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ORIGINAL PAPER
The occurrence of fluor-wagnerite in UHT granulites and its
implications towards understanding fluid regimes in the evolution
of deep crust: a case study from the Eastern Ghats Belt, India
Kaushik Das
1,2
&Naotaka Tomioka
2
&Sankar Bose
3
&Jun-ichi Ando
1
&Ichiro Ohnishi
4
Received: 1 May 2014 /Accepted: 18 September 2016 /Published online: 29 September 2016
#Springer-Verlag Wien 2016
Abstract We report the occurrence of a rare phosphate min-
eral, fluor-wagnerite (Mg
1.91–1.94
Fe
0.06–0.07
Ca
<0.01
)(P
0.99–
1.00
O
4
)(OH
0.02–0.17
F
0.98–0.83
) from the Eastern Ghats Belt of
India, an orogenic belt evolved during Meso- to
Neoproterozoic time. The host rock, i.e. high- to ultrahigh
temperature (UHT) granulites (~1000 °C, 8–9 kbar) of the
studied area was retrogressed after emplacement to mid-
crustal level (800–850 °C, 6–6.5 kbar) as deduced from their
pressure-temperature histories. Based on mineral chemical da-
ta and micro-Raman analyses, we document an unusual high
Mg-F-rich chemistry of the F-wagnerite, which occur both in
peak metamorphic porphyroblastic assemblages as well as in
the retrograde matrix assemblage. Therefore, in absence of
other common phosphates like apatite, fluor-wagnerite can
act as an indicator for the presence of F-bearing fluids for
rocks with high X
Mg
and/or fO
2
. The occurrence of F-rich
minerals as monitors for fluid compositions has important
implications for the onset of biotite dehydration melting and
hence melt production in the deep crust. We propose that
fluor-wagnerite can occur as an accessory mineral associated
with F-rich fluids in lower-mid crustal rocks, and F in
coexisting minerals should be taken into consideration when
reconciling the petrogenetic grid of biotite-dehydration
melting.
Keywords Fluor-wagnerite in Al-rich granulites .Biotite
dehydration melting .UHT granulites .Eastern Ghats
GranuliteBelt .India
Introduction
Accessory mineral phases in deep-crustal granulites often pre-
serve a plethora of information regarding the pressure (P)-
temperature (T)-fluid-time (t) evolution of the deep continen-
tal crust undergoing orogenesis (e.g. Harlov and Foster 2007;
Harlov et al. 2011). Minerals like zircon and monazite are
widely used for retrieving the age information of
tectonothermal events that the deep crust undergo during their
orogenic evolution. They have shown to be of immense im-
portance to monitor the geological evolution of continental
blocks through time. At the same time, there are accessory
minerals, which are significant in properly assessing the P–T
histories (Bingen et al. 1996; Bea and Montero 1999; Harlov
et al. 2006; Harley 2008; Harley and Nandakumar 2014).
Minor components present in the major and accessory phases
may play a crucial role in the topological relationships on
metamorphic petrogenetic grids (Bose et al. 2005). One such
minor component is fluorine (F), which is accommodated
within hydrous minerals like biotite in metapelitic granulites
of Bappropriate^bulk compositions (Motoyoshi and Hensen
2001;Boseetal.2005) and hornblende in high-grade
metabasites (Tsunogae et al. 2003). Experimental results
(Munoz and Ludington 1977; Hensen and Osanai 1994;
Icenhower and London 1997) and natural rock studies show
that F prefers incorporation in these minerals over melt over
the P-T range of anatexis. This stabilizes both minerals up to
Editorial handling: R. Milke
*Kaushik Das
kaushik@hiroshima-u.ac.jp
1
Department of Earth and Planetary Systems Science, Hiroshima
University, Higashi Hiroshima, Japan
2
Institute for Study of the Earth’s Interior, Okayama University,
Okayama, Japan
3
Department of Geology, Presidency University, Kolkata, India
4
JEOL Ltd., Tokyo, Japan
Miner Petrol (2017) 111:417–429
DOI 10.1007/s00710-016-0474-y
high temperature (HT) –ultra-high temperature (UHT) condi-
tions (Tsunogae et al. 2003;Boseetal.2005). The stabilizing
effect of F necessitates the proper characterization of the meta-
morphic conditions of F-containing rock systems. If F is not
derived fromthe protolith, then thereis a further needto assess
the exact stage when F enters the system (e.g. via fluid) during
the metamorphic evolution. Such information becomes impor-
tant in assessing phase relations at different stages during the
evolutionary history of a granulite-facies metamorphic ter-
rane. Hence, the proper textural and chemical characterization
of major or accessory phases containing F is of immense
importance.
Though a common accessory mineral like apatite generally
serves as a sink for F, minerals such as wagnerite,
(Mg,Fe,Ca,Mn)
2
[PO
4
(F,OH)], can also serve as a F sink in
Ca-poor metapelitic granulites in differentgeotectonic settings
(Braitsch 1960; Irouschek-Zumthor and Armbruster 1985;
Leroux and Ercit 1992; Novák and Povondra 1984; Brunet
et al. 1998) and also in regional granulite terranes (Grew 1981;
Simmat and Rickers 2000). There have been reports that wag-
nerite occasionally forms during the retrograde stage in Mg-Al
granulites when the ambient fluid is hydrous and P is not
partitioned into apatite (Simmat and Rickers 2000; Ren et al.
2003). Here, we report occurrences of F-rich wagnerite from
two aluminous granulite samples located in the Eastern Ghats
Belt of eastern India.
Geological background
The Proterozoic Eastern Ghats Belt exposes deep crustal sec-
tion along the eastern part of India bearing crucial evidence for
India- East Antarctica correlation (Dasgupta et al. 2013 and
references therein). Although composed entirely of granulite-
facies rocks, this belt has a complex evolutionary history with
different crustal domains and provinces amalgamated in the
present configuration (Rickers et al. 2001a; Dobmeier and
Raith 2003; Dasgupta et al. 2013). The Eastern Ghats
Province (Dobmeier and Raith 2003) which is a combination
of isotopicdomains 2 and 3 (Rickers et al. 2001a) occupies the
major part of the belt wherein most of the petrological, struc-
tural and geochronological data are available. Aluminous
granulites from the Domain 2 are reported to preserve UHT
metamorphism (T>1000°Cat8–10 kbar) that ensued
through anticlockwise P-T trajectory (Sengupta et al. 1990;
Dasgupta et al. 1995;Korhonenetal.2013a) at a time frame
of ca. 1030–990 Ma (Bose et al. 2011; Das et al. 2011). The
UHT metamorphosed lower crust was near-isobarically
cooled (T~ 800 °C at 6 kbar) before being reworked by a
second granulite grade event (T~ 850 °C at 7–8 kbar) at ca.
953 Ma (Das et al. 2011).
Fluor-wagnerite is identified from two samples of Domain
2intheEasternGhatsBelt(Fig.1), where rocks have
reportedly undergone at least four phases of deformation and
at least three phases of metamorphism (Bhowmik 1997;Das
et al. 2011). The P-T conditions of the UHT peak metamor-
phism (M
1
) and subsequent reworking (M
2
) are mentioned
before. The third phase of metamorphism (M
3
) occurred at
500–800 °C (Das et al. 2011). The first sample containing
fluor-wagnerite was collected from a locality near
Bondaguda (N 18°18′7.1″, E 82°57′59.2″). Here, the major
rock unit is a quartz-plagioclase-garnet-K-feldspar gneiss (lo-
cally known as leptynitic gneiss), which shows magmatic fea-
tures (Fig. 2a). This leptynitic gneiss locally contains
foliation-parallel elongated enclaves of aluminous granulite
(spinel-garnet-cordierite-orthopyroxene-sillimanite-biotite-K-
feldspar-quartz) and charnockite (quartz-K-feldspar-
orthopyroxene-plagioclase-garnet-ilmenite) (Fig. 2c). A peg-
matitic variety of quartzofeldspathic rock occurs as conform-
able layers along the gneissic foliation, but locally straddles
the regional foliated structure. From the textural and
compositional characteristics of the aluminous granulite, Das
et al. (2006) postulated the existence of an anomalously Al-
rich orthopyroxene at the peak of UHT metamorphism
(>1000 °C at 9–10 kbar; Das et al. 2006) which on breakdown
resulted in different intergrowth textures involving spinel, sap-
phirine and garnet, all enclosed within garnet porphyroblasts.
Using the sequence of mineral assemblages, this study docu-
mented UHT metamorphism at peak stage which was follow-
ed by near isobaric cooling and decompression in an overall
anticlockwise P-T path. The fO
2
evolution history of the entire
litho-assemblage was documented from intricate textural and
compositional characteristics of different Fe-Ti oxides along
with the fluid inclusion data (Bose et al. 2009; Das et al.
2013). Their results show that the cooling history occurred
at relatively high fO
2
conditions (3–4 log units higher than
FMQ buffer) when an oxygenated H
2
O fluid with a compo-
nent of possible F-rich brine was coexisting with the retro-
grade assemblage. Subsequently, a late low-fO
2
H
2
O–CO
2
fluid (evident from fluid inclusion data in Bose et al. 2009)
with fluoride-bearing mixture infiltrated the litho-assemblage
from the outside, which broadly coincided with the final crys-
tallization of the pegmatite (Bose et al. 2009). Zircon U-Pb
SHRIMP analyses of the aluminous granulite sample yielded
an age population of ca. 950–900 Ma, which possibly repre-
sents the timing for a pervasive metamorphic overprinting
(Bose et al. 2011). In the absence of more well-constrained
data (e.g. texturally constrained monazite and/or zircon), the
timing of the early UHT event could not be determined.
The second fluor-wagnerite occurs in Panirangini (N18°18′
26″, E82°54′28″), which is located approximately 10 km
north of Bondaguda. In the Panirangini, aluminous granulite
(garnet-spinel-sapphirine-cordierite-orthopyroxene-silliman-
ite-K-feldspar-quartz) occurs as a small pod within the
leptynitic gneiss (quartz-K-feldspar-plagioclase-garnet-bio-
tite-ilmenite) (Fig. 2b and d). Other rocks include
418 Das K. et al.
sillimanite-garnet-K-feldspar-quartz gneisses (locally known
as khondalites), calc-silicate granulites (wollastonite-scapo-
lite-clinopyroxene-plagioclase-quartz-calcite), charnockites
and mafic granulites (orthopyroxene-clinopyroxene-plagio-
clase-garnet-ilmenite-hornblende). Based on textural,
microstructural and compositional criteria, Das et al. (2011)
documented polymetamorphic signatures with an early
counter-clockwise metamorphic evolution culminating in
UHT metamorphism (>1000 °C) at 6.5–8.5 kbar. Thermal
relaxation of the crust was later achieved by cooling down
to ~800 °C. Prominent granulite-facies reworking occurred
during a compressive event which accompanied folding, deep
crustal shearing and melt extraction. Based on texturally well-
constrained monazite, the early UHT metamorphism has been
dated at ca. 990 Ma (Das et al. 2011). Zircon SHRIMP U-Pb
data yielded a concordant age of 953 ± 6 Ma, which is
interpreted as the timing of granulite reworking as the cooled
lower crust underwent loading and heating by a prominent and
separate orogenic pulse (Das et al. 2011). Zircon age data
further yielded a younger group age of ca. 900 Ma which
possibly signifies the cooling event when zircon grains finally
crystallized from the extracted melt (Das et al. 2011). On the
contrary, Korhonen et al. (2013a) argued that the entire
Domain 2 witnessed prolonged thermal high and the proposed
counter-clockwise path resulted from one single cycle of oro-
genesis that was operative over 160 million years. This was
based on the monazite data (Korhonen et al. 2013b)showing
age spread from ca. 1130 Ma to ca. 970 Ma. Whether the
Domain 2 evolved in a single stage or two stages is thus
debatable.
Methodology
Fluor-wagnerite grains were identified by microscopic study
followed by back-scattered electron imaging (BSE) using a
JEOL JSM-6390 A scanning electron microscope equipped
with JED-2300 energy dispersive system at the Hiroshima
University, Hiroshima, Japan. Point chemical analyses of
fluor-wagnerite are carried out on a JEOL-JXA- 8900
EPMA (Electron Probe Micro-analyzer) at the University of
Kobe, Kobe, Japan. Other minerals were analyzed by EPMA
technique using the JEOL JXA 8200 Superprobe at the
Natural Science Center for Basic Research and Development
(N-BARD), Hiroshima University, Hiroshima Japan. The op-
erating conditions were 15 kV accelerating voltage, 12 nA
beam current, and 1–2μm beam diameter. JEOL standards
(JEOL Datum Ltd.) are used for analyses. Problems in the
proper analysis of F using EPMA have been discussed in
detail by several workers (e.g. Pitra et al. 2008; Fialin and
Chopin 2006; Stormer et al. 1993; Goldoff et al. 2012). The
choice of proper analytical conditions and Bappropriate^stan-
dards has a broad influence on the actual quantification of F by
EPMA. We used fluorite (CaF
2
)astheFstandardandana-
lyzed for F in biotite, apatite and wagnerite using LDE1 crys-
tal. The time-dependent intensity of the FK
α
line for fluorite
shows differential changes using a 15 kV, 12 nA beam current
and0to50μmbeamdiameter(Fig.3). We chose a 15 kV,
12 nA and 10 μm beam as this represented the optimum con-
ditions with a very small change in intensity. Using this con-
dition, natural fluor-apatite (North Wales, UK, cf. page 665 of
Deer et al. 1992) yielded an F-content of 3.2 wt% (± 0.06). We
Fig. 1 Generalized map of
Eastern Ghats Belt (modified after
Ramakrishnan et al. 1998). Inset
shows the position of EGB in
present-day India. The present
study area (marked as star) be-
longs to the central migmatitic
zone according to Ramakrishnan
et al. (1998) and domain 2 of
Rickers et al. (2001a)
The occurrence of fluor-wagnerite in UHT granulites 419
Fig. 2 a Hand sketch of the Bondaguda quarry (modified after Das et al.
2006). bThe local geological map around Panirangini modified is after
Das et al. (2011). cOutcrop photograph of aluminous granulites and
associated rocks at Bondaguda. AG stands for aluminous granulite. The
pen cap is 4 cm long. (d) Outcrop photograph of Panirangini. The pen is
14.5 cm long. Darker patches are orthopyroxene-rich domains in the
aluminous granulite
Fig. 3 The time sequence analysis of fluorite using different probe
diameters to determine the best analytical conditions. During analysis of
standard F-apatite, and unknown wagnerite and apatite, a 15 kV
accelerating voltage, 12 nA beam current and 10 μm probe diameter
were used. The vertical bars represent the calculated errors in X-ray
counts (1 s.d.) for each data point
420 Das K. et al.
analyzed the same grain by the ISE (ion selective electrode)
method which also yielded an F-content of 3.1 wt%, though
the reported value of F in this fluor-apatite is 3.7 wt%. Pitra
et al. (2008) noted that measurement of F using an apatite
standard may underestimate the F-content in the unknown
sample compared to using other standards such as topaz.
There is a possibility that our estimation of the F-content is
also slightly on the lower side (i.e. 0.5 to 0.6 wt%). During
analysis of F in the CaF
2
standard, fluor-wagnerite, biotite and
fluor-apatite, peak and background values were measured for
40 s and 20 s, respectively. Other mineral phases were mea-
sured using a 15 kV of accelerating voltage, 20 nA beam
current and 1–2μmbeamdiameter.
Raman spectral analysis of fluor-wagnerite was done
in quasi-backscatter geometry using a home-built micro-
Raman spectrometer. An argon laser with wavelength of
488 nm (Ion Laser Technology Model 5500 A) was
focused with a 50× objective lense (NA 0.55) to a spot
size of 1 μm having a laser power of 10 mW on the
desired mineral grains in the polished thin section. The
system had been used in the confocal mode. Raman
signals from the samples were obtained using a
nitrogen-cooled CCD detector (Princeton Instruments
SPEC-10) with spectrograph (Acton SpectraPro-500i).
Wavenumber calibration was done using plasma dis-
charge lines of the Ar laser; the resulting wavenumber
accuracy was 0.1 cm
−1
. Peak fitting was done after ap-
propriate background correction, assuming Voigt-shaped
band profiles. The details of the method is described in
Kanzaki et al. (2012).
Results
Petrography
Mineral assemblages and petrographic textures in aluminous
granulites wherein fluor-wagnerite was observed are quite
similar for both samples. Mineral assemblages in aluminous
granulite include porphyroblastic garnet, cordierite,
orthopyroxene (± sapphirine, spinel), and quartz with minor
zircon and monazite representing the possible pre- to near-
peak conditions of metamorphism. The foliation- (i.e., trans-
posed S
2
/S
3
foliation during D
2
/D
3
) forming assemblage con-
sists of elongated quartz ribbons, perthitic K-feldspar, biotite,
garnet, orthopyroxene, sillimanite and cordierite (Fig. 4a).
Spinel is often associated with magnetite and presumed to
be exsolved from a spinel solid solution (Bose et al. 2009).
In the Panirangini sample, hemo-ilmenite is additionally pres-
ent as an opaque phase. Fine grained matrix minerals in a
mylonitic fabric (S
3m
) formed by skeletal intergrowth of
orthopyroxene + sillimanite + quartz which represents the
breakdown products of cordierite porphyroblasts (Fig. 4b).
Biotite is late, replacing all the ferromagnesian minerals
along their grain boundaries and fractures irrespective of their
textural associations. Such biotite grains are more euhedral in
shape and have sharp boundaries compared to the ferromag-
nesian minerals with which they occur. While late patchy bi-
otite tends to be associated with fluor-wagnerite, there is an
early variety of biotite (chemically and texturally distinctive)
in this rock. This early variety occurs inside the garnet,
orthopyroxene and cordierite porphyroblasts (away from frac-
tures in these grains), and has rounded grain boundaries.
Formation of ribbon quartz and flattened garnet
grains are conspicuous in this rock. Centimeter-scale,
massive domains of aluminous granulite also occur, pos-
sibly as low-strain zones. The fluor-wagnerite grains oc-
cur in close association with the garnet porphyroblasts;
while in the foliated high-strained zones augen of fluor-
wagnerite occur in the fine grained matrix (Fig. 4c–e).
Fluor-wagnerite is characteristically absent in the
porphyroblastic orthopyroxene-rich microdomains. In
the foliation (transposed S
2
/S
3
) perpendicular section,
grains of fluor-wagnerite are elongated parallel to the
foliation and exhibit boudinage to pinch- and swell mi-
crostructures (Fig. 4f). Apatite grains occur in fractures
in the fluor-wagnerite, in the neck zones of the boudins,
and as rims around recrystallized grains of fluor-
wagnerite (Fig. 4f). The estimated modal percentages
of different minerals within the garnet-bearing microdo-
mainsinboththesamplesare23.3–25.8 % garnet,
19.4–24.1 % biotite, 12.9–23.3 % quartz, 14.2–17.3 %
cordierite, 4.1–5.6 % sillimanite, 3.2–4.5 % feldspars,
2.2–4.1 % spinel-magnetite-hemoilmenite, 0.5–0.8 %
monazite, 0.1–0.3%zirconand0.1–0.2 % wagnerite.
Based on these observations, two types of fluor-wagnerite
grains can be defined. Type I occurs in close association with
garnet. Type Ia occurs as inclusions within garnet and is not
associated with fluor apatite (Fig. 4c). Type Ib occurs along
grain boundaries or along fractures in garnet and is partly
overgrown by apatite (Fig. 4e). Type II fluor-wagnerite occurs
as elongated grains in the matrix assemblage parallel to the
foliation associated with fluor-apatite and carbonates (Fig. 4f).
Raman spectroscopy
Type I and II fluor-wagnerite grains were analyzed using
micro-Raman spectroscopy and compared with available
spectra from the RRUFF database of the University of
Arizona (Downs 2006). All the analyzed grains have similar
spectra with prominent bands at ~460, 985, 1005, 1056, and
1100 cm
−1
and broader and smaller bands around 625, 812
and 1140 cm
−1
(Fig. 5a). Type I and type II fluor-wagnerite
grains show slight differences with respect to bands
>1000 cm
−1
. Type Ib fluor-wagnerite is characterized by the
reduction or non-appearance of the ~1056 and 1140 cm
−1
The occurrence of fluor-wagnerite in UHT granulites 421
bands. While comparing these data with those available at
RRUFF database, we observed that most of the band spectra
are strikingly similar to that of wagnerite (R050519).
Particularly, the band occurring at 1005 cm
−1
for types I and
II is similar to that of R050519 wagnerite (University of
Arizona Mineral Museum). This band is characteristically ab-
sent in other wagnerite samples from the RRUFF database
(Fig. 5b). Some of the differences in the positions of the
Raman bands among type I, type II, and the R050519 wag-
nerite could be caused by different F and Fe contents. The
calculated crystal structure for R050519 wagnerite from the
RRUFF database is that of a triploidite group wagnerite
polytype (Ma2bc) with space group of P2
1
/cor P2
1
/a(Coda
et al. 1967; Tadini 1981). We consider that the fluor-wagnerite
in this study has a crystal structure similar to the Ma2bc
polytype, unlike the structure reported for wagnerite which
contains less Mg and F from a granulite-facies terrane like
Larsemann Hills, Prydz Bay, East Antarctica, which has a
different polytype Ma5bc (Ren et al. 2003). This is also sup-
ported by R060456 wagnerite with the Ma5bc polytype,
which does not show a strong Raman band at ~1005 cm
−1
compared to R050519 wagnertite and the fluor-wagnerite in
these samples.Moreover, the spectral data of type Ia shows no
discernible O-H stretching vibration band at ~3580 cm
−1
re-
ported from Larsemann Hills sample (Ren et al. 2003).
Mineral chemistry
Wagnerite chemical compositions are F-rich (9.6 to 11.2 wt%).
The three petrographic types (types Ia, Ib and II) have subtle
chemical differences (Table 1). All types are very similar in
terms of magnesian content as X
Mg
ranges between 0.96 and
0.97 (type Ia being slightly lower than the other two types).
However, type Ia and II wagnerite grains are the most F-rich
[X
F
= F/(F + OH) ~ 0.91 to 0.98], while those which occur
along the fractures of garnet grains (type Ib) are less F-rich
(X
F
~ 0.83 to 0.85) (Fig. 6). The measured Ca, Mn and Ti
contents are low. The overall chemical compositions are close
to end-member stoichiometry. As mentioned above, type Ib
and type II wagnerite grains are replaced by fluor-apatite along
their fractures. In places, there is late replacement of fluor-
apatite (not affecting other minerals including adjacent fluor-
Fig. 4 Back-scattered electron
photomicrographs of rock
textures and different type
occurrences of fluor-wagnerite. a
Mylonitic fabric shown by
deformed porphyroclasts of
spinel, orthopyroxene, among
others in the Panirangini sample.
bSkeletal intergrowth of
orthopyroxene + sillimanite +
quartz surrounding
porphyroblastic cordierite. The
intergrowth phases are aligned
along the mylonitic foliation
shown by matrix quartz grains. c
Rounded grains occurring inside
garnet porphyroblasts (type 1a,
note that late fractures affect both
garnet and fluor-wagnerite). d
Fluor-wagnerite occurring along
the fractures of garnet
porphyroblasts (type Ib) and eal-
so along the grain boundaries. f
pinch-and-swell structured fluor-
wagnerite grains in the mylonitic
matrix of the aluminous granulite.
Note that apatite (white phase
with black arrow) develops on
fluor-wagnerite in (eand f). In (f),
a replacing phase (carbonate-
fluorapatite??) develops (slightly
grey, marked with white arrow)
on apatite
422 Das K. et al.
wagnerite) which results in a patchy phase with low oxide wt%
totals. Since the F and CaO contents are comparable with that
of fluor-apatite, the P
2
O
5
content is reduced, possibly partially
replaced by carbonate component during very low temperature
alteration of apatite (Shemesh 1990;Miyakeetal.1990).
The major associated minerals include garnet and bi-
otite (Table 2). Garnet is mainly a solid solution of
almandine and pyrope with a low grossular and spessar-
tine content (~Alm
60
Pyp
36
Grs
3
Sps
1
). Biotite grains oc-
cur texturally as two types. These form inclusions in
garnet porphyroblasts (namely, early biotite) and those
occurring along the fracture, grain boundaries and in the
matrix (namely, late biotite). The former contains higher
Mg (X
Mg
~0.88–0.85) and F contents (F ~ 3–4wt%),
while the latter contains less Mg (X
Mg
~0.68–0.71) and
a lower F content (F ~ 2 wt%). This variation thus
follows the Fe-F avoidance trend (Nash 1993). At the
same time, the role of earlier biotite and fluor-wagnerite
in locking F from the effective bulk rock composition
can also be important. Both types of biotite are rich in
Ti(nearly5to2.6wt%),butthevariationisnotsys-
tematic. Associated cordierite is Mg-rich (X
Mg
=0.83–
0.86), while in the matrix orthopyroxene (X
Mg
=0.61–
0.62) intergrowth with sillimanite and quartz developed
on cordierite is quite aluminous (Al
2
O
3
=6.3–6.9 wt%)
(cf. Das et al. 2006,2011;andBoseetal.2006).
Among the Fe-Ti oxide minerals, green spinel contains
1–2 mol% gahnite and 3–4 mol% magnetite components
while magnetite is almost pure. Ilmenite contains 10–
15 mol% hematite and 4–7 mol% geikielite component
while exsolved hematite contains ~16 mol% ilmenite
component.
Discussion
Generalities
The Mg and F-rich wagnerite (Mg
1.91–1.94
Fe
0.06–
0.07
Ca
<0.01
)(P
0.99–1.00
O
4
)(OH
0.02–0.17
F
0.98–0.83
) from the
Eastern Ghats Belt chemically and structurally differs
from an earlier reported, hydroxyl-rich variant of wag-
nerite (Simmat and Rickers 2000) from the nearby
Anakapalle region of the same terrane. This phase also
differs in composition and microstructurally from other
occurrences associated with high-grade assemblages
(e.g. Pitra et al. 2008), and from the Antarctic occur-
rence (e.g. Ren et al. 2003,2005;Grewetal.2007). In
the latter occurrence, wagnerite shows F/OH ratio of
1.56 compared to the present case (49–4.88) although
the Mg/Fe ratio is similar. For the Armorican occur-
rence (Pitra et al. 2008), the F/OH ratio varies widely
within the range 4.56–0.78 (with an exceptional value
of infinity; Analysis no. 301, sample VY11). The Mg/Fe
ratio for this wagnerite is much lower compared to the
present one.
Despite the known occurrence of F in the structure of
biotite and other accessory minerals, no direct evidence of
the source of F can be identified. It may either be in the
original protolith of the aluminous granulites or in the
coexisting halide-rich fluids present during the evolution
of these granulites (Higashino et al. 2013;Boseetal.
2005,2009; Pitra et al. 2008). Nevertheless, the mode of
occurrence and the characteristic mineral associations have
important implications for the petrogenesis of these fluor-
wagnerite-bearing rock suites. Both aluminous granulite
samples (Bondaguda and Panirangini) are characterized by
garnet porphyroblast microdomains and aluminous
orthopyroxene porphyroblast microdomains. Interestingly,
fluor-wagnerite occurs only within the garnet porphyroblast
domains, where rounded to prismatic grains of type Ia
fluor-wagnerite associated with F-rich biotite (3–4wt%F)
grains occur as mineral inclusions. These wagnerite grains
Fig. 5 a Representative micro-Raman spectra of fluor-wagnerite grains.
Spectra of different textural types are shown with vertical offset. Note the
two strong bands around 1000 cm
−1
.bRaman band spectra for three
samples from the RRUFF database (http://rruff.info/; Downs 2006).
Note the similarity of the band spectra for R050519 with the natural
samples
The occurrence of fluor-wagnerite in UHT granulites 423
are the most F-rich [X
F
=0.96–0.98] and possibly consti-
tute part of the prograde-to-peak metamorphic assemblage
in this rock. Larger fluor-wagnerite grains also occur as
boudinaged to pinch-and-swell grains (type II) in the matrix
of the Panirangini sample associated with the last metamor-
phic mylonitic fabric (i.e. S
3m
). This implies at least pre-D
3
crystallization of this variety of fluor-wagnerite (D
2
-M
1
stage identified as peak to early retrograde metamorphic
stage by Das et al. 2011, their Table 1). Type Ib fluor-
wagnerite (X
F
=0.83–0.85), on the other hand, occurs as
prismatic grains along fractures and at grain-boundaries of
garnet porphyroblasts partially replaced by apatite. All the
wagnerite grains are F-rich, but there are differences in F
contents among different textural types. As the type Ia and
type II wagnerite grains are more F-rich, we need to con-
ceive a process that can explain the preferential partitioning
of F over H
2
O in these wagnerites. Brunet et al. (1998)
experimentally showed that the synthetic hydroxyl equiva-
lent (β-Mg
2
PO
4
OH) is stable above 10 kbar and argued
that for F-bearing system, wagnerite would be stable at
lower pressure (<10 kbar) conditions. Therefore, F-
enrichment in wagnerite could imply a decrease in pressure.
Tabl e 1 Representative electron microprobe point analyses data of different textural types of wagnerite
Type I wagnerite Type II wagnerite
type Ia (grains inside grt) Ib (along fracture and boundary of grt) matrix grains
Sample No. 4Q_1 4Q_2 Average s.d. (n= 10) 4Q_13 4Q_15 Average s.d. (n= 7) Pan10A_32 Pan10A_33 Average s.d.(n = 8)
wt%
P
2
O
5
43.09 42.77 42.84 0.17 42.87 42.87 42.92 0.14 43.08 42.53 42.73 0.19
TiO
2
0.16 0.19 0.18 0.02 0.23 0.23 0.21 0.01 0.34 0.23 0.28 0.04
Al
2
O
3
0.01 0.01 0.01 0.01 0.02 0.01 0.01 0.01 0.01 0.02 0.01 0.01
FeO 3.68 3.35 3.10 0.27 2.84 2.97 3.04 0.16 2.70 2.54 2.98 0.24
MgO 46.80 46.83 47.25 0.43 47.38 47.82 47.43 0.21 47.67 47.37 47.27 0.20
MnO 0.11 0.08 0.08 0.02 0.03 0.10 0.08 0.02 0.05 0.05 0.07 0.02
CaO 0.26 0.25 0.25 0.02 0.28 0.31 0.26 0.02 0.21 0.15 0.19 0.03
F 11.25 10.55 10.99 0.26 9.62 9.87 9.56 0.17 10.52 11.23 10.67 0.32
Total 100.63 99.58 100.09 0.34 99.21 100.03 99.49 0.43 100.14 99.37 99.70 0.42
a.p.f.u.
P 1.00 1.00 1.00 0.00 1.00 1.00 1.00 0.00 1.00 1.00 1.00 0.00
Ti 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.01 0.00 0.01 0.00
Al 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
Fe 0.08 0.08 0.07 0.01 0.07 0.07 0.07 0.00 0.06 0.06 0.07 0.01
Mg 1.91 1.93 1.94 0.02 1.95 1.96 1.95 0.01 1.95 1.96 1.95 0.01
Mn 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
Ca 0.01 0.01 0.01 0.00 0.10 0.01 0.01 0.00 0.01 0.00 0.01 0.00
F 0.97 0.92 0.96 0.02 0.84 0.86 0.83 0.01 0.91 0.99 0.93 0.03
OH 0.03 0.08 0.05 0.02 0.17 0.15 0.18 0.01 0.10 0.02 0.07 0.03
F/OH 34.39 11.01 4.94 5.73 9.10 49.50
X
Mg
0.96 0.96 –– 0.97 0.97 –– 0.97 0.97 ––
X
F
0.97 0.92 –– 0.83 0.85 –– 0.91 0.98 ––
X
Mg
= [Mg/(Mg + Fe
2+
)]
X
F
=[F/(F+OH
calc
)]
OH is calculated on charge balance criteria
Fig. 6 Compositional variation for different types of fluor-wagnerite.
The square symbol is for type Ia fluor-wagnerite, while triangles
represent type Ib fluor-wagnerite, and solid diamonds represent type II
fluor-wagnerite
424 Das K. et al.
None of the presently studied samples shows any evidence
for decompression to substantiate this model. Based on
textural, chemical and stable isotopic data, Pitra et al.
(2008) presented an alternative model of shear zone en-
hanced metasomatism which can explain F-enrichment in
wagnerite due to long-term multistage fluid-rock interaction
occurring both going up- and down-temperature. Such
widespread fluid-rock interaction is interpreted as the most
viable mechanism for F-wagnerite formed by non-
magmatic process (Pitra et al. 2008). The first mechanism
involving P-dependent incorporation of OH can be applied
to explain the occurrence of OH-wagnerite from Dora
Maira Massif (Chopin et al. 1993). Similarly the OH-
wagnerite reported from the nearby area of the Eastern
Ghats Belt (Simmat and Rickers 2000) also follows this
trend as petrological data from the aluminous granulite of
Anakapalle also imply early high-pressure (>10 kbar) con-
ditions during the metamorphic peak (Dasgupta et al. 1994;
Rickers et al. 2001b). Contrary to the two mentioned mech-
anisms, occurrence of wagnerite in anatectic rocks like the
present samples can be explained by melt-solid interaction
during migmatization, or by later-stage interaction of rocks
Tabl e 2 Representative EPMA analysis data of phases associated to wagnerite
Phase Grt Grt Opx Opx Sil Bt Bt Bt Crd Crd Ap Ap
Texture Por Por fine grain fine grain fine grain matrix matrix included in grt matrix matrix on wagnerite
wt%
SiO
2
39.56 39.39 48.66 47.94 36.46 39.67 39.29 38.90 48.70 49.23 CaO 55.21 55.63
TiO
2
0.01 n.d. 0.05 0.05 0.01 2.60 2.66 5.07 n.d. n.d. Na
2
O n.d. n.d.
Al
2
O
3
22.12 21.97 7.16 7.13 62.09 13.75 14.40 14.20 32.96 33.25 MnO n.d. 0.07
Cr
2
O
3
n.d. 0.01 0.02 0.02 0.03 n.d. 0.06 0.06 n.d. 0.02 FeO 0.33 0.36
FeO 26.37 26.71 22.33 22.85 1.02 8.61 8.40 6.22 4.41 4.14 P
2
O
5
41.98 41.46
MnO 0.59 0.66 0.19 0.18 n.d. 0.00 0.00 0.00 0.05 0.07 Cl n.d. n.d.
MgO 11.39 10.90 20.08 20.43 n.d. 21.71 20.76 21.16 10.79 11.17 F 4.39 4.20
CaO 0.78 0.75 0.02 0.04 0.01 n.d. n.d. 0.17 n.d. n.d. O = F,Cl -1.85 -1.77
Na
2
O n.d. n.d. 0.01 0.01 0.02 0.33 0.28 0.38 0.03 0.01 Total 100.06 99.95
K
2
O n.d. 0.01 n.d. 0.01 n.d. 9.89 10.70 10.16 0.02 n.d.
F n.d. n.d. n.d. n.d. n.d. 4.18 4.27 2.53 n.d. n.d.
F = O -1.76 -1.80 -1.06
Cl n.d. n.d. n.d. n.d. n.d. n.d. 0.08 0.07 n.d. n.d.
Cl = O n.d. -0.02 -0.02
Total 100.83 100.40 98.53 98.65 99.63 99.03 99.08 100.84 96.97 97.90
O basis 12 6 5 22 18 O basis 25 25
a.p.f.u.
Si 2.99 3.00 1.84 1.82 0.99 5.68 5.64 5.54 4.99 4.99 Ca 9.53 9.65
Ti 0.00 0.00 0.00 0.00 0.00 0.28 0.29 0.54 0.00 0.00 Na 0.00 0.00
Al 1.97 1.97 0.32 0.32 1.99 2.32 2.44 2.39 3.98 3.97 Mn 0.00 0.01
Cr 0.00 0.00 0.00 0.00 0.00 0.00 0.01 0.01 0.00 0.00 Fe 0.04 0.05
Fe 1.67 1.70 0.71 0.72 0.02 1.03 1.01 0.74 0.38 0.35 P 5.72 5.69
Mn 0.04 0.04 0.01 0.01 0.00 0.00 0.00 0.00 0.00 0.01 Cl 0.00 0.00
Mg 1.28 1.24 1.13 1.15 0.00 4.63 4.45 4.50 1.65 1.69 F 2.24 2.15
Ca 0.06 0.06 0.00 0.00 0.00 0.00 0.00 0.03 0.00 0.00
Na 0.00 0.00 0.00 0.00 0.00 0.09 0.08 0.11 0.01 0.00
K 0.00 0.00 0.00 0.00 0.00 1.81 1.96 1.85 0.00 0.00
F––– – – 1.89 1.94 1.14 ––
Cl ––– – – 0.00 0.02 0.02 ––
Total 8.02 8.01 4.00 4.02 3.01 17.73 17.82 16.85 11.02 11.02
X
Mg
(Fe total) 0.44 0.42 0.62 0.61 –0.82 0.82 0.86 0.81 0.83
Oxide weight measured less than 0.005 % is represented as 0.00
Elements calculated less than 0.005 pfu are represented as –
BPor^indicates porphyroblasts
The occurrence of fluor-wagnerite in UHT granulites 425
with P-rich melts (Ren et al. 2005;Grewetal.2007)or
even fluids (Webster et al. 1997;Thomasetal.2003). In
absence of experimental data on partition coefficients (D)
between wagnerite-biotite and wagnerite-melt, we cannot
quantify the mechanism any further. However, from the fact
that F/OH ratio in wagnerite is much higher than that of
biotite, an extreme partitioning of F between wagnerite and
melt can be inferred.
Except for type 1a grains, which are included in the
metamorphic peak garnet porphyroblasts, the other two
types i.e., type Ib and type II responded to later
physico-chemical changes. These two latter types are ex-
tensively fractured with development of late-stage fluor-
apatite along those fractures. Apatite is a very common
phosphate mineral in geological environment when com-
pared to wagnerite and relative stability of these two phos-
phate minerals often depends on the Ca/P ratios of the
host rock vis-à-vis apatite (Pitra et al. 2008). The shift
of wagnerite to apatite stability in a rock therefore implies
increase in Ca/P ratio. Pitra et al. (2008) envisaged Na-
metasomatism as a viable mechanism to explain the sta-
bility of wagnerite over apatite in cordierite-gedrite-bearing
rocks of Armorican Massif, France. These authors argued
that Na metasomatism enhanced stability of plagioclase at
a later stage which helped in sequestering Ca and reducing
the stability of apatite. Stability of apatite over wagnerite
thus requires increased availability of Ca, but there is no
evidence of Ca-metsomatism in the present samples. The
source of Ca during this replacement in an overall Ca-
poor bulk composition is not well-understood, although
the very low modal presence of plagioclase in these rocks
may possibly act as a source. The following model bal-
anced reaction involving wagnerite, apatite and plagioclase
can be formulated.
3Mg2PO4F;OHðÞþ5CaAl2Si2O8þO2¼Ca5PO4
ðÞ
3F;OHðÞþ2Mg3Al2Si3O12þ3Al2SiO5þSiO2þH2O=F
wagnerite anorthite apatite pyrope sillimanite quartz
Textural and chemical characteristics, therefore suggest
that fluor-wagnerite was present as a part of the pro-
grade assemblage (Type Ia being pre-D
1
-D
2
/M
1
), be-
came part of the subsequent D
2
-M
1
assemblage (Type
II) and late post D
3
-M
2
assemblage (Type Ib) along
with late-stage biotite. The Fe-Ti-oxide assemblages
from the two samples are represented by the spinel-
magnetite solid solution along with ilmenite. Studying
oxide mineral assemblages from various rocks of the
Bondaguda locality, Bose et al. (2009) suggested that
oxygenated fluid coexisted with the mineral assemblages
all throughout the metamorphic history. Such high fO
2
conditions presumably helped to change Fe
2+
to Fe
3+
,
and effectively increase the bulk X
Mg
, eventually stabi-
lizing high-Mg phases including wagnerite. High-F ac-
tivities during the peak and retrograde stages have two
implications, its effect on fluid activities (e.g. reduced
a
H2O
in local-scale) and its effect on the stability of
biotite-bearing assemblages in deep crustal levels.
Possible implication for wagnerite-Ti-F-biotite-bearing
assemblages in Ca-poor metapelites
Type Ia and type II fluor-wagnerite and coexisting Ti-F-
rich early biotite included in garnet porphyroblasts im-
ply that F-availability during prograde metamorphism
was quite high in these microdomains. This stabilizes
the prograde biotite-bearing assemblages to comparative-
ly higher temperature (Hensen and Osanai 1994)com-
pared to the F-free biotite-dehydration melting region
which has been experimentally deduced to be ~800–
850 °C, in aluminous metapelites (Carrington and
Harley 1995; Das et al. 2003). The behavior of F in
aluminous melt is not well-known. However, theoretical
considerations (Bose et al. 2005) and experimental data
(Hensen and Osanai 1994; Tareen et al. 1998;Nairand
Chako 2002) indicate that biotite-dehydration melting
curves in an F-bearing system may shift as much as
100 °C towards higher temperatures at lower crustal
depths. In a simplified KFMASHTiF system where bio-
tite and partial melt are the only two potential F hosts,
the stability fields of F-biotite-bearing assemblages are
expanded towards higher temperatures. For a reaction
involving biotite, K-feldspar and orthopyroxene, the cal-
culated shift in temperature is 120–150 °C assuming an
ideal phlogopite mixing model. This implies that our
earlier estimates of initial biotite-dehydration melting
of the rocks (Das et al. 2006,2011;Boseetal.2009)
needs to be increased up to 900 °C which is close to
UHT conditions (see discussion in Bose et al. 2005).
This shift has been schematically shown in Fig. 7with
an experimentally constrained KFMASH petrogenetic
grid at relatively high oxygen fugacity (Das et al.
2003). Due to lack of experimental data, the exact
amount of shift is still unknown. Moreover, in more
complex natural pelitic systems with P-rich and Ca-
poor bulk compositions, wagnerite may act as another
competing solid for the incorporation of F. Although
this occurs as an accessory mineral in most of the alu-
minous granulites and amphibolites, the preferential
426 Das K. et al.
partitioning of F into wagnerite over coexisting biotite
can marginally enhance the production of melt at UHT
conditions (>900 °C). Obviously, the stability of other
common but accessory phosphate phases like monazite
at these P-T conditions is also important. Thus, depend-
ing on the relative stabilities of different phosphates and
F-bearing phases during biotite dehydration-melting P-T
space, and the relative contents of REE, P and F in the
bulk chemistry, fluor-wagnerite may act as a sink for F
and eventually help to stabilize HT/UHT assemblages
(e.g. sapphirine-quartz-Al-orthopyroxene ± spinel)
coexisting with partial melts in the deep continental
crust. However, the exact controlling factors cannot be
quantified without experimental data.
Conclusions
Textural and phase chemical data for F-wagnerite from two
samples of aluminous granulite of the Eastern Ghats Belt,
India indicate fluid/melt-rock interaction involving F-rich
agents during the evolution of lower crust. Stability of F-
wagnerite in the early stage of granulite metamorphism could
result either from an F-rich protolith, or by melt/fluid-solid
interaction during partial melting. F-rich conditions were
maintained during the early retrograde stage when F-biotite
was stabilized and this has direct bearing on the partial melt
productivity based on biotite dehydration melting in the deep
continental crust. Replacement of F-wagnerite by F-apatite
possibly indicates availability of Ca from plagioclase in local-
ized microdomains during the latter part of the evolutionary
history. Characterization of such unusual F-bearing accessory
minerals is important for proper assessment of the P-T-fluid
evolution of deep crustal granulites.
Acknowledgments We thank M. Kanzaki for his generous assistance
with the Raman spectroscopy at the Institute for Study of the Earth’s
Interior, Misasa, Japan. We thank Yasuhiro Shibata for assisting in
EPMA analysis. We thank Dirk Spengler, Tetsuo Kawakami and Silvio
Ferrero for their insightful comments on an earlier version of the manu-
script that certainly helped to improve the quality of the paper. Helpful
comments and suggestions from Peter Tropper, Simon Harley and one
anonymous reviewer significantly improved the manuscript quality.
Efficient editorial handing by Lutz Nasdala is greatly appreciated. This
work is partially supported by the Visiting Scientist program at ISEI,
Okayama University for KD. Partial funding for this research was pro-
vided by CSIR, Govt. of India, via grant 24(0333)/14-EMR II to SB.
Fig. 7 The experimentally constrained petrogenetic grid in the
KFMASH system (modified after Das et al. 2003) showing the possible
positions of invariant points and univariant lines. The low-temperature
sides of the thick univariant lines represent the stability of biotite-bearing
assemblages, which may shift towards the high temperature side along the
three univariant lines (thick dashed) towards the (Bt) absent invariant
points. The thin lines represent the reactions involving anhydrous solid
UHT assemblages. Note that the exact degree of shift for the biotite
dehydration melting reactions (the shift of dashed thick lines) in the F-
present system (Ca-poor) is not known, since other components like Ti, P
and REE would influence this shift in presence of different competing
accessory minerals such as wagnerite and monazite. The two P-T paths
represent the reported paths from the two study areas (dark shade is for
Bondaguda area, Bose et al. 2006; light shade is for Panirangini area, Das
et al. 2011)
The occurrence of fluor-wagnerite in UHT granulites 427
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