Intra-oceanic Subduction Zone Research Articles

Hydrous aluminosilicate metasomatism in an intra-oceanic subduction zone: Implications from the Kurancali (Turkey) ultramafic-mafic cumulates within the Alpine Neotethys Ocean

The Kurancali ultramafic-mafic cumulate body, an allochthonous ophiolitic sliver in central Anatolia, is characterized by the presence of abundant hydrous phases (phlogopite, pargasite) besides augitic diopside, plagioclase, and accessory amounts of rutile, sphene, apatite, zircon, and calcite. Based on modes of the essential minerals, the olivine-orthopyroxene-free cumulates are grouped as clinopyroxenite, hydrous clinopyroxenite, phlogopitite, hornblendite, layered gabbro, and diorite. Petrographical, mineralogical and geochemical features of the rocks infer crystallization from a hydrous magma having high-K calc-alkaline affinity with slightly alkaline character, and point to metasomatised mantle as the magma source. Our evidence implies that the metasomatising component, which modified the composition of the mantle wedge source rock in an intra-oceanic subduction zone, was a H2O, alkali and carbonate-rich aluminosilicate fluid and/or melt, probably derived from a subducted slab. We suggest that the metasomatic agents in the subarc mantle led to the generation of a hydrous magma, which produced the Kurancali cumulates in an island-arc basement in a supra-subduction-zone setting during the closure of the Izmir-Ankara-Erzincan branch of the Alpine Neotethys Ocean.

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.

Li, B and Be Contents of Harzburgites from the Dramala Complex (Pindos Ophiolite, Greece): Evidence for a MOR-type Mantle in a Supra-subduction Zone Environment

The Pindos ophiolite represents oceanic lithosphere obducted during theJurassic.The Dramala mantle section mainly consists of highly depleted spinel harzburgite and minor plagioclase-bearing harzburgite. Textural observations and major element compositions of minerals indicate that the harzburgites experienced impregnation by a mafic, depleted melt and subsequent high-temperature (high-T) hydration and cooling (47508C) forming pargasite and edenitic hornblende. During further cooling (from 350^4008C to 51008C), talcþ tremolite serpentine olivine, serpentineþ magnetite, and finally plagioclase alteration phases formed.To test the hypothesis of a supra-subduction zone origin for the Dramala mantle, we measured Li, B and Be contents of minerals by secondary ion mass spectrometry. Whole-rock contents were measured using inductively coupled plasma^mass spectrometry and prompt gamma neutron activation analysis. We observe low Li and B contents of primary minerals (olivine, orthopyroxene, clinopyroxene) consistent with values for unmetasomatized mantle minerals; only Li contents of clinopyroxene (up to 3 7 g/g) are slightly elevated. The bulk Li contents (0 5^1 1 g/g) are in the upper range of values for unmetasomatized mantle, whereas B contents (50 04^1 1 g/g) are variable and slightly elevated compared with the unmetasomatized mantle as a result of serpentinization. Beryllium abundances in all minerals are very low (50 005 g/g), except for pargasite, where a maximum Be content of 0 012 g/g was measured. The selective addition of Li to clinopyroxene can be related to the interaction with a depleted melt, and/or to partitioning of Li into clinopyroxene upon cooling. During high-T hydration and cooling, the fluid calculated to be in equilibrium with the pargasite or edenitic hornblende (based on Li, Be and B) could have been reactionmodified seawater. Low-T hydration may have led to a very minor increase in bulk B content of most samples and to the formation of serpentine with highly variable B contents (0 1^28 g/g). Low-T hydration decreased the Li content of orthopyroxene, and Li was probably leached from some samples.The lack of correlation between degree of serpentinization and bulk B contents as well as the presence of highand low-B serpentine can be explained by low fluid^rock ratios, decreasingTduring serpentinization and lack of equilibrium as a result of fast obduction^exhumation. The low light-element contents of primary minerals and whole-rock samples clearly argue against a supra-subduction zone (SSZ) origin of the Dramala mantle section, and against the previous hypothesis of hydrous melting of the Pindos mantle above a subduction zone.We therefore conclude that the Dramala harzburgites represent a mid-ocean ridge (MOR)-type mantle, and not an SSZ-type mantle, juxtaposed with MOR-type and SSZ-type oceanic crust, either in a back-arc or in an intra-oceanic subduction zone setting.

Rapid timescales of differentiation and evidence for crustal contamination at intra-oceanic arcs: Geochemical and U–Th–Ra–Sr–Nd isotopic constraints from Lopevi Volcano, Vanuatu, SW Pacific

The extent and geochemical impact of crustal contamination during magmatic evolution in intra-oceanic subduction zone settings is often assumed to be of minimal significance but remains poorly constrained. Acquiring such information is crucial if meaningful timescales of magma generation and crustal residence beneath volcanoes are to be determined. High-MgO basalts and differentiates erupted over the last 100 years at Lopevi volcano display a strong negative correlation between 87Sr/86Sr isotope ratio and indices of differentiation (e.g. SiO2) even though these lavas have a restricted Sr–Nd isotopic range compared to other Vanuatu arc lavas. This trend provides compelling evidence for the interaction of rising mafic magmas with ‘primitive’ sub-arc crust and so this volcano affords an excellent opportunity to investigate and ascertain timescales of crustal interaction using U-series data. Quantitative geochemical modelling of whole-rock trace element ratios, 87Sr/86Sr isotope compositions and U-series data shows that assimilation of a relatively small-degree partial melt of >380 Ka mafic oceanic crust (similar to Pacific- or Indian-MORB in 87Sr/86Sr isotopic composition) during fractional crystallisation of magma exerts major control on the (230Th/232Th) and (226Ra/230Th) activity ratios of the lavas. The incorporation of higher (230Th/232Th) and lower (226Ra/230Th) assimilated material drives compositions closer to secular equilibrium than simple closed-system differentiation, reducing calculated apparent timescales of closed-system differentiation from Th isotope composition (104–105) by orders of magnitude. Modelling suggests that assimilation occurs extremely rapidly at Lopevi with timescales for magma generation, differentiation and eruption on the order of 102 years.

Geochemistry and tectonic setting of mafic rocks from the Othris Ophiolite, Greece

We present new geochemical analyses of minerals and whole rocks for a suite of mafic rocks from the crustal section of the Othris Ophiolite in central Greece. The mafic rocks form three chemically distinct groups. Group 1 is characterized by N-MORB-type basalt and basaltic andesite with Na- and Ti-rich clinopyroxenes. These rocks show mild LREE depletion and no HFSE anomalies, consistent with moderate degrees (~15%) of anhydrous partial melting of depleted mantle followed by 30–50% crystal fractionation. Group 2 is represented by E-MORB-type basalt with clinopyroxenes with higher Ti contents than Group 1 basalts. Group 2 basalts also have higher concentrations of incompatible trace elements with slightly lower HREE contents than Group 1 basalts. These chemical features can be explained by ~10% partial melting of an enriched mantle source. Group 3 includes high MgO cumulates with Na- and Ti-poor clinopyroxene, forsteritic olivine, and Cr-rich spinel. The cumulates show strong depletion of HFSE, low HREE contents, and LREE enrichments. These rocks may have formed by olivine accumulation from boninitic magmas. The petrogenesis of the N-MORB-type basalts and basaltic andesites is in excellent agreement with the melting conditions inferred from the MOR-type peridotites in Othris. The occurrence of both N- and E-MORB-type lavas suggests that the mantle generating the lavas of the Othris Ophiolite must have been heterogeneous on a comparatively fine scale. Furthermore, the inferred parental magmas of the SSZ-type cumulates are broadly complementary to the SSZ-type peridotites found in Othris. These results suggest that the crustal section may be genetically related to the mantle section. In the Othris Ophiolite mafic rocks recording magmatic processes characteristic both of mid-ocean ridges and subduction zones occur within close spatial association. These observations are consistent with the formation of the Othris Ophiolite in the upper plate of a newly created intra-oceanic subduction zone.

A GEOCHEMICAL ATTEMPT TO DISTINGUISH FOREARC AND BACK ARC OPHIOLITES FROM THE “SUPRA-SUBDUCTION” CENTRAL ANATOLIAN OPHIOLITES (TURKEY) BY COMPARISON WITH MODERN OCEANIC ANALOGUES

The Central Anatolian Ophiolite (CAO) includes oceanic crust and mantle fragments and contains all the components of a typical ophiolitic sequence: metamorphic tectonites, cumulates, isotropic gabbros, plagiogranites, dolerite sheeted dykes, basaltic lavas and sedimentary cover. They are found as dismembered but partially preserved allochthonous bodies in the Central Anatolian Crystalline Complex (CACC) representing the metamorphosed passive northern edge of the Tauride-Anatolide Platform (TAP), central Turkey. Geochemically, the magmatic rock units of the CAO display part of a dominant co-magmatic differentiated series of island-arc tholeiites (IAT). In addition, IAT are overlain by a subordinate group of boninite-like basalts which are chemically and mineralogically intermediate between IAT and more depleted boninites. The variations in lava chemistry of the CAO reflect eruption of progressively more depleted magmas through time and point to diverse mantle source compositions and partial melting. Detailed chemical analyses of the magmatic units of the CAO revealed typical supra-subduction zone (SSZ) features with depleted high field strength elements (HFSE) and light rare earth elements (LREE: LaN/YbN: < 1) and enriched large-ion lithophile elements relative to normal mid-oceanic ridge (NMORB) and back-arc basin basalts (BABB). In this respect the CAO is similar to oceanic crust generated in the Izu-Bonin and Mariana fore arcs. A N-MOR or BAB spreading seems unlikely. However, progressive depletion in the lava sequence and absence of calc-alkali basalts and their differentiates indicate that the CAO formed at an initial stage of subduction from previously depleted MORB mantle (DMM) and oceanic lithosphere, prior to development of any island arc within the Vardar-I . zmir-Ankara-Erzincan (VIAE) ocean segment of Neotethys. Accordingly, a forearc setting proposed for the genesis of the CAO is inappropriate and misleading. It was generated above a short-lived north-dipping nascent intra-oceanic subduction zone during early-middle Turonian-early Santonian, then, it was rapidly emplaced southwards onto the CACC, soon after formation between post-early Santonian and pre-middle Campanian.

The Bangong Lake ophiolite (NW Tibet) and its bearing on the tectonic evolution of the Bangong–Nujiang suture zone

The Jurassic Bangong Lake ophiolite, NW Tibet, is a key element within the western part of the Bangong–Nujiang suture zone, which marks the boundary between the Lhasa and Qiangtang blocks. It is a tectonic mélange consisting of numerous blocks of peridotite, mafic lavas and dikes. The mantle peridotites include both clinopyroxene-bearing and clinopyroxene-free harzburgites. The Cpx-bearing harzburgite contains Al-rich spinel with low Cr#s (20–25), resembling peridotites formed in mid-ocean ridge settings. On the other hand, the Cpx-free harzburgite is highly depleted with Cr-rich spinel (Cr# = 69–73), typical of peridotites formed in subduction zone environments. Mafic rocks include lavas of N-MORB and E-MORB affinity and boninites. The N-MORB rocks consist of pillow lavas and mafic dikes, whereas the E-MORB rocks are brecciated basalts. The boninites have high SiO 2 (53.2–57.9 wt%), MgO (6.5–12.5 wt%), Cr (166–752 ppm) and Ni (63–213 ppm) and low TiO 2 (0.22–0.37 wt%) and Y (5.34–8.10 ppm), and are characterized by having U-shaped, chondrite-normalized REE patterns. The N-MORB and E-MORB lavas probably formed by different degrees of partial melting of primitive mantle, whereas the boninites reflect partial melting of depleted peridotite in a suprasubduction zone environment. The geochemistry of the ophiolite suggests that it is a fragment of oceanic lithosphere formed originally at a mid-ocean ridge (MOR) and then trapped above an intraoceanic subduction zone (SSZ), where the mantle peridotites were modified by boninitic melts. The Bangong–Nujiang suture zone is believed to mark the boundary between two blocks within Gondwanaland rather than to separate Gondwanaland from Eurasia.

SHRIMP dating of the Permo-Carboniferous Jinshajiang ophiolite, southwestern China: Geochronological constraints for the evolution of Paleo-Tethys

SHRIMP U–Pb zircon dating of gabbro, anorthosite, trondhjemite and granodiorite from the Jinshajiang ophiolitic mélange of southwestern China provides geochronological constraints on the evolution of Paleo-Tethys. The ophiolitic mélange is exposed for about 130 km along the Jinshajiang River where numerous blocks of serpentinite, ultramafic cumulate, gabbro, sheeted dikes, pillow lavas and radiolarian chert are set in a greenschist matrix. A cumulate gabbro-anorthosite association and an amphibole gabbro have ages of 338 ± 6 Ma, 329 ± 7 Ma and 320 ± 10 Ma, respectively, which constrain the time of formation of oceanic crust. An ophiolitic isotropic gabbro dated at 282–285 Ma has the same age as a trondhjemite vein (285 ± 6 Ma) cutting the gabbro. These ages probably reflect a late phase of sea-floor spreading above an intra-oceanic subduction zone. At the southern end of the Jinshajiang belt, a granitoid batholith (268 ± 6 Ma), a gabbro massif (264 ± 4 Ma), and a granodiorite (adakite) intrusion (263 ± 6 Ma) in the ophiolitic mélange constitute a Permian intra-oceanic plutonic arc complex. A trondhjemite dike intruded serpentinite in the mélange at 238 ± 10 Ma and postdates the arc evolution of the Jinshajiang segment of Paleo-Tethys.

The dynamics of intra-oceanic subduction zones: A direct comparison between fossil petrological evidence (Rio San Juan Complex, Dominican Republic) and numerical simulation

Dispersed blocks of various types of metamorphic rocks in serpentinite mélanges of the northern Dominican Republic (Hispaniola) provide fossil evidence for the dynamics of the subduction zone channel in the intra-oceanic Caribbean subduction zone system between 120 and 55 Ma. Comprehensive petrological and geochronological data on three exemplary samples of eclogite and blueschist are presented that allow a series of different but interrelated pressure–temperature–time paths to be delineated. Eclogites indicate a low P/T gradient during subduction and record conditions in the nascent stages of the subduction zone. Lu–Hf data yield 103.6 ± 2.7 Ma for peak metamorphic conditions of 23 kbar/750 °C. An anticlockwise P–T path is defined. Other blocks record the continuous cooling of the evolving subduction zone and show typical clockwise P–T-paths. Omphacite blueschists reach maximum P–T-conditions of 17–18 kbar/520 °C at 80.3 ± 1.1 Ma (Rb–Sr age data). The mature subduction zone is typified by jadeite blueschists recording very high (“cold”) P/T gradients. A Rb–Sr age of 62.1 ± 1.4 Ma dates peak metamorphic P–T conditions at 16–18 kbar/340–380 °C. The array of P–T–t data allows overall cooling rates of the subduction zone at depths of c. 60 km to be constrained at 9 °C/Ma. Cooling rates and exhumation rates (i.e., vertical component of retrograde trajectories) of the metamorphic blocks are 9–20 °C/Ma and 5–6 mm/a, respectively. The derived P–T–t array is compared with a 2-D numerical subduction-zone model published by Gerya et al. [Gerya, T.V., Stöckhert, B. and Perchuk, A.L., 2002. Exhumation of high-pressure metamorphic rocks in a subduction channel: a numerical simulation. Tectonics 142, 6-1-6-19.; 45° slab dip, 40 Ma lithosphere age, convergence rates of 10–40 mm/a], which incorporates weakening of lithospheric mantle of the hanging wall by fluids emanating from the downgoing slab, resulting in an increasingly more funnel-shaped subduction channel system with time. The numerically derived array of simulated P–T–t paths as well as the calculated rates of exhumation and cooling agree well with the P–T–t data derived from the metamorphic blocks of the Rio San Juan serpentinite mélanges when convergence rates of 15 to 25 mm/a are chosen. This value is also in accord with available paleogeographic reconstructions calling for a long-term average of 22 mm/a of orthogonal convergence. On the basis of the comparison, the onset of subduction in the Rio San Juan segment of the Caribbean Great Arc can be constrained to approximately 120 Ma. This segment was thus obviously active for more than 65 Ma. An orthogonal convergence rate of 15–25 mm/a requires that a minimum amount of 975–1625 km of oceanic crust must have been subducted. Both petrological/geochronological data and numerical simulation underscore the broad spectrum of different P–T–t paths and peak conditions recorded by material subducted at different periods of time as the subduction zone evolved and matured.

Crustal growth processes as illustrated by the Neoarchaean intraoceanic magmatism from Gadwal greenstone belt, Eastern Dharwar Craton, India

Geochemical studies on metavolcanic rocks of the Gadwal greenstone belt (GGB), eastern Dharwar craton, have documented several rock types that are indicative of subduction zone tectonics reflecting on the crustal growth processes in the Dharwar craton. The dominance of komatiites in the western Dharwar craton (WDC) and the arc volcanics in the eastern Dharwar craton (EDC) is an indication for the predominance of plume magmatism in the WDC and the intraoceanic subduction zone processes in EDC which together played a significant role in the growth and evolution of continental crust in the Dharwar craton. Boninites of GGB are high calcic type with high MgO (13–24 wt.%) and a characteristic MREE depleted U-shaped REE patterns whereas the basalts have flat REE patterns with no Eu anomalies. Nb-enriched basalts exhibit slightly fractionated REE patterns with high Nb (8–26 ppm) content compared to arc basalts. Adakites of GGB are Sr depleted with highly fractionated REE patterns and no Eu anomaly compared to rhyolites. The occurrence of boninites along with arc basalts, Nb-enriched basalts–basalt–andesite–dacite–rhyolites and adakites association in Gadwal greenstone belt indicate the intraoceanic subduction zone processes with a clear cut evidence of partial melting of metasomatized mantle wedge (boninites), melting of subducting slab (adakites) and residue of adakite–wedge hybridization (Nb-enriched basalts) which have played a significant role in the growth of continental crust in the Dharwar craton during the Neoarchaean.

Composite origin of an early Variscan transported suture: Ophiolitic units of the Morais Nappe Complex (north Portugal)

The two structural units that constitute the ophiolitic complex of the Morais allochthonous complex display different geochemical features. Amphibolites from the lower unit are nearly homogeneous, chemically similar to normal mid‐ocean ridge basalts, with low Th/Nb and highly radiogenic Nd isotope signature (ɛNd = +8.7), and might derive from igneous protoliths formed in an oceanic ridge setting 447 ± 24 Myr ago (Sm‐Nd whole rock isochron). In contrast, the upper unit consists largely of metaperidotites together with gabbroic rocks and minor amphibolites and felsic veins. U‐Pb zircon ages of the upper unit range from 405 ± 1 Ma to 396 ± 1 Ma. These rocks are geochemically more variable but share in common high Th/Nb ratios and high initial ɛNd (from +7.5 to +8.9) interpreted to reflect generation above an intraoceanic subduction zone. This implies that the Morais ophiolitic complex is composite and consists of two distinct, relatively thin tectonic slivers that were detached from contrasting oceanic domains before being finally imbricated. This occurred broadly coeval, at the regional scale, with eclogite‐facies metamorphism documented in several separate tectonic units of NW Iberia. On the basis of this space and time association, we interpret the early stage of the Variscan (in the broad sense) evolution (circa ≥ 405–400 Ma) in terms of intraoceanic subduction, followed by an arc‐continent collision, circa 395 Ma. This model may account for the long‐recognized polycyclic evolution of the north Portuguese catazonal complexes without resorting to the existence of a high‐grade Precambrian basement.

Transpression and strain partitioning in the Caribbean Island-arc: Fabric development, kinematics and Ar–Ar ages of syntectonic emplacement of the Loma de Cabrera batholith, Dominican Republic

An integrative structural and geochronologic study of the Loma de Cabrera batholith (LCB, Cordillera Central, Dominican Republic) and its country rocks reveals the interplay of deformation, metamorphism and plutonism produced in the Caribbean island-arc during Late Cretaceous oblique convergence. The results emphasize the interference between three contemporaneous strain fields: (1) a northern and southern domains produced by (<95 Ma) arc-perpendicular NE- and SW-vergent folding and thrusting, respectively; (2) arc-parallel sinistral strike-slip shearing along the La Meseta shear zone (LMSZ), active during the 88–74 Ma interval; and (3) the adjacent syn-kinematic emplacement of the LCB (90–74 Ma; 40Ar/ 39Ar in hornblende) during sinistral transpressional shearing. Comparison of the structural data with strain models of oblique plate convergence suggest that the LMSZ is a preserved ductile signature of strike-slip partitioning within a sinistral transpressional intra-oceanic subduction zone. In the LCB, microstructural data indicate that the magmatic to high-temperature solid-state deformation initially occurred over a wider band of heterogeneously distributed shear deformation, and was partitioned in narrow bands of mid- to low-temperature deformation connected with the LMSZ during the cooling of the batholith. Field and geochronologic studies also suggest that shortening across the southern domain took place concurrently with sinistral strike-slip movement along the crustal-scale La Guácara and Macutico fault zones, also consistent with a transpressional setting for the Late Cretaceous Caribbean magmatic arc. Shear and fault zones were variably reactivated during Upper Eocene-Oligocene thrusting and Miocene to Recent uplift of the Cordillera Central.

Geochemical and petrological evidence for a suprasubduction zone origin of Neoarchean (ca. 2.5 Ga) peridotites, central orogenic belt, North China craton

Research Article| July 01, 2006 Geochemical and petrological evidence for a suprasubduction zone origin of Neoarchean (ca. 2.5 Ga) peridotites, central orogenic belt, North China craton Ali Polat; Ali Polat 1Department of Earth Sciences, University of Windsor, Windsor, Ontario N9B 3P4, Canada Search for other works by this author on: GSW Google Scholar Claude Herzberg; Claude Herzberg 2Department of Geological Sciences, Rutgers University, New Brunswick, New Jersey 08903, USA Search for other works by this author on: GSW Google Scholar Carsten Münker; Carsten Münker 3Institut für Mineralogie, Universität Münster, Corrensstrasse 24, 48149 Münster, Germany, and Mineralogisch-Petrologisches Institut, Universität Bonn, Poppelsdorfer Schloss, 53115 Bonn, Germany Search for other works by this author on: GSW Google Scholar Rebecca Rodgers; Rebecca Rodgers 4Department of Geological Sciences, Rutgers University, New Brunswick, New Jersey 08903, USA Search for other works by this author on: GSW Google Scholar Tim Kusky; Tim Kusky 5Department of Earth and Atmospheric Sciences, Saint Louis University, St. Louis, Missouri 63103, USA Search for other works by this author on: GSW Google Scholar Jianghai Li; Jianghai Li 6Department of Geology, Peking University, Beijing 100871, China Search for other works by this author on: GSW Google Scholar Brian Fryer; Brian Fryer 7Department of Earth Sciences, University of Windsor, Windsor, Ontario N9B 3P4, Canada, and Great Lakes Institute for Environmental Research, University of Windsor, Windsor, Ontario N9B 3P4, Canada Search for other works by this author on: GSW Google Scholar Jeremy Delaney Jeremy Delaney 8Department of Geological Sciences, Rutgers University, New Brunswick, New Jersey 08903, USA Search for other works by this author on: GSW Google Scholar GSA Bulletin (2006) 118 (7-8): 771–784. https://doi.org/10.1130/B25845.1 Article history received: 17 May 2005 rev-recd: 14 Nov 2005 accepted: 28 Feb 2006 first online: 08 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 Ali Polat, Claude Herzberg, Carsten Münker, Rebecca Rodgers, Tim Kusky, Jianghai Li, Brian Fryer, Jeremy Delaney; Geochemical and petrological evidence for a suprasubduction zone origin of Neoarchean (ca. 2.5 Ga) peridotites, central orogenic belt, North China craton. GSA Bulletin 2006;; 118 (7-8): 771–784. doi: https://doi.org/10.1130/B25845.1 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 SocietyGSA Bulletin Search Advanced Search Abstract The 2.55–2.50 Ga Zunhua and Wutaishan belts within the central orogenic belt of the North China craton contain variably metamorphosed and deformed tectonic blocks of peridotites and amphibolites that occur in a sheared metasedimentary matrix. In the Zunhua belt, dunites comprise podiform chromitites with high and uniform Cr-numbers (88). Peridotites and associated picritic amphibolites are characterized by light rare earth element (LREE)–enriched patterns and negative high field strength element (HFSE: Nb, Zr, and Ti) anomalies. They have positive initial ϵHf values (+7.9 to +10.4), which are consistent with an extremely depleted mantle composition. Mass-balance calculations indicate that the composition of the 2.55 Ga mantle beneath the Zunhua belt was enriched in SiO2 and FeOT compared to modern abyssal peridotites. These geochemical signatures are consistent with a suprasubduction zone geodynamic setting. Metasomatism of the subarc mantle by slab-derived hydrous melts and/or fluids at ca. 2.55 Ga is likely to have been the cause of the subduction zone geochemical signatures in peridotites of the Zunhua belt.In the Wutaishan belt, chromitite-hosting harzburgites and dunites display U-shaped rare earth element (REE) patterns and have high Mg-numbers (91.1–94.5). These geochemical characteristics are similar to those of Phanerozoic forearc peridotites. The dunites might have formed by dissolution of orthopyroxene in reactive melt channels, similar to those in modern ophiolites. However, they differ in detail, and they might be residues of Archean komatiites. Following the initiation of an intra-oceanic subduction zone, they were trapped as a forearc mantle wedge between the subducting slab and magmatic arc. Slab-derived hydrous melts infiltrating through the mantle wedge metasomatized the depleted mantle residue, resulting in U-shaped rare earth element (REE) patterns. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Contrasting modes of ophiolite emplacement in the Eastern Mediterranean region

Abstract The Eastern Mediterranean region is characterized by one of the largest concentrations of ophiolites anywhere in the world. Many of these ophiolites are fragmentary or highly deformed, such that their initial mode of tectonic emplacement cannot easily be inferred from the local field relations. The emplacement of many of these ophiolites can usefully be compared with the intact Oman ophiolite, one of the largest and best-studied ophiolites in the world. The Oman ophiolite is commonly believed to have been created in Late Cretaceous time (c. 95 Ma) above an oceanward-dipping, intra-oceanic subduction zone. This was followed by collision of the subduction zone with the downflexed Arabian passive margin, facilitating the emplacement of the ophiolite onto the continental margin. A less likely alternative is that the Oman ophiolite formed at a mid-ocean ridge that then collapsed, initiating the emplacement of the ophiolite. An Oman-type model is applicable to many of the Mid-Jurassic and the Late Cretaceous ophiolites of the Eastern Mediterranean region that were thrust over former passive continental margins. These ophiolites are again mainly of suprasubduction-zone type. Such ophiolites include many of the Jurassic ophiolites of Greece, Albania and former Yugoslavia, and also the Late Cretaceous ophiolites of Turkey and northern Syria. These ophiolites were emplaced from both more northerly and southerly Neotethyan ocean basins. In contrast, the opposing (northerly) margins of these oceanic basins experienced a history of subduction-accretion, marginal arc volcanism and back-arc basin formation ('Cordilleran-type' ophiolites). Ophiolites that were emplaced associated with active margin settings range from large accreted thrust sheets to small slices within accretionary prisms and back-arc basins. Examples include the Late Cretaceous ophiolites that are related both to the northern margin of the southern Neotethys and to the northern margin of the northern Neotethys in Turkey. Not all ophiolites were emplaced in response to large-scale horizontal tectonic transport (e.g. Jurassic Guevgueli ophiolite, northern Greece), and several ophiolites experienced dominantly strike-slip or transpression (e.g. the Late Cretaceous Antalya ophiolites, SW Turkey). In general, the mode of ophiolite emplacement, especially the direction of emplacement relative to the orientation of the adjacent continental margin was influenced by the regional palaeogeographical setting.

Sinking slabs below fossil subduction zones

The fate of sinking lithospheric slabs, following the cessation of subduction, is examined using a two-dimensional numerical model of mantle convection. The model incorporates rigidly moving oceanic plates, which recycle through the underlying viscous mantle, and rigid continental plates, which cannot be subducted due to their inherent buoyancy. The model is used to study the effects of the cessation of subduction due to the accretion of a 4000 km wide block of continental crust to the margin of a large stationary continent (e.g. the India–Asia collision). We model a continental block (representative of greater India) that is carried along with an oceanic plate towards a subduction zone at the leading edge of a much larger stationary continent, which represents Asia. Prior to continental collision, subducted oceanic plate material under-rides the margin of the larger continent for several hundred kilometres before peeling away from the surface boundary layer and descending into the mantle as a cold narrow slab. Consequently, once a moving continental block arrives at the continental margin and closes over the original subduction zone the surface location of the continental suture lies several hundred kilometres seaward of the location of the cold remnant of the sinking slab. The ongoing convective flow in the subcontinental mantle adjusts to accommodate the change in mechanical boundary condition at the upper surface. A step increase in viscosity at 660 km depth slows the rate of slab descent substantially but cannot account for mid-mantle thermal anomalies attributed to subduction zones that became extinct more than 140 My ago. The horizontal displacement of the slab remnants relative to the continental crustal suture depends on the relative motion of the fossil plate boundary at the surface and the underlying mantle. Over-riding of an intra-oceanic subduction zone or the indenting of a smaller subcontinent into a larger continental mass can cause the tomographically observed cold anomalies to be displaced laterally by thousands of kilometers from the location of the former converging plate boundary.

Blueschists and Blue Amphiboles: How much Subduction do they Need?

We focus on blueschist formation in evolving, nascent intra-oceanic subduction zones where cessation of subduction, e.g., by collision with continental margins, preserves the first-formed blueschist products. The Caribbean area provides a case study for a tectonic regime that has led to obducted blueschist occurrences without obvious coeval volcanism, raising the question of the minimum subduction duration and amount of convergence necessary to produce them. Systematic numerical modeling shows that for slab ages of 20-100 Ma and subduction rates of 2-14 cm/yr blueschist-facies conditions can be attained in only 0.25 to 3 m.y., with amounts of total convergence ranging between 35 and 75 km. Because of the geometrical interplay between the subducting slab "nose" and the evolving array of isotherms, younger and hotter slabs can lead to earlier blueschist formation than older, cooler slabs. There is a distinct optimum for slabs between 40 and 60 m.y. In principle, Andean-type, continental margin models yield almost identical values. Depending on the specific exhumation scenario, the time required for exhumation must in general be added to the minimum life span of the subduction zone.

Paleocene–Eocene record of ophiolite obduction and initial India‐Asia collision, south central Tibet

Uppermost Cretaceous to Eocene marine sedimentary sequences occur both to the south and north of the Yarlung Zangbo suture in south central Tibet. They consist of Indian‐margin strata of the northern Tethyan Himalaya and Asian‐margin strata of the Gangdese forearc. Both assemblages are characterized by major changes in depositional environment and sedimentary provenance at ∼65 Ma and an appearance of detrital chromium‐rich spinel of ophiolite affinity (TiO2 generally &lt;0.1 wt%) during the Paleocene. Ophiolitic melange exposed along the suture could have provided a source for detrital spinel. The melange occurs in the hanging wall of a north dipping, south directed mylonitic shear zone which includes a tectonic sliver of mafic schist. Amphibole from the schist yields 40Ar/39Ar ages of ∼63 Ma, which we attribute to cooling during slip along the shear zone and southward obduction of the melange. Melange obduction was coeval with the development of an angular unconformity within the Gangdese forearc basin to the north (between late Maastrichtian time and ∼62 Ma). Upper Paleocene to middle Eocene sandstones in the northern Tethyan Himalaya yield 200–120 Ma U‐Pb detrital zircon ages and 190–170 Ma 40Ar/39Ar detrital mica ages. These detrital grains were most likely sourced from regions north of the Yarlung Zangbo suture, suggesting that onset of India‐Asia collision in south central Tibet is middle Eocene or older in age. Collectively, our results support previous suggestions that oceanic rocks were obducted onto the northern margin of India during latest Cretaceous–earliest Tertiary time. Coeval changes in Gangdese forearc sedimentation raise the possibility that this obduction event marks onset of tectonic interaction between India and Asia at ∼65 Ma. Alternatively, in concert with the conventional view of Eocene collision initiation, the obducted oceanic rocks may be of intraoceanic origin, while coeval changes in Gangdese forearc sedimentation may be a consequence of an increase in the rate of ocean‐continent convergence following the demise of the intraoceanic subduction zone.

Rift-Drift, Seafloor Spreading, and Subduction Tectonics of Albanian Ophiolites

Albanian ophiolites in the Dinaride-Albanide-Hellenide mountain system in the Balkan Peninsula represent oceanic remnants of the Mesozoic Pindos-Mirdita basin and structurally overlie the peripheral tectonic units, composed mainly of volcanosedimentary rift assemblages and conjugate passive margin sequences. These tectonic units and the ophiolites collectively record the rift-drift, seafloor spreading, and subduction evolution of the Pindos-Mirdita basin, which developed as a Tethyan seaway between the Apulian and Korabi-Pelagonian microcontinents. The peripheral tectonic units on the west include, from west to east, the Apulian platform (Sazani, Ionian, and Kruja zones), Apulian passive margin sequences (Krasta-Cukali Zone), volcanosedimentary unit (Triassic-Upper Jurassic), subophiolitic mélange, and metamorphic assemblages; the Korabi-Pelagonian platform and its passive margin sequences bound the ophiolites on the east. Metamorphic assemblages include amphibolite, micaschist, and marble that generally display inverted field gradients, analogous to metamorphic soles beneath other ophiolites in the eastern Mediterranean region. Some metamorphic assemblages with cooling ages >169 Ma in southern Albania may represent, instead, dynamothermally metamorphosed volcanosedimentary rocks developed during advanced stages of continental rifting, prior to the onset of seafloor spreading. The subophiolitic mélange contains blocks and clasts of material derived from ophiolites, metamorphic units, platform carbonates, and passive margin sequences, and represents a subduction-accretion complex formed during initial displacement and tectonic emplacement of the ophiolites. The supra-opholitic mélange and the overlying flysch deposits (Tithonian-Valanginian) and neritic limestones (Barramian-Upper Cretaceous) unconformably rest on the ophiolites and the platform carbonates, and constrain the timing of ophiolite emplacement onto the Korabi-Pelagonian continental margin as the latest Jurassic-Early Cretaceous. The Pindos-Mirdita basin initially developed as a broad transtentional, intra-continental rift system with highly oblique, ultraslow rifting and spreading. Ophiolites in the Mirdita region show a transition from MORB to IAT and boninitic affinities from west to east and structurally upwards, indicating magma genesis from highly depleted, refractory mantle sources increasingly affected by subduction-derived fluids. These spatial, temporal, and chemostratigraphic relations point to a suprasubduction zone origin of the eastern ophiolites above a west-dipping, intra-oceanic subduction zone, whose rollback resulted in lithospheric extension and the formation of a boninitic "proto-arc" in the upper plate. This subduction-accretion system was terminated by collision of the Korabi-Pelagonian margin with the trench that facilitated eastward emplacement of the ophiolites. Subsequent oblique collision of Apulia with Eurasia during the Maastrichtian-Paleogene transported the Albanian ophiolites westward onto the Apulian margin. The bidivergent emplacement tectonics of the Albanian ophiolites is hence a result of two separate collisional events at different times.

EXPERIMENTAL HYDROUS PARTIAL MELTING OF HARZBURGITE AT SHALLOW PRESSURES: IMPLICATIONS ON HYDROUS LATE-STAGE MAGMATIC PROCESSES WITHIN THE MANTLE/CRUST TRANSITION OF THE OCEANIC LITHOSPHERE

EXPERIMENTAL HYDROUS PARTIAL MELTING OF HARZBURGITE AT SHALLOW PRESSURES: IMPLICATIONS ON HYDROUS LATE-STAGE MAGMATIC PROCESSES WITHIN THE MANTLE/CRUST TRANSITION OF THE OCEANIC LITHOSPHERE

GEOCHEMISTRY AND TECTONIC SETTING OF METAMORPHIC SOLE ROCKS AND MAFIC DIKES FROM THE PINARBAŞI (KAYSERI) OPHIOLITE, CENTRAL ANATOLIA (TURKEY)

The Pinarbasi ophiolite is located in Central Anatolia and emplaced onto the Tauride platform in Late Cretaceous. It comprises remnants of lower part of oceanic lithosphere, namely mantle tectonites tectonically underlain by high-grade metamorphic sole rocks and ultramafic to mafic cumulates. Number of isolated microgabbro-diabase and pyroxenite dikes cut the mantle tectonites at different structural levels. The mantle tectonites are dominated by harzburgite and dunite, whereas the cumulates consist of wehrlite, clinopyroxenite, olivine gabbro, troctolite and gabbronorite. The metamorphic sole rocks in the Pinarbasi ophiolite crop out as thin slices beneath the sheared serpentinites and display inverted metamorphic gradient from amphibolite to greenschist facies. The rock types in the metamorphic sole are calcschists, epidote + plagioclase + amphibole schists, plagioclase + amphibole schists, amphibole schists, plagioclase amphibolites, amphibolites. The isolated microgabbro-diabase dikes are tholeiitic in character (Nb/Y = 0.03-0.07). The REE patterns, multi-element and tectonomagmatic discrimination diagrams suggest that the isolated dikes formed in a subduction-related environment. The metamorphic sole rocks exhibit two distinct geochemical features. The first group is alkaline (Nb/Y = 1.5-2.6), whereas the second group is tholeiitic (Nb/Y = 0.05-0.22) in nature. The REE patterns, multi-element and tectonomagmatic discrimination diagrams suggest that the protholith of the first group is similar to within-plate alkali basalts, whereas the second group is more akin to island arc tholeiitic basalts. All the evidences suggest that the Pinarbasi ophiolite and the isolated dikes formed in a suprasubduction zone tectonic setting. The alkaline amphibolites were formed as a result of metamorphism of the seamount type basaltic rocks in intraoceanic subduction zone whereas the tholeiitic amphibolites were formed as result of intraoceanic thrusting in a supra-subduction zone (SSZ) basin during the closure of the Inner Tauride Ocean in Late Cretaceous.