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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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Chapter # 3 Petrography
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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Chapter # 3 Petrography
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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Chapter # 3 Petrography
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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Chapter # 3 Petrography
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
Chapter # 3 Petrography
Fig. 3.2 Photomicrographs showing: (A) Spherulitic Texture – PPL,(B) Manganese Dendrites – PPL
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A
B
Spherulitic Texture
Manganese Dendrites
Chapter # 3 Petrography
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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Chapter # 3 Petrography
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