JOURNAL OF PETROLOGY
VOLUME 54
NUMBER 1
PAGES 41^74
2013
doi:10.1093/petrology/egs062
Fluid-induced Dehydration of the Paleoarchean
Sand River Biotite^Hornblende Gneiss, Central
Zone, Limpopo Complex, South Africa
H. M. RAJESH1*, G. A. BELYANIN1, O. G. SAFONOV1,2,3,
E. I. KOVALEVA3,4, M. A. GOLUNOVA2 AND D. D. VAN REENEN1
1
DEPARTMENT OF GEOLOGY, UNIVERSITY OF JOHANNESBURG, AUCKLAND PARK, JOHANNESBURG 2006,
SOUTH AFRICA
2
INSTITUTE OF EXPERIMENTAL MINERALOGY RAA, CHERNOGOLOVKA, MOSCOW REGION, RUSSIA
3
DEPARTMENT OF PETROLOGY, MOSCOW STATE UNIVERSITY, MOSCOW, RUSSIA
4
LABOR FU«R GEOCHRONOLOGIE, DEPARTMENT FU«R LITHOSPHA«RENFORSCHUNG, UNIVERSITA«T WIEN, AUSTRIA
RECEIVED NOVEMBER 18, 2011; ACCEPTED AUGUST 20, 2012
ADVANCE ACCESS PUBLICATION SEPTEMBER 29, 2012
A clear case study of local-scale, fluid-induced dehydration of the
Paleoarchean Sand River biotite^hornblende gneiss from the
Central Zone of the Limpopo Complex is presented here. Field and
petrographic examination of three adjacent zonesçdarker Sand
River orthogneiss with local occurrence of orthopyroxene and
clinopyroxene, a lighter intermediate gneissic zone with more orthopyroxene than the Sand River orthogneiss, and tonalitic veins
containing large orthopyroxene-bearing patchesçindicates the local
transformation of a light grey, fine- to medium-grained, hornblende^biotite gneiss into a greenish brown, medium- to coarsegrained orthopyroxene-bearing dehydration zone. Field evidence
indicates that the tonalitic veins were emplaced in discrete ductile
shear zones, with development of large orthopyroxene-bearing patches
in a sigmoidally transposed foliation bounded by shear planes.
Orthopyroxene-forming reaction textures after biotite and amphibole
together with the occurrence of microveins of K-feldspar along
quartz^plagioclase grain boundaries in the three adjacent zones,
and the higher modal abundance of orthopyroxene and K-feldspar
with lesser biotite and amphibole from the Sand River orthogneiss
to the intermediate gneissic zone to the orthopyroxene-bearing
patches, indicate that the three adjacent zones represent progressive
stages of the dehydration process. Such K-feldspar microveins along
quartz^plagioclase grain boundaries have been proposed as evidence
for the presence and passage of a low H2O activity fluid. Further,
the occurrence of monazite inclusions in fluorapatite in
orthopyroxene-bearing zones suggests dissolution and reprecipitation
involving a free fluid phase. Fluid inclusion studies indicate the presence of a fluid with CO2, NaCl and H2O components, with higher
salinity of the fluid (up to 29% NaCl) in the orthopyroxene-bearing
patches relative to the intermediate gneissic zone. The increase in Cl
content in amphibole, biotite and fluorapatite from the Sand River
orthogneiss to the orthopyroxene-bearing patches supports the presence of a Cl-rich brine fraction in the fluid responsible for the dehydration process. Further, the increase in An content of plagioclase at
the contact with the K-feldspar rims on quartz reflects an increase
in potassium activity in the fluid. The whole-rock major, trace and
rare-earth element enrichment or depletion patterns of the
orthopyroxene-bearing zones relative to the precursor support the
dehydration process.The diffuse contact relationship of a granite pegmatite occurring in the vicinity of the dehydration zones, together
with fluid inclusion and whole-rock major, trace and rare element
characteristics of samples collected along a traverse from the granite
pegmatite to the Sand River orthogneiss, suggests a scenario in
which the dehydrating fluids derived from an external source utilized
lithological contrasts, such as the gneiss^pegmatite boundaries, as
fluid conduits. Dehydration of the gneissic wall-rock occurred where
permeability was sufficient for fluid penetration. The occurrence of
orthopyroxene-bearing tonalitic veins along deformation-transposed
foliation planes further attests to a structural control to the channeling of the dehydrating fluids.
*Corresponding author. Fax: 27-11-559-4702. E-mail: hmrajesh@
uj.ac.za
ß The Author 2012. Published by Oxford University Press. All
rights reserved. For Permissions, please e-mail: journals.permissions@
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JOURNAL OF PETROLOGY
VOLUME 54
KEY WORDS: orthopyroxene-bearing patches; CO2-rich fluid with additional Cl-rich brine and H2O; fluid-induced dehydration; fluid infiltration along deformation-transposed foliation planes; Paleoarchean
Sand River biotite^hornblende gneiss; Limpopo Complex
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metasomatism either preceding or accompanying anatexis
(e.g. Sta«hle et al., 1987; Pattison, 1991; Hansen & Stuk,
1993; Perchuk et al., 2000; Blattner, 2005; Hansen &
Harlov, 2009).
This study picks up on this interesting debate by focusing on local-scale orthopyroxene-bearing patches within
the Paleoarchean migmatitic Sand River biotite^hornblende gneiss from the Central Zone of the Limpopo
Complex in South Africa. Based on a comprehensive
field, petrographic, mineral chemical, fluid inclusion and
geochemical characterization of the dehydration zones
and adjacent rocks, including a granite pegmatite in the
vicinity of the dehydration zones, compelling evidence is
provided for the involvement of an immiscible CO2- and
H2O-bearing Cl-rich brine in the dehydration process at a
local scale.
I N T RO D U C T I O N
Orthopyroxene-bearing, dark green metamorphic dehydration zones (popularly known as incipient charnockites)
occurring in amphibolite-facies rocks were first described
from Kabbaldurga, Karnataka, India by Pichamuthu
(1960). Such localized dehydration zones, occurring over a
scale of centimeters to a few meters, have been subsequently documented in various amphibolite- to granulitefacies terranes around the world (Newton et al., 1980; Santosh, 1986; Hansen et al., 1987; Sta«hle et al., 1987;
McLelland et al., 1988; Todd & Evans, 1994; Knudsen &
Lidwin, 1996; Van den Kerkhof & Grantham, 1999;
Perchuk et al., 2000; Harlov & Fo«rster, 2002; Rajesh, 2004;
Ravindra Kumar, 2004; Harlov et al., 2006). Although studies on dehydration zones have clearly shown that, depending on the composition of the protolith, the metamorphic
dehydration reaction involves the breakdown of hornblende, biotite or garnet, with the appearance of orthopyroxene and/or clinopyroxene, the mechanism of
dehydration continues to be a matter of debate. The
debate is polarized between a fluid-absent dehydration
melting alternative, in which hydrous minerals such as biotite and hornblende melt incongruently to form orthopyroxene and a H2O-bearing melt (Brown & Fyfe, 1970; Fyfe,
1973; Powell, 1983; Lamb & Valley, 1984; Waters, 1988;
Thompson, 1990; Clemens, 1992), and a fluid-present alternative, in which they are a product of dehydration by an
immiscible low-aH2O fluid (Newton, 1992) that is either
CO2-rich (e.g. Touret, 1971) or a Na and K concentrated
Cl-rich brine (e.g. Newton et al., 1998) or both as coexisting
immiscible fluids (e.g. Perchuk & Gerya, 1993; Touret &
Huizenga, 1999).
In most anatectic terranes, low-aH2O fluid-controlled
dehydration reactions do not seem to have been important
in the metamorphism, with fluid-absent melting widely
held as the dominant process responsible for stabilizing
granulite-facies mineral assemblages (e.g. Fyfe, 1973;
Powell, 1983; Lamb & Valley 1984; Waters, 1988; Thompson, 1990; Clemens, 1992; Brown, 1994; Stevens et al., 1997;
Daczko et al., 2001; Guernina & Sawyer, 2003; Clarke
et al., 2007). These different studies have contended that
the properties of granitic magmas imply that the generation of orthopyroxene-bearing patches in migmatitic
gneisses occurred in the absence of excess pervasive fluid.
However, several studies have argued, using mass-balance
calculations based on whole-rock chemical analyses, supported by mineral assemblages and/or structural evidence,
that in some orthopyroxene-bearing migmatites element
transport by a fluid phase played an important role, for
GEOLOGIC A L S ET T I NG
The Limpopo Complex, situated between the Archean
Kaapvaal Craton to the south and the Zimbabwe Craton
to the north (Fig. 1), is a high-grade metamorphic province
that has an exposed strike length of about 700 km and a
width of about 200 km. It is subdivided into three zones,
the Northern Marginal Zone, Central Zone and Southern
Marginal Zone, which are separated from each other and
also from the surrounding cratons by prominent shear
zones (Van Reenen et al., 1992; Kramers et al., 2006). The
Northern and Southern Marginal Zones, which are composed of reworked rocks of the respective cratons (Kreissig
et al., 2000; Blenkinsop et al., 2004), are thrust onto the adjacent cratons along 410 km wide, steep, inward-dipping,
2·65 Ga shear zones, the North Limpopo Thrust Zone
and the Hout River Shear Zone, respectively (Roering
et al., 1992; Smit et al., 1992; Blenkinsop et al., 1995). The
Central Zone is separated from the marginal zones by
reactivated 2·0 Ga crustal-scale SW^NE-trending shear
zones, the Triangle Shear Zone in the north and the
Palala Shear Zone in the south (McCourt & Vearncombe,
1992; Kamber et al., 1995a; Schaller et al., 1999; Kreissig
et al., 2001; Smit et al., 2001).
Available geochronological, structural and petrological
data have been interpreted by various researchers to indicate that the Central Zone was affected by at least three
high-grade tectono-metamorphic events at 3·2 Ga,
2·6^2·5 Ga and 2·0 Ga (Watkeys et al., 1983; Armstrong
et al., 1988; Barton & Sergeev, 1997; Jaeckel et al., 1997;
Holzer et al., 1998, 1999; McCourt & Armstrong, 1998;
Kro«ner et al., 1999; Schaller et al., 1999; Buick et al., 2003,
2006; Van Reenen et al., 2004, 2008; Zeh et al., 2007, 2010;
Millonig et al., 2008, 2010; Gerdes & Zeh, 2009). In
contrast, both the Southern and Northern Marginal
Zones are affected by a Neoarchean high-grade tectonometamorphic event within the same time interval
(2·7^2·6 Ga; Barton et al., 1983; Berger et al., 1995;
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RAJESH et al.
DEHYDRATION OF SAND RIVER GNEISS
Fig. 1. Generalized geological map of the Limpopo Complex overlain on a TM742 (as RGB) satellite image, showing the three sub-zones
(NMZ, Northern Marginal Zone; CZ, Central Zone; SMZ, Southern Marginal Zone), adjacent Archean cratons and the Causeway locality,
SE of Musina. Inset is a TM742 compilation for southern Africa showing the location of the Limpopo Complex with respect to the Kaapvaal
Craton to the south and the Zimbabwe Craton to the north. The prominent shear zones (NMTZ, Northern Marginal Thrust Zone; LMSZ,
Lethlakane^Magagapathe Shear Zone; TSZ, Triangle Shear Zone; SSZ, Sunnyside Shear Zone; TsSZ, Tshipise Straightening Zone; HRSZ,
Hout River Shear Zone), which separate the three sub-zones of Limpopo Complex from each other and from the adjacent Archean cratons,
are also shown. The TM742 band combination uses false colors to separate soil and different lithologies from vegetation (shades of green) and
water (dark blue to black). The violet-colored Tuli basin is mainly filled with rocks of the Karoo Supergroup. The blue-colored Sua Pan is one
of the three large salt pans within the Makgadikgadi region of Botswana. Soda ash (sodium carbonate) and salt are mined from the pan.
intruding into the deformed and metamorphosed Beit
Bridge Complex rocks, and considered to be the product
of crustal anatexis (Singelele-type gneisses; Fripp et al.,
1979; Watkeys et al., 1983). The weakly deformed Bulai
pluton, exposed NW of Musina (Fig. 1), with an 2·6 Ga
main porphyroblastic granitic phase (Millonig et al.,
2008), is an important time marker as it intrudes migmatitic gneisses of the Beit Bridge Complex, thus signifying
the end of the Neoarchean high-grade tectonometamorphic event that affected the Central Zone (Van
Reenen et al., 2008). The final magmatic event that affected
the Central Zone at 2·01 Ga (Jaeckel et al., 1997; Holzer
et al., 1998; Zeh et al., 2007; Millonig et al., 2010) is reflected
by completely undeformed granitic melt patches and veins
that obliterate the earlier gneissic fabric of the rocks from
within which they were generated, and undeformed intrusive granitic dykes and granite pegmatites. The 2·01 Ga
time frame coincides with the reactivation of the
crustal-scale SW^NE-trending shear zones, both those
Kamber et al., 1995b; Mkweli et al., 1995; Rollinson & Blenkinsop, 1995; Frei et al., 1999; Kreissig et al., 2001;
Blenkinsop et al., 2004; Zeh et al., 2009), with records of
Paleoproterozic reworking from the southern part of the
Northern Marginal Zone (Kamber et al., 1995a).
The Central Zone, occupying the largest area of the
three sub-zones of the Limpopo Complex (Fig. 1), comprises a variety of deformed supracrustal rocks (c. 3·3^2·6
Ga), collectively referred to as the Beit Bridge Complex,
associated with mafic to ultramafic rocks of the Musina
layered intrusion (3·3^3·1 Ga), and orthogneisses, including the c. 3·3^3·2 Ga banded and migmatitic Sand River
tonalite^trondhjemite^granodiorite gneisses and various c.
2·7^2·5 Ga granitoid gneisses (e.g. Alldays, Zanzibar,
Verbaard, Regina gneisses) (Brandl, 1983, 1992; Fripp,
1983; Horrocks, 1983; Watkeys et al., 1983; Van Reenen
et al., 1992; Barton, 1996; Kramers et al., 2006). The
2·7^2·5 Ga group also includes a variety of homogeneous
non-banded garnet-bearing quartzofeldspathic gneisses,
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JOURNAL OF PETROLOGY
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within the Central Zone and those that separate the
Central Zone from the marginal zones, with the movement
represented by transpressive strike-slip displacement
(McCourt & Vearncombe, 1992; Kamber et al., 1995a;
Scha«ller et al., 1999; Kreissig et al., 2001; Smit et al., 2001).
The orthopyroxene-bearing patches studied here occur
in the Sand River biotite^hornblende gneiss exposed at
the Causeway locality, SE of Musina (Fig. 1), within the
Central Zone.
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at 2·01Ga (Jaeckel et al., 1997). The local presence of
orthopyroxene-bearing patches within the Sand River
orthogneiss has been reported by various workers (e.g.
Jaeckel et al., 1997; Kro«ner et al., 1999), and Jaeckel et al.
(1997) reported a metamorphic zircon age of 2·01Ga for
the development of these orthopyroxene-bearing patches.
The Sand River orthogneiss is locally cut by deformed
and/or metamorphosed older mafic dykes (3·06 Ga;
Barton et al., 1990; Fig. 2f), undeformed younger mafic
dykes (Fig. 2g) and biotite-bearing granite pegmatites
(2·01Ga; Jaeckel et al., 1997; Fig. 2h).
SA N D R I V E R G N E I S S A N D OT H E R
RO C K T Y P E S E X P O S E D AT T H E
C AU S E WAY L O C A L I T Y
F I E L D A N D P E T RO G R A P H I C
C H A R AC T E R I S T I C S O F T H E
D E H Y D R AT I O N Z O N E S A N D
A DJ AC E N T RO C K S
The Sand River gneiss exposed in rock pavements at the
Causeway locality is generally well-banded, migmatitic
and interlayered with the Beit Bridge Complex paragneisses (Fig. 2a). The intrusive relation of the Sand River
gneiss with the Beit Bridge Complex supracrustal rocks
and rocks of the Musina layered intrusion is evident from
the presence of inclusions of the latter in the former
(Fig. 2b and c). Based on U^Pb zircon ages obtained from
the Causeway locality, Retief et al. (1990), Tsunogae &
Yurimoto (1995), Jaeckel et al. (1997), Kro«ner et al. (1998,
1999), Zeh et al. (2007) and Gerdes & Zeh (2009) constrained the magmatic crystallization age of the Sand
River gneiss at 3·31^3·28 Ga. Various studies showed
that the Sand River orthogneiss preserves evidence
(zircon overgrowths) for two anatectic eventsça dominant one at 2·64^2·61Ga and a minor one at 2·01Ga
(Retief et al., 1990; Jaeckel et al., 1997; Kro«ner et al., 1999;
Zeh et al., 2007, 2010; Gerdes & Zeh, 2009). A third
high-grade event at 3·14 Ga is probable, but is constrained only by Lu^Hf and U^Pb analyses from a single
zircon domain (Zeh et al., 2007).
The Neoarchean tectono-metamorphic event in the
Sand River orthogneiss correlates well with meter-sized
bodies of Neoarchean (2·68^2·57 Ga; Jaeckel et al., 1997;
Kro«ner et al., 1999; Zeh et al., 2007; Van Reenen et al.,
2008) garnet-bearing Singelele-type gneiss, considered as
products of local anatexis (Hoffmann et al., 1998), and
occurring interlayered and interfolded with Beit Bridge
Complex rocks (Fig. 2d). The Neoarchean overprint ages
have been reported from exposures within the Causeway
locality where the Singelele-type gneiss intrudes the Sand
River orthogneiss (e.g. Fig. 2e).
Both the garnet-bearing Singelele-type gneiss and the
supracrustal Beit Bridge Complex rocks exposed at
the Causeway locality have been overprinted by the
Paleoproterozoic tectono-metamorphic event, as suggested
by the presence of 2·01Ga granitic melt patches and
veins (Jaeckel et al., 1997). Such undeformed granitic
patches and veins also occur in the Sand River orthogneiss, and single magmatic zircons from these were dated
The exposure studied here is representative of the localscale dehydration in the Causeway locality and is characterized by three adjacent zones: the darker Sand River
biotite^hornblende gneiss with local occurrence of orthopyroxene and clinopyroxene (A in Fig. 3), the lighter
intermediate gneissic zone with more widespread orthopyroxene than the Sand River orthogneiss (B in Fig. 3),
and the tonalitic veins with large orthopyroxene-bearing
patches (C in Fig. 3). Representative samples were collected
from the three adjacent zones.
The Sand River orthogneiss (represented by GAB-7
series samples; GAB-7a, b and c; A in Fig. 3) is a granoblastic rock with local evidence of a weak gneissic fabric, indicated by elongated amphibole and biotite. It is fine- to
medium-grained and dominantly made up of plagioclase,
quartz, biotite and amphibole, with accessory magnetite,
ilmenite, allanite, chlorite, zircon and fluorapatite
(Fig. 4a). Amphibole and biotite commonly occur together,
with the former characterized by the occurrence of inclusions of quartz, plagioclase, zircon and magnetite^ilmenite. The rare occurrence of K-feldspar as thin rims along
quartz^plagioclase grain boundaries and around quartz,
biotite and amphibole (Fig. 4b), and rare orthopyroxene
and clinopyroxene developing on biotite and amphibole
(Fig. 4c), observed in one of the samples (GAB-7c), reflects
evidence of a local dehydration process. The latter
texture (Fig. 4c) indicates the dehydration reactions
hornblende þ quartz ¼ clinopyroxene þ orthopyroxene þ
plagioclase þ K-feldspar þ H2O and biotite þ quartz ¼
orthopyroxene þ K-feldspar þ H2O. Secondary amphibole
occurs along the margin of some of the orthopyroxene
grains (Fig. 4c).
The lighter intermediate gneissic zone (represented
by GAB-8 series samples; GAB-8a, b and c; B in Fig. 3)
is more leucocratic than the Sand River orthogneiss,
with a prominent gneissic fabric characterized by distinct
biotite^orthopyroxene-rich layers alternating with
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RAJESH et al.
DEHYDRATION OF SAND RIVER GNEISS
Fig. 2. Field photographs from the Causeway locality illustrating the well-banded, migmatitic nature of the Sand River orthogneiss (a); inclusion of Beit Bridge Complex supracrustal gneiss (b) and meta-anorthosite of the Musina layered intrusion (c) in the Sand River orthogneiss;
garnet-bearing Singelele-type gneiss interlayered with Beit Bridge Complex rocks (d); inclusion of Beit Bridge Complex supracrustal gneiss
within the Singelele-type gneiss (e); deformed and metamorphosed mafic dyke (f) in the Sand River orthogneiss; dolerite dyke (g) in the Sand
River orthogneiss; biotite-bearing granite pegmatite (h) in the Sand River orthogneiss. The orthopyroxene-bearing dehydration zones studied
here occur not far from the granitic pegmatite in (h).
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Fig. 3. Field photograph illustrating the exposure studied here, which is representative of the local-scale dehydration in the Sand River orthogneiss at the Causeway locality. A, B and C indicate the approximate locations where representative samples were obtained from the Sand
River orthogneiss, intermediate gneissic zone and the tonalitic veins containing orthopyroxene-bearing patches respectively. The pocket knife
in the image is 11cm long.
1969) was observed in some of the plagioclase. In places
secondary amphibole replaces orthopyroxene, and biotite^quartz symplectite occurs between biotite and orthopyroxene (Fig. 4f), indicating the late hydration reaction
orthopyroxene þ K-feldspar þ H2O ¼ biotite þ quartz. No
amphibole- and/or biotite-rich domains were found to surround the orthopyroxene-rich domains.
The 3^6 cm wide tonalitic veins (C in Fig. 3) trace a
discrete ductile shear deformation system, with the older
gneissic foliation sigmoidally transposed to the new direction bounded by shear planes. In contrast to the diffuse
margins of the intermediate gneissic zone and the Sand
River orthogneiss, the tonalitic veins have sharper margins
with the Sand River orthogneiss and less sharp margins
with the intermediate gneissic zone. Large greenish brown
orthopyroxene-bearing patches are recognizable as dark
quartzo-feldspathic layers. The contact between the intermediate gneissic zone and the Sand River orthogneiss is
gradational. The intermediate gneissic zone is medium
grained and dominantly made up of quartz, plagioclase,
biotite, orthopyroxene and K-feldspar, with accessory
magnetite, ilmenite, allanite, zircon and fluorapatite. The
progression of dehydration is indicated by the higher
modal abundance of orthopyroxene and K-feldspar with
lesser biotite and amphibole in the intermediate gneissic
zone relative to the Sand River orthogneiss (Table 1), the
occurrence of K-feldspar as microveins along quartz^
plagioclase grain boundaries (Fig. 4d), and the development of orthopyroxene and K-feldspar on biotite and
quartz (Fig. 4e and f). The latter texture indicates the
dehydration reaction biotite þ quartz ¼ orthopyroxene þ
K-feldspar þ H2O. Replacement antiperthite (Griffin,
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RAJESH et al.
DEHYDRATION OF SAND RIVER GNEISS
Fig. 4. Representative back-scattered electron (BSE) images illustrating the mineral assemblages and textures within samples from the
three adjacent zones. (a^c) Sand River orthogneiss (a, GAB-7a; b, c, GAB-7c); (d^f) intermediate gneissic zone (d, e, GAB-8a; f, GAB-8b);
(g^o) orthopyroxene-bearing patches within the tonalitic veins (g, k, m, GAB-6a; h, i, l, GAB-6b; j, n, o, GAB-6c). Pl, plagioclase; Kfs,
K-feldspar; Qtz, quartz; Amp, amphibole; Bt, biotite; Opx, orthopyroxene; Cpx, clinopyroxene; F-Ap, fluorapatite; Mt, magnetite; Ilm, ilmenite; Zrn, zircon; Cm, cummingtonite; Chl, chlorite. (See text for details.)
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Fig. 4. (Continued)
are coarse-grained, and are dominantly made up of
quartz, plagioclase, biotite, orthopyroxene and K-feldspar,
with accessory magnetite, ilmenite, amphibole, zircon,
monazite and fluorapatite. In comparison with the Sand
patches against the lighter-colored tonalitic veins (C in
Fig. 3). These greenish brown patches (represented by
GAB-6 series samples; GAB-6a, b and c; C in Fig. 3)
occur along the center and length of the tonalitic veins,
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RAJESH et al.
DEHYDRATION OF SAND RIVER GNEISS
Table 1: Modal composition of the three adjacent zones
based on representative samples
Sand River
Sand River
Intermediate
orthogneiss
orthogneiss
gneissic zone
Tonalitic vein with
orthopyroxenebearing patches
GAB-7a
GAB-7c
GAB-8a
GAB-6a
Plagioclase
51
41
44
45
Quartz
15
15
26
21
9
6
10
Amphibole
13
10
4
3
Biotite
19
12
7
5
4
9
11
4
5
K-feldspar
Orthopyroxene
Clinopyroxene
Fluorapatite
7
2
2
orthopyroxene, and is often altered (Fig. 4g). No biotite
selvages were observed to rim the large orthopyroxene
grains. Several features of the orthopyroxene-bearing
greenish brown patches point to a continuation of the dehydration process observed in the intermediate gneissic
zone, namely the occurrence of K-feldspar as microveins
along plagioclase^plagioclase and along quartz^plagioclase grain boundaries (Fig. 4h), lesser modal abundance
of biotite and amphibole with higher orthopyroxene and
K-feldspar relative to the gneissic zone (Table 1), and occurrence of large orthopyroxene grains with inclusions of
amphibole, plagioclase, quartz and biotite (Figs. 4i^k).
The latter textures indicate the dehydration reaction hornblende þ biotite þ plagioclase þ quartz ¼ orthopyroxene þ
K-feldspar þ H2O. Replacement antiperthite was observed
in some of the plagioclase (Fig. 4l). Orthopyroxene shows
replacement texture involving secondary amphibole
(Fig. 4m). The orthopyroxene-bearing greenish brown
patchy domains are surrounded by monzonitic (biotite þ
plagioclase þ K-feldspar þ quartz) domains (Fig. 4n), with
plagioclase dominating K-feldspar. Myrmekites are locally
developed along the contacts of plagioclase with
K-feldspar in these domains (Fig. 4o). These two domainsçthe dominant orthopyroxene-bearing patchy
domain and the relatively minor biotite-bearing monzonitic domainçcan be observed along the length of the
tonalitic veins. Field and petrographic examination clearly
indicates that the minor monzonitic domain is late with respect to the dominant orthopyroxene-bearing patchy
domain.
The dehydration zones studied in detail here occur not
far (3 m) from a large biotite-bearing granite pegmatite.
The contact relation between the pegmatite and host
Sand River orthogneiss is not uniform, with discrete and
unaltered margins in places, and a relatively diffuse
Fig. 4. (Continued)
River orthogneiss and the intermediate gneissic zone,
orthopyroxene-bearing greenish brown patches are more
altered, as indicated by the presence of secondary minerals
calcite, chlorite, allanite and sericite. Biotite occurs as
large grains and as inclusions in magnetite and
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metasomatic margin in other places where patchy irregular granitic material from the pegmatite makes its way
into the surrounding gneiss along foliation planes. The
fuzzy outliers near the diffuse margin preserve the faint
gneissic foliation of the host Sand River orthogneiss.
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content of amphibole increases from the Sand River
orthogneiss (0·02^0·06 wt %) to the orthopyroxenebearing patches (0·08^0·18 wt %) (Fig. 5e). The F content
of amphibole also shows a similar increasing trend (Sand
River orthogneiss 0·10^0·20 wt %; orthopyroxene-bearing
patches 0·03^0·27 wt %) (Fig. 5e). Orthopyroxene in the
three adjacent zones have similar XMg (Sand River
orthogneiss 0·51^0·52; intermediate gneissic zone 0·51^
0·54; orthopyroxene-bearing patches 0·51^0·54), whereas
the Al2O3 content is slightly higher in the orthopyroxenebearing patches relative to the gneissic rocks [XAl (Al/2)
is 0·009^0·016 in orthopyroxene-bearing patches and
0·007^0·012 in gneissic rocks]. Although orthopyroxene
does not show any regular zoning in the Mg and Fe content, at the contact with K-feldspar, the XAl of orthopyroxene decreases from the core to the rim (Fig. 5c), whereas
XMg increases slightly towards the rim (0·51^0·53).
M I N ER A L C H EM ICA L
C H A R AC T E R I S T I C S
Electron microprobe analyses of the various minerals in
representative samples from the three adjacent zones were
carried out using a CAMECA SX100, equipped with an
energy-dispersive spectrometry (EDS) detector, housed at
the central analytical facility (SPECTRAU) of the
University of Johannesburg. Analytical conditions of 15 kV
acceleration voltage, 20 nA beam current, counting times
of 10^20 s for most elements, except 50 s for F and 30 s for
Cl, and 1^5 mm beam spot size were used. Representative
mineral chemical data are reported in Tables 2 and 3.
The mole fraction of NaAlSi3O8 (XAb) of plagioclase
varies in the range 0·64^0·61 in the Sand River orthogneiss, 0·69^0·65 in the intermediate gneissic zone, and
0·68^0·63 in the orthopyroxene-bearing patches. Although
plagioclase does not show any regular zoning, adjacent to
K-feldspar rims on quartz or orthopyroxene, plagioclase
shows zoning with rims in all cases more Ca-rich than the
cores (Fig. 5a^c). Large biotite grains from the Sand
River orthogneiss and the intermediate gneissic zone have
similar compositions [XMg ¼ Mg/(Mg þ Fe) ¼ 0·46^0·48,
TiO2 ¼ 4·79^5·31wt % for the Sand River orthogneiss;
XMg ¼ 0·47^0·48, TiO2 ¼ 4·46^5·28 wt % for the intermediate gneissic zone], and are slightly different from
large biotite grains in the orthopyroxene-bearing patches
(XMg ¼ 0·51^0·55, TiO2 ¼ 3·73^3·98 wt %). Biotite occupying the symplectite (Fig. 4f) in the intermediate gneissic
zone has a higher XMg (0·55^0·57) and lower TiO2 (3·26^
3·71wt %) content. The three varieties of biotite occurring
within the orthopyroxene-bearing patches are characterized by different TiO2 contents (biotite inclusion in magnetite: 4·88 wt %; large biotite grains: 3·73^3·98 wt %;
altered biotite: 1·41^1·47 wt %). Significantly, the Cl content of large biotite grains increases from the Sand River
orthogneiss (0·04^0·06 wt %) through the intermediate
gneissic zone (0·06^0·10 wt %) to the orthopyroxenebearing patches (0·11^0·24 wt %), whereas the F content
decreases from the intermediate gneissic zone (0·31^
0·60 wt %; Sand River orthogneiss ¼ 0·23^0·45 wt %)
to the orthopyroxene-bearing patches (0·16^0·44 wt %)
(Fig. 5d).
Primary amphiboles in both the Sand River orthogneiss
(large grains: XMg ¼ 0·48^0·49, Al2O3 ¼ 9·81^10·28 wt
%) and the orthopyroxene-bearing patches (inclusions in
orthopyroxene: XMg ¼ 0·49^0·53, Al2O3 ¼ 8·86^10·97 wt
%) have similar compositions. As with the biotite, the Cl
M O N A Z I T E A N D F L U O R A PAT I T E
OCCU R R ENC E
The modal per cent of fluorapatite increases from the Sand
River orthogneiss to the intermediate gneissic zone to the
orthopyroxene-bearing patches, with the last containing
up to 5 vol. % of the mineral (Table 1). The size of fluorapatite grains is also larger in the intermediate gneissic
zone and the orthopyroxene-bearing patches compared
with the Sand River orthogneiss (Fig. 6a^i). The F content
in fluorapatite is similar in the three adjacent zones, with
the orthopyroxene-bearing patches characterized by the
highest Cl content (Table 3; Fig. 6j).
Monazite occurs in association with fluorapatite in the
three adjacent zones. Rare monazite inclusions occur in
fluorapatite within the Sand River orthogneiss (Fig. 6c).
This occurs in the same sample (GAB-7c) from the Sand
River orthogneiss in which both clinopyroxene and orthopyroxene were observed. Monazite occurs dominantly as
inclusions, with minor occurrences along rims of fluorapatite in both the intermediate gneissic zone and the
orthopyroxene-bearing patches (Figs. 6d^i). In elongated
fluorapatite grains, monazite forms elongate inclusions
parallel to the length of the host fluorapatite grain, with
their size larger in the orthopyroxene-bearing patches.
Relative to the orthopyroxene-bearing patches, monazite
rims are fewer in the intermediate gneissic zone, where allanite usually replaces monazite rims (Figs. 6d^i). No
monazite grain was observed along rim of fluorapatite
from the Sand River orthogneiss.
F L U I D C H A R AC T E R I S T I C S
Considering the occurrence of granite pegmatite not far
from the dehydration zones, a sampling traverse was carried out to understand the role of the pegmatite (if any)
in the dehydration process. Together with samples from
50
RAJESH et al.
DEHYDRATION OF SAND RIVER GNEISS
Table 2: Representative mineral chemical data for various minerals from the orthopyroxene-bearing patches in tonalitic
veins (GAB-6 series samples), intermediate gneissic zone (GAB-8 series samples), and the Sand River orthogneiss
(GAB-7 series samples)
Sample no.:
GAB-6a
GAB-6b
GAB-6c
GAB-6a
GAB-6b
GAB-6c
GAB-6a
GAB-6b
GAB-6c
GAB-6a
GAB-6b
GAB-6c
Mineral:
Opx
Opx
Opx
Bt
Bt
Bt
Amp
Amp
Amp
Pl
Pl
Pl
SiO2
50·22
50·55
50·78
37·07
36·71
36·97
41·74
43·64
42·95
60·85
61·35
TiO2
0·08
0·08
0·06
3·95
3·98
4·88
1·47
0·17
1·40
0·02
0·04
0·03
Al2O3
1·07
0·98
1·16
14·26
14·03
14·78
10·61
10·97
9·77
24·30
24·35
24·55
Cr2O3
60·91
0·01
0·02
0·02
0·03
0·06
0·04
0·04
0·03
0·07
0·02
0·01
0·00
FeO
28·23
28·15
28·76
18·98
19·67
17·89
18·58
17·93
18·12
0·36
0·12
0·18
MnO
1·77
1·87
1·7
0·27
0·2
0·31
0·50
0·38
0·34
0·00
0·02
0·00
MgO
16·87
16·50
16·75
12·17
11·63
11·23
9·80
10·63
10·32
0·01
0·00
0·02
CaO
0·77
0·84
0·77
0·03
0·04
0·02
11·25
11·14
11·40
6·94
6·68
6·61
Na2O
0·02
0·04
0·02
0·03
0·1
0·07
1·44
1·77
1·43
7·47
7·54
7·40
K2O
0·00
0·00
0·01
8·92
8·46
9·21
1·32
0·97
1·23
0·48
0·64
0·73
Cl
0·21
0·22
0·24
0·15
0·08
0·14
F
0·44
0·26
0·25
0·21
0·27
0·23
96·36
95·36
95·89
97·11
97·98
97·40
100·45
100·75
100·43
22
22
22
23
23
23
8
8
8
Total
99·04
99·03
100·03
O
6
6
6
Si
1·962
1·974
1·988
5·703
5·646
5·588
6·428
6·586
6·735
2·704
2·714
2·705
Ti
0·002
0·002
0·003
0·171
0·453
0·555
0·170
0·019
0·123
0·001
0·001
0·001
Al-total
0·049
0·045
0·034
2·521
2·521
2·633
1·926
1·951
1·598
1·273
1·270
1·285
Al(4)
0·038
0·026
0·012
2·297
2·354
2·412
1·572
1·414
1·265
Al(6)
0·011
0·019
0·021
0·224
0·167
0·221
0·353
0·537
0·333
Fe(iii)
0·034
0·006
0·000
0·000
0·000
0·000
Cr
0·000
0·001
0·002
0·005
0·001
0·005
0·005
0·004
0·002
0·001
0·000
0·000
Fe(ii)
0·886
0·912
0·918
2·507
2·447
2·261
2·393
2·263
2·199
0·013
0·004
0·007
Mn
0·059
0·062
0·073
0·058
0·034
0·040
0·065
0·049
0·064
0·000
0·001
0·000
Mg
0·983
0·961
0·947
3·108
2·672
2·531
2·250
2·392
2·464
0·001
0·000
0·001
Ca
0·032
0·035
0·029
0·023
0·005
0·003
1·856
1·801
1·861
0·330
0·317
0·314
Na
0·002
0·003
0·000
0·015
0·015
0·021
0·430
0·518
0·389
0·644
0·647
0·637
K
0·000
0·000
0·001
1·506
1·654
1·776
0·259
0·187
0·203
0·027
0·036
0·041
0·029
0·057
0·061
4·994
4·991
4·991
XCa
0·330
0·317
0·317
XK
0·027
0·036
0·042
Cl
F
0·088
0·078
0·119
Total
4·009
4·002
3·994
15·617
15·447
15·412
15·781
15·769
15·638
XMg
0·526
0·513
0·508
0·554
0·522
0·528
0·485
0·514
0·528
XAl
0·013
0·012
0·009
(continued)
shown in Figs 3 and 7, the darker Sand River orthogneiss
sample 173/1, which is farthest from the granite pegmatite,
is similar to the Sand River orthogneiss samples (GAB-7a
and b) from the three adjacent zones, with both preserving
the Sand River orthogneiss composition. The samples
173/2 to 173/6 from the metasomatic fuzzy outlier near the
margin of the granite pegmatite are more leucocratic
the intermediate gneissic zone and orthopyroxene-bearing
patches, seven samples (173/1 to 7; Fig. 7) along a traverse
from the contact of the granite pegmatite with the Sand
River orthogneiss were selected for fluid inclusion study.
Sample 173/7 was collected from the biotite-bearing granite
pegmatite, whereas 173/6 is nearest the pegmatite and
173/1 is the farthest in the Sand River orthogneiss. As
51
JOURNAL OF PETROLOGY
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NUMBER 1
JANUARY 2013
Table 2: Continued
Sample no.:
GAB-8a
GAB-8b
GAB-8c
GAB-8c
GAB-8
GAB-8a
GAB-8b
GAB-8a
GAB-8a
GAB-8b
GAB-8b
GAB-8a
GAB-8b
GAB-8c
Mineral:
Opx
Opx
Opx
Bt
Bt
Bt
Bt
Amp
Amp
Amp
Amp
Pl
Pl
Pl
SiO2
50·63
51·03
50·88
36·63
36·12
37·93
37·94
54·01
54·20
54·13
55·68
61·34
62·24
TiO2
0·05
0·04
0·05
5·28
4·67
3·71
3·42
0·12
0·16
0·12
0·10
0·03
0·02
0·01
Al2O3
0·86
0·77
0·76
13·98
13·73
14·43
14·33
2·41
2·89
2·42
1·74
24·44
24·36
23·93
Cr2O3
0·00
0·03
0·04
0·05
FeO
27·58
28·41
28·14
18·29
MnO
1·97
1·56
1·57
0·28
MgO
17·21
17·13
16·82
11·36
0·03
20·5
0·24
10·5
62·14
0·01
0·05
0·00
0·03
0·00
0·03
0·01
0·02
0·00
18·98
18·15
12·08
11·99
11·16
10·97
0·07
0·11
0·13
0·29
0·2
0·68
0·66
0·65
0·63
0·00
0·00
0·00
12·74
13·15
16·72
16·48
17·24
17·55
0·00
0·01
0·01
CaO
0·69
0·61
0·6
0·05
0·02
0·02
0·01
11·84
12·08
12·08
12·01
6·18
6·04
5·95
Na2O
0·03
0·02
0·01
0·09
0·1
0·04
0·05
0·15
0·13
0·09
0·07
7·95
8·01
8·08
K2O
0·00
0
0·01
9·27
9·43
7·5
8·32
0·13
0·17
0·12
0·09
0·52
0·66
0·77
0·06
0·1
0·07
0·09
0·02
0·01
0·00
0·02
101·47
101·02
Cl
F
Total
99·02
99·60
98·88
0·37
0·37
0·45
0·6
0·15
0·13
0·13
0·17
95·71
95·81
96·17
96·31
98·31
98·93
98·14
99·06
100·54
O
6·000
6·000
6·000
22·000
22·000
22·000
22·000
23·000
23·000
23·000
23·000
8·000
8·000
8·000
Si
1·972
1·978
1·985
5·583
5·567
5·678
5·688
7·717
7·690
7·713
7·833
2·716
2·730
2·740
Ti
0·001
0·001
0·001
0·518
0·541
0·418
0·386
0·013
0·017
0·013
0·011
0·001
0·001
0·000
0·406
0·483
0·406
0·288
1·276
1·259
1·244
0·283
0·310
0·287
0·167
Al-total
Al(4)
0·028
0·022
0·015
2·417
2·433
2·322
2·312
Al(6)
0·012
0·013
0·020
0·111
0·060
0·224
0·220
Fe(iii)
0·022
0·011
0·000
Cr
0·000
0·001
0·001
0·001
0·004
0·001
Fe(ii)
0·874
0·909
0·919
2·622
2·642
Mn
0·065
0·051
0·052
0·025
Mg
1·000
0·990
0·978
2·399
Ca
0·029
0·025
0·025
Na
0·002
0·002
K
0·000
0·000
0·123
0·173
0·120
0·122
0·000
0·000
0·000
0·000
0·006
0·000
0·003
0·000
0·003
0·000
0·001
0·000
2·376
2·275
1·443
1·422
1·330
1·291
0·003
0·004
0·005
0·031
0·037
0·025
0·082
0·079
0·078
0·075
0·000
0·000
0·000
2·412
2·843
2·939
3·561
3·486
3·662
3·681
0·000
0·001
0·001
0·002
0·003
0·003
0·002
1·812
1·836
1·844
1·810
0·293
0·284
0·281
0·001
0·021
0·030
0·012
0·015
0·042
0·036
0·025
0·019
0·683
0·681
0·691
0·000
1·869
1·854
1·432
1·591
0·024
0·031
0·022
0·016
0·029
0·037
0·043
0·026
0·026
0·018
0·023
5·001
4·998
5·005
XCa
0·292
0·283
0·277
XK
0·029
0·037
0·043
Cl
F
0·156
0·180
0·213
0·284
Total
4·006
4·003
3·997
15·568
15·574
15·344
15·452
15·100
15·083
15·094
15·028
XMg
0·533
0·521
0·516
0·478
0·477
0·545
0·564
0·700
0·699
0·722
0·729
XAl
0·007
0·006
0·004
(continued)
petrography involving the documentation of the nature of
occurrence of inclusions, their distribution pattern, size,
shape and phase categories was carried out under a petrographic microscope. Phase transitions in fluid inclusions
were investigated using a LINKAM THMSG 600 heating^
freezing stage cooled with liquid nitrogen and calibrated
using a set of synthetic fluid inclusion standards. The accuracy of the thermometric measurements is about 0·38C.
than the darker Sand River orthogneiss sample 173/1, and
are similar to the intermediate gneissic zone samples
(GAB-8a, b and c) from the three adjacent zones. The leucocratic portions, represented by samples 173/2 to 173/6,
are concordant with the gneissic fabric observed in the
darker Sand River orthogneiss.
For fluid inclusion studies, doubly polished sections
(150^200 mm thick) were prepared. Fluid inclusion
52
RAJESH et al.
DEHYDRATION OF SAND RIVER GNEISS
Table 2: Continued
Sample no.:
GAB-7a
GAB-7b
GAB-7b
GAB-7a
GAB-7a
GAB-7b
GAB-7a
GAB-7a
GAB-7a
Mineral:
Bt
Bt
Bt
Amp
Amp
Amp
Pl
Pl
Pl
SiO2
36·22
36·18
36·28
41·74
43·07
42·15
59·99
60·56
TiO2
5·31
5·26
5·21
1·88
1·63
1·99
0·02
0·00
0·00
Al2O3
13·75
13·72
13·72
9·96
10·15
9·87
24·87
24·72
24·51
0·05
0·01
0·05
0·01
0·03
0·00
FeO
20·68
20·37
20·44
18·38
18·06
18·28
0·11
0·13
0·13
MnO
0·18
0·23
0·19
0·44
0·42
0·42
0·01
0·00
0·00
MgO
10·54
10·60
10·62
9·54
9·65
9·67
0·00
0·01
0·00
CaO
0·01
0·01
0·00
11·49
11·60
7·38
7·04
6·99
Na2O
0·06
0·05
0·03
1·39
1·06
1·35
7·21
7·45
7·38
K2O
9·56
9·64
9·70
1·33
1·31
1·38
0·54
0·58
0·63
Cl
0·06
0·05
0·04
0·04
0·02
0·04
100·14
100·52
100·15
Cr2O3
F
11·4
60·51
0·32
0·36
0·30
0·18
0·17
0·2
Total
96·69
96·47
96·53
96·42
97·15
96·80
O
22·000
22·000
22·000
23·000
23·000
23·000
8·000
8·000
Si
5·529
5·568
5·543
6·467
6·574
6·495
2·676
2·690
2·697
Ti
0·610
0·583
0·599
0·219
0·187
0·231
0·001
0·000
0·000
1·819
1·826
1·792
1·308
1·294
1·287
Al(4)
2·471
2·432
2·457
1·533
1·426
1·505
Al(6)
0·003
0·015
0·014
0·000
Al-total
0·286
0·400
0·287
Fe(iii)
0·000
0·000
0·000
Cr
0·006
0·001
8·000
0·006
0·000
0·001
Fe(ii)
2·640
2·582
2·611
2·381
2·305
2·355
0·004
0·005
0·005
Mn
0·023
0·029
0·025
0·058
0·054
0·055
0·000
0·000
0·000
Mg
2·399
2·444
2·419
2·204
2·196
2·221
0·000
0·001
0·000
Ca
0·002
0·000
0·000
1·907
1·897
1·882
0·353
0·335
0·334
Na
0·018
0·018
0·009
0·418
0·314
0·403
0·624
0·642
0·638
K
1·862
1·915
1·891
0·263
0·255
0·271
0·031
0·033
0·036
Cl
0·016
0·013
0·010
4·996
5·000
4·996
XCa
0·350
0·332
0·331
XK
0·031
0·033
0·036
0·154
0·218
0·145
Total
15·556
15·586
15·567
15·741
15·609
15·712
XMg
0·476
0·486
0·481
0·481
0·488
0·485
F
XAl
Fluid inclusions in samples 173/1 to 173/7
along the traverse
pseudo-secondary inclusions could be a result of brittle deformation. The inclusions are characterized by different
shapes and sizes (5^20 mm). Two types of inclusions, which
have different compositions, size and morphology, occur:
CO2 and H2O^NaCl inclusions.
Carbonic inclusions are relatively small (usually510 mm;
rarely up to 20 mm), voluminous, dark and isometric, and
frequently exhibit negative crystal shapes (Fig. 8a).
Fluid inclusions occur in quartz grains. Usually they form
arrays along cracks, but large inclusions are also present
in the central parts of the grains. In the latter case they
were identified as primary inclusions, whereas those along
cracks are pseudo-secondary. The primary inclusions most
probably formed during crystal growth whereas the
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Table 3: Representative mineral chemical data for fluorapatites from the orthopyroxene-bearing patches in tonalitic veins
(GAB-6 series samples), intermediate gneissic zone (GAB-8 series samples), and the Sand River orthogneiss (GAB-7
series samples)
Sample
GAB-6a
GAB-6a
GAB-6b
GAB-6b
GAB-6b
GAB-6b
GAB-6
GAB-6c
GAB-6c
GAB-6c
GAB-6c
GAB-7a
GAB-7a
GAB-7a
GAB-7b
GAB-7b
CaO
55·26
56·31
55·43
56·27
57·04
56·46
56·81
57·03
57·31
55·88
55·65
55·81
55·54
57·11
Na2O
0·12
0·11
0·04
0·03
0·07
0·06
0·1
0·05
0·05
0·06
0·08
0·03
0·06
0·05
0·03
0·04
FeO
0·23
0·27
0·13
0·19
0·14
0·13
0·13
0·1
0·1
0·1
0·11
0·05
0·04
0·12
0·03
0·03
MnO
0·13
0·12
0·11
0·13
0·13
0·14
0·18
0·16
0·18
0·16
0·19
0·09
0·04
0·06
0·11
0·09
MgO
0·01
0·02
0
0·01
0·02
0·02
0·03
0·02
0·02
0
0
0
0·01
0·03
0
0
P2O5
40·79*
40·79
40·79
40·79
40·79
40·79
40·79
40·79
40·79
40·79
40·79
40·79
40·79
40·79
SiO2
0·2
0·17
0·27
0·23
0·24
0·27
0·08
0·14
0·1
0·22
0·27
0·09
0·1
0·04
0
0·08
Cl
0·43
0·42
0·68
0·63
0·55
0·64
0·61
0·59
0·42
0·86
0·48
0·2
0·13
0·15
0·14
0·16
F
2·51
2·61
3·1
3·2
2·29
2·47
2·67
2·51
2·4
3·25
3·17
4·17
5·08
2·76
3·38
3·57
58·89
100·82
100·55
101·48
101·27
100·98
101·32
100·74
101·23
101·79
101·11
101·78
100·91
no.:
Total
O
25
25
25
25
9·769
9·852
10·029
9·941
9·978
10·018
10·072
9·796
9·791
9·756
9·64
10·042
10·009
Na
0·039
0·035
0·013
0·01
0·022
0·019
0·032
0·016
0·016
0·019
0·025
0·009
0·019
0·016
0·009
0·013
Fe
0·032
0·037
0·018
0·026
0·019
0·018
0·018
0·014
0·014
0·014
0·015
0·007
0·005
0·016
0·004
0·004
Mn
0·018
0·017
0·015
0·018
0·018
0·019
0·025
0·022
0·025
0·022
0·026
0·012
0·005
0·008
0·015
0·012
Mg
0·002
0·005
0
0·002
0·005
0·005
0·007
0·005
0·005
0
0
0
0·002
0·007
0
0
P
5·729
5·682
5·681
5·643
5·667
5·675
5·661
5·662
5·665
5·651
5·671
5·635
5·594
5·667
5·63
5·658
0·028
15·674
25
0·044
15·73
15·54
25
0·038
15·588
25
0·039
15·8
25
25
25
25
25
40·79
9·926
0·033
25
101·37
40·79
9·821
Total
25
101·39
56·15
Ca
Si
25
101·4
57·3
0·044
0·013
0·023
0·016
0·036
0·044
0·015
0·016
0·007
15·722
15·734
15·759
15·813
15·538
15·573
15·435
15·283
15·763
25
9·857
0
0·013
15·668
15·558
Sample no.: GAB-7b GAB-7b GAB-7c GAB-7c GAB-7c GAB-7c GAB-8a GAB-8a GAB-8a GAB-8a GAB-8b GAB-8b GAB-8b GAB-8b GAB-8c GAB-8c GAB-8c
CaO
55·91
55·79
55·38
55·04
56·32
57·22
55·41
56·49
55·15
54·91
55·31
55·28
56·11
57·85
57·57
55·19
54·8
Na2O
0·02
0·02
0·03
0·03
0·04
0·06
0·13
0·05
0·1
0·15
0·13
0·13
0·09
0·1
0·06
0·08
0·13
FeO
0·2
0·08
0·05
0·02
0·1
0·05
0·11
0·16
0·15
0·08
0·19
0·11
0·06
0·07
0·07
0·08
0·14
MnO
0·09
0·07
0·08
0·07
0·06
0·07
0·1
0·06
0·1
0·1
0·11
0·1
0·08
0·11
0·13
0·11
0·08
MgO
0
0·01
0·03
0·01
0·01
0·01
0·03
0·01
0·01
0·03
0·02
0·02
0
0·01
0·03
0
0
P2O5
40·79
40·79
40·79
40·79
40·79
40·79
40·79
40·79
40·79
40·79
40·79
40·79
40·79
40·79
40·79
40·79
40·79
0·18
0·1
0·05
0·02
0·08
0·05
0·08
0·12
0·15
0·03
0·12
0·1
0·19
0·11
0·13
0·09
0·23
Cl
0·14
0·16
0·16
0·19
0·14
0·13
0·47
0·32
0·37
0·38
0·49
0·3
0·31
0·25
0·24
0·41
0·39
F
4·38
3·17
3·62
3·35
3·76
3·26
4·74
3·45
4·56
2·82
3·5
4·13
4·2
2·41
2·39
4·38
5·32
101·63
100·14
100·16
99·58
101·27
101·67
101·9
101·48
101·26
99·38
100·64
101·05
101·75
101·72
101·37
101·27
101·83
25
25
25
25
25
25
25
25
25
25
25
25
25
25
25
25
25
9·741
9·863
9·775
9·761
9·858
10·002
9·626
9·881
9·634
9·771
9·737
9·674
9·773
10·137
10·115
9·634
9·503
SiO2
Total
O
Ca
Na
0·006
0·006
0·01
0·01
0·013
0·019
0·041
0·016
0·032
0·048
0·041
0·041
0·028
0·032
0·019
0·025
0·041
Fe
0·027
0·011
0·007
0·003
0·014
0·007
0·015
0·022
0·02
0·011
0·026
0·015
0·008
0·01
0·01
0·011
0·019
Mn
0·012
0·01
0·011
0·01
0·008
0·01
0·014
0·008
0·014
0·014
0·015
0·014
0·011
0·015
0·018
0·015
0·011
Mg
0
0·002
0·007
0·002
0·002
0·002
0·007
0·002
0·002
0·007
0·005
0·005
0
0·002
0·007
0
0
5·615
5·698
5·689
5·716
5·642
5·634
5·6
5·638
5·631
5·735
5·674
5·641
5·614
5·648
5·663
5·627
5·589
P
Si
Total
0·016
0·008
0·003
0·013
0·008
0·013
0·019
0·024
0·005
0·02
0·016
0·031
0·018
0·021
0·015
0·037
0·029
15·418
15·599
15·502
15·514
15·545
15·687
15·322
15·592
15·338
15·607
15·515
15·421
15·453
15·865
15·847
15·35
15·192
*P2O5 is calculated after average content of P2O5 in fluorapatite.
along arrays as equant and irregular varieties with sizes
ranging from 5 to 20 mm.
H2O^NaCl inclusions (Fig. 8b) contain up to 24% of
NaCl-equivalent content. They are characterized by a lighter
color, with equant or irregular shapes, and range in size
They are localized as small arrays along healed cracks
(pseudo-secondary), and rarely as primary inclusions in
the central part of grains. The rare primary QN2-rich inclusions are of medium size (10^15 mm), dark color and
negative crystal shape. Pseudo-secondary inclusions occur
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RAJESH et al.
DEHYDRATION OF SAND RIVER GNEISS
Fig. 5. Systematic variation in the chemical composition of plagioclase (a) adjacent to K-feldspar rims on quartz from the Sand River orthogneiss; (b) in the intermediate gneissic zone and (c) adjacent to K-feldspar rims on orthopyroxene in orthopyroxene-bearing patches within
tonalitic veins. The compositional variation within the orthopyroxene is also shown in (c). The location of the composition profiles is indicated
in the corresponding BSE images to the right. Variation in Cl and F contents in biotite (d) and amphibole (e) respectively from the three adjacent zones.
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Fig. 6. Representative BSE images illustrating the occurrence of fluorapatite and its association with monazite as inclusions and/or rim grains
in the Sand River orthogneiss (a^c); intermediate gneissic zone (d^f) and orthopyroxene-bearing patches within tonalitic veins (g^i).
Variation in Cl and F contents in apatite from the three adjacent zones (j). The steady increase in Cl from the Sand River orthogneiss through
intermediate gneissic zone to orthopyroxene-bearing patches within tonalitic veins should be noted.
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RAJESH et al.
DEHYDRATION OF SAND RIVER GNEISS
Fig. 7. Field photograph illustrating the location of drill core samples collected along a traverse from a granite pegmatite into the Sand River
orthogneiss. Sample 173/7 was collected from the biotite-bearing granite pegmatite. The darker Sand River orthogneiss sample 173/1, which is farthest from the granite pegmatite, is similar to the Sand River orthogneiss sample (GAB-7) from the three adjacent zones, with both preserving
the Sand River orthogneiss composition. Samples 173/2 to 173/6, which are more leucocratic than the darker Sand River orthogneiss sample
173/1, are similar to the intermediate gneissic zone (GAB-8) and orthopyroxene-bearing tonalitic vein (GAB-6) samples from the three adjacent
zones. The contact relation between the granite pegmatite and Sand River gneiss (highlighted) is discrete and unaltered at places (e.g. to the
right of the traverse), and diffuse and metasomatic at other places (e.g. to the left of the traverse). The cellphone in the image is 7 cm long.
from 5 to 15 mm. Equant-shaped inclusions are relatively
small and dominant, whereas the less widespread large inclusions (15^20 mm) are irregular in shape. The H2O^
NaCl inclusions are always pseudo-secondary and are often
associated with pseudo-secondary CO2 inclusions.
No CO2-rich fluid inclusion was found in pegmatite
sample (173/7); this contains only pseudo-secondary H2O^
NaCl inclusions. The other samples contain both CO2
and H2O^NaCl inclusions, but in different proportions.
Melting temperatures (Tm) for the primary CO2 inclusions are at ^56·68C. Homogenization temperatures (Th)
of the primary CO2 inclusions range from ^11·2 to 18·88Q,
with densities up to 0·989 g cm3. Pseudo-secondary CO2
inclusions haveTh ranging from ^7·7 to 22·58C, with densities up to 0·971g cm3. Tm ranges from ^56·6 to ^56·78C.
No systematic variations exist among Tm of the CO2-rich
inclusions. Tm of the H2O^NaCl inclusions ranges from ^
13·1 to ^228Q (17·09^24·04% of NaCl-equivalent content).
Microthermometric data are presented for two representative samples 173/4 and 173/5 in Fig. 8c^e. For sample 173/7,
which contains only H2O^NaCl inclusions, Tm varies
from ^6 to ^13·18Q corresponding to 9·21^17·09% of
NaCl-equivalent content.
To summarize, there is a significant change in the fluid
composition along the profile. The relative amount of
CO2-rich inclusions increases from sample 173/7 in the pegmatite to 173/1 in the gneiss. The relative amount of NaCl
in the H2O^NaCl inclusions increases from 17% in the pegmatite sample (173/7) to 24% in the gneiss sample (173/1).
It is assumed that in addition to NaCl, the H2O^NaCl
fluid might have contained potassium, based on mineral
chemical data. Th of pseudo-secondary CO2 inclusions is
not high (on average ^7·5 to 208Q in the samples). It indicates a sufficiently high density of the fluid and therefore a
high temperature of their formation (compare with the
primary CO2 inclusions with Th up to ^11·28Q). This suggests their non-secondary nature. Lack of primary inclusions in the granite pegmatite sample (173/7) may indicate
a high level of recrystallization. A comparison of the fluid
inclusions along the profile from the granitic pegmatite
sample 173/7 to the Sand River orthogneiss sample 173/1 is
given in Table 3.
Fluid inclusions in samples from the
intermediate gneissic zone and
orthopyroxene-bearing patches
Only H2O^NaCl inclusions (Fig. 9a) are present in the
intermediate gneissic zone sample (GAB-8a). These occur
along healed cracks (probably pseudo-secondary) and are
characterized by lighter color, irregular shapes and sizes
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Fig. 8. Representative photomicrographs and homogenization Tdata obtained from the study of fluid inclusions in samples along the traverse
(173/7 to 173/1). (a) Carbonic inclusions with negative crystal shape in sample 173/4; (b) H2O^NaCl inclusions in sample 173/5; (c, d)
Histograms of Th distribution for the samples 173/4 and 174/5. 1, primary; 2, pseudo-secondary inclusions. (e) Isochores of different generations
of CO2 inclusions in the samples 173/4 and 173/5 [calculated after Duan et al. (1996)]. Respective Th and CO2 densities are indicated. The
lowest Th values were used for isochore plotting (i.e. inclusions having the highest density). For primary CO2 inclusions Rh ¼ ^11·28C, and for
pseudo-secondary Rh ¼ ^7·78C.
ranging from 5 to 10 mm (sometimes up to 15 mm). Tm
varies from ^1·58Q to ^78Q, corresponding to 2·56^
10·49% of NaCl-equivalent content.
H2O^NaCl and H2O-rich fluid inclusions are dominant
in the orthopyroxene-bearing patch sample (GAB-6a)
with minor CO2 inclusions. The H2O^NaCl inclusions
are lighter in color and irregular shaped, with sizes ranging from 5 to 20 mm (Fig. 9b). Equant varieties also
occur. Melting temperatures of the H2O^NaCl inclusions
vary from ^21·4 to ^308C, corresponding to 23·63^29·19%
NaCl-equivalent content. The H2O-rich inclusions are
larger (usually 10^20 mm, sometimes up to 30 mm), with
irregular shapes and a dark color, with Tm of ^0·1
to ^0·38Q. Both H2O^NaCl and H2O-rich inclusions
occur along cracks and are pseudo-secondary.
Rare primary CO2 inclusions occur in the central parts
of grains. They are of medium size (5^10 mm) and dark
color, and have negative crystal shapes (Fig. 9c). CO2 homogenization temperatures range from 3 to 13·48Q, corresponding to a density range from 0·909 to 0·835 g cm3.
Tm values are at ^588Q. Pseudo-secondary CO2 inclusions
(Fig. 9d) are localized along grain cracks and are characterized by a negative crystal shape (commonly 5 mm in
size, rarely up to 20 mm). Th varies from 15·1 to 268Q, corresponding to a density range of 0·82^0·695 g cm3, respectively; Rm varies from ^58 to ^58·78Q. The depression
inTm of these CO2 inclusions can be attributed to the presence of additional volatile species such as N2 and/or CH4.
A histogram of homogenization temperatures for the
orthopyroxene-bearing patch sample is given in Fig. 9e.
The minimum temperature (2·58Q) indicates the low
density of the inclusions and, consequently, a low pressure
of entrapment. It is quite possible that all the inclusions
found in this sample are secondary. In summary, fluid inclusion data indicate a much higher salinity of the fluid
(up to 29 % NaCl) in the orthopyroxene-bearing patches
relative to the intermediate gneissic zone.
W H O L E - RO C K G E O C H E M I C A L
C H A R AC T E R I S T I C S
Major and trace element contents for representative samples were determined by X-ray fluorescence spectrometry
at ACME Labs, Canada, on glass beads prepared from
powdered whole-rock samples with a sample-to-flux (lithium tetraborate) ratio of 1:10. Volatiles were determined
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RAJESH et al.
DEHYDRATION OF SAND RIVER GNEISS
Fig. 9. Representative photomicrographs and homogenizationTdata obtained from the study of fluid inclusions in samples from the intermediate gneissic zone (a) and orthopyroxene-bearing patches (b^d). (a) Pseudo-secondary H2O^NaCl inclusions; (b) primary carbonic inclusions;
(c) pseudo-secondary carbonic inclusions; (d) pseudo-secondary H2O^NaCl inclusions. Melting temperature values are also shown.
(e) Histogram showing the distribution of homogenization temperatures in primary (1) and pseudo-secondary (2) inclusions for samples from
the orthopyroxene-bearing patches.
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JOURNAL OF PETROLOGY
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by loss on ignition. Trace and rare earth elements (REE)
were analyzed by inductively coupled plasma mass spectrometry (ICP-MS), also at ACME Labs. The sample powders were dissolved in lithium metaborate flux fusion and
the resulting molten bead was rapidly digested in a weak
nitric acid solution. Precision and accuracy based on replicate analysis of international rock standards are 2^5%
(1s) for most elements and 10% for U, Sr, Nd, and Ni.
Major and trace element, and REE data for samples
from the three adjacent zones (GAB-7a, GAB-8a,
GAB-6a) and those from the Sand River orthogneiss traverse (173/7 to 173/1) are given in Table 4. Of the three adjacent zones, the Sand River orthogneiss is the least
siliceous (57 wt %), with the orthopyroxene-bearing
tonalitic vein having an intermediate silica content
(64 wt %) and the intermediate gneissic zone having the
highest silica content (72 wt %). In the normative An^
Ab^Or triplot, all the three adjacent zones are tonalitic
(Fig. 10a). This is further substantiated in the K2O^
Na2O^CaO triplot (not shown). The Sand River orthogneiss samples along the traverse are dominantly tonalitic,
with sample 173/2 falling in the granodiorite field and the
pegmatite sample in the granite field in Fig. 10a. Although
the three adjacent zones have medium K2O contents,
there is a slight increase in the K2O/Na2O ratio for the
intermediate gneissic zone (0·46) with respect to the Sand
River orthogneiss (0·29) and the orthopyroxene-bearing
tonalitic vein (0·27). The K2O/Na2O ratio of the Sand
River orthogneiss samples along the traverse range from
0·22 to 1·09, with the granite pegmatite having the highest
ratio (2·17). In terms of the aluminium saturation index,
both the Sand River orthogneiss and the orthopyroxenebearing tonalitic vein are metaluminous, whereas the
intermediate gneissic zone is peraluminous (Fig. 10b).
Samples from both the Sand River orthogneiss and granite
pegmatite along the traverse are peraluminous (Fig. 10b).
NUMBER 1
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In terms of Fe-number [total Fe/(total Fe þ MgO); Frost
et al., 2001], both the Sand River orthogneiss and the intermediate gneissic zone are magnesian, whereas the
orthopyroxene-bearing tonalitic vein is ferroan (Fig. 10c).
In terms of the modified alkali lime index (Na2O þ K2O
^ CaO; Frost et al., 2001), the Sand River orthogneiss is
calc-alkalic, whereas both the intermediate gneissic zone
and the orthopyroxene-bearing tonalitic vein are calcic
(Fig. 10d). The Sand River orthogneiss samples along the
traverse are magnesian and calcic, whereas the granite
pegmatite sample is ferroan and alkali-calcic (Fig. 10c and
d). In summary, based on the similarity of their major
element geochemical characteristics, three groups can be
defined among the samples (Fig. 10e): (1) the granite pegmatite sample 173/7; (2) the Sand River orthogneiss samples GAB-7a and 173/1; (3) the intermediate gneissic zone
sample GAB-8a, the orthopyroxene-bearing tonalitic vein
sample GAB-6a and the Sand River gneiss traverse samples 173/2 to 173/6.
The three adjacent zones are rich in Ba and Sr, similar
to high-Ba^Sr granitoids [see Rajesh & Santosh (2004) for
the usage of the terms high- and low-Ba^Sr granitoids]
(Fig. 10f). The Sand River orthogneiss samples along the
traverse are similar to high-Ba^Sr granitoids, with the
granite pegmatite sample similar to low-Ba^Sr granitoids
(Fig. 10f). In an upper continental crust normalized
multi-element diagram, the intermediate gneissic zone is
enriched in Rb, Ba, K and Th, and depleted in Cs, U, Nb,
Ta, Sr, Zr, Hf, Ti and Y with respect to the Sand River
orthogneiss (Fig. 11a). The orthopyroxene-bearing tonalitic
vein is enriched in Cs, Th, U, Ta, Sr, Zr, Hf and Y, and
depleted in Rb, Ba, K, Nb and Ti, with respect to the intermediate gneissic zone (Fig. 11a). Relative to the Sand
River orthogneiss samples along the traverse, the granite
pegmatite is enriched in Cs, Rb, Ba, K, Nb, Ta, Ti and Y,
and depleted in Sr (Fig. 11b).
Table 4: Comparison of fluid inclusions in samples along the profile from the granite pegmatite to the Sand River orthogneiss (173/7 to 173/1)
Fluid
173/1
173/2
173/3
173/4
173/5
173/6
173/7
—
Carbon dioxide
Carbon dioxide
Carbon dioxide
Carbon dioxide and
Carbon dioxide
Only water–salt
and water–salt
and water–salt
and water–salt
composition
CO2
—
Primary and
pseudo–secondary
Primary and
pseudo–secondary
Primary: Th from –11·2
water–salt
Pseudo-secondary:
to 18·88Q;
pseudo–secondary:
and water–salt
Primary and
—
pseudo–secondary
Th from –7·7 to 25·58Q
Th from –7·5 to 20·28Q
H2 O
—
Pseudo-secondary
Pseudo-secondary
Pseudo-secondary:
Pseudo-secondary:
NaCl-eq. from 17·09
NaCl-eq. from
to 24·04%
60
18·53 to 22·94%
Pseudo-secondary
Pseudo-secondary:
NaCl-eq. from
9·2 to 17·09%
RAJESH et al.
DEHYDRATION OF SAND RIVER GNEISS
Fig. 10. Major element geochemical characteristics of the three adjacent zones [Sand River orthogneiss (GAB-7a), intermediate gneissic zone
(GAB-8a), orthopyroxene-bearing tonalitic vein (GAB-6a)] and samples along the traverse [granite pegmatite (173/7), Sand River gneiss samples 173/6 to 173/1] in terms of: (a) normative An^Ab^Or; (b) molar A/CNK vs molar A/NK; (c) Fe-number [total Fe/(total Fe þ MgO)] vs
SiO2; (d) modified alkali index (Na2O þ K2O ^ CaO) vs SiO2; (e) (Na2O þ K2O)^total Fe^MgO triplot; (f) Rb^Sr^Ba. The fields in (c)
and (d) are from Frost et al. (2001). [See Rajesh (2008) for the fields plotted in (e), and Rajesh & Santosh (2004) for details of the usage of the
terms ‘high-Ba^Sr’ and ‘low-Ba^Sr’ granitoids in (f).]
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Fig. 11. Upper continental crust normalized [using the values of McLennan et al. (2006)] multi-element diagrams (a, b) and
chondrite-normalized [using the values of Sun & McDonough (1989)] REE patterns (c, d) for the three adjacent zonesçSand River orthogneiss (GAB-7), the intermediate gneissic zone (GAB-8) and orthopyroxene-bearing tonalitic vein (GAB-6) (a, c), and the samples along the traverse [granite pegmatite (173/7), Sand River gneiss samples 173/6 to 173/1 (b, d)].
62
RAJESH et al.
DEHYDRATION OF SAND RIVER GNEISS
Chondrite-normalized REE patterns for the three
adjacent zones show light REE (LREE) enrichment
[(La/Sm)N ¼ 2·86^9·78] with an almost flat heavy
REE (HREE) pattern for the Sand River orthogneiss
[(Gd/Yb)N ¼1·31], and relatively depleted HREE patterns
for the intermediate gneissic zone [(Gd/Yb)N ¼ 3·13] and
the orthopyroxene-bearing tonalitic vein [(Gd/Yb)N ¼
1·71] (Fig. 11c). Both the Sand River orthogneiss (Eu/
Eu* ¼ 0·90) and the orthopyroxene-bearing tonalitic vein
(Eu/Eu* ¼ 0·58) exhibit a negative Eu anomaly, whereas
the intermediate gneissic zone has a positive Eu anomaly
(Eu/Eu* ¼ 1·78) (Fig. 11c). Regarding the traverse samples,
the Sand River orthogneiss sample (173/1) farthest from
the granite pegmatite has a chondrite-normalized REE
pattern of almost constant HREE [(Gd/Yb)N ¼1·25] and
a negative Eu anomaly (Eu/Eu* ¼ 0·94), similar to that of
the Sand River orthogneiss from the three adjacent zones
(compare patterns of GAB-7a and 173/1 in Fig. 11c and d,
respectively). The remaining Sand River orthogneiss samples (173/2 to 173/6) along the traverse have REE patterns
with relative HREE depletion [(Gd/Yb)N ¼ 2·01^9·46]
and dominantly positive Eu anomalies (Eu/Eu* ¼ 1·24^
5·86), with one sample having a weak negative Eu anomaly
(Eu/Eu* ¼ 0·75) (Fig. 11d), similar to those of the intermediate gneissic zone and orthopyroxene-bearing tonalitic
vein in Fig. 11c. The granite pegmatite has a HREEenriched pattern [(Gd/Yb)N ¼ 0·72] with positive Eu
anomaly (Eu/Eu* ¼ 4·57) and LREE concentrations
lower than the Sand River orthogneiss samples along the
traverse (Fig. 11d).
1997) in the Sand River orthogneiss. A granite pegmatite
(2·01Ga; Jaeckel et al., 1997) occurring in the vicinity of
the orthopyroxene-bearing patches, and the Sand River
gneiss section into which the pegmatite intrudes were also
studied.
D I S C U S S ION A N D C ONC LU D I NG
REMARKS
Age of orthopyroxene-bearing patches and
related events in the Sand River
orthogneiss
Geochemical evidence for the progression
of dehydration
Field and petrographic evidence for
local-scale dehydration of the Sand
River orthogneiss
Field and petrographic examination of the three adjacent
zonesçthe Sand River orthogneiss with local presence of
orthopyroxene and clinopyroxene, the intermediate gneissic zone with more orthopyroxene than the Sand River
orthogneiss and the tonalitic veins containing large
orthopyroxene-bearing patches, together with the field relations of the Sand River orthogneiss grading smoothly
into the intermediate gneissic zoneçindicates the local
transformation of a light grey, fine- to medium-grained,
hornblende^biotite gneiss into a greenish brown, mediumto coarse-grained orthopyroxene-bearing dehydration
zone. A contrast in mineral proportion is evident with an
increase in modal abundance of K-feldspar and orthopyroxene and a decrease in modal abundance of biotite in the
orthopyroxene-bearing patches relative to the gneissic
zone (Table 1). Orthopyroxene- and/or clinopyroxeneforming reaction textures, similar to those observed in the
present study (Fig. 4), have been extensively documented
from vein and patchy dehydration zones in amphibolefacies terranes around the world (e.g. Chacko et al., 1987;
Hansen et al., 1987; Perchuk et al., 2000; Harlov & Fo«rster,
2002; Tsunogae et al., 2002; Rajesh, 2004).
The most noticeable geochemical characteristic supporting
the occurrence of the dehydration process is the depletion
of HREE in the intermediate gneissic zone and
orthopyroxene-bearing tonalitic vein relative to the Sand
River orthogneiss (Fig. 11c). This is similar to many studies
on orthopyroxene-bearing dehydration zones, which have
shown whole-rock depletion in HREE compared with
their precursors (e.g. Janardhan et al., 1982; Hansen et al.,
1987; Sta«hle et al., 1987; Friend & Nutman, 1992; Raith &
Srikantappa, 1993; Ravindra Kumar, 2004; Harlov et al.,
2006). Although the darker Sand River orthogneiss farthest from the granite pegmatite has an almost constant
HREE pattern (173/1 in Fig. 11d), similar to the darker
non-dehydrated Sand River orthogneiss from the three adjacent zones (GAB-7a in Fig. 11c), the Sand River gneiss
samples occurring in the lighter metasomatic zone (Fig. 7)
show relative HREE depletion (173/6 to 173/2 in Fig. 11d),
similar to the intermediate gneissic zone and tonalitic
orthopyroxene-bearing vein (GAB-8a and GAB-6a in
Fig. 11c). The enrichment or depletion patterns observed
between the three adjacent zones and along the traverse
Although the 3·31^3·28 Ga Sand River biotite^hornblende gneiss exposed within the Causeway locality preserves evidence (zircon overgrowths) for two anatectic
events, a dominant one at 2·64^2·61Ga and a minor one
at 2·01Ga (Retief et al., 1990; Tsunogae & Yurimoto,
1995; Jaeckel et al., 1997; Kro«ner et al., 1998, 1999; Zeh
et al., 2007, 2010; Gerdes & Zeh, 2009), the Neoarchean
overprint ages have been reported from exposures where
the 2·68^2·57 Ga Singelele-type gneiss intrudes the Sand
River orthogneiss. The Paleoproterozoic anatectic event is
related to the 2·01Ga undeformed granitic melt patches
and veins (Jaeckel et al., 1997) within the Sand River
orthogneiss, which are relatively minor with respect to the
larger Singelele-type anatectic event. This study concentrates on the orthopyroxene-bearing patches (2·01Ga;
Jaeckel et al., 1997), which occur locally associated with the
2·01Ga granitic melt patches and veins (Jaeckel et al.,
63
JOURNAL OF PETROLOGY
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are illustrated in Fig. 12. The progression of dehydration
(173/6 to 173/2) is characterized by increase in Si, K, Rb,
Ba, Th/U and Nb/Y, and decrease in Ti, Al, Ca, Na, Mg,
Fe, Sr and Sm/Nd with respect to the non-dehydrated
Sand River orthogneiss sample (173/1). Similar progression
of dehydration trends was observed in the intermediate
gneissic zone (GAB-8a) and tonalitic orthopyroxenebearing vein (GAB-6a) with respect to the nondehydrated Sand River orthogneiss (GAB-7a) (Fig. 12).
NUMBER 1
JANUARY 2013
dehydration process. Using the composition of biotite,
orthopyroxene and K-feldspar from the orthopyroxenebearing tonalitic vein, water activity was estimated (using
winTWQ 2.3 software) in the range of 0·28^0·26 (Fig. 13).
Such a low water activity is imposed by the coexisting
aqueous brine and CO2-rich fluids (e.g. Aranovich &
Newton, 1997). The NaCl concentrations in the fluid inclusions (up to 29 %) reported in this study fall short of
those required to lower the H2O activity sufficiently to stabilize orthopyroxene and K-feldspar (e.g. Aranovich &
Newton, 1996). Using the K/Na ratio of the K-feldspar in
the tonalitic vein for calculation of the K/Na ratio in the
fluid, and following the equations of Aranovich & Newton
(1997) for water activity in KCl^NaCl^H2O fluids, the estimated water activity would correspond to about 55^56%
NaCl for the dehydration process. It has been pointed out
in many fluid inclusion studies that, in general, brine inclusions are much smaller than CO2-rich inclusions and
therefore more difficult to detect (e.g. Touret & Huizenga,
1999; van der Berg & Huizenga, 2001). These studies
pointed out that, unlike CO2, brine remnants found in inclusions represent a major fraction of the fluid; this is supported by microtextures such as extensive K-feldspar
veining in the respective rocks. Except for the brine remnants preserved in inclusions, fluids were expelled from
the rock during post-metamorphic uplift, leaving only
traces that indicate that large fluid quantities must have
percolated through these ‘dry’ rocks (e.g. Newton et al.,
1998). Another possibility is that if the observed NaCl-rich
fluid inclusions are related to the dehydration (which
seems likely), then they may have been subsequently disrupted with separation of fluid from halite daughter crystals [see Srikantappa & Zargar (2009) for an example of
such a scenario].
In addition to the prominent occurrence of K-feldspar
microveining in the three adjacent zones, the increase in
Cl content in amphibole, biotite (Fig. 5d and e) and fluorapatite (Fig. 6j) from the Sand River orthogneiss to the
orthopyroxene-bearing patches supports the presence of a
Cl-rich brine fraction in the fluid responsible for the dehydration process. Further, the increase in An content of the
plagioclase at the contact with the K-feldspar rims on
quartz (Fig. 5a^c) reflects an increase in potassium activity
in the fluid. Experimental studies have shown that the mobility of fluids of different composition is largely determined by their wetting behaviour. Fluids consisting only
of Cl-rich brines have relatively small wetting angles,
whereas CO2-rich fluids have large wetting angles
(Watson & Brenan, 1987; Brenan & Watson, 1988; Holness,
1997; Gilbert et al., 1998). The implication is that, because
of their large wetting angles, CO2-rich fluids do not form
an interconnected network along grain boundaries, limiting their ability to infiltrate and thereby resulting in
local-scale (centimetres to few metres) dehydration zones.
Evidence for fluid-induced dehydration
The occurrence of K-feldspar microveins along quartz^
plagioclase grain boundaries in the three adjacent zones
(Fig. 5a^c), together with the increase in modal abundance
of K-feldspar from the Sand River orthogneiss to the intermediate gneissic zone to the orthopyroxene-bearing
patches (Table 1), indicates that K-feldspar formation was
associated with the formation of orthopyroxene. Such
K-feldspar microveins along quartz^plagioclase grain
boundaries have been proposed by various petrographic
studies as evidence for the presence and passage of a low
H2O activity fluid (e.g. Perchuk & Gerya, 1992; Todd &
Evans, 1994; Hansen et al., 1995; Harlov et al., 1998).
Experimental studies by Harlov & Fo«rster (2003) showed
that highly mobile, concentrated brines, specifically KCl,
play an important role in the dehydration process, being
involved in the breakdown of biotite and amphibole to
orthopyroxene as well as the formation of K-feldspar veins
along quartz^plagioclase grain boundaries. Further, the
fluorapatite^monazite textures observed in the present
study (monazite inclusions in fluorapatite in the three adjacent zones; Fig. 6), have been suggested as an important indicator of the involvement of a fluid with a brine
component in the dehydration process. Experimental studies by Harlov & Fo«rster (2003) and Harlov et al. (2005)
showed that the reaction of (Y þ REE)-bearing fluorapatite with fluids, including KCl brines, results in the formation of monazite inclusions in fluorapatite by
metasomatism of the host fluorapatite via dissolution and
reprecipitation.
Nature of the fluid responsible for the
dehydration
Fluid inclusion studies of the intermediate gneissic zone
and orthopyroxene-bearing patch samples indicate the
presence of a CO2-rich fluid with additional brine and
H2O. The orthopyroxene-bearing patch sample has a
higher salinity of the fluid (up to 29% NaCl) relative to
the intermediate gneissic zone. This higher salinity fluid,
acting during the evolution of the orthopyroxene-bearing
patches, is consistent with the higher Cl contents of amphibole, biotite and fluorapatite in this rock relative to the
intermediate gneissic zone. The presence of CO2 and
brine, responsible for the breakdown of biotite and amphibole to pyroxenes, implies low water activities during the
64
RAJESH et al.
DEHYDRATION OF SAND RIVER GNEISS
Fig. 12. Element enrichment or depletion trends between the three adjacent zones [Sand River orthogneiss (GAB-7a), intermediate gneissic
zone (GAB-8a), orthopyroxene-bearing tonalitic vein (GAB-6a)] and along the traverse from the granite pegmatite (173/7) to the Sand River
orthogneiss (173/1). Similar enrichment or depletion trends are observed moving from the non-dehydrated Sand River orthogneiss sample 173/1
to the gneiss samples along the traverse from 173/2 to 173/6, and from the non-dehydrated Sand River orthogneiss sample GAB-7a to the intermediate gneissic zone (GAB-8a) and orthopyroxene-bearing tonalitic (GAB-6a) samples, supporting fluid-induced dehydration.
65
JOURNAL OF PETROLOGY
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NUMBER 1
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Fig. 13. Water activity values calculated using winTWQ 2.3 software for orthopyroxene and biotite associated with K-feldspar and quartz in the
orthopyroxene-bearing tonalitic vein (see inset image). The calculation was carried out for 5·6 kbar, the average of pressures (5·4^6·2 kbar) obtained by the application of hornblende^plagioclase^quartz (Holland & Blundy, 1994) and clinopyroxene^plagioclase^quartz (Ellis, 1980) geobarometers to the partially dehydrated gneiss.
relation between water and potassium activities during the
dehydration process. A decrease of water activity and a
slight increase of potassium activity in the fluid resulted
in a transition (lower trend in Fig. 14) from biotite þ amphibole þ plagioclase (initial Sand River orthogneiss) through
biotite þ orthopyroxene þ plagioclase (intermediate gneissic
zone) to orthopyroxene þ K-feldspar þ plagioclase (orthopyroxene-bearing patches). At higher potassium activity
the initial Sand River orthogneiss (biotite þ amphibole þ
plagioclase) would be transformed to an orthopyroxene þ
clinopyroxene þ K-feldspar assemblage (the upper trend in
Fig. 14). The specific distribution of the clinopyroxene þ orthopyroxene assemblage in the Sand River orthogneiss indicates local variations in the water and potassium
activity in the fluid, which could be a consequence of immiscibility of aqueous brine and CO2 fluids, and unequal mobility of these fluid portions in the rock. The local presence of
a fluid component in the initial Sand River orthogneiss
(A in Fig. 3) is supported by the observation of rare monazite inclusion in fluorapatite (Fig. 6c) in the same sample
(GAB-7c) in which both clinopyroxene and orthopyroxene
was observed.
On the other hand, Cl-rich brines would flow along grain
boundaries over relatively large distances, because of their
low wetting angles (Aranovich et al., 1987; Aranovich &
Newton, 1998; Newton et al., 1998). Thus, based on the results of fluid inclusion and mineral chemical studies, it is
argued that a fluid with the composition of CO2 coupled
with a Cl-rich brine fraction and H2O caused the formation of the orthopyroxene-bearing patches over limited distances (centimeters to a few metres). The local scale of the
dehydration process is supported by the field evidence for
the local occurrence of the orthopyroxene-bearing patches
within the Sand River orthogneiss at the Causeway
locality.
Evidence for local variations of water and
potassium activity in the fluid
To account for the observation of the local occurrence of
clinopyroxene and orthopyroxene within the Sand River
orthogneiss (Fig. 4c), a paragenetic analysis of the mineral
assemblages in the three adjacent zones was carried out
using a mK2O^mM2O diagram (Fig. 14). The analysis in
Fig. 14 allows suggestion of two trends with an inverse
66
RAJESH et al.
DEHYDRATION OF SAND RIVER GNEISS
length of the tonalitic veins, together with the extent of alteration, is suggestive of hydrothermal vein growth along
deformation zones. The shear deformation along the
narrow linear zones reported here, now occupied by tonalitic veins, is similar to that reported as part of the
2·01Ga reactivation of crustal-scale shear zones from
various parts of Central Zone, including the Sand River
orthogneiss (McCourt & Vearncombe, 1992; Kamber
et al., 1995a; Scha«ller et al., 1999; Kreissig et al., 2001; Smit
et al., 2001).
The common spatial association of large biotite-bearing,
granite pegmatites with dehydration zones in the Sand
River orthogneiss suggests that they are an obvious candidate to test in terms of the fluid carrier. However, the lack
of CO2-rich fluid inclusions and the diffuse contact relationships with the Sand River orthogneiss rule out the
granite pegmatites as the carrier of the dehydrating fluids.
Granitic melts can contain very small quantities of CO2
and Cl (e.g. Webster et al., 1999), but these components
will be overwhelmed by the H2O given off by crystallization of the pegmatite.
The geochemical characteristics (e.g. HREE depletion
in Fig. 11d and enrichment or depletion patterns in Fig. 12)
of the Sand River gneiss samples collected from the lighter
metasomatized zone near the margin of the granite pegmatite (Fig. 7), together with the diffuse metasomatic contact relationships at places between the granite pegmatite
and Sand River gneiss, suggest that the dehydrating fluids
were derived from an external source and utilized lithological contrasts, such as the gneiss^pegmatite boundaries,
as fluid conduits. Based on the field occurrence, with the
Sand River gneiss samples 1736 to 173/2 collected from the
metasomatized zone, which is more leucocratic than the
darker Sand River gneiss sample 173/1 (Fig. 7), it can be
inferred that the fluids most probably moved along grain
boundaries parallel to the gneissic fabric observed in the
Sand River gneiss. Dehydration of the wall-rocks occurred
where the permeability was sufficient for fluid penetration.
It is thus suggested that during the Paleoproterozoic
tectono-metamorphic event, H2O^CO2-brine fluids
derived from an external source were lithologically or
structurally channeled; these then interacted with the
Sand River orthogneiss, triggering dehydration with the
formation of orthopyroxene. Being more mobile in comparison with the H2O^CO2 fluid, the supercritical brine
penetrated into the gneiss, provoking further progression
of the dehydration process under higher alkali activity
(upper trend in Fig. 14) with the localized formation of
the clinopyroxene þ orthopyroxene þ K-feldspar assemblage. The H2O^CO2 fluid most probably resulted in late
re-hydration with the formation of late amphibole and
biotite^quartz after orthopyroxene. The occurrence of
minor monzonitic domains surrounding the dominant
orthopyroxene-bearing domains in the tonalitic veins
Fig. 14. Paragenetic analysis of the mineral assemblages in the three
adjacent zones studied here in terms of the mK2O^mM2O diagram.
This suggests two trends with inverse relations between water and
potassium activities during the dehydration process. Decrease of
water activity and slight increase of potassium activity in a fluid results in a transition (lower arrowed trend) from biotite þ amphibole þ plagioclase (initial Sand River orthogneiss) through
biotite þ orthopyroxene þ plagioclase (intermediate gneissic zone) to
orthopyroxene þ K-feldspar þ plagioclase
(orthopyroxene-bearing
patches). At higher potassium activity the initial Sand River orthogneiss biotite þ amphibole þ plagioclase would be transformed to the
orthopyroxene þ clinopyroxene þ K-feldspar assemblage (upper
arrowed trend). (Cpx), (Pl), etc. show reactions (monovariant lines)
without the participation of these phases. [Cpx], [Opx], etc. show invariant points. Thus the line starting from [Opx] to (Hbl) shows a
monovariant reaction in which neither orthopyroxene nor hornblende
participates.
Evidence for structural or lithological control on channelling of the fluids
The contrasting contact relations of the orthopyroxenebearing tonalitic veins with adjacent zonesçsharper margins with the Sand River orthogneiss and less sharp
margins with the intermediate gneissic zoneçtogether
with their occurrence as narrow linear zones tracing a discrete ductile shear deformation system, indicate that the
orthopyroxene in the tonalitic veins formed in a sigmoidally transposed foliation bounded by shear planes. This attests to a structural control in the channeling of the fluids
responsible for the dehydration. The deformation association of orthopyroxene-bearing dehydration zones
(orthopyroxene-bearing tonalitic veins in the present
study) with the eradicated older fabric, with no pervasive
new fabric, and the deformation assistance of fluid infiltration have been widely reported in the literature (e.g.
Newton et al., 1980; Hansen et al., 1987; Sta«hle et al., 1987;
Santosh et al., 1990; Harlov et al., 2006). The conspicuous
central location of the orthopyroxene crystals along the
67
JOURNAL OF PETROLOGY
VOLUME 54
NUMBER 1
JANUARY 2013
Table 5: Major, trace and rare earth element data for samples from the granite pegmatite to the Sand River orthogneiss traverse (173/7 to 173/1), and from the three adjacent zones (GAB-7a, GAB-8a, GAB-6a)
Sample no.:
173/7
173/6
173/5
173/4
173/3
173/2
Lithology:
Gran peg
Sand River
Sand River
Sand River
Sand River
Sand River
Sand River
gn
gn
gn
gn
gn
gn
SiO2 (wt %)
173/1
58·82
70·02
74·39
75·81
71·88
72·39
TiO2
1·86
0·26
0·17
0·14
0·31
0·21
0·70
Al2O3
14·21
17·02
14·52
13·68
15·33
14·98
17·19
Fe2O3
11·73
1·99
1·28
1·15
2·15
1·62
5·86
MnO
0·10
0·02
0·01
0·01
0·02
0·02
0·08
MgO
3·81
0·60
0·40
0·33
0·69
0·50
2·22
CaO
2·36
4·25
3·80
3·46
3·72
2·87
5·26
Na2O
1·86
4·06
3·40
3·25
3·40
2·96
3·37
K 2O
4·04
0·90
0·95
1·06
1·38
3·22
1·80
P 2O5
0·01
0·02
0·02
0·02
0·02
0·02
0·19
LOI
0·88
0·74
0·61
0·68
0·74
0·55
0·72
99·68
99·88
99·55
99·59
99·64
99·34
98·61
28
Total
Cr (ppm)
61·22
33
16
13
12
13
15
Sc
b.d.
b.d.
b.d.
b.d.
b.d.
b.d.
75·0
Cl
669
91
115
75
154
154
360·0
Ba
1380
206
311
303
402
1372
229
Be
1·07
2·23
1·89
1·58
1·73
1·29
0·89
Co
44·21
24·86
26·09
22·30
26·57
24·92
32·64
Cs
Ga
9·87
66·4
0·58
55·0
0·41
0·44
50·5
45·9
1·01
55·1
0·88
48·3
Hf
2·51
2·54
4·14
5·17
6·47
2·38
Nb
38·88
8·39
8·38
4·09
12·80
8·01
Rb
Sn
Sr
281·1
4·84
125·3
39·1
1·76
238·0
Ta
2·188
0·444
Th
9·46
1·81
U
V
W
1·16
262·9
2·144
Zr
126
Y
29
0·33
63·7
0·567
31·8
36·2
1·05
0·99
235·6
186·3
0·281
0·281
29·42
10·32
0·69
0·48
57·2
50·3
0·605
124
210
8
10
0·501
252
7
59·6
1·87
205·6
0·837
15·98
0·64
75·7
2·425
296
10
91·7
1·21
235·7
0·413
20·06
3·40
71·0
0·545
143
6
1·72
52·6
4·35
7·63
98·7
2·03
272·2
0·847
6·61
1·89
208·0
0·449
202
21·2
La
6·25
10·30
68·64
26·32
45·55
14·32
16·95
Ce
8·78
13·06
114·35
41·53
72·35
20·91
35·52
Pr
0·73
1·07
10·44
3·69
6·52
1·79
4·35
Nd
2·66
3·40
33·92
12·27
21·14
6·19
19·71
Sm
0·50
0·52
4·28
1·66
2·63
0·89
4·39
Eu
0·90
1·06
1·04
1·10
1·09
1·37
1·36
Gd
0·73
0·59
4·20
1·65
2·73
0·97
4·37
Tb
0·135
0·081
0·370
0·166
0·260
0·123
0·745
Dy
1·00
0·43
1·49
0·73
1·19
0·59
4·47
Ho
0·22
0·09
0·17
0·11
0·16
0·11
0·87
Er
0·69
0·24
0·54
0·33
0·50
0·31
2·54
Tm
0·118
0·036
0·059
0·048
0·063
0·045
0·388
(continued)
68
RAJESH et al.
DEHYDRATION OF SAND RIVER GNEISS
Table 5: Continued
Sample no.:
173/7
173/6
173/5
173/4
173/3
173/2
Lithology:
Gran peg
Sand River
Sand River
Sand River
Sand River
Sand River
173/1
Sand River
gn
gn
gn
gn
gn
gn
Yb
0·817
0·232
0·354
0·384
0·408
0·281
2·788
Lu
0·116
0·047
0·056
0·123
0·069
0·048
0·425
Mo
Cu
0·433
10·45
0·697
0·164
0·164
0·503
0·181
8·04
7·14
9·01
9·37
5·28
0·412
38·88
Pb
13·8
16·7
20·9
16·9
21·1
26·3
12·6
Zn
165·2
28·7
18·7
17·1
34·1
39·8
73·0
K2O/Na2O
2·17
0·22
0·28
0·33
0·41
1·09
0·53
K/Rb
119·16
190·83
248·07
242·80
191·88
291·16
151·18
K/Sr
267·35
31·35
33·43
47·17
55·65
113·25
54·83
Rb/Sr
2·24
0·16
0·13
0·19
0·29
0·39
0·36
Th/U
8·17
5·56
42·38
21·37
25·01
5·90
3·50
Sm/Nd
0·19
0·15
0·13
0·14
0·12
0·14
0·22
Nb/Y
1·34
1·05
0·84
0·58
1·28
1·33
0·36
A/CNK
1·21
1·11
1·07
1·07
1·11
1·10
1·01
A/NK
1·91
2·22
2·19
2·11
2·16
1·79
2·29
Fe*/(Fe* þ MgO)
0·75
0·77
0·76
0·78
0·76
0·76
0·73
Na2O þ K2O – CaO
3·54
0·71
0·55
0·85
1·06
3·31
–0·09
(La/Sm)N
7·67
12·10
9·88
9·77
10·69
9·89
2·38
(Gd/Yb)N
0·72
2·02
9·46
3·43
5·33
2·75
1·25
Eu/Eu*
4·57
5·86
0·75
2·01
1·24
4·48
0·94
Sample no.:
GAB-7a
GAB-8a
Lithology:
Sand River
Intermediate
Opx-bearing
gn
gneissic zone
tonalitic vein
SiO2 (wt %)
GAB-6a
57·33
72·64
TiO2
0·68
0·27
0·12
Al2O3
18·92
14·48
16·98
Fe2O3
6·10
2·41
4·57
MnO
0·10
0·03
0·12
MgO
2·79
0·83
1·40
CaO
7·03
3·16
4·80
Na2O
4·83
3·98
4·66
K2O
1·38
1·83
1·28
P2O5
0·23
0·07
0·24
LOI
0·40
0·20
1·00
99·79
99·90
99·83
Cr (ppm)
13
15
28
Sc
13
3
7
Cl
154
154
360
Ba
172
461
212
Be
2
6
2
Co
17
5·4
7·8
0·3
0·6
Total
Cs
0·4
64·66
(continued)
69
JOURNAL OF PETROLOGY
VOLUME 54
NUMBER 1
Table 5: Continued
Sample no.:
GAB-7a
GAB-8a
Lithology:
Sand River
Intermediate
GAB-6a
Opx-bearing
gn
gneissic zone
tonalitic vein
Ga
22·3
15·6
18·7
Hf
7·7
2
3·8
Nb
7·7
4·2
1·9
Rb
31·6
51·1
25·7
Sn
1
51
51
Sr
372·4
242·5
274·6
Ta
0·5
0·2
0·3
Th
3·7
4·7
17·4
U
2
V
112
W
Zr
0·6
329
0·3
1·2
38
47
50·5
1
78
147·1
Y
22·3
2·3
La
19·4
21·4
37·5
Ce
40·2
34·6
75·2
Pr
Nd
4·87
18·2
18
3·55
8·08
10·7
27·6
Sm
4·19
1·35
4·68
Eu
1·25
0·71
0·85
Gd
4·29
1·02
4·07
Tb
0·7
0·12
0·54
Dy
3·79
0·67
2·91
Ho
0·84
0·06
0·63
Er
2·47
0·29
1·73
Tm
0·39
0·03
0·27
Yb
2·61
0·26
1·9
Lu
0·41
0·04
0·25
Mo
3·1
5·3
7·7
Cu
18·5
12·3
10·1
Pb
Zn
K2O/Na2O
1·2
32
1·3
4·4
29
35
0·29
0·46
0·27
K/Rb
362·06
296·91
412·92
K/Sr
30·72
62·57
38·65
Rb/Sr
0·08
0·21
0·09
Th/U
1·85
15·67
14·50
Sm/Nd
0·23
0·13
0·17
Nb/Y
0·35
1·83
0·11
A/CNK
0·85
1·01
0·96
A/NK
2·00
1·70
1·88
Fe*/(Fe* þ MgO)
0·69
0·74
0·77
–0·82
2·65
1·14
(La/Sm)N
2·86
9·78
4·94
(Gd/Yb)N
1·31
3·13
1·71
Eu/Eu*
0·90
1·78
0·58
Na2O þ K2O – CaO
LOI, loss on ignition; b.d., below detection limit.
70
JANUARY 2013
RAJESH et al.
DEHYDRATION OF SAND RIVER GNEISS
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AC K N O W L E D G E M E N T S
We are grateful to the late Leonid Perchuk for discussions
on the topic addressed in this study. Journal reviewers
Robert Newton, Zeb Page and an anonymous reviewer
provided detailed, thorough, encouraging and insightful
comments on the paper, which were instrumental in delineating the limits of the study and producing the present
state of the paper. Editorial comments by Alasdair Skelton
are highly appreciated.
FUNDING
This work was carried out as part of the Russian
Federation^South Africa scientific collaboration and was
supported by grants from the Russian Foundation for
Basic Research (project 10-05-00040 to O.G.S.), the
Russian President Grants for Young Scientists
(MD-222.2012.5 to O.G.S.) and the National Science
Foundation of South Africa (GUN: 2053192 to D.D.v.R.).
H.M.R. was also supported by a UJ/SASOL grant.
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