Eastern Ghats Mobile Belt Research Articles

The Jugsaipatna Anorthosite Complex, Eastern Ghats Belt, India: Magmatic lineage and petrogenetic implications

The Jugsaipatna Anorthosite Complex (JAC) in the Eastern Ghats Mobile Belt (India) comprises a central anorthosite – leuconorite – troctolitic norite core mantled by a rim of high-Al gabbronorite (HAG). Small dykes of ferrodiorite occur in the core region of the JAC. A lensoidal body of eucritic gabbronorite occurs in contact with leuconorite and outer gabbronorite rim. Baring ferrodiorites, the mafic members such as HAG, troctolitic norite and eucritic gabbronorite are not known in other anorthosite complexes of this high grade belt. The HAG is characterised by high Al 2O 3 content (17 wt.%), enriched ΣREE and LREE, absence of ‘Eu-anomaly’ and negative Sr anomaly and is, therefore inferred to be the source melt for the JAC suite of rocks. The eucritic gabbronorite comprising An-rich plagioclase and high-Al pyroxenes has a komatiitic affinity with high Cr–Ni contents and primitive REE patterns. Troctolitic norite, leuconorite and anorthosite are interpreted to be cumulate rocks fractionated from the HAG. The ferrodiorite with enriched Fe, Ti, P, Zr and REE abundances and a prominent ‘negative Eu’ anomaly corresponds to the residual melt of the plagioclase (+pyroxene) dominated fractionates that comprise anorthosite-leuconorite.

Coexisting ultramylonite and pseudotachylyte from the eastern segment of the Mahanadi shear zone, Eastern Ghats Mobile Belt

Pseudotachylytes occur associated with mylonite and ultramylonite in the Mahanadi shear zone (MSZ) in the Eastern Ghats Mobile Belt (EGMB). The MSZ is about 200 km long curvilinear high strain zone trending WNW-ESE in its eastern part that splays out in the west. In Kantilo-Ganian segment of MSZ in northern EGMB, an interbanded sequence of granulite facies lithoassemblage has undergone ductile shearing. Kinematic studies of mylonite and ultramylonite indicate MSZ to be a NE-dipping, extensional type ductile shear zone. Non-coaxial metamorphic growth of garnet and presence of truncated sillimanite-fish in ultramylonite suggest high temperature regime during shearing. Pseudotachylytes in MSZ occur as millimetre thick layers to decimetre thick zones containing fragments of mylonite, ultramylonite and lithic clasts. Pseudotachylyte generation veins are mostly sub-parallel to C-planes and the injection veins cross-cut at high angle to these. The presence of an isotropic glassy matrix, injection features, corroded grains and dendritic microlites can be evidences for the existence of a melt phase. The composition of pseudotachylyte matrix (by EPMA) indicates silica deficiency with higher normative hypersthene, plagioclase and lower quartz compared with average whole rock composition for host. Absence of overprinting of mylonitic fabric on pseudotachylytes indicates their formation by brittle failure without passing through a plastic deformation and thus a two stage development for mylonite-ultramylonite and pseudotachylyte generation is suggested.

Heavy Mineral Placer Sand Deposits of Kontiagarh Area, Ganjam District, Orissa, India

AbstractThe Kontiagarh placer deposit in the Ganjam district, Orissa, India extends in northeast direction having a width of 700–1000 m. A total of 187 samples were collected meterwise from 55 bore holes in a grid pattern from beach, frontal, intermediate and back dunes covering an area of approximately 1 km2. Light minerals decrease in size from the beach to the back dunes, whereas the size distribution of heavy minerals in the beach and dunes is more or less uniform. The average heavy mineral content in the beach and dunes vary from 9.38% to 24.20%. The heavy minerals are ilmenite, garnet, sillimanite, rutile, monazite, and zircon with trace amounts of magnetite, hornblende, diopside, sphene, tourmaline, and epidote. Heavy minerals are mostly less than 350 µm in size, with a peak distribution in the range between 180 and 125 µm. Ilmenite shows exsolution intergrowth with hematite. Mineral chemistry of ilmenite, hematite, leucoxene, magnetite, monazite and sillimanite are examined by EPMA. Leucoxene is lower in Fe and higher in Ti, Al, Cr and V than ilmenite. The litho‐units of the Precambrian Eastern Ghats Mobile Belt, comprising primarily khondalite, charnockite, calc‐silicate granulite and gneiss, are the source of heavy minerals for this deposit. The bulk sample has 7.30% ilmenite, 5.24% sillimanite, 9.16% garnet, 0.18% rutile, 0.14% monazite, 0.06% zircon and 0.52% other heavy minerals. The deposit has good potential for economic exploitation of ilmenite, rutile, sillimanite, monazite, zircon and garnet.

Sources of monazite sand in southern Orissa beach placer, eastern India

This paper intends to explore whether there is an important source for monazite beach placer of the Gopalpur-Chhatrapur-Rushikulya coast, Orissa, in the adjacent Eastern Ghat Mobile Belt (EGMB). Petrographic and mineralogical studies were conducted on all the rock types constituting the EGMB exposed over a stretch extended up to ∼20 km landward from the estuary of the River Rushikulya that is believed to transport the major bulk of sand to the Gopalpur-Chhatrapur-Rushikulya beach. Heavy mineral population was concentrated using bromoform and percentages of each heavy mineral constituting the population were estimated for all the potential source rock types. Isodynamic separation and XRD techniques were deployed for precision identification of every heavy mineral present. The study identified the granitoid (or migmatite) basement rock as by far the major contributor of monazite to the Chhatrapur beach sand. The study also reveals that charnockite is an important contributor of orthopyroxene as well as garnet, although the sillimanite-garnet-quartz schist (khondalites) is also an important source for the latter. On the other hand, garnet-quartz schist and garnet-biotite-quartz schist may also contribute substantial quantity of pyroxene and garnet. The high grade metasedimentary rocks, in general, could be the major sources for rutile, while ilmenite, magnetite and zircon in the beach sand have their sources perhaps in all the varieties of rocks constituting the EGMB.

Origin of graphite, and temperature of metamorphism in Precambrian Eastern Ghats Mobile Belt, Orissa, India: A carbon isotope approach

The carbon isotope composition of graphite and carbon and oxygen isotope composition of associated calcite from different locations of the Eastern Ghats Mobile Belt (EGMB) of Orissa have been measured in order to understand the origin of graphite. The δ 13C values of graphite range from −2.4‰ to −26.6‰. Forty-four of sixty-one samples have δ 13C values less than −20‰. Most of these low δ 13C values graphite corresponds to schists and disseminations in khondalite and calc-silicate granulites, thus indicating graphitization of organic matter. The remaining light-carbon–graphite occurs as veins which is the result of graphitization of transported organic matter. The graphite with intermediate δ 13C value (−13‰ to −19‰) indicates carbon contributions from both organic and carbonates sources and/or mantle sources. The higher δ 13C values graphite (−2.4‰ to −8.8‰) represent mantle carbon and/or carbonate sources without significant contribution from organic carbon. The temperatures of metamorphism have been estimated using carbon isotope ratios of graphite and associated calcite of calc-silicate granulites, where typical cation exchange thermometer assemblages are lacking and significant mineral reaction textures used to calculate pressure–temperature of metamorphic events are absent. Metamorphic temperatures obtained 945 °C are close to the ultrahigh-temperature reported from the EGMB. The minimum temperature estimated using the graphite–carbonate carbon isotope ratio is 90 °C. The lower estimates of temperatures probably indicate changes in the carbon isotope ratio of calcite by decarbonation reaction or armoring of carbonaceous matter in silicates during metamorphism preventing continuous exchange with calcite.

Permian–Jurassic Mahanadi and Pranhita–Godavari Rifts of Gondwana India: Provenance from regional paleoslope and U–Pb/Hf analysis of detrital zircons

The Permian–Jurassic Mahanadi and Pranhita–Godavari Rifts are part of a drainage system that radiated from the Gamburtsev Subglacial Mountains in central Antarctica. From 12 samples we analysed detrital zircons for U–Pb ages, Hf-isotopes, and trace elements to determine the age, rock type and source of the host magma, and T DM model age. Clusters, in decreasing order of abundance, are (1) 820–1000 Ma, host magmas felsic granitoids with alkaline rock, (2) 1500–1700 Ma felsic granitoids, (3) 500 to 700 Ma mafic granitoids with alkaline rock, (4) 2400–2550 Ma granitoids, and (5) 1000–1200 Ma felsic and mafic granitoids, mafic rock, and alkaline rock. T DM ranges from 1.5 to 3.5 Ga. Joint paleoslope measurements and zircon ages indicate that the Eastern Ghats Mobile Belt (EGMB) and lateral belts and conjugate Antarctica are potential provenances. Zircons from the Gondwana Rifts differ from those in other Gondwanaland sandstones in their predominant 820–1000 Ma and 1500–1700 Ma ages (from the EGMB and conjugate Rayner–MacRobertson Belt) that dilute the 500–700 Ma (Pan-Gondwanaland) ages. The 1000–1200 Ma zircons reflect the assembly of Rodinia, the 500–700 Ma ones that of Gondwanaland; the other ages reflect collisions in the region.

Petrogenesis of Pal Laharha granitic gneiss in Eastern Indian craton: Evidence from field relations and petrochemistry

A large segment of the southcentral part of the Eastern Indian Craton proximal to the contact of the Eastern Ghats Mobile belt is occupied by the Pal Laharha gneiss. Spatially associated with the lithoensemble of greenstone belts. This segment is a zone of high strain traversed by regional faults. The granite gneiss is mainly represented by stromatic migmatite having supracrustal rocks constituting the immobile components. Three different varieties of granite VIZ (i) fine to medium grained mesocratic, horriblende-magnetite bearing granite, (ii) banded, pink, leucocratic granitic gneiss, and (iii) muscovite-biotite±garnet bearing medium to coarse grained granitic gneiss have been identified in this segment. The granitic gneiss has a granite to monzogranite range of composition and compares well with anorogenic, metalummous A-type granite (A/CNK<1). Highly fractionated REE, LREE enrichment, flat HREE pattern and a -ve Eu-Anomaly characterize the granites. From field, petrographic and geochemical attributes, it is suggested that the Pal Laharha granitic gneiss does not form a discrete plutonic body. It possibly evolved by the synkinematic mid-crustal anatexis of the supracrustal sequences now occuring either as megaenclaves or as immobile components.

Palinspastic (pre-rift and -drift) fit of India and conjugate Antarctica and geological connections across the suture

The passive continental margins of India and conjugate Antarctica (30°–80° E) have been reconstructed quantitatively by eliminating the intervening ocean floor. Recently acquired seismic and gravity data from the margins define the boundary between continental and oceanic crust (COB) and indicate the thickness of the extended (rifted) continental crust. The COBs are restored to their pre-rift position by eliminating pre-drift extension and are fitted together. This fully palinspastic reconstruction reveals the Lambert and Mahanadi Rifts aligned in a Permian–Triassic rift system, the Napier salient draped by the bight of India, and strong connections within each of four sectors: (1) Southern Granulite Terrain–Sri Lanka–Lützow–Holm Terrane–Rayner Complex; (2) Napier–southern Eastern Ghats Mobile Belt–Dharwar Craton; (3) Kemp Land–MacRobertson Land–northern Eastern Ghats Mobile Belt–Bastar Craton; and (4) Prydz–Rauer–Vestfold Hills–Singhbhum Province. Major events registered are (a) a 0.9–1.3 Ga (Grenville) convergence that formed the Eastern Ghats–Rayner Mobile Belt in Rodinia, and (b) 0.50–0.60 Ga (Pan-Gondwanaland) events that accreted the Southern Granulite Terrain and Sri Lanka to the Antarctic–Indian region. Gondwanaland broke up at 0.14 Ga along the grain of the Eastern Ghats–Rayner Mobile Belt and across the composite Archean Dharwar–Napier Craton and the long axis of the Permian–Triassic rift system.

Structural fabric of the Southern Indian shield as defined by gravity trends

Southern Indian shield represents a mosaic comprised of several smaller structural domains separated by discrete shear zones. Here we present a horizontal Bouguer gravity gradient map of the Indian shield, south of 14 °N, to define a continental mosaic of gravity trends domains akin to structural domains. The gravity gradient image is based on 7862 newly collected observations merged with 6359 old gravity data. This combined dataset delineates structural boundaries of the five gravity domains related to the Eastern Dharwar Craton, the Eastern Ghats Mobile Belt, the extended Eastern Ghats Mobile Belt, the Southern Granulite Terrain, and the Western Dharwar Craton. Other belts of significant gravity gradients are found associated with the Eastern and the Western coasts. The loci of Closepet granite and Kolar schist belts do not manifest themselves as boundary zones between two distinct gravity domains of the Eastern Dharwar Craton. Lack of a gravity gradient across Karur–Oddanchatram–Kodaikanal and Karur–Kambam–Painavu–Trichur Shear Zones may be attributed to a lack of gravity measurements caused by difficulties in collecting data in topographically difficult terrain. The subdued gravity gradient across the Palghat–Cauvery Shear Zone and a weak gradient across the Achankovil Shear Zone indicates a lithological and/or morphological boundary rather than a terrane boundary. Alternatively, structural domains encompassing Palghat–Cauvery and Achankovil Shear Zones may have been in a neighbouring position during the Gondwana assembly, when Pan-African thermal perturbation reactivated the structures and reworked partly or totally obliterating earlier crustal fabric.

Aeromagnetic anomalies uncover the Precambrian basement in the Chhattisgarh basin area, Central India

This paper presents aeromagnetic images for the Chhattisgarh basin region, in Central India, to provide a new window on Precambrian basement geology and structure. On the basis of aeromagnetic patterns, the Chhattisgarh basin is sub-divided into a northern low (negative) anomaly zone and a southern high (positive) anomaly zone. The northern portion of the main Chhattisgarh basin has been further divided into two subbasins, the Hirri sub-basin in the west, and Baradwar sub-basin in the east. A prominent negative anomaly delineates a NW-SE trending greenstone belt separating these sub-basins. Positive magnetic anomalies delineate the extent of the Dongargarh granite and equivalents, while the weak magnetic anomaly in the southeast of the Dongargarh granite and equivalents reflect granulite gneisses of the Eastern Ghat Mobile Belt. By applying the reduced-to-the-equator filter we enhanced the possible magnetic sources and structural lineaments within the Chhattisgarh basin.

Transpressional tectonics during the emplacement of Pasupugallu Gabbro Pluton, Western margin of Eastern Ghats Mobile Belt, India: Evidence from AMS fabrics

The Pasupugallu Gabbro–Anorthosite Pluton intruded the transition zone between the Mesoproterozoic Eastern Ghats granulites and the Meso- to Paleoproterozoic amphibolites and migmatitic gneisses of East Dharwar Craton, southern India. The pluton is bound by shear zones at its entire eastern, northern and southwestern margins. The objective of this work is to study the internal fabric of the pluton to understand the tectonic setting during its emplacement, essentially by means of Anisotropy of Magnetic Susceptibility (AMS). A total number of 107 samples have been subjected to AMS and IRM studies. The magnetic fabric is principally ferromagnetic in origin as the magnetic susceptibility values are >10 −3 SI, which is further supported by IRM studies. The magnetic fabrics correspond well with the field measurements and petrographic studies. They show two lobes in both the northern and southern parts which display concentric inward dipping helicoidal foliation trajectories with interlayered steep and gently dipping domains. The magnetic lineation pattern suggests that the magma might have ascended through the central lobes and extended towards outer margins, particularly to the west. The constrictional strains occur at the eastern margin while flattening strains mark the western margin. AMS studies together with our field structural observations suggest that the magma has been emplaced into a chamber (dilational jog) induced by crustal-scale dextral transpressional tectonic regime, probably during the onset of Grenvilian event.

Timing and dynamics of the juxtaposition of the Eastern Ghats Mobile Belt against the Bhandara Craton, India: A structural and zircon U‐Pb SHRIMP study of the fold‐thrust belt and associated nepheline syenite plutons

Sensitive High Resolution Ion MicroProbe (SHRIMP) U‐Pb dating of zircon from basement granite gneisses and nepheline syenites of the Sinapalli Nappe, occurring along the northwestern margin of the Eastern Ghats Mobile Belt, indicate high grade regional metamorphism and associated folding accompanying juxtaposition of the nappe with the Bhandara Craton, to have taken place between 617 ± 85 Ma (lower intercept age of a reworked basement unit) and 517 Ma (age of the youngest syenite). This shows, for the first time, that the final juxtaposition of the northwestern parts of the Eastern Ghats Mobile Belt against the Bhandara Craton came about in the late Neoproterozoic and not, as previously thought, during the Mesoproterozoic. The northwestern part of the Eastern Ghats Mobile Belt comprises a fold‐ thrust belt consisting of a stack of northwesterly verging nappes that have been thrust over the Bhandara Craton. The Sinapalli Nappe is the lowermost nappe and rests over a tectonic contact on the Archean granites and gneisses of the craton. The basal décollement is exposed as a two‐km‐wide ductile‐brittle thrust, hosting nepheline syenite plutons that show fabrics consistent with a synkinematic emplacement during thrusting. The Sinapalli Nappe is comprised of a sequence of alternating mafic granulites and quartzofeldspathic gneisses with slivers of basement granites, which are folded in three phases of folding (F1, F2 and F3) and were subjected to granulite facies metamorphism during F1 folding. Thrusting is synkinematic to F2 folding and is responsible for the juxtaposition of the northwestern part of the Eastern Ghats Terrane over the Bhandara Craton during the assembly of parts of eastern Gondwana.

Structure of Charnockitic basement in a part of the Krishna-Godavari basin, Andhra Pradesh

A regional magnetic survey was carried out over an area of 8000 km2 in Godavari districts of Andhra Pradesh, India, which is covered by the rocks of Eastern Ghat Mobile Belt (EGMB)viz., the Khondalitic series and Charnockites in the northern half and Permian to Mesozoic and Cenozoic sediments in the southern half, and forms a part of the Krishna-Godavari (K-G) basin. The survey brought out a strong NE-SW trending anomaly in the area covered by the rocks of Eastern Ghat Mobile Belt (EGMB), and a mild ENE-WSW trending anomaly in the area covered by the sediments of the Krishna-Godavari (K-G) basin. The NE-SW trending anomaly in the northern half could be attributed to the exposed/near surface Charnockite basement that has come closer to the surface as a result of Eastern Ghat Mobile Belt (EGMB) tectonics. Explanation of the mild ENE-WSW trending anomaly over the sediments of the Krishna-Godavari (K-G) basin required a faulted magnetic basement at depth downthrown towards the south. It is therefore concluded that the Charnockitic basement together with the Khondalite group of rocks which are folded and faulted during the different phases of tectonics of Eastern Ghat Mobile Belt (EGMB) extend into the Krishna-Godavari (K-G) basin and further, were involved in faulting during the phases of formation and sedimentation in the Krishna-Godavari (K-G) basin.

Dynamic melting of the Precambrian mantle: evidence from rare earth elements of the amphibolites from the Nellore–Khammam Schist Belt, South India

The Nellore–Khammam Schist Belt (NKSB) in South India is a Precambrian greenstone belt sited between the Eastern Ghats Mobile Belt (EGMB) to the east and the Cratonic region to the west. The belt contains amphibolites, granite gneisses and metasediments including banded iron formations. Amphibolites occurring as dykes, sills and lenses—in and around an Archaean layered complex—form the focus of the present study. The amphibolites are tholeiitic in composition and are compositionally similar to Fe-rich mafic rocks of greenstone belts elsewhere. The NKSB tholeiites show highly variable incompatible trace element abundances for similar Mg#s, relatively constant compatible element concentrations, and uniform incompatible element ratios. Chondrite-normalized REE patterns of the tholeiites range from strongly LREE depleted ((La/Yb)N = 0.19) to LREE enriched ((La/Yb)N = 6.95). Constant (La/Ce)N ratios but variable (La/Yb)N values are characteristic geochemical traits of the tholeiites; the latter has resulted in crossing REE patterns especially at the HREE segment. Even for the most LREE depleted samples, the (La/Ce)N ratios are > 1 and are similar to those of the LREE enriched samples. There is a systematic decrease in FeOt, K2O and P2O5, as well as Ce and other incompatible elements from the LREE enriched to the depleted samples without any variation in the incompatible element ratios and Mg#s. Neither batch and fractional melting, nor magma chamber processes can account for the non-correlation between the LREE enrichment and HREE concentrations. We suggest that dynamic melting of the upper mantle is responsible for these geochemical peculiarities of the NKSB tholeiites. Polybaric dynamic melting within a single mantle column with variable mineralogy is the likely mechanism for the derivation of NKSB tholeiitic melts. It is possible that the NKSB tholeiites are derived from a source with higher FeO/MgO than that of present day ridge basalts.

A new high background radiation area in the Geothermal region of Eastern Ghats Mobile Belt (EGMB) of Orissa, India

A high natural radiation zone is investigated for the first time in a geothermal region of Eastern Ghats Mobile Belt (EGMB) of Orissa state in India. The surrounding area comprises a geothermal region which has surveyed using a portable pulsed Geiger–Muller counter. On the basis of findings of GM counter, an area was marked as a high radiation zone. Soil and rock samples collected from the high radiation zone were analyzed by γ -ray spectrometry (GRS) using NaI(Tl) detector. The radioactivity is found to be contributed mainly by thorium. Concentration of thorium is reported to be very high compared to their normal abundance in crustal rocks. Further, concentrations of 238U and 40K are also high compared to normal abundance in crustal rocks but their magnitude is comparatively less than that of thorium. The average concentrations of 238U (i.e. U ( β – γ ) ), 232Th and 40K are found to be 33, 459 ppm and 3%, respectively, in soils and 312, 1723 ppm and 5%, respectively, in the granitic rocks. Maximum concentrations of 238U, 232Th and 40K are found to be 95, 1194 ppm and 4%, respectively, in soils and 1434, 10,590 ppm and 8%, respectively, in the granitic rocks. Radioactive element emits various energies in its decay chain. High energies are utilized to estimate the concentration of actual 238U, 232Th and 40K using a NaI(Tl) detector, however, low energies are used for the same in an HPGe detector. Some of the rock samples (eight in number) were also analyzed using HPGe detector for studying the behavior of low energies emitted in the decay series of uranium and thorium. The absorbed gamma dose rate in air and external annual dose rate of the high radiation zone are calculated to be 2431 nGy/h and 3.0 mSv/y, respectively. It is approximately 10 times greater than the dose rates obtained outside the high radiation zone. The high concentration of uranium and thorium may be one of the possible heat sources together with the normal geothermal gradient for hot springs present in the region.

Proterozoic mountain building in Peninsular India: an analysis based primarily on alkaline rock distribution

Peninsular India was assembled into a continental block c. 3 million km2 in area as a result of collisions throughout the length of a 4000 km long S-shaped mountain belt that was first recognized from the continuity of strike of highly deformed Proterozoic granulites and gneisses. More recently the recognition of a variety of tectonic indicators, including occurrences of ophiolitic slivers, Andean-margin type rocks, a collisional rift and a foreland basin, as well as many structural and isotopic age studies have helped to clarify the history of this Great Indian Proterozoic Fold Belt. We here complement those studies by considering the occurrence of deformed alkaline rocks and carbonatites (DARCs) in the Great Indian Proterozoic Fold Belt. One aim of this study is to test the recently published idea that DARCs result from the deformation of alkaline rocks and carbonatites (ARCs) originally intruded into intra-continental rifts and preserved on rifted continental margins. The suggestion is that ARCs from those margins are transformed into DARCs during continental, or arc–continental, collisions. If that idea is valid, DARCs lie on rifted continental margins and on coincident younger suture zones; they occur in places where ancient oceans have both opened and closed. Locating sutures within mountain belts has often proved difficult and has sometimes been controversial. If the new idea is valid, DARC distributions may help to reduce controversy. This paper concentrates on the Eastern Ghats Mobile Belt of Andhra Pradesh and Orissa, where alkaline rock occurrences are best known. Less complete information from Kerala, Tamil Nadu, Karnataka, West Bengal, Bihar and Rajasthan has enabled us to define a line of 47 unevenly distributed DARCs with individual outcrop lengths of between 30 m and 30 km that extends along the full 4000 km length of the Great Indian Proterozoic Fold Belt. Ocean opening along the rifted margins of the Archaean cratons of Peninsular India may have begun by c. 2.0 Ga and convergent plate margin phenomena have left records within the Great Indian Proterozoic Fold Belt and on the neighbouring cratons starting at c. 1.8 Ga. Final continental collisions were over by 0.55 Ga, perhaps having been completed at c. 0.75 Ga or at c. 1 Ga. Opening of an ocean at the Himalayan margin of India by c. 0.55 Ga removed an unknown length of the Great Indian Proterozoic Fold Belt. In the southernmost part of the Indian peninsula, a line of DARCs, interpreted here as marking a Great Indian Proterozoic Fold Belt suture, can be traced within the Southern Granulite Terrain almost to the Achankovil-Tenmala shear zone, which is interpreted as a strike-slip fault that also formed at c. 0.55 Ga.

Emplacement History of Pasupugallu Gabbro Pluton, Eastern Ghats Belt, India: a Structural Study

Structural mapping of the Pasupugallu pluton, an elliptical intrusive gabbro-anorthosite body, emplaced into the western contact zone between the Eastern Ghats Mobile Belt and the Archaean East Dharwar Craton, along the east coast of India, reveals concentric, helicoidal and inward dipping magmatic and/or tectonic foliations. We identify a <1 km-wide structural aureole characterized by pronounced deflection of regional structures into margin parallel direction, mylonitic foliations with S-C fabrics, sigmoidal clasts, moderately plunging stretching lineations, non-cylindrical intrafolial folds, and stretched elliptical mafic enclaves in the aureole rocks. Our results suggest that the pluton emplacement is syn-tectonic with respect to the regional ductile deformation associated with the terrane boundary shear zone at the western margin of the Eastern Ghats. We present a tectonic model for the emplacement of the pluton invoking shear-related ductile deformation, rotation and a minor component of lateral expansion of magma. The intrusive activity (1450-800 Ma) along the western margin of the Eastern Ghats can be correlated with the significant event of recurring mafic, alkaline and granitic magmatism throughout the global Grenvillian orogens associated with the continent-continent collision tectonics possibly related to the amalgamation and the breakup of the supercontinent Rodinia.

Petrogenesis of granitoid rocks at the northern margin of the Eastern Ghats Mobile Belt and evidence of syn-collisional magmatism

The northern margin of the Eastern Ghats Mobile belt against the Singhbhum craton exposes granitic rocks with enclaves from both the high-grade and low-grade belts. A shear cleavage developed in the boundary region is also observed in these granitoids. Field features and petrography indicate syn-tectonic emplacement of these granitoids. Petrology-mineralogy and geochemistry indicate that some of the granitoids are derived from the high-grade protoliths by dehydration melting. Others could have been derived from low-grade protoliths. Moreover, microstructural signatures in these granitoids attest to their syn-collisional emplacement.

Emplacement kinematics of nepheline syenites from the Terrane Boundary Shear Zone of the Eastern Ghats Mobile Belt, west of Khariar, NW Orissa: Evidence from meso- and microstructures

Nepheline syenite plutons emplaced within the Terrane Boundary Shear Zone of the Eastern Ghats Mobile Belt west of Khariar in northwestern Orissa are marked by a well-developed magmatic fabric including magmatic foliation, mineral lineations, folds and S-C fabrics. The minerals in the plutons, namely microcline, orthoclase, albite, nepheline, hornblende, biotite and aegirine show, by and large, well-developed crystal faces and lack undulose extinction and dynamic recrystallization, suggesting a magmatic origin. The magmatic fabric of the plutons is concordant with a solid-state strain fabric of the surrounding mylonites that developed due to noncoaxial strain along the Terrane Boundary Shear Zone during thrusting of the Eastern Ghats Mobile Belt over the Bastar Craton. However, a small fraction of the minerals, more commonly from the periphery of the plutons, is overprinted by a solid state strain fabric similar to that of the host rock. This fabric is manifested by discrete shear fractures, along which the feldspars are deformed into ribbons, have undergone dynamic recrystallization and show undulose extinction and myrmekitic growth. The shear fractures and the magmatic foliations are mutually parallel to the C-fabric of the host mylonites. Coexistence of concordant solid state strain fabric and magmatic fabric has been interpreted as a transitional feature from magmatic state to subsolidus deformation of the plutons, while the nepheline syenite magma was solidifying from a crystal-melt mush state under a noncoaxial strain. This suggests the emplacement of the plutons synkinematic to thrusting along the Terrane Boundary Shear Zone. The isotopic data by earlier workers suggest emplacement of nepheline syenite at 1500 +3/−4Ma, lending support for thrusting of the mobile belt over the craton around that time.

Geochronology and Tectonic Evolution of Karimnagar and Bhopalpatnam Granulite Belts, Central India

The Karimnagar Granulite Belt (KGB) and the Bhopalpatnam Granulite Belt (BGB) occur along both flanks of the Pranhita-Godavari (PG) rift basin. We present a state-of-the-art overview on the geochronological and tectonic aspects of these belts and surrounding geologic domains, and report new age data on zircon, monazite and uraninite recovered from granulite facies assemblages from KGB and BGB based on electron microprobe analyses (EPMA). Zircons from KGB charnockites show core ages of up to 3.1 Ga mantled by rims of 2.6 Ga. Zircons from BGB have 1.9 Ga cores mantled by 1.7 Ga rims. Zircons with core ages of 1.6 to 1.7 Ga in BGB rocks suggest new growth at this time. Monazites and uranitite from KGB show clear peaks with well-defined ages in the narrow range between 2.42±0.08 Ga and 2.47°0.03 Ga. Rims of monazite show mean age of 2.21±0.08 Ga. Monazites from BGB define sharp linear trend in PbO vs. ThO 2* diagram delineating a clear isochron with age of 1.59±0.03 Ga. Age data from KGB and BGB presented in this report negate current models linking these terrains to "Godavari Granulite Belt" and considering them as single and contemporaneous entity. The mid-Archaean to early Palaeoproterozoic signature recognized from KGB is totally missing in BGB. On the other hand, KGB rocks do not record any evidence for major Mesoproterozoic thermal regime. The two granulite belts shouldering the PG rift basin have therefore evolved in different times under distinct P-T conditions and thermal regimes. Our results have important implications in evaluating models of supercontinent assemblies, particularly the older assemblies of Ur, Columbia and Rodinia. While tectonothermal events in KGB broadly match with those of East Dharwar, we propose that BGB represents a 1.6 Ga collisional mobile belt between the Bastar and the Dharwar cratons. The 1.6 Ga collisional mobile belt at the southern margin of the Bastar craton was superposed by rift activity along the PG basin at 1.5 Ga. This sequence of events goes against the existence of a 3.0 Ga old contiguous assembly of Ur but closely matches with the history of accretion and break-up of the Columbia. Further, parts of the PG basin located away from the influence of the Eastern Ghats Mobile Belt, neither recorded any Grenville ages (1.0 Ga) corresponding to the Rodinia accretion nor late Pan-African ages (ca. 550 Ma) relating to the Gondwana amalgamation, indicating that the region did not witness any of these younger tectonic events.