Chapter - IV
PETROGRAPHY
4.1. Introduction
Petrographic studies are an integral part of any structural or petrological studies in
identifying the mineral assemblages, assigning nomenclature and identifying the
deformational textures and metamorphic reactions which are crucial for reconstructing the
deformational and/or metamorphic events in an area. Petrography is also very vital in
intrusive complexes to distinguish the magmatic and tectonic fabrics. Microstructural
evidences are very much necessary to document the shear zone fabrics and to understand
their kinematics. The important rock units that are mapped in the study area include
peridotites, hornblendites, pyroxenites, ultramafic rock (garnet-clinopyroxene rock), metagabbros and amphibolites. The other rock units are BIF and plagiogranites. All of them have
been studied for their petrographic details with the help of Petrological microscope. Thin
sections from fifty representative rock samples were studied under a polarizing (petrological)
microscope and the results are described in the following.
4.2. Peridotites
These are generally medium to coarse-grained in the hand specimen and are dark
looking. Under the microscope, they contain mineral assemblages such as olivine (Ol) (4050%) and orthopyroxene (Opx) (40-50%) with accessory amphibole (Amph), spinel (Spl) and
opaque minerals. Clinopyroxene (Cpx) is absent. Subidioblastic olivine is also seen with
medium to coarse-grained (0.3-2.5mm), and is in contact with orthopyroxene which is also
subidioblastic and medium to coarse-grained (0.2-1.5mm). There are also rare amphiboles
(Fig. 4.1a & b) which are medium-grained (0.3-0.6mm) and xenoblastic, and occur along the
79
Fig. 4.1: Photomicrographs from peridotite (a-e) and hornblendite (f): (a) Cumulate texture with
spinels along the grain boundaries of orthopyroxene and olivine. (b) Coarse-grained sub idioblastic
olivine contact with xenoblastic orthopyroxene. (c) Amphibole is xenoblastic and occurs along the
grain boundaries, showing as retrograde mineral. (d) Subidioblastic olivine and orthopyroxene with
rare amphibole and spinel. (e) Spinel is seen along the grain boundary of olivine. (f) Subidoblastic
amphibole with quarzt and plagioclase along the grain boundaries.
grain boundaries of pyroxene indicating its retrograded nature (Fig. 4.1c). Cumulate texture
with spinels occurs along the grain boundary of orthopyroxene and olivine (Fig. 4.1d & e).
Greenish spinel, probably Fe-rich, show fine to medium-grained (~0.5mm) and is often
associated with opaque mineral. It is generally present either at the grain boundaries of
80
pyroxenes or as inclusions in olivine and orthopyroxene, probably representing primary
phase.
4.3. Hornblendites
In hand specimen, hornblendites are medium to coarse grained and show dark
brownish color. These rocks are composed mostly of amphibole (85-90%), accessory
plagioclase (Pl), quartz (Qtz), biotite (Bt), chlorite (Chl) and calcite (Cal). Subidioblastic
amphibole is medium to coarse-grained (0.5-3.2 mm) and contains inclusions of quartz (Fig.
4.1f). Fine to medium-grained quartz (up to 0.5 mm) are also seen. Plagioclase grains are
present along the grain boundaries (Fig. 4.2a) of amphibole and show xenoblastic texture.
Fine to coarse-grained biotite (0.1-1.4 mm) is partly replaced by retrograded chlorite. Calcite
fills grain boundary of most amphibole grains (Fig. 4.2b). Some samples show that cumulate
texture with triple junctions (Fig. 4.2c) and are composed mostly of hornblende (Hbl) (9095%), with accessory quartz (Fig. 4.2d). Subidioblastic hornblende is medium to coarsegrained (0.2~3mm). Fine-grained quartz is present along the grain boundary and is
xenoblastic and probably crystallized after hornblende and orthopyroxene (Fig. 4.2e). Some
other samples of hornblendites also show hornblende (80-90%) and orthopyroxene (10-20%),
with accessory plagioclase, rutile (Rt) and biotite (Bt). Hornblende is fine to coarse-grained
(0.1-1.5mm) and xenoblastic. Coarse-grained hornblende contains inclusions of rutile (Fig.
4.2f).
Subidioblastic orthopyroxene is fine to coarse-grained (0.1-2.5mm) and most of the
mineral grains are transformed to hornblende (Fig. 4.3a) and biotite is seen within the
amphiboles (Fig. 4.3b). Plagioclase is fine-grained (up to 0.1mm) and is seen as inclusion
within coarse-grained orthopyroxene as a relict (Fig. 4.3c) and this texture suggests
amphibole was probably formed by the following decompression/hydration reaction:
Orthopyroxene + Pl + H2O=> Amph.
81
Fig. 4.2 Photomicrographs from hornblendite: (a) Quartz and plagioclase are present along grain
boundary. (b) Biotite is partly replaced by retrograde chlorite and calcite fills the grain boundary of
amphibole grains. (c) Cumulate texture with triple junctions. (d) Fine-grained quartz crystallization
along the grain boundaries of hornblende and orthopyroxene. (e) Subidioblastic hornblende is with
accessory quartz. (f) Rutile forms in xenoblastic hornblende.
Some samples are composed mostly of hornblende (90%) and accessory plagioclase.
Subidioblastic to idioblastic (Fig. 4.3d) hornblende is fine to coarse-grained (0.1-1.3mm).
Plagioclase fills the matrix of hornblende (Fig. 4.3e). Foliation is not obvious.
82
4.4. Pyroxenites
These rock units are dark grayish in color and occur as boudins in metagabbros,
which probably corresponds to pyroxene-rich and plagioclase-poor portion of the cumulate.
Microscopically, the rocks show primary magmatic cumulus texture and are characterized by
coarse-grained subhedral clinopyroxene and orthopyroxene (Fig. 4.3f). This rock is
composed of clinopyroxene (50-60%), orthopyroxene (30-40%), and amphibole (10-20%)
with accessory opaque minerals. Olivine is absent. Clinopyroxene (0.3-4 mm) and
orthopyroxene (0.3-3 mm) are coarse-grained and subidioblastic. Amphibole (-1.8 mm) is
formed along grain boundary of pyroxenes as a retrograde mineral. Fine-grained opaque
mineral (possible ilmenite (Ilm)) is present along the margin of amphibole although the
amount of amphibole in this rock is more than those of other rocks.
Some of the samples show primary magmatic cumulus texture (Fig. 4.4a & b) and
triple junctions (Fig. 4.4c). They are characterized by coarse-grained subhedral clinopyroxene
and orthopyroxene that contain many very fine-grained solid inclusions (Fig. 4.4d). The rock,
in general, is composed of clinopyroxene (60-70%) and orthopyroxene (30-40%) with
accessory amphibole, quartz, and biotite. Olivine is absent. Clinopyroxene (0.3-3mm) and
orthopyroxene (0.3-3mm) are coarse-grained and subidioblastic. Amphibole is present along
the grain boundaries of pyroxenes as a secondary mineral (Fig. 4.4e). Clinopyroxene contains
inclusions of quartz, while orthopyroxene contains biotite possibly as primary minerals.
Some of the samples are medium- to coarse-grained. Although, they are tectonically highly
disturbed, they are free from any foliation fabrics. They show granular texture and exsolution
lamelle in clinopyroxene. Clinopyroxene crystals are very high and a little percentage of
orthopyroxene and no feldspars are seen. Clinopyroxenes are highly fractured and coarsegrained. At some places, cleavages are deformed. Some of the phenocrysts of clinopyroxenes
83
Fig. 4.3 Photomicrographs from hornblendite (a-e) and pyroxenite (f): (a) Subidioblastic
orthopyroxene mineral grains are transformed to hornblende. (b) Plagioclase as relict within
orthopyroxene. (c) Biotite is formed within the amphibole menerals. (d) Subidioblastic to idioblastic
texure. (e) Plagioclase fills the matrix of hornblende. (f) Primary magmatic cumulate texture with
subhedral clinopyroxene and orthopyroxene in pyroxenite.
show orthopyroxenes as relict granulations suggesting their derivation from the later. In some
places, orthopyroxenes are seen as broken sub grains at the margins of the clinopyroxene.
In some samples, strain effects are commonly observed with clinopyroxene and
orthopyroxene showing wavy extinction. Typical igneous textures show equilibrium of
84
Fig. 4.4 Photomicrographs from pyroxenite (a-e) and ultramafic rock (f): (a) Pyroxenite shows
primary magmatic cumulus texture. (b) Coarse–grained clinopyroxene and garnet formed within
secondary brownish hornblende. (c) Cumulate texture with triple junctions. (d) Clinopyroxene
contains inclusion of quartz and orthopyroxene contains biotite. (e) Amphibole is a secondary mineral
present along the grain boundary of pyroxenes. f) Idioblastic garnets and clinopyroxenes with
exsolusions.
crystallization. Garnet and secondary hornblendes are present. Garnet is surrounded by
hornblende, which shows wavy extinction. Reaction rims of hornblende are also present.
Feldspars are absent and opaque minerals are very few. In some samples, orthopyroxene and
clinopyroxene are available nearly in equal proportions. The amount of garnet is relatively
less compared to the total volume of orthopyroxene and clinopyroxene. Secondary quartz is
85
present in orthopyroxene showing exolution lamella. Quartz also shows wavy extinction.
Garnet contains inclusions. The textural features described above indicate that the rocks are
deformed.
In some of the samples, original grains are well preserved despite deformation.
Hornblende is seen as secondary mineral derived from orthopyroxene and clinopyroxene.
Secondary epidotes are also common and the opaque minerals represent magnetites (Mt).
Garnet is either absent or very less and either euhedral or subhedral within matrix of
hornblende. Some of the samples show the presence of exsolution lamella in clinopyroxene.
Secondary hornblende and opaque minerals are seen in small amounts mostly around
clinopyroxe. Igneous textures are well preserved although the grains are altered. Amphiboles
are coarse-grained, uniform in grain size and contain opaque inclusions, small amounts of
epidote, and are free from feldspars.
4.5. Ultramafic rock
It is medium- to coarse-grained garnetiferous rock. Microscopically, the rock is
characterized by the association of garnet (30-40%), clinopyroxene (30-40%), amphibole (1015%) and orthopyroxene (5-10%) with accessory rutile and other opaque minerals. Mediumto coarse-grained garnet (0.6-3.5mm) is subidioblastic and coexists with clinopyroxene and
orthopyroxene with exsolutions (Fig. 4.4f). Subhedral orthopyroxene and idioblastic garnet
(Grt) are seen with reaction rims (Fig. 4.5a) and in some places garnet lacks solid inclusions.
Clinopyroxene is medium- to coarse-grained (0.5-2.5mm) and subidioblastic. Orthopyroxene
is fine-grained (0.2-1mm) and less abundant than clinopyroxene and clinopyroxene contains
quartz inclusions. Coarse–grained clinopyroxene and garnet mineral grains are with
secondary brownish amphibole mineral matrix. The amphibole matrix is xenoblastic and
medium-grained (0.1-1mm). As the amphibole fills the matrix of pyroxenes and garnets (Fig.
86
4.5b), the amphibole is regarded as a retrograde mineral. Rutile and opaque minerals occur
together with amphibole (Fig. 4.5c) and therefore, they could also be retrograde minerals.
4.6. Metagabbros
In hand specimen, the rock is medium- to coarse-grained. It is common to see garnets
surrounded by fine grained plagioclase, defining layers within the pyroxene rich matrix.
Under the microscope, the rock is composed of clinopyroxene (40-50%), garnet (20-30%),
orthopyroxene (10-20%), amphibole (10-20%), and plagioclase (5%), with accessory zircon
(Zr) and opaque minerals, fine grained opaque (possible ilmenite) minerals along the
margin of amphibole (Fig. 4.5d).
Amphiboles are formed along grain boundaries of
pyroxenes as a retrograde mineral (Fig. 4.5e). Subidioblastic clinopyroxene is fine to coarsegrained (0.1-3mm) and surrounded by fine-grained (up to 0.2mm) idioblastic garnet (Fig.
4.5f). Fine- to coarse-grained garnet is subidioblastic; contain the inclusions of clinopyroxene
and plagioclase (Fig. 4.6a). Orthopyroxene is subidioblastic and fine- to coarse-grained (0.13mm). Garnet and ortho/clinopyroxenes are separated by symplectite of plagioclase +
amphibole, probably suggesting the progress of the following retrograde reaction (Fig. 4.6b):
Grt +clinopyroxene (Orthopyroxene) + Qtz + H2O=> Amph + Pl.
Some other samples show that they are plagioclase-bearing variety of garnetclinopyroxene rocks. It is composed of garnet (20-30%), clinopyroxene (20-30 %),
orthopyroxene (15-20%), amphibole (10-20%), plagioclase (10-20%), with accessory rutile
and ilmenite. The medium- to coarse-grained garnet (0.3-2.6mm) is subidioblastic and
contains fine-grained inclusions of plagioclase (0.1-0.3mm). Clinopyroxene (0.2-2mm) and
orthopyroxenes (0.1-1.6mm) are medium- to coarse-grained and subidioblastic.
Dark greenish amphibole is fine to coarse-grained (0.1~1.6mm), and plagioclase is
also fine to coarse-grained (0.1-1.5mm). They are subidioblastic to xenoblastic, and often fill
the grain boundaries of garnet, clinopyroxene, and orthopyroxene as a retrograde phase.
87
Fig. 4.5 Photomicrographs from ultramafic rock (a-c) and metagabbro (d-e): (a) Subhedral
orthopyroxene and idioblastic garnet are with reaction rims. (b) Retrograde mineral amphibole fills
the matrix of pyroxenes and garnet. (c) Idioblastic garnet and rulite occur within amphibole. (d) Finegrained opaque (possible ilmenite) mineral along the margin of amphibole. (e) Amphibole is formed
along grain boundary of pyroxenes as a retrograde mineral. (f) Clinoplyroxene surrounds idioblastic
garnet.
Rutile and ilmenite meneral are formed adjacent to the amphibole, and they are sometimes
exsolved from the amphibole. Garnet and orthopyroxene are often separated by symplectite
of amphibole (light greenish) + plagioclase probably formed by the following
decompression/hydration reaction:
Grt + Orthopyroxene + Qtz + H2O=> Amph + Pl
88
Fig. 4.6 Photomicrographs from metagabbro: (a) Garnet is subidioblastic and formed within
clinopyroxene and plagioclase. b) Ortho/clinopyroxenes are separated by symplectite of plagioclase
and amphiboles showing retrograde reaction. (c) Symplectite of amphiboles and plagioclases are
around the garnet. (d) Xenoblasitc plagioclase and amphibole filling the grain boundary of garnet. (e)
Amphibole and plagioclase are subidioblastic to xenoblastic; fill the grain boundaries of garnet,
clinopyroxene and orthopyroxene. (f) Accessory phase of opaque at grain boundaries of garnet and
clinopyroxene.
Symplectites of amphibole and plagioclases around the garnet (Fig. 4.6c) are
common. Xenoblasitc plagioclase and amphibole fill the grain boundaries of garnets (Fig.
4.6d). Amphibole and plagioclase are subidioblastic to xenoblastic and fill the grain
boundaries of garnet, clinopyroxene and orthopyroxene (Fig. 4.6e). The accessory phases of
89
opaques occur at grain boundaries of garnet and clinopyroxene (Fig. 4.6f). Medium-grained
subidioblastic garnet, clinopyroxene and orthopyroxene with secondary hornblende (Fig.
4.7a), and symplectites of plagioclase and amphibole formed between garnet and
orthopyroxene (Fig. 4.7b) are common.
Some other samples are composed of clinopyroxene (30-40 %), garnet (20-30%),
orthopyroxene (10-20%), amphibole (10-20%), and plagioclase (5-15%), with accessory
rutile and opaque mineral. Clinopyroxene is fine to coarse-grained (0.2-3mm) and
subidioblastic. Garnet is also fine to coarse-grained (0.1-4mm) and subidioblastic often
contain inclusions of plagioclase, clinopyroxene, and rutile. Symplectites of plagioclase +
amphibole can be seen around the garnet (Fig. 4.7c). As the symplectite texture occurs along
the grain boundaries of garnet + clinopyroxene or garnet + orthopyroxene or garnet +
amphibole, the progress of the following decompression/hydration reaction is probable (Fig.
4.7d): Grt +clinopyroxene (Orthopyroxene) + Qtz + H2O=> Amph + Pl.
Subidioblastic orthopyroxene is fine to coarse-grained (0.1-2.5mm) with triple junctions (Fig.
4.7e). Dark green amphiboles are fine to coarse-grained (up to 1.2mm) and xenoblastic. Some
amphiboles occur along the grain boundaries as a secondary mineral. Xenoblastic plagioclase
is fine to coarse-grained (0.1-1.2mm) and is also present along the grain boundaries. Rutile
(up to 0.5mm) and other opaque minerals are scattered.
4.7. Amphibolites
These rocks are coarse-grained and light brownish in color, often associated with
metagabbros and mafic granulites. Microscopically, they are generally composed of
amphibole (40-50%), plagioclase (20-30%), and quartz (20-30%), with accessory garnet,
apatite, biotite, calcite and opaque minerals. The amphiboles are greenish, fine to mediumgrained (up to 1 mm) and show subidioblastic to idioblasitc texture (Fig. 4.7f). Some
amphiboles also show inclusions of vermicular quartz and plagioclase. Idioblastic to
90
Fig. 4.7 Photomicrographs from metagabbro (a-e) and amphibolites (f): (a) Medium-grained
subidioblastic garnet, clinopyroxene and orthopyroxene are seen with secondary hornblende and
plagioclase. (b) Symplectites of plagioclase and amphibole observed between garnet and
orthopyroxene. (c) Symplectites of plagioclase and amphibole are seen around the garnet. d)
Dehydration reaction between garnet, amphibole and plagioclase. (e) Subhedral opx and with triple
junctions. (f) Amphibolite shows subidioblastic to idioblastic texture.
xenoblastic quartz (Fig. 4.8a) is fine to coarse-grained (0.1-1.5 mm) and fine-grained calcite
inclusions are common in amphibole (Fig. 4.8b). Plagioclase is subidioblastic to xenoblastic
and is fine to coarse-grained (1-1.2 mm). Rare fine-grained garnets (up to 0.1 mm) are also
seen within plagioclase, but most of the garnets were probably completely changed to other
minerals.
91
Fig. 4.8 Photomicrographs from amphibolite: (a) Quartz is idioblastic to xenoblastic formed within
the amphibole. (b) Fine grainded calcite inclusions are formed within the amphiboles. (c) Amphibole
is idioblastic to subidioblastic formed within the vermicular quartz. d) Subidioblastic to xenoblastic
plagioclase and quartz are present. (e) Garnet is surrounded by plagioclase. (f) Amphibole is mediumto coarse-grained and subidioblastic to xenoblastic texture.
Some samples consist of amphibole (40-50%), plagioclase (30-40%), and quartz (1020%), with accessory garnet, calcite, biotite, and opaque minerals. Greenish amphiboles are
fine to coarse-grained (up to 1.5mm) and are idioblastic to subidioblastic. Amphiboles show
inclusions of vermicular quartz (Fig. 4.8c) and plagioclase. Subidioblastic to xenoblastic
92
plagioclase and quartz (Fig. 4.8d) are fine to coarse-grained (0.1-1.2mm). Fine to mediumgrained garnet (0.1-0.9mm) is surrounded by plagioclase (Fig. 4.8e).
Some samples
are
comprised
of amphibole (50-60%),
quartz
(20-30%),
orthopyroxene (10-20%), and plagioclase (5-10%), with accessory biotite. Greenish
amphiboles are medium- to very coarse-grained (0.4-7 mm) and subidioblastic to xenoblastic
texture (Fig. 4.8f). Some amphiboles also show inclusions of fine to medium-grained quartz
(up to 0.3 mm). Fine- to coarse-grained (0.2-1.2 mm) quartz is subidioblastic to xenoblastic
(Fig. 4.9a). The xenoblastic quartz occurs along the grain boundaries of amphibole and
orthopyroxene (Fig. 4.9b) probably as a recrystallized mineral. Subidioblastic orthopyroxene
is fine to coarse-grained (0.1-5mm) and is often surrounded by fine-grained quartz and
amphibole. Plagioclase is fine to medium-grained (0.1-0.8 mm) and xenoblastic. Some finegrained (less than 0.2 mm) biotite grains occur as inclusions in amphiboles indicating that
they represent retrograde minerals. Some samples are composed of plagioclase (60-70%),
amphibole (20-30%), quartz (10-20%), and accessory orthopyroxene, biotite, apatite and
opaque mineral. Fine to coarse-grained plagioclases (0.2-1.2 mm) are subidioblastic to
xenoblastic and possess inclusions of amphibole and rutile (Fig. 4.9c). Greenish amphiboles
are subidioblastic to xenoblastic, fine to coarse-grained (0.1-1.5 mm), and show inclusions of
plagioclase. Xenoblastic quartz is fine to medium-grained (0.1-0.8 mm). Orthopyroxene is
fine grained (up to 0.2 mm), and partly transformed to amphibole (Fig. 4.9d). Biotite is fine
to medium-grained (up to 0.8 mm) and is present in the vicinity of amphibole (Fig. 4.9e).
93
Fig. 4.9 Photomicrographs from amphibolite: (a) subidioblastic to xenoblastic texture. (b) Quartz
occurs along the grain boundary of amphibole and orthopyroxene. (c) Amphiboles contain inclusions
of plagioclase, rutile and zircons. These inclusions are subidioblstic to xenoblastic. (d) Orthopyroxene
is fine-grained and partly transformed to amphibole. (e) Biotite with relict amphibole.
Some other samples consist of garnet (30-40%), quartz (30-40%), amphibole (2030%), orthopyroxene (5-15%), and plagioclase (5-15%), with accessory biotite, rutile and
opaque mineral. Idioblastic to subidioblastic garnet is mostly coarse-grained (0.1~5mm) and
shows poikiloblastic texture (Fig. 4.9f). Garnet contains inclusions of many quartz and rutile
grains (Fig. 4.10a). No reaction texture can be seen around garnet. Subidioblastic to
xenoblastic greenish amphibole is fine to medium-grained (0.1~0.8mm) and sometimes show
94
garnet and quartz as inclusions. Fine-, to coarse-grained (0.1~1.5mm) quartz is idioblastic to
xenoblastic. Some rare orthopyroxene grains are fine to medium-grained (0.1~0.5mm) and
xenoblastic. Plagioclase is fine to coarse-grained (0.1-1.2mm) and is subidioblastic to
xenoblastic. Amphibole, plagioclase and orthopyroxene are also present along cracks within
the garnet (Fig. 4.10b). Some samples show extensive weathering (Fig. 4.10c) and are
composed of amphibole (40-50%), calcite (40-50%), biotite, and quartz. Amphiboles are
green in color and subidioblastic to xenoblastic with deformed biotites and calcites (Fig.
4.10d), and calcite is seen filling the grain boundaries of minerals. Biotite is partly deformed
and transformed to secondary chlorite (Fig. 4.10e). A few grains of quartz are fine to
medium-grained (up to 0.5 mm) and xenoblastic. Some samples comprise dominantly of
calcic amphibole (70-80%), plagioclase (10-20%), and garnet (5-10%) with accessory calcite
and opaque mineral. Both coarse-grained porphyroblastic (~3.5mm) and fine-grained
(~0.2mm) amphiboles are present in the samples (Fig. 4.10f), although they have similar
greenish color.
The modal abundance of the fine-grained amphibole is up to 60 %. Plagioclase is
fine-grained (~0.2mm) and xenoblastic. It fills the grain boundaries of amphibole (Fig.
4.11a), or occurs as thin film around garnet. Garnet is fine- to coarse grained (0.1-1.6mm)
and subidioblastic. Garnet is often surrounded by plagioclase + fine-grained amphibole (Fig.
11b) and shows symplectite texture (Fig. 4.11c), suggesting that the progress of the following
retrograde reaction (Fig. 4.11e): Grt + Qtz + H2O => Amph + Pl
95
Fig. 4.10 Photomicrographs from amphibolite: (a) Subidioblastic garnet with quartz and rulite
inclusions. (b) Amphibole, plagioclase and orthopyroxene are present along cracks within garnet. (c)
weathered amphibolites with amphibole, biotite, calcite and quartz. (d) Subidioblastic to xenoblasitc
hornblende with deformed biotites and calcites in amphibole. (e) Deformed biotite, chlorite and
calcite. (f) Porphyroblastic and fine-grained amphiboles.
4.8. Banded Iron formation (BIF)
In the hand specimen, the rock looks brownish in colour and is mostly fine to –
medium-grained. Under the microscope, the rock shows mylonitic texture with alternate
bands of quartz and magnetite-rich layers (Fig. 4.11d & 4.12a). The quartz-rich layer is
96
Fig.4.11 Photomicrographs from amphibolites (a-c & e, f) and BIF (d): (a) Plagioclase and garnets are
formed within fine-grained hornblende. (b) Garnet is surrounded by plagioclase and amphibole. (c)
Coarse-grained hornblende and symplectite of plagioclase + calcic-amphibole surrounding garnet. (d)
Alteration of quartz and magnetite-rich layer. (e) Symplectite of amphibole showing retrograde
nature.
composed of recrystallized aggregates (Fig. 4.12b) of fine-grained (~0.3 mm) nature of quartz
grains, while the magnetite-rich layer is composed mostly of magnetite (Fig. 4.12c).
4.9. Plagiogranites/ Trondhjemites
Plagiogranites are leuocratic rocks with coarse to medum grained and show granular
texture (Fig. 4.12d) suggesting its igneous nature. This rock comprises dominantly of quartz
97
(50-60%) and plagioclase (30-40%) with accessory biotite, muscovite (Ms) and calcite.
Calcite grains are seen within the plagioclase (Fig. 4.12e). Subidioblastic quartz and
plagioclase with rare biotite are also seen (Fig. 4.12f). Quartz and plagioclase are medium- to
coarse-grained (0.2-3mm) and subhedral.
Fig. 4.12 Photomicrographs from BIF (a-c) and plagiogranites (d-f): (a) recrystallized aggregates of
the quartz-rich layer defining mylonitic foliation. (b) Quartz and magnetite-rich bands. (c) Finegrained recrystallized quartz. (d) Granular texture in plagiogranite with subhedral biotites along the
grain boundaries of quartz and plagioclase. (e) Calcite forms within the plagioclase (f) Subidioblastic
quartz and plagioclase are with rare biotite.
98
Biotite and muscovite are rare and medium-grained (0.1-2mm) and are mostly present along
the grain boundaries (Fig. 4.13a) of quartz and feldspars, or along secondary cracks in
plagioclase and quartz.
Some samples show granular texture (Fig. 4.13b) as an igneous rock and comprises
dominantly of plagioclase (70-80%) and quartz (20-30%) with accessory biotite, chlorite
muscovite, and calcite. Muscovite, calcite and chlorite minerals are within the feldspar and
quartz (Fig. 4.13c). Plagioclase is fine to coarse-grained (0.2-3 mm) and is euhedral to
subhedral. Fine to coarse-grained quartz (0.2-1.2 mm) is subhedral to anhedral. Biotite,
muscovite and calcite are rarely observed. They are fine to medium-grained (01-1 mm) and
occur along grain boundaries of quartz and feldspars (Fig. 4.13d), or along mineral cracks.
Biotite is partly replaced by secondary chlorite.
4.10. Quartzo-feldspathic gneiss
Quartzo-feldspathic metamorphic rock is a leucocratic and composed of plagioclase
(35-45%), quartz (35-45%), garnet (10-15%), clinopyroxene (5-10%), and orthopyroxene (13%) with accessory amphibole, biotite, apatite, opaque mineral and zircon. It is intensely
deformed (Fig. 4.13e) and all minerals are stretched along the foliation of the rock as a
mylonite (Fig. 4.13f). Plagioclase and quartz are dominant minerals and are fine to mediumgrained (~1.3mm) with subidioblastic to xenoblastic (Fig. 4.14a). Quartz shows ribbon
texture related to deformation (Fig. 4.14b). Garnet is subidioblastic, medium-grained (0.20.5mm), and contains quartz and opaque mineral. Clinopyroxene and orthopyroxene minerals
are fine to medium-grained (0.1-1.0mm) and subidioblastic. Clinopyroxene is more abundant
than orthopyroxene. Amphiboles occur around garnet, clinopyroxene, and orthopyroxene,
while fine-grained biotite (~0.1mm) occurs around garnet (Fig. 4.14c) and other minerals.
Opaque mineral and zircon are generally scattered in the rock.
99
Fig. 4.13 Photomicrographs from plagiogranites (a-d) and quartzo-feldspathic gneiss (e, f): (a) Biotite
and muscovite are present along the grain boundary. (b) Plagiogranite shows granular texture. (c)
Muscovite, calcite and chlorite are formed within the feldspar and quartz. (d) Muscovite and calcite
are present along the grain boundary of quartz and feldspars. (e) Weakly deformed texture. (f)
Amphibole, quartz, plagioclase, garnet and clinopyroxene are stretched along the foliation of the rock.
Some samples are composed dominantly of quartz (40-50%) and plagioclase (4050%), with accessory amphibole, muscovite, apatite, garnet, and opaque mineral. Both
Quartz and plagioclase occur as fine-grained (~0.2mm) or coarse-grained (~2.5mm) minerals.
The
coarse-grained quartz and plagioclase often show ribbon texture suggesting intense
100
deformation (Fig. 4.14d). Xenoblastic amphibole (Fig. 4.14e) is present at places. Finegrained garnet (~0.2mm) rarely occurs in the rock. Muscovite is present along grain boundary
or mineral cracks (Fig. 4.14f).
Fig. 4.14 Photomicrographs from quartzo-feldspathic gneiss: (a) Subidioblastic to xenoblastic
plagioclase and quartz are present. (b) Quartz show ribbon texture due to deformation. (c) Biotite
forms surrounding garnet. (d) The coarse-grained quartz and plagioclase often show ribbon texture
probably due to deformation. (e) Amphibole is xenoblastic f) muscovite is present along grain
boundary or mineral crack.
101