1

ABSTRACT

The magmatic bodies of Gujarat are well studied throughout the years. But the rocks of the isolated magmatic body of Bhanjada Bet, are not largely studied. The present work aims to study the field occurrence of the rocks, its petrography and the general tectonic setting of the formation of the rocks of the island. The main mass is largely composed of phonolite surrounded by nepheline-syenite at the periphery. From the modal analysis, the coarse grained rocks are named as foid-monzo-syenite, while its volcanic equivalent as phonolite. The major minerals of both the rock types are alkali feldspar, plagioclase feldspar, nepheline, aegirine etc. the presence of these types of rocks indicated silica under-saturated melt composition. These rocks typicaly represents the continental rift setting, which is evident from the fact that Bhanjada Bet is a part of the Kutch rift.

2

ACKNOWLEDGEMENT

It is my great pleasure to acknowledge all the persons who have raised their helping hands during my completion of the project paper. It is my pleasure to complete this work during my course of M.Sc. under the supervision of Dr. Arijit Ray, Assosiate Professor, Department of Geology, Presidency University, Kolkata. I would like to extend my gratitudes towards Dr. Ray whose constant help, supportive and encouraging nature, both in the field and in the laboratory made it possible to complete my dissertation.

I would also like to thank Dr. Joydip Mukherjee, Prof. and Head, Dept. of Geology, Presidency University, for providing the necessary infrastructure and helping me in all possible ways during the course of my work.

My deepest gratitude also goes to my classmate Kalyanbrata Hatui, because of his constant support and untiring help throughout my work.

Last but not the least, I would like to thank my respected and beloved parents, for their continuous support and encouragement.

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CONTENTS

1. Introduction 5-7

2. Geology of the Kutch Basin 9-10

3. Objectives 11

4. Petrography 12-19

5. Modal analysis 20-22

6. Conclusion 23

7. References 24

4

LIST OF FIGURES

Figure.1. Schematic geological map of the Gujarat 7

showing location of Banjada Bet

Figure.2. Figure showing the petrogenetic path of 15

the formation of the nepheline-monzo syenite

Figure.3 Figure showing the flow texture defined 18

by the fine grains in phonolite

Figure.4 Figure showing the photo micrograph of aegirine 18

grains at maximum absorption position within the groundmass.

Figure.5. Figure showing the photomicrograph of 19

perthite grain in fine grained matrix in foid- monzo-syenite

Figure.6. Figure showing the photomicrograph of 19

the formation of disseminated amphibole in phonolite

Figure.7. IUGS classification triangular diagram 21

(after Streckeisen, 1991) showing plot of 3 samples

Figures.8. IUGS classification triangular diagram 22

(after Streckeisen, 1991) showing plot of 2 samples

LIST OF TABLES

Table.1. Table showing the results of the modal 20

analysis of the 5 samples

5

INTRODUCTION

The Kutch region along the western margin of the Indian Peninsula forms an important peri-

cratonic basin between latitude 2230N to 2430N and longitude 68E - 72E which has

Mesozoic-Tertiary sediment fill. The topography of the area is undulating with number of

hillocks and ridges. Three major east-west trending uplifts characterize the geomorphic

features of the basin. These are Kutch Mainland Uplift (KMU), Wagad Uplift (WU) and

Island Belt Uplift (IBU) from south to north. The southern part of the KMU, the topography

is gentler with several flat-topped Deccan Trap hills. Tertiary and Quaternary rocks occur

towards south bordering peripheral plains. The rugged topography in the northern part of

KMU due to presence of a number of cone shaped volcanic alkali basalt plug stands out

intruding the Mesozoic rocks. The Island Belt Uplift (IBU) comprises of four detached small

uplands from west to east namely, Pachham Uplift (PU), Khadir uplift (KU), Bela uplift (BU)

and Chorar uplift (CU). Wagad Uplift and IBU are made up of Mesozoic rocks intruded by

several magmatic bodies and mafic dykes. Tertiary and Quaternary sediments fill the

intervening sub-basins. The sediment fill thickness increases from 500 m in NE to 2500 m in

SW (Biswas, 2005). The basin shallows towards the east and is delimited by a first order

ridge as named in Radhanpar-Barmer Arch that separated it from Cambay basin (Biswas,

1993) and towards west the rift merges with the offshore shelf. The region attracts attention

of several workers because of its excellent Mesozoic sedimentary rock records and very rich

Mesozoic –Tertiary fossil record.

IGNEOUS ACTIVITY

Igneous rocks occur in profusion in KMU and PU. The Mesozoic rocks in the uplifts are

intruded by mafic igneous rocks (Biswas, 1980) but the Tertiary rocks have not been affected

by the intrusives. This is evident from disposition of magmatic rocks of Northern Island Belt

that their emplacement is structurally controlled. They are localized mostly in the zones of

intense faulting. Igneous intrusions are fairly common in the major uplift areas of Kutch. On

the basis of modes of occurrence, petrology, magmatic rocks are classified into three groups.

Numerous outcrops of doleritic and gabbroic rocks occur as dyke, laccolith, cone-sheet and

sill in the Kutch Mainland. Dykes are the most common forms of intrusion present all around

the Kutch Mainland as well as in Northern Island Belts.

6

Figure.1.Schematic geological map of the Gujarat showing location of Bhanjada Bet

7

Stocks come next in order, sills are sometimes present in the uplifts, and laccoliths are seen

forming domes in the flexure zone. In the Kutch Mainland there is no physical connection

between these dykes and laccoliths with the alkali basalt plugs and tholeiitic flows. However

at some places dykes spread from the central alkali plug. Of particular regional significance is

the occurrence of Deccan tholeiitic flows which are restricted along southern part of Kutch

Mainland. The significant feature of the flows is their surprising horizontality.

Innumerable alkali basalt plugs of Kutch Mainland (De, 1980), lies in the central part of the

Kutch rift basin. These alkali basalts have been found occurring mostly as isolated volcanic plugs

intrusive into the Cretaceous Bhuj Formation. Plug like occurrences of alkali basalts are found at

Dhrubia, Sayra Devi, Vithon and Dinodhar, whereas cone-sheet like bodies occur at Bhujia. Some of

these alkali basalt plugs are associated with pyroclastic materials as in Vithon and Dhrubia. Alkali

basalt flows are exposed to the southwest of Nakhtrana around Jadai, Pioni area. These are

characterized by well-developed columnar joint. Almost all of these alkali basalt plugs contain

mantle derived ultramafic xenoliths. These xenolith bearing alkali basalts are not found elsewhere in

the Deccan Volcanic Province. Except near Wamoti moti, nowhere in this region alkali basalt and

tholeiitic basalt flows are found to be in direct contact. The tectonic control for their emplacement

can be suggested from the general disposition of these plugs and the Kutch Mainland Fault.

Peridotitic mantle xenoliths in alkali basalts are first reported by De (1964). Presence of ultramafic

xenoliths within the alkali basalt and associated pyroclastics at some place indicates their forceful

intrusion into the Mesozoic country rock. The Kutch Mainland Fault marks the northern limit of the

volcanic field. This is corroborated by the absence of tholeiitic basalt-alkali basalt association further

north beyond the Kutch Mainland Fault. Among the Island Belt Uplifts, Pachham Island has the

record of occurrence of magmatic rocks in profusion.

Magmatic rocks of Kutch Mainland

Tholeiitic Basalt: Tholeiitic basalt flows are exposed only in the southern part of Kutch

Mainland in a 10 km wide belt forming the Dhola Hills, having a NW-SE trend with a convexity

towards south. The tholeiitic basalt flows disconformably overlie the Mesozoic rocks. The Deccan

Traps continue beneath the Tertiary sediments towards south in the Gulf of Kutch and across the

North Kathiawar Fault.

Stocks and Plugs: A large number of cylindrical bodies of intrusive having circular

outline are seen all over the Kutch Mainland. They are Plutons of irregular shape with

discordant contacts. The stocks and plugs of the Kutch Mainland are Vithon alkali basalt

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plug, Dinodhar alkali basalt plug, Dhrubiya alkali basalt plug, Lodai alkali basalt plug,

Nakhatrana stock, Bhujia stock, Sayra Stock, Alkali basalt flows.

Magmatic rocks of Pachcham Island

Detached from the Kutch Mainland by the Banni Half Graben at the northern extreme of four

E-W trending uplifts are separated by N-S trending minor cross faults. Pachham Island is the western

most uplift of the Island Belt Uplift occurring in the Great Rann of Kutch. The magmatic bodies of

the Pachcham island includes Sadara mafic sill, Kaladongar mafic dyke, Kuran gabbro, Nir Wandh

igneous complex

The magmatic bodies of the Gujarat islands are well studied. Bhanjada Bet is an isolated

magmatic body in the Rann of Kutch within the Khadir Island, which is less studied. The

present study is to prepare the lithological map of the area, to know the rock character and the

mode of occurrence of the rocks in the field.

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GEOLOGY OF KUTCH BASIN

The stratigraphy of Kutch comprises sediments range in age from middle Jurassic

(Bathonian) to recent (Biswas, 1971). These Mesozoic sediments were laid down on a granitic

basement (Erinpura granite), which is exposed in Meruda hill in the Great Rann of Kutch

(Biswas and Deshpande, 1968). The sedimentation started synchronous with the rifting event on

late Triassic. Fossiliferous Jurassic rocks represent the transgressive phase of marine deposition

during the early rift stage (Biswas, 2005). Deccan Trap flows limit the southern extent of these

sedimentary rocks while in the north these are bordered by saline marsh of the Rann of Kutch.

Tertiary and Quaternary sedimentation occurs along the periphery of the Mesozoic uplifts.

Wynee (1872) had published the first geological map of Kutch. Waagen (1875) had first

published the classification of the Jurassic rock after Stoliczka’s classification in an unpublished

report. The Mesozoic stratigraphy of Kutch subsequently discussed by Spath (1933), Rajnath

(1942), Agarwal (1957) etc. However a very little attempt has been made earlier to describe and

interpret the structural style of the region. The salient features of the structure of Kutch-

Kathiawar region with detail morphology of various structural elements have been presented by

Biswas (1980). The Mesozoic rocks have been classified as Patcham, Charee, Katrol and Oomia

Group by Waagen (1873). Biswas, (1971) proposed a new stratigraphic nomenclature by

classifying different lithostratigraphic unit into Jhurio, Jumara, Jhuran and Bhuj. The revised

Mesozoic stratigraphy of Kutch and other areas by Biswas is reproduced below.

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11

OBJECTIVES

The main objectives of the present study are as follows:

To prepare the lithological map of the magmatic body

To collect the representative samples from different lithological -units

To study the petrography of the samples from Bhanjada bet in order to identify the

mineral composition and texture of the rocks

To analyse the modal value for classification and characterisation

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DEFINATION AND OCCURRENCE OF

SYENITES

Syenites are defined as intrusive igneous rocks that contain alkali-feldspars as the dominant

mineral. Nepheline-syenites are syenites from silica-undersaturated magmas that do not

crystallize quartz: they consist dominantly of alkali-feldspar and nephelinephenocrysts and a

groundmass with minor amounts of mafic minerals, namely alkali amphiboles, pyroxenes.

The finer variety of nepheline-syenite, having same composition termed phonolite is also

present.

Phonolite covers the upper portion of the plug shaped igneous body while the periphery is

covered by the nepheline-syenite. The contact between the two varieties is not observed.

PETROLOGY OF NEPHELINE SYENITE

The given thin section is of a coarse grained holocrystalline igneous rock. The grains in the

sample are between 5 and 10 mm in size with intermediate material in the 0.1 to 1 mm size

range. The section is dominated by perthitic, poikilitic, subhedral alkali feldspar which

composes nearly half of the mineral assemblage.

The petrographic description of the rock is given below:

(A) Mineralogy:

The minerals present in the rock in order of decreasing abundances are alkali feldspar,

perthite, aegirine, nepheline, biotite, plagioclase feldspar and calcite.

(i) Major minerals:

Alkali feldspar,aegirine and nepheline are present as the major minerals. The optical

properties by which they are identified are described below:

Alkali feldspar:

Alkali feldspar occurs as orthoclase andperthite which can be identified by the following

properties:

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Orthoclase: orthoclase grains are euhedral in shape and are present in two sizes;

coarse grained and medium grained. They are identified by colourless appearance in

the plane polarized light, very low refractive index, 1st order grey interference colour

and 1st order yellow interference colour with a dusty apperence.

Perthite: Perthite can be identified by its low refractive index and relief, 1st order grey

to blue interference colour. They are coarse grained and anhedral in shape.

Aegirine:Aegirine grains are almost euhedral in shape and are relatively coarse

grained. They are identified by their distinct pleochroic scheme from light green to

deep green colour, low refractive index, higher order interference colour, extinction

angle with respect to the elongation direction is about 17̊.

Nepheline: The next dominant mineral is the nepheline. They are identified by their

1st order grey interference colour with characteristic sericitic alteration along cracks

and also show low refractive index value and very low birefringence. They are

anhedal in shape and coarse in size.

(ii) Minor minerals:

Biotite and plagioclase feldspars are present as the minor minerals whose optical properties

are given below:

Biotite: Biotite grains are subhedral in nature, elongated and medium to coarse

grained in size. They are identified by their pleochroic scheme(light brown-dark

brown), high refractive index,interference colour masked by body colour, one set of

cleavage and with straight mottled extinction.

Plagioclase feldspar: the feldspar grains are subhedral and some are lath-shaped with

two size variance, one larger and one smaller. They are identified by colourless

appearance in plane polarized light, low refractive index, one set of cleavage, 1st order

grey interference colour, one set of lamellar twinning.

(iii) Accessory minerals:

Calcite is present as the accessory mineral.

Calcite: They are identified by their high refractive value, anhedral shape and

showing lamellar twinning. The boundaries of the calcite grains are embayed.

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(B) Texture:

The rock shows coarse grained, holocrystalline and inequigranular texture. Most of the grains

areeuhedral to subhedral in naturethus the overall texture isidiomorphic. At places it shows

perthitic texture and poikilitic texture.Perthitic texture is characterised by the presence of

very fine lamellae and patches of plagioclase in orthoclase host.Poikilitic texture is defined as

large grains (oikocrysts) of alkali feldspar grains containing numerous smaller sized

inclusions (chadacyrsts) of nepheline and aegirine.

(C) Paragenetic sequence:

Very few plagioclase grains are present within the orthoclase i.e. they are earlier; but most of

the plagioclase grains are present as independent grains. Feldspar (both alkali and

plagioclase) are coarse grained and nepheline also, and as they are sharing common

boundary, therefore they are simultaneously crystallised according to their position in

Bowen’s reaction series. They are followed by biotite and calcite at the, later stage of

crystallisation.

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(D) Petrogenetic history:

In this thin section a large amount of feldspars are present but quartz is totally absent, from

this we can say that the source magma is silica undersaturated. Biotite is present in the rock in

a considerable amount which indicates that the source magma was hydrous. Presence of

calcite grains indicates PCO2 of the magma was high. Presence of perthite indicates that it

was a subsolidus cooling process. Phenocryst of nepheline is present in the rock in a large

amount so, the initial melt was nepheline rich (L).Along with the crystallisation of nepheline

grains the liquid follow the dotted line and when it reaches the nepheline-alkali feldspars

cotectic, feldspar grains begin to crystallize and finally the residual liquid touch the eutectic

point R and stops cyrstallization.

Figure.2. Figure showing the petrogenetic path of the formation of the nephelinesyenite

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PETROGRAPHY OF PHONOLITE

The given thin section is of a fine grained rock. The phenocrysts are of three types: 1) alkali

feldspar,2) elongate aggregates of elongated fibrous mafic material(amphibole and biotite),

3) plagioclase feldspar. The matrix is fine grained. It is composed of alkali feldspar,

amphibole, opaques etc. The mafic material defines a trachytic texture all over the section.

(A) Mineralogy:

The phenocryst constitutes25-30%of the thin section. The major phenocrysts in order of

abundance are alkali feldspar, aegirine, nepheline and amphibole etc.

Alkali feldspar: grains are perfectlyeuhedral in shape,about 10mm in length. They are

identified by their colourless nature in the plane polarized light, low R.I, dirty

appearance, 1st order interference colour in the cross polarized light, lath shaped

nature.

Aegirine: they are slender and elongated in nature, about 4mm in length. They are

identified by their pleochroic nature from deep green to yellowish brown, one set of

cleavage, 30 degree extinction angle with respect to the cleavage.

Amphibole: the grains are fibrous in nature, which are pleochroic from straw colour to

dirty straw colour, with high R. I., 2 sets of cleavage at 120degree, 2nd order green

interference colour, inclined extinction (extinction angle 22degree with respect to the

cleavage). They are of relatively smaller size about 3-4 mm.

Nepheline: they occur as subhedral coarse grains. They are identified by their 1st order

grey interference colour with characteristic sericitic alteration along cracks and also

show low refractive index value and very low birefringence.

Groundmass: The groundmass is composed of 70% grains of feldspar and remaining by

the mafic minerals. A trachytic texture in the groundmass is defined by the sub-parallel

orientation of the feldspar and mafic grains.

(B) Texture:

The rock shows fine grained inequigranular texture. Most of the phenocyrsts are euhedral in

nature, thus the overall rock is idiomorphic. Trachytic texture defined by the preferred

orientation of the grains of the groundmass. The grains are dominantly of two types, large

ones known as phenocrysts (feldspar, aegirine, nepheline and amphibole) and the smaller

17

ones collectively known as the groundmass is mainly formed by the feldspar and mafic

aggregates. At places, clusters of feldspar phenocrysts, defines theglomero-porphyritic

texture.

(C) Paragenetic History:

Alkali feldspar is present all over the thin section as phenocrysts, thus it is the earliest formed

phase. Alkali feldspar crystallised throughout the crystallisation history of the rock, evident

from the presence of alkali feldspar both as phenocryst and groundmass. Next to form was

the nepheline, evident from the altered grains of nepheline as phenocrysts. Plagioclase grains

are present within the groundmass showing preferred orientation around the phenocrysts,

indicating itsformation after the formation of the phenocrysts. Small grains of lath shaped

aegirineand biotite are also present as phenocrysts and groundmass indicating its growth as

the intermediate phases. Fibrous amphibole presents the alteration products of the mafic

counterparts. Calcite is present with its boundaries embayed, indication nb its formation prior

to that of the groundmass.

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Figure.3. Figure showing the flow texture defined by the fine grains in phonolite

Figure.4 Figure showing the photo micrograph of aegirine grains at maximum absorption position within the groundmass.

19

Figure.5. Figure showing the photomicrograph of perthite grain in fine grained matrix in foid- monzo-syenite

Figure.6. Figure showing the photomicrograph of the formation of disseminated amphibole in phonolite

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MODAL ANALYSIS

Modal proportion of minerals has been determined by automatic point counter. Modal

analysis has been done from thin sections of various, widely spaced samples in order to get

idea about the mineralogical variation in the samples.

For modal analysis at first modal counting is done. Number of count for each slide varies

from 1300-1600 with varying shifts. The space shift along X and Y axis is 5mm.Fivesamples

of magmatic rocks of different locations have been used for modal analysis.

The modal analysis indicates that the plagioclase feldspars, alkali feldspars, nepheline,

aegirine and opaques are the main mineral constituents of the rock. If we plot the plagioclase

feldspar-alkali feldspar-nepheline modal compositions in IUGS Classification FAP triangle

(after Streckeisen, 1991) we can get that the composition of rock samples.

Table.1. The results of five samples of mafic rocks are given in the following table:

Slide

number

Plagioclase

feldspar

Alkali

feldspar

Nepheline Aegerine Amphibole Ground-

mass

Opaques

BB-11 6.6 50.8 13.5 20.9 8.2

BB-9 18 52.6 11.1 15.1 3.5

BB-

17A/09

13.2 54.2 16.6 12.2 3.4

BB-8 4.4 29 7 0.8 1.6 57.6

BB-18 4.3 20.5 8.6 2.0 64.6

21

Figure.7. IUGS classification triangular diagram (after Streckeisen, 1991) showing plot of 3

samples

From the above diagram we can conclude that the name of the rock is foid-monzo-syenite.

22

Figure.8.IUGS classification triangular diagram (after Streckeisen, 1991) showing plot of 2

samples

From the above diagram we can conclude that the name of the rock isphonolite.

23

CONCLUSION

From our study, we can conclude the following:

I. The magmatic body occur as isolated mass without any association with the

nearby rock-bodies

II. The main body is phonolite with the periphery covered by nephelinesyenite

III. The mineralogy of both phonolite and nephelinesyenite are almost equivalent and

composed of plagioclase feldspar, alkali feldspar, nepheline, aegirine .

IV. The main textures are flow texture, porphyritic texture and perthitic texture

V. The rocks are intermediate-alkaline, undersaturated magmatic rocks which are

volcanic to hypabyssal in origin

VI. According to the modal values, the coarse grained rocks are named as Foid-

monzo-syenite and their volcanic equivalent as phonolite

VII. These rocks are characteristic of the continental rift zone

VIII. The Bhanjada bet is a part of the Kutch rift and the presence of these rocks is

indicative of the rift related magmatic zone.

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REFERENCES

Biswas, S. K., 1971. Note on the Geology of Kutch. Quat. Jour. Geol. Min. met. Soc. India, 43 (4),

223-235

Biswas, S. K., 1980. Structure of Kutch-Kathiawar region, Western India. Proc 3rd Indian Geol.

Cong. Pune, 255-272

Biswas, S. K., 1993. Geology of Kutch. KDM institute of Petrleum Exploration (ONGC), Dehradun

Biswas, S. K., 2005. A review of structure and tectonics of Kutch basin, western India, with

special reference to earthquakes. Curr. Sci., 88 (10), 1592-1600

Biswas, S. K., and Deshpande S. V., 1968. The basement of the Mesozoic sediments of Kutch,

Western India. Bull. geol. min. met. Soc. India, 40, 7 p

De, A. 1980. Late Mesozoic –Lower Tertiary magma types of Kutch and Saurashtra. In: Deccan

Volcanism and Related Basalt Provinces in other parts of the world. K. V. Subbarao and R. N.

Sukheswala (Eds.), Mem. Geol. Soc. India, 3. 327-339

Krishnan,M.S.( 1982) Geology of India and Burma, 6th edition, (Delhi: CBS Publishers and

distributors)

Spath, L. F., 1933. Revision of the Jurassic cephalopod fauna of Kachh (Cutch), Mem. Geol. Surv.

India, Palaeont. Indica, 9, mem. 2, pt. 6, pp. i-vii + 659-945, pl. 125-130

Streckeisen,(1991) The IUGS classification of igneousrocks. Journal of the Geological

Society, London, Vol. 148,pp. 825-833

Waagen, W., 1873. Jurassic fauna of Kutch. The cephalopoda. Mem. Geol. Surv. India, Palaeont.

Indica, 9, 1, pt. 1, pp. Introduction 1-22, pl. 1-4

Waagen, W. 1875. Jurassic fauna of Kutch. Mem. Geol. Surv. India, Palaeont. Indica, 9 (1): 247

Wynne, A. B., 1872. Memoir on the geology of Kutch, to accompany the map compiled by A. B.

Wynne and F. Fedden during the seasons of 1867-1868 and 1868-1869, Mem. Geol. Surv. India, 9,

Pt. 1, 293, 6 plates and maps