Ophiolitic Melange Research Articles

Geochemistry of lamprophyre dykes, Wadi Sikait area, South Eastern Desert, Egypt

The Wadi Sikait area lies at about 95 km southwest of Marsa Alam City along the Red Sea Coast, Eastern Desert, Egypt. It is occupied by Precambrian rocks of ophiolitic melange, metamorphosed sandstones (MSS), gabbros and monzogranites which were later intruded by lamprophyre dykes and quartz veins.

Palaeozoic tectonic evolution of the Tianshan belt, NW China

The Chinese Tianshan belt is a major part of the southern Central Asian Orogenic Belt, extending westward to Kyrgyzstan and Kazakhstan. Its Paleozoic tectonic evolution, crucial for understanding the amalgamation of Central Asia, comprises two stages of subduction-collision. The first collisional stage built the Eo-Tianshan Mountains, before a Visean unconformity, in which all structures are verging north. It implied a southward subduction of the Central Tianshan Ocean beneath the Tarim ac-tive margin, that induced the Ordovician-Early Devonian Central Tianshan arc, to the south of which the South Tianshan back-arc basin opened. During the Late Devonian, the closure of this ocean led to a collision between Central Tianshan arc and the Kazakhstan-Yili-North Tianshan Block, and subsequently closure of the South Tianhan back-arc basin, producing two su-ture zones, namely the Central Tianshan and South Tianshan suture zones where ophiolitic melanges and HP metamorphic rocks were emplaced northward. The second stage included the Late Devonian-Carboniferous southward subduction of North Tianshan Ocean beneath the Eo-Tianshan active margin, underlined by the Yili-North Tianshan arc, leading to the collision between the Kazakhstan-Yili-NTS plate and an inferred Junggar Block at Late Carboniferous-Early Permian time. The North Tianshan Suture Zone underlines likely the last oceanic closure of Central Asia Orogenic Belt; all the oceanic domains were consumed before the Middle Permian. The amalgamated units were affected by a Permian major wrenching, dextral in the Tianshan. The correlation with the Kazakh and Kyrgyz Tianshan is clarified. The Kyrgyz South Tianshan is equivalent to the whole part of Chinese Tianshan (CTS and STS) located to the south of Narat Fault and Main Tianshan Shear Zone; the so-called Middle Tianshan thins out toward the east. The South Tianshan Suture of Kyrgyzstan correlates with the Central Tianshan Suture of Chinese Tianshan. The evolution of this southern domain remains similar from east (Gangou area) to west until the Talas-Ferghana Fault, which reflects the convergence history between the Kazakhstan and Tarim blocks.

EARLY JURASSIC TO EARLY LATE CRETACEOUS RADIOLARIANS FROM THE SANTA ROSA ACCRETIONARY COMPLEX (NORTHWESTERN COSTA RICA)

In the circum-Pacific ophiolitic belts; when no other biogenic constituents are found; radiolarians have the potential to provide significant biostratigraphic information. The Santa Rosa Accretionary Complex; which crops out in several half-windows (Carrizal; Sitio Santa Rosa; Bahia Nancite; Playa Naranjo) along the south shores of the Santa Elena Peninsula in northwestern Costa Rica; is one of these little-known ophiolitic melanges. It contains various oceanic assemblages of alkaline basalt; radiolarite and polymictic breccias. The radiolarian biochronology presented in this work is mainly based by correlation on the biozonations of Carter et al. (2010); Baumgartner et al. (1995b); and O’Dogherty (1994) and indicate an Early Jurassic to early Late Cretaceous (early Pliensbachian to earliest Turonian) age for the sediments associated with oceanic basalts or recovered from blocks in breccias or megabreccias. The 19 illustrated assemblages from the Carrizal tectonic window and Sitio Santa Rosa contain in total 162 species belonging to 65 genera. The nomenclature of tectonic units is the one presented by (Baumgartner and Denyer; 2006). This study brings to light the Early Jurassic age of a succession of radiolarite; which was previously thought to be of Cretaceous age; intruded by alkaline basalts sills (Unit 3). The presence of Early Jurassic large reworked blocks in a polymictic megabreccia; firstly reported by De Wever et al. (1985) is confirmed (Unit 4). Therefore; the alkaline basalt associated with the radiolarites of these two units (and maybe also Units 5 and 8) could be of Jurassic age. In the Carrizal tectonic window; Middle to early Late Jurassic radiolarian chert blocks associated with massive tholeitic basalts and Early Cretaceous brick-red ribbon cherts overlying pillow basalts are interpreted as fragments of a Middle Jurassic oceanic basement accreted to an Early Cretaceous oceanic Plate; in an intra-oceanic subduction context. Whereas; the knobby radiolarites and black shales of Playa Carrizal are indicative of a shallower middle Cretaceous paleoenvironment. Other remnants of this oceanic basin are found in Units 2; 6; and 7; which documented the rapid approach of the depocentre to a subduction trench during the late Early Cretaceous (Albian-Cenomanian); to possibly early Late Cretaceous (Turonian).

Palaeozoic tectonics and evolutionary history of the Qinling orogen: Evidence from geochemistry and geochronology of ophiolite and related volcanic rocks

The tectonic framework and the evolutionary history of the Qinling orogenic belt are keys for understanding the convergent processes between the North China and South China blocks. The widely exposed ophiolitic and subduction-related volcanic melange along the Shangdan and Erlangping belts provides important constraints on the tectonic evolutionary processes of the Qinling orogen. The melange in the Shangdan zone is predominantly composed of ultramafic and mafic rocks that can be divided into three geochemical groups: (1) N- MORB type; (2) E-MORB type; and (3) island-arc/active continental margin-related basalts. The samples with N- and E-MORB affinity are characterized by depletion or slight enrichment of LREE without fractionation of HFSE and no negative Nb–Ta anomaly. The island-arc/active continental margin-related basalts are typically depleted in Nb–Ta and Ti. It is inferred that the melange within the Shangdan suture represents remnants of an oceanic crust and associated volcanics. An age for the melange can be constrained by a U–Pb zircon age of 517.8±2.8Ma, obtained from the gabbro within the E-MORB-type ophiolite in the Yanwan area.To the north of the suture zone, the Erlangping melange consists similarly of ultramafic and mafic rocks, andesites and rhyolites. The mafic and andesitic rocks exhibit strong depletion of Nb–Ta and Ti indicating a subduction-related affinity. However, their depletion in Nb–Ta is weaker than that of the island-arc/active continental margin-related volcanic rocks. Taken together, the two ophiolitic melange zones indicate the existence of an early Palaeozoic Shangdan Ocean that was associated with a back-arc basin on the northern North Qinling Island arc terrane, separating the South China from the North China blocks.

Zircon ages, Sr-Nd-Hf isotopic compositions, and geochemistry of granitoids associated with the northern ophiolite melange of Central Cuba: Tectonic implication for Late Cretaceous magmatism in the Northwestern Caribbean

<h3>Abstract</h3> Tuberculosis remains one of the leading causes of death from a single infectious agent, surpassing both AIDS and malaria. In recent years, two bicyclic nitroimidazole drugs, delamanid and pretomanid have been approved to treat this airborne infection. This has spurred a renewed interest in developing new and improved nitroimidazole analogs. We have previously identified a new bicyclic heteroaromatic subclass, the nitroimidazopyrazinones, with substituted analogs showing promising activity against <i>Mycobacterium tuberculosis</i> under both aerobic and hypoxic environments. A second generation of nitroimidazopyrazinones with extended biaryl side chain also possessed good antiparasitic activity against <i>Trypanosoma brucei brucei</i> and <i>Trypanosoma cruzi</i>, suggesting the utility of this new scaffold for development into potential candidates against both tuberculosis and the kinetoplastid parasites which cause neglected tropical diseases. In this study, we further evaluated the properties of nitroimidazopyrazinone derivatives by assessing their selectivity against different mycobacterial species, measuring their reduction potential, and determining the kinetic parameters as substrates of the deazaflavin-dependent nitroreductase (Ddn), which is the activating enzyme of delamanid and pretomanid in <i>M. tuberculosis</i>. We also conducted an <i>in vivo</i> evaluation of a lead compound, MCC8967 that demonstrated a favorable pharmacokinetic profile, with good oral bioavailability and efficacy in an acute <i>M. tuberculosis</i> infection model. Two other promising compounds MCC9481 and MCC9482, with good <i>in vitro</i> activity (IC₅₀ = 0.016 and 0.10 µM, respectively) against <i>T. cruzi</i>, the causative agent for Chagas diseases, were similarly tested for <i>in vivo</i> activity. These compounds also exhibited good oral bioavailability, and transiently reduced the acute-stage parasite burden by &gt;98‒99% at doses of 50 mg/kg once or twice daily, similar to benznidazole at 100 mg/kg once daily. Overall, we have demonstrated that active nitroimidazopyrazinones have potential to be developed as clinical candidates against both tuberculosis and Chagas disease. <h3>Author Summary</h3> Tuberculosis and parasitic infections continue to impose a significant threat to public health and economic growth worldwide. Most of the efforts to control these diseases still rely on drug treatments with limited effectiveness and significant side effects. There is now an urgent need to develop new treatments to combat these infections. Here, we report the <i>in vitro</i> and <i>in vivo</i> profile of a new bicyclic nitroimidazole subclass, namely nitroimidazopyrazinones, against mycobacteria and <i>Trypanosoma cruzi</i>. We found that derivatives with monocyclic side chains are selective against <i>Mycobacterium tuberculosis</i>, the causative agent of tuberculosis, but not active against other nontuberculosis mycobacteria. In an acute mouse model, they were able to reduce the bacterial load in lungs via oral administration. From a biochemistry perspective, we demonstrated that deazaflavin-dependent nitroreductase (Ddn) could act effectively on nitroimidazopyrazinones, indicating the potential of Ddn as an activating enzyme for these new compounds in <i>M. tuberculosis</i>. We also showed that derivatives with extended biaryl side chain were effective in suppressing infection in an acute <i>T. cruzi</i> infected murine model, with satisfactory oral bioavailability. These findings improve the understanding of the biological profile of nitroimidazopyrazinones for further development as potential antitubercular and antiparasitic agents.

Timing of ophiolite obduction in the Grampian orogen

This study addresses the timing and pressure-temperature (P-T) conditions of ophiolite obduction, one of the proposed causes of the ca. 470 Ma Grampian orogeny of Scotland and Ireland. This event gave rise to the main structural and metamorphic characteristics of the Grampian terrane-the type area for Barrovian metamorphism, the cause of which remains enigmatic despite a century of research. Zircons from the Highland Border ophiolite, Scotland, define a 499 +/- 8 Ma U-Pb concordia age, which is interpreted as dating magmatism. Its metamorphism is dated by a 490 +/- 4 Ma Ar-40-Ar-39 hornblende age, and a 488 +/- 1 Ma Ar-40-Ar-39 muscovite age from a metasedimentary xenolith within it, from which P-T estimates of 5.3 kbar and 580 degrees C relate to ophiolite obduction. Metamorphism of the Deerpark complex ophiolitic melange (Irish correlative of the Highland Border ophiolite) is constrained by a 514 +/- 3 Ma Ar-40-Ar-39 hornblende age, while mica schist slivers within it yield detrital zircon U-Pb ages consistent with Laurentian provenance and Rb-Sr and Ar-40-Ar-39 muscovite ages of ca. 482 Ma. P-T values of 3.3 kbar and 580 degrees C for the mica schist constrain the conditions of ophiolite obduction. Metamorphic mineral ages from the Grampian terrane (Dalradian Supergroup) are substantially younger (ca. 475-465 Ma) than those from the ophiolites. If conductive heating in overthickened crust was the cause of Barrovian metamorphism, then collisional thickening must have started soon after ophiolite obduction at ca. 490 Ma in order to generate the ca. 470 Ma metamorphic peak in the Grampian terrane. (Less)

Origin and emplacement of Archean ophiolites of the central orogenic belt, North China craton

Understanding Archean crustal and mantle evolution hinges upon identification and characterization of oceanic lithosphere. We report and update here more than 10 years work on a complete, yet dismembered and metamorphosed Archean ophiolite sequence in the North China craton, in the Dongwanzi (东湾子)-Zunhua (遵化) structural belt and correlatives in the Wutaishan (五台山) area. Banded iron formation structurally overlies several tens of meters of variably deformed pillow lavas, mafic flows, and picritic amphibolites. These are in structural contact with a 2 km thick mixed gabbro and dike complex with gabbro screens, exposed discontinuously along strike for more than 20 km. The dikes consist of metamorphosed diabase, basalt, Hb-Cpx-gabbro, and pyroxenite. The dike/gabbro complex is underlain by several kilometers of mixed isotropic and foliated gabbro, which preserve compositional layering approximately 2 km below the dike complex, and then over several hundred meters merge into strongly compositionally layered gabbro and olivine-gabbro. The layered gabbro becomes mixed with layered pyroxenite/gabbro marking a transition zone into cumulate ultramafic rocks including serpentinized dunite, pyroxenite and wehrlite, and finally into strongly deformed and serpentinized olivine and orthopyroxene-bearing ultramafic rocks interpreted as depleted mantle harzburgite tectonites. A U/Pb zircon age of 2.505 Ga from gabbro of the Dongwanzi ophiolite makes it one of the world’s oldest recognized, laterally-extensive complete ophiolite sequences, though older dismembered ophiolites are recognized in South Africa and Greenland, extending back to 3.8 Ga. This age is confirmed by a ca. 2.6 Ga Re-Os isochron from chromites from the belt, and a number of dated 2.5–2.4 Ga cross-cutting younger igneous units. The Dongwanzi ophiolite is one of the largest well-preserved greenstone belts in the central orogenic belt that divides the North China craton into eastern and western blocks. More than 1 000 other fragments of gabbro, pillow lava, sheeted dikes, harzburgite, and podiform-chromite bearing dunite occur as tectonic blocks (tens to hundreds of meters long) in a biotite-gneiss and BIF matrix, intruded by tonalite and granodiorite, in the Zunhua structural belt. Blocks in this metamorphosed Archean ophiolitic melange preserve deeper levels of oceanic mantle than the Dongwanzi ophiolite. The ophiolite-related melange marks a suture zone across the North China craton, traced for more than 1 600 km along the central orogenic belt. Many of the chromitite bodies are localized in dunite envelopes within harzburgite tectonite, and have characteristic nodular and orbicular chromite textures, known elsewhere only from ophiolites. The chromites have variable but high chrome numbers (Cr/(Cr+Al)=0.74−0.93) and elevated P, also characteristic of suprasubduction zone ophiolites. The high chrome numbers, coupled with TiO2<0.2 wt.% and V2O5<0.1 wt.% indicate high degrees of partial melting from a very depleted mantle source and primitive melt for the chromite. A Re-Os isochron from the chromites indicates an age of 2.6 Ga, showing that they are the same age as the Dongwanzi ophiolite. The range in initial Os isotopic compositions in the chromites in these ophiolitic blocks is small and well within the range seen in modern ophiolites. The ultramafic and ophiolitic blocks in the Zunhua melange are therefore interpreted as dismembered and strongly deformed parts of the Dongwanzi ophiolite. We suggest the name “Dongwanzi-Zunhua ophiolite belt” for these rocks. Geochemical and structural features of the Dongwanzi ophiolite suggest that it formed in a forearc environment and was incorporated in an accretionary prism soon after it formed. Neoarchean and Paleoproterozoic (2.50 and 1.90 Ga) high-pressure granulites form a belt more than 700 km long along the western side of the central orogenic belt. Several Neoarchean sedimentary basins consisting of conglomerate, greywacke, and shale are located along the eastern side of the central orogenic belt, and are interpreted as remnants of a foreland basin. The three belts record the Neoarchean subduction and collision between an arc terrane and eastern blocks of the North China craton in the Neoarchean, and further deformation and metamorphism in the Paleoproterozoic related to collisions on the northern margin of the already amalgamated North China craton.

Mineralogy and major-trace element geochemistry of the Haymana manganese mineralizations, Ankara, Turkey

The study area comprises Cayirli, Harapaǧillari and Ortabel (CHO) districts in the Haymana town 25 km SW of the Ankara city. In this study, geologic setting, mineralogical and geochemical characteristics of manganese mineralizations are examined and their origin is discussed. Data were obtained from field observations, various polished and thin section determinations and chemical analyses of 25 ore samples. Major and trace element analyses were conducted with ICP-ES and ICP-MS methods at ACME Laboratories (Canada). CHO manganese ores are found in reddish radiolarites and partly interbedded marls of the Ankara ophiolitic melange of lower Cretaceous-upper Senonian age. In these extremely folded rocks, manganese is observed as band and lenses. All samples are composed of pyrolusite and quartz. Circular pyrolusites around the gangue form a ring texture and radiolarite ores yield spherical sections. In addition, breccia-like texture is observed between pyrolusite and quartz. Geologic setting, mineralogical and textural data indicate an exogene environment. Major and trace element abundances and correlation among them imply that CHO mineralizations are of hydrothermal origin and linked to the same source. Evaluation of all these data shows that CHO manganese mineralizations are of hydrothermal type sedimentary mineralization and they were deposited in distal part of the source.

Tectonic implications of the geochemical data from the Makran igneous rocks in Iran

AbstractMakran is one of the largest accretionary prisms on Earth, formed by the closure of the Neotethys ocean which is now represented by its remnant, the Gulf of Oman. Tectonic evolution of the Makran island‐arc system is explored within the context of a north dipping subduction zone, with temporal variations in slab dip arrangement. In a Middle Jurassic–Early Paleocene steep slab dip arrangement, the Mesozoic magmatic arc and the Proto‐Jaz Murian depression, which was an intra‐arc extensional basin, were developed. This was associated with development of outer‐arc ophiolitic mélange and oceanward migration of the Bajgan–Durkan continental sliver, which is the continuation of the Sanandaj–Sirjan zone of the Zagros orogenic belt into the Makran region. In a Late Paleocene to Late Pliocene moderate to shallow slab dip arrangement, compression and tectonic inversion of the Proto‐Jaz Murian extensional basin into the Jaz Murian compressive basin was associated with the uplift of the southern part of the Jaz Murian Depression along the South Jaz Murian Fault, and emplacement of the Paleogene–Neogene magmatic arc, behind the Jaz Murian compressive basin. A shallow slab dip arrangement in the Quaternary led to the emplacement of a third magmatic arc inland, over the southern part of the Yazd–Tabas–Lut micro‐continental block. It is envisioned that the Makran island‐arc system will pass through similar tectonic events in the future, as the Zagros island‐arc system did in the past. However, a future remnant and/or residual basin similar to the present Gulf of Oman will continue to survive to the east.

Study on the geochemical characteristics of ocean-ridge and oceanic-island volcanic rocks in the Nan-Uttaradit zone, northern Thailand

Field investigations and laboratory integrated research as indicated that ophiolite melange in the Nan-Uttaradit zone, northern Thailand, consists of fragments of tectonites such as metamorphic peridotite (extremely silicified serpentinite), cumulates (pyroxenolite, gabbro, and gabbro-diorite), ocean-ridge basalt, oceanic-island basalt and radiolarian silicalite, and it was formed during D3-P. The rock series, rock types and petrogeochemical characteristics of metamorphic tholeiites in the Nan area of the Nan-Uttaradit zone are similar to those of ocean-ridge basalts (C1) in China’s Ailaoshan zone. As for the Hawaiites in the Nan area of the Nan-Uttaradit zone, their major elements, REEs and trace elements are similar to those of oceanic-island basalts in China’s Jinshanjiang zone (P1 1). In the Uttaradit area of this zone the metamorphic alkaline basalts show transitional petrogeochemical characteristics between ocean-ridge basalts and oceanic-island basalts, which were still formed in oceanic-island environments. The above-described basalts are all oceanic volcanic rocks and they are the most important part of the Paleo-Tethys oceanic crust in the Nan-Uttaradit zone.

Caracterização química de cromitas nos maciços de Cromínia e Mairipotaba, Goiás, Brasil

Metaultramafic bodies tectonically emplaced within the metasedimentary sequence of the Araxa Group are associated with an ophiolitic melange in southeast Goias. In the region of Crominia - Mairipotaba, they occur as lenticular bodies aligned E-W. Cumulate textures and geochemical data indicate that the parent rocks had harzburgitic to dunitic compositions. Relicts of primary crystals of olivine and orthopyroxene are suggestive of amphibolite facies metamorphic re-equilibration fabrics, even though the paragenesis and mineral associations of these metaultramafic rocks are typical of greenschist facies (T < 550oC and P = 5.5 kbar). The chromitites exhibit massive to breccioid structure and pull-apart texture, with chromite crystals around 0.5 mm in size. Chromite concentrations in the chromitite levels reach 70 to 85% by volume of the rock. The crystals are dispersed in the matrix, which is composed essentially of serpentine, and subordinately of chlorite and talc. The textures and geochemical data (Cr2O3 x TiO2 and Mg x Cr ratios present in the chromitite) are similar to those observed in ophiolitic complexes. Hence, they correspond to allochthonous bodies (Alpine type) associated with an ophiolitic melange.

Metamorphism and metasomatism in the Jurassic Nain ophiolitic melange, Central Iran

Metamorphism and metasomatism in the Jurassic Nain ophiolitic melange, Central Iran

Oceanization of the northern Neotethys: Geochemical evidence from ophiolitic melange basalts within the İzmir–Ankara suture belt, NW Turkey

The remnants of the Neotethyan Izmir–Ankara Ocean, the main branch of Neotethys in the eastern Mediterranean are represented by the Dagküplü Melange Complex in Central Sakarya, NW Turkey. It comprises several blocks or tectonic slices of pillow lavas, some of which include mudstones and radiolarian cherts as intra-pillow-fillings or interlayers. In the Igdecik area, a huge basaltic block has been studied in detail. Geochemical data reveal three distinct basalt types separated by sheared contacts. The first of these groups is an enriched mid-oceanic ridge basalt (E-MORB) type which is enriched in the most incompatible trace elements relative to normal MORB (N-NORB) in addition to having heavy rare earth elements (HREE) depletion, suggesting the influence of residual garnet in their mantle source region. The second is back-arc basin basalt (BABB) type with relatively depleted trace element compositions with respect to N-MORB together with a negative Nb anomaly, suggesting generation above an intra-oceanic subduction zone where partial melts are derived from a depleted (MORB-like) mantle. The final group is island-arc tholeiite (IAT) type, displaying the most depleted trace element abundances among the studied groups in addition to marked Nb depletion, reflecting intra-oceanic supra-subduction zone (SSZ) signatures similar to the BABB-type but requiring a depleted mantle source which has experienced a previous melt extraction. Combined with a previously ascribed Late Triassic age of Tekin et al. (2002) (221 Ma, Late Carnian; based on the radiolarian fauna found in a chert layer alternating with mudstones), the associated basalts with E-MORB-type geochemical signatures, suggest formation of oceanic crust as early as Late Carnian. This age is the oldest thus far obtained from the basalts of the Izmir–Ankara Ocean. This new data provides constraints on tectonic models for the opening the Izmir–Ankara Ocean and its relationship to other branches of the Neotethyan ocean in the Eastern Mediterranean area.

Rodingite with Ti- and Cr-rich vesuvianite from the Sartuohai chromium deposit, Xinjiang, China

Rodingite containing vesuvianite with the world's highest Cr2O3 and TiO2 contents was found to occur in the ultramafic body of the Early to Middle Devonian Darbut-Sartuohai ophiolite zone belonging to the Sartuohai alpine-type ophiolitic melange, west Junggar district, Xinjiang, China. Many blocks of massive and vein rodingite were located at the Sartuohai chromium deposit in the serpentinized ultramafic body along the suture zone near Karamay. The rodingite is mainly composed of vesuvianite, grossular-andradite series garnet, diopside, and chlorite. The Cr2O3 content of the rodingite masses and veins is up to 0.2 wt% and 0.1 wt%, respectively. While the Cr2O3 content (wt%) of each mineral species in rodingite is up to 0.04 for vesuvianite, 0.4 for garnet and 0.4 for diopside, respectively. Tiny sporadic inclusions of vesuvianite and garnet of 5 μm to 35 μm in diameter also disseminate along the cleavage or in the grain of diopside of the massive rodingite. The vesuvianite inclusions are characterized by a high concentration in chromium of Cr2O3 = 6.21 wt% and titanium of TiO2 = 7.43 wt% of all the reported data, in contrast to the low concentration in the host diopside. The garnet inclusions belong to the grandite series. The morimotoite and uvarovite components are 18% and 11% at most, respectively. The rodingite was considered to have been formed by two stages of metasomatism between gabbroic rock and a calcium-rich solution, which was probably produced in the process of serpentinization of peridotite under Cr- and Ti-rich, and Cr- and Ti-free environments.

Early Cretaceous migmatitic mafic granulites from the Sabzevar range (NE Iran): implications for the closure of the Mesozoic peri-Tethyan oceans in central Iran

Terra Nova, 22, 26–34, 2010 Abstract The ophiolitic melange of the Sabzevar Range (northern Iran) is a remnant of the Mesozoic oceanic basins on the northern margin of the Neotethys that were consumed during the Arabia–Eurasia convergence history. Occurrence of km-scale, dismembered mafic HP granulitic slices is reported in this study. Granulites record an episode of amphibole-dehydratation melting and felsic (tonalite/throndhjemite) melt segregation at c. 1.1 GPa and 800 °C. In situ U(-Th)–Pb geochronology of zircon and titanite grains hosted in melt segregations points to an Early Cretaceous (Albian) age for the metamorphic climax. Results of this study (i) impose reconsideration of the current palaeotectonic models of the Neothetyan convergent margin during the Early Cretaceous and (ii) argue that punctuated events of subduction of short-lived back-arc oceanic basins accompanied the long-lasting history of the Neotethyan subduction in the region.

The basal tectonic mélange of the Cabo Ortegal Complex (NW Iberian Massif): a key unit in the suture of Pangea

Recent field work and mapping in the lower units of the Cabo Ortegal Complex provided new data about the tectonic melange that appears in the lowest structural position: the Somozas Melange. This melange unit with average thickness of 1800 m is restricted to the eastern part of the complex, and is located at the advancing front of the allochthonous complexes of NW Iberia. Three rock units are involved in the melange: 1) an ophiolitic melange consisting of igneous rocks mixed with serpentinites; 2) a metasedimentary unit with phyllites and phyllonites, with scarce conglomerates, marbles and quartzites; 3) high-T metamorphic rocks with varied types of amphibolites and orthogneisses. Two granitic rocks within the ophiolitic melange were dated using U-Pb zircon geochronology at 527 ± 2 Ma and 499 ± 1 Ma. Two different series of igneous rocks can be distinguished in this melange. The first series consists of gabbros, diorites, granitoids and basalts-basaltic andesites with calc-alkaline affinities. The second series contains common basaltic rocks, diabasic dikes and gabbros with chemical compositions typical of island-arc tholeiites. Both igneous series shared a common geographic setting, but the island-arc tholeiites are younger than the calc-alkaline igneous rocks. The two igneous series were probably generated in a mature volcanic arc located along the periphery of Gondwana. In the metasedimentary unit, a conglomerate from a large tectonic block included in serpentinites yielded age populations of detrital zircons suggesting that the sediments were deposited along the periphery of the West-African Craton. This conglomerate contains a large number of zircons (n = 24) with ages ranging 630-464 Ma, probably representing the chronology of the Pan-African event, including the magmatic activity in the volcanic arc where the igneous lithologies involved in the melange were generated. The maximum age of sedimentation for this conglomerate is estimated as latest Cambrian – earliest Ordovician, and constraints the end of the magmatic activity in the volcanic-arc. Within the unit of high-T rocks, an orthogneiss yields a U-Pb protolith age of 485 ± 6 Ma, which is similar to other ages of igneous rocks in the basal allochthonous terrane in NW Iberia. The three rock assemblages forming part of the Somozas Melange may be linked to the evolution of a mature peri-Gondwanan volcanic arc. This volcanic arc was affected by pronounced extension which caused the opening of intra-arc basins, culminating the rifting and subsequent drift of the external parts of the arc during the opening of the Rheic Ocean. This opening started during a time interval constrained by the peak activity in a mature volcanic arc (c. 527-499 Ma) and the generation of intraarc basins around the Cambrian-Ordovician boundary. Tectonic melanges including high-P rocks have been classically related to subduction zone environments. Regional relationships in NW Iberia and the nature of the rock units involved in the Somozas Melange, suggest that two different subduction zones generated during oblique convergence and collision between Gondwana and Laurussia were active during the final stages of the assembly of Pangea. The first related to the underthrusting beneath Laurussia of the most external Gondwana margin (c. 370 Ma). The second subduction zone was a new one which accreted later remnants of a peri-Gondwanan arc and sediments of the continental margin below a layer of exhumed high pressure rocks. This oblique collision finished the closure of the Rheic Ocean and contributes to define the new oceanic domain located to the East of Pangea, the Palaeotethys.

Upper Triassic and Middle Jurassic radiolarians from the ophiolitic melange of the Dinaridic Ophiolite Belt, SW Serbia

Upper Triassic and Middle Jurassic radiolarians from the ophiolitic melange of the Dinaridic Ophiolite Belt, SW Serbia

Geochemical studies of granitoids from Shyok tectonic zone of Khardung-Panamik section, Ladakh, India

The Shyok tectonic zone lies to the north of Ladakh magmatic arc or the Ladakh batholith in the Trans-Himalaya of Ladakh district, J & K. Investigations were carried out on the granitoids exposed along Leh-Siachan highway between Khardung and Panamik villages. The granitoid bodies under study are: Khardung granite (KG), Tirit granite (TG) and Panamik granite (PG) belonging to Ladakh batholith, Shyok ophiolitic melange and Karakoram batholith respectively. Though the granitoids belong to different litho-tectonic units, yet they have subduction related geochemical characters typical of Andean-type granitoids. Re-melting of crustal rocks of volcanic arc affinity has played an important role for the origin of KG rocks which are more evolved, while the TG and PG rocks represent transitional tectonic environment from primitive to mature arc.

Paleozoic multiple subduction-accretion processes of the southern Altaids

The formation and development of the southern Altaids is controversial with regard to its accretionary orogenesis and continental growth. The Altay-East Junggar orogenic collage of North Xinjiang, China, offers a special natural laboratory to resolve this puzzle. Three tectonic units were juxtaposed, roughly from North to South, in the study area. The northern part (Chinese Altay), composed of variably deformed and metamorphosed Paleozoic sedimentary, volcanic, and granitic rocks, is interpreted as a Japan-type island arc of Paleozoic to Carboniferous-Permian age. The central part (Erqis), which consists of ophiolitic melanges and coherent assemblages, is a Paleozoic accretionary complex. The southern part (East Junggar), characterized by imbricated ophiolitic melanges, Nb-enriched basalts, adakitic rocks and volcanic rocks, is regarded as a Devonian-Carboniferous intra-oceanic island arc with some Paleozoic ophiolites, superimposed by Permian arc volcanism. A plagiogranite from an imbricated ophiolitic melange (Armantai) in the East Junggar yields a new SHRIMP zircon age of 503 ± 7 Ma. Using published age constraints, we propose the presence of multiple subduction systems in this part of the Paloasian Ocean in the Paleozoic. The intraoceanic arcs became accreted to the southern active margin of the Siberian craton in the middle Carboniferous-Permian. During the long accretionary processes, in addition to large-scale southward-directed thrusting, large-scale, orogen-parallel, strike-slip movements (for example, Erqis fault) in the Permian translated fragments of these intraoceanic arcs and associated accretionary wedges. This new tectonic model has broad implications for the architecture and crustal growth of Central Asia and for other ancient orogens.

GEOCHEMISTRY AND TECTONO-MAGMATIC AFFINITY OF MAFIC EXTRUSIVE AND DYKE ROCKS FROM THE OPHIOLITE MELANGE OF THE SW ZAGORJE-MID-TRANSDANUBIAN ZONE (MT. MEDVEDNICA, CROATIA)

Mafic extrusive rocks occasionally intersected by dolerite dykes are the most abundant fragments of oceanic crust in the lower Callovian to upper Valanginian ophiolite melange exposed on the Mt. Medvednica, at the SW tip of the Zagorje-Mid-Transdanubian Shear Zone. The melange includes a chaotic mixture of various extrusive rocks which may be analogues of the oceanic rocks derived from (at least) two different tectonic settings. The extrusive and dyke rocks of this melange are geochemically classified into three magmatic groups: high-Ti group of N-MORB-like with arc signatures (Ti/V = 26.3-38.5), medium- Ti group (Ti/V = 20.2-23.0) and low-Ti group showing untypical LREE depleted [(La/Sm)cn = 0.49-0.62] IAT-like signatures (Ti/V = 13.2-18.0). All three geochemical groups of magmas were successfully monitored by clinopyroxene geochemistry. The magmas have consistent initial Nd-isotopic values (eNd(T=163 Ma) +5.9 to +6.3) and initial 87Sr/86Sr ratios (0.703748-0.704286) suggesting a similar parental source related to the initial stage of intraoceanic subduction of a back-arc basin ridge. It represents the new crust formed from melts derived from LREE extremely depleted MORB-type mantle which was affected by concomitant metasomatism and partial melting in a suprasubduction forearc setting. K-Ar ages on plagioclase separates indicate uppermost Bathonian - lower Oxfordian magmatic activity between 165±3.3 Ma and 160±3.3. A geodynamic model for the Mt. Medvednica ophiolites, part of the Repno oceanic domain of the Meliata-Maliak oceanic system, is here proposed. There are many common geochemical and geological correlation features between the Mt. Medvednica and Szarvasko ophiolites, located in NE Hungary that allow to integrate them in the same oceanic domain, whereas this is not obvious in the case of the Dinaric ophiolites.