Ophiolitic Rocks Research Articles

Jurassic back-arc and Cretaceous hot-spot series In the Armenian ophiolites — Implications for the obduction process

The identification of a large OIB-type volcanic sequence on top of an obducted nappe in the Lesser Caucaus of Armenia helps us explain the obduction processes in the Caucasus region that are related to dramatic change in the global tectonics of the Tethyan region in the late Lower Cretaceous. The ophiolitic nappe preserves three distinct magmatic series, obducted in a single tectonic slice over the South Armenian Block during the Coniacian–Santonian (88–83 Ma), the same time as the Oman ophiolite. Similar geological, petrological, geochemical and age features for various Armenian ophiolitic massifs (Sevan, Stepanavan, and Vedi) argue for the presence of a single large obducted ophiolite unit. The ophiolite, shows evidence for a slow-spreading oceanic environment in Lower to Middle Jurassic. Serpentinites, gabbros and plagiogranites were exhumed by normal faults, and covered by radiolarites. Few pillow-lava flows have infilled the rift grabens. The ophiolite lavas have hybrid geochemical composition intermediate between Arc and MORB signatures: (La/Yb) N = 0.6–0.9; (Nb/Th) N = 0.17–0.57; ( 143Nd/ 144Nd) i = 0.51273–0.51291; ( 87Sr/ 86Sr) i = 0.70370–0.70565; ( 207Pb/ 204Pb) i = 15.4587–15.5411; ( 208Pb/ 204Pb) i = 37.4053–38.2336; ( 206Pb/ 204Pb) i = 17.9195–18.4594. These compositions suggest they were probably formed in a back-arc basin by melting of a shallow asthenosphere source contaminated by a deeper mantle source modified by subducted slab-derived products. 87Sr/ 86Sr ratios and petrological evidence show that these lavas have been intensely altered by mid-oceanic hydrothermalism as well as by serpentinites, which are interpreted as exhumed mantle peridotites. The gabbros have almost the same geochemical composition as related pillow-lavas: (La/Yb) N = 0.2–2.3; (Nb/Th) N = 0.1–2.8; ( 143Nd/ 144Nd) i = 0.51264–0.51276; ( 87Sr/ 86Sr) i = 0.70386–0.70557; ( 207Pb/ 204Pb) i = 15.4888–15.5391; ( 208Pb/ 204Pb) i = 37.2729–37.8713; ( 206Pb/ 204Pb) i = 17.6296–17.9683. Plagiogranites show major and trace element features similar to other Neo-Tethyan plagiogranites (La/Yb) N = 1.10–7.92; (Nb/Th) N = 0.10–0.94; but display a less radiogenic Nd isotopic composition than basalts [( 143Nd/ 144Nd) i = 0.51263] and more radiogenic ( 87Sr/ 86Sr) i ratios. This oceanic crust sequence is covered by variable thicknesses of unaltered pillowed OIB alkaline lavas emplaced in marine conditions. 40Ar/ 39Ar dating of a single-grain amphibole phenocryst provides a Lower Cretaceous age of 117.3 ± 0.9 Ma, which confirms a distinct formation age of the OIB lavas. The geochemical composition of these alkaline lavas is similar to plateau-lavas [(La/Yb) N = 6–14; (Nb/Th) N = 0.23–0.76; ( 143Nd/ 144Nd) i = 0.51262–0.51271; ( 87Sr/ 86Sr) i = 0.70338–0.70551; ( 207Pb/ 204Pb) i = 15.5439–15.6158; ( 208Pb/ 204Pb) i = 38.3724–39.3623; ( 206Pb/ 204Pb) i = 18.4024–19.6744]. They have significantly more radiogenic lead isotopic compositions than ophiolitic rocks, and fit the geochemical compositions of hot-spot derived lavas mixed with various proportions of oceanic mantle. In addition, this oceanic + plateau sequence is covered by Upper Cretaceous calc-alkaline lavas: (La/Yb) N = 2.07–2.31; (Nb/Th) N = 0.08–0.15; ( 144Nd/ 143Nd) i = 0.51271–0.51282; ( 87Sr/ 86Sr) i = 0.70452–0.70478), which were likely formed in a supra-subduction zone environment. During the late Lower to early Upper Cretaceous period, hot-spot related magmatism related to plateau events may have led to significant crustal thickening in various zones of the Middle-eastern Neotethys. These processes have likely hindered subduction of some of the hot and thickened oceanic crust segments, and allowed them to be obducted over small continental blocks such as the South Armenian Block.

Structural and geochronological relationships of metamorphic soles of eastern Mediterrranean ophiolites to surrounding units: indicators of intra‐oceanic subduction and emplacement

This paper is a synthesis of structural and geochronological data from eastern Mediterranean ophiolitic metamorphic rocks and surrounding units to interpret the intra‐oceanic subduction and ophiolite emplacement mechanism. Metamorphic rocks occur as discontinuous tectonic slices at the base of the ophiolites, generally between the peridotite tectonites and volcanic‐sedimentary units, and locally in fault zones in the overlying peridotites. They consist essentially of amphibolite, and in lesser quantities, micaschist, quartzite, epidotite and marble. Geological and geochronological data indicate that recrystallization of the metamorphic rocks occurred in the oceanic environment. The contact between the metamorphic rocks and the hanging‐wall is parallel to the foliation of the metamorphic rocks, and is interpreted as the fossil plane of intra‐oceanic subduction. Structural relationships suggest that intra‐oceanic subduction was situated between two lithospheric blocks separated by an oceanic fracture zone. Therefore the Neotethyan ophiolites with metamorphic soles represent the remnants of the overriding oceanic lithosphere's training slices of the metamorphic rocks at the base. In the Anatolian region, radiometric dating of metamorphic rocks from the Taurus and Izmir‐Ankara‐Erzincan zone ophiolites yield nearly identical ages. Besides, palaeontological and structural data indicate coeval opening and similar oceanic ridge orientation. Consequently it is highly probable that Taurus and Izmir‐Ankara‐Erzincan zone ophiolites represent fragments of the same oceanic lithosphere derived from a single spreading zone. Palaeontological data from underlying volcanic and sedimentary units point out that the opening of the Neotethyan ocean occurred during Late Permian‐Middle Triassic time in the Iranian‐Oman region, during Middle Triassic in Dinaro‐Hellenic area, and finally during Late Triassic in the Anatolian region. Radiometric dating of the metamorphic rocks exhibit that the intra‐oceanic thrusting occurred during late Lower‐early Late Jurassic for Dinaro‐Hellenic ophiolites, late Lower‐early Late Cretaceous for Anatolian, Iranian and Oman ophiolites well before their obduction on the Gondwanian continent. Neotethyan ophiolites were obducted onto various sections of the Gondwanian continent from late Upper Jurassic to Palaeocene time, Dinaro‐Hellenic ophiolites during late Upper Jurassic‐early Lower Cretaceous onto the Adriatic promontory, Anatolian, Iranian and Oman ophiolites from late Lower Cretaceous to Palaeocene onto the Aegean, Anatolian and Arabic promontories.

Mineral Deposits of Turkey in Relation to Tethyan Metallogeny: Implications for Future Mineral Exploration

The Tethyan metallogeny of Turkey, shaped by the interplay between subduction, collision, postcollision, and rifting processes, is mainly associated with Late Cretaceous to Cenozoic volcanoplutonic and ophiolitic rocks. A wide spectrum of ore deposits is represented from those occurring in island arcs to those associated with continental settings. Though Turkey has been one of the leading producers of chromite, the country is an emerging producer of precious and base metals. This assessment of the metallogeny of Turkey is based on a comprehensive GIS database compilation that contains known mineral deposits and prospects with relevant descriptive data. The data set contains information on major deposit types of economic importance such as porphyry, skarn, epithermal including both high-and low-sulfidation, polymetallic volcanic-associated massive sulfide (VMS) deposits including both Kuroko and Cyprus types, podiform chromite, lateritic nickel, carbonate-hosted lead-zinc with nonsulfide zinc including sedimentary-exhalative (SEDEX) and Mississippi Valley type (MVT), karstic and lateritic bauxite, orogenic gold including both mesothermal and listwanite-hosted types, and placer deposits. Well known deposit types that are less well represented in the database include sediment-hosted Cu, Carlin-type gold, iron oxide copper-gold (IOCG), and detachment-fault related gold systems. Exploration programs in the last two decades have started to reveal the true mineral potential of the country. Turkey is an underexplored country by today’s standards, with a large prospective area with a wide spectrum of mineral deposits reflecting the diverse geological environments that are present. Turkey is the least well explored portion of the Tethyan belt, which hosts Au and Cu endowments comparable with the Andes and the southwest Pacific metallogenic belts.

Ophiolite-Hosted Copper and Gold Deposits of Southeastern Turkey: Formation and Relationship with Seafloor Hydrothermal Processes

The paper documents evidence about the physical and chemical controls on the behaviour of Au and Cu sulphide deposits in fossil seafloor hydrothermal systems of SE Turkey. Observations from actively forming sulphides at mid-ocean ridges (MOR) and in back-arc environments are compared briefly with ancient analogues of gold and pyritic copper deposits such as Kisecik, Ergani and Siirt-Madenköy, formed at mid-oceanic ridges, or possibly at seamounts and back-arc settings. Many ophiolite-hosted Au- and Cu-deposits, of various sizes, are known to exist along the Bitlis-Zagros Suture Zone (BZS), the boundary between Anatolian and Arabian plates in SE Turkey. These deposits are associated with Cretaceous intra-oceanic supra-subduction zone ophiolitic slabs, such as Kızıldağ, the Bäer-Bassit massif (the southern extension of the Kızıldağ Massif in Syria), and the Yüksekova and Berit ophiolite bodies, which tectonically overlie the Mesozoic platform carbonates and Palaeozoic sediments of the Arabian plate. East of the BZS, ophiolitic segments of Tertiary age are strongly mineralized. Several Au and Co-Ni bearing pyritic Fe-Cu oxide and sulphide deposits associated with chloritized and spilitized basaltic pillow lavas or sheeted dykes crop out along the BZS. Volcanics associated with mineralisation closely resemble MOR sequences, sedimented ridges and back-arc environments in spreading centres of island arc systems. Ophiolitic rocks and the mineralogy of associated alteration are similar to characteristics of modern mid-oceanic ridge mineralization along the East Pacific Rise (EPR), Mid-Atlantic Ridge (TAG hyrothermal field) and Red Sea (Atlantis-II Deep).

Melange genesis and ophiolite emplacement related to subduction of the northern margin of the Tauride–Anatolide continent, central and western Turkey

Abstract The Tauride–Anatolide continent, stretching for c . 900 km across western and central Turkey, is one of the world's best example of a subducted, exhumed passive margin within a collisional orogen. Twelve widely separated areas were studied and correlated to develop a new plate-tectonic model. A metamorphosed, rifted continental margin of Triassic–Lower Cretaceous age (Tauride–Anatolide platform) is overlain by Upper Cretaceous (Cenomanian-Lower Maastrichtian) pelagic sediments and then by both tectonic melange (subduction complexes) and sedimentary melange (foredeep gravity complexes). The melanges are overthrust by unmetamorphosed ophiolitic rocks, commonly peridotites with swarms of diabase/gabbro dykes, and are underlain by metamorphic soles. New geochemical evidence from basaltic blocks in the melange indicates predominantly subduction influenced, within-plate and mid-ocean ridge-type settings. The dykes cutting the ophiolites were probably intruded during early-stage intra-oceanic arc genesis. The metamorphosed continental margin, melanges and ophiolites in the north (Anatolides) are correlated with unmetamorphosed equivalents in the Taurides further south (e.g. Beyşehir and Lycian nappes). Oceanic crust of Triassic–Late Cretaceous age formed between the Gondwana-related Tauride–Anatolide continent in the south and the Eurasia-related Sakarya microcontinent in the north. Following Late Triassic–Early Cretaceous passive margin subsidence, the continental margin was covered by Cenomanian-Turonian pelagic carbonates ( c . 98–90 Ma). Ophiolites formed in an intra-oceanic subduction zone setting in response to northward subduction, probably within a two-stranded ocean, with the Inner Tauride ocean in the SE and the İzmir–Ankara–Erzincan ocean in the north/NW. Metamorphic soles relate to intra-oceanic subduction ( c . 95–90 Ma). Oceanic sedimentary/igneous rocks accreted to the advancing supra-subduction oceanic slab. The Tauride–Anatolide continental margin then underwent diachronous collision with the trench ( c . 85 Ma), deeply subducted and metamorphosed at HP/LT ( c . 80 Ma). Accretionary, ophiolitic and exhumed HP/LT rocks were gravity reworked into a southward-migrating flexural foredeep and progressively overridden ( c . 70–63 Ma). Slices of the upper part of the platform and its margin detached and were thrust southwards as the (Tauride) Lycian and Beyşehir nappes, together with regional-scale ophiolites. The continental margin and melange were simultaneously exhumed during Maastrichtian–Early Paleocene (70–63 Ma) and transgressed by shallow-water sediments, beginning in the Late Maastrichtian in the east ( c . 64 Ma) and the Mid?-Late Paleocene ( c . 60 Ma) further west. Remnant oceanic crust was consumed during Early Cenozoic time, followed by Mid Eocene (45–40 Ma) diachronous continental collision and a second phase of regional deformation. Rather than being progressive there were two stages of collision: first, Upper Cretaceous ophiolite emplacement driven by continental margin-subduction trench collision, and secondly Eocene collision of the Tauride and Sakarya/Eurasian continents.

Petrogenesis of the Refahiye Ophiolite and its Tectonic Significance for Neotethyan Ophiolites Along the İzmir-Ankara-Erzincan Suture Zone

The Refahiye ophiolite, situated near Erzincan in the eastern part of the İzmir-Ankara-Erzincan Suture Zone (İAESZ), is one of the best exposures of oceanic lithosphere in the northern branch of the Neotethyan Ocean. The ophiolite, mainly thrust southwards, transported on an ophiolitic mélange, over the Early Triassic-Campanian Munzur Limestone of the Anatolide-Tauride Platform, was also emplaced by north-directed backthrusting onto the Pontides in the Late Cretaceous. It displays an almost complete ophiolite series unconformably overlain by Tertiary volcano-sedimentary units. The mantle peridotites are dominated by harzburgites, with dunite bands and lenses locally cut by isolated diabase dykes, whereas the mafic-ultramafic cumulates of the mantle-crust transitional zone consist of dunites, wehrlites, pyroxenites and gabbros. The diabase dykes of the sheeted-dyke complex show a magmatic boundary with the underlying massive gabbros which are in places crosscut by plagiogranite dykes. The basic and leucocratic rocks exhibit greenschist facies assemblages of ocean floor metamorphism. The spilitic pillow basalts and cover sediments (radiolarites, pelagic limestones) of the upper units of the oceanic crust are preserved as megablocks in the ophiolitic mélange. Petrological features of the ophiolitic rocks show that the ultramafic, mafic and leucocratic rocks belong to a co-magmatic differentiated tholeiite series. The crystallization order is olivine-clinopyroxene (locally with orthopyroxene)-plagioclase. The gabbros and diabases are tholeiitic (Nb/Y= 0.05-0.5). Whereas the LIL elements (K, Sr, Rb, Ba), with the exception of Th, show variable scatter because of ocean floor hydrothermal alteration, the HFSE (Nb, Ti, Zr, Y) and the LREE (La, Ce, Nd) have been depleted relative to N-MORB. The ratios of the selected trace elements (Ti/V, Zr/Y, Th/Y, Ti/Zr) and the tectonomagmatic discrimination diagrams for Ti-poor gabbros and dykes suggest that the Refahiye ophiolite has island arc tholeiitic (IAT) and possibly boninitic affinities. The mantle peridotites have very low REE concentrations, showing U-shaped REE patterns. The isotropic gabbros and plagiogranites exhibit high REE concentrations with positive and negative Eu anomalies respectively, patterns typical of SSZ magmatism. The ultramafic-mafic and leucocratic rocks of the Refahiye ophiolite were formed in the earliest stages of island arc development in a suprasubductional, forearc tectonic setting in the northern branch of the Neotethyan Ocean, similar to most of the Cretaceous Eastern Mediterranean ophiolites.

Structural and tectonic evolution of the Jabal Sumeini – Al Ain – Buraimi region, northern Oman and eastern United Arab Emirates

ABSTRACT Four main Late Cretaceous and Tertiary phases of crustal shortening and thrust-related deformation are recognised in the northern Oman Mountains, each phase ending with a regional unconformity throughout the Oman Mountains and the UAE foreland. The earliest is the major thrust emplacement of the Semail Ophiolite, and underlying Haybi, Hawasina and Sumeini duplexes from NE to SW onto the depressed northeastern continental margin of the Arabian Plate during the Cenomanian to Campanian-early Maastrichtian (D1). A prominent widespread unconformity that places Maastrichtian Qahlah Formation laterite, sandstones and conglomerates and Simsima Formation rudist - Acteonellid gastropod limestones over all underlying allochthonous units is recognised throughout the Oman Mountains. SW-verging folds and thrusts in Triassic to Cretaceous carbonate slope facies rocks exposed in Jabal Sumeini (Sumeini Group and Hamrat Duru Group) have been emplaced over uppermost Cretaceous Juweiza Formation conglomerates at the highest level of the Aruma foreland basin. These Sumeini D1 structures are abruptly truncated by an unconformity, above which are Middle Maastrichtian beds showing up to 95% ‘death assemblage’ rudists and gastropods just below the Cretaceous – Tertiary boundary (top of Simsima Formation). A second deformation event (D2) affected the Simsima Formation and underlying Sumeini Group, Hamrat Duru complex and Semail Ophiolite rocks. This phase shows gentle folding about NW-SE fold axes (e.g. Jabal Rawdah), along a similar trend to the Late Cretaceous D1 event. This trend is also parallel to a regional set of NW-SE aligned fractures in the UAE foreland. A major angular unconformity occurs beneath the Upper Palaeocene – Eocene shallow-marine limestones (Umm Er Radhuma, Rus and Dammam formations). Many foreland jabals in eastern Abu Dhabi (Jabals Qatar, Malaqet, Mundasa) show gentle post-Eocene folding formed during the third stage of crustal shortening (D3). The large pericline of Jabal Hafit is a double-plunging, east-verging box fold that formed after deposition of the Oligocene Asmari Formation limestones and Miocene Fars Formation gypsum and clays, the youngest beds affected by the fold. This Late Miocene – Pliocene phase of crustal shortening (D4) is the youngest phase of deformation in the eastern Al Ain-Buraimi region.

Springs and streams of the Taro–Ceno Valleys (Northern Apennine, Italy): Reaction path modeling of waters interacting with serpentinized ultramafic rocks

In the area of the Taro–Ceno Valleys (Northern Apennine, Emilia-Romagna region, Italy), waters of meteoric origin interact with ophiolite rocks of the External Ligurides. Fresh water springs issuing from basalts have a Ca–HCO 3 composition, whereas freshwater springs from ultramafites vary in composition from Ca–HCO 3 or Mg–HCO 3 to Na–OH or Na–SO 4 types and in pH values from 7.3–8.8 up to pH 11, respectively. In addition, the boron content of the alkaline waters is up to 13 mg/L, which is unusually high for freshwaters in general and ultramafites that have undergone oceanic serpentinisation in particular and gives a boric alkalinity to the waters. The springs waters show evidence of recent low-temperature continental serpentinisation and the process is modeled by reaction paths using an updated geochemical thermodynamic database, consistent with the local primary and secondary serpentinite paragenesis. For the model, bicarbonate waters evolve to alkaline waters supersaturated in Ca–(Mg)-carbonate, based on the assumption that the dissolution of serpentinite results in supersaturation with respect to kaolinite, ferrihydrite, vermiculite, Fe 2+–Mg 2+-saponite, and poorly crystalline serpentine. The alkaline composition and the chloride content of the waters suggest a prolonged interaction with the rocks at depth that led to dissolution of albite and leaching of olivine-hosted fluid inclusions. A similar evolution is also proposed for the more developed springs issuing from the ultramafic rocks of the Voltri Group (Liguria region), where solutions are supersaturated in brucite and are in equilibrium with enstatite and/or chlorite.

Tectonic significance of the geochemistry and petrology of ophiolites in southeast Anatolia, Turkey

The southeast Anatolian ophiolites form discontinuous linear belts of oceanic fragments immediately north of the Bitlis–Zagros suture, which marks a continental collision zone between the Arabian platform to the south and the Taurides to the north. The Late Cretaceous ophiolites in the southeast Anatolia are represented by the Kızıldağ in the Hatay area, the Göksun and the Berit to the north of Kahramanmaraş, İspendere in the Malatya area and Kömürhan–Guleman in the Elazığ area. The Göksun, Berit, İspendere, Kömürhan and Guleman ophiolites were attached to the base of the Tauride platform (i.e. the Malatya–Keban) along its northern active margin and intruded by the I-type calc-alkaline granitoids of Late Cretaceous age. In contrast, the Kızıldağ (Hatay) ophiolite was thrust over the Arabian passive margin in the south. The cumulate rocks in the ophiolites are represented by dunite, wehrlite, lherzolite, olivine clinopyroxenite, olivine gabbronorite, olivine gabbro, gabbronorite and gabbro. Highly magnesian olivines (Fo 88 to 74) and pyroxenes (Mg# 95 to 60) as well as highly calcic plagioclases (An 95 to 68) from the cumulate rocks differ from oceanic equivalents, which mainly formed from a MOR basaltic melt. The order of crystallization in mineral phases, whole rock and mineral chemistry data from the cumulates suggest that the primary magma is compositionaly similar to that observed in modern island arc tholeiitic sequences. The volcanic and subvolcanic rocks exhibit tholeiitic compositions. Chondrite normalized REE, N-MORB normalized multi-element patterns and tectonic discrimination diagrams suggest the existence of two main types of parental basic magmas in the crustal rocks of the SE Anatolian ophiolites. These are (i) IAT series that characterize the Kızıldağ, Göksun, İspendere, Kömürhan and Guleman ophiolites and (ii) Low-Ti boninitic series known only in the Kızıldağ ophiolite. The geochemistry of the crustal rocks suggests that they formed in a suprasubduction zone tectonic setting, including arc–forearc environments in the southern Neotethys. These well-preserved plutonic oceanic crustal remnants mainly exhibit an intact ophiolite pseudostratigraphy and are overlain by a volcanic-sedimentary unit made up of alternations of basic to acidic extrusive rocks, debris flows, volcaniclastic sandstones and pelagic limestones, interpreted as a tholeiitic ensimatic island arc assemblage built on the suprasubduction zone type crust. The metamorphism of some of the ophiolites (i.e. Berit) in the SE Anatolia might be related to later stages of intraoceanic subduction during the evolution of the southern Neotethys.

Cenozoic extensional tectonics of the Western Anatolia Extended Terrane, Turkey

The Western Anatolia Extended Terrane in Turkey is located on the eastern side of the Aegean Extended Terrane and contains one of the largest metamorphic core complexes in the world, the Menderes massif. It has experienced a series of continental collisions from the Late Cretaceous to the Eocene during the formation of the Izmir-Ankara-Erzincan suture zone. Based our field work and monazite ages, we suggest that the north-directed postcollisional Cenozoic extension in the region is the product of three consecutive stages, triggered by three different mechanisms. The first stage was initiated about 30 Ma ago, in the Oligocene by the Orogenic Collapse the thermally weakened continental crust along the north-dipping Southwest Anatolian shear zone. The shear zone was formed as an extensional simple-shear zone with listric geometry at depth and exhibits predominantly normal-slip along its southwestern end. But, it becomes a high-angle oblique-slip shear zone along its northeastern termination. Evidence for the presence of the shear zone includes (1) the dominant top to the north-northeast shear sense indicators throughout the Menderes massif, such as stretching lineations trending N10E to N30E; and (2) a series of Oligocene extensional basins located adjacent to the shear zone that contain only carbonate and ophiolitic rock fragments, but no high grade metamorphic rock fragments. During this stage, erosion and extensional unroofing brought high-grade metamorphic rocks of the Central Menderes massif to the surface by the early Miocene. The second stage of the extension was triggered by subduction roll-back and associated back-arc extension in the early Miocene and produced the north-dipping Alaşehir and the south-dipping Büyük Menderes detachments of the central Menderes massif and the north-dipping Simav detachment of the northern Menderes massif. The detachments control the Miocene sedimentation in the Alaşehir, Büyük Menderes, and Simav grabens, containing high-grade metamorphic rock fragments. The third stage of the extension was triggered by the lateral extrusion (tectonic escape) of the Anatolian plate when the North Anatolian fault was initiated at about 5 Ma. This extensional phase produced the high-angle faults in the Alaşehir, Büyük Menderes and Simav grabens and the high-angle faults controlling the Küçük Menderes graben.

Podzolization over ophiolitic materials in the western Alps (Natural Park of Mont Avic, Aosta Valley, Italy)

Soil surveys carried out on ophiolitic rocks and till under subalpine forest in the Natural Park of Mont Avic (Aosta Valley, West Italian Alps) revealed the unexpected occurrence of soils showing a horizon set typical of the podzols. Podzolization is uncommon on mafic and ultramafic substrata: the translocation of organo-metallic compounds in soils developed from such parent materials slows down considerably due to the high Fe and Mg contents, the high base saturation and the close to neutral pH. In order to determine the real origin and evolution of such soils, 6 pedons, selected among 83 profiles with a similar “podzol” morphology, were sampled and analyzed. Chemical, mineralogical, and petrographic analyses of the sampled soils and parent materials were performed, and correlation analysis of the obtained data was carried out. Results show that, in favourable environmental conditions (mainly climate and vegetation), even mafic and ultramafic parent materials can undergo different degrees of podzolization. Due to the adopted diagnostic criteria the most common classification systems (particularly the WRB) do not allow satisfactory taxonomic arrangement of such soils.

The late Early Miocene Acıpayam piggy-back basin: Refining the last stages of Lycian nappe emplacement in SW Turkey

Alluvial fan to shallow-marine sedimentation in the Acıpayam piggy-back basin took place in the hinterland zone of the SE-facing Lycian nappe stack and constrains the age and style of late-stage advance of these nappes. Syn-orogenic sedimentation commenced in Burdigalian time and is marked by coarse-grained alluvial fan deposits that directly overlie Mesozoic ophiolitic rocks. Palaeocurrent data obtained from these coarse-clastic units suggest a south-southeastward drainage. The succession fines upwards and passes into fluvial deposits that become increasingly enriched in carbonate clasts derived from tectonically deeper-seated nappes, suggesting progressive unroofening of the nappe pile. Upwards, the succession is transitional to a Late Burdigalian–Early Langhian marginal marine succession, consisting of off-shore fines with coal seams that overly both the coarse-clastic sediments and basement rocks. Clastic sedimentation ceased at the end of Early Langhian based on reef carbonates that cap the succession. Shallow-marine deposits and overlying reef carbonates confirm a marine transgression that has been reported from elsewhere in the Tauride Belt. Detailed sedimentology of the Acıpayam basin fill succession, its age and overall tectonic setting imply that during Burdigalian time the succession was deposited in a piggy-back basin, located in the hinterland zone of actively moving thrust sheets. We propose a model in which thick-skinned thrusting invoked inversion of early extensional faults in the autochthon and may explain the generation of large open folds with thick sedimentary sequences in their core. The formation of these large folds may have importantly influenced the frontal accretion such that active shortening moved into the hanging wall resulting in episodic nappe advanced of structurally higher nappes in a mode similar to thin-skin thrusting. The tectonosedimentary evolution of the Acıpayam piggy-back is strongly dependent on its position relative to the hinge of the large open fold. The absence of any compressional signatures in the basin fill succession indicates that these thrust translations ceased shortly after Aquitanian time. By Serravallian, the succession was interrupted and the region was uplifted to its present elevation, probably by gravitationally isostatic (orogenic) rebound of the nappe stack. After these episodes, tectonic extension predominated and the succession became dissected by nappe-front parallel normal faults that bound the present Çameli Basin.

Middle-Late Ordovician magmatism and Late Cretaceous collision in the southern Maya block, Rabinal-Salama area, central Guatemala: Implications for North America-Caribbean plate tectonics

The Rabinal-Salama area in central Guatemala provides critical data bearing on the relationships between the North American and Caribbean plates because it lies within the Polochic-Motagua fault zone that separates the two plates. The cumulative Cenozoic sinistral displacement across this zone that separates the Maya and Chortis terranes has been variously estimated to be ~125 km or ~1100 km, evidence for which should be recorded in the rocks of the studied area. The Rabinal-Salama area lies between two of the east-west faults within the Motagua fault zone, the Polochic fault, and the Baja Verapaz shear zone. The shear zone separates the Maya block from eclogitic rocks of the Chuacus Complex that pass southward into ophiolitic rocks and melanges that define a suture between the Chuacus Complex and the Chortis block. The following sequence of events is recorded in the Rabinal-Salama area: (1) low-grade, pre-Silurian siliciclastic metasedi-mentary rocks (San Gabriel unit), that are intruded by (2) ca. 462–453 Ma calc-alkaline, peraluminous, S-type Rabinal granite suite, and unconformably overlain by (3) very low grade clastic and calcareous metasedi-mentary rocks (Santa Rosa Group) containing Mississippian conodonts and pebbles of granite, sandstone, and phyllite derived from the older units. The Rabinal granite suite is inferred to be rift related, inheriting its calc-alkaline signature from its source, along with the ca. 1 Ga xenocrystic zircons (upper intercept U-Pb data). Deformation in all these Paleozoic rocks produced a steeply south-southwest–dipping cleavage (chlorite and sericite) and a stretched quartz lineation. These fabrics become more intense adjacent to the Baja Verapaz shear zone, where C-S fabrics and rotated porphyroclasts indicate a reverse sense of motion with a sinistral component. White mica in the shear zone yields 74–65 Ma K-Ar ages, which are inferred to closely postdate the time of crystallization. Thus, although evidence for major sinistral displacement is absent, the kinematics are consistent with uplift and exhumation of the Chuacus Complex during obduction of the Baja Verapaz ophiolite onto the Paleozoic rocks of the Rabinal-Salama area in latest Mesozoic-Paleocene. This is inferred to have been produced during collision of the Cuban arc and Chortis block with the southern Maya block. Restoration of the Early Mesozoic ~70° anticlockwise rotation of the Maya block places the Rabinal-Salama area adjacent to northeastern Mexico, where comparable continental-shallow marine, Paleozoic rocks occur near Ciudad Victoria overlying the ca. 1 Ga Oaxaquia basement.

Structure and petrology of the Altan Uul Ophiolite: new evidence for a Late Carboniferous suture in the Gobi Altai, southern Mongolia

Altan Uul and Nemegt Uul are mountain ranges in southern Mongolia containing polydeformed Mid- to Late Palaeozoic sedimentary, volcanic and metamorphic rocks uplifted at a restraining bend along the Gobi–Tien Shan left-lateral strike-slip fault system. Altan and Nemegt Uul contain part of an intra-oceanic island arc that was active during the Carboniferous. Thrust-bound sequences of highly fractured pillow basalt, cumulate gabbro, peridotite, serpentinite and jasperoid occur directly north of the arc rocks in at least three discrete belts and are interpreted to be fragments of an ophiolite. Metamorphosed marine sediments that have undergone intense contractional strain lie north of, and structurally below, the ophiolitic rocks. The gabbros within the ophiolitic sequences have undergone prograde metamorphic reactions consistent with an ocean-floor environment, and trace element analyses are similar to those from cumulate gabbros of the Oman ophiolite. Combined field and petrological evidence suggests that there is an east–west-trending south-dipping Late Carboniferous suture in Altan and Nemegt Uul that may be regionally correlated in the southern Gobi Altai region. This proposed suture represents a new and important tectonic boundary between tectonostratigraphic terranes in southern Mongolia, requiring modification of existing models of Late Palaeozoic terrane accretion in the Central Asian Orogenic Belt.

Geochemistry of Cretaceous to Eocene Ophiolitic Rocks of the Central Cordillera: Implications for Mesozoic-Early Cenozoic Evolution of the Northern Philippines

The Central Cordillera in northern Philippines is underlain mainly by ophiolitic basement of Cretaceous to Eocene age. In the central portion of the range, these complexes consist mainly of pillow basalt and basaltic feeder dikes that are unconformably overlain by epiclastic rocks, including turbidites. Field and geochemical evidence reveals that these rocks have a supra-subduction signature, and were generated in a back-arc setting. These relationships are similar to those exhibited by the Cretaceous to Eocene Lepanto Metavolcanics and the Pugo Formation, comprising the ophiolitic basement in the southern portions of the range. This result suggests that the Central Cordillera is floored by a common volcanic basement, possibly forming part of the Philippine Sea plate, proto-Philippine Sea plate, or proto-Molucca Sea plate.

Waziristan Ophiolite: A back-arc basin caught in continental collision, Waziristan, NW Pakistan

The western margin of the Indian plate is a classical example of continent-continent collision. The Afghanistan-Kabul-India continental blocks are here welded to each other by a network of sutures defined by the occurrence of ophiolites. The Waziristan Ophiolite (WO) is one of a series of ophiolites sandwiched between the western margin of the Indian plate and Afghanistan block. It is highly dismembered, but contains all the segments of an ideal ophiolite suite. It can be internally divided into three nappes which, from east to west, are: Vezhda Sar nappe, comprising entirely of pillow basalts; Boya nappe, a tectonic melange with an intact ophiolite section in the basal part at Mami Rogha; and Datta Khel nappe, consisting of sheeted dykes with variable proportions of other components. The intact ophiolite section consists of 1) basal ultramafics followed upward by 2) isotropic gabbros, and 3) pillow basalts, intercalated with and capped by pelagic sediments (chert, shale, limestone) (Figure 1). Isolated mafic dykes (doleritic), characterised by chilled margins, intrude all types of ophiolitic rocks, except the pillow basalts of Vezhda Sar nappe. Trondhjemites mostly intrude the ultramafics and rarely gabbroic rocks. Layered gabbros, missing in the ophiolite section, are sporadically distributed as fault-bounded blocks in the two western nappes.

Collisional emplacement history of the Naga-Andaman ophiolites and the position of the eastern Indian suture

Himalayan Journal of Sciences Vol.2(4) Special Issue 2004 p.87-8

NEW EVIDENCE FOR SUPRA-SUBDUCTION ZONE OPHIOLITES IN THE VARDAR ZONE OF NORTHERN GREECE: IMPLICATIONS FOR THE TECTONO-MAGMATIC EVOLUTION OF THE VARDAR OCEANIC BASIN

The Vermion Massif (northern Greece) is located across the boundary between the Pelagonian and Vardar Zones and includes several tectonic units bearing ophiolitic rocks, which represent remnants of the oceanic lithosphere formed in the Neotethyan Vardar Ocean, between the Pelagonian and the Serbo- Macedonian continental realms. This massif consists of tectonic units belonging to the Pelagonian Domain, which are tectonically overlain by units associated with the Almopias sub-Zone (Vardar Zone). Ophiolitic rocks consist of mantle harzburgites and ophiolitic melanges. The ophiolitic melanges incorporate rocks exhibiting a wide range of composition, including various intrusive rocks and volcanic rocks ranging from basalts, basaltic andesites, andesites, dacites, to rhyolites. Incompatible elements and rare earth elements analyses indicate that a number of different rock-types formed in distinct tectonic settings can be identified. Mantle harzburgites have a very depleted nature and represent portions of the supra-subduction (SSZ) mantle developed in an intra-oceanic arc setting. The melanges units include six rock types variably distributed in the Pelagonian and Almopias Units. They are: (1) calc-alkaline rocks with marked depletion in Nb, Ta Ti and enrichment in LREE, Th; (2) LREE-depleted N-MORB; (3) LREE-enriched E-MORB; (4) alkaline within-plate basalts showing marked enrichments in Th, Ta, Nb, LREE; (5) low-Ti island arc tholeiites featuring depletion in HFSE; and (6) very-low-Ti boninites characterized by strong depletion in HSFE and REE. Previous interpretations have referred the Vardar ophiolites to a MORB-type oceanic setting and to a MORB-type backarc setting; however, the widespread occurrence of SSZ ophiolites has never been documented in the Vardar Zone before and is particularly important as it testifies for the existence of an intra-oceanic arc basin in the Vardar oceanic domain. The results presented in this paper compared with literature data on other magmatic rocks within the Vardar Zone suggest that the opening and closure of the Vardar Ocean record several distinct accretion events in this basin, that is oceanic crust generation at mid-ocean ridge, alkaline seamounts in the oceanic domain and SSZ setting, as well as two accretion events in the western realm of the Serbo-Macedonian continent, that is volcanic arc and backarc settings. Based on the comparison between the modern west Pacific subduction system and the results from this study a new model for a multistage tectono-magmatic evolution of the Vardar Ocean is proposed.

Geology and Lithostratigraphy of the Ultramafic-Mafic Rocks and Associated Mineralizations, Wadi Khamal Area, West-Central Arabian Shield

Field, petrography and major elements geochemistry of the ultramafic -mafic rocks of Wadi Khamal area led to the recognition of the following rock groups, namely: 1) ophiolitic suite, melagabbro- norite and cumulate gabbronorite to the south of Wadi Mahalah (southern ultramafic-mafic rocks) and 2) Wadi Khamal anorthosite- gabbro complex to the north. The ophiolitic rocks in the southern area include serpentinites, peridotites and pyroxenites. The serpentinites contain large blocks of amphibolites, schists, gneisses, and metagabbros. These rocks were in- truded by melagabbronorite and cumulate gabbronorite. On the AFM diagram, these rocks plot along the MgO-FeO* side in the tholeiitic field except those of the melagabbronorite which plot within the calc- alkaline field. The Wadi Khamal complex consists of four major units; mar- ginal gabbro unit, anorthosite unit, central gabbroic unit and northern gabbronorite unit. The marginal gabbro unit and anorthosite unit have a calc alkaline differentiation trend while those of the central gabbro unit (olivine gabbro, melagabbronorite and Fe-Ti oxides-apatite gab- bronorite) and the northern gabbronorite show a distinct different trend parallel to the FeO*-MgO side line and belong to the tholeiitic differentiation trend. Based on major element geochemistry, the southern ultramafic - mafic rocks were derived from a more mafic liquid source rather different from magma source(s) which formed the Khamal anorthosite-gabbro complex.

Mesoproterozoic plate tectonics: A collisional model for the Grenville-aged orogenic belt in the Llano uplift, central Texas

Research Article| January 01, 2008 Mesoproterozoic plate tectonics: A collisional model for the Grenville-aged orogenic belt in the Llano uplift, central Texas S. Mosher; S. Mosher 1Department of Geological Sciences, Jackson School of Geosciences, University of Texas at Austin, Austin, Texas 78712, USA Search for other works by this author on: GSW Google Scholar J.S.F. Levine; J.S.F. Levine 1Department of Geological Sciences, Jackson School of Geosciences, University of Texas at Austin, Austin, Texas 78712, USA Search for other works by this author on: GSW Google Scholar W.D. Carlson W.D. Carlson 1Department of Geological Sciences, Jackson School of Geosciences, University of Texas at Austin, Austin, Texas 78712, USA Search for other works by this author on: GSW Google Scholar Geology (2008) 36 (1): 55–58. https://doi.org/10.1130/G24049A.1 Article history received: 01 May 2007 rev-recd: 05 Sep 2007 accepted: 06 Sep 2007 first online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation S. Mosher, J.S.F. Levine, W.D. Carlson; Mesoproterozoic plate tectonics: A collisional model for the Grenville-aged orogenic belt in the Llano uplift, central Texas. Geology 2008;; 36 (1): 55–58. doi: https://doi.org/10.1130/G24049A.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract The Llano uplift of central Texas, United States, exposes the core of a Mesoproterozoic orogenic belt that formed along the southern margin of Laurentia during Grenville time. A new collisional model is proposed that reconciles differences in structural stacking, apparent tectonic transport, and deformation conditions between the eastern and western portions of the uplift and explains uplift and exhumation of high-pressure eclogitic rocks, emplacement of ophiolitic rocks, and subsequent late-stage to postcollisional plutonism. Our model proposes that subduction with southward polarity resulted in collision of an exotic arc with Laurentia, emplacement of ophiolitic rocks, and telescoping of the intervening basinal sediments, followed by overriding of the arc and margin of Laurentia by a southern continent with transport toward Laurentia. The model further proposes that convergence led to subduction of the Laurentian margin, resulting in high-pressure metamorphism, but buoyancy forces due to subduction of continental crust under the southern continent resulted in uplift and retrotransport away from Laurentia, in a manner similar to that proposed for the Alpine orogeny. Slab breakoff resulted in upwelling of the asthenosphere, leading to intrusion of juvenile granitic plutons. Subduction along strike caused continued contraction that waned with time. The eastern uplift records continent-arc-continent collision, whereas the western uplift records continent-continent collision; the two regions also expose different crustal levels in the orogen. The striking similarity with Phanerozoic orogens, including emplacement of ophiolites and formation of high-pressure rocks, implies that plate tectonic processes including subduction were active prior to the Neoproterozoic. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.