Petrogenesis of Subduction Zone and Dunite Bodies

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  Abstract: The dunite bodies, which extend as the direction of W-E, are exposed to the southeast of Elaz?g located within the Eastern Taurus Belt of Turkey. Mafic-Ultramafic section in the Guleman ophiolite consists of dunite which containing disseminated chromites, wehrlite, gabbros (isotrope gabbro and layered gabbro) and clinopyroxenite. Dunite blocks above the harzburgite massif have irregular contacts with the enclosing peridotites. Dunite blocks are generally around a few of meters. Dunite blocks consist of gabbro and pyroxenite patches. The origin of dunite blocks are belong to the transition zone of harzburgitic ophiolites which is located at the base of the mafic layered section. They are entirely or largely magmatic formed by olivine and chromite ponds at the base of the crustal magma chamber. The rather around of rock pieces within dunite bodies are foliated such as features have been ascribed to the ophiolite being impregnated by and reacting with a melt. Rocks in the bodies show depleted in incompatible trace elements such as Ba, Nbet al., characteristic of subduction related magma. Furthermore, the high LREE/HREE and high Rb/Th ratios indicates a mantle that has been enrichmented by subduction. As a result, isotopic data, petrographic and geochemical of bodies’s result suggest a parental magma derived from an enrichmed source of subduction zone. A few meters of the large dunite bodies, and ascribes to the central dunites a cumulative origin by fractionation from a picritic melt.
  Key words: Neotethyan, Turkey, dnite bodies, subduction zone, ophiolite, geochemistry.
  1. Introduction
  Anatolia comprises a significant part of the Alpine-Himalayan folded belt, which is one of the most remarkable collision zones on the earth. The Neotethies ocean opened in the Triassic and closed in the Upper Cretaceous, played a significant part in the evolution of Turkey [1, 2]. The remnants of Neotethys, in a structurally descending order are characterized by ophiolites, metamorphic soles and ophiolitic melanges. These ophiolites and related subduction accretion units were generated during the closing stages of the Neotethys oceanic basins in the Late Cretaceous [1]. Well-preserved Neotethys ophiolites in Turkey are subduction zone type [1-6] and display a consistent sequence of events during their formation and emplacement (Fig. 1).
  Late Cretaceous ophiolites in Turkey are located in five zones which are from north to south, (1) Pontide ophiolite, (2) Anatolian ophiolite belt, (3) Tauride ophiolite belt, (4) Southeast Anatolian ophiolites and(5) The peri-Arabian platform ophiolites [1].
  The Guleman ophiolite is one of southeast Anatolian ophiolites and the best preserved Neotethies oceanic lithospheric remnants in southeast Turkey.
  Recent field studies on ophiolites (examples: Tulameen complex in British Columbia, Trinity) [7, 8]. The petrological and geochemical studies of Brouxel et al. [8, 10] mapped in detailed the ultramafics which in the China Mountain-Mt Eddy Quadrangle within the frame of petrological and geochemical study are supported. Boudier and co-workers [7, 11, 12] covered structurally the entire masif. They support Lindsley-Griffin’s conclusion about the ophiolitic association of the mafic and ultramafic sections. Ref.[13] shows the same discrepancy between mafic-ultramafic section of the ophiolites.
  They tend to consider them as independent intrusions and surrounded by peridotites. Dunite bodies within the Guleman ophiolite consist of serpantinit,gabbro, clinopyroxenite around a dunite core. These bodies are emplacemented by faulting in the magma chamber or are tectonically emplacemented of ophiolite. As magma crystallizes and differentiates, cyclically layered sequences form [14]. An ideal cycle is consisted of basal dunite followed upward by a harzburgite or ortopyroxene layer [15]. These cyclic units vary in thickness. For example, in the Bushveld complex they are present on a milimetre scale, at Great Dyke on centimetre scale [15]. Olivine composition of this complexs ranges of Fo 77 and Fo 35. The specific characters of them show that they formed along an environment similar to a modern island arc or a back arc basin as proposed by Yodogzinski et al. [16, 17]. This paper aims to present petrographical and geochemical properties of dunite bodies.
  2. Geological Background
  The Guleman ophiolite is one of the best southeast Turkey ophiolites and is exposed to the northeast of Maden and the east of Hazar Lake. It is thrust over by the well-exposed sedimentary succession of the Hazar Group, Maastrichtian-Paleocene in age [17-19]. The Guleman ophiolite comprises mantle tectonites and ultramafic to mafic cumulates, layered gabbros, isotrope gabbros, plagiogranite, klinopyroxenite. Diabase dikes cut through the above mentioned rocks at different structural levels [18] determined the age by Ar-Ar dating in plagioclase 72.4 Ma for Guleman ophiolite. In outside of study area, the Guleman ophiolite covers Maden complex (Middle Eocene) tectonically (Fig. 2). Middle Eocene Maden complex uncomformably overlies the Hazar Group. The Maden complex comprises mudstone, sandstone, lots of limestone olistolithes, agglomerate, basaltic and andesitic lavas. The Hazar groupe (Upper Meastrichtian-Middle Eocene) is largely comprised of siltstone, limestone, sandstone, claystone and marl.
  3. Analytical Tchniques
  Major and trace element analyse of nine samples from dunite bodies is given in Table 1. Samples were carried out at AAL (Acme Analitical Laboratories) in Canada [18]. A total of 2 g of powdered sample was mixed with 0.9 g of wax, the mixture was pressed at 20 N in a automatic pres machine to get a pressed disc. REE (rare earth element) analyses of 9 samples were performed using ICP-MS (iductively coupled plasma-mass spectrometry) following a LiBO2 fusion and dilute nitric digestion on representative powders. Plagioclases were seperated from the gabro sample(Table 2), which has very fresh and chemically homogeneous grains. Geochronology of plagioclase was undertaken by laser 40Ar/39Ar dating (Fig. 3).
  4. Petrography
  The Guleman ophiolite comprises mantle tectonites and ultramafic to mafic cumulates, layered gabbros, isotrope gabbros, throctolite, plagiogranite and Klinopyroxenite. Diabase dikes cut through the above mentioned rocks at different structural levels. Mantle tectonites are more than 4 km thick and form at least 60% of the whole ophiolite body [6]. They are dominated by harzburgites which contain dunite and podiform chromite, generally 0.5-10 m in thick [20]. The mantle tectonites are gradually overlain by layered cumulates, which starts with dunites containing disseminated and banded chromites, followed by alternations of gabbros, wehrlite and clinopyroxenite.
  The gabbroic level is represented by throctolite, layered gabbro, isotrope gabbros and plagiogranite. The transition between layered gabbro or foliated gabbro and isotropic gabbros in posibility ophiolites show that it is difficult to draw a line between them. The layered gabbros progressively lose their layering and grade into poorly foliated gabro, where it is enrichment on favor of pyroxenes. The isotropic gabbros are characterized by agreat heterogenity in grain size. The gabbros exhibit adcumulate and heteradcumulate textures suggesting a slow rate of crystallization [20, 21]. The products of anatexis are leucocratic tonalites and plagiogranites which invade the gabbros as a dike or vein-like network and locally dismember them into a magmatic breccia. Distinguishing plagiogranites produce by this anatexis from others ascribed to the crystallization of highly evolved melt at the top of the chamber [22, 23]. A sheeted-dike complex in the Guleman ophiolite was not preserved, possibly because of erosion after emplacement [18, 19]. Dunite bodies such as transition zone characteristic below the layered gabbros and above mantle tectonites. These bodies having approximatelly 1.5-2 m size which comprises clinopyroxenite around dunite core, serpantinized harzburgite and gabbros (Fig. 4a). The rocks of bodies characterized by well fabrics (Fig. 4b) are found in random orientations within apparently cumulates gabbbro and dunite core from the base of the magma chamber (such as trinity ophiolite) [10, 24]. The rather around of rock pieces within dunite bodies are foliated(Fig. 5). In the gabbros diffuse aggregates of feldspar and clinopyroxene are observed. Such features have been ascribed to the ophiolite being impregnated by and reacting with a melt [24, 25]. Another characteristic of dunite bodies is including rich in Mg-silicate rocks. Serpentinite and forsteritic olivine are common silicates with high magnesium contents. The dunite core of Fo-olivine riches consists of diopsitic augite and chromite. Alteration products comprise carbonate, magnetite and talc (Fig. 6).
  The clinopyroxenite around of dunitic core is medium to coarse grained and has black green clinopyroxene and partialy serpentinized olivine. The clinopyroxenites commonly exhibit mineral foliations. Magnetite is very found as a blotchy. It is suggested that possibly oxidation of the magma chamber. The gabbros within dunite bodies comprises of plagioclase, clinopyroxene and accessory chlorite (Fig. 6). Fluids made chemical remobilization of minerals and it caused sosuritization of plagioclase, kaolinitization of feldispar.
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