The Eastern Ghats Mobile Belt,India: A Collage of Juxtaposed Terranes (?)

Examination of Landsat TM images, reconnaissance field traverses and the published geological maps from the Eastern Ghats Mobile Belt (EGMB), India, reveal a network of major ductile shear zones both within and at the margins. These shear zones are characterized by mylonitic foliation, grain size reduction, metamorphic retrogression, stretching lineations and distinct signatures of alkaline, anorthositic and granitic magmatism. These shear zones divide the EGMB into distinct terranes, which are heterogeneously deformed with extensive tracts of foliated mylonitic gneisses and ultramylonites. The main gneissic foliation in all the terranes is refolded in near non-coaxial manner generally about the axis subparallel to the elongation of the terrane.Structural history in each terrane is distinct in the orientation of stretching lineations, attitude of gneissosity and early fold axial planes, lithological assemblages and available geochronological data. It is possible to recognise nine large terranes within the EGMB and the characteristics of each terrane have been described. The terrane distribution in the EGMB could well fit a thrust tectonic, allochthonous model of amalgamation and accretion. Different terranes could be different thrust nappes or allochthonous tectonic sheets representing tectono-stratigraphic terranes.     

P–T conditions of the cratonic rocks and Eastern Ghats granulites along the Eastern Ghats Frontal Thrust,Jeypore (Orissa), India

The Eastern Ghats Frontal Thrust (EGFT) demarcates the boundary between the Archaean/Paleoproterozoic cratonic rocks to the west, and the Meso/Neoproterozoic granulites of the Eastern Ghats Mobile Belt (EGMB) to the east. At Jeypore (Orissa, India), mafic schists and granites of the cratonic domain document a spatial increase in the metamorphic grade from greenschist facies (garnet, clinozoisite – absent varieties) in the foreland to amphibolite facies (clinozoisite- and garnet-bearing variants) progressively closer to the EGFT. Across the EGFT, the enderbite–charnockite gneisses and mafic granulites of EGMB preserves a high-grade granulite facies history; amphibolite facies overprinting in the enderbite–charnockite gneisses at the cratonic fringe is restricted to multi-layered growth of progressively Al, Ti – poor hornblende at the expense of pyroxene and plagioclase. In associated mafic granulites, the granulite facies gneissic layering is truncated by sub-centimeter wide shear bands defined by synkinematic hornblende + quartz intergrowth, with post-kinematic garnet stabilized at the expense of hornblende and plagioclase. Proximal to the contact, these granulites of the Eastern Ghats rocks are intruded by dolerite dykes. In the metadolerites, the igneous assemblage of pyroxene–plagioclase is replaced by intergrown hornblende + quartz ± calcite that define the thrust-related fabric and are in turn mantled by coronal garnet overgrowth, while scapolite is stabilized at the expense of recrystallized plagioclase and calcite. Petrogenetic grid considerations and thermobarometry of the metamorphic assemblages in metadolerites intrusive into granulites and mafic schists within the craton confirm that the rocks across the EGFT experienced prograde heating (T_max value ∼650–700 °C at P ∼ 6–8 kbar) along the prograde arm of a seemingly clockwise P–T path. Since the dolerites were emplaced post-dating the granulite facies metamorphism, the prograde heating is correlated with renewed metamorphism of the granulites proximal to the EGFT. A review of available age data from rocks neighboring the EGFT suggests that the prograde heating of the cratonic granites and the re-heating of the Eastern Ghats granulites are Pan – African in age. The re-heating may relate to an Early Paleozoic Pan-Gondwanic crustal amalgamation of older terrains or reactivation along an old suture.     

Intrusion age,geochemistry and metamorphic conditions of a quartz-monzosyenite intrusion at the craton–Eastern Ghats Belt contact near Jojuru,India

The boundary between the Archean cratons and the Eastern Ghats Belt in peninsular India represents a rifted Mesoproterozoic continental margin which was overprinted by a Pan-African collisional event associated with the westward thrusting of the Eastern Ghats granulites over the cratonic foreland. The contact zone contains a number of deformed and metamorphosed nepheline syenite complexes of rift-related geochemical affinities. In addition to the nepheline-bearing rocks, metamorphosed quartz-bearing monzosyenitic bodies can also be identified along the suture in the region between the Godavari-Pranhita graben and the Prakasam Igneous Province. One such occurrence at Jojuru near Kondapalle is geochemically comparable to the nepheline syenites and furnishes a weighted mean concordant U–Th–Pb SHRIMP zircon age of 1263 ± 23 Ma (2σ), which provides a lower age bracket for the rift-related magmatic activity. The original igneous mineral assemblage in the monzosyenite was partially replaced by the formation of coronitic garnet during the Pan-African metamorphism of the rocks. P–T estimates of garnet corona formation at the interface between clinopyroxene–orthopyroxene–ilmenite clusters and plagioclase indicate mid to upper amphibolite facies condition (5.5–7.0 kbar and 600–700 °C) during the thrust induced deformation and metamorphism associated with the Pan-African collisional tectonics.     

Petrotectonic framework of granulites from northern part of Chilka Lake area,Eastern Ghats Belt,India: Compressional vis-à-vis transpressional tectonics

Granulite-facies rocks occurring north-east of the Chilka Lake anothosite (Balugan Massif) show a complex metamorphic and deformation history. The M₁–D₁ stage is identified only through microscopic study by the presence of S₁ internal foliation shown by the M₁ assemblage sillimanite–quartz–plagioclase–biotite within garnet porphyroblasts of the aluminous granulites and this fabric is obliterated in outcrop to map-scale by subsequent deformations. S₂ fabric was developed at peak metamorphic condition (M₂–D₂) and is shown by gneissic banding present in all lithological units. S₃ fabric was developed due to D₃ deformation and it is tectonically transposed parallel to S₂ regionally except at the hinge zone of the F₃ folds. The transposed S₂/S₃ fabric is the regional characteristic structure of the area. The D₄ event produced open upright F₄ folds, but was weak enough to develop any penetrative foliation in the rocks except few spaced cleavages that developed in the quartzite/garnet–sillimanite gneiss. Petrological data suggest that the M₄–D₄ stage actually witnessed reactivation of the lower crust by late distinct tectonothermal event. Presence of transposed S₂/S₃ fabric within the anorthosite arguably suggests that the pluton was emplaced before or during the M₃–D₃ event. Field-based large-scale structural analyses and microfabric analyses of the granulites reveal that this terrain has been evolved through superposed folding events with two broadly perpendicular compression directions without any conclusive evidence for transpressional tectonics as argued by earlier workers. Tectonothermal history of these granulites spanning in Neoproterozoic time period is dominated by compressional tectonics with associated metamorphism at deep crust.     

Mesoproterozoic arc magmatism in SE India: Petrology,zircon U–Pb geochronology and Hf isotopes of the Bopudi felsic suite from Eastern Ghats Belt

The southern segment of the Eastern Ghats Mobile Belt (EGMB) in India was an active convergent margin during Mesoproterozoic, prior to the final collision in Neoproterozoic during the assembly of the Rodinia supercontinent. Here we present mineralogical, whole-rock geochemical, zircon U–Pb and Hf isotopic data from a granitoid suite in the Bopudi region in the EGGB. The granitoid complex comprises quartz monzodiorite with small stocks of rapakivi granites. The monzodiorite, locally porphyritic, contains K-feldspar megacrysts, plagioclase, quartz, biotite and ortho-amphibole. The presence of mantled ovoid megacrysts of alkali feldspar embaying early-formed quartz, and the presence of two generations of the phenocrystic phases in the rapakivi granites indicate features typical of rapakivi granites. The K-feldspar phenocrysts in the rapakivi granite are mantled by medium-grained aggregates of microcline (Ab₇ Or₉₃), which is compositionally equivalent to the rim of Kfs phenocryst and Pl (An_23–24 Ab₇₅). The geochemistry of both the granitoids shows arc-like features for REE and trace elements. LA-ICP-MS zircon analyses reveal ²⁰⁷Pb/²⁰⁶Pb ages of 1582 (MSWD = 1.4) for the rapakivi granite 1605 ± 3 Ma (MSWD = 3.9) for the monzodiorite. The zircons from all the granitoid samples show high REE contents, prominent HREE enrichment and a conspicuous negative Eu anomaly, suggesting a common melt source. The zircons from the monzodiorite have a limited variation in initial ¹⁷⁶Hf/¹⁷⁷Hf ratios of 0.28171–0.28188, with εHf(t) values of −2.2 to +2.8. Correspondingly, their two-stage Hf isotope model ages (T_DM2) ranging from 2.15 to 2.47 Ga probably suggest a mixed source for the magma involving melting of the Paleoproterozoic basement and injection of subduction-related juvenile magmas. The prominent Mesoproterozoic ages of these granitoids suggest subduction-related arc magmatism in a convergent margin setting associated with the amalgamation of the Columbia-derived fragments within the Neoproterozoic Rodinia assembly.     

Deformation Characteristics and Tectonic Evolution of the Eastern Qinling Orogenic Belt

The Qinling Mountains separating the northern from the southern China plate is a key region for the study of structural evolution of eastern Asia. It is composed of the Palaeozoic fold belt in its northern part and the Variscan and Indosinian fold belts in its southern part. The evolution of the former is marked by the closure of a northward subducting oceanic basin in the early stage, followed by southward obduction of ophiolites and intracontinental thrusting during the Variscan; whereas that of the latter is represented by intracontinental, shallow crustal deformation on the basis of a large-scale detachment structure(with a horizontal slip of at least of 100 km). Since the late Palaeozoic, however, both of the belts have been cut by a series of east-west sinistral strike-slip faults.     

Comment on: ‘High-temperature dehydration melting and decompressive P–T path in a granulite complex from the Eastern Ghats India’ by S. Bhattacharya and R. Kar

Thermodynamic and chemographic modelling of complex reaction textures observed in Mg-Al-rich pelitic granulites is an important tool to unravel the P–T evolutionary history of high-grade rocks. In the Eastern Ghats Belt, India, several studies have been carried out on these fascinating aluminous granulites, and the results of these studies have revealed complex P–T–t histories (Dasgupta and Sengupta 1995; Sengupta et al. 1999; Rickers et al. 2001a, 2001b; Gupta et al. 1999; Dobmeier and Simmat 2002; Dobmeier and Raith 2003). In recent communication, Bhattacharya and Kar (2002) reported reaction textures from a suite of Mg-Al granulites from the Paderu area of the Eastern Ghats Belt. Combining the textural relations and thermodynamic calibration of some construed reactions, the authors have put forward a single phase metamorphic evolution of the area along a clockwise pressure–temperature trajectory. Combining the petrological features from the Paderu area with those reported from the Chilka Lake complex, the authors proposed a general tectonic model for the entire Eastern Ghats Belt. Incidentally, the rocks in and around Paderu have been studied in some detail by several other workers (Lal et al. 1987; Mohan et al. 1997; Sengupta et al. 1997). The purpose of this comment is to demonstrate that the conclusions made in the paper are inconsistent with the petrological features described in the text. Further, the thermodynamic treatment used in the paper has serious errors in many places, and hence, is often in complete disagreement with the existing experimental data and theoretical analyses on the Mg-Al-rich assemblages. There are also significant problems arising from the poor quality of the analytical database. Unfortunately, the authors cite only a few published works (mostly their own) ignoring many other relevant studies from this belt (cited above). Our observations are organised according to the sections of the paper.Editorial responsibility: T.L. Grove     

Relationship between the TanLu Fault and Dabie Orogenic Belt in Eastern China

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Petrological evolution of silica-undersaturated sapphirine-bearing granulite in the Paleoproterozoic Salvador–Curaçá Belt,Bahia, Brazil

The Salvador–Curaçá Belt, located in São Francisco Craton, Brazil, was subjected to granulite facies metamorphism during the Paleoproterozoic orogeny (c. 2.0 Ga). Well preserved in enclaves of silica-undersaturated sapphirine-bearing granulite occur in a charnockite outcrop located along a kilometric-scale shear zone. The sapphirine-bearing granulite preserves domains with distinct mineral assemblages that record interactions between melt and peritectic phases (orthopyroxene₁ + spinel₁ + biotite₁). Sapphirine was crystallized in the Si-poor cores of the enclaves, sillimanite and spinel–cordierite symplectites in the intermediate Si-rich domains between cores and margins, and garnet and quartz-bearing cordierite/biotite symplectites in Si-rich margins of the enclaves. Melt-rock interactions and metamorphism occurred at ultrahigh temperatures of 900–950 °C at 7.0–8.0 kbar pressures. The mineralogical evolution of the domains reflects not only the influence of changes in bulk composition in the equilibrium volume of the reactions but also P–T changes during orogeny evolution. Electron microprobe dating of monazite both in the sapphirine-bearing granulite and charnockite indicates UHT metamorphism timing at c. 2.08–2.05 Ga that is related to global Paleoproterozoic UHT metamorphic events that occurred during the Columbia supercontinent assembly.     

The Niassa Gold Belt,northern Mozambique – A segment of a continental-scale Pan-African gold-bearing structure?

The Niassa Gold Belt, in northernmost Mozambique, is hosted in the Txitonga Group, a Neoproterozoic rift sequence overlying Paleoproterozoic crust of the Congo–Tanzania Craton and deformed during the Pan-African Orogeny. The Txitonga Group is made up of greenschist-facies greywacke and schist and is characterized by bimodal, mainly mafic, magmatism. A zircon U–Pb age for a felsic volcanite dates deposition of the sequence at 714 ± 17 Ma. Gold is mined artisanally from alluvial deposits and primary chalcopyrite-pyrite-bearing quartz veins containing up to 19 ppm Au have been analyzed. In the Cagurué and M’Papa gold fields, dominantly N–S trending quartz veins, hosted in metagabbro and schist, are regarded as tension gashes related to regional strike-slip NE–SW-trending Pan-African shear zones. These gold deposits have been classified as mesozonal and metamorphic in origin. Re–Os isotopic data on sulfides suggest two periods of gold deposition for the Cagurué Gold Field. A coarse-crystalline pyrite–chalcopyrite assemblage yields an imprecise Pan-African age of 483 ± 72 Ma, dating deposition of the quartz veins. Remobilization of early-formed sulfides, particularly chalcopyrite, took place at 112 ± 14 Ma, during Lower Cretaceous Gondwana dispersal. The ～483 Ma assemblage yields a chondritic initial ¹⁸⁷Os/¹⁸⁸Os ratio of 0.123 ± 0.058. This implies a juvenile source for the ore fluids, possibly involving the hosting Neoproterozoic metagabbro. The Niassa Gold Belt is situated at the eastern end of a SW–NE trending continental-scale lineament defined by the Mwembeshi Shear Zone and the southern end of a NW–SE trending lineament defined by the Rukwa Shear Zone. We offer a review of gold deposits in Zambia and Tanzania associated with these polyphase lineaments and speculate on their interrelation.     

The Neotectonic Movements of the Changle-Zhao''an Fault Belt in Fujian

The Changle-Zhao' an fault belt is an important belt bordering the southeast coast of China. It wasformed in pre-Mesozoic time and has undergone polyphase activities of different modes ever since. Through astudy of the data concerning the Cenozoic strata, volcanism. seismicity, hydrothermal activity and crustaldeformation, the characteristics of the neotectonic movement of this fault belt were preliminarily ascertained,the amplitudes and rates of recent faulting in some places were calculated, and the intensities of the activities invarious structural basins along the fault zone were compared. Besides, this paper also discusses the main causefor the origin of the Quanzhou-Shantou strong earthquake tectonic zone.     

Feldspar Lead Isotopic Composition of Granitoids from the Eastern Qinling Orogenic Belt and Their Tectonic Significance

This paper reports 48 feldspar lead isotope analyses from 27 granitic intrusions,which formed from the Late Proterozic to Mesozoic within the Eastern Qinling oregenic belt. Itis found that the granitic rocks of South Qinling are characterized by a strong block-effect anddepletion in U-Pb and Th-Pb, showing that these rocks came from the same lead isotopetectono-geochemical province, while those of North Qinling are characterized by higher U-Pband Th-Pb for Late Proterozoic to Early Paleozoic ones and lower U-Pb and Th-Pb forLate-Palaeozoic and younger ones in their feldspar lead isotopic composition. In the NorthQinling block, lead isotopic signatures reflect that the source of granitic magma had changedsince the Late Palaeozoic. Comparison of feldspar lead isotopic composition between SouthQinling and North Qinling shows that there is marked difference in lead isotopic compositionfor pre-Palaeozoic granitoids, indicating that the South Qinling and the North Qinling blocksbelong to different tectonic units, but the similarities in lead isotopic composition are quiteclear, which indicates that the South Qinling block had been welded with the North Qinlingblock and that the magma sources of both blocks were identical. The analysis provides directevidence for underplating of the continental crust of South Qinling beneath the North Qinlingblock in the continent-continent interaction stage of the Eastern Qinling oregenic belt.     

The link between partial melting,granitization and granulite development in central Ribeira Fold Belt,SE Brazil: New evidence from elemental and Sr–Nd isotopic geochemistry

Elemental and Sr–Nd isotopic data on metatexites, diatexites, orthogneisses and charnockites from the central Ribeira Fold Belt indicate that they are LILE-enriched weakly peraluminous granodiorites. Harker and Th–Hf–La correlation trends suggest that these rocks represent a co-genetic sequence, whereas variations on CaO, MnO, Y and HREE for charnockites can be explained by garnet consumption during granulitic metamorphism.Similar REE patterns and isotopic results of ?_Nd⁵⁶⁵ = ?5.4 to ?7.3 and ⁸⁷Sr/⁸⁶Sr₅₆₅ = 0.706–0.711 for metatexites, diatexites, orthogneisses and charnockites, as well as similar T_DM ages between 2.0 and 1.5 Ga are consistent with evolution from a relatively homogeneous and enriched common crustal (metasedimentary) protolith.Results suggest a genetic link between metatexites, diatexites, orthogneisses and charnockites and a two-step process for charnockite development: (a) generation of the hydrated igneous protoliths by anatexis of metasedimentary rocks; (b) continuous high-grade metamorphism that transformed the “S-type granitoids” (leucosomes and diatexites) into orthogneisses and, as metamorphism and dehydration progressed, into charnockites.     

The West Spitsbergen Fold Belt: The result of Late Cretaceous-Palaeocene Greenland-Svalbard convergence?

The West Spitsbergen Fold Belt, together with the Eurekan structures of northern Greenland and Ellesmere Island, are suggested to be the result of Late Cretaceous-Palaeocene intracontinental compressional tectonics. The Late Palaeozoic –Mesozoic rocks of western Spitsbergen are characterized by near-foreland deformation with ramp-flat, top-to-the east thrust trajectories, whereas structurally higher nappes involving Caledonian complexes are typified by more listric thrusts and mylonite zones. A minimum of 40 km of shortening is estimated for the northern part of the West Spitsbergen Fold Belt. The axial trends in the West Spitsbergen and the North Greenland Eurekan fold belts parallel the principal fault zones which accommodated the separation of Greenland and Svalbard after Chron 25/24. In northern Greenland, north directed Eurekan thrusts associated with mylonites and cleavage formation represent at least 10 km of shortening. Between 50 and 100 km of shortening is estimated for the markedly arcuate Eurekan Fold Belt of Ellesmere Island, but the principal tectonic transport is eastwards. Kinematic reconstructions suggest that Svalbard was linked to North America before the opening of the Eurasian Basin and Norwegian — Greenland Sea. In the Late Cretaceous — Palaeocene interval, the relative motion between Greenland and North America was convergent across the Greenland — Svalbard margin, giving rise to the West Spitsbergen Fold Belt and the Eurekan structures of North Greenland.     

Isotope Geochemistry of the Xinchang -Yongjia Silver (Lead-Zinc)Ore Belt in Eastern Zhejiang Province

In the Xinchang-Yongjia silver (lead-zinc) ore belt, there mainly occur the large to medium-sized Haoshi, Bamao, Dalingkou and Wubu silver deposits or silver-bearing lead-zinc deposits. On the basis of researches on these typical deposits, the mechanism of leaching-drawing mineralization of Mesozoic geothermal water and the related model are put forward in this paper in the light of the time interval between rock and formation ages as well as hydrogen, oxygen, sulphur and lead isotope geochemical characteristics. The major metallogenic process occurred in volcanic rock layers. The ore-forming fluids are geothermal water coming from meteoric water and circulating at shallow layers. This geothermal water leached and absorbed ore-forming materials from its country rocks during its flowing (such metallogenic elements as silver, lead-zinc and sulphur mainly came from consolidated volcanic rocks), leading to the formation of meso - epithermal silver deposits.     

Zircon geochronology of the Koraput alkaline complex: Insights from combined geochemical and U–Pb–Hf isotope analyses,and implications for the timing of alkaline magmatism in the Eastern Ghats Belt,India

Zircon formation and modification during magmatic crystallization and high-grade metamorphism are explored using TIMS and LA-ICP-MS U–Pb geochronology, Lu–Hf isotope chemistry, trace element analysis and textural clues on zircons from the Koraput alkaline intrusion, Eastern Ghats Belt (EGB), India. The zircon host-rock is a granulite-facies nepheline syenite gneiss with an exceptionally low Zr concentration, prohibiting early magmatic Zr saturation. With zircon formation occurring at a late stage of advanced magmatic cooling, significant amounts of Zr were incorporated into biotite, nearly the only other Zr-bearing phase in the nepheline syenite gneisses. Investigated zircons experienced a multi-stage history of magmatic and metamorphic zircon growth with repeated solid-state recrystallization and partial dissolution–precipitation. These processes are recorded by complex patterns of internal zircon structures and a wide range of apparently concordant U–Pb ages between 869 ± 7 Ma and 690 ± 1 Ma. The oldest ages are interpreted to represent the timing of the emplacement of the Koraput alkaline complex, which significantly postdates the intrusion ages of most of the alkaline intrusion in the western EGB. However, Hf model ages of TDM = 1.5 to 1.0 Ga suggest an earlier separation of the nepheline syenite magma from its depleted mantle source, overlapping with the widespread Mesoproterozoic, rift-related alkaline magmatism in the EGB. Zircons yielding ages younger than 860 Ma have most probably experienced partial resetting of their U–Pb ages during repeated and variable recrystallization events. Consistent youngest LA-ICP-MS and CA-TIMS U–Pb ages of 700–690 Ma reflect a final pulse of high-grade metamorphism in the Koraput area and underline the recurrence of considerable orogenic activity in the western EGB during the Neoproterozoic. Within the nepheline syenite gneisses this final high-grade metamorphic event caused biotite breakdown, releasing sufficient Zr for local saturation and new subsolidus zircon growth along the biotite grain boundaries.     

Tectonic Nature of the Northern Segment of the Jiao-Liao-Ji Belt: A Rift or Continental Collision Belt?

<正>Objective The tectonic characteristics and evolution of the Paleoproterozoic Jiao-Liao-Ji belt have been extensively studied in recent decades(Fig.1 a).Two main models have been proposed for the formation of this belt:a continental-or arc-continent collisional belt,and the opening and closure of an intra-continental rift.The main reasons for these ongoing debates are own to the complex composition,including metamorphosed volcano-sedimentary rocks,multiple pulses of granitic magmatism,meta-mafic intrusions,and tectonometamorphic history.In addition,earlier work focused on the geochronology and metamorphic evolution,whereas the     

The Relationship between Granitic Rock and Skarn Fe Deposit: A Case Study from the Chengchao Fe Deposit, Edong Ore District, Middle–Lower Yangtze River Valley Metallogenic Belt, Eastern China

Please?refer?to?the?attachment(s)?for?more?details.     

Contrasting Peridotite Types in the Dabie UHP Belt, Eastern China: the Raobazhai and Bixiling Ultramafic-mafic Complex

The Dabie UHP metamorphic belt, central China,contains two contrasting types of mafic-ultramafic complex. The Bixiling peridotite in the southern Dabie terrane contains abundant garnet (21.1-32.2 vol% )and thus has high CaO + Al2O3 (9.81-15.9 wt% ).