The southern Indian granulite terrane is known for granulite - facies rocks which is formed during the ‘Pan-African orogeny. The region is composed of Neoproterozoic to Cambrian crustal blocks, dissected by large-scale shear zones (Palghat-Cauvery and Achankovil). The Palghat-Cauvery Shear Zone System (PCSZ), separates the terrane into two parts, Archean Dharwar Craton in the north and the Neoproterozoic Madurai Block in the south. The southern margin of this block is defined by the Achankovil Shear Zone. Geology of the study area The Achankovil Shear Zone (ACSZ) is major lineament of 8-10 km width and >100 km length. The rocks in the zone display a prominent NW-SE trending foliation with steep dips to southwest. Estimation of pressure and temperature (P-T) of this lithology was first carried out by Santosh (1987) based on conventional geothermometers and mineral equilibrium, that gave 700-800◦ C at 5.5-7.0 kbar. Later study done by Nandakumar and Harley (2000) which is slightly higher 925 ± 20◦ C at 6.5-7.0 kbar . 3.1. Grt- Opx- Crd Gneiss The Grt-Opx-Crd gneiss is a coarse-grained, granulite-facies rock with a probable pelitic protolith. The mineralogy of a representative sample (KR19-5G1) is plagioclase (30-40%), ortho-pyroxene (20-30%), garnet (10-20%), K-feldspar (10-20%), quartz (5-10%), and cordierite (2-5%) with accessory of biotite, spinel, and sillimanite (Fig. 2a). Garnet is very coarse-grained (3-6 mm) ,subidioblastic, and contains numerous fine-grained inclusions of sillimanite (0.05-0.2 mm), biotite (0.05-0.4 mm), spinel (0.05-0.1 mm), and quartz (0.05-0.1 mm). The most significant feature of this rock is the direct contact relation of fine grained spinel and quartz (Spl + Qtz), which occur only as inclusions in garnet. Spl + Qtz association has been regarded as one of the indicators for decompression at UHT conditions. This is the first finding of such an assemblage from the ACSZ.

1. ultrahigh-temperature metamorphism (UHT)
A cause study of Achankovil Shear Zone
(ACSZ)
Omer M .Ahmed
University of Kerala
India
MSc geology
2016

2. Spinel + quartz assemblage in
granulites from the Achankovil
Shear Zone,
southern India:
Implications for ultrahigh-
temperature metamorphism
By; Hisako Shimizu, Toshiaki Tsunogae and
M. Santosh
Journal of Asian Earth Sciences,
2009

3. Introduction
• The southern Indian granulite terrane is known
for granulite - facies rocks which is formed
during the ‘Pan-African orogeny.
• The region is composed of Neoproterozoic to
Cambrian crustal blocks, dissected by large-scale
shear zones (Palghat-Cauvery and Achankovil).
• The Palghat-Cauvery Shear Zone System (PCSZ),
separates the terrane into two parts, Archean
Dharwar Craton in the north and the
Neoproterozoic Madurai Block in the south.
• The southern margin of this block is defined by
the Achankovil Shear Zone.

4. Fig. 1.
Simplified geological map of the southern part of Kerala and Tamil Nadu (after GSI, 1995a,b)
PCSZ: Palghat-Cauvery Shear Zone; DC: Dharwar Craton; MGB: Madurai Block;
ACSZ: Achankovil Shear Zone;

5. Geology of the study area
• The Achankovil Shear Zone (ACSZ) is major
lineament of 8-10 km width and >100 km length.
• The rocks in the zone display a prominent NW-SE
trending foliation with steep dips to southwest.
• Estimation of pressure and temperature (P-T) of
this lithology was first carried out by Santosh
(1987) based on conventional geothermometers
and mineral equilibrium, that gave 700-800◦ C at
5.5-7.0 kbar.
• Later study done by Nandakumar and Harley
(2000) which is slightly higher 925 ± 20◦ C at 6.5-
7.0 kbar .

6. • Available geothermobarometric results from the
ACSZ thus demonstrate that the area underwent
UHT metamorphism.
• Geochronological investigation of the high-Al
orthopyroxene-bearing rocks suggests that the
peak UHT metamorphism took place at 580-600
Ma.
• The rocks show NW-SE foliation dipping steeply
to southwest, which is with the regional trend of
the ACSZ.

7. Petrography
• Most of the representative rock samples were
collected from the Pakkandom open quarry.
• The major lithology is comprises of
 Charnockite
 Khondalite
 Leptynite
 Different types of Gneiss
 Granulite
 Quartzite
 Metacarbonate rocks.
• The mineral assemblages and approximate
modal abundances of minerals are listed in
Table 1.

8. Table 1
Mineral assemblages of studied granulites with approximate abundance.
+++, abundant; ++, moderate; +, rare; ‘‘, inclusion in garnet.
1: Grt gneiss; 2: charnockite; 3: Spl-Sil gneiss; 4: Grt-Bt-Spl gneiss

9. 3.1. Grt- Opx- Crd Gneiss
• The Grt-Opx-Crd gneiss is a coarse-grained, granulite-facies rock
with a probable pelitic protolith.
• The mineralogy of a representative sample (KR19-5G1) is
plagioclase (30-40%), ortho-pyroxene (20-30%), garnet (10-20%),
K-feldspar (10-20%), quartz (5-10%), and cordierite (2-5%) with
accessory of biotite, spinel, and sillimanite (Fig. 2a).
• Garnet is very coarse-grained (3-6 mm) ,subidioblastic, and contains
numerous fine-grained inclusions of sillimanite (0.05-0.2 mm), biotite
(0.05-0.4 mm), spinel (0.05-0.1 mm), and quartz (0.05-0.1 mm).
• The most significant feature of this rock is the direct contact
relation of fine grained spinel and quartz (Spl + Qtz), which
occur only as inclusions in garnet.
• Spl + Qtz association has been regarded as one of the indicators for
decompression at UHT conditions. This is the first finding of such an
assemblage from the ACSZ.

10. The coexistence of
spinel and quartz
Grt, Pl-Qtz assemblage in
Grt-Opx-Crd gneiss
PPL view

11. Back-scattered electron image photographs of granulites from
PKDM showing detailed textures of minerals.
(a) The two minerals show direct grain contact with no
reaction texture between spinel and quartz

12. • Brownish orthopyroxene is sub-idioblastic and very
coarse-grained (up to 3cm) occurs adjacent to coarse
garnet. It contains inclusions of quartz, K-feldspar,
and Fe-Ti oxide. It is describe the equilibrium.
• Biotite is also present as an inclusion phase and
sometimes occurs in contact with quartz grains
coexisting with spinel.
• Cordierite is present as a matrix phase coexisting with
quartz- and sillimanite-bearing garnet (Fig. 2e).

13. Cordierite around garnet with numerous
sillimanite inclusions
XPL view .

14. 3.2. Grt charnockite
• Garnet-bearing charnockite occurs as layers parallel
to the foliation defined by Grt-Opx-Crd gneiss.
• A representative sample contains
 K-feldspar (20-30%), quartz (20-30%), plagioclase (10-15%),
garnet (5-10%), orthopyroxene (5-10%), and biotite (5-7%).
 With accessory spinel, Fe-Ti oxide, and zircon (Fig. 2g).
• Garnet (0.02-6.8 mm) is subidioblastic and contains numerous
inclusions of quartz (0.05-0.75 mm), biotite (0.05-0.75 mm),
zircon (0.04-0.1 mm), and Fe-Ti oxide.
• Orthopyroxene is in direct contact with garnet (Fig. 2g) and in
part slightly aligned along a weak foliation.

15. Grt-Opx-Pl-Qtz assemblage in Grt charnockite
PPL view

16. 3.3. Spl-Sil Gneiss
• Spl-Sil gneiss is characterized by the occurrence of numerous
spinel and sillimanite grains enclosed in plagioclase.
• The mineralogy of a representative sample is K-feldspar (20-
25%), quartz (20-25%), plagioclase (20-25%), garnet (25%)
and spinel (10%)
• With accessory biotite, sillimanite, and Fe-Ti oxide.
• Spinel (0.05-2.5 mm)
 is subidioblastic to idioblastic
 occurs as aggregates mostly in plagioclase and rarely in
garnet (Fig. 2h and i).
 It does not show any contact relation with quartz.
 The spinel in plagioclase is aligned parallel to the matrix
foliations and sillimanites in plagioclase are randomly
oriented.
• Coarse-grained garnet (up to1.5 cm) is subidioblastic and often
also elongated along the foliation (Fig. 2i).

17. Coarse-grained garnet (up to 1.5 cm) is subidioblastic and
occasionally elongated along the foliation
PPL viewsillimanite occurs as aggregates mostly in
plagioclase

18. 3.4. Grt-Bt-Spl Gneiss
• The rock type is composed dominantly of quartz (25%),
plagioclase (10%), garnet (10%) and biotite (7-10%) with
accessory K-feldspar, spinel, sillimanite, cordierite, Fe-Ti oxide,
and zircon (Fig. 2j).
• Garnet (0.63-5 mm) is subidioblastic and contains numerous
inclusions of sillimanite (0.025-0.25 mm), biotite (0.025-0.5
mm), spinel (0.025-0.25 mm), and quartz (0.05-0.5 mm).
• The inclusion sillimanites are aligned along the matrix foliation
defined by biotites.
• Biotite is mostly present as inclusions in garnet and is rare in
the matrix.
• Fine-grained symplectic aggregates of cordierite and quartz
occur around garnet (Fig. 2k).
• Fe-Ti oxide (complex intergrowth of magnetite and ilmenite) is
also present in contact with garnet and/or the symplectite (Fig.
2l).

19. (j) Grt-Spl-Pl assemblage in Grt-Bt-Spl
(k) Fine-grained aggregates of cordierite
and quartz occur around garnet
(l) The occurrence of Fe-Ti oxide with
the symplectite.
PPL view
XPL view .

20. Mineral chemistry
• Chemical analyses of all the minerals were carried
out using a WDS electron microprobe analyzer
[EMPA] at the University of Tsukuba, Japan.
• The data were regressed using oxide-ZAF correction
method.
• Below, the description of mineral chemistry of
examined gneiss.

21. 4.1. Garnet
[X3Y2(SiO4)3 X = Ca, Fe2+, Mg, Mn2+; Y = Al, Cr, Fe3+, Mn3+, Si]
• Garnet in Grt-Opx-Crd gneiss and Grt charnockite is
essentially a solid solution of pyrope and almandine
(0.39-0.44) with low contents of spessartine (<3 mol.%)
and grossular (<2 mol.%) (Table 2).
• The mineral shows a general rim ward increase of
almandine content and slightly pyrope-rich core .

22. Table 2
Representative electron microprobe analyses of garnet (O = 12).

23. 4.2. Spinel [XY2O4]
X may be (Mn,Fe2+,Mg,Ni,Zn) Y may be (Al,Fe3+,Cr)2
• The composition of spinel in the studied rocks varies
depending on its occurrence and textural association.
• Matrix spinel coexisting with ilmenite in (Grt-Opx-Crd gneiss)
has the highest ZnO content of 3.10-3.29 wt.% and the
highest XMg ratio of 0.47.
• In contrast, spinel coexisting with quartz in the same sample
shows lower ZnO content of 1.35-1.79 wt.% and negligible
Cr2O3 (<0.10 wt.%), and lower XMg ratio (0.39-0.40). Low ZnO
is indicator to UHT
• Spinel coexisting with plagioclase in (Spl-Sil gneiss) shows the
lowest XMg (0.34) and ZnO content (1.32 wt.%).

24. Table 3
Representative electron microprobe analyses of spinel (O = 4).

25. 4.3. Orthopyroxene
• All orthopyroxene grains in Grt-Opx-Crd gneiss are
compositionally nearly homogeneous in terms of Al2O3
and XMg content.

26. Table 4
Representative electron microprobe analyses of orthopyroxene.

27. 4.4. Biotite
• Biotite is Mg-rich and characterized by high-TiO2
content (up to 6.4 wt.%) (Table 5).
• The inclusion of biotite within garnet is relatively Mg-
rich (XMg = 0.75-0.78) compared to the matrix phase
(0.68-0.75)
• Greenish biotite with higher XMg (0.80) and lower
TiO2 values (2.5-2.6%) occurs together with Spl +
Qtz association in garnet (Fig. 2c).

28. Table 5
Representative electron microprobe analyses of biotite (O = 22).

29. 4.5. Other minerals
• Plagioclase in the examined samples is albite-rich
Ab66-73.
• K-feldspar has a composition of Or90.
• Cordierite in all the samples shows a uniform
magnesian composition with XMg = 0.85.
• Ilmenite, Magnetite and Sillimanite are close to their
ideal composition.

30. 5. Metamorphic P-T conditions
If we know the pressure (P) and temperature (T) at which
metamorphic rock equilibrated, we can determine where and
how the rock has been formed.
Methods of determine the P-T
• Index minerals: characteristic minerals that provide an
indication of the temperature and pressure conditions at
which a rock formed (e.g kyanite). Not all rocks have a
suitable bulk composition to produce index minerals.
• Metamorphic facies: assemblages of minerals, each
characteristic for a particular bulk composition and
indicating the range of pressure-temperature conditions at
which the rock equilibrated For example, blue schist facies,
indicator to high-pressure - low-temperature conditions.
Qualitative methods don't necessarily provide information
about both pressure and temperature.

31. Thermobarometry
• Is the quantitative determination of the temperature and
pressure at which a metamorphic or igneous rock
reached chemical equilibrium.
• The study in the area done by using different
geothermobarometers calibrated on the basis of
experimental, empirical, thermodynamic parameters
and using different composition-activity models for the
various minerals.
• Several geothermobarometers are applicable for the
Grt-Opx-Crd Gneiss and Grt Charnockite from the
studied area.

32. 5.1. Grt-Opx Geothermobarometers
• The Grt-Opx geothermometer was applied to
poikiloblastic garnet and matrix orthopyroxene in
 Grt-Opx-Crd Gneiss
 Grt Charnockite.
• Application of the method of Lee and Ganguly (1988),
which is based on experimental calibration of Fe-Mg
fractionation between garnet and orthopyroxene
• Gave temperature ranges of 920-990oC (Grt-Opx-Crd
gneiss) and 910-930oC (Grt charnockite) at 9 kbar.
P-T diagram (Fig. 5).

33. Fig. 5. P-T diagram
Showing P-T path
(gray array) for
Grt-Opx-Crd gneiss
and Grt charnockite

34. 5.2. Grt-Crd geothermobarometers
• The Grt-Crd geothermometer of Bhattacharya et al.
(1988) was applied to estimate the conditions of
retrograde metamorphism.
• Application of this method to cordierite and adjacent
garnet yielded a temperature range of 660-670oC at
4 kbar. for (Grt–Bt–Splgneis)
• Retrograde pressure was estimated by using an
experimental geobarometer of Nichols et al. (1992)
yielded a pressure range of 4.0-4.2 kbar at 650oC.
Fig. 5.

35. Fig. 6.
Composition of core
of orthopyroxene in
Grt-Opx-Crd gneiss
y(opx) (= Al + Si2)
isotherms of Kelsey
et al. (2003b) at 9
kbar are also plotted
in the diagram.

36. Conclusion
• Based on detailed petrographic and mineralogical data in
the ACSZ of Spl+Qtz bearing Grt-Opx-Crd granulites, it
has been estimated P-T of 920-980C at 8-10 kbar as
the peak metamorphic condition for the granulite-facies
• Such high-temperature condition is provided
evidence for UHT metamorphic
• Generally, UHT metamorphic rocks are characterized
by diagnostic minerals assemblages such as :
Spr + Qtz, Spl + Qtz, Opx + Sil + Qtz

37. • The Spl + Qtz association reported in this study is a
robust evidence for UHT metamorphism within the
ACSZ.
• The assemblage should not be regarded as a
diagnostic evidence of UHT metamorphism when
 (1) the two minerals are not observed as stable major
phases or
 (2) they are separated, and shows no contact relation
with each other .
• The rocks within the accretionary belt in the ACSZ
have undergone a rapid decompression history and
retrograde mineral growth

38. Reference
Hisako Shimizu,Toshiaki Tsunogae and M.Santosh. Spinel + quartz assemblage in granulites from
the Achan kovil Shear Zone, southern India: Implications for ultrahigh- temperature metamorphism.
Journal of Asian Earth Sciences 36, 209–222 (2009).

39. THANKS
Omer M .Ahmed, omerupto3@gmail.com