Download Petrography of Granitic Rocks

Chapter-4
Petrography of Granitic Rocks
4.1 Sampling
During the field work, 176 samples of rocks and minerals were collected from the study
area for various observat~onsand measurements in the laboratory. the details of which follow. 48
samples of metasediments (mica schist, meta-arkose, conglomerate) and 4 samples of
metavolcanics representing the Mesoproterozoic Barotiya Group which occur in proximity to SG
and GG were collected for petrographic study. 18 samples of SG, 27 samples of medium-grained
GG and 17 samples of come-grained GG were collected for petrographic study as well as
geochemical analysis (5 to 15 kg of each sample). 34 mineral samples from quartz-rich veins
(some of which are known to be W-bearing) including quartz. tourmaline, muscovite and
wolframite were collected for microscopic and other studies. 6 samples of altered wall rock mica
schist enbeloping W-bearing quartz vein were collected for petrographic study. 22 samples of
rocks from ductile and brittle shear zones in SG and mica schist were also collected for
petrographic study.
Thin sections were prepared for all the rock types to decipher the texture, mineralogy,
alteration and deformation. Petrographic study can be useful in classify the rocks and its
correlation with the whole rock chemical data. 72 thin sections of various rock and mineral
samples were prepared for petrographic study. This includes 22 samples of Barotiya rocks, 16
Sewariya granite, 18 medium-grained Govindgarh granite, 11 coarse-grained Govindgarh
granite, and 5 tourmaline from quartz-rich veins. Polished sections of wolframite from Pipaliya
tungsten prospect were prepared to study the ore and gangue mineral assemblage. Petrographic
characteristics of the two granitic rocks SG and GG have been studied in detail. Particular
emphasis was given to understand the characteristics of tourmaline present in different granites
and related rocks. Some of the thin sections and polished sections were also used for electron
micro-probe analysis of selected mineral grains of tourmaline, feldspars, garnet and wolframite.
Two samples each of SG, MGG and CGG, and 6 samples of muscovite separated from MGG (2)
and CGG (4) were reduced to -200 mesh size and scanned in powder X-ray diffractometer by
Philips PW1710 at Wadia Institute of Himalayan Geology, Dehradun and PANalytical XPert Pro
at the Department of Earth Sciences, Pondicherry University to supplement microscopic studies
for mineral identification.
4.2 Petrographic characteristics of rocks in the study area
In Govindgarh-Sewariya area there are different lithologies belonging to the
Mesoproterozoic Delhi Supergroup and two distinct types of post-Delhi granites called Sewariya
granite and Govindgarh granite. In this chapter, petrographic characteristics of these rocks are
explained in detail.
4.2.1 Barotiya group of rocks
The Barotiya Group of rocks of Mesoproterozoic age include an assemblage of
metasediments (dominantly mica schist, along with conglometrate, meta-arkose) and
metavolcanics (dominantly mafic with only one outcrop of felsic rock in the study area).
Mica schist is medium to coarse grained with foliation plane defined by alignment of
micas. It consists of biotite, muscovite and quartz as essential minerals with minor amount of Kfeldspar and rare sodic plagioclase. Presence of garnet in mica schist is restricted to few outcrops
near Sagarmati river, where mica schist is intruded by stock-like body of medium-grained GG.
Mica schist from Pipaliya prospect shows development of quartz ribbons (Fig. 4. l), indicating
the effect of mylonitisation which is more prevalent in Sewariya granite. Mica schist from wall
rock area of tungsten mineralised quartz veins contain tourmaline with large number of quartz
inclusions (Fig. 4.2). These tourmaline grains show irregular colour zoning in patches of orange
to yellow, and dichroism from orange/yellow to colourless.
Another dominant country rock in the study area is a fine to medium grained meta-basic
volcanic rock which shows distinct and alternate layers rich in diopside or hornblende and
hornblende-plagioclase assemblage (Fig. 4.3). Replacement of diopside by hornblende is
commonly observed in this rock. Meta-felsic volcanic rock is fine grained, well foliated in
outcrop as well as microscopic scale, and consists of microcline, orthoclase, quartz and garnet
along with minor amount of plagioclase and diopside (Fig. 4.4). X-ray diffraction analysis has
shown the garnet from this rock to be a grossularite.
Fig. 4.1: Dex.elopment of quartz ribbons parallel to foliation in mica schist from Pipaliya
prospect. Crossed polars. Width of photo = 2. lmm.
Fig. 4.2: Mica schist from wall rock of tungsten mineralized quartz \.ein contain
tourmaline with large number of quartz inclusions. Crossed polars. Width of
photo = 2.1 mm.
Fig. 4.3: Meta-basic volcanic rock shox5-salternate layers rich in diopside or llornhlende
and hornblende-plagioclase. Plane light. Width of photo = 2.lmm.
Fig. 4.4: Meta-felsic volcanic rock consists of microcline, orthoclase, and quartz along
with minor amount of plagioclase and diopside. Crossed polars. Width of photo =
2.1 mm.
4.2.2 Sewariya granite
SG is a gray coloured, medium to coarse grained gneissic rock consisting of quartz,
microcline and sodic plagioclase as major constituents, along with accessory biotite and
muscovite (Fig. 4.5). Quartz is fine to coarse grained shows undulose extinction and occur as
porphyroblast in some of the sections. The anhedral, recrystallised quartz crystals are common in
SG with the interfacial angle of 120' among quartz grains at triple junctions. Unlike the
tourmaline leucogranites of the study area (MCG and CGG) which contain more plagioclase
than K-feldspar, Sewariya granite contains more K-feldspar than plagioclase.
Microcline is
dominant mineral among the feldspars, showing characteristic cross-hatched twining, followed
by plagioclase of albite composition and less orthoclase. Chemical composition of plagioclase
from SG analysed by EPMA is given in Table 4.1. In some instances, repiacement of K-feldspar
by muscovite and quartz is observed in thin section (Fig. 4.6).
Both biotite and muscovite are invariably present in SG as accessory minerals, with
relatively more biotite than muscovite. Biotite shows pleochroism from colourless to light and
dark shades of brown, while muscovite is colourless in thin section. Fine to coarse flakes of
biotite and muscovite are generally aligned along a plane, which defines the gneissic foliation of
the Sewariya granite. Presence of kink bands (Fig. 4.7) in both these micas and swerving of
micas around porphyroblasts of K-feldspar in Sewariya granite are commonly observed.
In outcrops of Sewariya granite, tourmaline is noticed only within leucocratic patches (Fig.
3.1 of Chapter-3) which are few cm wide and occur in SG close to intrusive contact with dykes
of tourmaline leucogranite, indicating their metasomatic origin. The inferred reaction is:
Biotite + B-rich fluid
--+Tourmaline
(schorl) + quartz
These leucocratic patches in SG consist of quartz. albite, K-feldspar, tourmaline, and are
totally devoid of biotite and muscovite. In adjoining portions of SG near to these leucocratic
patches, few instances of tourmaline replacing biotite have been observed. In all these instances,
tourmaline from SG contains inclusions of quartz and shows irregular colour zoning in patches
of blue and greenish yellow (Fig. 4.8).
Sewariya granite shows evidence of ductile and brittle deformation in several outcrops
(Fig. 3.1 of Chapter-3) resulting in development of protomylonite (north of Sewariya village),
ultramylonite (near Bijathal and Richmaliyan), breccia (near Bijathal), thin and dark coloured
Fig. 4.5: Sewariya granite consisting of quartz, microcline and sodic plagioclase as major
constituents, along with accessory biotite and muscovite. Crossed polars. Width
of photo = 2.lmrn.
Fig. 4.6: Replacement of K-feldspar by muscovite and quartz in wall rock- Sewariya
granite near tungsten-mineralised quartz vein. Crossed polars. Width of photo =
4.2mm.
Fig. 4.7: Musco~itein Sewariya granite showing kink bands. Crossed polars. Width o f
photo = 1.05mm.
Fig. 4.8: Replacement of biotite in SG by tourmaline showing irregular colour zoning in
patches of blue and greenish yellolv. Plane light. Width of photo = 0.53mm.
cataclasite bands (near Kalni and Kotariya) and pseudotachylite (Phutia Bala nala near Kotariya).
Petrographic characteristics of SG from these shear zones are described below.
SG from protomylonite is medium grained with few megacrysts (upto few cm) of
microcline which appear to be porphyroblasts (Fig. 4.9) around which micas swerve, and shows
the development of quartz ribbons. SG from uitramylonite contains about 90% of fine to very
fine grained matrix in which porphyroblastic grains of quartz, K-feldspar and micas (in the size
range of 4-20 mm) are disseminated (Fig. 4.10). Thin quartz ribbons containing fine grained
quart^ are also observed in ultramylonite (Fig. 4.1 1). Cataclastic bands in SG, as small as less
than a mm wide, show intense brecciation and size reduction of constituent minerals and whose
effect also extends into the adjoining coarse mineral grains of SG (Fig. 4.12). Thin sections of
pseudotachylite show a mixture of glassy matrix and very fine grained to fine grained clasts
mostly comprising quartz (Fig. 4.13), with sharp contact with medium to coarse grained SG.
SG is intruded by a number of quartz veins near Kalni and Kotariya, where these quartz
veins show evidence of brittle deformation and displacement upto few 10s of cm along fault
planes of different orientation. These are mono-mineralic quartz veins in which quartz is
macroscopically grey coloured.
4.2.3 Govindgarh granite
Govindgarh granite (GG) is the younger acid magmatic rock in the study area, which is
found in two varieties of medium- and coarse-grained tourmaline leucogranite. The present study
has revealed the occurrence of a swarm of dykes and few relatively large stock-like bodies of GG
along a NNE-SSW trending zone in the western margin of the South Delhi Fold Belt.
Layered Medium grained GG
The early formed medium grained variety of GG occumng as stock-like bodies shows
layering in some instances, with alternate dark coloured bands rich in schorl variety of
tourmaline and light coloured bands rich in sodic plagioclase, perthite and quartz. Tourmaline
crystals are relatively fine grained (mm size), randomly oriented and closely spaced in darker
bands, whereas these are relatively larger (cm size) and disseminated among felsic minerals in
light coloured bands. Tourmaline from both the layers show colour zoning in thin section, in
Fig. 4.9: Protornylonite in SG with few megacrysts of microcline. Plane light. Width of
photo =2. lmm.
Fig. 4.10: Ultramylonite in SG with fine to very fine grained matrix in which
porphyroblastic grains of quartz, K-feldspar and micas are disseminated.
Crossed polars. Width of photo = 2. lrnm.
Fig. 4.11: Thin quartz veins in SG ultramylonite. Crossed polars. Width of photo = 4.2rnx-n.
Fig. 4.12: Cataclastic bands in SG showing intense brecciation and size reduction of
constituent minerals (quartz and microcline) whose effect also extends into the
adjoining coarse mineral grains of SG. Crossed polars. Width of photo = 2. lmm.
shades of blue. with pale blue cores and dark greenish blue rims (Fig. 4.14), which is a unique
feature of this variety of GG. Tourmaline in this rock shows dichroism from blue to pink colour.
Medium grained GG
The medium grained GG is more commonly massive and consists of quartz, sodic
plagioclase, microcline perthite, tourmaline and muscovite; garnet and apatite are often found as
accessory minerals (Fig. 4.15). Quartz is fine to coarse grained, subhedral to anhedral and
occasionally shows undulose extinction. Quartz grains containing inclusions of tiny flakes of
muscovite are seen in some of the sections.
Plagioclase is subhedral to anhedral, shows lamellar twinning and showing 18" extinction
angle with reference to twin plane (010) suggesting a composition of Abo95Ano.osto A b o . s o Alo
~
(albitic) and shows kink bands in some instances. Powder X-ray diffraction analysis of samples
of this rock also showed that plagioclase is albite in composition in medium grained GG.
Chemical composition of plagioclase from MGG analysed by Electron Probe Micro Analysis
(EPMA) is given in Table 4.1.
Table 4.1: Chemical composition of plagioclase feldspar from Govindgarh granite and Sewariya
granite
Fig. 4.13: Pseudotachylite showing a mixture of glassy matrix and very fine grained to
fine grained clasts mostly comprising quartz. Crossed polars. Width of photo =
2.lmm.
Fig. 4.14: Tourmaline from layered GG showing colour zoning in shades of blue, with
pale blue core and dark greenish blue rim. Plane light. Width of photo = 2.lrnm.
Microcline is the common alkali feldspar occurring in MGG and shows characteristic
cross-hatched twinning (albite and pericline laws). Presence of orthoclase is also noticed in some
instances, which is also corroborated by XRD analysis. Muscovite is an essential constituent of
MGG and it is the only mica species present in this rock. Muscovite is colourless in thin section,
subhedral, and shows interference colours of pink, yellow and dark blue. From X-ray diffraction
analysis of mica separated from two samples of MGG it is confirmed to be 'muscovite' (JCPDS
7-25 muscovite lM, and 7-42 muscovite 3T).
Presence of black coloured tourmaline as primary mineral is also ubiquitous to MGG, and
therefore it is a tourmaline leucogranite. It is present in various shapes, mostly elongated in
nature with striated prism faces. Tourmaline is invariably the coarsest mineral in MGG. In thin
section it shows dichorism from colouriess to dark greenish blue. Through, there are crystals of
tourmaline observed without fracture, many tourmaline crystals are fractured perpendicular to caxis and these fractures are filled with quartz. Tourmaline in medium gained leucogranite shows
coiour zoning in thin section with a small blue coloured core while rest of the grain is greenish
yellow up to the rim (Fig. 4.16).
Garnet is present as one of the accessory phases in MGG. This is often fine grained, brown
coloured and euhedral. Chemical composition of garnet present in MGG was determined by
EPMA analysis and data given in Table 4.2. Calculation of the mineral composition of garnet by
using the formula given by Deer et a1 (1992) shows that garnet occumng in medium grained GG
,~ 8
is dominantly of aimandine composition with significant spessartite component ( P Y ~A1n-1~~
Grol 1 Sp193). Garnet of higher almandine and spessartine components are typical of
peraluminous granitoid rocks (Clark, 1981). Apatite in blue and green colours is another
accessory phase found in medium grained GG.
Coarse grained GG
Presence of coarse grained GG (CGG) is observed in number of places in the study area,
including all the tungsten prospects, where it occurs as dykes. The mineral assemblage of CGG
is same as medium grained GG, with minor differences in the composition of garnet and
tourmaline. The coarse grained granite consists of quartz, sodic plagioclase, alkali feldspar
(commonly microcline and rare orthoclase), muscovite and tourmaline as the essential
constituents and rarely contains garnet. Mica separated from 4 samples of CGG were identified
as "muscovite" from X-ray diffraction data.
Fig. 4.1 5: Medium grained GG consists of quartz, sodic plagioclase, microcline perthite,
tourmaline and muscovite. Crossed polars. U7idthof photo = 4.2mm.
Fig. 4.16: T o m a l i n e in medium grained GG shows colour zoning with a small blue
coloured core while rest of the grain is greenish yellow up to the rim. Plane
light. Width of photo = 2. lmm.
Tourmaline from CGG shows colour zoning in thin section with a large core of blue colour and a
thin rim showing greenish yellow colour (Fig. 4.17). Bhattacharya et al., (1993) have reported
the occurrence of two lithium phosphate minerals, namely ferrisicklerite and triphylite in dykes
of coarse grained GG found near to Pipaliya tungsten prospect.
Table.4.2: Chemical composition of garnet from medium grained variety of Govindgarh granite
Fig. 4.17: Tourmaline from CGG shows colour zoning ~vitha large zone of blue colour
and a thin rim shorn-ing greenish yellow colom. Plane light. Width of photo =
2.lmm.
Petrographic characteristics of Sewariya granite and Govindgarh granite shows the following
distinctions between these two rocks:
1 ) Biotite is ubiquitously present in SG, and totally absent in GG.
2) GG contains tourmaline as essential mineral and minor amount of garnet and apatite,
whereas S G contains metasomatic tourmaline and no garnet or apatite. This shows that
the granitic melt from which GG crystallised was distinctly peraluminous and enriched in
3 and P.
3 ) Primary tourmaline occurring in various lithologies of GG, a n d metasomatic tourmaline
from SG and mica schist are all black coloured, but show definite patterns of colour
zoning in thin section which is unique to each of these lithologies.
4) Shear zones with ductile and brittle deformation are restricted to SG and adjoining mica
schist.