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~ons and 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 K-

feldspar 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-s alternate 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~ite in 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 (0 10) suggesting a composition of Abo 95Ano.os to Abo . soA~ lo

(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 c-

axis 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

is dominantly of aimandine composition with significant spessartite component ( P Y ~ , ~ A1n-1~~ 8

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. U7idth of photo = 4.2mm.

Fig. 4.16: Tomaline 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 ~vith a 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.