Ophiolitic Melange Research Articles

GEOLOGICAL CONTRIBUTION TO THE TECTONO- STRATIGRAPHY OF THE NAFPLION AREA (NW ARGOLIS, GREECE)

The geological mapping in scale 1:5.000 in the greater Nafplion area indicated a Tertiary nappe stack of different Pelagonian-originated tectonic units structurally overlying the Subpelagonian series of Argolis Peninsula. The Subpelagonian series as lower unit is characterized by a shallow-water carbonate platform of Middle Triassic to Early Jurassic age, locally deep-water ammonitico-rosso facies and red cherts and is overlain by a tectono-sedimentary ophiolitic melange of Malm age. After the compressive tectonic phase of late Jurassic, the Nafplion area at that time records a severe extensional intra-Cretaceous syn-rift phase leading to the deposition of diachronous Meso-autochthonous Cretaceous limestone deposits rich in faulted-derived limestone breccias series, topped by deep-water limestone of Campanian-Maastrichtian and then from Lower Tertiary pelagic limestone facies passes upwards into post-Ypresian flysch. The different Pelagonian telescoped tectonic units were contemporaneously overthrusting northwestward, over the Subpelagonian post-Ypresian flysch sequence, during the Late Eocene compressive phase, are successively characterized by: (a) a middle tectonic unit of a flyschoidal melange of Late Cretaceous-Early Tertiary age, like the Adheres Melange surfaces in Southern Argolis, associated with various carbonate and ophiolite tectonosomes trapped and carried within this highly disrupted terrigenous flyschoidal melange and, (b) an upper unit consists of Cretaceous carbonate slivers bearing serpentinite sole (Palamidi, Akronafplia, Profitis Ilias, Aria) and/or of Middle Triassic-Early Jurassic carbonate platform slices. The above nappe stacking may be connected with the Eocene continental collision of the Hellenides.

NEW EARLY TUVALIAN (CARNIAN, TRIASSIC) RADIOLARIANS FROM THE HUǦLU-PINDOS SUCCESSION IN THE SORGUN OPHIOLITIC MÉLANGE, SOUTHERN TURKEY

Three new radiolarian genera ( Paramonocapnuchosphaera , Monocoronella and Minicrampus ) and 8 new species are described herein from one of the most diverse and best preserved Carnian (Late Triassic) radiolarian faunas of the world, from the early Tuvalian sequence of the Sorgun Ophiolitic Melange, southern Turkey. The radiolarian fauna belongs to the Spongotortilispinus moixi Zone (equivalent to Paragondolella postinclinata - Paragondolella noah conodont Zone). The very similar late Julian-early Tuvalian radiolarian faunas from the Northern Calcareous, Alps through the Lagonegro Basin, Sicily, Rhodes Island, Greece and farther east in the Antalya nappes, in the Mersin Melange and in Elbistan, Turkey may confirm that the Pindos-type succession found in different units of early Mesozoic oceanic basins formed in a single oceanic basin.

The formation mechanism of accretionary wedge at Karamay in West Junggar, NW China

Accretionary wedge is the typical product of subduction-zone processes at shallow depths. Determining the location, composition and mechanism of accretionary wedge has important implications for understanding the tectonism of plate subduction. The Central Asian Orogenic Belt (CAOB) is one of the world’s largest accretionary orogenic belts, and records the bulk evolution of Paleo-Asian Ocean from opening to closure, with multi-stages and multi-types of crust-mantle interaction in the Paleozoic. West Junggar (western part of Junggar Basin), located in the core area of CAOB, is characterized by a multiple intra-oceanic subduction system during the Paleozoic. In its eastern part crop out Devonian-Carboniferous marine sedimentary rocks, Darbut and Karamay ophiolitic melanges, alkali oceanic island basalts, island arc volcanic rocks and thrusted nappe structure. Such lithotectonic associations indicate the occurrence of accretionary wedge at Karamay. In order to decipher its formation mechanism, this paper presents a synthesis of petrography, structural geology and geochemistry of volcanic rocks. In combination with oceanic subduction channel processes, it is suggested that the accretionary wedge is a composite melange with multiple stages of formation. The application of oceanic subduction channel model to the Karamay accretionary wedge provides new insights into the accretion and orogenesis of CAOB.

FOREARC MANTLE METASOMATISM BY 11B-DEPLETED FLUIDS FROM A HIGHLY DEHYDRATED SLAB: A SNAPSHOT OF SLAB ROLL-BACK?

Accretionary orogens form along convergent plate margins due to the ongoing subduction of oceanic lithosphere, and comprise accretionary prisms, magmatic arcs, back-arc domains, ophiolitic melanges and possibly oceanic plateaus and continental fragments [Condie, 2007; Cawood et al., 2009]. Based on the dips and velocities of subducting slabs, accretionary orogens can be divided into retreating and advancing type, as exemplified by modern SW Pacific and Andes, respectively [Royden, 1993; Cawood et al., 2009].

DIACHRONOUS EVOLUTION OF BACK-ARC BASINS IN THE SOUTH TIANSHAN: INSIGHTS FROM STRUCTURAL, GEOCHRONOLOGICAL AND GEOCHEMICAL STUDIES OF THE WUWAMEN OPHIOLITE MÉLANGE

The South Tianshan is located to the north of the Tarim block and defines the southern margin of the Paleozoic Central Asian Orogenic Belt (CAOB). This study presents new structural data, geochronological and geochemical results for the Wuwamen ophiolite melange in the Chinese segment of the South Tianshan. In the south, the Wuwamen ophiolite melange shows typical block-in-matrix fabrics and occurs in the footwall of a south-dipping thrust fault, hanging wall of which is composed of weakly metamorphosed and deformed Lower Paleozoic marine to deep marine sequences from the South Tianshan. In the north, a southdipping thrust fault juxtaposes the Wuwamen ophiolite melange in its hanging wall against the high-grade and strongly deformed metasedimentary rocks from the Central Tianshan in its footwall.

Lithological Analysis of Nain Ophiolitic Zone Using ASTER Data

The Mesozoic ophiolitic Melange, north of Nain in the Central-East Iran Microplate (CEIM) comprises serpentinized ultramafic rocks, harzburgites, dunite, gabbro, peridotite, pelagic limestone and other carbonate rocks. The excellent and vast exposure of this desert region is well suited for geologic mapping of this rock suite using remote sensing, especially using data from the satellite-borne advanced Space borne Thermal Emission and Reflection Radiometer (ASTER) imaging system which was designed for mapping mineral information. In this study, data processing methods like Method Minimum noise fraction (MNF), Feature Oriented Principal Components Selection (FPCS), Band Ratios (BR) and Optimum Index Factor (OIF) were used to process ASTER data to optimize the mapping of ophiolite rock types. For example, a simple color composites of OIF (Red: B3, Green: B4, and Blue: B8) and Band ratios (e.g. Red: (B2 + B4)/B3, Green: (B5 + B7)/B6, Blue: (B7 + B9)/B8) were useful for discriminating serpentinite, meta-basalt and granite rock types. It is concluded here that proposed ASTER data has the potential for mapping similar ophiolites elsewhere using the global archive of ASTER imagery.

The tauride ophiolites of Anatolia (Turkey): A review

The Tauride ophiolites lie on the northern and southern flanks of an E-W-trending Tauride carbonate platform. They mainly consist of three tectonic units namely in ascending order, ophiolitic melange, sub-ophiolitic metamorphic sole and oceanic lithospheric remnants. They were generated above intra-oceanic subduction zones and emplaced over the Tauride carbonate platform from different Neotethyan oceanic basins in the Late Cretaceous. Tauride ophiolites from west to east are described and reviewed. All are underlain by well-preserved dynamothermal metamorphic soles of varied structural thicknesses up to 500 m that have a constant structural position between ophiolitic melange below and harzburgitic mantle tectonites above and display typical inverted metamorphic sequences from amphibolite facies above to greenschist facies below. The metamorphic soles are shown to have evolved during the initiation of subduction and emplacement processes. In the Pozanti- Karsanti area the contact between the metamorphic sole and the overlying serpentinized harzburgites is characterized by a 1.5–2-m-thick zone of sheared serpentinized harzburgitic mantle intercalated with amphibolites and cut by thick mafic dykes (7–8 m) which postdate intraoceanic metamorphism and high-temperature ductile deformation. This contact is interpreted as an intra-oceanic decoupling surface along which volcanics from the upper levels of the down-going plate were metamorphosed to amphibolite facies and accreted to the base of the hanging wall plate. The metamorphic soles and overlying ophiolitic rocks were intruded by numerous isolated post-metamorphic diabase dykes filled by island arc tholeiitic magma. Subduction initiation and roll-back processes best explain the structural and petrological relationships of Late Cretaceous ophiolite genesis, metamorphic sole formation and subsequent dyke emplacement of the Tauride ophiolites.

Sediment melting during subduction initiation: Geochronological and geochemical evidence from the Darutso high‐Mg andesites within ophiolite melange, central Tibet

AbstractIn addition to fluids, the concept of sediment‐derived melts infiltrating the fore‐arc mantle during subduction initiation has been proposed based on studies of modern subduction zones and ophiolite mélange. However, outcrops that contain the products of such melts are rare, especially in conjunction with boninite. New U‐Pb zircon dating reveals that the Darutso volcanic rocks (DVRs) within ophiolitic mélange in the Beila area, central Tibet, crystallized at ∼164–162 Ma. This is the first recognition of Jurassic volcanic rocks in the middle section of the Bangong‐Nujiang Suture Zone. Geochemically, the DVRs are high‐Mg andesites with moderate SiO2 (59.03–63.62 wt %) and high MgO (3.74–6.53 wt %), Cr (up to 395 ppm), and Mg# (50.3–67.9). They also have high Th contents, (La/Sm)N ratios, and (87Sr/86Sr)i values (0.7085–0.7147); low Ba/Th, U/Th, and Sr/Y ratios; and negative values of εNd(t) (−8.7 to −9.8) and zircon εHf(t) (−7.4 to −9.9). The εNd(t) values of the DVRs overlap those of regional sediments. Detailed analyses of these geochemical characteristics indicate that the DVRs were derived from partial melting of subducted sediments and subsequent interaction with overlying mantle peridotite in a shallow and hot setting. In combination with the regional geology, in particular adjacent ophiolites that contain MORB‐like and boninite mafic lavas, these rocks collectively recorded the evolution of a fore‐arc setting during the initiation of the northward subduction of the south branch of the Bangong‐Nujiang Ocean. Therefore, the results provide direct evidence for sediment melting during subduction initiation and constrain the Jurassic tectonic evolution of the Lhasa terrane.

Genesis of oceanic island basalt in ophiolitic mélange from core area of Central Asian Orogenic Belt

The Central Asian Orogenic Belt (CAOB) is one of the largest orogens in the world and comprises island arcs, seamounts, accretionary wedges, oceanic plateaux and microcontinents accreted during the closure of the Paleo-Asian Ocean which is a long-lived ocean that existed for ~800 Ma from the late Mesoproterozoic to the Mesozoic. The CAOB extends from the Urals in the west, through Kazakhstan, northern China, and Mongolia to the eastern Russian coast. The core area, which extends from central Kazakhstan to Xinjiang (northwestern China), are divided into three main geological domains. To the northeast, (1) the Altai range is composed by Early and Late Palaeozoic units that were accreted and docked to the Siberian margin and affected by high-grade metamorphism. To the south, the convergence between the Tarim Block and several micro continents such as Yili and Central Tianshan formed the (2) Palaeozoic Tianshan Orogen. The central and northwestern parts of Central Asia display a horseshoe shape that can be followed from North Tianshan to West Junggar around the Balkash Lake area. This megastructure is termed the (3) Kazakh Orocline. Until now so many studies focused on this area. However, the architecture and orogenic processes of the CAOB are still hotly debated. Remnants of oceanic island basalts (OIB) in association with ophiolites have been identified in several accretionary complexes in core area of CAOB. The nature of occurrence, geochemical characteristics of these OIBs and ophiolites has been taken to indicate extensive crust-mantle cycling magmatism. Therefore, in this paper, we provide an overview of the salient features of twelve OIB-hosting with age of Late Neoproterozoic to Early Carboniferous in core area of CAOB, and distinguish rock association of typical seamount from ophiolitic melanges, including pillow basalt, volcanic breccia, reef limestone, olistostromes rocks, and terrestrial clastic rocks. Combined with unique structural features of seamount, we preliminary reconstruct stratigraphic framework of paleoseamount. Geochemically, the volcanic rocks from paleoseamount are mainly alkaline basalts with litte tholeiitic basalts. The alkaline basalts are characterized by LREE enrichment and HREE depletion, and no obvious Nb, Ta and Ti negative anomalies, suggesting typical OIB affinity. The Nd, Sr and Pb isotopic features indicate that the magma source have enriched components and show fluid metasomatism. In short, this are consistent with geochemical features of Louisville, Ontong Java and Hawaiian seamounts which are related to mantle plume, but not develop high magnesium picrite and komatitic basalt. The OIB in core area of CAOB are at least two possible sources: (1) seamount fragments were scraped and retained in the accretionary wedge; (2) partial melting of mantle metasomatism rocks. However, we can not rule out the possibility of partial melting of subducted oceanic crust and terrigenous sediments. Therefore, the source of OIB in core area of CAOB are very complex and show crust-mantle interaction. This is similar to mantle plume activity and seamounts evolution in modern ocean.

Basin Analysis of the Albian–Santonian Xigaze Forearc, Lazi Region, South-Central Tibet

Abstract The Xigaze forearc basin records the evolution of the southern Lhasa terrane convergent margin, largely affected by Neo-Tethyan subduction processes, prior to the Paleocene Tethyan Himalaya–Eurasia collision. New geologic mapping and U-Pb detrital-zircon geochronologic data from the Lazi region, 340 km southeast of Lhasa, show that forearc basin sedimentation began ca. 110 Ma conformably atop the Yarlung–Tsangpo ophiolitic melange. By this time, the arc–trench system along the southern margin of the Lhasa terrane (Eurasia) consisted of an accretionary complex, overlying ophiolitic melange, the Xigaze forearc basin, and the Gangdese magmatic arc. There is no geological evidence in the Lazi region for more than one subduction zone between the southern Lhasa terrane margin and India. Sedimentological facies analysis from Albian to Santonian clastic and carbonate sedimentary rocks preserved in the Xigaze forearc basin indicate deep-marine sedimentation characterized by hemipelagic carbonate and volcanogenic sediment-gravity-flow deposits. Sandstone modal petrographic and U-Pb detrital-zircon geochronologic data reveal Asian continental margin, Gangdese magmatic arc, and central to northern Lhasa terrane provenance. During this time, basin fill was deposited in cycles of high and low sediment flux characterized by alternating successions of clastic turbidite inner- to outer-fan deposits and hemipelagic limestone and marlstone sequences. Along-strike differences in the timing of initial forearc basin sedimentation are likely the result of intra-basin topography and/or diachronous development of ophiolitic forearc basement.

The Liuqu Conglomerate, southern Tibet: Early Miocene basin development related to deformation within the Great Counter Thrust system

The Liuqu basin formed during the early Miocene between ophiolitic melanges (to the south) and uplifted Cretaceous forearc deposits (to the north) along a central, 150-km-long segment of the India-Asia suture zone in southern Tibet. Sedimentological analysis shows the Liuqu Conglomerate to be composed of mixed fluvial and sediment-gravity flow lithofacies assemblages locally interbedded with mature paleosols. We interpret the Liuqu Conglomerate as coarse-grained fluvial and alluvial-fan deposits. Structural analysis indicates that the Liuqu Conglomerate was deposited in a contractional setting. Paleocurrent and provenance data demonstrate that sediment was transported north-northwest from the hanging wall of a coeval thrust fault system along the southern limit of Liuqu outcrops. Detrital zircon U-Pb ages (and Hf[ t ] isotope ratios) cluster around 80–110 Ma (eHf[t] = −23.5–14.6), 120–135 Ma (eHf[t] = −12.6–13.1), 150–170 Ma (eHf[t] = −14.1–14.7), 500–600 Ma (eHf[t] = −26–3.4), and 1100–1200 Ma (eHf[t] = −27.6–2.9), requiring input from both Gangdese and melange sources. Asian zircons were recycled northward after being incorporated into accretionary melanges along the southern Asian margin prior to India-Asia collision. The age of the Liuqu Conglomerate is still somewhat uncertain, but new chronologic data, including biotite 40Ar/39Ar data, detrital zircon fission-track analyses, and δ13C compositions of soil carbonates, all converge on ca. 20–19 Ma as the most probable age. Together, these results indicate that part of the north-to-south sediment transport system that existed prior to India-Asia collision and into the Eocene was reversed by ca. 20 Ma. The Liuqu Conglomerate may represent deposits associated with the paleo–Yarlung River.

Tracing of hydrothermal ore forming process in the Sorkhvand manganese deposit, Kermanshah Province, Iran

The Sorkhvand manganese deposit is located in the Zagros thrust belt, close to the Kermanshah ophiolite melange. This ophiolite records the early stage of Neo-Tethys evolution in the western part of Tethyan realm. In the study area, Mn-mineralization occurs as irregular lenses or interlayers with radiolarian cherts within the Cretaceous radiolarite-bearing mudstones. The present work is an attempt in order to characterize the geochemical process involved in manganese mineralization. Mineralogically and texturally, the Sorkhvand manganese deposit is analogous to that of Tethyan manganese occurrences, comprising three generation of Mn-ores including syngenetic (pyrolusite), diagenetic (braunite-pyrolusite), and late-stage epigenetic mineralization (pyrolusite). The significant geochemical characteristics such as high Mn content (6.35–67.64 wt%; average 33.4 wt%), low concentration of Fe (0.18 to 8.4 wt%; average 2.26 wt%), high Ba (14–3445 ppm, average 1038.3 ppm), LREE > HREE, and negative Ce anomalies (Ce/Ce* = 0.58–1.02; average 0.74) with a negative Eu anomalies (Eu/Eu* = 0.48–0.91, average 0.67) reveal a primary distal hydrothermal source for mineralization. Also, the prevailing of oxic conditions (V/V + Ni = 0.11–0.52), absence of sulfide minerals coupled with relative enrichment of some bioessential elements (e.g., Co) suggest that manganese segregation from hydrothermal ore-bearing solutions may be related to microbial activity influenced by bacterially mediated reactions.

Structure and Petrology of the Nagaland-Manipur Hill Ophiolite Melange Zone, NE India: A Fossil Tethyan Subduction Channel at the India – Burma Plate Boundary

Structure and Petrology of the Nagaland-Manipur Hill Ophiolite Melange Zone, NE India: A Fossil Tethyan Subduction Channel at the India – Burma Plate Boundary

Structure and Petrology of the Nagaland-Manipur Hill Ophiolite Melange Zone, NE India: A Fossil Tethyan Subduction Channel at the India – Burma Plate Boundary

Structure and Petrology of the Nagaland-Manipur Hill Ophiolite Melange Zone, NE India: A Fossil Tethyan Subduction Channel at the India – Burma Plate Boundary

Structure and petrology of the Nagaland-Manipur Hill Ophiolitic Mélange zone, NE India: A Fossil Tethyan Subduction Channel at the India – Burma Plate Boundary

The Nagaland-Manipur Hill ophiolite belt in NE India represents the southern extension of the Neotethyan Yarlung-Zhangbo suture zone in Southern Tibet, and connects this on-land exposure of the late Mesozoic collision front in the north with a modern trench-arc system in the Andaman Sea region in the south. Ophiolitic subunits in the Nagaland-Manipur Hill area in the Indo-Myanmar Ranges occur as blocks or thrust sheets within a melange with a serpentinite or fine-grained greywacke matrix, and are spatially associated with eclogitic and blueschist rock assemblages. This ophiolitic melange zone is tectonically sandwiched between an older (Triassic - Cretaceous) accretionary prism complex (Nimi Flysch) to the east and a younger (Late Cretaceous -Miocene) accretionary wedge (Disang Flysch) to the west. The Nagaland-Manipur Hill ophiolitic melange is thus part of a progressively westward migrated subduction-accretion complex, and it represents a typical subduction channel melange evolved during the fast subduction of the Neotethyan oceanic lithosphere beneath Asia - Sundaland. The exhumation of this melange was facilitated by return flow within a subduction channel at the plate interface between the downgoing Indian continental margin, subducting at a shallow angle during the Eocene, and the Triassic - Early Cretaceous accretionary prism of the overlying Burma plate. The Upper Eocene - Miocene Pokhpur sedimentary strata uncomformably overlying the ophiolitic melange and the accretionary prism metasedimentary rocks constitute a wedge-top or slope-basin sequence, rather than a postcollisional molasse deposit. The occurrence of ophiolitic subunits in the Nagaland-Manipur Hill area represents an accretionary-type ophiolite, derived mainly from the downgoing plate, and does not characterize a Penrose-type, complete ophiolite sequence.

A newly found Paleozoic Japan-type subduction–accretion system

Accretionary orogenic belts are complicated collages composed of magmatic arcs, accretionary complexes, ophiolitic melanges, fore-arc/back-arc basins and microcontinents formed at convergent plate margins, which are key sites of continental growth through earth history [1]. Arcs are important orogenic components of accretionary orogenesis, but the types of active margin with old continental basement and intraoceanic arcs without continental basement and their roles in accretionary orogenesis are different. The Japan-type arc is a special kind of arc rifted from active margin, a special intraoceanic arc with continental basement that has been neglected in the anatomy of accretionary orogeny [2]. It is noted that the Qilian orogenic belt has been considered to be developed from a Japan-type or west Pacific-type-arc–back-arc system [3]. The Central Asian Orogenic Belt, one of the largest accretionary orogenic systems in the world, is characterized by the appearance of various kinds of magmatic arcs and associated accretionary complexes that are formed diachronously from ca. 1000 Ma to ca. 250 Ma [4, 5], although some authors consider that the final collision occurred as late as 230–220 Ma [6]. Such long-evolved orogenic belts have been found elsewhere in China indicated by accurate geochronologic research, such as the Tongbai orogen [7] and the Trans-North China orogen [8, 9]. The Beishan orogenic collage, which connects the Tianshan orogenic belt to the west and the Inner Mongolia– Solonker suture to the east [4], is a key area for understanding the accretionary tectonics of the southern Central Asian Orogenic Belt. Extensive metamorphic complexes exist in the central segment of the Beishan, which are pivotal for understanding the subduction–accretion processes of the Beishan orogen. However, the timing and tectonic attribute for metamorphic terranes from the Beishan orogenic collage is highly ambiguous. Up to date, there has been no detailed study on the metamorphic complexes and associated volcanic rocks, which has hampered man’s understanding of the architecture of the Beishan orogen. The Tectonics Research Group led by Wenjiao Xiao from the Tethys Research Center, Institute of Geology and Geophysics, Chinese Academy of Sciences, have carried out detailed studies from field mapping and geochronology for four cross sections on the metamorphic complexes and volcanic rocks from the central segment of Beishan [10] (Fig. S1). Field mapping shows that all the metamorphic complexes have undergone polyphase ductile deformation (Fig. S2), with mainly NNE-dipping foliations, while the volcanic rocks are weakly deformed and the primary pyroclastic structures are preserved [10]. High-resolution geochronological data reveal that the ages of the protolith of the metamorphic complexes range from *465 to *400 Ma, but were not formed in the Precambrian as previously regarded, which probably represent the deeper parts of arcs or accretionary complexes. The volcanic rocks are formed at 446–441 Ma, with Precambrian detrital zircon ages [10]. The age information from these rocks may indicate continental materials in the generation of the magmatic arcs. However, it excludes the possibility that large-scale crystallized basement exists in the Beishan orogenic collage. Combined the temporal and spatial distribution of ophiolites and fore-arc complex in this area, they proposed that during the Paleozoic there was a JapanElectronic supplementary material The online version of this article (doi:10.1007/s11434-015-0956-z) contains supplementary material, which is available to authorized users.

Oldest Paleo-Tethyan ophiolitic mélange in the Tibetan Plateau

An early Paleozoic ophiolitic melange has recently been documented in the west Gangma Co area, north-central Tibetan Plateau. It is composed of serpentinite, isotropic and cumulate gabbros, basalt, and plagiogranite. Whole-rock geochemical data suggest that these rocks were formed in an oceanic-ridge setting, and the depletion in Nb and Ti indicates their suprasubduction zone nature. Furthermore, whole-rock e Nd ( t ) (+3.5 to +10.6) and zircon e Hf ( t ) values (+11.4 to +14.5) suggest that these rocks were derived from a long-term depleted mantle source. These geochemical features, combined with the rock types and their field relationships, suggest the rocks represent an ophiolite suite. Zircon U-Pb dating of isotropic gabbros and plagiogranites yielded weighted mean ages ranging from 437 to 501 Ma, which makes the west Gangma Co ophiolitic melange the oldest Paleo-Tethys ophiolitic melange in the Tibetan Plateau. The main Paleo–Tethys Ocean basin probably opened in the Middle Cambrian; it continued to grow throughout the Paleozoic and closed in the later Triassic. The Paleo–Tethys Ocean was formed by the breakup of the northern margin of Gondwana, with southward subduction of the proto-Tethys oceanic lithosphere along the northern margin of the supercontinent.

Syn- to post-orogenic exhumation of metamorphic nappes: Structure and thermobarometry of the western Attic-Cycladic metamorphic complex (Lavrion, Greece)

The Lavrion peninsula is located along the western boundary of the Attic-Cycladic metamorphic complex in the internal zone of the Hellenic orogenic belt. The nappe stack is well exposed and made, from top to bottom, of (i) a non-metamorphic upper unit composed of an ophiolitic melange, (ii) a middle unit mainly composed of the Lavrion schists in blueschist facies, (iii) and a basal unit mainly composed of the Kamariza schists affected by pervasive retrogression of the blueschist facies metamorphism in greenschist facies. The middle unit is characterized by a relatively steep-dipping foliation associated with isoclinal folds of weakly organized axial orientation. This foliation is transposed into a shallow-dipping foliation bearing a N-S trending lineation. The degree of transposition increases with structural depth and is particularly marked at the transition from the middle to the basal unit across a low-angle mylonitic to cataclastic detachment. The blueschist facies foliation of the Lavrion schists (middle unit) is underlined by high pressure phengite intergrown with chlorite. The Kamariza schists (basal unit) contains relics of the blueschist mineral paragenesis but is dominated by intermediate pressure phengite also intergrown with chlorite and locally with biotite. Electron probe micro-analyzer chemical mapping combined with inverse thermodynamic modeling (local multi-equilibrium) reveals distinct pressure–temperature conditions of crystallization of phengite and chlorite assemblages as a function of their structural, microstructural and microtextural positions. The middle unit is characterized by two metamorphic conditions grading from high pressure (M1, 9–13kbar) to lower pressure (M2, 6–9kbar) at a constant temperature of ca. 315°C. The basal unit has preserved a first set of HP/LT conditions (M1–2, 8–11kbar, 300°C) partially to totally transposed-retrogressed into a lower pressure mineral assemblage (M3, 5–8.5kbar) associated with a slight but significant increase in temperature (∼350°C).These structural, petrologic and thermobarometric data document the tectonic evolution from construction to destruction of the Hellenic orogenic belt. The steep-dipping blueschist facies foliation, preferentially preserved at high structural level in the middle unit, is considered to reflect tectonic accretion marked by successive burial (D1M1) and syn-orogenic exhumation (D2M2) without thermal relaxation. The transposition-retrogression of the blueschist facies mineral assemblage into a shallow-dipping greenschist facies foliation associated with an increase in temperature corresponds to lateral flow of the thermally relaxed nappe stack (D3M3). The development of a low-angle detachment, accommodating post-orogenic exhumation of the orogenic root, is attributed to gravitational collapse of the Hellenic belt.

Geochemistry of volcanic and subvolcanic rocks and biostratigraphy on radiolarian cherts from the Almopias ophiolites and Paikon Unit (Western Vardar, Greece)

The Almopias and Paikon “sub-zones” in northern Greece represent the easternmost part of the Vardar Domain. Volcanic sequences in the Paikon sub- zone largely consist of calc-alkaline volcanic rocks with very minor normal-type (N-) mid-ocean ridge basalts (MORB) and boninites. These data confirm that the Paikon volcanics have originated in a volcanic arc setting developed during the Middle to Late Jurassic times on the westernmost continental margin of the European plate. The Almopias “sub-zone” consists of several ophiolite-bearing tectonic units showing a complex combination of different metamorphic, litho-stratigraphical, and age characteristics. The Liki, Nea Zoi, and Vrissi units (central Almopias) are ophiolitic melanges, the Mavrolakkos and Krania units (eastern Almopias) consist of volcanic sequences, and the Ano Garefi unit (eastern Almopias) includes serpentinized peridotites and overlying basaltic lavas. The Liki unit underwent metamorphism and deformation under amphibolite-facies condition and includes rocks showing N-MORB and low-K island arc tholeiitic affinities. The Nea Zoi unit incorporates N-MORBs of unknown age and Late Triassic calc-alkaline rocks. The Vrissi unit includes an incomplete volcanic sequence consisting of MORB-type volcanics linked with and crosscut by Middle-Late Jurassic arc-type volcanic rocks and dykes. The Mavrolakkos unit consists of sheeted dykes, lavas, and individual dykes with N-MORB affinity, which are crosscut by dykes showing volcanic arc affinity. The Krania unit consists of Middle-Late Jurassic N-MORB series tectonically associated with calc-alkaline volcanics and dykes. Basalts of the Ano Garefi unit have alkaline affinity, whereas dikes cutting the peridotites show enriched-type MORB chemistry. Our data show the occurrence in the Almopias units of Middle-Late Jurassic N-MORB lavas associated with overlying coeval volcanic arc-type lavas and/or intruded by volcanic arc-type dykes. A major point arising from this paper is that some of the Almopias ophiolitic units do not correlate with the other ophiolitic units in the Dinaride-Hellenide belt and some could correlate with the northernmost extension of the Vardar zone in the South Apuseni - Transyl- vanian ophiolitic belt in Romania.

Rare earth elements and stable isotope geochemistry of carbonates from the mélange zone of Manipur ophiolitic Complex, Indo-Myanmar Orogenic Belt, Northeast India

Carbonates that occur in the melange zone of Manipur ophiolitic complex (MOC), Indo-Myanmar Orogenic Belt, Northeast India contain diverse fauna with dominance of foraminifera assemblages (planktonic and benthic) and calcareous nanofossils. Rare earth element (REE) contents (~51 ppm average value) in the MOC carbonates are high compared with the average value of typical marine carbonate (~28 ppm). The Post-Archean Australian Shale (PAAS)-normalized REE patterns of these carbonates exhibit seawater-like REE patterns with light rare earth elements (LREE) depletion and relative heavy rare earth elements (HREE) enrichment with negative Ce anomalies (Ce/Ce* = 0.28–0.69) and positive Eu anomalies (Eu/Eu* = 1.07–2.08). The carbonates show depleted values of δ13C ‰ (PDB) (0.89–2.74 ‰) which characterize marine precipitates while depletion in δ18O ‰ (PDB), reaching minimum values (−6.29 to −11.40 %). The negative anomalies of Ce in these carbonates suggest that they were deposited under oxygenated environment. The observed shale-normalized positive Eu anomalies and spread in negative δ18O values to a lesser extent of δ13C ‰ (PDB) values of these carbonates suggest their formation were altered by diagenesis in shallow marine environment. Field, petrographical studies in conjunction with REE and stable isotope characteristics suggest that these carbonates form part of the ophiolitic melange zone that emplaced during subduction and obduction processes of the Indian plate and Myanmar plate collision.