Pelagic Sedimentary Rocks Research Articles

The early history of the Neotethys archived in the ophiolitic mélange of northwestern Croatia

Ophiolite exposures in NW Croatia have been attributed to the Western Vardar Ophiolitic Unit and interpreted as derived from the Meliata–Maliac–Vardar branch of the Neotethys Ocean. Blocks within the ophiolitic mélange on Mt Ivanščica were investigated to determine the petrological and geochemical characteristics of the effusive rocks and to carry out radiolarian dating of the associated pelagic sedimentary rocks. The analysed effusive basic rocks represent chemostratigraphically uniform subalkaline high-Ti massive tholeiitic basalts characterized by an enriched composition typical of incompatible element-enriched mid-ocean ridge basalt (E-MORB). These basalts are compatible with c. 9–11% of partial melting of an enriched mantle source transitional between primitive and depleted MORB-type mantle and were formed in the non-subduction geotectonic setting of E-MORB-type rocks. This reflects an initial succession of oceanic protocrust formation and the onset of ocean spreading. Radiolarians from the chert and shale successions associated with the basalts indicate a Late Anisian–Early Ladinian age of initial ocean floor spreading, which continued into the Langobardian. The obtained data are correlative with reported blocks interpreted as remnants of the Triassic Neotethys crust from the ophiolitic mélange of the Western Vardar Ophiolitic Unit and reaffirm a common origin from a single ocean basin located east of the Adria microplate. Thematic collection: This article is part of the Ophiolites, melanges and blueschists collection available at: https://www.lyellcollection.org/topic/collections/ophiolites-melanges-and-blueschists

Frictional melting and thermal fracturing recorded in pelagic sedimentary rocks of the Jurassic accretionary complex, central Japan

Deep ocean drilling revealed that shallow seismic slip in subduction zones is localized along weak pelagic clay, resulting in devastating tsunamis. However, the actual conditions of earthquake faulting in the deeper portions of subduction zones are largely unknown. The Jurassic chert–clastic complex in central Japan was formed under a cold subduction zone, within which the termination stage of superanoxia in the Middle Triassic was recorded in the pelagic sedimentary rocks. We conducted structural and Raman spectroscopic analyses of the fault rocks distributed along the thrust fault branched from the plate-boundary décollement. The results show that preferential melting of illite, thermal erosion, wall-rock fracturing, and increased carbonization are recorded along a few millimeter-thick pseudotachylyte derived from the black carbonaceous mudstone. A comparison of the clast volume fraction between the pseudotachylyte and black carbonaceous mudstone indicates that half amount of the clasts in the pseudotachylyte originate from the incorporation of fractured wall rocks composed of pelagic chert. Thermal modeling using the thermal diffusivities of wall rocks and the measurement of wall-rock fracture density suggest that fractured wall rocks represent thermal fracturing associated with the alpha–beta transition of quartz in the chert. Our pseudotachylyte study suggests that seismic slip at the deeper portions of the subduction zones is localized along the black carbonaceous mudstone flanked with chert. The intensely fractured wall rocks may be characteristic of pseudotachylyte, which accompanies quartz-rich wall rocks with high thermal diffusivities. The coseismic thermal fracturing of wall rocks potentially affects the dilatancy-induced fluid pressure drop in the slip zone and the reduction of wall-rock stiffness, resulting in the generation of frictional melt and acceleration of seismic slip.

Early Palaeozoic reduction of the Palaeo‐Asian Ocean: Evidence from the Ordovician strata and ophiolitic mélanges of northern West Junggar, NW China

West Junggar is an important part of the Central Asian Orogenic Belt, which is the evolutionary product of the Palaeo‐Asian Ocean (PAO). In this area, sporadically distributed ophiolitic mélanges and associated Palaeozoic stratal succession provided research materials revealing the reduction and closure of the PAO. Based on the systematically reviewed Ordovician strata in the northern West Junggar, we find that the Lower Ordovician is mainly composed of ultramafic and mafic rocks of ophiolitic mélanges, the Middle Ordovician is made up of island arc volcanic rocks and pelagic sedimentary rocks from the ophiolitic mélanges, and the Upper Ordovician consists of coarse volcaniclastic turbidites. The geochemical characteristics of the Middle Ordovician radiolarian‐bearing siliceous rocks show that these deposits formed in a limited oceanic basin controlled by terrestrial sources, and that the basin‐filling succession began with the occurrence of the Upper Ordovician proximal turbidites. The above shows that the Ordovician PAO underwent an evolutionary process from oceanic basin to limited oceanic basin and to marginal sea in northern West Junggar, and constrains the age of the ocean–continent transformation to the Middle–Late Ordovician transition, with northern West Junggar entering the evolutionary stage of marginal sea in the Late Ordovician.

On the age of the Ballantrae Complex, SW Scotland

The Ballantrae Complex, SW Scotland, is an ophiolitic assemblage of mostly Early and early Middle Ordovician age (Tremadoc–Arenig in terms of the British Ordovician Series). Its varied components were generated and assembled in the Iapetus Ocean, then obducted on to the Laurentian continental margin by the earliest Llanvirn. The timing of obduction is constrained by biostratigraphic and radiometric data. It was most probably a polyphase process initiated at about the beginning of the Arenig, at around 478 Ma. However, parts of the Complex are significantly younger, with some recent evidence taken to suggest an earliest Llanvirn age of about 464 Ma for the emplacement of some of the volcanic and pelagic sedimentary rocks. The oldest strata in the succession that now unconformably overlies the Ballantrae Complex were deposited at about 463 Ma. Hence there may have been as little as one million years available for the final stages of the Complex's tectonic assembly, obduction, uplift, erosion and downfaulting. Obduction of the Complex has been invoked as a factor in the initiation of the Grampian Orogeny and, whilst there is a broad correlation in timing, the detail from Ballantrae militates against a causal relationship.

Progressive changes in lithological association of the Sanbagawa metamorphic complex, Southwest Japan: Relict clinopyroxene and detrital zircon perspectives

AbstractThe main tectono‐stratigraphic unit (Shirataki unit) of the Sanbagawa metamorphic complex in central Shikoku is characterized by abundant mafic schist layers that show the mid‐ocean ridge basalt (MORB) affinity. These MORB‐derived schist layers are absent in a southern (structurally lower) domain within the unit. Instead, sporadic occurrences of small metabasite lenses that contain relict igneous minerals (Ti‐rich augite and kaersutite) indicative of alkali basalt magmatism are newly recognized in the southern domain. Compositions of relict clinopyroxene in metabasalt are useful to identify the tectonic setting and origin of the protolith basalt, and those in each unit of the Sanbagawa metamorphic complex are presented. The metamorphic grade of the Shirataki unit generally increases structurally upwards in the southern side of the highest‐grade zone, and metamorphic zonation is subparallel to lithostratigraphic succession. The protolith assemblage of the Shirataki unit shows a distinct change from the southern low‐grade domain (lower Shirataki subunit) composed of terrigenous sedimentary rocks (mudstone and sandstone) with minor alkali basalt to the northern higher‐grade domain (upper Shirataki subunit) consisting of terrigenous and pelagic sedimentary rocks with abundant MORB. The youngest detrital zircon U–Pb ages (ca 95–90 Ma) suggest that both domains have Late Cretaceous depositional ages at the trench. Progressive peeling of oceanic plate stratigraphy during subduction can account for the observed change of lithological association in the Shirataki unit.

Structural architecture and low‐grade metamorphism of the Mikabu‐Northern Chichibu accretionary wedge, SW Japan

ABSTRACTTo understand tectono‐metamorphic processes within or close to the brittle–ductile transition of quartz‐rich crustal rocks in an accretionary wedge, an integrated field, petrological, geochronological and Raman spectroscopic study was conducted on the Mikabu‐Northern Chichibu belt in SW Japan. Field mapping in central Shikoku reveals that the Northern Chichibu belt is comprised of a pile of four tectono‐stratigraphic units, referred to as A, B, C and D units. The A unit (dominated by pelagic sedimentary rocks) represents the structurally lowest and youngest accretionary complex that forms a composite unit with the Mikabu ophiolitic suite. The B unit (consisting of chert‐clastic rock sequences) overlies the A unit and is overlain by the C and D units (mudstone‐matrix mélange units). Raman spectroscopy of carbonaceous material constrains the peak temperature of each unit to be ~290°C for the A unit, 270–290°C for the B unit, 230–250°C for the C unit and ~220°C for the D unit. Ductile deformation and pervasive metamorphism are limited to rocks in the Mikabu, A and B units. Alkali pyroxene and sodic amphibole occur in metabasite from the Mikabu, A and B units, and the widespread occurrence of prograde veins containing lawsonite+quartz pseudomorphs after laumontite was newly recognized from the C unit. Phase petrological data constrain the peak pressure of each unit to be ~0.65 GPa for the Mikabu‐A unit (aragonite stable), ~0.45–0.6 GPa for the B unit (jadeite+albite stable in the structurally lower part), and ~0.35 GPa for the C unit (prehnite+lawsonite stable). The peak metamorphic pressure increases towards structurally lower and younger accretionary complexes, but the thickness of the preserved strata is insufficient to account for the inferred pressure range. The structural–metamorphic relations imply thickening of the accretionary wedge by underplating was followed by a significant phase of thinning by both ductile and brittle processes.

Fault zone structure in pelagic sedimentary rocks: an example from the thrust fault in the Jurassic accretionary complex, central Japan

Fault zone structure in pelagic sedimentary rocks: an example from the thrust fault in the Jurassic accretionary complex, central Japan

Structure, age, and mechanism of emplacement of the Amur and Kiselevka–Manoma accretionary complexes of the lower Amur region, Russian Far East

The Amur and Kiselevka–Manoma accretionary complexes belong to the Cretaceous Khingan–Okhotsk active continental margin, which was formed in the east of Eurasia as a result of the subduction of the Pacific oceanic plates. The Kiselevka–Manoma complex is composed of oceanic pelagic and hemipelagic sedimentary rocks and intraplate oceanic basalts. It is located to the southeast, along the ocean-faced front of the Amur complex, which is predominantly composed of turbidites of the convergent boundary of lithospheric plates. The biostratigraphic study of radiolarians from rocks of the frontal part of the Amur complex allowed us to correlate them with rocks of the Kiselevka–Manoma complex and to define the period of accretion to be from the Late Aptian to the Middle Albian. The tectonostratigraphic interrelations of these two contrasting lithotectonic complexes are established and two possible models of their common emplacement are suggested. Both models suppose a continuous spatiotemporal relation of complexes with the primary paleolocation of the Kiselevka–Manoma complex in front of (on the ocean side) the Amur complex. The frontal part of the Amur complex and the Kiselevka–Manoma complex were emplaced synchronously with the western part of the East Sakhalin accretionary complex. This scenario defines the Early Cretaceous tectonic zonation of the region and the choice of the appropriate paleotectonic model. At the end of the Early Cretaceous, a single convergent boundary of the lithospheric plates is suggested with the position of the Sakhalin island arc system south of the Khingan–Okhotsk active continental margin.

Magma–sediment interaction during the emplacement of syn-sedimentary silicic and mafic intrusions and lavas into and onto Triassic strata (Circum-Rhodope Belt, northern Greece)

Abstract Within the Circum-Rhodope Belt in northern Greece, Middle Triassic neritic carbonate metasediments are locally intercalated with quartz-feldspar-phyric metarhyolites. In the same belt, Upper Triassic pelagic lime-marl-layered metasediments are similarly intercalated with low-grade metamorphosed basalt, dolerite and minor andesite and trachydacite. We interpret these sequences as due to magmatism active during the rifting event that eventually led to the opening of the Vardar Ocean. Despite the overprint of Late Jurassic deformation and low greenschist metamorphism, peperitic textures produced by magma-wet sediment interaction are well preserved at the contacts between the silicic volcanic rocks and the originally wet unconsolidated neritic carbonate sediments, suggesting contemporaneous magmatism and sedimentation. The mafic and intermediate volcanic rocks lack peperitic textures at their contacts with the pelagic sedimentary rocks. Thin margin parallel banding in the sedimentary members of the sequence indicates thermally affected original contacts with the mafic volcanic rocks only locally and at a microscopic scale. The absence of peperite in this case is attributed to the consolidated state of the sediments at the time of the mafic magma emplacement.

Geochemistry and geochronology of the Troodos ophiolite: An SSZ ophiolite generated by subduction initiation and an extended episode of ridge subduction?

New trace-element, radiogenic isotopic, and geochronologic data from the Troodos ophiolite, considered in concert with the large body of previously published data, give new insight into the tectonic history of this storied ophiolite, as well as demonstrating the variability of suprasubduction-zone ophiolites, and differences between them and commonly used modern analogs. Similar to earlier studies, we find that island-arc tholeiite of the lower pillow lava sequence erupted first, followed by boninite. We further divide boninitic rocks into boninite making up the upper pillow lava sequence, and depleted boninites that we consider late infill lavas. We obtained an Ar-Ar age from arc tholeiite of 90.6 ± 1.2 Ma, comparable to U-Pb ages from ophiolite plagiogranites. New biostratigraphic data indicate that most of the basal pelagic sedimentary rocks that conformably overlie the boninitic rocks are ca. 75 Ma. This suggests that voluminous eruption of boninitic rocks persisted until ca. 75 Ma. Limited eruption of boninitic lavas may have continued until 55.5 ± 0.9 Ma, based on the Ar-Ar age we obtained. The duration of arc magmatism at Troodos (at least 16 m.y., with some activity perhaps extending 35 m.y.) without the development of a mature arc edifice greatly exceeds that of other well-studied suprasubduction-zone ophiolites. We propose that Troodos was formed over a newly formed subduction zone, similar to many proposed models, but that the extended period of magmatism (boninitic) resulted from a prolonged period of ridge subduction.

Classification and origin of the Neoproterozoic ophiolitic mélanges in the Central Eastern Desert of Egypt

The Eastern Desert of Egypt is part of the Neoproterozoic Arabian–Nubian Shield and displays different occurrences of Neoproterozoic ophiolitic mélanges. The mélanges contain exotic and native blocks and fragments of variable sizes and types set in a sheared and schistose volcaniclastic matrix. The main exotic blocks are ophiolitic and include metamorphosed ultramafic rocks, metagabbros, massive and pillowed metabasalts and pelagic sedimentary rocks. Based on the mode of occurrences of the ophiolitic components and the processes of mélange formation, the ophiolitic mélanges of the Central Eastern Desert are classified and mapped into tectonic mélange, olistostrome and olistostromal mélange. The whole rock associations of these mélanges were subjected to two different events of deformation (D1 and D2) and contemporaneous metamorphism (M1 and M2).The metagreywackes of the mélange matrix are similar to oceanic island arc sandstone and appear to have been deposited in a back-arc or inter-arc basin. The blocks of ophiolitic metabasalts within the mélange show tholeiitic affinity and have back-arc basin (BAB) tectonic origins. Metamorphosed ultramafic blocks in the mélange display both mid-ocean ridge (MOR) and suprasubduction zone (SSZ) affinities. The ophiolitic components represent fragments of oceanic lithosphere that formed in a back-arc/arc environment, and were incorporated into the mélange through tectonic and/or sedimentary processes. Both tectonic and sedimentary processes played a major role during mélange formation in a back-arc or inter-arc setting.

The importance of early crustal architecture for subsequent basin-forming, magmatic and fluid flow events. The Granites-Tanami Orogen example

The Paleoproterozoic Granites-Tanami Orogen (GTO) consists of tightly folded, predominantly greenschist facies rocks. The orogen, a component of the North Australian Craton includes a ca. 1864 Ma succession of turbiditic and pelagic sedimentary rocks interbedded with volcanic rocks and abundant dolerite sills in a back-arc basin setting, which is intruded by collision-related ca. 1800 Ma granitic rocks. Integrated 4D interpretation of deep seismic profiles, gravity and magnetic field observations associated with available surface geology and igneous isotope geochemistry suggest that the GTO forms an imbricated crust developed on a partially inverted S-dipping, rifted Archean basement. The rifting took place in the Paleoproterozoic around 1864 Ma, and was followed by E–W orientated compressional deformation ca. 1850 Ma with the development of northerly trending isoclinal folds and associated faults. Orthogonal collisional tectonics followed at ca. 1800 Ma, which is associated with S-verging kilometre-scale, folds, faults, and orogenic lode-gold deposits in the region. Recognition of the preserved early rift architecture in the GTO has major implications for the subsequent evolution of the orogen. The early architecture has influenced all subsequent deposition, deformation, magmatic and fluid events, including spatial control of the ca. 1864 Ma rock succession to the location of intrusive bodies and fluid transfer during formation of ca. 1800 Ma orogenic gold mineralization.

An example of a Palaeoproterozoic back-arc basin: Petrology and geochemistry of the ca. 1864 Ma Stubbins Formation as an aid towards an improved understanding of the Granites–Tanami Orogen, Western Australia

The Palaeoproterozoic Stubbins Formation is a newly defined succession of turbiditic and pelagic sedimentary rocks interbedded with volcanic and abundant dolerite sills. Based on preliminary SHRIMP U–Pb data from various parts of the Stubbins Formation, the age of the formation is ca. 1864 Ma, which is about 30 million years older than that interpreted for other successions included in the Granites–Tanami Orogen of Western Australia and the Northern Territory. A study of the geochemistry of dolerite and spatially restricted basalt from the top 200 m of the Stubbins Formation indicates that these rocks: (1) have tholeiitic to calc-alkaline affinities; (2) are related to the same magmatic events; and (3) based on comparisons with modern analogues, were deposited in a back-arc basin to island-arc setting between the Halls Creek and Arunta Orogens. Deposition in the basin ceased during compressional (collisional) deformation about 1850 Ma. This was followed by collisional tectonics at ca. 1800 Ma, which is associated with orogenic lode-gold deposits in the region.

Search for traces of the late heavy bombardment on Earth—Results from high precision chromium isotopes

High precision mass spectrometric analyses of the chromium isotopic composition of metamorphosed turbiditic and pelagic sedimentary rocks and banded iron stones from the ∼3.7 Gyr Isua Supracrustal Belt (ISB) in West Greenland cannot be distinguished from the standard terrestrial 53Cr / 52Cr ratio at our present level of resolution. As a consequence, our search for chemical traces of possible impact-derived meteoritic components (asteroidal and/or cometary material, or accreted cosmic dust) in the Earth's oldest chemical and detrital sediments was negative. Our results, based on the 53Mn– 53Cr short-lived radionuclide system (half-life of 3.7 Myr), cannot confirm the recent findings by [1] [R. Schoenberg, B.S. Kamber, K.D. Collerson, S. Moorbath. Tungsten isotope evidence from approximately 3.8-Gyr metamorphosed sediments for early meteorite bombardment of the Earth. Nature 418 (2002) 403–405.] of tungsten isotope anomalies (based on the 182Hf– 182W short-lived radionuclide system; half-life of 9 Myr) in these sediments, which were interpreted as indicating a component derived from meteorites. Possible reasons for the failure to trace cosmic material in the ISB metasediments are various: 1. The samples studied are not representative; 2: The sedimentation period did not overlap with the period of late heavy bombardment of the Moon; and 3. The potential chromium anomalies, if present, are too small to be traceable by our present levels of detection. Unequivocal evidence of a late heavy bombardment on the early Earth therefore remains elusive and uncertain.

Geochemical characteristics, 40Ar– 39Ar ages and original tectonic setting of the Band-e-Zeyarat/Dar Anar ophiolite, Makran accretionary prism, S.E. Iran

The Makran accretionary prism in southeastern Iran contains extensive Mesozoic zones of melange and large intact ophiolites, representing remnants of the Tethys oceanic crust that was subducted beneath Eurasia. To the north of the Makran accretionary prism lies the Jaz Murian depression which is a subduction-related back-arc basin. The Band-e-Zeyarat/Dar Anar ophiolite is one of the ophiolite complexes; it is located on the west side of the Makran accretionary prism and Jaz Murian depression, and is bounded by two major fault systems. The principal rock units of this complex are a gabbro sequence which includes low- and high-level gabbros, an extensive sheeted diabase dike sequence, late intrusive rocks which consist largely of trondhjemites and diorites, and volcanic rocks which are largely pillow basalts interbedded with pelagic sedimentary rocks, including radiolarian chert. Chondrite- and primitive-mantle-normalized incompatible trace element data and age-corrected Nd, Pb, and Sr isotopic data indicate that the Band-e-Zeyarat/Dar Anar ophiolite was derived from a midocean ridge basalt-like mantle source. The isotopic data also reveal that the source for basalts was Indian-Ocean-type mantle. Based on the rare earth element (REE) data and small isotopic range, all the rocks from the Band-e-Zeyarat/Dar Anar ophiolite are cogenetic and were derived by fractionation from melts with a composition similar to average E-MORB; fractionation was controlled by the removal of clinopyroxene, hornblende and plagioclase. Three 40Ar– 39Ar plateau ages of 140.7±2.2, 142.9±3.5 and 141.7±1.0 Ma, and five previously published K–Ar ages ranging from 121±4 to 146±5 Ma for the hornblende gabbros suggest that rocks from this ophiolite were formed during the Late Jurassic–Early Cretaceous. Plate reconstructions suggest that the rocks of this complex appear to be approximately contemporaneous with the Masirah ophiolite which has crystallization age of (∼150 Ma). Like Masirah, the rocks from the Band-e-Zeyarat/Dar Anar ophiolite complex represent southern Tethyan ocean crust that was formed distinctly earlier than crust preserved in the 90–100 Ma Bitlis-Zagros ophiolites (including the Samail ophiolite).

Age, geochemistry and tectonic setting of Buqingshan ophiolites, North Qinghai-Tibet Plateau, China

The Buqingshan ophiolite complex is a sector of the A'nyemaqen ophiolite belt in the East Kunlun southern marginal suture zone in the northern part of the Qinghai-Tibet Plateau. The WNW-trending ophiolite complex consists of metaperidotite, gabbro, diabase, pillow basalt, massive basalt and pelagic sedimentary rocks including radiolarian chert. We have determined that Early Paleozoic and Early Carboniferous–Early Permian ophiolites are present in this ophiolite complex that were previously thought of Permian–Middle Triassic age. A zircon U–Pb age of 467.2±0.9 Ma for gabbro, and a Rb–Sr isochron age of 481±130 Ma for diabase and gabbro have been obtained from the Early Paleozoic ophiolite slice, and Middle–Late Ordovician acritarchs have been found in the mélange matrix that envelopes the sliver. We interpret the first two ages to represent the formation age of the ophiolite. Granodiorite–tonalite plutons with a zircon U–Pb age of 402±24 Ma intrude the slice. Early Carboniferous–Early Permian radiolarians have been discovered in cherts and argillaceous cherts in the second ophiolite slice. Geochemistry indicates that metaperidotites in the first slice represent a depleted ocean lithosphere mantle. Most mafic rocks in the Early Paleozoic ophiolite are of N-MORB type with small amount of T-MORB, and they resulted from the magma from the depleted oceanic lithosphere mantle with little crustal contamination. The basalts in the Early Carboniferous–Early Permian ophiolite have similar characteristics to the mafic rocks and also show DUPAL anomaly and a mixed source of DMM and EM II. The original formation environments of the two-stage ophiolites are both mid-ocean ridges. These lines of evidence suggest that a mature Early Paleozoic Kunlun–Qilian–Qinling ocean basin and a mature Paleotethyan ocean basin once existed in the study area.

Basaltic sheet intruding into Middle-Late Permian pelagic sedimentary rocks at Arrow Rocks, Waipapa Terrane, North Island, New Zealand

Basaltic sheet intruding into Middle-Late Permian pelagic sedimentary rocks at Arrow Rocks, Waipapa Terrane, North Island, New Zealand

Long-lived early Cretaceous seamount volcanism in the Mariana Trench, Western Pacific Ocean

An alkaline volcanic rock sample, peralkaline rhyolite pillow lava, in addition to some radiolarian-bearing pelagic sedimentary rocks, were collected by the Japanese submersible Shinkai 6500 from Quesada Seamount on the oceanward slope of the Mariana Trench. 40Ar– 39Ar dating resulted in a plateau age of 129.3±2.6 Ma, which is approximately 10 Myr younger than the radiolarian age of the oldest intercalated tuffaceous radiolarian claystone of early Berriasian age (approximately 140 Ma). Fragments of volcanic glass that are of alkali-basalt (hawaiite) composition may indicate that this tuffaceous claystone was formed during the shield-building volcanic stage. Because the peralkaline rhyolite is a very differentiated volcanic rock and commonly erupts in the post-shield stage, we interpret that the activity lasted for a long period (approximately 10 Myr). This long-lived shield-building stage may be due to the slow moving rate of the Pacific Plate during the Early Cretaceous.

Evolution of a submerged composite arc volcano: volcanology and geochemistry of the Normétal volcanic complex, Abitibi greenstone belt, Québec, Canada

The 4 km-thick Archean Normétal volcanic complex (NVC), composed of basaltic andesite, dacite, and rhyolite, is represented by five distinct volcanic phases and one sedimentary phase. Initial volcanic construction features effusive mafic volcanism characterized by massive, pillowed and pillow breccia flows and local massive dacite (phases 1 and 2a). Prominent felsic volcanism of phase 2 commences locally with tuffs, lapilli tuffs and lapilli tuff breccias derived either from hydroclastic or autoclastic fragmentation processes (phase 2b). The principal constructive phase of the NVC (phase 2c) is composed of pillowed andesite, massive dacite, and dominant massive, flow banded and lobate rhyolite flows. Autoclastic or hydroclastic brecciation of the former have produced rhyolitic tuff, lapilli tuff and lapilli tuff breccia. Rhyolitic volcanism continued with eruption of lava flows (phase 3) and the intrusion of dykes and felsic endogenous domes (phases 3 and 4). A subsequent 20–70 m-thick sedimentary unit, composed of volcaniclastic turbidites and pelagic background sediments, constitutes a marker horizon indicating volcanic quiescence. Renewed volcanism of phase 5 is characterized by mafic to felsic turbiditic lapilli tuffs and tuffs, and mafic to felsic flows or intrusions. The felsic lapilli tuffs, tuffs and flows host the Normétal VMS deposit. The geometry and volcanic stratigraphy of the NVC suggests emission of viscous, phenocryst-rich felsic flows from three principal centers, including a parasitic western vent, the major central 6 km-wide cauldron structure and an eastern vent. Voluminous viscous felsic lava over a large area supports the inference of numerous vents whereby individual centers coalesced to produce a composite or complex stratovolcano. Proximal to distal facies changes, variable rhyolitic unit and lobe closures argue for multiple conduits. The VMS deposits are located at the western edge of the central cauldron. Geochemical analyses show two complete compositional spectrums (phases 1, 2, 4 and 5) from basaltic andesite to rhyolite. The Zr/Y and LaN/YbN ratios of phases 1, 2, 4 and 5 show a transitional affinity whereas phase 3 is tholeiitic to slightly transitional. Multi-element diagrams suggest that all phases are consistent with subduction-related processes. The mafic-felsic NVC, a composite volcano that formed upon a shield type volcano, displays subaqueous effusive dominant volcanic construction at depth below storm wave base, as indicated by pillowed flows, turbiditic and pelagic sedimentary rocks, and massive sulphide deposits. Geochemistry and physical volcanology of the NVC are consistent with construction of an immature arc volcano. The submerged Izu-Bonin arc volcanoes may be modern analogues.

Petrology, geochemistry and tectonic setting of the Khoy ophiolite, northwest Iran: implications for Tethyan tectonics

The Khoy ophiolite in northwestern Iran represents a remnant of oceanic lithosphere formed in the Mesozoic Neo-Tethys. This northwest–southeast trending ophiolite complex consists from bottom to top (east to west) of a well-defined basal metamorphic zone, peridotites (dunite, harzburgite) and serpentinized peridotite, gabbros, sheeted dikes, pillow and massive lava flows, and pelagic sedimentary rocks, including radiolarian chert. The rocks of the metamorphic zone have an inverse thermal gradient from amphibolite facies to greenschist facies. The high-grade metamorphic rocks are immediately adjacent to the peridotite and the gabbros and the low-grade rocks are in contact with the Precambrian Kahar Formation. Based on mantle-normalized incompatible trace element diagrams there are two distinct types of basalt flows present at the Khoy ophiolite: (1) massive basalts that have patterns virtually identical to E-MORB, and (2) pillow basalts that have more primitive chemical composition whose trace element patterns plot between E-MORB and N-MORB. The chondrite-normalized REE patterns for the pillow basalts are LREE-depleted [(La N/Sm N) ave=0.70], similar to patterns for the mean diabase composition for the Oman ophiolite and LREE-depleted basalts of the Band-e-Zeyarat ophiolite of southern Iran. The REE patterns for the massive basalts are similar in general REE abundances to the pillow basalt patterns, but they are slightly LREE-enriched [(La N/Sm N) ave=1.09] and their patterns cross those of the pillow basalts. The REE patterns for the gabbros and diorites indicates that the crustal-suite rocks were most likely derived by a process of fractional crystallization from a common basaltic melt. This basaltic melt was most likely generated by approx. 20–25% partial melting of a simple lherzolite source and had REE concentrations of roughly 10× chondrite. A comparison between the results from the Khoy ophiolite and the data from other Iranian ophiolites reveals geochemical evidence to suggest a tectonic link between the Khoy ophiolite and the rest of the Iranian ophiolites. Our results suggest that Khoy ophiolite is equivalent to the inner group of Iranian ophiolites (e.g. Nain, Shahr-Babak, Sabzevar, Tchehel Kureh and Band-e-Zeyarat) and was formed as a result of closure of the northwestern branch of a narrow Mesozoic seaway which once surrounded the Central Iranian microcontinent.