petrography

Chapter # 3 Petrography

PETROGRAPHY

3.1. General Statement

Petrography and mineralogical composition plays an important role for the study of

engineering properties of the rocks. The activities of the engineering geologists,

invariably, are directly or indirectly associated with rocks and rock forming minerals.

Petrography includes very minor level (microscopic) to mega level (field) studies i.e. the

study of grain size, shape, orientation, crystallinity, granularity and textural relationship

etc. Gohati rhyolites are exposed at several localities as mentioned above. These rocks

show flow structures, secondary epidote, quartz and ore (Shah et al., 1997). According to

them plagioclase and alkali feldspar occurs as phenocrysts (<5%), which in case of

Rhyolites exposed at Gohati village are not so prominent. Instead they consist of

phenocrysts of quartz floating in very fine grained matrix. The phenocrysts are not

properly recrystallized because the boundaries of the original mineral grains can be

identified. The process of quartz recrystallization occurs in three phases; bulging

recrystallization, sub-grain rotation recrystallization and grain boundary migration

recrystallization (Stipp, M. et al., 2002). According to them bulging recrystallization is

characterized by bulges and small recrystallized grains along grain boundaries and to

some extent microcracks. The temperature extent for this recrystallization is about 250 –

400 0C. With increasing T the dominant texture changes to one marked by the presence

of distinct sub-grains, recognized in thin section by a more polygonized texture. While in

grain boundary migration, the highest T of the three textures grain boundary migration

become the dominant mechanism but it generally take place at relatively high T ranging

from 500 – 550 0C (Stipp, M. et al., 2002). It shows that in Gohati Rhyolites the

phenocrysts have sub-grains with distinct grain boundaries which clarify that phenocryst

were in sub-grain rotation recrystallization regime. The ground mass is mostly glassy, but

locally devitrified to very fine grained matrix having >90% felsic constituents (Shah et

al., 1997).

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3.2. Samples and Methods

As the overall mass of Gohati rhyolite is almost uniform, therefore one representative

bulk sample were collected during the field work. Four cylindrical core samples were

drilled for the detailed petrographic observations and determination of the mechanical

properties. Thin chips were obtained from these which were grinded and glued on the

glass slides to make their thin sections. Thin sections were then studied with the help of

Polarizing microscope in the Department of Geology, University of Peshawar;

3.3. Petrographic Observation

Petrographic studies are carried out both at outcrop scale and microscopic scale. A small

description of the observations is given as under;

3.3.1. Field Observations

The studied rhyolites are mostly jointed due to deformation, but massive portions are also

observed at certain places with light to dark grey color. The bulk sample was acquired

from massive portion as to drill the core of desired length. The rock seems very compact

and resistant in hand specimen most probably because of the fine grained nature and

resistant mineralogy. The most common feature found in Gohati rhyolite is the

Manganese Dendrites which are dark grey dendritic structures consisting of manganese

formed due to Manganese rich solution intrusions along the weak zones.

3.3.2. Microscopic Observations

3.3.2.1. Major Mineral Constituents

The Gohati rhyolites are dominantly characterized by porphyritic texture (Fig. 3.1a, 3.4a)

which is the foremost characteristic of its volcanic origin. Quartz is the most abundant

mineral present in Gohati rhyolites with modal proportion 70 – 77% present mostly in

form of phenocryst (Table 3.1). These phenocrysts appear to be floating in the fine grain

matrix or randomly arranged within the matrix consisting of fine grains of recrystallized

quartz (Fig. 3.1b). Spherulitic texture (Fig. 3.2a) is also observed in the ground mass

which is the other important clue for the volcanic origin.

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Alkali feldspar is the second most abundant mineral present with modal mineralogy in

the range of 14-17%.

Plagioclase is third most abundant mineral present in the rhyolite ranging 9 – 14% in

modal composition. Almost all the plagioclase is very fine grained and present in the

matrix and gives cloudy appearance because of alteration. This alteration is because of

the reason that wherever quartz and alkali feldspar present in association are altered, then

any plagioclase present in association with them will undergo alteration as well. The

modal abundance of these minerals is presented in Table 3.1 and plotted on the relevant

IUGS Classification Triangle (Fig. 3.3).

3.3.2.2. Accessory Mineral Constituents

Along with above mentioned major minerals which constitute the volcanic rock,

accessory minerals are also observed during the petrographic studies of Gohati Rhyolites

including Sphene (Fig. 3.4b), Rutile, Tourmaline (Fig. 3.5b), Garnet, Epidote (Fig. 3.4b),

Monazite (Fig. 3.5a) and Oxidized Minerals (Fig. 3.4b). According to Shah et al., (1997)

epidote and ore minerals occur as secondary phases. Among these accessory minerals,

sphene {CaTi [SiO4](O, OH, F)} is the most abundant ranging 1-11% in modal

composition, which suggest that original magmatic source was rich in TiO2. Since these

rocks have intruded the Jafar Kandao Formation, which is mostly limestone with some

argillites and sandstone. Most probably, during extrusion, the highly silicic lava due to its

high temperature might have deformed, recrystallized or altered the chemical

composition of the overlaying formation (CaCO3) so these conditions might have let the

transformation of Rutile (TiO2) to Sphene {CaTi[SiO4](O, OH, F)}. This may also lead to

the possibility of metasomatism within these rocks. Detailed chemical analysis is required

to confirm the either possibilities.

Trace amount of Rutile also occurs in Gohati rhyolite ranging 0.2 – 1% in modal

composition and present in association with Sphene.

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3.3.2.3. Manganese Dendrites

Manganese dendrites are the characteristic feature of Gohati rhyolite. These dendrites

appear as stains of brown color under the microscope. These are present exclusively in a

random manner (Fig. 3.2b). Manganese dendrites develop with a typical multi-branching

tree-like form. This type of crystal growth is very common and is in the form of

snowflake and ice patterns in Gohati rhyolite. These dendrites might have formed from

growth instabilities that occur when the growth rate is limited by the rate of diffusion of

solute atoms to the interface (Potter and Rossman, 1979).

Table 3.1 Modal mineralogical composition of rhyolites from Gohati Area

Samples Qtz Alkf Plg Sph Tour Epd Grt Mnz Rut OxdGRH-2 62.75 14.125 12 10.75 --- --- --- --- 0.375 ---GRH-5 69.875 13 8.375 7.875 0.875 --- --- --- --- ---GRH-6 71.625 13 8.375 1.125 --- 0.375 0.25 1.125 1 1.25GRH-8 65.375 15 10 5.375 3.125 --- --- 1.375 0.25 0.75

Qtz = Quartz, Alkf = Alkali Feldspar, Plg = Plagioclase, Sph = Sphene,Tour = Tourmaline, Epd = Epidote, Grt = Garnet, Mnz = Monazite, Rut = Rutile,

Oxd = Oxidized Minerals

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Fig. 3.1 Photomicrographs showing:(A) Porphyritic Texture - PPL,(B) Floating Texture - PPL

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A

B

Quartz Phenocryst

(Recrystallized)Matrix

Floating Texture


Fig. 3.2 Photomicrographs showing: (A) Spherulitic Texture – PPL,(B) Manganese Dendrites – PPL

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A

B

Spherulitic Texture

Manganese Dendrites


Q = Quartz, A = Alkali Feldspar, P = Plagioclase

Fig. 3.3 Modal Composition of the studied volcanic rocks, plotted on the

IUGS classification diagram (Le Bas and Streckeisen, 1991)

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Fig. 3.4 Photomicrographs showing: (A) Porphyritic Texture – XPL(B) Sphene, Epidote and Oxidized Mineral – XPL

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A

B

Porphyritic Texture

Sphene

Epidote

Oxidized Mineral


Fig. 3.5 Photomicrographs showing: (A) Monazite present in phenocryst of Quartz – XPL(B) Tourmaline– XPL

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A

B

Tourmaline

Monazite

petrography

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