Zone Ophiolites Research Articles

PETROLOGY AND ORIGIN OF PLAGIOGRANITES FROM THE DAĞKÜPLÜ (ESKİŞEHİR) OPHIOLITE ALONG THE İZMİR-ANKARA-ERZİNCAN SUTURE ZONE, TURKEY

The Izmir-Ankara-Erzincan Suture Zone (IAESZ) formed through the closure of the northern branch of the Neotethyan Ocean that once separated the Sakarya Continent and the Anatolide-Tauride Platform and the subsequent collision of these two continental blocks during Late Cretaceous-Paleocene time. Mafic-ultramafic rocks representing oceanic lithosphere, metamorphic sole rocks, and ophiolitic melange comprising oceanic and continental fragments and continental margin metamorphic and sedimentary rocks are preserved along the IAESZ. A northward dipping subduction zone developed within the Izmir-Ankara-Erzincan Ocean, a northern branch of Neotethyan Ocean, in the Early Cretaceous. The suprasubduction zone (SSZ) ophiolites formed in this intra-oceanic setting. The Dagkuplu Ophiolite was, like other ophiolite nappes in the region, thrust over the southern Anatolide-Tauride Platform. The Dagkuplu Ophiolite, exhibiting an incomplete and inverted ophiolite suite, consists mostly of refractory peridotites (harzburgite and dunite) representing mantle unit and lesser dunite and wehrlite cumulates, pyroxenites, and massive-layered gabbros. The plagiogranite volumetrically constitutes about 10% of the entire ophiolitic rocks. Petrographic and geochemical data suggest that plagiogranites were generated from TiO2-poor cumulate gabbros by fractional crystallization at a suprasubduction zone spreading center.

西モンゴル，アルタイ地域のハンタイシルオフィオライトの岩石学的特徴と形成場のテクトニックセッティング

Precambrian-Cambrian Hantaishir ophiolite complex locates in the Altai region of the West Mongolia. The complex is approximately 50 km long and 12 km wide, and its entire configuration is elongated from northwest to southeast. The complex consists of two major massifs: they are Naran massif and Taishir massif. Although the massifs are continuous and no boundary is recognized, both consist of altered harzburgite and dunite with cumulated websterite, clinopyroxenite (Taishir massif) and gabbro with volcanic rocks. Pillow lavas are dominant both in the Naran and Taishir massifs. Sheeted dikes are also major rocks in both massifs, but they are apparently subordinate volcanics in the Taishir massif to the pillow lavas. Sheeted dikes of the Taishir massif consist of boninites, tholeiitic andesites and tholeiitic basaltic andesites. A small amount of the sheeted dikes of the Naran massif are solely tholeiitic andesite in composition. Both massifs are sandwiched in the large serpentinite melange. The dominant ultramafic rock is harzburgite in the ophiolite complex and it is highly altered. Chromian spinel commonly remains as fresh mineral in the harzburgite, and they have similar composition to the chromian spinel in the harzburgite occurring in the other supra-subduction zone (SSZ) ophiolites. The mineral chemistries of rarely found highly magnesian olivine in the harzburgite and dunite are consistent with residual upper mantle origin in combination with the highly chromian spinel chemistry. The ultramafic body of the Hantaishir ophiolite could be formed in the mantle below the zone of plate collision. The REE patterns of the gabbro and volcanic rocks of the Naran massif seem almost flat and significantly differ from those of the Taishir massif: both the tholeiitic andesites and boninites of the latter are substantially depleted in LREE. The spider diagrams for both massifs show clearly different patterns as well. No significant chemical difference between boninites and tholeiitic andesites from the Taishir massif was noticed as far as trace elements are concerned. The chemical characteristics of the boninite from the ophiolite are common to the modern boninites from the West Pacific island arcs excluding REE patterns: geochronologically recent boninites have U-shaped forms. On the basis of the discrimination diagrams, the gabbros and volcanic rocks from the Hantaishir ophiolite complex can be considered to have originated near plate margin.

Platinum-Group-Element Systematics of Peridotites from Ophiolite Complexes of Northwest Anatolia, Turkey: Implications for Mantle Metasomatism by Melt Percolation in a Supra-subduction Zone Environment

Platinum-group-element (PGE) studies of peridotites from the supra-subduction zone (SSZ) ophiolites of northwest Anatolia provide evidence for the nature of melt extraction within the uppermost mantle, and interactions between subduction-related magma and oceanic lithosphere. The peridotite samples from the mantle section of the ophiolites are mainly spinel-harzburgites and dunites, accompanied by subordinate amounts of spinel-lherzolite. Whole-rock major-trace element and mineral chemical characteristics indicate that the peridotites originated as the solid residues of varying degrees of partial melting (~5 to ~20%), and were subsequently modified by interaction with metasomatizing melts. The samples have non-chondritic, fractionated chondrite-normalized PGE patterns. Melt-depleted (e.g., low Al2O3 and CaO contents) mantle harzburgites and dunites show moderate to strong enrichments in the palladium group relative to the iridium group PGEs (PdN/IrN = 1.81±0.23; N = CI-chondrite normalized), and in most samples, pronounced Rh and Pd enhancements relative to Pt (RhN/PtN = 2.31 ± 0.66; PdN/PtN = 1.93 ± 0.20). These signatures cannot be reconciled with a simple in situ melt extraction and removal of sulfide phases, but most likely reflect a multi-stage petrogenetic process that selectively enriched the local mantle environment in incompatible and less refractory siderophile elements that are mobilized during continuous melt percolation, while relatively depleting the mantle wedge in Pt, which was not as effectively mobilized by silicate melts (or fluids). The results of quantitative model calculations indicate that the addition of sulfides that originated from interaction between solid mantle and percolating hydrous basaltic melts may account for the strongly supra-chondritic ratios of both Pd/Ir and Ir/Os, as well as for the formation of abundant chromite deposits within the ophiolite complex. The peridotites show no systematic variation of Ir-group PGE (Ir, Ru, Os; I-PGE) abundances relative to melt depletion indices such as Mg#, Al2O3, CaO, or spinel Cr#, despite their remarkable inter-element PGE variations. These along with elevated values of strongly incompatible lithophile elements (e.g., Ba, U, and LREE) in the reactive harzburgites and dunites suggest a post-melting metasomatism and melt impregnation in a suprasubduction zone environment. Enrichment in various incompatible elements (Hf, U, Ta, Sr) relative to the expected values for melt-depleted mantle residues and pronounced negative anomalies in fluid-insoluble high-field-strength elements (Ta, Nb, Zr, Hf, Ti) further suggest that both siliceous melts and slab-derived hydrous fluids were involved in mantle metasomatism.

Late Neoproterozoic proto-arc ocean crust in the Dariv Range, Western Mongolia: a supra-subduction zone end-member ophiolite

An unusual late Neoproterozoic ( c . 572 Ma) ophiolite is exposed in the Dariv Range (western Mongolia), which contains intermediate to acidic lavas and sheeted dykes, and an igneous layered complex consisting of gabbro–norites, websterites, orthopyroxenites and dunites underlain by serpentinized mantle harzburgites. Based on the compositions of the crustal units and the crystallization sequences in the mafic and ultramafic cumulates we conclude that the entire oceanic crust, including the cumulates, was made from arc magmas with boninitic characteristics. The Dariv rocks bear a strong resemblance to rocks recovered from the modern Izu–Bonin–Mariana fore-arc, a fragment of proto-arc oceanic basement, and we propose that the Dariv Ophiolite originated in a similar tectonic setting. A metamorphic complex consisting of amphibolite- to granulite-facies metasedimentary and meta-igneous rocks was thrust over the ophiolite. This metamorphic complex probably represents a Cambrian arc. Thrusting started before 514.7 ± 7.6 Ma as constrained by new sensitive high-resolution ion microprobe U–Pb zircon analyses from a syn- to post-tectonic diorite. The Dariv Ophiolite is a type-example of a proto-arc ophiolite, a special class of supra-subduction zone ophiolites.

Tethyan ophiolite emplacement, Africa to Europe motions, and Atlantic spreading

Abstract Final emplacement of the mid-Jurassic and mid-Cretaceous supra-subduction zone (SSZ) ophiolites onto adjacent continental areas in the Mediterranean region is synchronous with reductions in the rate of motion between Africa and stable Europe. The Apennine-Ligurian-Alpine ophiolites lack SSZ chemistry, are mid-ocean ridge basalt (MORB)-like, range in age from c. 169 to 148 Ma, and were emplaced in late Cretaceous and Cenozoic time. The Hellenic-Dinaric SSZ ophiolites include some MORB, ranging from 173 to 168 Ma, and were emplaced, eroded, and covered by younger sediments by c. 140 Ma. The creation of the Apennine-Ligurian-Alpine and Hellenic-Dinaric suites is attributed to the motion of Adria, which formed a promontory on Africa, or essentially moved with it, as the central Atlantic opened. Extension to the west of Adria gave rise to the Ligurian Sea, generating MORB crust; to the east, a pre-existing Triassic ocean was subducted, with rollback creating Jurassic SSZ ophiolites that were emplaced onto adjacent continental margins in the later stages of convergence. The two episodes of slower motions between Africa and Europe are attributed to two episodes of attempted subduction of a passive continental margin. This speculation suggests that emplacement of some SSZ ophiolites may exert a significant control on oceanic spreading patterns.

Lower Permian formations of the Buqingshan Mountains in the A'nyemaqen ophiolitic zone (Eastern Kunlun, Qinghai Province, China): On the beginning of the Paleotethys closure

Lower Permian formations of the Buqingshan Mountains in the A'nyemaqen ophiolitic zone (Eastern Kunlun, Qinghai Province, China): On the beginning of the Paleotethys closure

MORB-type vs. BARB-type ophiolites of the Dinarides: geologic and geochemical data

The Dinaridic Alpine ophiolites can be divided into two groups: 1) those of the Jurassic Dinaride Ophiolite Zone (DOZ) and 2) those of the Cretaceous-Early Paleogene (?) Sava-Vardar Zone (SVZ). However, geologically and petrologically they have so far been treated as a whole, and their geotectonic setting has not been reliably established. We have investigated ophiolite terranes from both regions in order to determine their tectonic setting. Our comparison is based on trace element data and characteristic elemental ratios correlated with ophiolite settings in recent oceans. We show that the DOZ tectonite peridotites are enriched in LREE and have high Ti/V ratios (up to 23), whereas the SVZ mantle rocks are LREE-depleted and have low Ti/V ratios (1-4). The DOZ cumulate gabbros and peridotites are characterized by increased Ni content and high Ba/Sr ratio, positive Eu anomalies and lower Cr# of spinels, comparable with MORB cumulates. The SVZ cumulates do not show a positive Eu anomaly and are characterized by higher concentrations of V, Zr and Y, and higher K/Nb, Ti/Zr, Zr/Sm ratios, which correlate with those from some recent back-arc settings. Essential differences between DOZ and SVZ basaltic rocks are as follows: high-Ti vs. low-Ti ophiolites: Ti/V = 20-50 vs. <20; higher Mg#, Ni, Cr, Nb and Ta vs. higher Rb and Ba; elemental ratios of Sr/Zr, Ti/Zr, Y/Nb, Zr/Nb, Th/Nb, Th/Ta, La/Nb, La/Ta and Sm/Hf are similar to those from recent back-arc settings. These geochemical data confirm the earlier geologic model of Pamić et al. (2002). These contrasting geochemical signatures are the result of different geochemical processes taking place in two different settings: a) MORB along an accretionary plate margin underlain by oceanic upper mantle, and b) BARB along a supra-subduction zone characterized by an obducting upper mantle wedge underlain by subducted oceanic crust, which was a source of fluids.

40Ar/ 39Ar mineral ages from basement rocks in the Eastern Kunlun Mountains, NW China, and their tectonic implications

40Ar/ 39Ar dating and estimates of regional metamorphic P– T conditions were carried out on the basement rocks of the Eastern Kunlun Mountains, Western China. Samples from the Jinshuikou, Xiaomiao, Kuhai, Wanbaogou, and Nachitai groups revealed distinct metamorphic events and four age groups. The age group in the range from 363 to 439 Ma is interpreted to represent cooling after Middle Silurian–Late Devonian granulite(?) and amphibolite facies metamorphism, which is dominated by low–middle pressure/high temperature conditions. This tectono-thermal event is related to the closure of an oceanic basin or marginal sea. An age group of 212–242 Ma represents cooling after Triassic metamorphic overprint, which is probably associated with magmatic intrusions. This thermal event, together with the Permo-Triassic ophiolite zone along the South Kunlun Fault, relates to the closure of a major ocean (between India and Eurasia) and the eventual N-ward accretion of the Qiangtang block in Permo-Triassic times. The significance of the age group of 104–172 Ma may be related to the ductile deformation along the Xidatan fault due to the northward-directed accretion of the Lhasa block. Biotites from Nachitai record a partial isotopic resetting at ca. 32 Ma that is interpreted to represent a late-stage exhumation caused by further crustal shortening.

Neotethys and the India–Asia collision: Insights from a palaeomagnetic study of the Dazhuqu ophiolite, southern Tibet

Since 1999, independently derived geophysical and geological models have been published arguing for an intra-oceanic subduction system along essentially the entire width of the India–Eurasia collision belt. This idea conflicts with earlier proposals, where in the eastern part of the convergence zone Neotethyan mid-ocean ridge-generated lithosphere directly north of the Indian craton was consumed beneath Eurasia in Tibet in an Andean-type plate configuration. New palaeomagnetic data are reported from three Barremian–Aptian (∼ 120 Ma) sequences of chert, siliceous mudstones and volcaniclastic rocks. These rocks form the uppermost part of the Dazhuqu supra-subduction zone ophiolite terrane, which crop out along substantial portions of the India–Eurasia (= Yarlung Tsangpo) suture zone in southern Tibet. The declination data provide little regionally-useful tectonic information; they are dominated by local rotations, presumably related to the Dazhuqu terrane's initial obduction onto the India plate in the Palaeocene and subsequent movement(s) as India later collided and indented into Eurasia. The inclination data are, however, useful because they yield consistent sub-equatorial formation sites (2.7 °S ± 3.0°, 7.9 °N ± 2.7°, 1.4 °N ± 5.7°), which correspond with the location of the Neotethyan intra-oceanic subduction system inferred from the seismic tomographic data. At the time these Dazhuqu terrane rocks formed, the intra-Tethyan subduction zone would have been about 2500 km south of Eurasia.

Radioisotopic and biostratigraphic age relations in the Coast Range Ophiolite, northern California: Implications for the tectonic evolution of the Western Cordillera

The Coast Range ophiolite (CRO) in northern California includes two distinct remnants. The Elder Creek ophiolite is a classic suprasubduction zone ophiolite with three sequential plutonic suites (layered gabbro, wehrlite-pyroxenite, quartz diorite), a mafi c to felsic dike complex, and mafi c-felsic volcanic rocks; the entire suite is cut by late mid-oceanic-ridge basalt (MORB) dikes and overlain by ophiolitic breccia. The Stonyford volcanic complex (SFVC) comprises three volcanic series with intercalated chert horizons that form a submarine volcano enclosed in sheared serpentinite. Structurally below this seamount are melange blocks of CRO similar to Elder Creek. U/Pb zircon ages from plagiogranite and quartz diorites at Elder Creek range in age from 165 Ma to 172 Ma. U/Pb zircon ages obtained from CRO melange blocks below the SFVC are similar (166‐172 Ma). 40 Ar- 39 Ar ages of alkali basalt glass in the upper SFVC are all younger at ≈164 Ma. Radiolarians extracted from chert lenses intercalated with basalt in the SFVC indicate that the sedimentary strata range in age from Bathonian (Unitary Association Zone 6‐6 of Baumgartner et al., 1995a) near the base of the complex to late Callovian to early Kimmeridgian (Unitary Association Zones 8‐10) in the upper part. The SFVC sedimentary record preserves evidence of a major faunal change wherein relatively small-sized, polytaxic radiolarian faunas were replaced by very robust, oligotaxic, nassellarian-dominated faunas that included Praeparvicingula spp. We suggest that CRO formation began after the early Middle Jurassic (172‐180 Ma) collision of an exotic or fringing arc with North America and initiation of a new or reconfi gured east-dipping subduction zone. The data show that the CRO formed prior to the Late Jurassic Nevadan orogeny, probably by rapid forearc extension above a nascent subduction zone. We infer that CRO spreading ended with the collision of an oceanic spreading center ca. 164 Ma, coincident with the oldest high-grade blocks in the structurally underlying Franciscan assemblage. We further suggest that the “classic” Nevadan orogeny represents a response to spreading center collision, with shallow subduction of young lithosphere causing the initial compressional deformation and with a subsequent change in North American plate motion to rapid northward drift (J2 cusp) causing sinistral transpression and transtension in the Sierra foothills. These data are not consistent with models for Late Jurassic arc collision in the Sierra foothills or a back-arc origin for the CRO.

Paleoproterozoic arc and ophiolitic rocks on the northwest-margin of the Trans-Hudson Orogen, Saskatchewan, Canada: their contribution to a revised tectonic framework for the orogen

A new lithotectonic framework for the northwestern Reindeer Zone of the Trans-Hudson Orogen divides rocks into five northwest- to north-dipping volcano-sedimentary assemblages: (1) at the structural base, the 1.92–1.87 Ga largely sedimentary Levesque Bay Assemblage (partly equivalent to former ‘MacLean Lake gneisses’), which lies within the confines of the Kisseynew Domain and is tectonically imbricated with metasedimentary rocks of the <1.85 Ga McLennan and Burntwood groups; (2) the turbiditic Duck Lake Assemblage, also located along the northern edge of the Kisseynew Domain; it contains detrital zircons ranging in age between 1.92 and 1.87 Ga; (3) the ?1.92 Ga mafic–ultramafic volcano-plutonic Lawrence Point Assemblage of the La Ronge Domain; (4) the ≥1.88 Ga felsic to intermediate volcano-plutonic Reed Lake Assemblage of the La Ronge Domain; and (5) the turbiditic Milton Island Assemblage of the Rottenstone Domain, which contains detrital zircons ranging in age between 2.83 and 1.86 Ga. The assemblages are intruded by a variety of 1.91–1.78 Ga mafic to felsic plutons. The Lawrence Point Assemblage is interpreted as a dismembered supra-subduction zone ophiolite. High-MgO refractory harzburgite (‘Group 1’ ultramafic rocks), at the structural base of the assemblage, is geochemically identical to the upper mantle section of selected supra-subduction zone ophiolites and mantle tectonites. Chromite and olivine compositions of the ‘Group 1’ ultramafic rocks are also comparable to those of ophiolitic harzburgite and mantle tectonite. Mafic metavolcanic rocks of the assemblage are classified as subalkaline tholeiitic basalts. Their trace element patterns and Hf, Ta, Th, Y, Nb, and La element ratios resemble those of modern back-arc basin basalts. The Reed Lake Assemblage represents a subduction-generated arc complex that was built on top of the Lawrence Point Assemblage; its mafic metavolcanic rocks are subalkaline basalts, with calc-alkaline trends, and elevated Th and Ce concentrations and negative Nb anomalies. Feldspar porphyry dykes intruding the Lawrence Point and Duck Lake assemblages constrain timing of Lawrence Point ophiolite emplacement onto the Duck Lake Assemblage to 1.86–1.84 Ga. The trace element geochemistry of the dykes suggests continued arc volcanism after ophiolite emplacement. Mafic metavolcanic rocks of the Levesque Bay Assemblage are geochemically similar to those of the Lawrence Point Assemblage. Other ultramafic rocks (peridotite to pyroxenite) are abundant in the Lawrence Point Assemblage, but have similar geochemistry to small ultramafic bodies intruding the Reed Lake, Duck Lake and Levesque Bay Assemblages. They represent a separate, later phase (?1.86 Ga) of ultramafic plutonism, which post-dates ophiolite emplacement. Timing of Lawrence Point ophiolite emplacement (between 1.86 and 1.84 Ga) and geochemistry of later felsic and mafic/ultramafic volcanism suggest that the Lawrence Point ophiolite and overlying Reed Lake arc assemblage were not accreted to the Hearne Craton prior to 1.86 Ga, but were first accreted to the Flin Flon–Glennie Complex after 1.86 Ga.

Petrological and geochemical evidence for the origin of the Yarlung Zangbo ophiolites, southern Tibet

The Early Cretaceous Yarlung Zangbo Suture Zone (YZSZ) ophiolites form a nearly continuous E–W-trending belt in southern Tibet. They are remnants of the Neo-Tethys ocean-floor, marking the suture between the Indian and Eurasian plates. Seven ophiolitic massifs in the Xigaze area (from west to east: Lhase, Jiding, Beimarang, Qunrang, Bainang, Dazhuqu and Jinlu), the focus of this study, consist of crustal (volcanic and subvolcanic rocks and cumulates) and mantle (peridotites) units. Crustal samples (basalts and diabases) are subdivided into two groups. The rocks of the first group are LREE-depleted, with [La/Yb] CN=0.6±0.1 and with slight negative Ta–Nb anomalies. They have back-arc basin affinity and occur in the western portion of the study area (Jiding, Beimarang, Qunrang, Bainang massifs and part of the Dazhuqu massif). The second group includes rocks that are LREE-enriched, with [La/Yb] CN=4.6±1.4, and display distinct negative Ta–Nb anomalies. These rocks have an intraoceanic arc affinity and occur in the eastern segment of the study area (the Jinlu massif and part of the Dazhuqu massif). Rocks from the mantle parts of the massifs are mainly harzburgites and can also be subdivided into two groups. Harzburgites of the first group occur in the western part (the Lhase massif and part of the Qunrang massif); they are LREE-depleted with [La/Sm] CN=0.3±0.2, and their spinel composition shows a limited compositional range. The rocks represent the residue after 7–12% melting of a N-MORB-like mantle source. The mantle rocks of the second group occur in the eastern segment of the area (part of the Qunrang massif and in the Bainang and Dazuhuqu massifs). They are LREE-enriched, with [La/Sm] CN=2.8±0.8, and their spinels have a highly variable composition (Cr# varies from 0.13 to 0.82 and Mg# from 0.75 to 0.27). These rocks are inferred to be residue after 30–40% melting of a depleted mantle source but were metasomatically modified by a Ti-poor fluid or magma.

Tethyan vs. Cordilleran ophiolites: a reappraisal of distinctive tectono-magmatic features of supra-subduction complexes in relation to the subduction mode

Supra-subduction zone (SSZ) ophiolites deserve special attention because they represent fundamental markers of intraoceanic convergence and generation of new lithosphere above subduction zones. Moreover, owing to their structural characteristics and location in the overriding plate, these complexes are far better represented and preserved than Mid-Ocean-Ridge-Basalt (MORB) ophiolites in orogenic belts. In terms of their structure, tectonics, and magmatic features, SSZ ophiolites may be classified in two main types: (1) “Tethyan complexes” (such as those of the Albanide-Hellenide belt), which mostly consist of complete and extensive volcanic, dyke, plutonic, and mantle sections with prevalent Island Arc Tholeiitic (IAT) affinity and the significant presence of Boninites, obducted as relatively intact lithospheric slabs onto collisional continental margins; (2) “Cordilleran complexes” (including the circum-Caribbean ophiolites of Guatemala, Cuba, and Venezuela), mostly represented by dismembered sections of arc volcanic, plutonic, and subordinate mantle sequences with tholeiitic to Calc-Alkaline (IAC) magmatic affinity and acidic differentiates, commonly associated with metamorphic “subduction complexes” and tectonically emplaced onto or juxtaposed against the continental margin within polygenetic terranes. These two types appear to be related to significantly different subduction modes and intraoceanic plate dynamics whereby SSZ ophiolites were generated. The Tethyan complexes can best be compared with west Pacific-type subduction with accentuated steepening and retreat of the subducted slab, accompanied by progressive decoupling of the converging plates, intense mantle diapirism, and tensional events in the upper plate with generation of large IAT-sheeted dike complexes in “open” oceanic spreading systems. Continuous slab sinking and roll back allow increasing asthenospheric diapirism and interarc spreading from the arc axis to the forearc region with generation of boninites and Back Arc Basin Basalts (BABB) with intermediate MORB/IAT composition. By contrast, the genesis of the Cordilleran complexes requires a subduction mode characterized by a steady state regime, with moderate and constant dip of the subducted slab and limited extension in the backarc region. The magmatic evolution of these ophiolites from IAT to IAC and the significant presence of rhyodacite (and tonalite) differentiates coherently indicate a more mature stage of arc magmatism, as well as the occurrence of efficient differentiation processes developing under nearly “closed-system” conditions in independent magma chambers. Tethyan complexes are likely to be obducted as large and relatively intact slabs onto the continental margins through the interposition of metamorphic soles, which represent relics of the MORB lithosphere underplating the SSZ ophiolites. On the other hand, the common emplacement of Cordilleran complexes within polygenetic terranes appears to be controlled by prolonged accretionary mechanisms that trap, against the continental margin, parts of the arc structure, subduction complexes, melanges, and volcanoclastic products.

Geochemical evidence for the tectonic setting of the Harmancik ophiolites, NW Turkey

The Harmancik, Orhaneli and Tavşanlı ophiolites of NW Turkey are remnants of oceanic lithosphere formed in the Neo-Tethys during the Mesozoic. These ophiolites constitute part of the İzmir–Ankara suture zone which separates the Pontides to the north from the Anatolides and Taurides to the south. The Harmancik area includes a metamorphic basement, ophiolitic melange, peridotite (dunite, harzburgite), serpentinized peridotite, doleritic dykes and Neogene cover sediments. Metamorphic rocks in the Harmancik area are psammites, pelites and marbles, metamorphosed under blueschist facies conditions. They are overlain tectonically by peridotites cut by basic dykes. The dykes are dolerites, and based on texture and mineralogy, can be divided into three petrographic groups, coarse, medium-grained and porphyritic. These groups have similar chemical compositions with respect to major, trace and REE elements. When plotted on a basalt discrimination diagram the dykes plot in the IAT (Island Arc Tholeiite) field and are considered to have been intruded during subduction zone magmatism. Chondrite-normalized REE patterns for the dykes are Eu-depleted, similar to patterns for the mean dolerite composition of the NE Anatolian ophiolites, and are slightly LREE-enriched. Our results suggest that the Harmancik ophiolite is a supra-subduction zone ophiolite on which a volcanic arc sequence was built, and that the dolerite dykes were formed by the partial melting of the underlying mantle.

Fragments of Vendian-Early Carboniferous Oceanic Crust of the Paleo-Asian Ocean in Foldbelts of the Altai-Sayan Region of Central Asia: Geochemistry, Biostratigraphy and Structural Setting

Detailed geological, geochemical and biostratigraphic studies of rocks from basaltic-sedimentary terranes in the Kurai and Katun accretionary wedges (Vendian-Middle Cambrian units), the Charysh-Terekta strike-slip zone (Late Cambrian-Early Ordovician units), and the Chara ophiolite-bearing strike-slip zone (Late Devonian-Early Carboniferous units) have been undertaken. The Early Cambrian accretionary wedges record a stage of the Kuznetsk-Altai island arc evolution. The Charysh-Terekta strike-slip zone records evidence of the Late Devonian collision of the Gondwana-derived Altai-Mongolian terrane and the Siberian continent. The Chara ophiolitic zone was formed during the Late Carboniferous-Permian collision of the Siberian and Kazakhstan continents. Our study of these fragments of oceanic crust led us to conclude that intra-plate volcanism was active at the early stages of the Paleo-Asian oceanic evolution, in a period from the Vendian to the Early Carboniferous. Fragments of weakly to strongly differentiated oceanic and island-arc basalts have been preserved in accretion-collision zones and give information about chemical composition, petrology and tectonic setting of the oceanic crust at these times. The geochemical data indicate that the Altai and East Kazakhstan metabasalts could have been formed at mid-oceanic ridges, oceanic islands or oceanic plateau of the Paleo-Asian Ocean. Our interpretation of structural, lithological, geochemical and biostratigraphic data shows that the structure and composition of the oceanic lithosphere of the Paleo-Asian Ocean were similar to those of the present Pacific Ocean.

Multi-Stage Origin of the Coast Range Ophiolite, California: Implications for the Life Cycle of Supra-Subduction Zone Ophiolites

The Coast Range ophiolite of California is one of the most extensive ophiolite terranes in North America, extending over 700 km from the northernmost Sacramento Valley to the southern Transverse Ranges in central California. This ophiolite, and other ophiolite remnants with similar mid-Jurassic ages, represent a major but short-lived episode of oceanic crust formation that affected much of western North America. The history of this ophiolite is important for models of the tectonic evolution of western North America during the Mesozoic, and a range of conflicting interpretations have arisen. Current petrologic, geochemical, stratigraphic, and radiometric age data all favor the interpretation that the Coast Range ophiolite formed to a large extent by rapid extension in the fore-arc region of a nascent subduction zone. Closer inspection of these data, however, along with detailed studies of field relationships at several locales, show that formation of the ophiolite was more complex, and requires several stages of formation. Our work shows that exposures of the Coast Range ophiolite preserve evidence for four stages of magmatic development. The first three stages represent formation of the ophiolite above a nascent subduction zone. Rocks associated with the first stage include ophiolite layered gabbros, a sheeted complex, and volcanic rocks with arc tholeiitic or (more rarely) low-K calc-alkaline affinities. The second stage is characterized by intrusive wehrlite-clinopyroxenite complexes, intrusive gabbros, Cr-rich diorites, and volcanic rocks with high-Ca boninitic or tholeiitic ankaramite affinities. The third stage includes diorite and quartz diorite plutons, felsic dike and sill complexes, and calc-alkaline volcanic rocks. The first three stages of ophiolite formation were terminated by the intrusion of mid-ocean ridge basalt dikes, and the eruption of mid-ocean ridge basalt or ocean-island basalt volcanic suites. We interpret this final magmatic event (MORB dikes) to represent the collision of an active spreading ridge. Subsequent reorganization of relative plate motions led to sinistral transpression, along with renewed subduction and accretion of the Franciscan Complex. The latter event resulted in uplift and exhumation of the ophiolite by the process of accretionary uplift.

Geology and petrology of the Austroalpine Châtillon slice, Aosta valley, western Alps

Slices of continental crust pertinent to the lower Austroalpine domain of the western Alps, crop out within the ophiolitic Piemonte Zone. Among them, the Châtillon slice was studied in detail. The slice consists of orthogneiss with subordinate metabasics and very minor paraschist. The garnet-phengite-epidote-albite orthogneiss is characterised by polyphase garnet porphyroclasts. Metabasics consist of prasinite lenses and eclogite relics. Phengite-clinozoisite eclogite is characterised by small garnet idioblasts with prograde zoning; jadeite content in omphacite increases towards the rim; Si content in phengite decreases towards the rim. Garnet-glaucophane-phengiteparagonite micaschist is characterised by polymetamorphic garnet porphyroclasts, and small Alpine garnet idioblasts. A pre-Alpine amphibolite-facies metamorphism is inferred for the polymetamorphic rocks of the Châtillon slice. Paragneiss and micaschist probably derive from pre-Alpine “kinzigites”; the orthogneiss protolith was a late-Variscan porphyritic granitoid. Thermobarometry in the eclogite constrains the metamorphic peak at T ≤ 560 °C and P = 16 kbar. The HP minerals were partly retrogressed to greenschist-facies assemblages during the late Alpine tectono-metamorphic recrystallisation. The inferred Alpine P-T conditions are consistent with those for other Penninic and Austro-Alpine nappes of the northwestern internal Alps. The Châtillon slice is very similar to the Eclogitic Micaschists Complex of the Sesia-Lanzo Zone and to the other eclogite-facies Austroalpine slices of the Dent Blanche Nappe, but it could also represent a portion of the Sesia-Lanzo Zone basement, which experienced a somewhat different subduction depth. The tectonic position of the Châtillon slice within the Piemonte Zone is essential to reconstruct the geometric relationships in the Austroalpine-Piemonte nappe stack of the northwestern internal Alps.

PETROGENESIS AND TECTONO-MAGMATIC SIGNIFICANCE OF THE ALBANIDE - HELLENIDE SUBPELAGONIAN OPHIOLITES

The Mirdita-Subpelagonian ophiolites of the Albanide-Hellenide orogen are parts of a continuous belt extending from the former Yugoslavia to Greece, and share common geological, litho-stratigraphical, geochemical, and metallogenic features. In the Albanian sector, two distinct ophiolitic belts can be clearly identified: the Western Belt, mainly composed of mid-ocean ridge (MORB) ophiolites, and the Eastern Belt characterized by supra-subduction zone (SSZ) ophiolites with prevalent island arc tholeiitic (IAT) and minor boninitic affinity. In the easternmost border of the Western Belt (Central Mirdita), a transitional zone with MORB/IAT intermediate basalts and boninitic dykes also occur. In the Greek sector, a definite distinction into two ophiolitic belts cannot be made, and MORB-type ophiolites (western type) are subordinate, being represented only by the intrusive and lower volcanic sequences of the Pindos Massif. By contrast, SSZ- ophiolites (eastern type) are predominant and well-represented by the IAT and boninitic sequences of the Vourinos Massif, as well as by MORB/IAT intermediate basaltic-andesitic suites and boninites of the upper part of the Pindos volcanic sequence. Petrological and geochemical modelling suggest that the different Albanide -Hellenide ophiolitic sequences originated from distinctly different parental magmas by partial melting of mantle sources progressively depleted by previous melt extractions. MORB may have derived from 10 - 20% partial melting of an undepleted lherzolitic source, while MORB/IAT intermediate basalts may have generated by ca. 10% of H2O-assisted partial melting of a cpx-poor lherzolite that had previously experienced MORB extraction. IAT magmas and boninites may, in turn, have derived from 10 – 20 % and ca. 30% partial melting of the same source, variably enriched by subduction-derived fluids and related incompatible elements. The favoured tectono-magmatic model for the genesis of the Albanide-Hellenide ophiolites implies a low plate-convergence velocity with: 1) intra-oceanic subduction within a pristine MORB lithosphere, resulting in SSZ magmatism with IAT affinity, and generation of a nascent arc by nearly open-system supply of undifferentiated basalts (sheeted dyke complexes); 2) progressive slab sinking and retreat coupled with mantle diapirism and extension from the arc axis to the forearc region, with generation of boninites and/or very low-Ti tholeiites from depleted sub-arc sources, leaving highly depleted harzburgitic residua; 3) contemporaneous generation at the spreading axis of IAT/MORB intermediate basalts resulting from the interference of MORB-source diapirs with suprasubduction mantle sources; 4) convergence processes leading to obduction of large and relatively intact lithospheric sections of SSZ ophiolites onto the Pelagonian continental margin, often with the interposition of metamorphic soles. The latter have prevalent MORB affinity and represent relics of the pristine MORB lithosphere overthrust by the still hot ophiolitic slab.

Geochemistry of ophiolite cherts from the Qinling orogenic belt and implications for their tectonic settings

Paleozoic cherts from the Mianlue and the Erlangping ophiolite zones of the Qinling orogenic belt are characterized by low Si/Al ratios (52.14－683.52 in the Mianlue cherts, 12.29－58.62 in the Erlangping cherts), Fe 2 O 3 (0.01－0.35 and 0.02－1.24) and high Al 2 O 3 /(Al 2 O 3 +Fe 2 O 3 ) ratios (0.82－0.99 and 0.83－0.99). The negative correlation between Si 2 O and Al 2 O 3 in the cherts reflects the important role of terrigenous components. The Erlangping cherts have La n /Ce n =0.9－1.15 and Ce/Ce * =0.95－1.15 with low contents of V, Ni and Cu, consistent with those of cherts forming on the continental margin. In contrast, the Ce/Ce * ratios of the Mianlue cherts range from 0.71 to 1.18 and La n /Ce n from 0.88 to 1.43 with slightly high V, Ni and Cu, which are similar to cherts found in the mid-ocean ridges and pelagic basins. Combined with the features of basic lavas associated with the cherts, it is suggested that during the Paleozoic, when the back-arc basin represented by the Erlangping ophiolite commenced shrinking in size in the mid-Ordovician, the southern Qinling was still in an extensional regime and finally grew into a new limited oceanic basin in the early Carboniferous.

Mineralization Controls in Island Arc Settings: Insights from Philippine Metallic Deposits

A review of the gold-copper, volcanogenic massive sulfide and ultramafic-hosted (i.e., chromitite, nickel sulfide, platinum-group minerals) deposits in the Philippines is presented. It is critical that a thorough understanding of the spatial and temporal relationship among magmatism, structures and mineralization must be gained if the correct evaluation of the economic potential of a particular deposit is to be done. Structural features conducive to precious and base metal mineralizations are associated with shear zones, extensional jogs and collision zones. In Northern Luzon, alkali and adakitic magmatism are considered good markers for gold-copper mineralization. Volcanogenic massive sulfide deposits are hosted by either ophiolites of marginal basin origin or metamorphic terranes. Exploration works on these deposits have been geared in determining the gold content of the massive sulfides. Chromitite deposits are related with ultramafic rock-hosted deposits. Their occurrence is attributed to crystallization, magma mixing and mantle-melt interaction processes in subduction-related settings. The multiple stages of partial melting responsible for the formation of supra-subduction zone ophiolites result in the generation of second to third stage melts that are enriched in nickel sulfides and platinum group minerals. On the basis of structural, geochemical and tectonic controls, Panay, Mindoro and Central Mindanao and the Sierra Madre, Leyte, and Samar are good exploration targets for precious and base metal deposition in the western and eastern sides of the Philippines, respectively.