Bhavani Shear Zone Research Articles

Spectral pathways for effective delineation of mafic-ultramafic rocks by using PRISMA hyperspectral data

The Spectral Pathway Analysis (SPA) by using hyperspectral remote sensing data has a significant role in the demarcation of litho boundaries. The SPA is a 5 step continuous process 1) The reflectance spectral characterization through a controlled environment to the direct field sample along with satellite data acquisition 2) The generation of ICA and MNF 3) The endmember spectral extraction through the sub-pixel-based classification 4) The extraction of mineral characters from spectral absorption behavior 5) Mapping of litho units based on the integrative approaches by the data from the field, geology, and geochemical characters. In the present study, mafic and ultramafic complexes of the Bhavani shear zone which have a greater significance in the economically viable mineral aspects are taken into the consideration of lithological delineation by using PRISMA hyperspectral data. As part of SPA, ICA and MNF were generated using the PRISMA data and validated with geology, which shows a tremendous result in demarcating the lithological boundaries. Subpixel classification (PPI, SAM, LSU, MF, MTMF, TCIMF, and MITTCIMF) was performed and evaluated using ancillary data from field observation, XRF, and laboratory-based ASD spectra.

Combined analysis of remote sensing, gravity and magnetic data across Moyar Bhavani Shear Zone, Southern Granulite Terrane (SGT), India: appraisals for crustal architecture and tectonics

We present a combined analysis of remote sensing, gravity and magnetic data across Moyar Bhavani Shear Zone (MBSZ), Southern Granulite Terrane (SGT), India to study crustal architecture and tectonics. The horizontal gradient of gravity data shows strong density inhomogeneity across the MBSZ. The magnetic data and residual gravity analysis shows corroboration of a NE–SW trending linear magnetic anomaly zone and residual gravity high strip along the MBSZ. The low magnetic anomalies dominate the northern part of the MBSZ, while the southern part is dominated by moderate to high anomalies. The integration of potential data and remote sensing data clearly reveals that ENE–WSW is the dominant structural trend that coincides with the Mettur Shear Zone (MeSZ) trend. The joint gravity and magnetic modelling suggest that the MBSZ, is characterized by the disposition of mélange of moderate to high-density rocks of diamagnetic nature.

Contrasting kinematics of brittle-shears within the Salem–Attur and Bhavani shear zone, south India: Tectonic implications

We document kinematics and rheological behaviour of brittle shears (~50 cm wide) postdating solid-state tectonic fabric in the Salem–Attur (SASZ) and Bhavani (BSZ) shear zone that constitute a Paleoproterozoic (~2500 Ma) suture juxtaposing disparate granulite blocks in south India. We constrain brittle deformation mechanisms from established relationship between changing orientation of deflected strain marker (quartz vein) and foliation within the shear band with respect to their orientation outside the shear band. Quartz c-axis orientation in charnockite (host lithology) and phyllonite (reworked charnockite) from the SASZ show presence of mixed basal 〈a〉 (low-T) and prism 〈a〉 (high-T) slip, and single basal 〈a〉 slip mechanism, respectively. This suggests considerable cooling of the granulite block prior to the onset of brittle shearing. Distribution of strain parameters – effective shear strain (Γ), shear strain (γ), stretch K2 along intermediate strain axis Y – from margin to the centre of the shear band, show peaked distribution with a single maximum at the shear zone centre. This implies rheological-weakening/strain-softening induced localizing shear zone character. Kinematically heterogeneous strain distribution during brittle shearing varies from transpression dominated for the BSZ to transpression-to-transtension switchover for the SASZ. Demonstrably, contrasting cooling-exhumation, hitherto unexplored, characterizes post-accretionary tectonics along the paleo-suture zone.

Tectonothermal evolution of a garnet-bearing quartzofeldspathic gneiss from the Moyar shear zone, south India and its bearing on the Neoarchean accretionary tectonics

We present mesoscale structural development across the Nilgiri Block and the flanking Moyar and Bhavani shear zones in south India, and detailed mineral–chemical and geothermobarometric studies of a garnet-bearing quartzofeldspathic gneiss from the easternmost part of the Moyar shear zone. Barring a narrow (<100μm) rim domain, major element distribution within garnet porphyroblasts reveals complete chemical homogenization. The absence of growth zoning in garnet porphyroblasts may suggest a protracted post-garnet growth residence period of the rock at elevated temperatures. Chemical zoning near garnet rim reflects the signature of both retrograde net-transfer (ReNTR) and retrograde exchange (ReER) equilibria. The ReNTR-equilibrium is recognized by prominent Mn kick-up in garnet, whereas the ReER-equilibrium is identified by divergence of Fe and Mg between garnet and biotite. Diffusion modelling, though qualitative, of the observed chemical zoning in garnet suggests an initial phase of rapid (~150°C/Ma) cooling, which may have been achieved by tectonic-extrusion-induced exhumation. Pressure–temperature conditions for peak, ReNTR and ReER are constrained, respectively, at 900°C; 9–11kbar, 735°C; 8kbar and 685°C; 7.8kbar. Analyses of structural fabrics establish oppositely verging nature of the Moyar and Bhavani shear zone and may suggest a doubly vergent orogenic development, with the former as prowedge and the latter as retrowedge. The presence of the Nilgiri Block as a topographically elevated region between these oppositely dipping thrust faults indeed corroborates a doubly vergent orogenic setup. The tectonic scenario is comparable with a continent–continent collision type accretionary tectonics. Peak high-P granulite facies metamorphism and post-peak long residence period of the studied quartzofeldspathic gneiss at deep crustal level suitably fit into the Neoarchean crustal dynamics resulting in crustal thickening, in the order of ~41km, within the Nilgiri Block.

Reactivation and Strain Localisation in Bhavani Shear Zone, South India

Abstract: Shear markers and shear sense indicators from the Bhavani shear zone, a member of the shear system separating the northern Archaean and southern Pan African granulite terranes of the South Indian shield, with differently exhumed crustal blocks, suggest multiphase reactivation. This is revealed by textural and geochemical characteristics of mylonites which indicate an event of prograde epidote-amphibolite facies metamorphism of the previously retrograded protolith of shear zone rocks. The amphibole varies progressively from an initial magnesio-hornblende to ferrotschermakitic hornblende with increasing Al and K while the plagioclase maintains oligoclase composition (An21-25). P-T estimates suggest metamorphic conditions of 634°C - 720°C at 3.6±0.5 to 5.8±0.5 kbar. Strain partitioning during the reactivation of the shear zone prior to the Pan African event, is responsible for the gradation in the intensity of mylonitic fabric as well as some of the opposing shear sense indicators.

Genetic aspects of gold mineralization in the Southern Granulite Terrain, India

Gold mineralization in Southern Granulite Terrain (SGT) of India has close spatial relationship with the shear zones (Moyar–Bhavani) present in Cauvery Suture Zone. Gold is found to be associated with primary quartz veins, placers and laterites. The gold prospects in SGT can be broadly grouped into three provinces i) Wynad-Nilgiri, ii) Malappuram and iii) Attappadi. The auriferous quartz veins are within the deformed biotite/hornblende bearing gneisses and amphibolite. Wall rock alteration is conspicuous around the mineralized veins and gives an assemblage of muscovite–calcite–ankerite–chlorite–biotite–pyrite related to fluid–rock interaction at the time of vein formation. Fluid inclusion studies of vein quartz gives an idea of the nature of the ore forming fluids, the fluid involved in gold mineralization is of low saline and aqueous-carbonic in composition and quite similar to the orogenic lode gold deposits reported world-wide. Micro-thermometric data indicates fluid immiscibility (phase separation) during trapping of fluid inclusions and this must have played an important role in gold deposition. Geochronological studies of mineral separates from Wynad-Nilgiri province using Rb–Sr and Sm–Nd isochron methods of the auriferous quartz veins gave an age of approximately 450Ma for the vein formation. The present studies on SGT gold mineralization indicate 1. During the Pan-African orogeny, extensive fluid influx from mantle and metamorphism extracted gold from a mafic source and were focused along major structural discontinuities of Moyar–Bhavani Shear Zone, 2. The aqueous–carbonic ore fluid interacted with rocks of the upper crust and triggered a set of metasomatic changes responsible for the dissolved components such as Ca, Si and Fe and finally precipitating in the veins and 3. The mineralizing fluid with dissolved gold in sulphide complex got destabilized due to fluid immiscibility and wall rock alteration leading to the deposition of gold with associated sulphide minerals in the vein system.

A Gravity and Bathymetric Study in the South East Continental Margin of India

The Eastern Continental Margin of India (ECMI) has formed as an upshot of separation of India from Antarctica during the early cretaceous period and subsequent seafloor spreading led to the evolution of Bay of Bengal. The study area, which lies between latitude 8° to 14°N and longitude 77.5° to 81°E, has been selected to delineate the width of the continental shelf region of the south east coast of India. GEBCO bathymetry data and Satellite gravity data has been used for the present study. The bathymetry contour map generated by GEBCO bathymetry data shows gradual increase in the depth from the coastal region (~100 m) to the central basin (~3700 m) and nearly follows a N-S trend from Karikal to Chennai. 24 profiles were extracted from the bathymetric as well as satellite gravity grid. The profiles were created perpendicular to the coastal margin. The maximum width of shelf (~45 km) is observed along the coast near Mamallapuram which is the river mouth of Palar River. Shelf width is gradually decreasing from Chennai to Karikal and the continental slope is also very steep. On the southern part of it, the continental shelf of India merges with that of Srilanka. Further south, in the Mannar basin, the shelf width ranges from 25 to 33 km. The continental shelf region is marked by a relatively high (-40 to 40 mGal) compared to the low gravity anomaly (-40 to -180 mGal) observed towards the basinal area. A local gravity high compared to the surroundings centered at 80.6A‹ÂsE and 11.8A‹ÂsN can be associated with the offshore extension of Moyar- Bhavani shear zone, which dissects south Indian terrain.

Metamorphism and deformation of mafic and felsic rocks in Bhavani Shear Zone, Tamilnadu, India

Metamorphism and deformation of mafic and felsic rocks in Bhavani Shear Zone, Tamilnadu, India

Zircon U–Pb geochronology and Hf isotope of felsic volcanics from Attappadi, southern India: Implications for Neoarchean convergent margin tectonics

The Attappadi area on the south-western flanks of the Archean Dharwar Craton in southern India is located along the E–W trending Bhavani Shear Zone which marks the trace of a Neoarchean suture zone. The dominant rock types in the area include meta-ultramafics, amphibolites, TTG (tonalite–trondhjemite–granodiorite) gneisses, metapelites, and sulphidic banded iron formation (BIF). Here we report the occurrence of felsic volcanic rocks preserving primary textures from the Anaikatti area in eastern Attappadi. The felsic volcanics are interbanded with the BIF, amphibolite, metapelite, metapyroxenite and hornblende gneisses. The felsic volcanics are divided into two types based on their textures. The Type-1 rock is medium grained with gneissic texture and lack unequivocal primary volcanic textures. Type-2 felsic volcanics are thinly laminated, fine grained and at places preserve relict soft-sediment deformation structures. They also contain relict volcanic clasts or lapilli and are interpreted as felsic tuff. Geochemically, the Attappadi felsic volcanics are rhyolitic in composition and have arc-related trace element signatures. They also possess a calc-alkaline volcanic affinity.We report LA-ICPMS U–Pb ages from zircons in four samples of the felsic tuffs which show weighted mean ages of 2567±18Ma and (MSWD=1.4), 2499±19Ma (MSWD=0.57), 2555±24Ma (MSWD=1.7) and 2576±64Ma (MSWD=5.8). The late Neoarchean–early Paleoproterozoic ages obtained in our study correlate well with the zircon U–Pb ages reported in recent studies from ophiolites and other suprasubduction suites from Attappadi and surrounding regions. The zircon εHf values range from −11.1 to 7.6 suggesting heterogeneous source material involving both juvenile and older reworked components. We build a tectonic model for the SW margin of the Dharwar Craton with an oceanic realm characterized by island arcs and widespread submarine tholeiitic as well as komatiitic ultramafic and mafic volcanism during the Neoarchean. This predominantly mafic volcanism on the ocean floor is represented by primitive komatiitic lavas, oxide and sulphide facies BIF. The birth of volcanic arc at the convergent margin is marked by felsic volcanism and the deposition of felsic volcanics and volcano-sedimentary successions. In the final stage of ocean closure, the ocean-plate and continental arc assemblages were brought in juxtaposition including the accretion of the ophiolitic fragments. Our study confirms the recent models of arc–arc and arc–continental accretion to the southern margin of the Dharwar Craton and major continental growth at the end of the Archean.

Suprasubduction zone ophiolite from Agali hill: Petrology, zircon SHRIMP U–Pb geochronology, geochemistry and implications for Neoarchean plate tectonics in southern India

The southern margin of the Dharwar Craton in Peninsular India preserves the records of an active convergent margin during the Neoarchean. Here we report the discovery of a relatively well-preserved suprasubduction zone ophiolite suite from the Agali hill in Attappadi, along the western extension of the Bhavani Shear Zone. In the Agali hill, from base to top, the rock sequence 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. Fragments of metabasite (dolerite) dykes occur within the gabbroic horizon. Elongate bands of metamorphosed banded iron formation in association with amphibolite occur proximally. The lithological distribution in the area represents a typical ‘Ocean Plate Stratigraphy’ sequence with arc and exhumed sub-arc mantle material toward the north-west, followed by accreted remnants of suprasubduction zone ophiolites, large tracts of TTG gneisses, and amphibolites in association with BIF bands. The central domain is occupied by a granite batholith. Toward the south-east, the dominant lithology grades to a continental shelf sequence represented by metamorphosed psammitic and pelitic rocks (trench) and metacarbonates. 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.We present major, trace and REE data on the Agali Ophiolite Complex which clearly suggest magma derivation in a suprasubduction setting in the absence of any significant crustal contamination. The internal structure as imaged from CL images of zircons separated from metagabbro, trondhjemite and metagranite show features typical of magmatic crystallization, with the grains mantled by bright structureless thin rims developed during a dominantly dry metamorphic event. The U–Pb concordia ages of 2547±17Ma (MSWD 0.84) and 2547±7.4Ma (MSWD 2.4) obtained from zircons in the metagabbro and trondhjemite are indistinguishable; the zircons in the metagranite also show comparable magmatic age of 2532±8.6Ma (MSWD 2.5) with metamorphic overgrowth at ca. 2470Ma. These ages correlate well with similar age data reported recently from suprasubduction zone and arc-related rocks elsewhere along the southern margin of the Dharwar Craton. We propose a tectonic model that envisages 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.

Crustal accretion and metamorphism of Mesoarchean granulites in Palghat-Cauvery Shear Zone, Southern India

This work provides unequivocal evidence of the existence of Mesoarchean granulite facies metamorphic event in the Palghat-Cauvery Shear Zone (PCSZ) of South India. Charnockite samples from two prominent hills at Kollaimalai (KM) and Pachchaimalai (PM) as well as from two quarries within the Bhavani Shear Zone (BSZ) have been analyzed for their Sm-Nd and Rb-Sr ages to investigate the existence or otherwise of the Archean granulite facies events within the PCSZ. The Rb-Sr whole-rock isochron ages for massive charnockites from both the hills appear to be contemporaneous at 2.9 Ga with the initial Sr isotopic ratios of 0.7012 and 0.7014, respectively. However, the Rb-Sr data for whole-rock samples of basic granulites from one of the quarries within the BSZ indicate open system behavior, while the charnockites from the other quarry have insufficient spread in 87Rb/86Sr ratios and do not yield any isochron. The Sm-Nd data, on the other hand, do not distinguish between the massive charnockite and the lowland charnockite and yield Depleted Mantle model ages in the range 2.98±0.3 Ga for all of them. The ɛTCHUR for all of these rocks are highly positive. Both the Sr isotopic ratios and positive ɛTCHUR values for these rocks strongly suggest a mantle source for all of them. An upper age limit of ∼3.28 Ga may be assigned to the crustal accretion of the protolith of all these rocks on the basis of their Nd model ages. The Rb-Sr isochron ages of 2.9 Ga for the two massifs could be the age of granulite facies metamorphism. Thus, the metamorphism in the KM and PM Hills took place within ∼100 Ma of crustal accretion of these rocks and probably was part of the same geological event of crust formation and metamorphism. The open system behavior with respect to Rb-Sr isotopes in the basic granulite from Bhavani is possibly due to the migration of Sr isotopes, triggered during the later shearing of these rocks.

A Unified Model of Neoarchean-Proterozoic Convergence and Rifting of Indian Cratons: Geophysical Constraints

Neoarchean and Proterozoic sutures and collision zones are identified in the Indian Peninsular Shield based on high seismic velocity; gravity highs and high conductivity in the upper crust due to thrusting while sub- ducted side are demarcated based on geophysical signatures of crustal thickening and back arc type basins. Some of them appear to form triple junctions. The Bouguer anomaly map of the south Indian shield when transformed to apparent density map through harmonic inversion, provided high density linear zones coin- ciding with the shear zone and the transition zone-the Moyar Bhavani Shear Zone (MBSZ) between the Eastern Dharwar Craton (EDC) and the Western Dharwar Craton (WDC) and the Dharwar cratons and the Southern Granulite Terrain (SGT), respectively. It is supported by high seismic velocity and high conductiv- ity suggesting them to be caused by high grade granulite rocks related to Neoarchean-Paleoproterozoic su- tures and collision zones. These investigations also suggest thick crust (~40 - 50 km) under the WDC and the SGT forming crustal root of 50 - 52 km in the south western part and thin crust of 31 - 32 km under the EDC indicating direction of convergence and subduction as E-W and N-S between the EDC and the WDC and Dharwar cratons and the SGT, respectively. It gave rise to contemporary lower crustal granulite rocks in the northern part of the SGT and Cauvery shear zone (CSZ) as collision related central core complex of various deep seated intrusive rocks of Paleo-Mesoproterozoic period. The second case belonging to Meso-proterozoic period is related to the collision of the Bundelkhand craton and the Bhandara-Bastar craton (BBC) and the Dharwar craton (DC) in Central India along the Satpura Mobile Belt (SMB) and the BBC and the DC along the Godavari Proterozoic Belt due to N-S and NE-SW convergences, respectively. This process has given rise to lower crustal granulite rocks of high density, high velocity and high conductivity along the SMB and the GPB. An upper mantle conductor delineated south of the western part of the SMB under Dec- can Volcanic Province and a regional gravity gradient almost sub parallel to it indicate an interface with flu- ids separating rocks of different densities that appears to demarcate the trace of the Proterozoic subduction and suture related to the SMB collision zone during Mesoproterozoic period. High reflectivity of the lower crust along seismic profiles across the SMB indicate an extensional phase prior to this convergence. The SMB is connected to the Aravalli Delhi Mobile Belt (ADMB) in the western part that is another collision zone of Meso-proterozoic period, forming an arcuate shaped collision zone between the Bundelkhand craton and Rajasthan block with E-W convergence. There are indications of a prior phase of convergence during Paleo-Proterozoic period followed by rifting during Paleo-Meso-proterozoic period (~1.9 - 1.6 Ga) along the SMB, the ADMB and the GPB that gave rise to large scale contemporary intrusive in these sections. The contemporary Mahakoshal-Bijawar and Pakhal group of rocks of Paleo Proterozoic period (~1.9 - 1.6 Ga) were deposited over the rifted platform of the Bundelkhand craton along the SMB and cratons along the GPB, respectively during the extensional phase as suggested above based on high reflectivity of the lower crust. It is followed by deposition of the Vindhyan sediments of Meso-Neoproterozoic period (~1.6 - 0.7 Ga) along the SMB and the ADMB as foreland basins during Meso-Neoproterozoic convergence. Simultaneous N-S and E-W directed convergences in the two cases, viz., the SMB and the ADMB that are connected forming an arcuate shaped collision zone suggest NE-SW directed primary stress direction similar to the GPB that is supported by NW-SE oriented large lineaments in Bundelkhand craton and Peninsular shield. The Eastern Ghat Mobile Belt (EGMB) also shows signatures of E-W or NE-SW directed Mesoproterozoic (~1.5 - 1.0 Ga) convergence with East Antarctica. This convergence was preceded by Paleo-Mesoproterozoic rifting (~1.9 - 1.6 Ga) that gave rise to contemporary activities of the EGMB and large scale volcanic activity that formed several basins west of it.

Crustal structure of the western part of the Southern Granulite Terrain of Indian Peninsular Shield derived from gravity data

The Southern Granulite Terrain (SGT) is composed of high-grade granulite domain occurring to the south of Dharwar Craton (DC). The structural units of SGT show a marked change in the structural trend from the dominant north–south in DC to east–west trend in SGT and primarily consist of different crustal blocks divided by major shear zones. The Bouguer anomaly map prepared based on nearly 3900 gravity observations shows that the anomalies are predominantly negative and vary between −125 mGal and +22 mGal. The trends of the anomalies follow structural grain of the terrain and exhibit considerable variations within the charnockite bodies. Two-dimensional wavelength filtering as well as Zero Free-air based (ZFb) analysis of the Geoid-Corrected Bouguer Anomaly map of the region is found to be very useful in preparing regional gravity anomaly map and inversion of this map gave rise to crustal thicknesses of 37–44 km in the SGT. Crustal density structure along four regional gravity profiles cutting across major shear zones, lineaments, plateaus and other important geological structures bring out the following structural information. The Bavali Shear Zone extending at least up to 10 km depth is manifested as a plane separating two contrasting upper crustal blocks on both sides and the gravity high north of it reveals the presence of a high density mass at the base of the crust below Coorg. The steepness of the Moyar and Bhavani shears on either side of Nilgiri plateau indicates uplift of the plateau due to block faulting with a high density mass at the crustal base. The Bhavani Shear Zone is manifested as a steep southerly dipping plane extending to deeper levels along which alkaline and granite rocks intruded into the top crustal layer. The gravity high over Palghat gap is due to the upwarping of Moho by 1–2 km with the presence of a high density mass at intermediate crustal levels. The gravity low in Periyar plateau is due to the granite emplacement, mid-crustal interface and the thicker crust. The feeble gravity signature across the Achankovil shear characterized by sharp velocity contrast indicates that the shear is not a superficial structure but a crustal scale zone of deformation reaching up to mid-crustal level.

Application of SEM-EBSD to regional scale shear zone analysis: A case study of the Bhavani Shear Zone, South India

The Bhavani Shear Zone (BSZ), part of the Palghat-Cauvery shear system of southern India, comprises a number of subparallel shear zones ranging in width from a few centimetres to tens of metres. These shear zones are characterised by progressive mylonitisation and shear fabrics corresponding to increasing strain, as documented by grain size reduction, flattening, elongation and dimensional preferred orientation of constituent minerals (quartz, hornblende, biotite, plagioclase and orthoclase). However, SEM-EBSD measured crystal preferred orientations (CPO) of these minerals are more complex and indicate interchange of maximum (X) and intermediate (Y) tectonic axes during different phases of deformation, with the minimum axis (Z) remaining approximately constant. The tectonic evolution of the BSZ and the disposition of the surrounding Archaean and Pan-African granulites are interpreted therefore in terms of polyphase deformation involving both dip-slip and strike-slip movement events in a multiply reactivated setting. SEM-EBSD CPO analysis provides clearer indication of potentially conflicting movement directions that are otherwise poorly constrained by shear markers and shear sense indicators.

Precambrian mafic magmatism in south Indian granulite terrain

South Indian granulite terrain had witnessed significant part of Precambrian mafic igneous activity in the form of episodic mafic dyke intrusions of the Palaeoproterozoic period. Strike trends of these dykes are not uniform over the region and the dykes are generally fresh, massive, black dolerites except in the Bhavani shear zone bordering the southern fringes of Nilgiri massif. In Agali-Coimbatore area of our study in the western Bhavani shear zone, the dykes appear to be penecontemporaneous with shearing. Isotopic data place age of Agali-Coimbatore dyke intrusions at about 2.1 Ga. The age of these dykes is significant to constrain an early Palaeoproterozoic age for major shearing event in the Bhavani shear zone. Other dyke emplacement ages are placed at about 1.8 Ga and 1.65 Ga based on the Ar/Ar and K-Ar isotopic results of dykes in Dharmapuri and Tiruvannamalai areas. Older ages comparable to those of the Dharwar craton are not known and in this respect future isotopic dating is vital. Geochemically, these dykes are quartz/hypersthene normative subalkalic tholeiites. An attempt is made here to provide insights into the general petrogenetic history of the Precambrian dykes. Compositional trends are explained by the fractional crystallization of ferromagnesian phases and plagioclase control is conspicuous at the advanced stages of fractionation. Geochemical characteristics suggest that the dykes have tapped Fe-rich non-pyrolite mantle sources with LIL and LREE enrichment as in many continental basalts. The data suggest that role of crustal contamination is limited in petrogenesis; crustal signatures are noticed in the more mafic end members formed in early stage of evolution suggesting that contamination was temperature controlled with most primitive high temperature magmas being most vulnerable to the process. Nd-Sr isotopic data, at present restricted to Agali-Coimbatore dykes, suggest that Palaeoproterozoic magmas tapped subcontinental lithosphere that may have stabilized in the Archaean times at about 3 Ga during the major crustal building activity in the shield region. Further work coupled with isotopic and mineral chemistry will improve our knowledge on the petrological evolution of the dyke magmas and mafic magmatism in general.

Detection of a collision zone in south Indian shield region from magnetotelluric studies

Magnetotelluric (MT) investigations were carried out along a profile in the greenschist–granulite transition zone within the south Indian shield region (SISR). The profile runs over a length of 110 km from Kuppam in the north to Bommidi in the south. It covers the transition zone with 12 MT stations using a wide-band (1 kHz–1 ks) data acquisition system. The Mettur shear zone (MTSZ) forms the NE extension of Moyar–Bhavani shear zone that traverses along the transition zone. The regional geoelectric strike direction of N40°E identified from the present study is consistent with the strike direction of the MTSZ in the center of the profile. The 2-D conductivity model derived from the data display distinct high electrical resistivity character (10,000 Ω m) below the Archaean Dharwar craton and less resistive (< 3000 Ω m) under the southern granulite terrain located south of the MTSZ. The MTSZ separating the two regions is characterized by steep anomalous high conductive feature at lower crustal depths. The deep seismic sounding (DSS) study carried out along the profile shows dipping signatures on either side of the shear zone. The variation of deep electrical resistivity together with the dipping signature of reflectors indicate two distinct terrains, namely, the Archaean Dharwar Craton in the north and the Proterozoic granulite terrain towards south. They got accreted along the MTSZ, which could represent a possible collision boundary.

Structure and evolution of the Cauvery Shear Zone system, Southern Granulite Terrain, India: Evidence from deep seismic and other geophysical studies

The Southern Granulite Terrain with exposed Archean lower crustal rocks is studied using various geophysical tools. The crustal structure derived from seismic reflection and refraction/wide-angle reflection studies is used to understand the tectonic evolution of the region. Deep seismic reflection section along the Kolattur–Palani segment shows an oppositely dipping reflection fabric near the Moyar–Bhavani shear zone, which is interpreted as a signature of collision between the Dharwar craton and another crustal block in the south. The thickened crust due to collision was delaminated during the orogenic collapse and modified the central part, covering the Cauvery Shear Zone system, located between the Moyar–Bhavani and Karur–Oddanchatram shear zones. The delaminated lower crust is altered by magmatic underplating as evidenced by the high velocity layer just above the Moho. The velocity model of the region indicates crustal thickening at the boundary of the Dharwar craton and Moyar–Bhavani shear zone and thinning further south. Back-scattered seismic wave field with negative moveout and the Moho-offset indicate the spatial location and strike-slip nature of the shear zones. Present study suggests that the late Archean collision and suturing of the Dharwar craton with the southern crustal block at the Moyar–Bhavani shear zone may be responsible for the evolution of late Archean granulites. Late Neoproterozoic rifting is observed along the paleo-fault zones. The seismic studies constrained by gravity, magnetic and magnetotelluric data suggest that the Moyar–Bhavani and Karur–Oddanchatram shear zones of the Cauvery Shear Zone system mark terrane boundaries/suture zones.

A palaeomagnetic study of charnockites from Madras Block, Southern Granulite Terrain, India

The South Indian Craton is composed of low-grade and high-grade metamorphic rocks across different tectonic blocks between the Moyar–Bhavani and Palghat–Cauvery shear zones and an elongated belt of eastern margin of the peninsular shield. The Madras Block north of the Moyar–Bhavani shear zone, which evolved throughout the Precambrian period, mainly consists of high-grade metamorphic rocks. In order to constrain the evolution of the charnockitic region of the Pallavaram area in the Madras Block we have undertaken palaeomagnetic investigation at 12 sites. ChRM directions in 61 oriented block samples were investigated by Alternating Field (AF) and Thermal demagnetization. Titanomagnetite in Cation Deficient (CD) and Multi Domain (MD) states is the remanence carrier. The samples exhibit a ChRM with reverse magnetization of D m = 148.1, I m = + 48.6 ( K = 22.2, α 95 = 9.0) and a palaeomagnetic pole at 37.5 °N, 295.6 °E (dp/dm = 7.8°/11.8°). This pole plots at a late Archaean location on the Indian Apparent Polar Wander Path (APWP) suggesting an age of magnetization in the Pallavaram charnockites as 2600 Ma. The nearby St. Thomas Mount charnockites indicate a period of emplacement at 1650 Ma (Mesoproterozoic). Thus the results of Madras Block granulites also reveal crustal evolution similar to those in the Eastern Ghats Belt with identical palaeopoles from both the areas.

Analysis of airborne magnetic and gravity anomalies of peninsular shield, India integrated with seismic and magnetotelluric results and gravity anomalies of Madagascar, Sri Lanka and East Antarctica

Modelling of gravity and airborne magnetic data integrated with seismic studies suggest that the linear gravity and magnetic anomalies associated with Moyar Bhavani Shear Zone (MBSZ) and Palghat Cauvery Shear Zone (PCSZ) are caused by high density and high susceptibility rocks in upper crust which may represent mafic lower crustal rocks. This along with thick crust (44–45 km) under the Southern Granulite Terrain (SGT) indicates collision of Dharwar craton towards north and SGT towards south with N–S directed compression during 2.6–2.5 Ga. This collision may be related to contemporary collision northwards between Eastern Madagascar–Western Dharwar Craton (WDC) and Eastern Dharwar Craton (EDC). Arcuate shaped N and S-verging thrusts, MBSZ-Mettur Shear and PCSZ-Gangavalli Shear, respectively across Cauvery Shear zone system (CSZ) in SGT also suggest that the WDC, EDC and SGT might have collided almost simultaneously during 2.6–2.5 Ga due to NW–SE directed compressional forces with CSZ as central core complex in plate tectonics paradigm preserving rocks of oceanic affinity. Gravity anomalies of schist belts of WDC suggest marginal and intra arc basin setting. The gravity highs of EGFB along east coast of India and regional gravity low over East Antarctica are attributed to thrusted high-density lower crustal/upper mantle rocks at a depth of 5–6 km along W-verging thrust, which is supported by high seismic velocity and crustal thickening, respectively. It may represent a collision zone at about 1.0 Ga between India and East Antarctica. Paired gravity anomalies in the central part of Sri Lanka related to high density intrusives under western margin of Highland Complex and crustal thickening (40 km) along eastern margin of Highland Complex with several arc type magmatic rocks of about 1.0 Ga in Vijayan Complex towards the east may represent collision between them with W-verging thrust as in case of EGFB. The gravity high of Sri Lanka in the central part falls in line with that of EGFB, in case it is fitted in Gulf of Mannar and may represent the extension of this orogeny in Sri Lanka.

Gold Mineralization in Kottathara Prospect, Attappadi Valley, Kerala, India: a Preliminary Appraisal

The Kottathara gold prospect of Attappadi Valley in Kerala is located within the Southern Indian Granulite Terrain comprising charnockite and gneisses with enclaves of high-grade supracrustals. The gold mineralization associated with the basic members of the Attappadi supracrustals and the quartz veins traversing them are confined within the Bhavani Shear Zone. Primarily the gold-quartz lode is emplaced in rheologically preferred zones along the contact of the basic members with the enclosing gneisses subsequent to a period of retrogression and shearing. Ore-mineralogical studies reveal that gold got remobilized and this remobilization is identified with the regional Bhavani Shear. SEM studies indicate that gold occurs in free state and also within sulphides especially pyrite. Variation in grain morphology is clearly discernible in gold occurring within oxidised and in non-oxidised zones. Sequencing of deformational events with associated emplacements of known ages suggests the age of gold mineralization of Attappadi area as between 2 Ga. and 2.5 Ga. The secondary mobilization has to be <2.0 Ga or younger possibly of younger Pan-African age related with the Moyar-Bhavani Shear System. The inherent gold content of the komatiitic metapyroxenites together with the auriferous quartz lodes assigns a lithological control on gold mineralisation. Subsequent folding and remobilization due to the regional shear constrained the geometry of the lode zones implying structural control.