new chapter-ii geology of southern granulite terrane 2.1....
Post on 22-Oct-2020
1 Views
Preview:
TRANSCRIPT
-
10
CHAPTER-II
Geology of Southern Granulite Terrane
2.1. Introduction
This chapter provides a summary of geological and tectonic aspects of Southern
Granulite Terrane (SGT), which hosts varied lithologies of high grade rocks like
charnockites, khondalites and other granulite facies metamorphic rocks with granitic
intrusions of different ages. The SGT extending from 80 to 110 N latitude of India (Tamil
Nadu, Karnataka, and Kerala states) is one of few terranes in the world that has preserved
Archaean and Proterozoic crust with extensive high-grade granulite facies rocks (Fig 1). The
transitional boundary between the high-grade and low-grade terranes is demarcated by well
known ‘‘Fermor line’’ representing a fundamental crustal discontinuity along which there
must have been considerable vertical displacement (Fermor, 1936). This line separates the
exposed charnockitic region from non-charnockitic region. The SGT is believed to be of
lower crustal origin through a complex evolutionary history with multiple deformations,
anatexis, intrusions and polyphase metamorphic events. More than 80% of the terrane is
covered by varied lithologies of Archaean and Proterozoic age groups namely,
Sathyamangalam Group (>3200 Ma), layered mafic and ultramafic complexes, Bhavani
Group (~3000 Ma), Kolar Group (~2900 Ma), Khondalite Group, Charnockite Group (~2600
Ma) and Migmatitic complex (2200-2250Ma) etc. The Proterozoic rocks include younger
granulites/charnockies, granites, alkali syenites, corbonatites, mafic and ultramafic intrusives
mostly occurring in and around the Cauvery suture/ shear zone (CSZ) (GSI, 2006).
-
11
Fig 2.1: Geological frame work of Southern India (after Santosh and Sajeev, 2006)
2.1.1. Sathyamangalam Group
The Sathyamangalam group of rocks is considered to be equivalents of ‘Sargurs’ of
Dharwar craton exposed in the central and northwestern part of the Indian Peninsula. The
-
12
group consists of quartzite ±fuchsite ± kyanite ± sillimanite and banded iron formation,
sillimanite schist ± garnet, kyanite- schist, corundum bearing mica schist and talc-tremolite
schist; calc granulite, crystalline limestone/marble, ortho-and para amphibolite
(Gopalakrishnan, et al., 1975).
2.1.2. Layered mafic and ultramafic complexes
In the proximity of Sathyamangalam group, ultrabasic rock sequence of dunite,
peridotite, websterite, garnetiferous gabbro, gabbroic anorthosite occur near Mettupalayam
(NW part) and other areas. They generally occur as enclaves within the migmatitic gneisses
as a part of the dismembered sequence. Large volume of garnetiferous gabbro and
hornblendic anorthosite with chromitite layers as well as small lenses of eclogitic rocks are
the characteristic features of this suite (Gopalakrishnan, 1994b). The well known Sittampundi
anorthosite yielded 3000-2900Ma by Sm-Nd systematics (Bhaskara Rao et al., 1996).
Sittampundi layered anorthosites define two populations : older yield a Concordia age of
2541± 13 Ma, younger belonging to a high-grade metamorphic event at 2461± 15 Ma by
zircon LA-ICPMS U–Pband Hf isotope data (Ram Mohan et al. 2013). Similar complexes
also occur as small bodies, lenses and bands, in the northeastern sector of the SGT around
Cuddalore, Vellore, Tiruvannamalai and Dharmapuri areas. They consist of dunite, peridotite,
hypersthenite, enstatite, augitite, hornblendite, websterite, gabbro and anorthosite (GSI,
2006). These are also considered to be intrusive into precursor rocks of Charnockite group
and later deformed and metamorphosed along with the host rocks under granulite facies
conditions (Sugavanam et. al., 1978).
2.1.3. Bhavani Group
The rocks belonging to Bhavani Group occur around Bhavani town (north of Palar
River) in the form of typical exposures of peninsular gneissic group of rocks. The gneissic
rocks also occur extending from Kerala border in the west through parts of Coimbatore,
-
13
Erode, Salem, Namakkal, Tiruchirapalli and Perambalur districts towards the east coast. The
rock types include fissile mica gneiss, quartzo-felspathic gneiss, augen gneiss, hornblende
gneiss, hornblende-biotite gneiss, biotite gneiss, granitic gneiss and pink migmatite (GSI,
2006).
2.1.4. Khondalite and Charnockite Groups
The khondalite and charnockite group of rocks (also equivalents of Eastern Ghats
Super group) and their reworked equivalents are the dominant variety of rocks in the SGT.
The Khondalite Group essentially consists of rocks of sedimentary parentage such as
quartzite and garnet-sillimanite gneiss ± graphite ± cordierite (metapelites) and occur mostly
to the south of Palghat-Cauvery Shear Zone (PCSZ). They are often interfolded and inter
banded with mafic granulite/amphibolite and charnockite. The Charnockite group,
comprising charnockite (hypersthene bearing granite), two-pyroxene granulite, banded
quartz-magnetite granulite/banded magnetite quartzite and thin pink quartzo-feldspathic
granulite, are extensively developed in the north-eastern sector of the SGT. They are also
well exposed in many prominent hill ranges such as Pallavaram-Chengleput, Javadi,
Shevaroy, Chitteri, Kalrayan, Kollimalai, Pachchamalai, Nilgiris, Kodaikanal, Palani,
Sirumalai, Varushanad, Gasthiarmalai and Hills around Nagercoil and are described as
charnockitic massifs (Fig. 2.2). The pyroxene granulites of Charnockite Group are considered
to represent mafic volcanics, while the banded magnetite quartzites indicates a volcanic
exhalative origin and the pink granulite is interpreted to represent the associated acid
volcanics (Gopalakrishnan et al., 1976, Suganvanam et al., 1978). The geochronological data
available for charnockites of the terrane show different ages ranging from ca 3000Ma to
550Ma. These data indicate that the charnockites occurring north of Palghat-Cauvery
Lineament (PCL)/Noyil-Cauvery rivers show prominently Neoarchaean ages of 3000-
2600Ma, while those occurring south of PCL yield younger ages of 550 Ma. However, in
-
14
recent years several mixed ages from 2200Ma to 600 Ma have been described (Thomson et
al., 2006; Plavsa et al., 2012). The charnockite interbands, which are rich in diopside, are
considered to be metamorphosed sediments, while mafic granulites/amphibolites probably
represent mafic volcanics (Gopalakrishnan et al., 1976; GSI, 2006). Based on U-Pb Zircon
dating, Ghosh et al. (1998) described that the Khondalite Group in Palaiyam area is younger
to the Mid-Archaean Sathyamangalam Group. However, the garnet-sillimanite gneiss from
adjoining Kerala has given Rb/Sr whole rock age of 3065 ± 75 Ma (Crawford, 1969). Recent
Nd-isotope studies have yielded model Nd ages (TDM) ranging from 2.60 Ga to 1.34 Ga
indicating a Palaeoproterozoic age for the Khondalite Group in Kerala (Harris et al., 1994).
Fig.2.2. Shaded relief image of southern India showing the distribution of major charnockite massifs (after Rajesh, 2004); TB-Trivandrum Block; NB-Northern Block
-
15
2.1.5. Migmatite Complex
The SGT has witnessed two major periods of granitic activity, initially during
Neoarchaean to Palaeoproterozoic and another during Neoproterozoic period. The older
granitic emplacement event are restricted to the northern part of Terrane, north of Palghat-
Cauvery Shear Zone (PCSZ), while the younger Pan-African event is widespread in the
region south of PCSZ. The time of emplacement of these granitoids has been well
constrained by Rb-Sr isotopic systematics (Nathan et al., 2001). The Neoarchaean granitoids
were developed mostly to the north of the CSZ viz Tiruttani, Sholingar, Bisanattam, Ebbari
and Krishnagiri (Ca 2500 Ma) (Krogstad and Hanson, 1988; GSI 1991), while
Paleoproterozoic granitoids are recognized around Gingee, Tiruvannamalai and Tirukovilur
(2254 Ma; Balasubrahmanyan et al., 1979). The rocks of the Khondalite and Charnockite
groups have been subjected to regional migmatisation and retrogression with influx of
quartzo-feldspathic material resulting in the formation of different types of gneisses such as
biotite gneiss, hornblende gneiss, augen gneiss, garnetiferous biotite gneiss, garnetiferous
quartzo-feldspathic gneiss depending upon the parent rock (GSI, 2006). These rocks are
grouped under migmatite complex. The formation of these gneissic rocks took place in
different stages of formation from meta-texites to diatexites (Gopalakrishnan et al., 1976).
These rocks have also experienced multiple deformations and polymetamorphism with
concomitant anatexis giving rise to a range of migmatites. The Migmatite Complex mostly
belongs to Archaean age and a few belong to Paleoproterozoic age i.e. 2250- 2100 Ma,
(Crawford 1969; 2250 Ma, GSI, 1978, Balasubramanyan et.al., 1979).
2.1.6. Alkaline magmatism
The SGT has witnessed two events of alkaline magmatism, during Paleoproterozoic
period and Neoproterozic periods. In the northern part of the SGT, several syenite-
carbonatite, nepheline-syenite, theralite and camptonite bodies have intruded in a concordant
-
16
linear fashion with in the country rocks around Pikkili and Hogenakkal areas. Along this
NNE-SSW trending lineament, some pyroxenite, syenite and carbonatite plutons are also
reported around Hogenakkal area. The Hogenakkal and Pikkili Syenite intrusives range in
age from 1994 Ma and 2371Ma (Natarajan et al., 1994; NGRI, 1994). The alkaline related
plutonism was also extensive around Vellore, Dharmapuri and Salem districts, where a
number of ultramafic-syenite-carbonatite bodies of Elagiri, Koratti, Samalpatti and
Pakkanadu occur in the form of a NNE-SSW trending zone extending for about 200km from
Gudiyattam in the north to Bhavani in the south (GSI, 2006). A number of smaller
ultramafic-syenite-carbonatite bodies also occur along sub-parallel NNE-SSW trending shear
zones on both sides of the main zone of alkaline activity. The ultramafics also occur in large
proportion around the Samalpatti pluton, Koratti pluton, Pakkanadu pluton and they carry
large chunks of ilmeno-rutiles at many places. Small carbonatite bodies of both sovite
Benstonite (barium bearing carbonatite) and beforsite composition occur within the
Samalpatti pluton. In Salem district, many ultramafic bodies are also well known for hosting
the famous magnesite deposits (G.S.I, 2006). Reddy et al., (1995) described an age of
808±18Ma for the Salem ultramafic complex.
2.1.7. Mafic Dykes
A number of mafic dyke swarms traverse the northern part of the SGT and intrude the
charnockite and migmatite group of rocks with varying trends from WNW-ESE and NNE-
SSW and rarely N-S and NNW-SSE. In the central part of terrane, they trend in ENE-WNW
to NE-SW. The textural characteristics of dykes show dolerite, gabbroic/basaltic in nature.
Petrochemical studies (Krishna Rao and Nathan, 1999) indicate that the majority of these
dykes are quartz normative tholeiites, while olivine-dolerite dykes show basaltic-komatiite
chemistry. K-Ar isotopic ages of these mafic dykes are interpreted to be around 1700Ma
(Radhakrishna and Mathew Joseph, 1993; Sarkar and Mallick, 1995).
-
17
2.1.8. Ultrabasic/basic rocks
The anorthosites and ultramafic rocks occur in the southern part of the terrane
(Oddanchatram, Kadavur and Tirunelveli), south of the PCSZ indicating that they were
possibly emplaced within a Palaeoproterozoic crust (GSI, 2006). The host charnockite for the
Oddanchatram anorthosite has been dated as 550Ma (Bartlett et.al., 1995; Jayananda et.al.,
1995), and Oddanchatram anorthosite yielded ca.600Ma (Ghosh et.al., 1998) suggesting its
ambiguity of the host and the intrusive. The Kadavur anorthosite complex of hornblende
gabbro body has been reported as ca. 600Ma from K-Ar mineral age by Balasubramanyan
and Sarkar, 1981. Recently Teale et al., (2011) documented U-Pb ages of 829 ± 14Ma for the
metamorphic zircons from Kadavur gabbro anorthosite complex and the metmorphic zircons
from the surrounding quartzite yielded 843± 23.
2.2. Metamorphism
The SGT broadly witnessed two granulite facies metamorphic events: Neoarchaean
(ca. 2.5 Ga) cratonic part in the north and a Neoproterozoic (ca 1.0–0.55 Ga) terrane in the
south. The Charnockites from Pallavaram and Shevroy hill massifs show quartz- K-feldspar-
plagioclase feldspar- clinopyroxene- garnet- hornblende- biotite- apatite- zircon- magnetite-
ileminte- rutile- pyrope (Rajesh and Santosh, 2004). However, the mafic charnockites from
the Shevroy hill massif has a dominant mineralogy of plagioclase- clinopyroxene-
orthopyroxene- hornblende- magnetite- ilemenite- rutile- K-feldspar- biotite- apatite- zircon.
The non-garnetiferous enderbites from both the Biligirirangan and Nilgiri hill massifs have a
similar mineralogy of quartz- plagioclase- feldspar- clinopyroxene- garnet- hornblende-
biotite- apatite- zircon- magnetite- ilmenite- rutile- pyrope. The felsic type Cardamom hill
massif has a dominant mineralogy of quartz- K-feldspar- plagioclase feldspar-
orthopyroxene- magnetite- ilmeinite- apatite, - hornblende- biotite while the intermediate
type Cardamom hill massif has mineralogy of quartz- plagioclase- K-feldspar-
-
18
orthopyroxene- clinopyroxene- hornblende- biotite- magnetite- rutile- zircon- apatite. The
Nagercoil massif charnockites show a common mineral assemblage of quartz- plagioclase-
K-feldspar- orthopyroxene- clinopyroxene- hornblende- biotite-magnetite- rutile- zircon-
apatite- garnet (Rajesh and Santosh, 2004). The Nilgiri block was metamorphosed under
medium-high pressure granulite facies conditions (6-10 kbar; Harris et al 1982). The ages of
high-grade metamorphism for the Nilagiri massif charnockites are ca. 2496 ± 15Ma
(Jayananda and Peucat 1996). The geochemical characters of the charnockitic massifs in the
Madurai Block were interpretedto show strong calc-alkaline affinity (Chacko et al., 1992-
Cardamom Hill Charnockites; Thomson et al., 2006) and also described (Rajesh, 2007) a
subduction related origin for the charnockites of the Cardamom Hill. Whereas, Ishii et al.
(2006) who described P-T estimates up to 8.5-9 kbars and 940-1040°C from cordierite and
orthopyroxene-bearing ultra high temperature (UHT) granulites within the AKSZ of
Trivandrum block. The metamorphism along Kerala Khondalite Block (KKB) is
characterized by a clockwise P-T path with post peak isobaric cooling followed by isothermal
decompression (Santosh, 1987; Fonarev et al., 2000; Cenki et al., 2002, 2004). Some studies
(Braun et al., 1996; Chacko et al., 1996; Satish Kumar and Harley, 1998; Nandakumar and
Harley, 2000; Cenki et al., 2002) indicate that the P-T estimation from KKB represents the
highest-grade assemblages record temperatures in the range of 8400-10700C and pressures up
to 9.5 kbar.
2.2.1. Ultra high temperature metamorphism and eclogite facies rocks
Extreme crustal metamorphism at T=900-11500C and P= 7-13 kbar occur in some
areas of the SGT, which are generally designated as ultra-high temperature (UHT)
metamorphic rocks (Harley, 1998; 2004). There have been several reports of granulite facies
assemblages, such as orthopyroxene-bearing granulites, sapphirine-bearing pelites, and calc-
silicate rocks, which suggest high to ultra high temperature metamorphism (e.g., Chacko et
-
19
al., 1996; Raith et al., 1997; Satish Kumar and Harley, 1998). In the eastern part of the CSZ
and in Madurai Granulite Block several workers (e.g., Shimpo et al., 2006; Collins et al.,
2007b; Clark et al., 2009; Tsunogae and Santosh, 2006; Nishimiya et al., 2010) have
documented the evidence for a prograde high-pressure (HP) event and subsequent ultra high-
temperature (UHT) metamorphism along a clockwise path based on geochemical and
petrological studies. Fluid inclusion studies of some metamorphic rocks within the CSZ
include those on Mg-Al-rich granulites (Ohayama et al., 2008) and mafic granulites
(Nishimiya et al., 2008; Santosh et al., 2010) show the occurrence of abundant CO2 rich
inclusions in Mg-Al-rich rocks and retrogressed eclogites (high-pressure granulites). Similar
primary fluid inclusions with markedly low-density CO2 have also been reported from some
of UHT terranes elsewhere such as from the Trivandrum Block (Fonarev et al., 2001) and
Madurai Block (Tsunogae et al., 2008). In a recent study, Nishimiya et al. (2010) reported
equilibrium sapphirine + quartz assemblage from Panangad area within the PCSZ providing
unequivocal evidence for extreme crustal metamorphism at UHT conditions associated with
the collisional assembly of the Gondwana supercontinent in the Neoproterozoic-Cambrian
period. The results indicate peak UHT conditions of 940-990°C at 7-8 kbar followed by a
retrograde event of 600-700°C along a clockwise P-T path (Santosh et al., 2009a). Recently
Sajeev et al. (2009) described regtrogressed eclogites from the well known Sittampundi
anorthosite complex with in the garnet gabbro layer in anorthosite. The P-T conditions
represent that garnet-rutile-melt was the peak metamorphic assemblage that eclogites
developed at ca 20 kbar and above 10000C.
2.3. Tectonic bocks in SGT
The Southern Granulite Terrane has a complex evolutionary history from the
Paleoarchaean to Neoproterozoic (3500–550 Ma) with repeated multiple deformations,
anatexis, intrusions and polyphase metamorphism (Bartlett et al., 1998; Bhaskar Rao et al.,
-
20
2003). The SGT comprises juxtaposed crustal blocks, which are fragmented and
dismembered (Chetty et al., 2006; and the references there in). These blocks are separated by
two major shear zones namely the Cauvery Shear Zone (CSZ) and Achankovil Shear Zone
(AKSZ). These shear zones separate distinct geological domains in terms of lithology,
structural style, and crust forming age of the basement gneisses (Bhaskar Rao et al., 2003).
Essentially, the CSZ divides the SGT into discrete tectonic blocks, viz., the Archaean
granulite blocks to the north and the Neoproterozoic granulite blocks to the south (CSZ,).
Fig.2.3: Regional tectonic framework of CSZ showing various Shear zones within it. The present two study areas are marked as red ellipses. WDC-Western Dharwar Craton, EDC-Eastern Dharwar Craton, Tz- Transition Zone, CSZ-Cauvery Suture Zone, Mo-Moyer Shear Zone, Bh-Bhavani Shear Zone, PCSZ-Palghat Cauvery Shear Zone and MSZ- Mettur Shear Zone (Modified after Chetty et al., 2003).
2.3.1. Cauvery suture zone
A crustal-scale (suture) shear zone system divides the Southern granulite terrane
(SGT) of southern India into discrete tectonic blocks (Drury and Holt, 1980; Gopalakrishnan
et al., 1990; Chetty, 1996). The most prominent of these is an east–west trending zone of
intense planar fabrics separating the northern late Archaean granulite block(s) from the late
Neoproterozoic Madurai granulite block (MGB), which is well known as the Cauvery shear
-
21
(suture) zone system (CSZ). The geology and the age relationships of the CSZ have been
reviewed recently by Bhaskar Rao et al. (2003), Chetty et al. (2003), Jain et al. (2003),
Mukhopadhay et al. (2003) and Ramakishanan (2003). The CSZ has been variously described
as: (i) a collision zone and cryptic suture, evident from the occurrence of remnants of
probable ophiolitic sequence (Gopalakrishnan, 1994a), (ii) dextral shear zone as exemplified
by the deflection of north-south Archean fabrics to near east-west disposition along the
MBSZ (Drury et al., 1984), (iii) an analogue of the central part of Limpopo mobile belt
(Ramakrishnan, 1993), (iv) the Archean-Proterozoic Terrane boundary (Harris et al., 1994),
(v) a zone of Paleo- and Neoproterozoic reworking of Archean crust (Bhaskar Rao et al.,
1996), and (vi) a Neoproterozoic dextral-ductile transpressive tectonic zone (Meissner et
al,2002; Chetty et al., 2003, Chetty and Bhaskar Rao, 2006). The CSZ is considered as the
trace of the Cambrian suture zone of Gondwana (Collins et al., 2007a).
Based on the U-Pb SHRIMP ages from charnockites of Salem-Madras Block, Clark et
al., (2009) described the magmatism at ca.2530 Ma age and subsequent high-grade
metamorphism and partial melting at ca. 2480 Ma. Available geochronological data on the
protoliths of the Salem Block indicate Meso- to Neoachaean rocks stretching as far south as
the PCSZ with metamorphic ages indicating a granulite facies event in the latest Archaean to
earliest Proterozoic (Peucat et al., 1993).
The CSZ has been interpreted as a dextral transcurrent shear zone and its crustal
architecture modelled in terms of a regional ‘flower structure’ (Chetty et al., 2003; Chetty and
Bhaskar Rao, 2006) typical of collisional orogens (Roure et al. 1989). It has been emphasized
that the CSZ essentially forms a zone of late Neoproterozoic reworking of Archaean
protoliths. The history of crustal reworking here is considered contemporaneous with the Pan
African orogenic processes linked to the amalgamation of continental fragments in eastern
Gondwana (Meert, 2003). Although, there is no consensus on the correlations of CSZ with
-
22
the shear zones in other Gondwana fragments, a recent view favours its correlation with the
Betsimisaraka suture zone in the eastern Madagascar (Collins and Windley, 2002), while
earlier models linked it to the sinistral Ranatsora shear zone of southern Madagascar to the
west and the boundary between Napier complex and Rayner complex of east Antarctica
(Chetty, 1995; Harris, 1997; Janardhan, 1999). All the geological and geophysical studies
across the CSZ described in the literature suggest that the CSZ comprises a complex tectonic
zone of dismembered and imbricated crustal blocks.
Several major shear zones have been delineated from Landsat interpretation from the
SGT (Drury and Holt, 1980). They include: (1) the Moyar-Attur Shear Zone, (2) the Bhavani
Shear Zone, (3) the Palghat-Cauvery Shear Zone (PCSZ), and (4) the Achankovil Shear
Zone. The most important shear zones of the region are the Moyar-Bhavani (MBSZ), the
Palghat-Cauvery (PCSZ) and the Achankovil (AKSZ) shear zones. The MBSZ branches into
several curvilinear shear zones in the NE-SW direction. Prominent among them is the Mettur
Shear Zone (MTSZ). Based on satellite data interpretation and subsequent ground follow-up,
Chetty et al. (2003) termed the network of these crustal-scale shear zones as the Cauvery
Suture Zone (CSZ). They divided the region between the Biligiri Rangan and Kodaikanal
high-grade charnockite massifs (see Fig. 2.3) into the Moyar-Bhavani shear zone (MBSZ),
the Chennimalai-Noyil shear zone (CNSZ), the Dharapuram shear zone (DSZ), the Devattur-
Kallimandayam shear zone (DKSZ), the Karur-Oddanchatram shear zone (KOSZ). All the
shear zones of the CSZ exhibit dextral strike-slip movement with a maximum lateral
displacement of ~ 80 km (Drury et al., 1984; Chetty et al., 2003). The maximum width of the
shear zones is around 20 km.
2.3.2. Moyar-Attur Shear Zone (MASZ)
It is an E-W trending wide shear zone extending for about 200 km from east of Attur
through Bhavani, Salem, Bhavanisagar, Moyar, and south of Gundlupet in the west. Ghosh et
-
23
al. (2004) described the kinematic history in and around the shear zone (MASZ). The
structural fabric within the MASZ represents a flattening type of deformation. The MASZ
links with the Bhavani Shear Zone near Bhavanisagar and with the Cauvery Bhavani Shear
Zone, just west of Bhavani. A number of charnockite massifs occur adjacent to the MASZ.
Coorg Massif, the Biligirirangan Hill Massif and the Shevaroy Hills Massif occur from east
to west to the north of the MASZ, while Nilgiri Hill Massif and the Kollimalai Hill Massif
occur to the south of MASZ. The structural studies by Ghosh et al., (2004) from the Salem
area, the Bhavani area, and the Bhavanisagar area indicate that the regional strike of the
lithologic and tectonic fabrics (S0 and S1, respectively) in north and south of the MASZ is N-
S to NNE-SSW. Inter layered BIF, mafic granulites and TTG gneisses occur along the shear
zone, which are highly deformed and mylonitized. Minor folds and lineations, two periods of
folding were recognized in the shear zone (Ghosh et al., 2004). The lineations are subparallel
to the axes of sub-vertical to moderately east plunging F2 folds, suggesting sub vertical
extension along the shear planes.
2.3.3. Bhavani Shear Zone (BSZ)
The Bhavani Shear Zone is a NE-SW trending shear zone marking the southern
boundary of the Nilgiri massif that occurs at the western part of the CSZ. Similar to the
MASZ, the BSZ is also characterized by a zone of intense mylonitic fabrics, in turn
overprinted by late brittle to brittle ductile structures with sheared gneisses of steeply dipping
with sparse sub-vertical stretching lineations (Ghosh et al., 2004).
2.3.4. Cauvery-Bhavani Shear Zone
It is about 100 km long and 10 km wide shear zone that follows the NW-SE course of
the Cauvery river from south of Namakkal to just west of Bhavani town, where it joins the
MASZ. This shear zone also marks a prominent metamorphic boundary between
predominantly granulite facies rocks to the north east and the retrogressed equivalents in the
-
24
amphibolite facies to the south west. Around Namakkal area, the general strike of the
lithological units and their gneissic fabric trends from NE-SW to NW-SE along the shear
zone. This change in strike is in part due to a major late E-W trending fold (F2), (Ghosh et
al., 2004) described as the Namakkal Fold well developed in the Nainarmalai Hill and the
Saruva Malai Hill, respectively. The limbs of the F2 Namakkal fold preserve early kilometer-
scale Z-shaped folds (F1) within an early dextral shear zone (S1). Two pyroxene granulites,
granitic gneiss, amphibolites, BIF and metacherts are the major lithologies which are strongly
deformed and trending NE-SW in the north to the E-W and NW-SE in the south is related to
four deformational episodes.
2.3.5. Palghat-Cauvery Shear Zone (PCSZ)
The PCSZ is an E-W trending shear zone along the Palghat-Cauvery Lineament
(following the Noyil and Cauvery rivers) and also the southern boundary of the CSZ
extending from the east coast to the west coast of India (Drury and Holt, 1980;
Ramakrishnan, 1993; GSI and ISRO, 1994). Regional trends of the structural fabric,
however, suggests that the PCSZ is a crescent shaped shear zone extending from Kazhikode
at the west coast to the east coast through Mallapuram and Palghat and southwest of
Namakkal, where it merges with the Cauvery-Bhavani Shear Zone. Thus the PCSZ, together
with the Bhavani Shear Zone and the Cauvery-Bhavani Shear Zone delineating the western
part of the CSZ with an overall dextral sense of movement. Horizontal sheets of granitic and
mafic granulite gneisses dominate in this part of the CSZ. In the Palghat Gap area, at least
four phases of granitic intrusions, each related to discrete shearing events, have been
identified (Ghosh et al., 2004).
2.3.6. Cauvery Suture Zone: a ‘flower structure’
A 100km wide corridor in the central part of the CSZ has been studied as a part of
geotransect recently. The structural studies reveal many E-W trending sub parallel shear
-
25
zones. From north to south, they are: (i) Moyar- Bhavani Shear Zone (MBSZ), marking the
northern boundary; (ii) Chennimalai-Noyil Shear Zone (CNSZ), (iii) Dharapuram Shear Zone
(DSZ), (iv) Devattur-Kallimandayam Shear Zone (DKSZ), and (v) Karur-Oddanchatram
Shear Zone (KOSZ) and all of them are dextrally displaced (Chetty and Bhaskar Rao, 2003).
These shear zones delineate distinct domains of contrasting geological characteristics. Chetty
and Bhaskar Rao, (2006) described that large-scale north-verging thrusts and related
structures accompanied by north–south shortening are dominant during D1, while D2 is
characterized by extensive dextral shearing, migmatisation and the emplacement of granitoids
and alkaline intrusive. Both gneissic foliation(S1) and mylonitic foliation(S2) show dip
values that vary consistently from north to south in the region; southward dipping in the
MBSZ and northward dipping in all the other shear zones south of it. The disposition,
regional geometry, dramatic variations in foliation fabrics, persistent dextral kinematic
indicators on all scales of observation, the apparent contemporaneous development of
mylonitic fabrics based on Rb–Sr mica ages (Meissner et al., 2002; Bhaskar Rao et al., 2003)
and the presence of possible convex upward reversed or thrust faults suggest that the CSZ
could reflect a crustal-scale flower structure displayed (Fig. 2.4.) (Chetty and Bhaskar Rao,
2006).
Fig. 2.4: A 3D cartoon shows crustal scale flower structure of CSZ (after Chetty and Bhaskar Rao, 2006).
-
26
2.3.7. Madurai granulite block
The Madurai Granulite Block occurs immediately south of the CSZ and is the largest
crustal block in southern India. This block comprises dominantly of charnockite massifs
intercalated with tonalitic/granodioritic gneisses and elongated narrow belts and slivers of
metasedimentary rocks including quartzites, metamorphosed carbonates, iron formations and
pelites, all suggesting an accretionary realm (Santosh et al., 2009a). A N-S trending Karur-
Kambam-Painavu-Trissur (KKPT) Shear Zone runs with in the MGB (Ghosh et al., 1998,
2004). The MGB can be lithologically divided into a western region and an eastern region;
Madurai Block in Kerala (MBK) and Madurai Block in Tamil Nadu (MBTN) (Cenki and
Kriegsman, 2005). The MBK is characterized by two different groups of hornblende-biotite
and orthopyroxene-biotite (charnockite) gneisses, one being quartz rich and the other feldspar
rich. While the eastern part, MBTN is composed of massive charnockites and enderbites with
heterogeneously distributed quartzites and calc silicate series of rocks (Cenki and Kriegsman,
2005).
2.3.8. Achankovil shear zone (AKSZ)
The AKSZ separates the Madurai Block from the Trivandrum Block (see Fig.1). The
AKSZ shows NW-SE trending foliation fabrics with steep dips to southwest. The adjacent
Madurai Block and Trivandrum Block show contrasting lithological and structural
characteristics. The southern boundary of the Madurai Block is marked by the Achankovil
Shear Zone (AKSZ), which also separates the Madurai Block from the Trivandrum Block to
the south. The Trivandrum Block is subdivided on lithological grounds into three tectonic
units; the Kerala Khondalite Belt (KKB), the Nagercoil unit and the Achankovil
metasediments. Trivandrum Block comprises dominantly of metasedimentary gneisses
including garnet-bearing felsic gneisses (known locally as leptynites) and granulite facies
garnet+spinel+cordierite+sillimanite metapelites (termed khondalites). These lithologies
-
27
constitute a vast sequence of continental margin sediments originally defined as the Kerala
Khondalite Belt (KKB) by Chacko et al. (1987).
The AKSZ is 10-20 km wide and 120 km long and extends along the southern edge of
the charnockite massifs of Cardamom Hills (Drury and Holt, 1980). The AKSZ holds a key
position in juxtaposing the member terranes in the East Gondwana supercontinent
reconstructions. The AKSZ has been correlated with the sinistral Ranotsara shear zone of
Madagascar (Ramakrishnan, 1991; Kriegsman, 1995). However, Sacks et al. (1997) reported
field evidences in support of dextral movements along AKSZ discarding its correlation with
sinistral Ranotsara shear zone. Rajesh et al., (1998) reported structures indicating a sinistral
sense of movement along AKSZ and reiterated its correlation with Ranotsara, which led to
the suggestion of repeated movements along AKSZ (Sacks et al., 1998). The important
lithologies in AKSZ include leptynitic garnet-biotite gneiss, khondalite, cordierite-gneiss, and
minor mafic granulite, calc-silicates, quartzites, and massive granites. Guru Rajesh and
Chetty, (2006) brought out a number of kinematic indicators from the shear zone such as
asymmetric boudins, porphyroblasts, flanking folds, “S” shaped folds, S-C′ fabrics, shear
bands, sub-horizontal stretching lineations, and porphyroclasts etc. They also described two
events of deformation with in the AKSZ; D1- initial dextral deformation and D2- reactivated
and superimposed by sinistral kinematics. The important feature of this shear zone is the
variation in structural trends across the lineament is from N-S on the north east to NW-SE on
the southwest. Santosh et al., (1992) and Bartlett et al., (1995) described that it is a major
Pan-African shear zone in South India as evidenced by 539 Ma (Sm/Nd isochron ages) and
533 Ma (U-Pb ages for zircons) of cordierite-bearing Charnockites.
2.4. Ophiolites
Ophiolites, the remnants of oceanic lithosphere, provide important information on the
evolution of ancient arcs, petrogenetic processes, opening and destruction of ocean basins,
-
28
and the nature of subduction–accretion–collision tectonics in major orogenic belts (Nicolas,
1989; Şengör, 1990; Searle and Cox, 1999; Dilek and Newcomb, 2003; Beccaluva et al.,
2004). The occurrence of ophiolitic rocks has been reported from various terranes of different
ages on the globe. Although most of the well documented examples come from Phanerozoic
belts (Ishikawa et al., 2002; Dilek and Robinson, 2003; Dilek and Newcomb, 2003; Vaughan
and Scarrow, 2003; Hara et al., 2009; Braid et al., 2010; Isozaki et al., 2010; Pearce and
Robinson, 2010; Zhang et al., 2010), ophiolites of Archean age have also been recorded such
as those from the 2.5 Ga ophiolites in the Dongwanzi, Zunhua and Wutaishan areas in the
North China Craton (Kusky et al., 2001; Polat et al., 2005, 2006; Kusky,2010),and the~3.8
Ga ophiolites from Isua supra crustal belt in southwest Greenland (Furnes et al., 2007, 2009).
Whereas the ophiolites in younger orogenic belts preserve a complete sequence/stratigraphy,
those from Archean and Proterozoic terranes are highly dismembered, preserving only partial
sequences (Kroner, 1985; Barhe, 1990; Dann, 1991; Kusky, 2004). Some of the world's best
examples of Proterozoic ophiolites have been reported from the Arabian–Nubian shield,
Kareliean shield, Capesnot Belt of West Africa and the southwest USA (St Onge et al., 1989;
Abhouchami et al., 1990; Scott et al., 1991, 1992; Boher et al., 1992; Dann, 2004). Some of
the well studied examples for ophiolitic assemblages from Proterozoic terranes include the
Mesoproterozoic Kandra (Vijayakumar et al., 2010), Kanigiri ophiolites (Dharma Rao and
Reddy, 2009; Dharma Rao et al., 2011), and the Neoproterozoic Manamedu ophiolite
complex (Santosh et al., 2009; Yellappa et al., 2010) reported from Peninsular India.
Recently, three important Precambrian ophiolite complexes are reported from the
CSZ that include: Manamedu, Devanur and Agali complexes. Their lithologies, structural
styles, geochemical signatures and geochronological data are described below.
-
29
2.4.1. Manamedu ophiolitic Complex (MOC)
The Manamedu ophiolite complex (MOC) within the southeastern part of the Cauvery
Suture Zone in Southern India (Yellappa et al., 2010) comprises metamorphosed equivalents
of the following lithological units: (1) an ultramafic group comprising dominantly of
pyroxenite, and dunite, locally preserving cumulate textures; (2) gabbroic rock types
consisting of gabbro, gabbro norite, and anorthosite; (3) sheeted mafic dykes of amphibolite
to andesite categories and (4) plagiogranites and a thin pile of ferruginous cherts. The
succession displays dismembered ophiolite succession comprising actinolite-hornblendite,
hornblendite, pyroxenite, gabbro-norite, gabbro, anorthosite, amphibolite, plagiogranite,
mafic dykes, and associated pelagic sediments such as chert-magnetite bands and carbonate
horizons. The structural studies reveal the anatomy of imbricate thrust sheets and slices of a
dismembered ophiolite suite and pelagic sediments (Chetty et al., 2011). The foliation
trajectory map of MOC reveals inward dipping foliations both in the east and west and shows
isoclinal fold structures in the north and south. A major detachment zone occurs at the
western margin with the development of high amplitude tight folds and intrusive
plagiogranites. Based on the geometry of fold styles, foliation trajectories and large variations
in N-S trending hinges, the MOC can be interpreted as a deformed large scale sheath fold,
associated with south verging back thrust system in a crustal-scale ‘flower structure’
described by Chetty and Bhaskar Rao (2006). Petrological and geochemical characteristics
suggest that these rocks represent the remnants of oceanic crust, developed at shallow levels
from mantle-derived arc magmas probably within a suprasubduction zone tectonic setting
(Yellappa et al., 2010). Geological setting and field observations suggest that these rocks
were obducted on to the continental margin with the closure of ocean basin during
Neoproterozoic period. The dominant population of zircons in the two plagiogranite samples
of MOC yielded weighted mean 206Pb/238U ages of 737 ± 23 Ma and 782 ± 24 Ma. Zircons
-
30
in the gabbroic anorthosite and gabbro samples show well-defined single clusters on the
concordia with weighted mean 206Pb/238U ages of 744 ± 11 Ma and 786 ± 7.1 Ma (Santosh
et al., 2012). The Manamedu ophiolite units may represent the remnants of the Mozambique
Ocean crust developed during Rodinia breakup which was destroyed during Cambrian period
at the time of Gondwana amalgamation (Santosh et al., 2009).
2.4.2. Devanur ophiolitic Complex (DOC)
The other ophiolitic Complex located at Devanur, about 20km north of MOC,
comprises dismembered outcrops along an east-west trending shear zone and represents
typical oceanic sequences/ocean plate stratigraphy with mafic-ultramafic components,
overlying felsic groups and the hornblende gneisses (Yellappa et al., 2011). The DOC occurs
in the form of a lensoid body and comprises rock types such as pyroxenites, gabbros,
anorthosites, actinolite-hornblendites, amphibolite dykes, dolerites, pyroxene granulites,
trondhjemites/quartz keratophyres and thin layers of ferruginous cherts with varied
dimensions. The DOC extends for over a strike length of ~ 15km and with a maximum width
of less than a km. The field and petrographic studies indicate that these lithologies are highly
altered, sheared and metamorphosed and obducted along shear/thrust planes with in the CSZ.
The geochemistry of mafic dykes shows basaltic-andesitic type magmas with tholeiitic to
calc-alkaline characteristics suggesting that these rocks were generated along a
suprasubduction zone tectonic setting with island arc affinities. They are inferred to have
been incorporated within a Neoarchean accretionary belt associated with continental
collision. Two trondhjemite samples of Devanur complex have yielded SHRIMP zircon
238U-206Pb ages of 2528±61 and 2545±56Ma (Yellappa et al., 2011). Similar ages have
been obtained from magmatic zircons in charnockites, anorthosites and orthogneisses in the
adjacent regions (e.g, Ram Mohan et al., 2012). The suprasubduction zone rock assemblages
-
31
and arc magmas suggest Neoarchean ocean closure along the southern margin of the Dharwar
craton.
2.4.3. Agali Ohiolite Complex (AOC)
Recently, the discovery of a relatively well-preserved suprasubduction zone ophiolite
suite has been reported from the Agali hill in Attappadi, along the western extension of the
Bhavani Shear Zone (Santosh et al., 2013). The rock sequence from bottom upwards
includes altered ultramafics with vestiges of dunite, thin layer of cumulate pyroxenite, a thick
unit of metagabbro with the upper part grading into anorthositic gabbro and carrying thin
layers of hornblendite, capped by metavolcanics (amphibolites) carrying veins and pools of
trondhjemite. Several fragments of metabasite (dolerite) dykes occur within the gabbroic
body. Metamorphosed banded iron formation also occurs in association with amphibolites.
The lithological distribution in the area has been interpreted to represent a typical ‘Ocean
Plate Stratigraphy’ sequence with arc and exhumed sub-arc mantle material (Santosh et al.,
2013). The common occurrence of magnesite in association with ultramafic units in the area
suggests CO2-induced metasomatism of peridotites in the mantle wedge through fluids
released within the subduction zone. Major, trace and REE data on the Agali Ophiolite
Complex suggest magma derivation in a suprasubduction setting. The U–Pb concordia ages
of zircons in the metagabbro and trondhjemite yielded ~2.5Ga, which are correlatable with
similar age data reported recently from DOC. A tectonic model was proposed envisaging
accretion of oceanic arcs and micro-continents onto the margin of the Dharwar Craton during
Neoarchean, marking an important event of continental growth, and broadly coinciding with
the global crustal growth event at this time.
In the light of the significance of the CSZ in terms of suture zone tectonics and its
correlation with other comparable shear zones in the adjacent fragments of the Gondwana
continent, a small corridor between Namakkal and Mohanur in the south central part of the
-
32
CSZ has been chosen for the present study. Further, it was also a great opportunity to
investigate the recently dug Rail cutting section near Anniyapuram exposing excellent
geological sections, which otherwise would not have been possible. Detailed field
observations, structural studies, petrographic and geochemical characteristics have been
attempted in the present study. The study area would be described as Namakkal-Mohanur
Corridor (NMC) in the present thesis. The significance of these results in terms of
subduction-accretion and collision in an overall plate tectonic regime has been highlighted.
top related