East Gondwanaland Research Articles

The anatomy of the 750 Ma Bavali shear zone in South India: did the integration of India into East Gondwanaland initiate in the mid-Neoproterozoic?

The anatomy of the 750 Ma Bavali shear zone in South India: did the integration of India into East Gondwanaland initiate in the mid-Neoproterozoic?

Constraints on the expanse of Greater India in the Early Cretaceous from radiolarians

Defining the original size of Greater India is a critical parameter for reconstructing the kinematics and timing of the India–Asia collision. Early Cretaceous paleontology-based paleoceanographic and paleogeographic reconstruction for the northern coast of East Gondwanaland provides independent constraint for the extent of Greater India. In this study, newly discovered Early Cretaceous radiolarian faunas from the Tethys Himalaya were objectively compared with non-Tethyan austral faunas from the Argo Abyssal Plain (AAP) and the typical Tethyan fauna from Italy using the Hayashi's Quantification Method Type III. We found that (1) The occurrence of austral cold-water radiolarian species at the western coast of Australia and northern coast of Greater India is attributable to northward intrusion of circumantarctic cold water via a seaway newly opened during the breakup of East Gondwanaland from ∼136 Ma; (2) Gyangze was under shallow warm-water conditions with typical Tethyan radiolarian fauna (Cluster 3) until the late Valanginian when cold-water species (Cluster 1) arrived from the south. A water mass boundary shift from Tethyan species dominant to austral cold-water species dominant in Gyangze indicates its lower paleolatitude than Ocean Drilling Program (ODP) Leg 123, Site 765 in the AAP; (3) The same austral cold-water radiolarian-bearing intervals from the late Berriasian to Valanginian indicate that Kangma in southern Tibet was proximal to ODP Site 765; (4) Different radiolarian faunas in Italy–Zhongba, Kangma, and the AAP reveal complex water conditions under different degrees of effect by Tethyan warm water and austral cold water at different water depths during the Barremian to Aptian. This radiolarian biogeography is highly consistent with the paleomagnetic reconstruction and indicates that eastern Greater India extended to the Exmouth Plateau (>2400 km) during the Early Cretaceous.

Structure and chronology across the Achankovil terrain boundary shear zone system (South India), and its Madagascar connection in the Gondwanaland

In South India, the WNW-striking ~ 130-km-long Achankovil Shear Zone (AKSZ) separating the Trivandrum Block and the Madurai Block is speculated to continue within southern Madagascar in the East Gondwanaland. Two aspects continue to be debated, whether the AKSZ is a terrain boundary shear zone, and if so, when the accretion occurred. We present, for the first time, a detailed field-based analysis of contrasting mesoscale structures and deformation kinematics in the granulite facies rocks across the AKSZ. The mineralogical segregation layering (S2T) in the Trivandrum Block is sub-horizontal to gently inclined and describes F3T regional-scale open asymmetric WNW-trending sub-horizontal folds. By contrast, the S2M granulite facies layering in the Madurai Block describes regional-scale steeply plunging F3M folds. Both blocks share the NNW/NNE-trending sub-parallel sinistral and dextral S3 shears of the AKSZ. Deformation microstructures and garnet-bearing melt-hosted syn-S3 extensional shears attest to the persistence of high temperature during D2–D3 deformations. Monazite chemical dates and existing Pb–Pb zircon dates are overwhelmingly Pan-African in and across the AKSZ, but the Paleoproterozoic (2.1–1.8 Ga) dates in the Trivandrum Block, equivalent to those reported in Anosyan and Androyan domains of southern Madagascar, are lacking in the Madurai Block. By contrast, the Mid Neoproterozoic (800–750 Ma) Pb–Pb zircon dates in the Madurai Block, equivalent to the Antananarivo domain (Central Madagascar), are lacking in the Trivandrum Block. The structural–chronological contrasts between the two blocks suggest the AKSZ to be a Pan-African terrain boundary shear zone system that is continuous with the Ranotsara shear zone in Madagascar. [248]

Gondwana biostratigraphy and geology of West Bengal Basin, and its correlation with adjoining Gondwana basins of India and western Bangladesh

Integrated biostratigraphic and geological studies on the drilled exploratory boreholes of West Bengal Basin have revealed the presence of very thick distinctive Permo-Triassic Gondwana successions in the subsurface, and widely recognized across the basin below the latest Jurassic–Early Cretaceous Rajmahal Traps. Precise geological age of identified Gondwana lithounits of this basin is inferred on the basis of associated palynofloral assemblages along with their correlation with the adjoining Gondwana basins. In addition, two Gondwanic grabens are recognized below the pericratonic set-up whose presence in this basin, till now, are poorly understood. Detailed palynological studies on the Gondwana successions in the key boreholes of this basin enable recognition of 12 established palynological zones ranging from Early Permian (Asselian) to Middle Triassic (Ladinian). Lower Gondwana (Permian) palynofloras of this basin resemble earlier recorded palynofloras from the Talchir, Barakar and Raniganj formations of Indian Gondwana basins, suggesting the occurrence of well-developed above Lower Gondwana lithounits in this basin and the absence of Karharbari and Barren Measures formations which are marked by the non-depositional hiatuses. Upper Gondwana successions (Triassic) of the basin are represented by the Early and Middle Triassic palynofloras only that resemble Panchet and Supra-Panchet (Molangdighi Formation) palynofloral assemblages, and indicate the absence of Late Triassic succession of Upper Gondwana. Newly acquired geological and Bouguer anomaly data, and spatial distribution of Gondwana bearing boreholes across the basin suggest that the boreholes of CHK-A, GB-A, HRP-A, AMD-A, PLS-A and MNG-A fall in a well-defined NNE–SSW aligned graben, defined as the ‘Chandkuri–Palasi–Bogra Gondwanic Graben’ which includes Chandkuri and Palasi Gondwana basins in Indian part and Bogra Gondwana Basin in western Bangladesh where thick Gondwana successions are recognised in the subsurface sections of Singra (Singra-1X, Kuchma-X1, Bogra-X1), Jamalganj (EDH-1, 6) and Barapukuria (GDH-40, 43) coalfields. Gondwana bearing boreholes in the western margins of West Bengal Basin (GLS-A, GLS-B, GLS-C) represent a quite separate Gondwana basin of ‘Galsi’ which is located in the southern part of the N–S trending ‘Purnea–Rajmahal–Galsi Gondwanic Graben’. Latest Jurassic–Early Cretaceous Rajmahal Traps cap the Gondwana successions in the above Gondwana basins of West Bengal Basin along with Bogra and Rajmahal Gondwana basins. Rajmahal Traps and its infra- and intertrappean beds are exlcuded from the Gondwana cycle as these beds are marked by the Early Cretaceous marine dinoflagellate cysts, and the Rajmahal volcanism took place after a very long time gap from Late Triassic to Late Jurassic (ca. 78 my). Rajmahal Traps and its infra- and intertrappean beds indeed represent the post-Gondwana syn-rift sequence in West Bengal, Mahanadi and Rajmahal basins, and closely related with the break-up of Indian Plate from the East Gondwanaland during Tithonian (ca. 150 Ma). The post-trappean Late Cretaceous successions mark the initiation of passive-margin phase in this basin with the deposition of marine Bolpur and Ghatal formations.

The current status of orogenesis in the Central Indian Tectonic Zone: A view from its Southern Margin

The Central Indian Tectonic Zone, because of its critical position in the tectonic framework of East Gondwanaland, holds insights into our understanding of the assembly and dispersal of Columbia and Rodinia supercontinents through the growth of the Greater Indian Landmass. The tectonic zone, which is now a collage of different supracrustal and granulite‐gneiss belts, bears testimony to a multistage orogenic evolution from Late Palaeoproterozoic to Early Neoproterozoic time. In this contribution, the current status of orogenic evolution in the Central Indian Tectonic Zone has been reviewed using petrological and geochronological constraints from the Sausar Mobile Belt, its southernmost constituent. It reveals a three‐stage evolution leading to the growth of the Greater Indian Landmass: (a) c. 1.6–1.5‐Ga accretionary orogenesis produced the first nucleus of Greater India through the assembly of arc, back arc, and continental margins; (b) a phase of Middle Proterzoic extension led to the disintegration of the Proto‐Greater India; and (c) classical Himalayan‐style continental collision between 1.06 and 0.93 Ga finally stitched the North and South India blocks to produce the final configuration of the Greater Indian Landmass. It is argued that the Late Palaeoproterozoic to Early Mesoproterozoic orogenesis in India and former Gondwanaland and Laurentian fragments is a globally significant amalgamation event separate from the Columbia Supercontinent assembly.

Geochemistry and U–Pb SHRIMP zircon geochronology of microgranular enclaves and host granitoids from the South Khasi Hills of the Meghalaya Plateau, NE India: evidence of synchronous mafic–felsic magma mixing–fractionation and diffusion in a post-collision tectonic environment during the Pan-African orogenic cycle

Abstract Felsic magmatism in the South Khasi Hills of the Meghalaya Plateau, NE India, referred herein as the South Khasi granitoids (SKG: 519.5±9.7 Ma), invariably contains rounded to elongate, fine- to medium-grained, mafic to porphyritic microgranular enclaves (ME: 515±13 Ma) showing sharp to crenulate contacts with the host SKG. Compositions of plagioclase, amphibole and biotite in the ME are slightly distinct or similar to those of the host SKG, which appear re-equilibrated through diffusion mechanisms during partial liquid (semi-solid) conditions prior to the complete solidification of the mafic–felsic interacting system maximum at shallow continental crustal depths of approximately 9.5 km ( c. 250 MPa) under oxidizing conditions. Although the ME are chemically modified, both the ME and SKG exhibit a wide chemical variation as high-K 2 O metaluminous (I-type) granitoids. Linear to near-linear variations of chemical elements against SiO 2 may suggest the origin of the ME as the result of the mixing of crystal-charged mafic and felsic magmas in various proportions. However, the data scatter and ill-defined chemical variations can be attributed to chaotic chemical mixing, diffusion and, to some extent, mechanical sorting of the crystals. The identical trace element patterns of the ME and the respective SKG have strengthened the idea of chemical re-equilibration at varying levels between them through diffusion during synchronous mixing–fractionation and mingling. Mean zircon 207 Pb/ 206 Pb ages from the ME (515±13 Ma) and SKG (519.5±9.7 Ma) underline the co-existence of Cambrian mafic and felsic magmas formed during the later stages of the assembly of East Gondwanaland as an integral part of the Pan-Indian–African–Brasiliano orogenic cycle. The ME in SKG thus represent mingled, undercooled, heterogeneous hybrid magma globules formed by linear to chaotic mixing that was synchronous with fractionation of coeval crystal-charged mafic (enclave) and felsic (SKG) magmas, which experienced differential degrees of chemical exchange through diffusion with the surrounding felsic host in an open magma system.

Zircons traced from the 700–500 Ma Transgondwanan Supermountains and the Gamburtsev Subglacial Mountains to the Ordovician Lachlan Orogen, Cretaceous Ceduna Delta, and modern Channel Country, central-southern Australia

We test the hypothesis that the Transgondwanan Supermountains at the collision of East and West Gondwanaland were the provenance of a vast turbiditic fan that stretched alongside the East Gondwanaland margin to eastern Australia which, in turn, became the provenance of sediment shed into interior Australia to the Cretaceous Ceduna Delta in central-southern Australia and the modern Channel Country of central Australia. We employ an integrated analysis (U–Pb, Lu–Hf isotopes and trace elements) of detrital zircons in the Ceduna Delta and Channel Country. The main properties of the detrital zircons are U–Pb ages of 700–500 Ma (model ages TDMC 2.5–1.0 Ga; εHf +10 to −20) and 1300–1000Ma ages (TDMC 2.7–1.3Ga; εHf +4 to −17), in hosts of mafic granitoids with alkaline affinity. Zircons with these properties can be traced back through the drainage/paleo-slope to the intermediate provenances of the Ordovician turbidites and S-type granitoids of the Lachlan Orogen, then up-paleoslope to the primary or secondary provenance of the ancestral Gamburtsev Subglacial Mountains, and finally to the primary provenance of the Transgondwanan Supermountains atop the 700–500Ma East African–Antarctic Orogen. Another primary provenance, the 140–95Ma Whitsunday Volcanic Province/New Caledonia arc in northeastern Australia, also shed sediment across Australia to the Ceduna Delta.We suggest that the primary sediment from the 700–500Ma East African–Antarctic Orogen and the ancestral Gamburtsev Subglacial Mountains was shed into a deep-sea super-fan to (1) Ordovician turbidites in southeast Australia, recycled by melting of the turbidites to (2) 450MaS-type granites in the Lachlan Orogen, and (3) finally deposited, together with volcanogenic sediment from northeast Australia, in the Ceduna Delta. Zircons in the Channel Country and the Ceduna Delta have essentially the same properties, and indicate that the northeastern Australian provenance was largely unchanged over the past 100Ma.

Paleoproterozoic/Mesoproterozoic tectonism in the northern fringe of the Western Dharwar Craton (India): Its relevance to Gondwanaland and Columbia supercontinent reconstructions

AbstractA newly discovered suite of Paleoproterozoic/Mesoproterozoic amphibolite facies supracrustals and foliated granitoids between Pernem and Phonda along the northern Konkan coast of western India limits the northern boundary of the Archean Western Dharwar Craton (WDC). Top‐to‐the south transport of the suite is associated with progressively southward tightening of folds with down‐dip stretching lineation, obliteration of pre‐existing fabrics, and widespread stabilization of garnet (lacking in the WDC) along a mid‐crustal counterclockwise pressure‐temperature path. Th‐U‐total Pb ages in metamorphic monazites in the WDC supracrustals are 2.5 Ga; chemical ages of metamorphic monazites (2.3 Ga, 2.2 Ga, 1.9 Ga, and 1.7 Ga) in the WDC‐limiting Pernem‐Phonda tectonic zone decreases northwards. Based on the finding, we suggest that the southern boundary of the Central Indian Tectonic Zone (CITZ)—the accretion zone between the North and the South Indian Blocks—should be relocated ~ 550 km south from its perceived location near the Gulf of Cambay. In spite of the apparent geometric fit, we suggest the CITZ and the Trans North China Orogen may not be continuous as has been suggested in some Paleoproterozoic reconstructions of the Columbia supercontinent. The monazite ages in the footwall schists are similar to 2.5, 2.25, and 2.10 Ga Pb‐Pb ages in detrital zircon in the Sahantaha Group located north of the Paleoarchean Antongil‐Masora Block in Madagascar. We suggest that the Paleoproterozoic/Mesoproterozoic shear zone system extended westward into Madagascar north of the Antongil Block in East Gondwanaland, and this may clarify the unexplained source of zircon in the Sahantaha quartzites.

Meso/Neoarchean crustal domains along the north Konkan coast, western India: The Western Dharwar Craton and the Antongil-Masora Block (NE Madagascar) connection

Synthesis of mesoscopic structures and microscopic analyses of fabric superposition and deformation microstructures are combined with Th–U–Pb (total) ages in monazites to constrain the Meso/Neoarchean crustal domains in the Western Dharwar Craton (WDC) along the western coast of India; the domains are correlated with those in NE Madagascar to configure the assembly of crustal domains in the East Gondwanaland prior to the Mesozoic break up. In the WDC, para-schists and phyllites of the Shimoga schist belt, SSB (unmixed age components, 3067±26 and 3158±110Ma) and the Peninsular gneisses (3138±35Ma) are intruded by granitoid plutons (2924±50Ma) deformed at low-T. Five generations of deformation events are identified in the SSB and Peninsular gneisses. But the two early tectonic fabrics in Peninsular gneisses formed at anatectic amphibolite facies conditions are in sharp contrast to fabrics in SSB formed at greenschist facies. The two lithodemic units are inferred to share a pre-3.2Ga tectonic contact.By contrast, in the Goa schist belt (GSB), phyllites/schists (unmixed ages – 2436±34, 2543±66 and 2625±36Ma) and polymict conglomerates (unmixed ages – 2458±34 and 2566±53Ma) possess two generations of tectonic fabrics and are the youngest dated metamorphic unit in the WDC. The deformed Quepem granitoid pluton located along the axial zone of the regional-scale N-trending gently plunging upright folds in GSB yields comparable chemical age (2565±33Ma). Angular to sub-rounded and boulder to granule sized polymict clasts of Peninsular gneisses (3128±60Ma), foliated granitoids, amphibolites, BIFs and low-grade quartzite/chlorite-muscovite schists in GSB para-conglomerates suggest the sediments deposited in shallow basin were derived from neighboring sources, e.g. the Peninsular gneisses and SSB supracrustals. The Neoarchean deformation-metamorphism marks the closure of the Goa basin synchronous with the 2.4–2.6Ga accretion of the West and the East Dharwar Cratons.Based on the results of this study, the following correlation among Mesoarchean–Neoarchean crystallites in the Western Dharwar Craton and the Antongil-Masora Block, NE Madagascar (in italics below) appears realistic, e.g. 2.4–2.6Ga Goa schist belt≈Mananara Group; 2.5Ga Quepem granitoid≈Masoala Suite; 3.1Ga Shimoga schist belt≈Fenerivo Group; 3.1Ga Peninsular gneiss suite≈Nosy Boraha suite.

Geology of the Monapo Klippe, NE Mozambique and its significance for assembly of central Gondwana

The Monapo Klippe in north-east Mozambique is an ovoid-shaped outcrop measuring approximately 35×40km and is clearly visible on satellite and geophysical images. Based on recent field mapping, geochemical studies and new geochronological data, we present a revision of the lithostratigraphy of the klippe and offer a model for its origin and emplacement in the framework of regional tectonics. There are three main groups of rocks within the klippe: (1) the Metachéria Metamorphic Complex; (2) the Mazerapane Intrusive Suite; and (3) the Ramiane Intrusive Suite. The Metachéria Metamorphic Complex consists of a mélange of granulite gneiss, including mafic, felsic, pelitic and carbonate rocks, characterised by a strong penetrative shear fabric. The largely undeformed Mazerapane and Ramiane Suites have intruded into the Metachéria Metamorphic Complex. The Mazerapane Suite consists of foid-bearing ultramafic and mafic gneisses and intrudes into the western half of the complex, whereas the Ramiane Suite is dominated by alkaline granitic rocks, contains no foid-bearing units and intrudes into the eastern half of the complex. In addition to these three main units, there are a number of minor but structurally important units, the main ones of which include amphibolite-facies tonalitic gneisses and the Evate calcite carbonatite. Underlying all of these units is a narrow, high strain mylonite zone. Undeformed pegmatite bodies and dykes cross-cut all rock types of the Monapo Klippe including the marginal mylonite. Near identical dates for the intrusion of the Ramiane Suite at 637±5Ma and metamorphism of the Metachéria Complex at 634±8Ma indicates a major episode of granulite-facies metamorphism and crust generation at this time. The ∼635Ma age for the granulite-facies metamorphism is comparable to granulite-facies events identified in other parts of the East African Orogen in Tanzania, Madagascar and other parts of northern Mozambique to the north of the Lúrio Belt. The absence of granulite-facies rocks in the underlying Nampula Block is consistent with structural arguments that the Monapo Klippe is the remnant of an allochthonous thrust sheet. In this context, the Monapo Complex is very similar to other granulite-facies “klippe” in East Africa, Antarctica and Sri Lanka, lending support to the idea of a Pan-African mega-nappe formerly existing across greater East Gondwanaland.

Exotic crustal block accretion to the eastern Gondwanaland margin in the Late Cambrian–Tasmania, the Selwyn Block, and implications for the Cambrian–Silurian evolution of the Ross, Delamerian, and Lachlan orogens

Reconstructions of the Cambrian–Silurian tectonic evolution of eastern Gondwanaland, when the Australian Tasmanides and Antarctic Ross Orogen developed, rely on correlation between structural elements in SE Australia and Northern Victoria Land (NVL), Antarctica. A variety of published models exist but none completely solve the tectonic puzzle that is the Delamerian–Lachlan transition in the Tasmanides. This paper summarizes the understanding of Cambrian (Delamerian) to Silurian (Lachlan) geological evolution of the eastern Tasmanides, taking into account new deep seismic data that clarifies the geological connection between Victoria and Tasmania — the ‘Selwyn Block’ model. It evaluates previous attempts at correlation between NVL, Tasmania and Victoria, and presents a new scenario that encompasses the most robust correlations. Tasmania together with the Selwyn Block is reinterpreted as an exotic Proterozoic microcontinental block – ‘VanDieland’ – that collided into the east Gondwanaland margin south of western Victoria, and north of NVL in the Late Cambrian, perhaps terminating the Delamerian Orogeny in SE Australia. Subsequent north-east ‘tectonic escape’ of VanDieland in the Early Ordovician explains the present-day outboard position of Tasmania with respect to the rest of the Delamerian orogen, the origin of the hiatus that separates the Delamerian and Lachlan orogenic cycles in Australia, and how western Lachlan oceanic crust developed as a ‘trapped plate-segment’. The model establishes a new structural template for subsequent Lachlan Orogen development and Mesozoic Australia–Antarctica separation.

Mid-Carboniferous Centralian uplift linked by U–Pb zircon chronology to the onset of Australian glaciation and glacio-eustasy

In 1987 C. McA. Powell and J. J. Veevers postulated that the mid-Carboniferous uplift of the Centralian fold-and-thrust belt led to the growth of an ice sheet that covered much of Australia as it drifted from low to high latitudes. They presumed that the voluminous sediment shed from the final disruption and stripping to basement of the Centralian Superbasin was transported by ice streams out of the foreland basin to the edge of the ice sheet in the eastern and west-northwestern margins. Geochronological data accrued since 1987 confirm the 330–320 Ma synchronicity of uplift and exhumation in Centralia, deposition of glacial sediment on the margins and the onset of glacio-eustasy, as indicated by cyclothemic sedimentation patterns in the Viséan platform carbonates of the British Isles. These events are by-products of the 330–320 Ma merging of Gondwanaland and Laurussia in Pangaea that provided the global setting for glaciation. Probably more influential in initiating glaciation was the greater uplift in the Gamburtsev Subglacial Mountains of East Antarctica that became the focus of Permian post-glacial dispersal of sediment in East Gondwanaland.

Early Permian East Australian Rift System

The Early Permian to Middle Triassic Bowen and Gunnedah Basins in eastern Australia developed in response to a series of interplate and intraplate tectonic events that occurred to the east of the basin system. The initial event was extensional and stretched the continental crust to form part of the major Early Permian East Australian Rift System that occurred at least from far-north Queensland to southern New South Wales. The most important of the rift-related features, in a commercial sense, are a series of half-grabens that form the Denison Trough, now the site of several producing gasfields. The eastern part of the rift system commenced at about 305 Ma and was volcanic dominated. In contrast, the half-grabens in, and to the west of, the Bowen Basin were non-volcanic, and appear to have initiated significantly later, at about 285 Ma. These half-grabens are essentially north–south in length with an extension direction of approximately east-northeast. Mechanical extension appears to have ceased at about 280 Ma, when subsidence became driven by thermal relaxation of the lithosphere. The extension occurred in a backarc setting, in response to far-field stresses that propagated from the west-dipping subduction system at the convergent plate margin of East Gondwanaland that was located to the east of the East Australian Rift System.

Magnetic Evidence for Volcanism at Eastern Continental Margin of India: Juxtaposition with Elan Bank (Southern Indian Ocean)

The rifted Eastern Continental Margin of India (ECMI) has evolved as a result of breakup of East Gondwanaland. Previous geophysical studies of the continental margin have not elucidated upon its volcanic nature. Magnetics plays a useful role in the study of continental margins, particularly in identifying the volcanic units. The aeromagnetic map of the offshore Mahanadi basin of ECMI displays a conspicuous linear anomaly along the continental shelf. A comprehensive study of the published aeromagnetic, marine magnetic and gravity data of the offshore Mahanadi basin reveals the existence of a seaward dipping volcanic unit in the offshore Mahanadi basin bordering the Hinge zone. This inference suggests that the ECMI is a volcanic rifted margin. The study further indicates the deepening of the basement towards the sea. In addition, the existing geological studies on the ECMI demarcated the probable limit of the continental crust by studying the basement detached tectonic style of the sedimentation in sub-surface configuration of the East coast basins of India. The probable continental crustal limit, the Hinge zone, and the inner edge of the presently inferred volcanic unit conform to one another spatially in the offshore Mahanadi region. These features characterize the inferred volcanic body as seaward dipping reflectors (SDRs) that usually occur at the rifted continental margins. The deepening of the basement towards the sea and the presence of the volcanic body on the continental margin are indicative of the transitional nature of the crust. It is generally accepted that Antarctica and India were juxtaposed before the breakup of Gondwanaland. But the microcontinents in the southern Indian Ocean are neglected in the reconstruction of Gondwanaland continents. The recent studies of the discovery of continental crust within the Elan Bank (EB) microcontinent show that the EB was contiguous with the East coast of India before the breakup of Gondwanaland. Moreover, it is reported that the upper igneous crust of the EB consists of a 2–3 km thick layer of accumulated lava flows originating from the Kerguelen hotspot. An estimate shows that the total volume of volcanic and plutonic component of the Elan Bank is about 0.3 million cubic kilometers. The present inference of a volcanic body from the offshore Mahanadi basin is in agreement with the above observations of the juxtaposition of EB with ECMI.

Ultra high temperature-metamorphism and its significance in the Central Indian Tectonic Zone

In the present study from the southern margin of the Central Indian Tectonic Zone, it is demonstrated how the metamorphic P– T path of ultrahigh-temperature granulite terranes can be reconstructed using the metamorphic transition in corundum granulites from early biotite melting to later FMAS solid–solid reaction. The extreme metamorphism in these rocks caused two-stage biotite melting, resulting in initial porphyroblastic garnet 1 and later sapphirine–spinel 1 incongruent solid mineral assemblages. During this process, the leucocratic and melanocratic layers in the corundum granulites evolved from an initial silica-oversaturated to a later silica-undersaturated domain. In the melanocratic layer, this allowed localized concentration of sapphirine-spinel 1 and residual sillimanite 1, producing an extremely restitic assemblage, at the culmination of peak metamorphism, BM 1. BM 1 is constrained at ∼ 1000 °C at relatively deep crustal levels ( P ∼ 9 kbar) from the stability of ferroaugite in a co-metamorphosed Iron Formation granulite. During subsequent metamorphism (BM 2), the reaction path and history in the corundum granulites shifted to the restitic domain allowing reacting sapphirine, spinel 1 and sillimanite to produce coronal garnet 2–corundum assemblage via a FMAS univariant reaction. In the final stages of reaction history, biotite 2–sillimanite 2–spinel 2 assemblage was produced after garnet 2–corundum due to localized melt–crystal interaction. The metamorphic sequence, when interpreted with the help of a newly constructed, qualitative KFMASH petrogenetic grid, reveals successive stages of heating, increasing pressure and cooling around the KFMASH invariant point, [Opx,Crd], which is consistent with a counterclockwise metamorphic P– T path. The near isobaric nature of post-peak cooling (Δ T ∼ 250–300 °C) is also evident from multistage pyroxene exsolution and by the appearance of lamellar and coronal garnets in the Iron Formation granulites. This study provides the first tight constraint for ultrahigh- T metamorphism along a counter clockwise P– T trajectory in the Central Indian Tectonic zone, and has important bearing for terrane correlations in this part of East Gondwanaland. In addition, the new KFMASH grid allows evaluation of metamorphic phase relations in ultrahigh- T, corundum-bearing and corundum-absent aluminous granulites.

Characterization of the Mefjell plutonic complex from the Sør Rondane Mountains, East Antarctica: Implications for the petrogenesis of Pan‐African plutonic rocks of East Gondwanaland

Abstract The Mefjell plutonic complex consists of 500–550‐Ma Pan‐African plutonic rocks, which intrude into the Precambrian crystalline basement in the Sør Rondane Mountains, East Antarctica, and forms part of the Sør Rondane Suture Zone. The complex comprises syenitic and granitic (mostly monzogranitic) rocks, and is characterized by the presence of iron‐rich hydrous mafic minerals and primary ilmenite, both of which imply its formation at high temperature and under low oxygen fugacity conditions. The syenitic rocks are metaluminous, and are high in alkalis, K2O/Na2O, Al2O3, FeOt/(FeOt + MgO) (0.88–0.98), K/Rb (800–1000), Ga (18–28 p.p.m.), Zr (up to 2100 p.p.m.) and Ba. They also have a low Mg♯ (Mg/[Mg + Fe2+]), Rb, Sr, Nb, Y and F, low to moderate light rare earth element (LREE)/heavy rare earth element (HREE) ratios and positive Eu anomalies in their rare earth element (REE) patterns. The granitic rocks are metaluminous to peraluminous, and have a high Rb content, high Sr/Ba and LREE/HREE ratios, low K/Rb and negative Eu anomalies. Most of the syenitic and granitic rocks have Y/Nb ratios greater than 1.2, and are depleted in Nb, Ti and Sr on the primitive mantle‐normalized spider diagrams, indicating a crustal origin with subduction zone signatures. We interpret both the syenitic and granitic rocks to be derived from an iron‐rich lower crustal source by dehydration melting induced by the heat of mantle‐derived basaltic intrusion, after which they then underwent limited fractional crystallization. The Mefjell plutonic complex has a high Zr content and tectonic discrimination diagram signatures indicative of normal A‐type granitic rocks. Both rock suites may have been generated under the same postorogenic tectonic setting. The Mefjell syenitic rocks are chemically comparable to charnockites in the Gjelsvikjella and western Mühlig‐Hofmannfjella areas of East Antarctica, whereas the granitic rocks are comparable to aluminous A‐type granitic rocks in South India, which were emplaced during formation and evolution of the Gondwanaland supercontinent.

Opening of the Tethys in southwest China and its significance to the breakup of East Gondwanaland in late Paleozoic: Evidence from SHRIMP U-Pb zircon analyses for the Garzê ophiolite block

SHRIMP U-Pb zircon analyses for a gabbro sample from the Garze ophiolite block yielded a mean206Pb/238U age of 292 ±4 Ma, which indicated that the spreading time of the Garze-Litang Tethys was most likely at the earliest Permian. Combined with previous studies, we suggest that the opening of the Tethys in southwest China was derived from breakup of the East Gondwanaland in the late Paleozoic.

Opening of the Tethys in southwest China and its significance to the breakup of East Gondwanaland in late Paleozoic: Evidence from SHRIMP U-Pb zircon analyses for the Garz� ophiolite block

Opening of the Tethys in southwest China and its significance to the breakup of East Gondwanaland in late Paleozoic: Evidence from SHRIMP U-Pb zircon analyses for the Garz� ophiolite block

Rb‐Sr age and strontium isotope characteristics of the Greenland Group, Buller Terrane, New Zealand, and correlations at the East Gondwanaland margin

Rb‐Sr whole‐rock isochron ages of early Paleozoic, Greenland Group (Buller Terrane) metasediments from 17 localities range from 458 ± 9 to 271 ± 26 Ma, in a pattern similar to, but frequently 20–30 m.y. older than, corresponding K‐Ar ages of slates. The ages reflect an original low‐grade regional metamorphism at 450 ± 10 Ma in Late Ordovician times and subsequent cooling, which has then been pervasively overprinted by younger thermal events, most probably associated with the emplacement of Devonian‐Carboniferous granitoids. Initial 87Sr/86Sr ratios (i) at the time of metamorphism (t) increase from c. 0.717 at c. 450 Ma to c. 0.722 at c. 350 Ma—this evolution reflecting a bulk sediment Rb/Sr ratio of c. 1.0. The (t)‐(i) data suggest that the Greenland Group source area is extensive granitoid (mostly S‐type) and older metamorphic complexes, similar to the Wilson Terrane of the Ross and Adelaide Foldbelts of Antarctica and Australia. The New Zealand early Paleozoic data follow the (t)‐(i) trend of other late Precambrian to early Paleozoic metagreywacke sequences at the East Gondwanaland margin, in North Victoria Land and Marie Byrd Land, Antarctica, and Victoria and New South Wales, Australia. This indicates a large uniform source, providing well‐mixed detritus, and deposition at a passive continental margin. Preliminary Rb‐Sr ages from Cambrian‐Ordovician Takaka Terrane metasediments show no evidence for the Late Ordovician metamorphism recorded in the Buller Terrane, and reflect only post‐Devonian thermal overprints. However, their (t)‐(i) characteristics follow the Greenland Group trend (but along the less radiogenic part), and indicate a generally similar continent‐derived provenance.

Pan-African is Pan-Gondwanaland: Oblique convergence drives rotation during 650–500 Ma assembly

Gondwanaland was assembled by 650–570 Ma oblique subduction of ocean floor between cratons, including the Mozambique Ocean between West and East Gondwanaland. Oblique convergence set the cratons into a series of counterrotating cogs that sheared the intervening fold belts. The 550–490 Ma oblique subduction of paleo-Pacific ocean floor beneath Antarctica extended the system of counterrotating cogs and shear deformation to the Transantarctic Mountains and northern Australia. Terminal subduction was followed by epeirogenic uplift and concomitant stripping indicated by ubiquitous ca. 500 Ma K-Ar and apatite fission-track dates. These deep-seated events, recorded by zircons of 650–500 Ma age, and first noted in Africa (“Pan-African”), are now recognized as “Pan-Gondwanaland.”