Indian Precambrian Research Articles

Tectonic implications of seismic reflectivity pattern observed over the Precambrian Southern Granulite Terrain, India

The Southern Granulite Terrain (SGT) constitutes a large part of the South Indian Precambrian shield. A study of the crustal reflectivity and tectonic framework of the Madurai Block of the SGT has been carried out using multifold seismic reflection survey along the Vattalkundu–Kalugumalai transect. The seismic stack sections along the profile, showing conspicuous changes in reflectivity pattern, indicate the presence of several structurally distinct crustal blocks. It is apparent from the north dipping reflectors near Vattalkundu that the Kodaikanal massif extends 5 km further south of Vattalkundu, where the dip direction changes towards south. The reflectivity pattern in the crustal column of central part of the profile, between Sedapatti and Sarvathapuram suggests block uplift accompanied by internal domal deformation of the crust in Madurai Block. The dip on the southern flank of the dome is steeper than that of northern side. The south dipping reflections in the southern part of the profile, between Sivakasi and Kalugumalai in the upper crust are strong and continuous. The presence of the ultrahigh-temperature–pressure (UHT–P) metamorphic layering reported in the region might be contributing to the increased reflectivity. However, the relationship between the reflectivity and the UHT–P is enigmatic and yet to be established. In general, the region is characterized by distinct upper and mid crustal reflectivity, but the Reflection Moho identifiable at 13.0–13.5 s two-way travel time (TWT) is diffuse.

Crustal configuration of the Dharwar craton, India, based on joint modeling of regional gravity and magnetic data

Regional gravity and magnetic investigations were conducted along a 600 km transect across the northern part of the Dharwar craton in the Precambrian Indian peninsular shield from Vasco-da-gama (latitude 15°23′40″N and longitude 73°48′55″E) to Jadcharla (latitude 16°41′40″N and longitude 78°08′18″E). Joint modeling of the two data sets was carried out utilizing GM-SYS (Geosoft) software assuming a four-layer model. A tectonic re-classification of the craton is proposed, prompted by the location of four inferred deep-seated faults, into the western and eastern Dharwar Blocks with an intervening upthrown block. This classification is at variance with the current perceptions of the configuration of the Dharwar craton, some of which hold that the craton consists of only two sub-cratons, an eastern and a western, divided by the Chitradurga thrust fault. However, in the present study, this thrust fault is found to extend no deeper than the basement peninsular gneisses. Furthermore, according to the proposed classification, the thrust fault falls within the intervening block that forms the divide between the sub-units of the craton rather than representing a tectonic boundary. In addition, twenty-six near surface geological members in the region (supracrustals) consisting mainly of schists and younger granites were identified along the transect and modeled for the representative near-surface crustal configuration of the craton.

Structural inferences from satellite image in and around Gadag, in the Dharwar Craton.

The geological interest of the Dharwar craton in the Precambrian Indian peninsular shield stems from its complexity. A satellite image (IRS-ID LISS-III) of an area of approximately 12,000 sq km corresponding to the region in and around Gadag in the Dharwar craton, was analyzed to determine the geological/structural configuration of the region. The disposition of schist belts, shear zones, and various faults and other lineaments in the region were delineated. The findings are significant in view of the paucity of structural data in the region and form a geo-structural database based on which ground surveys can be planned.

Palaeomagnetic and rock magnetic studies on the dykes of Goa, west coast of Indian Precambrian Shield

Abstract Detailed alternating field (AF) and thermal demagnetization techniques on 120 samples belonging to 10 dykes of the Goa region along the west coast of the Indian Precambrian Shield, have yielded characteristic remanent magnetization (ChRM) directions from 9 dykes of which 6 dykes exhibited a reverse magnetic polarity direction (D=150°, I=44.30°, k=58, α95=5.8, N=6 dykes), while 3 dykes exhibited normal polarity D=351°, I=−50°, k=21, α95=14.9, N=3 dykes) direction. The remaining 1 dyke has shown stable direction, but with a strong overprint of the present earth field (PEF) direction, which could not be removed by either AF or thermal cleaning techniques. A Palaeomagnetic pole, computed by averaging the VGPs of 9 dykes is situated at 41.2°N 78.1°W (N=9 dykes; A95=9.81°), which is close to that of the Deccan super pole (36.9°N, 78.8°W), suggesting that the dykes in the Goa region are related to the Deccan basaltic eruptions. Recently published 40 Ar / 39 Ar dating on the normal polarity dykes of the studied area have assigned the ages as 62.8±0.2 Ma. These results demonstrate that the Deccan magnetism continued to affect western India for at least 1–2 Ma after the peak Deccan eruption during the K–T boundary. On the basis of published geological and geochemical information along with our Palaeomagnetic results, these intrusive bodies could be attributed as the feeders for the uppermost formations of Wai subgroup of the Western Ghat Deccan basalts, or to the major geodynamic events such as the opening of the Arabian Sea and the rifting of the Seychelles–Mascarene oceanic plateau, which occurred soon after the cessation of the Deccan volcanism. Rock magnetic experiments such as the Lowrie–Fuller test along with low-temperature magnetic susceptibility and isothermal remanent magnetization (IRM) measurements have identified single domain (SD) type titano-magnetite as the major characteristic remanent magnetization carrying magnetic mineral in the dyke samples.

Crust–mantle interaction in the genesis of siliceous high magnesian basalts: evidence from the Early Proterozoic Dongargarh Supergroup, India

Siliceous high magnesian basalts (SHMB) represent a rock type chemically distinct from other common volcanic rocks. The petrogenesis of this magma, reported at or near the Archean–Proteorzoic transition across the world, is controversial. In this paper, we present chemical (XRF, INAA) and mineralogical data on a SHMB suite from the Early Proterozoic (2.1–2.5 Ga) Dongargarh Supergroup, Central India, the first of its kind reported from the Indian Precambrian. This suite of basaltic rocks is unusually high in SiO 2 (54 wt.%) and enriched in incompatible elements. The SHMB melts discussed here can neither have formed by partial melting of the Earth's mantle nor by fractional crystallisation of a mantle-derived melt. It is shown here that excess SiO 2 and incompatible elements in SHMB are supplied by a crustal component to a basaltic komatiitic parent magma of mantle origin. Major and trace elements abundances and geochemical mass balance calculations suggest that a basaltic komatiite melt assimilated 15–20% of acid volcanics, the immediately underlying unit to these rocks in Dongargarh, before erupting as SHMB. Mantle-derived rocks have Ta/Th ratios of around 0.5 whereas crustal rocks have ratios of about 0.2. The Ta/Th ratios of Dongargarh SHMB and acid volcanics are nearly identical but both unusually low (<0.1) when compared to the normal upper continental crust (Ta/Th ∼0.2), supporting the view that the acid volcanics are the source for incompatible elements and SiO 2 in SHMB. It is also shown that there is an overall compositional similarity of such temporarily unique but spatially unrelated SHMB magmas occurring in different continents including the Dongargarh SHMB.

The Raniganj Coal Basin: an example of an Indian Gondwana rift

Abstract Rifting occurred in peninsular India during Permian–Triassic Gondwana sedimentation. The rocks of the Paleozoic–Mesozoic Indian Gondwana Supergroup succeed the Mesoproterozoic Vindhyan sedimentary rocks and the Indian Precambrian shield unconformably, and are renowned for their coal resources. The rocks of the Gondwana Supergroup occur mainly in trough-like depressions arranged in a more or less en echelon fashion along distinct shear/fault zones, which are parallel to ancient structural lineaments in the Indian peninsular shield. Thus, the Gondwana group of basins occurs along the three major directions: (a) Son–Narmada and Damodar–Koel Valley, running East–West, (b) Son–Mahanadi Valley, with a NW–SE trend, and (c) Rewa–Pranhita–Godavari Valley, which trends in a NNW–SSE direction. The important common structural imprints in most of the above basins are: (i) a ‘half-graben’ basin geometry with a strong master boundary fault on one side and a homoclinal tilt of the basin towards it; (ii) a network of intrabasinal faults, one set oriented along the basin axis and which formed during the tensional stage and another, (iii) transverse to it. Basin marginal oblique faults and slips coincide with both the tensional and compressional stages and have different manifestations. Syndepositional subsidence of the basin floor through loss of support from below due to the lateral transport of sub-crustal/mantle materials caused faulting at the axial zones of the basins/sub-basins to accommodate a thick pile of sediments. Volcanic activities in the region or in adjacent areas provide further support for these inferred processes, and are confined within comparatively narrow and linear zones defined by the crustal weaknesses of considerable antiquity. All these stages have their signatures imprinted in the sediments, which have been studied in detail. This paper presents relevant data on the Raniganj Coalfield of eastern India, in order to examine whether these linear depressions are of ‘rift’ origin. The process of rifting was a prelude to the rafting of Gondwana continents after the early Cretaceous fragmentation of ‘Gondwanaland’. Thus, the Gondwana rift valleys flanked the periphery of the peninsular Indian stable blocks, to collide ultimately with the Sino–Eurasian plate in the north.

Special Issue on “The Indian Precambrian”

Special Issue on “The Indian Precambrian”

Structural evidence supporting a remnant origin of patchy charnockites in the Chilka Lake area, India

AbstractDark patches of charnockitic rocks characterized by orthopyroxene occur within garnetiferous granite gneisses (leptynites) in a granulite-migmatite suite around the Chilka Lake, Orissa, within the Eastern Ghats belt in the Indian Precambrian. Analysis of structures of different scales observed in this terrain establishes the presence of three phases of deformation. S1 is pervasive in the metapelitic granulites (mainlykhondalite), while in the migmatite complex composed of leptynites, charnockites and quartzofeldspathic veins, S1 is present exclusively within the charnockite lenses and bands, and shows different stages of obliteration in the associated leptynites. Thus, the charnockite patches must be earlier than the surrounding migmatitic rocks. The charnockite patches and the surrounding leptynitic gneisses are chemically quite different and the two rock types are not related by any prograde or retrograde transformation. The shapes and disposition of charnockite patches in the mixed exposures are found to be largely controlled by the third phase of folding and locally associated shearing. The kinematics of this late deformation are not favourable for fluid ingress from deeper levels.

The Himalayan passive margin from Precambrian to Cretaceous times

The Himalaya passive margin should be separated into the Indian Himalaya and the Pakistan Himalaya, divided by the Nanga Parbat-Haramosh basement uplift in the northwestern syntaxes. The Indian passive margin shows a complex history involving splitting of microcontinents off the northern Gondwana margin from early Paleozoic times until the Jurassic. In contrast, the Pakistan Himalaya formed part of the stable Indian shelf until separation from Africa started in the Jurassic. Each margin has a distinct stratigraphic history until the mid-Mesozoic after which they show similar histories until the Tertiary.The Himalaya mountains in India consist of three distinct tectonic units juxtaposed during the Neogene by southward thrusting of the northern Indian Precambrian to Mesozoic passive margin. The Lesser Himalaya on the south forms a late Precambrian passive margin usually directly overlain by late Cretaceous to Tertiary clastics: its stratigraphy is closest to that of the Indian Shield. In contrast, the High Himalaya has a thin Ordovician to Carboniferous shelf sequence deposited after early Ordovician deformation and granite intrusion. Above this, thick Permian and Mesozoic shelf sequences mark the separation of continental blocks off the northern Indian margin and the opening of the Neotethys ocean. The North Himalaya formed the slope and basin of this ocean and consists of reactivated Paleozoic gneiss domes overlain by thick Mesozoic sediments which pass, in the northeastern Himalaya, into an enormously thick sedimentary sequence resting on oceanic crust.Paleomagnetic and structural evidence indicates at least 500 km of southward thrusting along the Main Central Thrust between the Lesser and High Himalaya. This thrusting, together with a further 250 km of erosion at the front of the nappes, has removed the entire inner shelf of the Himalaya passive margin and can explain the startling contrast between the High and Lesser Himalaya stratigraphic sequences. But there are still discrepancies, particularly in the eastern Himalaya, where Precambrian basement is juxtaposed with the North Himalaya Mesozoic slope. These discrepancies can be partly resolved by strike-slip movements roughly parallel to the Himalaya which have removed parts of the northern Indian passive margin. Such faults of the requisite orientation, displacement and age occur in southeast Asia where their cumulative displacements can add up to several thousand kilometres. Similar faults, of undoubted pre-Tertiary age, occur in the Pakistan Himalaya. Here, the passive margin shows a much simpler history of Jurassic to Miocene subsidence.

Metaphyte and Metazoan fossils from Precambrian sediments of India: a critique

Evidences of multicellular life from the Indian Precambrian sediments have been reviewed. Their nature, morphology, biogenecity and syngenecity are evaluated in the light of associated evidences. The published records have been grouped under 13 categories, viz., Archaeocyatha, Chuaria·Tawuia group, frordoid forms, Hyolithoides, Longfengshania, medusoids, metaphytic algae, Sekwia excentrica, shelly forms, spiral forms, trace fossils, trilobitoid and eurypteroid and enigmatic group. Contrary to the world-wide records of multicellular organisms only at the Terminal-Precambrian (≈600 Ma), some of the Indian reports are 1,000-2,500 Ma old. This older antiquity needs proper explanation. In the present review after reassessment, all the records have been classed as true fossil, non-fossil and dubiofossil. In few cases, it has not been possible to comment upon the structure reported due to non-availability of specimen and poor photographic reproduction in print, such records have been grouped separately without any comments.&#x0D; In several cases synaeresis and mud cracks have been described as trace fossils. Sedimentary structures have also been described as Sekwia, Longfengshania, Cyclomedusa, Archaeocyatha, Dasycladaceae algae, Epiphyton, lamellibranchs and such other forms Some records though of undoubted biogenic nature, viz., Katnia singhi and Vindhyavasinia misrai and Ajaicicyatha (from Krol sediments) need reassessment concerning their taxonomic affinity.&#x0D; The oldest authentic biogenic structures in this review are considered to be about 1,000 Ma old. These records of metaphytes and metazoans-Ramapuraea vindhyanensis, vendotaenid forms, trace fossils, megascopic spiral algal forms and Chuaria·Tawuia assemblage, are mostly from the base of Kaimur and top of Semri groups. Trace fossils from Bhander Limestone of Vindhyan Supergroup; a metaphytic alga Renalcis from the Calc-Zone of Pithoragarh, and frondoid forms, trace fossils as well as small shelly fauna from the Krol-Tal sequence belonging to younger sediments of probable Vendian-Tommotion age are authentic records.&#x0D; The multicellular plants and animals undoubtedly proliferated during the Vendian, but there are a few authentic exceptional records of multicellular life prior to 600 Ma from India. Similar earlier records are also known from China and Canada. These records are important in understanding the antiquity of multicellular life.

Comparison of electric and elastic parameters of indian precambrian rocks

Examination of the relation between electric resistivity, compressional wave velocity and density of Indian Precambrian rocks is presented.

Geochronology and geochemistry of precambrian granitic rocks of goa, sw india

The Precambrian rock succession in Goa, representing the NW extremity of the South Indian Precambrian Craton, includes: (1) a low grade supracrustal sequence known as the Goa Group; and (2) several acid granitic and gneissic bodies occurring in close spatial association with the supracrustals. The Anmod Ghat trondhjemitic gneisses of NE Goa, which form the basement to the Goa Group, yield a Rb-Sr whole-rock isochron age of 3400 ± 140 Ma. The pervasive late-Archaean acid plutonism post-dates evolution of the Goa Group supracrustal sequence and spans syn- to post-tectonic regimes with respect to the last major penetrative folding in the segment. The Rb-Sr whole-rock ages of the granitic plutons and migmatites (Chandranath granodiorite-granite: 2650 ± 100 Ma; Dudhsagar granite: 2565 ± 95 Ma; Londa migmatites; 2650 ± 130 Ma; Canacona graniteporphyry: 2395 ± 390 Ma) suggest a minimum age of the Goa Group of c. 2.6 Ga and confirm the correlation of the Goa Group with the Chitradurga Group, which occurs to the south of the craton. Geochemical data presented bring out essential differences between the c. 3.4 Ga old basement trondhjemitic gneisses and the younger group of granites (c. 2.6-2.4 Ga). The early Archaean trondhjemitic gneisses show low Rb/Sr (0.20), Th/U (1.45–1.93), highSr/Ba ( >0.5), low ΣREE (24.14–52.81) with a slightly fractionated REE pattern moderately depleted in HREE (CeN/YbN=2.1–6.5). Compared with the early Archaean gneisses, the younger granites are distinctly potassic with higher Rb/Sr (0.50–0.87), Th/U (4.05–14.91), lower Sr/Ba (0.22–0.43), a LREE enriched pattern (CeN/YbN=15.1–26.6) and much higher ΣREE (132.06–180.84).

Precambrian Continental Crust of India and Its Evolution

The Indian Precambrian continental crust exhibits a variety of geological features fashioned at different times by different geotectonic processes. The bulk of this crust was formed prior to 2600 m.y. ago and remobilized at least twice between 2600-2000 m.y. ago (early Proterozoic Mobile Belt, EPMB) and 2000-1500 m.y. ago (middle Proterozoic Mobile Belt MPMB). Three early Precambrian nucleii: Karnataka (KN), Jeypore-Bastar (JBN), and Singhbhum (SN) appear to have survived in the craton and are characterized by low-grade supracrustals and tonalitic trondhjemite gneisses, formed 3800-2600 m.y. ago. The EPMB event involved sedimentation, amphibolite-granulite facies metamorphism, and metasomatism and produced amphibolite facies rocks and K-granites in the north, and charnockite and other granulite facies rocks in the south. Gold sporadically distributed in the supracrustal rocks of the craton was remobilized during the EPMB event. K-granites form a garland around the central Dharwar craton, suggesting some type of collision between two blocks. The compressional stress directions in the craton and the surrounding mobile belts were EW, producing almost identical structures in all the regions. The supracrustals of the Indian Archean are broadly divisible into an older and a younger sequence. Older belts are characterized by argillites and chemogenic sediments of high Mg, Fe, Al, Cr, and Ni abundances, while younger belts are characterized by graywacke shale suites with abundant Na, K, Rb, and Sr. The REE, U, and Th abundance patterns of the two groups show significant differences. The small amount of ultramafic rocks in the Indian Precambrian necessitates alternative sources for the high Ni and Cr contents in the supracrustals. Cr and Ni contents are high even in gneisses of this region. The available data provide constraints for a model which suggests that older schist belts were developed in shallow water basins on a simatic crust. On the other hand, the platformal components of the younger greenstone belts were laid down in rifted basins on a sialic basement. Crustal deformation and thickening gave rise to the EPMB. At 2000-1500 m.y. ago, another intensive mobile belt event occurred in which subduction and flexure at the eastern and northern margins of the Dharwar-Singhbhum Protocontinent gave rise to Proterozoic sedimentary basins, rift valleys, and igneous and metamorphic suites. Plate tectonic regimes had clearly set in by 2000 m.y. ago; the middle Proterozoic orogeny shows clear evidence of modern-style collision tectonics.

Geological features and origin of the chromite deposits of sukinda valley, Orissa, India

The chromiferous ultramafic rocks of Sukinda Valley (21°0'–21°5'N:85°43'–86°0'E) of Orissa are intrusive into the Iron-Ore Supergroup (2,950–3,200 Ma) at the eastern periphery of the Indian Precambrian shield. Both laterally and vertically, chromite occurs as persistent layers, lenses or pockets in the serpentinized and silicified dunite-peridotite extending over a strike length of 25 km. The ultramafic rocks and the chromitite layers are cofolded with the Iron-Ore Supergroup into a plunging syncline. Primary layering, ball and pillow structures, cross laminations, graded bedding etc. can also be detected. The different varieties of chromite ore present in the area are massive, banded and spotted, laminated and friable. The grain size of chromite varies between 0.25 and 4 mm, and the fineness of the grain increases from the bottom to the top layers. The cell dimension of chromite (8.23–8.32 A) decreases with the increase of Al2O3.

Thermo-mechanical structure of the southern part of the Indian Shield and its relevance to Precambrian basin evolution

The thermal and mechanical structures of the southern part of the Precambrian Indian Shield have been estimated using available heat flow data and shear stress profiles from olivine rheology. These and other geological, geochronological and geophysical data including deep seismic studies (DSS) profiles of Proterozoic Cuddapah basin on South Indian Shield, are utilized to examine thermal models for the evolution of Precambrian intracratonic, platform basins on the Archean lithosphere of Indian Shield. Evidence of mantle perturbations and cycles of thermal events are documented to be important in the Cuddapah basin's evolution. Haxby et al.'s (1976) thermal model has been shown to explain the Cuddapah basin's flexuring and magnitude of subsidence.

Applicationof remote sensing techniques in geological studies of various mineralized belts in India. (National paper on IGCP — 143)

Geological interpretation, analysis and evaluation of LANDSAT and airborne MSS data, panchromatic and colour infrred photographs and spectral reflectance data for a number of test sites in different mineralized belts in India have led to certain significant results. Lithological discrimination on digitally processed enhanced MSS data products has helped in further subdivisions of major rock groups. Lineament pattern analyses reveals three prominent sets of fractures in Indian Precambrian Shield with predominant ENE-WSW megalineament probably representing earliest deep crustal fractures and subsequent NNW-SSE and NE-SW fractures, associated with polyphase tectonic movements. Correlation of structures and mineralization indicate structural control for most of the mineralized belts with intersecting locii type concentration of ore deposits in some cases. In a few cases control of mineralization is lithology and stratigraphy. Spectral groundtruth and laboratory studies indicate that major rock types have characteristic spectral patterns (signatures) which would be useful for lithological mapping by automatic classification techniques. The work also has led to the improvement of enhanced MSS data products for geological studies.

Post-Precambrian tectonism in the Delhi—Aravalli belt, Precambrian Indian shield — evidences from LANDSAT images

Multispectral LANDSAT images of the Precambrian shield of Rajasthan, India, indicate rejuvenation of Precambrian lineaments in later geological times. Lineaments in the southern part of the area seem related to the formation of the Cambay graben (Mesozoic) and the Narmada—Son megalineament (late Mesosoic). In addition, there are a few relatively young structural features traversing the Quaternary deposits in the northern part of the belt that could be manifestations of intraplate tectonics of the Himalayan orogeny.

Ore genesis and its relationship to volcanism, tectonism, granitic activity, and metasomatism along the Singhbhum shear zone, eastern India

The Singhbhum shear zone is a 200-km-long arcuate mineralized belt in the eastern part of the Indian Precambrian Shield, well known for copper, uranium, apatite-magnetite, and kyanite deposits. This zone evolved from a preexisting zone of deep-seated fracture and volcanism that separated an Archean granitic platform to the south from a Proterozoic mobile belt to the north. It progressively evolved in time, between 2,000 to 850 m.y. ago, as rocks of the northern mobile belt were deformed, metamorphosed, and thrust southward against the platform in four successive phases. Higher temperature and pressure conditions prevailing within this zone, during the course of its evolution, set up geochemical gradients which caused granitic activity in the western and southeastern sectors during the first two phases of deformation, and albite metasomatism, biotitization, chloritization, and sericitization, mainly in the central sector, during the third phase. The ore materials, which appear to have been introduced into this zone during the pre-deformational phase of volcanism, were subsequently mobilized by the chemically active fluids, permeating through the crushed and sheared rocks of this zone, to give rise to the copper, uranium, and apatite-magnetite deposits at shallower levels. The central sector of this zone, which was affected by intense deformation and metasomatism during the third phase, was more extensively mineralized than the western or southeastern sectors where intensity of deformation and metasomatism was lesser. Kyanite deposits were localized along the northern boundary of the shear zone along which somewhat higher pressure prevailed. Ore genesis thus appears to be related to the total evolutionary history of this zone and cannot adequately be described merely as epigenetic hydrothermal or syngenetic metamorphosed as proposed by other workers. These ore deposits may be better described as remobilized syngenetic.

Fossil marginal bassin from the Indian shield: A model for the evolution of Singhbhum Precambrian belt, Eastern India

In the eastern part of the Indian shield covering Shinghbhum and adjoining districts of Bihar, occurs the fossil marginal basin which opened up during the early Proterozoic, flanked by Archaean continent to the south. Volcanism played a significant role in filling up the basin. The spine of the basin is occupied by a basaltic lava pile representing a narrow oceanic sector. BIF and possible ‘marginal basin ophiolites’ are other significant members of the basin cover. The structural features and sedimentary history of the basin are characteristic for marginal basins in general. Identification of this marginal basin emphasises the role of plate tectonics in the evolution of the Indian Precambrian shield.

Assigning a magmatically defined tectonic environment to Chitradurga metabasalts, India, by geochemical methods

The major and trace-element patterns in a set of 45 analyses of the Precambrian Chitradurga metabasalts have been interpreted by geochemical methods to classify the magma-types and to determine the tectonic environment of eruption. During amphibolitization of the tholeiites, the order of relative chemical stability is: SiO 2 > MgO > Al 2 O 3 > FeO > CaO > Na 2 O > K 2 O > P 2 O 5 . On the ACFN plot the Chitradurga amphibolites approximate the unaltered basalts in their N component, show considerable depletion in F and A, and enrichment in the C components. With respect to the Keweenawan metadomains (pumpellyite and epidote), the Chitradurga amphibolites show considerable enrichment in N and depletion in the C constituents. The calc-alkaline (mol.props.) index, SiO 2 (wt.%) and “F”MA plots have shown the differentiation of the parent olivine normative tholeiite through the transitional calc-alkalic basalts to basaltic andesites; the ferrofemic index being 67. Discriminant functional analysis of the major-element patterns has classified the magma-types into ocean-floor basalts (9 analyses) and the volcanic arc series containing low-potassium tholeiites (24 analyses), calc-alkalic basalts (6 analyses) and basaltic andesites (6 analyses). The effective discrimination of the ocean-floor basalts from low-potassium tholeiites is accomplished due to the relatively low eigenvalues in functions F 1 and F 2 for SiO 2 , K 2 O and high values for MgO, TiO 2 in the former as compared to the latter. The low-K tholeiites are discriminated from calc-alkalic and basaltic andesites by the gradational increase in eigenvalues for K 2 O, SiO 2 and Al 2 O 3 in functions F 2 and F 3 . The discriminant analyses of the trace-element patterns have classified the ocean-floor basalts from low-K tholeiites on TiCr, TiZr and Ti(× 10 −2 ) Zr  Y (× 3) plots. The TiO 2 K 2 OP 2 O 5 plot has discriminated ocean-floor basalts, low-K tholeiites and the calc-alkalic group containing the basaltic andesite members. Thus the discriminant analysis of the major and trace-element patterns have shown noteworthy consistency, thereby attesting to the high success rate of classification. The Ca-Mg rich, low SiO 2 , K 2 O and alumina, olivine normative characteristics, and the normative pyroxene content of 36.15–45.88% of the ocean-floor basalts compare closely with those of the oceanic magma type of the Dalma volcanic suite of the north-eastern part of the Precambrian Indian shield of Bihar. These compositional features indicate their mantle origin, amphibolite and plagioclase—pyrolite assemblage of the upper mantle and relatively low-temperature, moderate pressure and hydrous environment of pyrolite. The presence of ocean-floor basalts at Chitradurga implies the existence of an oceanic rise and their eruption through its axial rift region. The mean Ti content (0.77%) of ocean-floor basalts compares closely with that of the Dalma suite (0.76%). The low-alumina content indicates faster spreading of the ocean floor. Identification of the magma-types of volcanic arc series containing a wide range of Zr and Y is attributed to quartz-normative fractionation of the magma and its differentiation to calc-alkalic and basaltic andesite members due to incorporation of sialic material by accretion in the marginal regions of the volcanic belt. Attention is focussed on the synonymous tectonic evolutionary trends of the two typical volcanic belts of the two geographically widely separated regions of the Indian Precambrian shield; the basis being: (1) low-alumina, enriched Ca-Mg and almost identical concentrations of Ti in the ocean-floor magma type, and (2) the presence of a pronounced volcanic arc magma type in the Chitradurga area and two analyses of a volcanic arc magma type in the Dalma area.