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) (40-50%) 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) (90-95%), 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 (10-15%) 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 (0.1-3mm). 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 (10-20%), 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 (20-30%), 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 (1-3%) 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 (0.2-0.5mm), 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 (40-50%), 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