and dehydration of hydrated, upper continental lithosphere. However, this does not explain the
high Mg content of the fluid, which is likely to result
from its interaction with a mafic component. Possibilities
for this include the serpentine-rich portions of the Ophiolite Terrane or a hidden greenstone belt similar to that
found in the Archaean sequences of the nearby Kalahari
Craton (Fig. 1). The latter also have significant deposits
of banded iron formations and if flakes of these have
somehow been subducted under the Chewores, they
It is interpreted that the peak of high-pressure metamorphism in the Kadunguri Whiteschists was synchronous with the development of S–L fabrics within both
the Foliated Whiteschists and the underlying Ophiolite
Terrane. Peak metamorphic conditions within garnetbearing meta-basalt lithologies of the Ophiolite Terrane
are calculated at 650–700°C at 10·5 ± 0·5 kbar ( Johnson
& Oliver, 1998; Johnson, 1999). Other lithologies from
the Chewore Inliers are interpreted to have formed
during similar high-pressure and moderate-temperature
P–T conditions (Goscombe et al., 1994, 1997, 1998,
2000). If the maximum pressure of the Kadunguri Whiteschists was between 13 and 15 kbar (>55 km depth)
then the Kadunguri Thrust represents a pressure gap of
2·5–4·5 kbar or a loss in vertical section of 9–17 km, and
could represent the major detachment surface along
which the whiteschists were exhumed. The progressive
nature of metasomatic alteration from a similar parental
composition, random arrangement of high-pressure
phases (in the Orthoamphibole and Unfoliated Whiteschists), the lack of ductile fabrics between lithologies
and the similar peak P–T conditions suggest that the
whiteschists were exhumed as a coherent, undeformed
block structurally bounded by ductile thrusts or shear
zones. The parallelism and synchronous development of
the high-pressure fabrics at the margin of the Kadunguri
Whiteschist block with the Kadunguri Thrust, and the
lack of retrogression, suggest that peak metamorphism
occurred during exhumation.
The peak of this high-pressure, moderate-temperature
metamorphism has been dated in the Chewore Inliers by
Goscombe et al. (1997, 1998, 2000). They have obtained
SHRIMP ages of 526 ± 17 Ma for zircon overgrowths
in granulites from the Granulite Terrane some 38 km
further to the north, and have interpreted this to represent
peak metamorphism during the high-pressure [M2 event
of Goscombe et al. (1997, 1998, 2000)] event. Preliminary
SHRIMP analyses of very low U/Th metamorphic zircon
overgrowths on older zircon cores separated from the
Kadunguri Whiteschists (S. P. Johnson, unpublished data,
2001) give an upper estimate of 590 ± 20 Ma. The bulk
of this data is discordant but suggests slightly younger
ages that are in the region of 520–550 Ma. This indicates
that the high-pressure whiteschist metamorphism and
synchronous exhumation occurred within a similar time
frame to that indicated by Goscombe et al. (1997, 1998,
2000), of between >550 and 520 Ma.
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Regional implications
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The Kadunguri Whiteschists are direct evidence for
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ACKNOWLEDGEMENTS
This research was funded by a scholarship to St. Andrews
University, grants from the Irvine and Welch bequests
to St. Andrews and as part of an IREX fellowship at the
University of Western Australia. Thanks are given to A.
Calder, who carried out the XRF major and trace
element analyses, and D. Herd, who supervised the
microprobe analyses. Professors C. Powell and B. Windley
are thanked for their helpful discussions, and we also
thank Geoff Clark and an anonymous reviewer, who
greatly improved the manuscript. Special thanks are
given to the Research Council and National Parks of
Zimbabwe for appropriate permits, and to the University
of Zimbabwe for logistical support. This is a contribution
to IGCP 418. TSRC manuscript number 155.
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