06_chapter 1

OUTLINE OF THE REGIONAL GEOLOGY OF
THE STUDY AREA
CHAPTER I
OUTLINE OF THE REGIONAL GEOLOGY OF THE STUDY AREA
1.1 Introduction
The reasons for uncommon and unabated interest in the potassium-rich and
associated rocks at Salem, despite their small aerial significance are:
(i) their exciting and extremely variable mineralogy,
(ii) variety of chemical types,
(iii) unusual REE patterns,
(iv) association of mantle derived rocks and minerals and
(v) their usefulness in providing clues in understanding the petrogenesis of
the rocks of their region.
An unusual occurrence of potassic rocks in association with dunite and
at. $4EM
peridotite was first reported by Ramanathan in the year 1954. Based mainly on
the chemical analysis, this rock type was compared with the shonkinite occurring
in Shonkin Sag Laccolith of Montana, U.S.A. described by Weed and Pirsson
(1901). The mineralogy of the shonkinite reported by Weed and Pirsson (a.cit.)
shows the presence of augite, olivine, biotite, apatite and magnetite in a matrix of
potash feldspar, zeolites and carbonates with pseudo-leucite appearing
conspicuously in a chilled marginal phase.
Ramanathan (1954) observed a somewhat similar series of rocks and
reported that the zeolites and carbonates are absent. Leucite and sporadic pseudoleucite occur in the dyke phases. However, Ramanathan (1954) and Naidu (1963)
retained the term shonkinite on account of their close resemblance, both
mineralogically and chemically to the shonkinite of the type area.
-: 2 : Nearly ten years later, P.R.J.Naidu (1963) presented a special paper on the
potassic rocks of Salem occurring as dykes and sheets within or marginal to a
dunite-periodthite body. According to him, the potassic rocks are black to dark
grey in colour and the texture resembles that of lamprophyres. His mineralogical
investigations showed that the rocks exhibit a panidiomorphic texture with
euhedral to subhedral crystals of olivine and augite set in a groundmass consisting
of felsic constituents.
Apart from understanding the nature of the mantle, the study of potassic
rocks provide us the tectonic setting, as major, trace element and Sr-Nd-Pb
isotopic signatures are associated with different magma generation environments
(Wilson, 1989).
The alkaline rocks occur in all tectonic environments with the exception
of mid-oceanic ridges; their occurrence may be grouped into three distinct
categories in relation to present day tectonism (Fitton and Upton, 1987).
(1)
Continental rift related magmatism: Example-East African rift
system, a classical region for the study of a complete spectrum of
unusual alkaline rocks of Mid-Tertiary to Recent age.
(2)
Oceanic and continental intraplate magmatism
(i) Oceanic: Tristan da cunha
(ii) Continental:
(a) Kola Peninsula (Kagarko, 1987)
(b) Cameroon line in West Africa.
-: 3 : (3)
Subduction related alkaline magmatism: These are of calc-alkaline
affinity and tend to become more potassic with time and may give
rise to shoshonitic association (Morrison, 1980).
1.2 Geology of South India and Tamil Nadu
The South Indian shield is composed of Archaean and Proterozoic rocks
of Tamil Nadu, Kerala, Karnataka and parts of Andhra Pradesh. It is a classical
region for the study of different stages of crustal evolution and is also one of the
extensively studied Precambrian terrains in the world. The South Indian shield is
flanked by the Mesozoic, Tertiary and Quaternary sediments to the east and west.
It is truncated by the Godavari graben towards north and partially masked by the
late Proterozoic basins (Cuddapah, Kaladgi, Badami and Bhima) towards northeast and Cretaceous to Tertiary flood basalts (Deccan Traps) towards north-west
(Fig.!.!).
The terms like non-charnockitic and charnockitic provinces were in vogue
as early as 1936 (Fermor, 1936) corresponding to the present day terms 'GraniteGre,enstone belt' and 'high-grade terrain' respectively. Layered complexes of
Archaean age occur in Sittampundi, Tamil Nadu (Subramaniani, 1956) and
Karnataka (Janardhan et a/,1978).
Innumerable ultramafic and potassic rocks of the South Indian Shield have
attracted the attention of many geologists in recent years. A review of the
Precambrian alkaline magmatism in eastern and southern peninsular India is
available in Memoir No.15 published by the Geological Society of India and
edited by Leelanandam (1989). Very recently, Lithos, an International Journal of
-: 4 : -
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GEOLOGICAL FRAMEWORK OF SOUTH INDIA
(modified from Drury et al, 1984)
8•
-: 5 : Mineralogy, Petrology and Geochemistry has published a special issue of Potassic
and ultrapotassic magmas and their orgin' edited by Peccerillo and Foley (1992).
1.2.1 Geology of Tamil Nadu
Tamil Nadu is situated in the southern part of Indian shield and contains
a major part of the high grade mobile belt terrain. It comprises a variety of
lithological units ranging from the oldest known Sathyamangalam supracrustals to
the younger sediments. Following is the broad geological succession in Tamil
Nadu:
Younger sediments
Younger granites (440 - 390 Ma)
a
Ultramfic and alkaline complexes (1200 - 800 Ma)
4
Basic dykes - Dolerites
Proterozoic anorthosites
Kadavur and Oddanchattram
Migmatitic gneisses
Arcot granite
Granulite facies rocks
Charnockites and Khondalites
Peninsular gneisses
Layered complex
Sittampundi and Gobichettipalayam
anorthosite - gabbros
Sathyamangalam
Amphibolites-BIF-sillimanite bearing
supracrustals
pates and quartzites.
Gnessic Basement ?
The high grade terrain of Tamil Nadu is broadly divisible Along the
Bhavani - Cauvery valley into two distinct physiographic provinces, i.e., a hilly
-:
6 :-
northern region and a hater southern region. Most of the high rising hill ranges
like Nilgiris, Athani-Andhiyur Hills, Shevaroy - Chitteri - Kalrayan Hills, Javadi
Hills, Kollaimalai - Pachaimali Hills are all located in the northern region, whereas
the region south of Cauvery is mostly a plain country with the exception of KodaiPalani Hills. The major rivers draining the fault-controlled valleys of Tamil Nadu
are Bhavani, Cauvery, Ponnaiyar, Vaigai and Tambraparni, all _flowing in the
easterly direction.
The geology of Tamil Nadu comprises a wide spectrum of lithologies
ranging from the oldest supracrustals to the youngest Cenozoic sediments (Fig.1.2).
The oldest supracrustals are represented by deformed enclaves of sillimanite /
fuchsite quartzites, amphibolites, banded iron formations and ultramafic-mafic
assemblages found in charnockites and gneisses. These are termed as
Sathyamangalam supracrustals by Gopala Krishnan et a/.(1975). Selvan (1981)
has compared these supracrustals with the oldest Sargur supracrustals in Karnataka
( > 3000 Ma ).
The well known chromite bearing anorthosite complex of Sittampundi
(Aiyangar, 1946; Subramaniarn,1956) has been compared with the Messina (South
Africa) and Fiskaenesset (Greenland) anorthositic complex by Windley (1973).
The most ubiquitous rock types in Tamil Nadu are granulites (twopyroxene granulites and charnockites). Holland (1900) was the first to describe
charnockitic rocks from hills around Pallavaram, Madras city. He proposed the
term `charnockite series' for the hypersthene bearing rocks of acid, intermediate,
basic and ultrabasic compositions. Significant contributions were made by many
-: 7 :-
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FIG.1.2 GEOLOGICAL MAP OF TAMIL NADU
(Source: Geological Survey of India)
-: 8 : investigators (Pichamuthu, 1961, 1967; Subramaniam, 1959; Naidu, 1963;
Narayanaswamy, 1971, 1975) towards the understanding of the charnockite
problem. For a long time, based on Narayanaswami's (1975) work, chamockitekhondalite system of rocks were considered to be the oldest in Tamil Nadu
geology. But recent works, including mapping of G.S.I. workers in 1980's and
geochronological data have brought out the fact that the charnockite-khondalite
group of rocks ( 2600 Ma) are not the oldest rocks; and that they are not new
additons to the crust, but the reflection of high metamorphism superimposed on
the pre-existing low grade gneisses (Peninsular gneisses). The orgin of chamockite
has been one of the most debated problems in South Indian geology.
Much work has been done on the retrogression of chamockites into
hornblende-biotite gneisses along major lineaments in Tamil Nadu (Moyar and
Bhavani lineaments - Srikantappa et ca,1988) and along Attur lineament,
retrogression and carbonatisation has been reported by Saleem Ahmed Khan and
Janardhan (1989).
Proterozoic massive anorthosites occur at Kadavur, Oddanchattram and at
other places. The basic intrusives are represented by swarms of dolerite dykes.
The conspicuous feature of these dolerite dyke swarms is that they occur
abundantly in northern Tamil Nadu and are scanty or absent in southern Tamil
Nadu.
The most important (both academic and economic view points) are the
ultramafic and alkaline occurrences confined to the late Proterozoic deep seated
lineaments (Fig.1.3). Grady (1971) and Ahmed et al. (1986) conclude that there
-: 9 : -
--- LINEAMENT
45 INTRUSIVE
COMPLEX
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Fig. 1.3 MAJOR LI NEAMENTS AND THE INTRUSIVE COMPLEXES
I AFTER SRINIVASAN I$77)
1
EL AGIR1
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2
ToRAPADI ULTRA BASIC COMPLEX
3 KAN)ANUR ULTRABAS1C CARBONATITE COMPLEX
I.
TENMuDIYANUR ULTRABASIC
$ HOGENAKAL CARBONATITE
6
PAR KANADu CAR BONATITE AND KUNDAMALAI ULTRA BASICS
CHALK HILLS u LT ILABASIC
I KANJAmAL Al
ULTRABASICS
TAmmAmPATTY ULTRA BASICS
ID
THEYUR ULTRABAS ICS
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13 TOLUR K AR ADu JAKKAHANAICKANPATTI AND NADANDAI
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14 VIMANAYAKKANUR ULTRABASICS
1$ VALASIRAMANI ULTRABASICS
II BNAYAN I ULTRABASICS
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KuNDA ULTRA BASICS
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22 PuNALUR ULTRABASICS
-: 10 : -
is a close relation between deep seated lineaments and magmatic bodies. Deans
and Powell (1968) recognised carbonatites at Koratti and subsequently several
other occurrences were discovered around Tiruppattur and now it is popularly
known as `Koratti carbonatite complex'. It is confined to NNE-SSW lineament.
Ramaswamy (1984) reported vogesties from Tiruppattur. Borodin et al.
(1971) described carbonatite - syenite complex near Paldcanadu, Salem District.
Srinivasan (1977) recognised carbonatites near Hogenalckal. All these occurrences
are confined to the NNE-SSW lineaments.
According to Murthy (1979), the ultramafic and potassic association which
is known as 'Chalk Hills ultramafic complex' occurs at the intersection of NW-SE
and NE-SW trending deep seated fault system.
Grady (1971) contended that this complex is situated at the intersection of
NNE-SSW and E-W trending Godumalai shear zone (the Attur lineament). The
ultramafic - syenite association has been described by Saleem Ahmed Khan
(1989).
It is clear from the ultramafic and alkaline rock occurrences throughout
Tamil Nadu and similar occurrences reported elsewhere in the world (Sorensen,
1974; Ulbrich and Games, 1980; Black et a/„1985) that there is a close relation
between deep seated lineaments and alkaline magmatism.
1.3 Location and Regional Geology of the Study Area
The potassium-rich and associated ultrwnafic complex of Nagaramalai is
-
11 : -
located about 7 km .north-west of Salem town covering an area of about 30 Sq,km
and contains one of the world's largest magnesite deposits. The area under study
is bounded by north latitudes 11 0 40 and 110 46' and east longitudes 78° 05 • and
78° 11' (Fig. 1.4). The oval shaped ultramafic complex is aligned along ENEWSW trend with maximum length of 20 km.between Nerinjipatti in the north-east
and Kamanayakanpatti in the south-west and the foot hills of Shevaroys on the
north-east.
The ultramafic complex has a maximum width of 3 km. between
Sanikavundanur on the north-west and Kusamalai (4545) on the south-east. The
entire ultramafic complex is bounded on all sides by well foliated and intensely
folded high grade granulites and gneisses. High grade granulites are represented
by Charnockite and pyroxene granulite together with concordant mafic bands and
lenses comprising gabbro, pyroxenite and norite. The associated gneisses are
represented by distinctly banded hornblende-biotite rich quartzofeldspathic rocks.
It is also very significant to note that the ultramafic bodies of Chalk Hills
are located at the intersection of the prominent deep seated faults (Fig 1.5)
trending in NW-SE and NE-SW directions (Murthy,1979).
The regional tectonics of Peninsular India has been dealt with at length by
Narayanaswami (1964, 1975); Grady, (1971); Katz (1978); Drury and Holt (1980)
and Drury et a/.(1984). Their works have brought out the fact that the
Precambrian shield of South India is dissected by a number of deep seated mega
lineaments. Drury et al.(1984), based on geophysical, geochemical, geological and
remote sensing data, divided the Precambrians of South Indian shield which occur
-: 12 : -
Fig .1•4 LOCATION MAP OF THE STUDY AREA
76
76
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LOCATION OF THE STUDY AREA
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-: 13 :-
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FIG. 1.5 PRINCIPAL DEEP SEATED FAULTS TRENDING IN NW-SE AND
NE-5W AND LOCATION OF CHALK HILLS ULTRAMAFIC COMPLEX OF
SALEM, TAMIL NADU.
(after Murthy, 1979)
: 14 : to the west of Eastern Ghats into two blocks viz., northern and southern, the
separating zone being the E-W trending Palghat-Cauvery lineament (Fig 1.6). Part
of Tamil Nadu which constitutes the southern portion of the northern block is
characterised by most of the high rising hill ranges viz., Nilgiris, Athani-Andiyur,
Shevaroys, Kalrayan, Kollaimalai and Pachaimalai hills.
Based on Landsat Imageries, Ahmed et a1.(1986) have prepared a lineament
map of Tamil Nadu and have divided Tamil Nadu area into three tectonic blocks
(1) Northern block, north of Moyar and Cauvery lineament,(2) Central Block,
between Moyar-Cauvery and Vaigai lineaments and (3) Southern block, west of
Vaigai lineament.
Individual lineaments in all these blocks have been given local names. The
density of lineaments is more in the northern block of Tamil Nadu, in contrast to
the other two blocks. These lineaments are important as major ultramafic and
alkaline rocks are located close to their proximity. From critical examination of
lineament maps of Tamil Nadu, it is seen that there is a close relationship between
the lineaments and the ultramafic occurrences.
1.4 Geomorphology
The general topography of the study area exhibits a gently raising high
ground towards Nagaramalai and Kurumbapatti reserved forest for the greater part.
The study area is bounded by a plain country with detached hills raising to 609
mts. from a general ground level of 340 mts. Nagaramalai is the prominent hill of
the study area with approximately EW alignment. The maximum elevation of the
Nagaramalai is 619 mts.
(Plate I, Fig.1). On the North, -eastern side of
Phi/101'otCrIC cover
ProlerozOiC eupracrustals
Migmatites and granitoid
plutons
Archaean granitoid gneiss**
Possible Archaean thrust/
she-ars
Boundaries of high Archaean
strains In western Karnataka
( LS • Low strain)
Northern limits oh granulites.
Maio( Proterozoic dykes
Boundaries of higk lee
Proterozoic a reeds,
li•gional movements in
late Proterozoic
Lat• Proterozoic fabric*
FIG.1.6 TECTONIC MAP OF SOUTH INDIA
(after Drury et al, 1984)
-: 15 :-
MADRAS
Phanoresec cover
Preierecoic auerecruss•I,
040Oineffies
PAlit0441
end graniioie
Atcsmose eacirscrual are
Alciwwba Otani
gneiaarie
Possible Archaean Moser
hounderies of high ArCh41111111
strains in western KI/Aalaill
( LS • Low streig
Northers Oahe ol granulites
Oda*, Proisanoic *ekes
illoundareog
reelerromsaiC $ (A
lei
**yaw moimakonts la
TRIVANDAthe
hale Porferatoic
Late Plefeeet04c terms
I.
74
t
o
76
FIG.1.6 TECTONIC MAP OF SOUTH INDIA
(after Drury et el, 1984)
-: 16 : Nagaramalai hill, the Shevaroy hillocks abruptly rises to a maximum height of
1625 mts. The other important hillocks found around Nagaramalai are as follows.
(Plate I, Fig.2 and plate II Figs. l and 2) .
(a)
Kusamalai (A545)
(b)
Karangankaradu (A435)
(c)
Ayyankaradu (A445)
(d)
Ammankaradu (A439)
(e)
Red Hills (A460)
(f)
Penimalkaradu (A460)
(g)
Veerappankaradu (A335)
(h)
Ammikundukaradu (A460)
In addition to the above hillocks, the study area is comprised of a number
of small boulders and rock knobs which are marked as outcrops.
The important villages located in the study area are:1.
Chettichavadi
2.
Gorimedu
3.
Karuppur
4.
Mungilpadi
5.
Tekkampatti
6.
Vellalappatti
7.
Sanikavundanur
8.
Tyagampatti
9.
Vinayagampatti
-: 17 : 10.
Pudur
11.
Kollappatti
12.
Kombalpatti
13.
Mettupatti
The extreme north of the study area is covered by eucalyptus plantation
and fairly dense scrub.
1.4.1 Drainage pattern
The general drainage pattern is radial in the ultramafic complex, while in
areas where chronockites and gneissic country rocks occur, the drainage pattern
is aligned roughly parallel to the regional foliation forming consequent valley.
The latter type of drainage is structurally controlled.
A number of seasonal streams are located in the study area. The supply
I water during monsoon and dry seasons vary widely. Streams running along the
lear planes show almost a straight line course. Along the southern slopes of the
hevaroy, dendritic type of drainage is noticed. Generally, the density of drainage
high on the Shevaroy Hills and it is sparse around Nagaramalai. The drainage
pattern of the study area is given in the figure 1.7.
In and around the study area, there a number of magnesite mines. Some
of the important working mines are as follows (Plate III, Figs. 1 and 2)
1. Dalmia Magnesite mine
2. Tamil Nadu Magnesite mine
3. Burn Standard and Company
-: 18 :-
r4
9 3
Kattura
Topographic map of Nagaramalai area
t•' contours
'PI* Metalled Road
Unenstalled Road
Telegraph line
-.-e-
Trolley lint
%I Village
• Hills
a Chimney
7E:
ef Streams /Nola
IN& Tanks
wow.. •■••••■•■•••••••••1 •■•■•••■
I
•• Mb SONO* ••=1,IM• . •••••••■
FIG.1.7 TOPOGRAPHIC MAP OF NAGARAMALI AREA SHOWING
THE DRAINAGE PATTERN.
: 19 : -
4. Ponkumar Magnesite mine
5. Ramakrishna Magnesite mine
6. Universal Mining Corporation
1.4.2 Weathering
The ultramafic rocks generally show evidence of greater weathering in
comparison to the adjacent gneissic country rocks. The depth of weathering in
dunite extends upto 40 mts., while in the gneissic country rocks, it ranges from 5
to 15 mts. The degree of weathering in these rocks are controlled by irregular
fracture patterns. The regional ground water table is confined to the sum of
weathering. The weathered soils of ultramafic rocks like dunite and peridotite
show yellowish red colour.
Potassium-rich rocks show mostly spheroidal weathering. Many well
sections show spheroidal boulders of shonkinite
1.4.3 Weather and Climate
Weather is a condition of the atmosphere in terms of heat, humidity,
pressure and wind movement at a given time and place. Climate is much broader
than weather. Climate is an expression of the syntheses of day to day changes of
meterological elements. Temperature and precipitation form the general backbone
of the climate. Hot and dry climate is prevailing around Nagaramalai. The hottest
months in the area under investigation are March, April and May. The heat
declines to some extent on the outbreak of the south-east monsoon. The southwest monsoon prevails during the months of June to September, whereas the
North-east monsoon occurs during the months of October, November and
: 20 : December. The average rainfall in the study area during South-west monsoon
period is 465mm.and during the North-east monsoon it is 302 mm. The rainfall
data of Salem district has been presented in the Table 1.1. The maximum
temperature ranges from 25°C to 40°C and the minimum temperature varies from
12°C to 20°C. The relative humidity percentage ranges from 60% to 90%.
Highest relative humidity values are during south-west monsoon period.
1.4.4. Vegetation
A very large area of Salem district is covered by forest. There are two
important reserved forest areas viz., (1) Nagaramalai reserved forest area and
(2) Kurumbapatti reserved forest area. In the extreme north of the study area
eucalyptus plantation is found. The other important trees found in the forest area
are (1) Prosopin (Karunkatau) (2) Acacis tencophoie (Vel-velam).
Nagaramalai and Chalk Hills areas are generally devoid of any vegetation.
At the foot hills and along the flanks of Shevaroys, thick vegetation in the form
of shrubs and thorny bushes are found. Within the ultramafic belt, vegetation is
very scanty, whereas the adjoining gneissic and charnockitic country rocks support
fairly thick vegetation. The important cultivation found in these areas are
sugarcane, paddy and groundnut.
1.4.5 Soil types
The soils of Salem District have been classified into five types. They are
1. Red Soil, 2. Black Soil, 3. Brown Soil, 4. Alluvial Soil and 5. Mixed Soil. On
the basis of parent material, texture, permeability and alkalinity, the above soil
types are further classified into five soil series. (Table 1.2). Figure 1.8 shows the
-: 21 : -
TABLE 1.1. RAIN FALL DATA OF SALEM DISTRICT, TAMIL NADU
SW MONSOON
NE MONSOON
June
79 mm
July
102 mm
November
94 mm
August
142 mm
December
30 mm
September
142 mm
TOTAL
465 mm
October
178 mm
302 mm
TABLE 1.2. SOIL SERIES OF SALEM TALUIC,
SALEM DISTRICT, TAMIL NADU.
S.No
Soil Series
Area
Parent material
Characteristic Features
1.
Salem Series
45.5 %
Weathered gneiss
with magnetite
quartzite.
Colour: Reddish brown
to dark brown.
Texture: Loamy to sandy
clayey loam.
Lime status: Non-calcareous.
2.
Somayanur
Series
89.5 %
Transported Soil
Colour: Dark reddish brown
to dark blown.
Texture: Sandy loam to
clayey loam.
Lime status: Calcareous.
3.
Mallur
Series
86.89 %
Lacustrine Origin
Colour: Greyish brown to
very dark brown.
Texture: Clayey loam to silty
clay.
Lime status: Calcareous.
4.
Tululdcanur
Series
85.63 %
Weathered
Quartzofeldspathic rocks
Colour: Dark brown to dark
reddish brown.
Texture: Sandy loam to
clayey loam.
Lime status: Calcareous.
5.
Peelamedu
Series
81.84 %
Granitic gneiss
Limestone
Colour: Dark grey to very
dark brown.
Texture: Clayey loam to clay.
Lime status: Calcareous.
—: 23 :-
Fg .1•8 SOIL MAP OF SALEM TALUK SALEM DISTRICT.
AM IL NADU
C SCALE 1: 250000
REFERENCE
----- DISTRICT BOUNDARY
TALUS BOUNDARY
RIVER •r STREAM
TANK
v".
ROCK
alb
SOIL SERIES BOUNDARY
OUTCROPS
•
ALKALINE
IIP
,,,.,, .1,1011 i.•
,-,.....,,...
...,
\■,N,
11
.1414, ,4. ..441101.1/0A. ../ta amp
41•1611.
4/116
..,...
,...1..•
ION . AAA
Ameme,..
•
VII IN
%GODUINAW
4
011 vii
nr
Us
•••••.,'
Imo
doiciri,
•
-,• —
.
ammemdIN"(11.v. 'IP W. (K. or 0 Jr-op
•
••.'
%
..
ad
III , •
C
—.
i'sil .,r
,•■2k
.........4=419
;,:! ■r.71C.:11.....
%,..r. `,44.444.4.4.4.11 •
.......-.„.%
VeS. N
411=1111.1./111
•:...-ommonm. Num= . 0TO V e
r—
1 141,■:47,11
Pdtt
111•11
,
■
t
i
#
.11:it-3
sze:
4.•
4
LEGEND
SAL E N
Iii
TULUKKNUR
SONAVANuK
EAR
PILANEDU
NALLUII
MAGNESITE QUARRY
Tlk
(
MISCELLANEOUS LAND
TYPE (141.7)
RESERVED FOREST
F.
SALEM TOWN
-: 24 : distribution of different types of soil of Salem district. However, the area under
investigation mostly contain red and brown soil which have been formed as a
result of weathering of ultramafic and gneissic country rocks.
1.4.6.- Communication and Accessibility
The study area is well connected by road, rail and air. Salem is 368 km
South-west of Madras and 200 km.south of Bangalore. The area can be easily be
reached either by Salem - Omalur road or by Salem - Yercaud road. Salem
Junction and Karuppur railway station are situated on the Jolarpet - Erode broad
gauge section of Southern Railway. The potassium-rich shonkinitic outcrops can
be reached by Salem to Danishpet road, Salem to Omalur road and Karuppur to
Tekkampatti road.
1.5. A Review of Previous Literature
Ramanathan, in the year 1954, collected two samples at Salem which were
dark coloured with almost porphyritic texture. According to him, these rocks had
phenocrysts of augite and other femic minerals embedded in a felsic matrix of
orthoclase and sanidine with occasional microcline grains. It has comparable
chemical analyses to that of shonkinites of Shonkin Sag, Montana, U.S.A. Nearly
ten years later, P.R.J.Naidu (1963) presented a special paper on the potassic rocks
of Salem occurring as dykes and sheets within or marginal to a dunite - peridotite
body. According to him, the potassic rocks are black to dark grey in colour and
their texture resembles that of lamprophyres. His mineralogical investigations show
that the rocks exhibit a panidiomorphic texture with euhedral to subhedral crystals
of olivine and augite set in a groundmass which may consist of the following
minerals:
- 25 : I. Leucite, pseudoleucite and
sanidine
- nearly 1.3% of groundmass
2. Nepheline, sanidine and
intergrowths of nepheline
and sanidine
- nearly 0.2% of groundmass
3. Sanidine laths or massive
sanidine and
- nearly 3.0% of groundmass
4. Sanidine or orthoclase
cryptoperthites
- nearly 95.5% of groundmass
Naidu (1963) is of the opinion that the presence of olivine and augite in
all the potassic rocks relates them to peridotites and dunites of Chalk Hills and the
larger proportion of felsic constituents is parallel with the increase of augite. He
quotes Bowen (1956) that the early abundant crystallisation of olivine and later
diopside -augite had rendered the residuum siliceous and alkalic. He proposed that
the magma with a system (Si02 - NaAl SiO4 - K Al SiO4) crystallised under high
water vapour pressure and that the field of leucite was restricted, (Fig. 1.9),
Consequently, the pseudoleucite reaction was restricted to a narrow range in time,
and the crystallisation of nepheline and sanidine and nepheline - sanidine
pobtssi
intergrowths was the end product of the differentiation of these pfertssie rocks.
Krishnamurthy (1962) indicated that the ultrabasic rocks of the Chalk Hills
consist mainly of dunite, peridotite and pyroxenite following an incomplete zonal
pattern with pyroxenite in the rim followed by peridotite and dunite towards the
core. He also suggested that the zoning may be due to successive intrusion on
fractional crystallisation and differentiation of ultrabasic magma.
-: 26 : a
SiO2
10
90
20
80
30
70
40
60
50
50
NaAl 5i3os
O. 2&3KAISIO8
60
30
KAI SI.0 6
20
Ne
Kp
(Neal Sipis )
10
20 30
40
50 60
70 60
90
( K At 5W )
4
01 LEUCITE
02 SANIDINE LATHS
0
3
04
MASSIVE SANIDINE
SANIDINE
PER THITE
(1/2 NEPHELINE AND SANIDINE
•
PSEUDOLEUCI TE
REACT ION
• rt INVARIANT POINT
• A,B,C,D ANALYSES OF POTASSIC ROCKS
Op
AVE RAGE OF PERIDOTITE
OB AVERAGE OF BASALT
Fig. 1.9
PHASE DIAGRAM OF THE SYSTEM Si02 (a) No AI 5;04 C Ne)- K AI SiO 4 ( Kp)
( AFTER
P.R.J
NAIDU , 19 6 3 )
-: 27 : Kadirvelu (1964) recognised in potassic rocks of Salem six components
namely, (1) Missourite, (2) Shonkinite, (3) Shonkinite-dunite, (4) Melasyenodiorite, (5) Yogoite and (6) Olivine-augite syenite and proposed an origin where
assimilative reaction of alkaline olivine basalt and granitic rocks of the basement
was suggested.
Murthy (1972) considered the potassic rocks of Salem as belonging to
alkali feldspar syenite in the IUGS classification (Fig. 1.10). Further, he observed
that olivine grains in the pyroxene do not show any characteristics of the relict
type and hence it is of a second generation.
In another paper, Murthy (1979) related the location of the ultrabasic rocks
and the cross cutting shonkinites to an intersection of two major fault systems
trending NE-SW and NW-SE. He also correlated all the other ultrabasic bodies
located along NE-SW trending fault system with Chalk Hills ultramafics.
Drury and Holt (1980) considered the intersection of major deep seated
structures and were of the view that the NE trending stnictrue is an extension of
the Proterozoic Moyar-Bhavani shear (Refer Fig.1.6) • According to Friend and
Janardhan (1984), the above shear could be the controlling factor in the location
of the ultrabasic rocks of Salem and was probably responsible for allowing the
ingress of fluids which gave rise to the formation of magnesite. While Ramanathan
and Naidu related the potassic rocks to the ultrabasic rocks in which they are
found, Friend and Janardhan (1984) considered that they are not related to each
other based on new petrographic and mineralogical data. Some of the evidences
given by them are
-: 28 :-
53 11N3X O UA 4
S31110 13 1U 34
OLIVINE
CPX--
OPX
OfITHOPYROXINITE
CLINOPYROXENITE
Fig . 1 •10 MODAL CLASSIFICATION OF ULTRAMAF1C ROCKS
( AFTER JUGS SYSTEMATICS OF IGNEOUS ROCKS)
-: 29 : (i) Deformational textures are seen in ultrabasic rocks,
Examples:
(a) foliation on an outcrop scale
(b) in thin section it shows mortar texture,
strain lamellaetetc. and
(c) kink bands in dunite.
(ii)
(iii)
Lack of deformational texture in the potassic rocks.
The felsic minerals of potassic rocks indicate a range in composition from
silica undersaturated feldspathoidal types to oversaturated quartz bearing
types.
(iv) In the Salem shonkinites, the finger print textures occupy interstitial areas
between K-feldspar, clinopyroxene and olivine phases. Therefore, they are
late, crystallising after K-feldspar. The intergrowth textures represent two
distinct phases,
(1) silica rich - potassium rich - sodium poor phase and
(2) silica poor - sodium rich phase. Such compositional finger print
textures have been reported in other similar alkaline rocks also. Their
petrographical and mineralogical data support the progressive crystallisation of a
relatively magnesian-alkaline magma to form a variety of ultramafic to gabbroic
textured rocks of broadly shonkinitic composition.
In 1984, Leelananda Rao and Periakali presented a paper on the
-: 30 : petrochemistry and genesis of the ultrabasic suite of rocks of Salem and included
potassic rocks along with the zoned plutonic complex with a core of dunite
mantled by peridotite and potassic members. They argue that the intimate relation
of the ultrabasic suite of rocks to one another prove a common source of origin.
According to them, the ultrabasic suite of rocks (including potassic rocks) appear
to have resulted due to the diapiric rise of the potash enriched upper mantle
material comprised of crystal mush that has consolidated at a depth corresponding
to about 1 K-bar where Pip°
Total
Detailed structural analysis by Srivastava (1990) brought out multiple
deformation pattern in charnockite and associated rocks at Kusamalai, south-east
of Chalk Hills ultramafics, clearly emphasising the complex structural frame work
surrounding the ultramafic complex, while the complex itself is devoid of any
structural fabric.
Based on detailed geological, petrological and chemical evidences, Murthy
(1988) envisaged a continental environment for the generation and emplacement
of the ultramafics of the Chalk Hills. He advocated the partial melting of upper
mantle below the continental crust generating a magma of picritic composition,
more basic than tholeiites and their emplacement through the transcrustal shears
at the intersection of Bhavani lineament and Attur fault.
Manjunatha Reddy (1991) presented the relation between leucite, potash
feldspar and nepheline and the origin of nepheline-K-feldspar intergrowth
(Manjunatha Reddy et al..1987). They considered that the intergrowths are not
pseudoleucites as non-sodic leucites (Na20 < 6-7%) cannot form pseudoleucites
-: 31 : (Bowen et a/,1937) and perhaps formed later than leudite and were then succeeded
by discrete nepheline. Later assemblages significantly contain albite and
microcline. They suggested that leucite might have formed in dry or CO2-rich
condition (Wendlandt and Eggler, 1980) which is more probable as carbonatites
and syenites ocotr along the NE-SW lineament cutting across Salem.
The age of the carbonatite complex has been estimated to be around 750 900 Ma (Dean and Powell, 1968; Borodin, 1971 and Stibramanian, 1983). The
age of potassic rocks of Salem is around 808 ± 18 Ma when plotted in 87Sr/86Sr
vs 87Rb/86Sr Isochron diagram (Manjunatha Reddy, 1991) (Fig. 1.11).
1.6. Scope and aims of present study
The present study aims at the following:
(1)
A scrutiny of the previous literature clearly shows that a variety of
potassic rocks occur and their field relations are very important
to understand the distribution of potassic rocks.
Therefore, the author undertook an intensive survey with detailed mapping
of the study area. He has established the field relations between different rock
types and is proposing a chronological succession of geological events in the area.
(2)
The mineral assemblages and textures in the potassic and associated
ultrafamic rocks reveal information on the physical conditions of
their formations and mode of emplacement of these rocks. The
author attempts a detailed study of the mineral assemblages and
textures.
-: 32 :-
Q.J.P - RENNES.Dec.69)
r
IONION.■••■•■•111ilimMialr
" sr/ "sr
0.740
Shonkinites from
9990
South India
9989
99794 /t• 5185 •
808 18M(2o)
10=0.7051 4_1 (2a)
t MSWD 3.9
"Rb/ "Sr
9986 (11.8.)
1
FIG.1.11 ISOCHRON DIAGRAM OF WHOLE ROCK SAMPLES OF
SHONKINITE GROUP OF ROCKS
(after Manjunatha Reddy, 1991)
-: 33 : (3)
To establish a petrogenetic model, the complete chemistry of rocks
must be known. Therefore, the author has utilised major oxide, trace
element and Rare Earth Element (REE) analyses for complete
geochemistry of the rock samples.
(4)
During the course of study, a relationship was noticed between the
deep seated major fault running through the study area and covering
a distance of nearly 150Icm (including carbonatite and alkali syenite
of Sevattur) and the pota.ssic rocks of Salem. This deep seated
major fault and structure of this region were
studied in detail.
(5)
Based on the field, petrography, structural, geophysical and
geochemical data, the petrogenesis, the possible mode of
emplacement and the time of emplacement of potassic and
associated ultramafic rocks have been attempted.
r Research methodology adopted
A detailed geological map on 1:25,000 scale was prepared by the author,
,g. 1.12),An area of about 30 sq.km.in the northwestern part of the Salem town
ng between north latitudes 11 040' and 11 046' 1. and east longitudes 78°05' and
'11' forming a part of Survey of India toposheet No. 58 1/2 was chosen for
Ailed mapping. Since it is an active mining area for magnesite and the rock
Dosures are being covered by mine dumps and soils, the author has taken
.ulicient care not to miss even a single outcrop. In addition to barren outcrops,
A
nine cut, mine faces, railway and road cuttings and well cuttings were also
-: 34 : carefully studied. Extensive cultivation has been made in the high grade gneissgran ulite terrain.
About 500 rock samples were collected considering the textural and
mineralogical variations of different rock types for detailed petrographic and
geochentical studies, More than 250 thin sections were prepared and examined
under the petrological microscope to understand their texture and mineralogy.
Samples were crushed upto - 240 mesh size using agate mortar and pestle for the
chemical analyses of major, trace and REE. Nearly 10 major oxide analyses and
trace element analyses and 10 REE analyses were done at the National
Geophysical Research Institute (NGRI), Hyderabad and Tamil Nadu Agricultural
University Research Station, Yercaud, Salem District. Trace and REE data were
obtained for selected potassic and ultramafic rocks with the help of ICP MS/VG
plasmaquad from National Geophysical Research Institute, Hyderabad.
PLATE - I
FIG. 1 A DISTANT VIEW OF THE EASTERN SIDE OF
NAGARAMALAI (A 619)
FIG.2 A VIEW OF KUSAMALAI (A 545) WHICH FORMS A
STRUCTURAL BASIN.
PLATE -1
Fig -2
PLATE - II
FIG.1
A VIEW OF THE NORTHERN SIDE OF AYYANKARADU (e445).
FIG. 2
A PANORAMIC VIEW OF CHALK HILLS WITH MAGNESITE
MINES.
PLATE - II
Fig • 1
Fig - 2
PLATE - III
FIG. 1.
A VIEW OF RAMAKRISHNA MAGNESITE MINES LOCATED ON
SALEM - DANISHPET ROAD.
FIG. 2.
A VIEW OF TAMIL NADU MAGNESITE MINE (TANMAG) AT
KURUMBAPATTI. A PART OF SHEVAROY HILLS IS SEEN IN
THE BACKGROUND.
•
Fig 2

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