Supra-subduction Zone Affinity Research Articles

Ophiolites in the Central Asian Orogenic Belt record Cambrian subduction initiation processes

Subduction initiation remains elusive because no present example exists. Ophiolites formed over nascent subduction zones in the past provide the key to constraining the processes of subduction initiation. Here we document three Cambrian ophiolites with supra-subduction zone affinity, which likely reflect the inception of a plate-boundary scale subduction zone within the Paleo-Asian Ocean. Our findings, together with a compilation of Cambrian ophiolites in the Central Asian Orogenic Belt, indicate diachronous subduction initiation(s) along a > 6000 kilometer zone within the Paleo-Asian Ocean between 536 and 528 million years ago. The subduction initiation of the Paleo-Asian Ocean coincides with the closure of the Mirovoi Ocean following the collision of a series of microcontinents with the Siberian craton, likely representing a typical record of collision-induced subduction jump. Our observations and numerical modeling provide a new scenario that subduction initiations would locate at oceanic weak zones rather than passive margins of accreted microcontinents during collision-induced subduction process.

Compositional signatures of ophiolitic rocks from the Dongco massif: Novel insights into the evolution of the central Tibetan Meso-Tethyan oceanic plateau

The role of the Meso-Tethyan oceanic plateau in the growth of the Tibetan Plateau is debated. In this paper, we report coexistence of large-scale mid-ocean ridge basalt (MORB)-like and enormous ocean island basalt (OIB)-type mafic rocks from the Dongco ophiolitic rocks of the Bangong–Nujiang Suture Zone. The MORB-like rocks host tholeiitic geochemical characterizes, and have supra-subduction zone affinity such as enrichments in large ion lithophile elements (LILEs), depletions in high field strength elements, high εNd(t) values (+6.5 to +9.5) and low initial 87Sr/86Sr ratios (0.7033–0.7050). The OIB-type rocks have an alkaline affinity and are characterized by enrichments in LILEs and light rare earth elements, strongly negative Sr anomalies, variable εNd(t) values of −3.7 to +7.3, and low initial 87Sr/86Sr ratios ranging from 0.7034 to 0.7050. Metamorphic zircons from amphibolites in the sole of the ophiolitic rocks yielded UPb ages of 104–100 Ma. Detrital zircons from the upper Cretaceous sandstone of the Abushan Formation (K2ab) yielded young UPb ages of 100–86 Ma. Considering the regional geology, we suggest that the Dongco MORB-like rocks were derived from a depleted mantle source and formed from an intra-oceanic subduction-related setting, representing remnants of normal oceanic crust, whereas the OIB-type rocks were derived from an enriched mantle source representing remnants of a Meso-Tethyan oceanic plateau. Our preferred hypothesis is that the Meso-Tethyan oceanic plateau experienced a phase of accretion at 104–100 Ma, which probably increased local subduction erosion of the upper plate and providing provenance for the deposition of the K2ab sandstone. The maximum depositional ages of K2ab sandstone constraint the minimum ages for closure of the Bangong–Nujiang Tethys Ocean most likely at 100–86 Ma.

Early Cretaceous subduction initiation beneath southern Tibet caused the northward flight of India

Collision between the Indian and Eurasian plates formed the ~2500 ​km long Yarlung Zangbo Suture Zone and produced the Himalaya mountains and Tibetan plateau. Here we offer a new explanation for tectonic events leading to this collision: that the northward flight of India was caused by an Early Cretaceous episode of subduction initiation on the southern margin of Tibet. Compiled data for ophiolites along the Yarlung Zangbo Suture Zone show restricted ages between 120 ​Ma and 130 ​Ma, and their supra-subduction zone affinities are best explained by seafloor spreading in what became the forearc of a north-dipping subduction zone on the southern margin of Tibet. The subsequent evolution of this new subduction zone is revealed by integrating data for arc-related igneous rocks of the Lhasa terrane and Xigaze forearc basin deposits. Strong slab pull from this new subduction zone triggered the rifting of India from East Gondwana in Early Cretaceous time and pulled it northward to collide with Tibet in Early Paleogene time.

Late Carboniferous ophiolites from the southern Lancangjiang belt, SW China: Implication for the arc–back-arc system in the eastern Paleo-Tethys

Abstract The Paleo-Tethyan orogenic record is well preserved in the Sanjiang area of SW China. However, ophiolites are commonly dismembered in orogenic belt, and complete ophiolite sequences are rare in the Sanjiang area. In this manuscript, we present a newly discovered ophiolite in the Yakou-Banpo area of the southern Lancangjiang belt in the Sanjiang area, SW China. The ophiolite displays a complete Penrose-type ophiolite sequence, composed of serpentinized peridotite, isotropic and cumulate gabbros, massive and pillow basalts, and plagiogranite. Whole-rock geochemical data indicate that these rocks were formed in an oceanic ridge setting, and they show depletions in Nb, Ta and Ti, and enrichment in Pb, suggesting a supra-subduction zone affinity. Furthermore, positive Ɛ Nd (t) (+4.5 to +6.7) and zircon Ɛ Hf (t) values (+12.4 to +14.3), as well as mantle-like δ 18 O values (~5.5‰), suggest that these rocks were derived from a long-term depleted mantle source. SHRIMP zircon U Pb dating yield weighted mean ages of 305 ± 3 Ma, 310 ± 2 Ma, and 313 ± 6 Ma. Hence, the Yakou-Banpo ophiolite belt formed during the Late Carboniferous (313–305 Ma) and records the existence of a mature ocean basin. We propose that an arc – back-arc system developed in the Sanjiang Paleo-Tethyan orogenic belt of SW China during the Late Carboniferous.

Origin and evolution of Suru Valley ophiolite peridotite slice along Indus suture zone, Ladakh Himalaya, India: Implications on melt-rock interaction in a subduction-zone environment

In this paper, we present whole-rock and mineral geochemistry of serpentinized peridotites from the Suru Valley ophiolite slice Ladakh Himalaya, in an attempt to put constraints on their petrogenesis and tectonic evolution in the context of Mesozoic Neo-Tethys Ocean. On the basis of petrographic study, Suru Valley serpentinized peridotites can be identified as serpentinized harzburgites. Relative to primitive mantle these rocks have depleted major and rare earth element (REE) geochemical characteristics comparable to ocean floor mantle rocks reflecting their mantle residual nature. However, higher abundance of highly incompatible large ion lithophile elements (e.g., Rb, Ba, Th, U, Pb and Sr), reflect metasomatism in a subduction zone environment. The presence of silicate assemblage includes Mg-rich olivine (Fo90-92) and orthopyroxene (En91-93 Fs6.4-8.7) of supra-subduction zone affinity. Evaluation of mineral and whole-rock geochemistry suggests that the Suru Valley ophiolitic peridotites represent residues left after moderate degrees of partial melting thereby underwent metasomatism in a supra-subduction zone environment related to north dipping intra-oceanic island arc during Cretaceous in the context of Mesozoic Neo-Tethys ocean.

Geochemical cycling during subduction initiation: Evidence from serpentinized mantle wedge peridotite in the south Andaman ophiolite suite

The ophiolite suite from south Andaman Islands forms part of the Tethyan Ophiolite Belt and preserves the remnants of an ideal ophiolite sequence comprising a basal serpentinized and tectonised mantle peridotite followed by ultramafic and mafic cumulate units, basaltic dykes and spilitic pillow basalts interlayered with arkosic wacke. Here, we present new major, trace, rare earth (REE) and platinum group (PGE) element data for serpentinized and metasomatized peridotites (dunites) exposed in south Andaman representing the tectonized mantle section of the ophiolite suite. Geochemical features of the studied rocks, marked by Al2O3/TiO2 > 23, LILE-LREE enrichment, HFSE depletion, and U-shaped chondrite-normalized REE patterns with (La/Sm)N > 1 and (Gd/Yb)N < 1, suggest contributions from boninitic mantle melts. These observations substantiate a subduction initiation process ensued by rapid slab roll-back with extension and seafloor spreading in an intraoceanic fore-arc regime. The boninitic composition of the serpentinized peridotites corroborate fluid and melt interaction with mantle manifested in terms of (i) hydration, metasomatism and serpentinization of depleted, MORB-type, sub-arc wedge mantle residual after repeated melt extraction; and (ii) refertilization of refractory mantle peridotite by boninitic melts derived at the initial stage of intraoceanic subduction. Serpentinized and metasomatized mantle dunites in this study record both MOR and intraoceanic arc signatures collectively suggesting suprasubduction zone affinity. The elevated abundances of Pd (4.4–12.2 ppb) with high ΣPPGE/ΣIPGE (2–3) and Pd/Ir (2–5.5) ratios are in accordance with extensive melt-rock interaction through percolation of boninitic melts enriched in fluid-fluxed LILE-LREE into the depleted mantle after multiple episodes of melt extraction. The high Pd contents with relatively lower Ir concentrations of the samples are analogous to characteristic PGE signatures of boninitic magmas and might have resulted by the infiltration of boninitic melts into the depleted and residual mantle wedge peridotite during fore-arc extension at the initial stage of intraoceanic subduction. The PGE patterns with high Os + Ir (2–8.6 ppb) and Ru (2.8–8.4 ppb) also suggest mantle rejuvenation by infiltration of melts derived by high degree of mantle melting. The trace, REE and PGE data presented in our study collectively reflect heterogeneous mantle compositions and provide insights into ocean-crust-mantle interaction and associated geochemical cycling within a suprasubduction zone regime.

Temperatures and cooling rates recorded in REE in coexisting pyroxenes in ophiolitic and abyssal peridotites

Mantle peridotites from ophiolites are commonly interpreted as having mid-ocean ridge (MOR) or supra-subduction zone (SSZ) affinity. Recently, an REE-in-two-pyroxene thermometer was developed (Liang et al., 2013) that has higher closure temperatures (designated as TREE) than major element based two-pyroxene thermometers for mafic and ultramafic rocks that experienced cooling. The REE-in-two-pyroxene thermometer has the potential to extract meaningful cooling rates from ophiolitic peridotites and thus shed new light on the thermal history of the different tectonic regimes. We calculated TREE for available literature data from abyssal peridotites, subcontinental (SC) peridotites, and ophiolites around the world (Alps, Coast Range, Corsica, New Caledonia, Oman, Othris, Puerto Rico, Russia, and Turkey), and augmented the data with new measurements for peridotites from the Trinity and Josephine ophiolites and the Mariana trench. TREE are compared to major element based thermometers, including the two-pyroxene thermometer of Brey and Köhler (1990) (TBKN). Samples with SC affinity have TREE and TBKN in good agreement. Samples with MOR and SSZ affinity have near-solidus TREE but TBKN hundreds of degrees lower. Closure temperatures for REE and Fe–Mg in pyroxenes were calculated to compare cooling rates among abyssal peridotites, MOR ophiolites, and SSZ ophiolites. Abyssal peridotites appear to cool more rapidly than peridotites from most ophiolites. On average, SSZ ophiolites have lower closure temperatures than abyssal peridotites and many ophiolites with MOR affinity. We propose that these lower temperatures can be attributed to the residence time in the cooling oceanic lithosphere prior to obduction. MOR ophiolites define a continuum spanning cooling rates from SSZ ophiolites to abyssal peridotites. Consistent high closure temperatures for abyssal peridotites and the Oman and Corsica ophiolites suggests hydrothermal circulation and/or rapid cooling events (e.g., normal faulting, unroofing) control the late thermal histories of peridotites from transform faults and slow and fast spreading centers with or without a crustal section.

Early Paleozoic magmatic record from the northern margin of the Tarim Craton: Further insights on the evolution of the Central Asian Orogenic Belt

The Southern Tianshan Orogenic Belt (STOB) is an important region to understand the prolonged evolution of the Central Asian Orogenic Belt (CAOB). A comprehensive study of zircon U–Pb ages and Lu–Hf isotopes, whole-rock elements and Sr–Nd isotopes was carried out for two intermediate-mafic intrusions in the Baicheng and Heiyingshan areas of Southwest Tianshan. LA–ICP-MS zircon U–Pb dating yielded crystallization ages of 431±3Ma (MSWD=0.073, n=31) and 430±3Ma (MSWD=0.104, n=30) for the Baicheng diorites, and 423.9±2.1Ma (MSWD=0.5, n=12) for the Heiyingshan gabbros. These data, in combination with other Late Silurian ages previously reported for the intrusive suites from the STOB, indicate an Early Paleozoic magmatic event in the region. In-situ zircon Hf isotope data on two samples from the Baicheng diorites show εHf(t) values of −2.4 to 4.9 and 0.2 to 6, and the Heiyingshan gabbros exhibit similar εHf(t) values from −1 to 1.8. The dioritic pluton exhibits Na-rich, arc-type geochemical and calc-alkaline affinity, and shows trace element patterns characterized by enrichment in large ion lithophile elements (Rb, Ba, K and Sr) and depletion in high strength field elements (Nb, Ta, Ti, Zr and Hf). These features resemble those of arc-type igneous rocks. Combined with initial 87Sr/86Sr ratios of 0.7047 to 0.7064 and εNd(t) of −1.6 to −5.06, we infer that the magma from which the Baicheng dioritic pluton formed was sourced from a mantle wedge which was subjected to metasomatism by fluids and melts related to subducted oceanic slab and the overlying sediments. The Heiyingshan gabbro shows a Na-rich nature and tholeiitic affinity. They not only display supra-subduction zone affinity, but also are geochemically similar to E-MORB, possible implying a back-arc basin setting. The initial 87Sr/86Sr ratios of the Heiyingshan samples range from 0.7046 to 0.7051 and εNd(t) values vary from 1.76 to 5.19. We speculate that the upwelling of the asthenospheric mantle to the supra-subduction wedge, facilitated by subduction roll-back, provided the source for the Heiyingshan gabbroic rocks. Together with other geologic evidence, we correlate the generation of the Baicheng diorite with the southward subduction of the South-Tianshan Ocean during the Early Paleozoic. The Heiyingshan gabbroic plutons were probably formed in a back-arc basin in response to the southward subduction of the Paleo-Tianshan Ocean. In combination with coeval arc-type magmatic rocks widely exposed in the southern margin of the Central Tianshan Block, a double-sided subduction model is proposed to explain the evolution of the South-Tianshan Ocean in the Silurian.

Geochemistry and tectonic evolution of the Late Cretaceous Gogher–Baft ophiolite, central Iran

The Late Cretaceous Gogher–Baft ophiolite is one of the best preserved remnants of Neo-Tethyan oceanic lithospheric within the inner Zagros ophiolite belt. The ophiolite comprises from bottom to top, harzburgites, pegmatite and isotropic gabbroic lenses within the mantle sequence, pillowed to massive basalts to dacites and pyroclastic rocks associated with blocks of pelagic limestone and radiolarite. Basaltic to dacitic sills crosscut the pyroclastic rocks. The ophiolite sequence is overlain by Turonian–Maastrichtian pelagic limestones (93.5–65.5Ma). Mineral compositions of harzburgites are similar to those of fore-arc peridotites and overlap with abyssal peridotites. Most Gogher–Baft ophiolite magmatic rocks show supra-subduction zone affinities, except for some E-MORB type lavas. The geochemical characteristics suggest that Gogher–Baft ophiolite magmatic rocks were generated during subduction initiation. These show progressive source depletion leading to the formation of MORB to boninitic magmas. Early E-MORB-type pillow lavas may have originated by melting mantle that was not affected by subduction components as the Tethyan oceanic plate began to sink beneath Eurasia as subduction began in the Late Cretaceous. Initial εNd (t) values range from +2.6 to +9 for Gogher–Baft magmatic rocks. Samples with radiogenic Nd overlap with least radiogenic MORBs and with Oman and other Late Cretaceous Tethyan ophiolitic rocks. The initial 87Sr/86Sr ratios range from 0.7048 to 0.7057, indicating modification due to seafloor alteration. Radiogenic 207Pb/204Pb isotopic compositions (systematically above the NHRL) and less radiogenic Nd isotopic compositions suggest the involvement of sediments in the mantle source in some magmatic rocks. Our results for Gogher–Baft ophiolite and the similarity of these to other Iranian Zagros ophiolites suggest a subduction initiation setting for the generation of these magmatic rocks.

Geodynamic evolution of Upper Cretaceous Zagros ophiolites: formation of oceanic lithosphere above a nascent subduction zone

AbstractThe Zagros fold-and-thrust belt of SW Iran is a young continental convergence zone, extending NW–SE from eastern Turkey through northern Iraq and the length of Iran to the Strait of Hormuz and into northern Oman. This belt reflects the shortening and off-scraping of thick sediments from the northern margin of the Arabian platform, essentially behaving as the accretionary prism for the Iranian convergent margin. Distribution of Upper Cretaceous ophiolites in the Zagros orogenic belt defines the northern limit of the evolving suture between Arabia and Eurasia and comprises two parallel belts: (1) Outer Zagros Ophiolitic Belt (OB) and (2) Inner Zagros Ophiolitic Belt (IB). These belts contain complete (if disrupted) ophiolites with well-preserved mantle and crustal sequences. Mantle sequences include tectonized harzburgite and rare ultramafic–mafic cumulates as well as isotropic gabbro lenses and isolated dykes within the harzburgite. Crustal sequences include rare gabbros (mostly in IB ophiolites), sheeted dyke complexes, pillowed lavas and felsic rocks. All Zagros ophiolites are overlain by Upper Cretaceous pelagic limestone. Limited radiometric dating indicates that the OB and IB formed at the same time during Late Cretaceous time. IB and OB components show strong suprasubduction zone affinities, from mantle harzburgite to lavas. This is shown by low whole-rock Al2O3and CaO contents and spinel and orthopyroxene compositions of mantle peridotites as well as by the abundance of felsic rocks and the trace element characteristics of the lavas. Similarly ages, suprasubduction zone affinities and fore-arc setting suggest that the IB and OB once defined a single tract of fore-arc lithosphere that was disrupted by exhumation of subducted Sanandaj–Sirjan Zone metamorphic rocks. Our data for the OB and IB along with better-studied ophiolites in Cyprus, Turkey and Oman compel the conclusion that a broad and continuous tract of fore-arc lithosphere was created during Late Cretaceous time as the magmatic expression of a newly formed subduction zone developed along the SW margin of Eurasia.

Geochemistry and tectonic setting of Matakaoa Volcanics, East Coast Allochthon, New Zealand: Suprasubduction zone affinity, regional correlations, and origin

In north-eastern New Zealand, the Late Cretaceous to Eocene submarine volcanic rocks of the East Coast Allochthon referred to as Matakaoa Volcanics formed a dominantly mafic melange with minor abyssal sediments during the subduction of oceanic crust. Accreted and melanged pillow lavas and dolerites display the geochemical and isotopic features of tholeiites with some diversity referred to as midocean ridge basalts, island-arc tholeiites and back-arc basin basalts. The association of supra-subduction tholeiitic magmas, abyssal sediments and polymetallic volcanogenic sulphide deposits is typical of many back-arc basins; therefore, a correlation with the alkaline and transitional basalts of the Hikurangi-Manihiki- Ontong Java Plateau may be ruled out. These petrological features and the fossil ages as well, are closely similar to those of the Tangihua Complex (Northland Allochthon) and allow Matakaoa and Tangihua mafic volcanic rocks, now separated by the modern Havre Trough, to be correlated within one single Late Cretaceous to Eocene 2 basin. The consideration of possible younger ages (Oligocene) is also discussed. The occurrence of Late Cretaceous-Eocene oceanic crust with supra-subduction zone affinities to the east of the Norfolk Ridge poses the problem of the intervening southwest-dipping subduction, the missing Late Cretaceous-Eocene arc and their relationship with Southeast Gondwana marginal break-up.

Structural architecture of the sheeted dike complex and extensional tectonics of the Jurassic Mirdita ophiolite, Albania

The Jurassic Mirdita ophiolite in northern Albania occurs in a ∼ 40 km-wide zone bounded by the passive margins of Apulia (W) and Pelagonia (E). The ∼ 3 km-thick Western Mirdita ophiolite (WMO) contains lherzolitic peridotites and gabbros overlain by basaltic pillow lavas, whereas the ∼ 12 km-thick Eastern Mirdita ophiolite (EMO) represents a Penrose-type oceanic crust with suprasubduction zone affinities. The 1 km-thick sheeted dike complex (SDC) in the EMO shows mutually intrusive relations with the underlying gabbros, is overlain by a 1.1-km-thick extrusive sequence, and records a history of complex seafloor spreading and rift tectonics in a suprasubduction zone environment. Crosscutting relations within the SDC indicate four generations of dike intrusions becoming progressively younger to the east. Early D1 and D2 dikes have basalt and basaltic andesite compositions and NNE and NNW attitudes, respectively, with moderate to gentle dips to the east. They are cut by mineralized, dike-parallel normal faults defining local grabens. Younger D3 dikes display andesitic and boninitic compositions and have WNW strikes with steep dips. These dikes are cut by both dike-parallel and dike-orthogonal faults that form locally well-developed graben structures with extensive epidosite and chalcopyrite mineralization. The youngest D4 dikes with 240°–290°orientations occur as isolated swarms intruding earlier dike generations in the eastern part of the SDC, range in composition from quartz–microdiorite to rhyodacite and rhyolite, and are cut by ∼ NW-oriented faults and shear zones. Changes in dike compositions from basalt and basaltic andesite to andesite, boninite, rhyodacite and rhyolite through time in the SDC are consistent with changes in the lava chemistry stratigraphically upward and eastward in the extrusive sequence, indicating significant chemical variations in melt compositions as the EMO evolved. Development of the SDC and its extensive normal faulting were a result of upper plate extension and rifting in the protoarc–forearc crust caused by rapid slab rollback in the Mirdita basin, reminiscent of the late Cenozoic extensional tectonics of the Izu–Bonin–Mariana system in the Western Pacific. Shifting of dike attitudes to more easterly orientations in later stages of SSZ magmatism signals a gradual change in the spreading direction likely caused by impingement of the clockwise rotating Pelagonia on the arc–trench rollback system in the south. The SSZ Mirdita basin evolved similarly to the last 100 km of recorded spreading in the West Philippine Basin, where the rotation of the seafloor spreading axis was caused by the incipient collision between the northern part of the Philippine arc with rifted fragments of Eurasia.

Neoproterozoic and Cambro-Ordovician magmatism in the Variscan K?odzko Metamorphic Complex (West Sudetes, Poland): new insights from U/Pb zircon dating

The Klodzko Metamorphic Complex (KMC) in the Central Sudetes consists of meta-sedimentary and meta-igneous rocks metamorphosed under greenschist to amphibolite facies conditions. They are comprised in a number of separate tectonic units interpreted as thrust sheets. In contrast to other Lower Palaeozoic volcano-sedimentary successions in the Sudetes, the two uppermost units (the Orla-Gologlowy unit and the Klodzko Fortress unit) of the KMC contain meta-igneous rocks with supra-subduction zone affinities. The age of the KMC was previously assumed to be Early Palaeozoic–Devonian, based on biostratigraphic findings in the lowermost tectonic unit. Our geochronological study focused on the magmatic rocks from the two uppermost tectonic units, exposed in the SW part of the KMC. Two orthogneiss samples from the Orla-Gologlowy unit yielded ages of 500.4±3.1 and 500.2±4.9 Ma, interpreted to indicate the crystallization age of the granitic precursors. A plagioclase gneiss from the same tectonic unit, intimately interlayered with metagabbro, provided an upper intercept age of 590.1±7.2 Ma, which is interpreted as the time of igneous crystallization. From the topmost Klodzko Fortress unit, a metatuffite was studied, which contains a mixture of genetically different zircon grains. The youngest 207Pb/206Pb ages, which cluster at ca. 590-600 Ma, are interpreted to indicate the maximum depositional age for this metasediment. The results of this study are in accord with a model that suggests a nappe structure for the KMC, with a Middle Devonian succession at the base and Upper Proterozoic units at structurally higher levels. It is suggested here that the KMC represents a composite tectonic suture that juxtaposes elements of pre-Variscan basement, intruded by the Lower Ordovician granite, against a Middle Palaeozoic passive margin succession. The new ages, combined with the overall geochemical variation in the KMC, indicate the existence of rock assemblages representing a Gondwana active margin. The recognition of Neoproterozoic subduction-related magmatism provides additional arguments for the hypothesis that equivalents of the Tepla-Barrandian domain are exposed in the Central Sudetes.

ISLAND ARC ORIGIN OF THE VARIEGATED FORMATIONS FROM THE EAST RHODOPE, BULGARIA - IMPLICATIONS FOR THE EVOLUTION OF THE RHODOPE MASSIF

The Variegated Formations (VF) of the eastern Rhodope Mountains (SE Bulgaria) form part of the pre-Alpine basement of the region. They are composed of alternating sediments and igneous rocks with a high-grade metamorphic overprint. Numerous ophiolitic slivers are associated with the VF and include metamorphosed peridotites, ultramafic cumulates, and amphibolitized eclogites. The dismembered ophiolites usually form the base of the VF successions. The metasedimentary rock types contain terrigenous materials (metapsammites and quartzites) that frequently alternate with metapelites and marbles. The nature of this sedimentary package, along with the field relations and sedimentary features, reflects its flysch character. The metaigneous rocks of the VF occur either as layers interbedded with the metasediments, or as intrusive bodies that intersect the ultramafic rocks. The principal mineral phases in the metabasites are amphibole + plagioclase + quartz + epidote ± garnet ± chlorite. We calculate temperatures of 630°C to 520°C at 6-2 kbar pressures, indicating moderate amphibolite facies metamorphism. Major rock-forming minerals (amphibole, plagioclase, and garnet) exhibit zoning typical of retrograde P-T conditions. When plotted on tectonic-setting discrimination diagrams, the metabasic rocks of the VF fall mainly in the fields of modern boninites and arc tholeiites. They show low Ti and Zr contents and key elemental ratios of CaO/TiO2, Al2O3/TiO2, Ti/Zr, Ti/Y and Zr/Y, all transitional between island arc tholeiites and boninites. Chondrite-normalized REE patterns reveal the existence of two different trends: U-shaped REE patterns (for the majority of samples) and LREE depleted patterns. Regardless of the existence of these two trends, the [La/Sm]N ratios of the metabasites perfectly coincide with the same ratios for many Cenozoic boninite series. The flysch character of the sedimentary sequences, as well as the clear supra-subduction zone affinities of the igneous rocks, indicates that the VF formed in an oceanic island-arc setting. The boninitic affinities of the meta-igneous rocks indicate possible origin in an immature arc. The character of the VF and its association with the dismembered ophiolite slivers shows the presence of a suture zone. The East Rhodope suture zone distinguishes the VF from the rocks structurally below it, which consist of orthogneisses typical of continental crust. Existing U-Pb zircon data indicate that the orthogneisses are of Variscan age. New U-Pb zircon age data for the VF suggest Late Neoproterozoic ages for some protoliths. Based on regional correlations, the interpretation of the VF as a fossil accretionary prism can be useful for elucidating the structure of the whole Rhodope composite terrane, and for tracing the suture itself to the Central and Western parts of the Massif.

Formation of the Late Palaeozoic–Early Mesozoic Karakaya Complex and related ophiolites in NW Turkey by Palaeotethyan subduction–accretion

Several pre-Jurassic tectonic units in NW Turkey are crucial to the current debate regarding the timing and direction of subduction of Palaeotethys, a major ocean that separated Eurasia and Gondwana in Late Palaeozoic–Early Mesozoic times. The most critical unit is the Karakaya Complex, a deformed, low-grade assemblage of oceanic origin which comprises a NW-verging, SE-dipping stack of tectonostratigraphic units, here interpreted as a Palaeotethyan accretionary complex. The units display lithologies consistent with origins in seamount, trench, abyssal and rifted carbonate platform settings. Clastic basin sequences developed on top of the complex. Other relevant tectonostratigraphic units in the area include ultrabasic rocks of supra-subduction zone affinity, which tectonically overlie Permian carbonate platform units with intervening metamorphic soles and mélanges. Restored structural trends suggest the presence of a southward-dipping Palaeotethyan subduction zone bordering Gondwana-related units during the Late Permian–Triassic. This was probably additional to more important regional northward subduction along the southern margin of Eurasia from the Eastern Mediterranean to the Himalayas.

Geochemical and isotopic (Nd, O, and Pb) constraints on granite sources in the Humber and Dunnage zones, Gaspésie, Quebec, and New Brunswick: implications for tectonics and crustal structure

Siluro–Devonian granitoids span a wide compositional range (~50–76% SiO2) and can be subdivided into two groups: (i) monzonitic or incompatible element enriched with affinities to within-plate magmatism (WPG); and (ii) calc-alkalic or incompatible element depleted with supra-subduction zone affinities (VAG). Granitoid εNd(T = 0.4 Ga) values range from −1 to +5.5; most lie between +3 and +5.5. 207Pb/204Pb isotopic compositions range from 15.52 to 15.61; most fall between ~15.55 and 15.59. Most δ18O values lie between +5.5 and +8‰. No well-established trends exist between SiO2 and isotopic composition, and isotopic compositions do not differ between the two trace element defined granitoid groups.Though Pb isotopic data are consistent with a major contribution to the granitoids from Proterozoic-aged Laurentian plate rocks (i.e., Grenville basement), Nd and O isotopic data are not. These isotopic data are consistent with major source components derived from early Paleozoic depleted or supra-subduction zone affected mantle and (or) crustal rocks derived from the early Paleozoic mantle(s). These protoliths would not have seen significant interaction with time-integrated old crustal material or surficial processes. Granitoid Pb isotopic data can be reconciled with an early Paleozoic mantle–crust origin, but it may also be that the Pb isotopes are decoupled from other isotopic systems. In either case, Nd and O isotopic data clearly prohibit the involvement of significant amounts of Grenville crust and suggest that seismic-reflection data do not define crustal blocks, or at least not blocks having a tectonic and geologic history easily related to the surface geology.