geology page no. 2.1 introduction 28 2.2 geographical location...
TRANSCRIPT
CHAPTER 2
GEOLOGY
Page No.
2.1 Introduction 28
2.1.1 Selection of the Study Area 28
2.2 Geographical Location 30
2.2.1 Topography 30
2.3 Geology 30
2.3.1 Granites and Gneisses 31
2.3.2 Porphyritic Granite 35
2.3.3 Pegmatite, Aplites and Quartz Veins 35
2.3.4 Gabbros 36
2.3.5 Epidiorite 36
2.4 Lithology 37
2.5 Stratigraphy 38
2.6 Structural Features 39
2.7 Rift, Grain and Hardway 40
2.8 Sheeting 40
2.8.1 Joints 40
2.8.2 Fractures 41
2.8.3 Lineaments and Faults 41
2.8.4 Strike and Dip 42
2.9 Regional Setting 42
3.0 Dolerite Dykes 44
3.1 References 46
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CHAPTER 2
GEOLOGY
2.1 Introduction
The Indian Peninsular shield is infested by a large number of dyke swarms.
The geological investigations of an area form an important basis for various
geophysical surveys to have a comprehensive picture under study. A classification of
hard consolidated rocks like granites and dykes according to their mineral
composition. Granites are also characterized by horizontal joints (Sheeting) which are
parallel to the surface. Granites are deeply weathered either by mechanical or
chemical processes resulting in formation of a thick covered mantle of residual
material commonly designated as disintegrated rock, which do not have primary
porosity and permeability and hence are poor water bearing formations. Joints,
fractures and other structural features can be developed.
2.1.1 Selection of the Study Area
The study area, which forms part of Greater Hyderabad and Ranga Reddy
District covering an area of about 2500 s.q. km is situated at about 50km north of
Hyderabad, Andhra Pradesh, India and is well connected by road. The area is covered
mainly by granites of Archaean age which are highly weathered and fractured. A large
number of out crops are available to map the structures and fractures. Number of
intrusive like dolerite dykes, Pegmatite veins and Quartz reefs/veins are common in
the area. Detailed geological and hydrogeological investigations were carried out with
an objective to demarcate different rock outcrops, exposures in the study area
measurement of structural elements and general assessment of the hydrogeological
conditions prevailing in the area. The map showing the study area is presented in Fig.
2.1
29
Fig 2.1 Location map of the Study Area
30
2.2 Geographical Location
The Mandals of Ibrahimpatnam, Keesara, Ghatkesar and Maheshwaram in
Greater Hyderabad and Ranga Reddy district have been selected for the present study.
These areas are continuously extended towards SSE, ESE, NE, SE, East ENE of
Hyderabad city. The study area forms part of Quadrangle Map or Topo sheet No 56k.
Hyderabad being the state capital and is also the headquarter of Ranga Reddy district,
is very well connected with all other state capitals by air, rail and road. Two National
high ways, namely NH-7 and NH-9 pass through these districts.
Ibrahimpatnam, Keesra, Ghatkesar and Maheshwaram Mandals are very well
connected by road. Ibrahimpatnam is on main road of Nagarjuna Sagar ring road and
SSE of Hyderabad road. Keesara and Ghatkesar are situated on National high way
No. 9 towards east south east. Maheshwaram is situated on National high way No.9
towards southwest. Maheshwarm is the near Shamshabad Airport, which was
connected with the number of buses from Greater Hyderabad and Ranga Reddy
District. Fair weather and network of macadamized roads and a few pedal path and
foot tracks connecting the local villages provide accessibility to the interiors of the
area.
2.2.1 Topography
Topographically the study area is generally undulating or flat tracts characterize
with isolated relief hillocks and hill ranges with intervening streams. Dolerite dykes,
quartz reefs have developed their own prominence in the investigated area.
The weathering plays major role in the development of the topographical
features. Joints plains have added advantage during the weathering process on account
of which smooth conical tors, rounded hills have developed into characteristic land
forms in this area. The long hillock ranges are in the western part of the study area.
2.3 Geology
The Indian Peninsular shield is infested by a large number of igneous dykes
which constitute swarms in a few localities. These dykes which may be of different
ages strike in different directions, NE-SW, ENE-WSW or NW-SE and are seen as
31
persistent linear ridges extending for several kilometers with width varying between 3
to 60 meters.
2.3.1 Granites and Gneisses
The granitic rocks of the world are the subject of intense investigation
regarding their origin and as read as put it “There are granites and granites”.
The study of Precambrian shield granites present many problems as they are
subjected to many phases of deformation, metamorphism and igneous activity. The
granites of Hyderabad belong to the Precambrian shield type and consist of many rock
types from syenites to Alaskites.
Precambrian rocks, such as Granite, Adamellite, Tonalite, Amphibolites,
Hornblende biotite schist occupy a major part of the area. These formations were
subjected to tectonic and green schist facies metamorphism. Except for portion in the
western part of the district to understand its geological significance most of the area is
occupied by granites. Based on mineral composition they were further classified as
alkali feldspar granite, Magmatite granite gneiss, Adamellite, Granodiorite, Tonalite
and Trondjimite.
Granites and gneissic complexes, occupy vast expenses of Peninsular India.
Granitic rocks of the Hyderabad form part of the Peninsular Gneissic Complex. Pink
and Grey granites are the major rock types occurring in and around Greater
Hyderabad. Dolerite dykes, basic enclaves, aplites, pegmatite, quartz veins,
migmatites and alaskite traverse these granites. At places alaskites and leucogranites
are emplaced within the pink and grey granites (Krishna, 2004). Mineralogy, textures,
structure and major intrusive of these areas are
The major units are:
1). Grey series: coarse porphyritic grey granites, biotite granites and pyroxene
granite.
2). Pink series: coarse porphyritic pink granite, fine to medium grained pink granite,
coarse porphyrite pink alaskite, fine grained pink alaskite, white alasklite.
3). Dykes and veins: Dolerite dykes (both fine and coarse grained)
Quartz veins and epidote veins
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4). Enclaves: Fine grained
5). Migmatitic zone: Inter mingling zones of pink and grey granites.
Investigated study area. Fig. 2.2
Fig. 2.2 Geology Map of the Study Area
and it covers the cluster of Mandals i.e. Ibrahimpatnam, Keesara, Ghatkesar and
Maheshwaram Greater Hyderabad and Ranga Reddy district. Almost all the Mandals
are covered with Peninsular granites. The Granites of Hyderabad have been
extensively studied during last three decades by a number of researchers,
(Balakrishna, 1964; Janardhan Rao, 1965 and Sitaramayya, 1968). The rock types
covered in the area are classified as grey and pink granites and they are intruded by
younger formation i.e. dolerite dyke, quartz, epidote and Pegmatite veins.
33
Raja (1959) attempted systematic studies on the petrography of the granites
occurring in and around the twin cities of Hyderabad & Secunderabad. According to
him, soda metasomatism of pre-existing hornblende – biotite schist gave rise to grey
granites. The parental rock occurs as enclaves and the pink granites were derived from
the granites by potash metasomatism. Rao (1963) suggested that there might not be
any age difference between grey granites and pink granites. He opined that the
difference in the colour of the two granites could be due to the difference is the
parental material, which was subjected to process of granitization. Rao (1965) opined
that both grey and pink granites are metasomatic in origin. The grey granites were
formed by granitization of parental metasediments like quartz mica schists and related
rocks. The pink granites were supposed to have formed along shear zones in grey
granites by potash metasomatism.
There are several researchers who have expressed different opinions regarding
their origin. Geological Survey of India, Hyderabad has concluded that the granites of
Hyderabad are of magmatic origin. Balakrishna, (1964) studied in detail the Petrology
and Petrogenesis of these granites. They came to the opinion that the granites have
formed due to metasomatism of the country rock.
According to (Balakrishna, 1964) granitc rocks predominate the peninsular
complex and are represented in the city of Hyderabad. He studied the granitic rocks of
Hyderabad with special reference to tectonics, petrography and textural features. He
noticed two types of granites, pink and grey, and said that it has been found difficult
to demarcate them separately. On the basis of his studies he concluded that the pink
could be a derivative from the grey series of granites and gneisses. He also concluded
that the pyroxene bearing granites seem to have all characteristics of the conventional
charnokites and they appear to be a sort of basis front formed due to late stage of
potash feldspar. He further said that, the pink granites are found to be rich in perthite
and myrmekitic structure and are seen in all types of granites particularly in hybrid or
magmatic zone. He also studied the elastic constants pointed out that the pink granites
are younger than the grey granites.
Janardhan Rao and Sitaramayya, (1968) has extensively studied the
Geological features like structures, the petrology and geochemistry of granites of
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Ghatkesar area near Hyderabad and opined that the pink alakites and basic dykes are
of anatectic origin. Janardhan Rao, (1965) has studied the origin of Hyderabad
granites.
Janardhan rao, (1965) is of opinion that these granites could ever passed
through a magmatic stage. The parallelism of the structural elements of the granites
with the Dharwar meta-sediments, the textural characters between the major mineral
constituents, the enormous heterogeneity in the composition of the granites in a very
small area, all go against magmatic origin. He also opines that all the granitic rocks
and their associated pegmatite and quartz reefs are metasomatic origin and also says
pink granites are associated with shear zones in grey granites. Age of this granite,
2490± 115 m.y. is very close to closepet granites (Crawford, 1969). Studied
peninsular granites and gneisses and the ages of many of the granites in peninsula are
similar.
According to (Baleequddin Hussain, 1968), Grey granites are due to the soda
metamorphism. The pink granites were formed during the stage of potash
metamorphim. The pink granites follow certain shear zones in the masses of grey
granites. He also indicated parallelism in the structural elements like foliation,
lineation, basic enclaves and dykes and the NNW-SSE trend is parallel to the
Dharwarian trend. He is of the opinion that the granites of Hyderabad are of
metasomatic origin derived from a parent rock of hornblende-biotite- schists type and
suggest that the granites might have passed through a magmatic stage. Work done by
(Raghava Rao, 1974), and (Satyanarayana, 1983) in different part of Hyderabad area,
together concluded that the granites are formed due to metasomatic processes.
These rock types in this region is characteristic of the Archaean granite terrain.
Due to similar rock type mineralogy of both are described together. The rocks of the
areas in grain size from medium to coarse grained. Feldspar is the predominant
mineral and occurs as phenocrysts. These rocks are essentially composed of quartz,
potash feldspar, plagioclase hornblend and biotite and accessory minerals as
magmatite, epidote and sphene. Few specks of pyrite and chalcopyrite are noticed in
the rock samples of the area.
Gnaneshwar (1986) concluded in his litho geochemical surveys of Hyderabad
granites that porphyritic granites, white alaskite and pink alaskite are potentially the
35
ore-bearing rock types. The porphyritic granites contain higher amount of U, Th and
Mo. In the vicinity of pink alaskite and these elements were mobilized and emplaced
in fracture systems at higher structural levels.
The geological Survey of India (2006) have investigated this area and
suggested that these granite rocks are of magmatic origin and pink series are intrusive
into grey series. A very large area of the investigation is underlain by Peninsular
gneissic complex of Archaeans age, consisting of pink and grey granites, granitic,
magmatites, Pegmatites, quartz veins and dolerite dykes, occurring in the forms of
domes, scaprs, massive columnar blocks and ‘tor’scattered over a partly undulating
country.
2.3.2 Porphyritic Granite
These are the dominant rock type of pink and grey colour in this area. Grey
porphyritic granite is predominating in the area of investigation. They are mainly
comprised of microcline perthite, plagioclase and biotite. At some places hornblend
occurs in addition to biotite. Magmatite, sphene, zircon and pyrite are the important
accessories in these granites. The variation in texture and composition is attributed
due to the assimilation of the pre-exisiting rocks of porphyritic granite. Band of
composite gneiss occurring at places in the porphyritic granite might have formed
earlier than the porphyritic granite though injection of granitic fluids along the
foliation planes of the pre-existing schist and reaction with them.
The structure of the porphyritic granite is generally massive, but where it is
composite in character it shows foliation. The foliation is derived from the pre-
exisiting schist’s which are granitised. This has a NW-SE strike and vertical dips. The
porphyritic granite is traversed by many joints. They generally belong to NE-SW and
NW-SE set, and the dip of the joints is vertical. Apart from these are sheet joints,
curved and low dipping in the porphyritic granite.
2.3.3 Pegmatite, Aplites and Quartz Veins
Porphyritic granites are traversed by numerous veins of pegmatites with
different trends. The common trends are NE-SW, NNE-SSW, NNW-SSE and NW-
SE. They are found in white, grey, pink and milk white colours. Feldspar is generally
36
microcline perthite. Apart from feldspar and quartz the other constituents are biotite,
magnetite, muscovite, sphene and zircon. Among the sulphides are pyrite,
chalcopyrite and molybdinite. The molybdinite bearing pegmatites traversing the
porphyritic granites are older than the medium grained granites (Kittu, 1968). Aplites
which are exposed in a few places occur either as narrow veins or small lenses.
The trend of the quartz veins is NE-SW. These veins are found in the
porphyritic granite and at some places quartz veins passes into pegmatites. Generally
they are entirely made up of quartz.
The pegmatites contain mostly feldspar and quartz and quartz veins traversing
these granites occasionally contain pyrite, chalcopyrite and malachite. These are
minor intrusive & traversing the host rocks granites. They are a few centimeters to
few meters wide and run over a few hundreds of meters of distance.
2.3.4 Gabbros
Gabbro is a coarse-grained, dark-colored, intrusive igneous rock. It is usually
black or dark green in color and composed mainly of the minerals plagioclase and
augite. It is the most abundant rock in the deep oceanic crust.
Gabbro is composed mainly of calcium-rich plagioclase feldspar and
clinopyroxene (augite). Minor amounts of olivine and orthopyroxene might also be
present in the rock.
This mineral composition usually gives gabbro a black to very dark green
color. A minor amount of light-colored mineral grains may be present. Unlike many
other igneous rocks, gabbro usually contains very little quartz
2.3.5 Epidiorite
Epidiorite dykes are noticed in this area. They are mostly in NE-SW direction they are
generally composed of amphibolites and andesine. Hornblende generally occurs as
pseudo-morph of pyroxene. The feldspar is very much altered.
Flourite, Epidote, Calcite, Feldspar, Quartz and Jaspar veins are travelling the
porphyritic granites, numerous veins of epidote, calcite, feldspar; quartz and jasper are
37
noticed with less than a centimeter in width. In few cases pyrite, chalcopyrite, biotite
and sphence are present. Epidote veins traverse the epidiorite, pegmatites and quartz
veins. The veins in general have NE-SW strike and steep dips.
The structure of the porphyritic granite is generally massive, but where it is
composite in character it shows foliation. The foliation is derived from the pre-
exisiting schist’s which are granitised. This has a NW-SE strike and vertical dips. The
porphyritic granite is traversed by many joints. They generally belong to NE-SW and
NW-SE set, and the dip of the joints is vertical. Apart from these are sheet joints,
curved and low dipping in the porphyritic granite.
2.4 Lithology
Peninsular rocks, a major part of the area are covered by Pink and Grey
granites. The variations in the granites, the country rocks, are quite apparent through
the two major types, pink and grey are being recognized, each one of them show
several minor variations. These variations are mainly due to mineralogical
composition textural difference, coarse to medium grained, fabric, and enclosers. The
enclosers may be schistose pockets which are referred xenoliths. These xenoliths are
considered to be remanents of pre-existing rock masses. Besides xenoliths, aplites,
pegmatites epidote veins and quartz veins are also responsible for lithological
variations. On a large scale the enclosers are formed from a dolerite cutting across the
lineation and foliation of the granite. The size of dolerite dykes both in width and
length is variable. Some of the dykes traverse this granite for kilometers together with
definite trends. Some times these dykes show enclave pattern indicating that they are
followed the joints plains. Branching, piching and swelling nature is a characteristic
feature of some of these dykes.
All these variations exert considerable influence on weathering and
consequent evolution of land forms. The major contrast is found that the grey granites
are not generally prone to weathering while pink masses are less resistant.
Consequently one finds large terrain being covered with black soils. Most probably
some of these are derived from dolerite dykes. As such the importance of lithology is
to be recognized in the development of soils and evolution of land form which has an
explicit control on occurrence.
38
Apart from these rocks the younger intrusive of dolerite dykes, quartz,
Pegmatite veins, Basalts are present. Geologically the area consists of Predominantly
Peninsular suite of rocks. Based on mineral composition, they were further classified
as alkali feldspar granite, migmatite granite gneiss, adamellite, granodiorite, tonalite
and trondjhemite.
A few patches of amphibolites and pyroxene granulites are also seen around
the Hyderabad. All these rock formations are traversed by NE-SE and N-S trending
dolerite dykes. The area experienced volcanism at the time of eruption of the Deccan
Traps. The Deccan Traps and intertrapean beds are seen in the western part of the
study area.
2.5 Stratigraphy
The investigated area is occupied mainly granite rocks of Archaean age
comprising grey and pink with medium to coarse grained texture cover a major
portion of the study area. In palaeomagnetic study, Proterozoic to Archaean age acid
intrusive pegmatite, quartz reef/veins and migmatites occur in the study area. The
granites were also intruded by Paleocene to Cretaceous age dolerite dykes and basalts
and represented in geology map of the area.
The stratigraphic succession is given below:
Basic dykes
Basic enclaves/Quartz reefs
Peninsular gneissic complex
Dykes Porphyritic type
Fine grained type
Leucocratic type
Mesocratic type
Melanocratic type
Pink granites
Fine grained
Medium grained
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Grey granites
Coarse grained
Gneissic type
2.6 Structural Features
The investigated area is mainly occupied with hard rock formations. In such
cases which are devoid of any primary porosity, secondary structures like faults,
joints, cleavage fractures, lineation, and foliation serve as channel ways for the
circulation of palaeomagnetism and dykes. Hence, the study of these features is
essential for proper understanding of the occurrence and behavior of dykes in these
formations.
First, many exposed granitic bodies display three sets of joints disposed at
right angles to each other or mutually. These are together referred to as orthogonal
system and they effectively divided the mass Granites. Even though granites are quite
massive and impermeable in their original unmodified state; they are generally
rendered pervious with the development of joints and fractures.
In the investigated area the regional structures and fracture pattern control the
topography. The trend of the main structural features is NNW-SSE and the alignment
of bed rock exposures in the study area mainly follows the same trend.
National Remote Sensing Agency (1978) employed the satellite imaginary
techniques to unravel the lineament pattern over the granite-gneiss terrain of Andhra
Pradesh. Their studies indicate “lineaments” in almost all directions of A.P. However,
the intensity and direction vary from area to area.
The most important directions are as follows:
a). NNE to SSW
b). WNW to ESE
c). ENE to WSW
d). NW to SE and
e). NE to SW
The two trends NNW-SSE and E-W are best displayed in the study area
40
The granitic rocks of the area possess certain characteristic structures in great
perfection.
2.7 Rift, Grain and Hardway
These planes approximately at right angles to one another along which granite
falls most easily under tension. The rift is due to the peculiar property of quartz and is
approximately horizontal in most of the granites in the investigated area. The grain is
usually in the direction to the rift and may be determined by tectonic cracks or other
features. The joints trend to be more consistent in altitudes than the cleavage. When
there are systems, they are equally spaced. The fractures planes give rise to a structure
of cuboidal blocks. However in many instances the jointing is much less regular. This
features is commonly seen in most of the outcrop areas.
2.8 Sheeting
This is the most important structure, particularly well developed at shallow
depth and usually observed in the outcrops of Ibrahimpatnam, Ghatkesar and
Maheshwaram and other areas. Sheeting is kind of horizontal feature. Though it
shows curved aspect on domal hills and resembles exfoliation shells. It will be nearly
horizontal at ground level. All types of granites are sheeted.
2.8.1 Joints
The concept of joints are studied & explained by different researchers in
several types. The simplest of all classification is based on the criteria of degree of
openness (Babuskin, 1972). Joints can be classified as 1). Hidden joints, these are not
seen by naked eye, 2). Closed joints, these are seen by naked eye, 3). Open joints with
the walls clearly separated.
According to (Balk, 1937) classified joints into 1). Cross joints, 2).
Longitudinal joints and 3). Diagonal joints.
Based on the origin (Babuskin, 1972) classified the joints 1). Tectonic 2). Weathered
3). Separater 4). Drained 5). Gravitational.
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The joints directions can be explained & obtained by various surface and as
sub-surface geophysical methods. Among the surface geophysical methods vertical
electrical resistivity method is used for delineating major joint directions and fractures
occurring up to a depth.
2.8.2 Fractures
These may be called closely spaced joints developed due to tectonic forces.
Intense shearing often produces potential fracture zone which would constitute highly
productive aquifers.
2.8.3 Lineaments and Faults
The area reveals several lineaments and faults identified running different
strikes. The lineaments and faults in the study area show NE-SW, NW-SE and N-S
trend is mainly associated with these lineaments and faults. Fig.2.3
Fig. 2.3 Structural Features Map of Study Area
42
The outcrops of the rock units have been demarcated by trekking the area.
Rocky outcrops of granites in the study area are well exposed in the central, north
eastern and south western part. Detached boulders of pink and grey granite are seen
all over the place due to intense fracturing.
2.8.4 Strike and Dip
Strike and dip refer to the orientation or attitude of a geologic feature. The
strike line of a bed, fault or other planar feature is a line representing the intersection
of that feature with a horizontal plane. Strike (or strike angle) can be given as either a
quadrant compass bearing the strike line (N25˚E for example) or in terms of east or
west of true north or south.
The dip gives the steepest angle of descent of a titled bed or feature relative to
a horizontal plane and is given by the number with degrees (0˚-90˚) as well as a letter
(NS-EW) with direction in which the bed is dipping.
2.9 Regional Setting
These areas are covered under the investigations come under hard rock
terrains as far as palaeomagnetic study investigations are concerned. The chief rock
types are plutonic complex consisting of peninsular granites and gneiss of the
Archaean age. Which form a part of the vast Precambrian Peninsular shield.
Subsequently Balakrishna, (1964) suggested that the pink granite could be
metasomatic derivatives of the grey series of granite and gneiss. Janardhan Rao,
(1965) has argued against magmatic origin of these granites based on the parallelism
of the structural elements in the granites with Dharwarian metasediments and certain
mineralogical and textured relationships.
The different rock types occurring in this area have been broadly classified
(Sitaramayya, 1968) into three groups. First two groups belong to granites while the
third group comprises the dolerite dykes. Among the first two groups, grey and pink
series have been identified.
43
The grey granites are fine to medium grained, dark coloured and appear
homogeneous from a distance. However, on close observation, they show small
clusters of mafic minerals, pyroxene and biotite. These clusters are arranged in
parallel planes and constitute the lineation causing pronounced gneissic banding. The
quartz grains in these rocks have light brown colour. They have greasy appearance.
Feldspar crystals are light brown colour and plagioclase grains are semi opheic with
white or light or light green colour.
Sometimes these rocks show patches of porphyroblasts of potash feldspar. The
gneissic bonding shows dominant trend of N50˚W. These rocks are traversed by
aplitic veins and dykes of width ranging from a few centimeters to 100 meters. Many
epidote veins come across these granites.
The pink granites are usually fine to medium grained but coarse grained
porphyritic varieties are not uncommon. These are characterized by a very low mafic
mineral content and have pink colour. The minerals are quartz and potash feldspars.
Minor amounts of plagioclase, biotite, magmatite and zircon are present. Usually
these rocks occur as ridges sometimes rising to a high top, more than 80m. They are
having consistent structural trends, the important trend is N 30˚-40˚W followed by the
N-S N60˚-70˚E. These rocks sometimes are characterized by pronounced foliation
due to the parallel arrangement of quartz grains.
These rocks exhibit a characteristic weathering which is controlled by closely
placed joints in more than one set. Many narrow quartz veins also traverse these
rocks. These rocks are to be considered younger to the grey series of rocks but to
dolerite dykes. It is considered that the emplacement of basic dykes followed weak
structural planes in the country rocks.
Both the pink and grey granites are traversed by quartz and pegmatite veins.
Intrusive bodies were more in to pink granites than in grey granites. The demarcation
between pink and grey granites is difficult. In pink granite itself there are several
distinct types. Some of them are brick red in colour, others are bright pink and still
others show all shades of pink red and reddish brown.
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3.0 Dolerite Dykes
Dolerite is coarse grained variety of basalt, if present in the dorm of a dyke
(discordant igneous body), it is called a dolerite dyke. Dolerite is the medium grained,
intrusive, equivalent of a basalt (link to basalts). It usually occurs as dykes. Being
intruded into country rocks at shallow levels, the magma has more time to cool than if
extruded. Dolerites may be porphyritic, containing phenocrysts of plagioclase, olivine
and even quartz (an olivine analcite dolerite is known as a crinanite).
Dolerite dykes, like many other magmatic intrusions, develop by rapid
hydraulic fracturing via the propagation of a fluid-filled open fissure, resulting in a
massive magmatic intrusion with a neat and transgressive contact with the country
rock. This fracturing mechanism is in contrast to the slow mode of hydraulic
fracturing responsible for breccia-intrusions (kimberlite). For the intrusion to develop
the magma pressure at the tip of the fissure must overcome the tensile strength of the
surrounding rock. Dykes can develop vertically upwards or laterally along-strike over
very long distances, as long as the magma pressure at the tip of the fissure is
maintained. The intrusion of dolerite and basaltic dykes are therefore never
accompanied by brecciation, deformation or shearing of the host-rock, at least during
their propagation.
The main gabbroic body is intruded by a member of dolerite dykes running
North- South. It is composed of clinopyroxene, Plagioclase, Hornblende,
Orthopyroxene and Ilmenite. Hornblend occurs as large Poikilitic grain and
constitutes around 60% of the rock. Both gabbro and associated dolerite dykes
(Aditimaddal et al., 2012). The Shillong Plateau occupies a unique position in the
Precambrian history of the Indian plate both geodynamically and seismically. The
Precambrian framework of Meghalaya and tectonics of NE India were given by
(Mazumber, 1986) and (Acharya, 2005). In the central and northwestern shilling
plateau a number of Mesozoic dolerite/basalt and ultra basics have intruded the
basement rocks in NW-SE. Greenstone are also exposed in NE-SW direction within
the gneisses (Nivarani Devi and Sarma, 2006). Palaeomagnetic and geochemical data
of the Proterozoic Khasi greenstone dykes to understand the evolutionary history of
the Shillong basin vis-à-vis the Shillong plateau (Mallikharjuna Rao et al., 2009).
45
In the present work of palaeomagnetism, we have selected prominently visible
20 dykes in and around the Greater Hyderabad and Ranga Reddy District and they are
shown in the Fig. 2.4
Fig. 2.4 Map showing the Dykes in the Study Area
46
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