جستجوی مقالات مرتبط با کلیدواژه « tethys » در نشریات گروه « زمین شناسی »

Significance of paleoecological calcified sponges (Neuropora lusitanica, Cladocoropsis mirabilis) in Esafandiar carbonate platform in Gazu area( east central Iran)
To progress in the knowledge of Upper Jurassic, evolution of the central Tethys realm an integrated approach which includes calcified sponges and microfacies has described in east central Iran.A rich poriferan that are compsed of Neuropora lusitanica, Cladocoropsis mirabilis and Chaetetid was identified within the Esfandiar Carbonate Platform of Iran. The excellent preservation state of most poriferans here led to recognize the some sponges and to the identification of previously unknown diagnostic features .N. lusitanica for the first time from the Upper Jurassic carbonates of the Esfandiar Carbonate Platform. The sedimentary input fluctuations and the nutrient competition had an important role in understanding the morphological adaptations of the analysed species. The existing palaeoecological and palaeoenvironmental conditions generated different distribution patterns of sponges. The absence of reef framework in Esfandiar platform may be contribute to high-temperature drying and the fluctuation of nutrient in middle Oxfordian- lower

The study of Faunal biodiversity distribution in the strata close to the Middle and Upper Permian boundary in the Hambast Valley section was carried out. Considering obvious environmental and faunal changes in the topmost horizon of the subunit 4a of the Abadeh Formation, the end-Guadalupian extinction has happened before Guadalupian–Lopingian boundary. The significant changes near the Guadalupian-Lopingian boundary in this section are the presence of dolomite and stromatolitic limestone at the topmost part of the subunit 4a, implying the shallowness of the depositional environment as well as the noticeable decline of faunal biodiversity. The cause of end-Guadalupian bio-crisis in the Hambast Valley section is more likely due to the falling sea level. The examination of the end-Guadalupian extinction horizon in South China and Japan shows its position at the end of Capitanian. In South China (Penglaitan section) and Abadeh (Hambast Valley section), the end-Guadalupian eustatic regression has only left shallowing effects with environmental and faunal changes implying the end-Guadalupian extinction evidence. In the Hambast Valley section, the latest Capitanian Hemigordius irregulariformis Zone with some Codonofusiella species in the basal part of the subunit 4b are representative of the re-occurrence of the foraminiferal fauna after the end-Guadalupian extinction, while fusulinid Zone of early Wuchiapingian Codonofusiella-Reichelina is indicative of post-extinction foraminiferal recovery.

The mineralogical composition and geothermobarometry of the peridotites from the Marivan-Kamyaran ophiolitic complex, as a part of the Neo-Tethyian ophiolites in the Zagros Mountains, western Iran, are investigated. The studied samples are belonged to mantle and crustal peridotite types. Results of electron probe microanalysis in peridotites indicate that olivine is forsterite (Fo87-Fo92), clinopyroxene is augite-diopside (En48-56Fs3-6Wo39-49) and orthopyroxene is enstatite (En90-91). Cr-number (Cr#) and Al-number (Al#) in spinels of the mantle peridotites are 33-59 and 41-67, respectively. The same values in spinels of the crustal peridotites are 67-98 and 2-32, respectively. Geothermobarometry calculations for the mantle peridotites show average temperature of ~1104°C and average pressure of ~14 kbar. The crustal peridotites are formed at ~900 °C and ~7 kbar. Two distinct mineral chemistry of the mantle peridotites indicate existence of two types of mantle peridotites in the area, suggesting the possibility of the influence of melt-rock interaction on the chemical composition of the mantle peridotites in the area.

Abstract. The biostratigraphy and paleoecology of Paleocene - Early Eocene deposits in the west of Rafsanjan city (Kerman Province) were studied for the first time. The Kuh – e - Ketabi sequence, the measured profile has about 161 m thick, mainly consists of limestone, dolostone, marl, shale, dolomitic limestone, and calcareous sandstone. Based on lithological characteristics, the studied section could be divided into 5 informal rock units. Forty genera and species of benthonic and planktonic foraminifera have been obtained from the section. One planktonic biozone (Morozovella angulata Lowest occurrence zone) and three shallow-water Tethyan foraminiferal biozones (SBZ4 – SBZ6) have been recognized based on the foraminiferal distribution in the stratigraphic column. From the view of abundance, diversity and distribution of foraminifera the Late Paleocene - Early Eocene part of the studied section is comparable with an equivalent sequence of the Tethyan Basin. Also, environmental depth changes are discussed based on faunal changes.

One of the most prominent cases of the study of alteration processes and their effect on geochemistry is the ophiolite investigation. The Marivan-Palangan ophiolite (MPO) complex is a part of the Neotethyan ophiolites, which situated in the Kurdistan Province, western Iran. The studied samples reveal the occurrence of the secondary minerals such as epidote group, chlorite, zeolites, and less calcite and iron hydroxides in the form of veins, vesicles and fractures infilling, and replacement phases. The element concentration variation versus Loss on ignition (LOI), normalization of the samples as to fresh sample in the study area and standard sample, and immobile element modeling indicate that element contents changes vary depending on the degree of alteration so that the concentration of Large-Ion Lithophile Elements (LILE) such as Rb, Cs, K, Ba, and Na shown a significant increase. CaO and FeO oxides are relatively depleted. Light Rare Earth Elements (LREE) display slight enrichment, while Middle Rare Earth Elements (MREE) and Heavy Rare Earth Elements (HREE) are almost constant. Therefore, in evaluation of the geochemical characteristics of extrusive section of the MPO complex, the effect of alteration on the changes in the concentration of elements, in particular LILEs, CaO, FeO and also LREEs should be considered.

The Posht-e-Badam ophiolite is situated in the 15 KM of the southwest of the Posht-e-Badam Robate village and in the northeast of Yazd province. The Posht-e-Badam ophiolite belongs to the Paleozoic and is a remnant of the Paleo-Tethys ocean. This ophiolite has passed three phases of metamorphism and consists of metaperidotite, metagabbro, amphibolite (ortho-amphibolite), rodingite and listwaenite. Metalherzolite is the most intact unites of the metaperidotite. Petrography study shows that metalherzolites consist of metamorphic minerals of olivine, tremolite, serpentine , chlorite, chromian magnetite and magnetite. The main textures of these rocks are nematoblastic, poikiloblastic, jack-straw and mesh texture. Mineral chemistry studies of metalherzolites indicate that olivines have Mg# (0.79-0.80), and are chrysolite in composition. Amphiboles are tremolite which present high values of Mg# (0.93-0.96). Chlorites are peninite in composition with Mg# (0.92-0.93). Spinels have Cr# 0.97, and are chromian magnetite in composition. Magnetite inclusions in metamorphic olivines reveals that the studied rocks were partly altered in greenschist facies. Tremolite with Jack-straw texture, as well as presence of Mg-chlorite and antigorite indicate progressive metamorphism in the amphibolite facies P-T conditions. Late-stage retrograde metamorphism led to formation of chlorite around of some of tremolites and partial serpentinization of metamorphic olivines. The phase relationships of metalherzolites of Posht-e-Badam ophiolite, in the CMASH (C = CaO, M = MgO, A= Al2O3, S = SiO2, H = H2O) model system, indicated that studied metalherzolites metamorphosed under lower amphibolite facies P-T conditions, followed by a retrograde metamorphism under greenschist facies conditions.

The present tectonic activity and seismicity of Makran subduction zone is affected northward movement of Neo-Tethys oceanic plate under Eurasia plate. According to scientific and historical observations, Makran fault has high seismicity and is completely capable to create tsunami hazard on Iranian southeastern coasts and Pakistanian western coasts. Topography, Free-Air and Bouguer gravity anomalies show critical changes on this region, which related to the changes in the earth deep structures. Western Makran experiences more negative Free-Air gravity values rather than eastern parts, this indicates that dip of subducting plate in the western Makran is more than eastern part. Many of earthquakes which magnitudes are 6 and above happened in the regions Free-Air gravity change abruptly positive to negative values, these regions might predict big earthquakes. Bouguer gravity values change 150 mGal in the oceanic crust of Oman sea to the -90 mGal in the Eurasian continental region. Moho depth is about 14 km beneath Oman sea and increases up to 32 km under Eurasian continent. Subducting plate dip is around 5.70. Studying and numerical simulation of run up due to tsunami, by consideration of earthquakes, is very important. In this research, the run up of tsunami on Makran fault in Sistan and Baluchestan province coastlines (Beris region) by using numerical methods has been studied. The results were tested by earthquakes magnitude of 8,8.5 (Pakistan, 1945) and 9 (Japan, 2010).Therefore the maximum flood of model was up to 0.5m (for earthquake magnitude of 8), 3.4m (for earthquake magnitude of 8.5) and 14m (for earthquake magnitude of 9). The maximum flood width would be 1.4Km and arrival time of the first tsunami waves would be 22 minute. These results could be possible to determine high risk region to optimize development planning.

The occurrence of blueschist metamorphic facies is believed to mark the existence of former subduction zones. This facies is represented in the main constituents of subduction-accretion complexes, where it occurs in separate tectonic sheets, imbricated slices, lenses, or exotic blocks within a serpentinite mélange (Volkova et al., 2011). The evidence of the presence and maturity of Paleo- Tethys oceanic crust in the CEIM (define this) in Paleo-Tethys branches, subduction and collision has been studied by various authors (Bagheri, 2007; Zanchi et al., 2009; Bayat and Torabi, 2011; Torabi 2011). Late Paleozoic blueschists have recognized in the western part of the CEIM (e. g. Anarak, Chupanan and Turkmeni) in linear trends. Metamorphic rocks of the Turkmeni area (SE of Anarak) are composed of blueschist and meta-greywacke and are situated along the Turkmeni-Ordib fault associated with Paleozoic rock units and serpentinized peridotite bodies. Turkmeni blueschist and meta-greywackes have not been studied by previous workers. The Turkmeni blueschists consist of albite, winchite, actinolite and epidote. Granoblastic, nematoblastic and lepidoblastic are main textures in these rocks. Winchite is found in the matrix and around epidote grains. This sodic-calcic amphibole serves as an index mineral in blueschist facies. Actinolite and epidote formed during retrograde metamorphism of blueschists in the greenschist facies. The mineral assemblage of albite, epidote, chlorite and phengite ± garnet is present in meta-greywackes in the Turkmeni blueschists. Veins of garnet, muscovite, quartz and opaque minerals are extensive in these rocks. Epidote and chlorite formed in meta-greywackes by retrograde metamorphism in the greenschist facies. The aim of the present study is to determine the petrological and geochemical characteristics, P-T condition of blueschists and meta-greywackes, as well as the geotectonic setting of primary basaltic rocks of the Turkmeni blueschists. This study is based on field observations and petrographical and analytical studies. Satellite images and a geological map were prepared. About 20 thin sections were supplied for petrological studies. Mineral chemical analyses were carried out by a JEOL JXA-8800R electron probe micro-analyzer (EPMA) at the Cooperative Center of Kanazawa University, Japan. The analyses were performed under an accelerating voltage of 15 kV and a beam current of 15 nA with 3µm probe beam diameter. The Fe3+ contents of minerals were estimated by assuming ideal mineral stoichiometry. The representative mineral compositions are given in Tables 1-3. Major oxides, rare earth elements (REE) and trace elements of five blueschists samples were analyzed by the ICP-MS method (Kanpanzhouh Research Company, Tehran, Iran) of the SGS laboratory of Canada. Whole rock chemical data are presented in Table 4. Petrographical and geochemical characteristics of Turkmeni blueschists reveal that they were derived from a similar mantle source and underwent analogous melt extraction and post magmatism occurrences. According to the trace and rare earth elements contents, the protolith of blueschists should be formed by crystallization of tholeiitic basalt and have sub-alkali basalt nature. Blueschists have LREE values more than HREE. High amounts and evident variations of LIL elements are obvious. Negative anomalies of HFSE such as Nb, Hf, Zr and Ti are evident in Turkmeni blueschist. REE trends of these rocks resemble as the back arc basin basalts. Based on the Nb/La ratio and REE contents, the original magma has been generated by low to medium degree of partial melting of a lithospheric mantle spinel lherzolite. Geochemical characteristics and normalized diagrams reveal that primary magma of protolith has been nature near to IAB and E-MORB. The related processes to subduction of Paleo-Tethys oceanic crust led to mantle enrichment and carbonate metasomatism. The Paleo-Tethys Ocean spreading in CEIM commenced in Late Ordovician and terminated in the Late Paleozoic-Triassic. Association of meta-greywackes with blueschist and LILE/HFSE contents shows that Paleo-Tethys oceanic crust subduction zone at Turkmeni region was been immature. Mineral chemistry and assemblages of the blueschists and meta-greywackes units reveal that they suffered different metamorphic evolution: (M1) greenschist metamorphism by existence of actinolite and albite in basaltic rocks, and then they passed a prograde metamorphism in the blueschist facies by existence of winchites (M2) which is followed by a retrograde metamorphism P-T condition in the greenschist facies (M3). Variscan tectono-metamorphism occurrence has been main metamorphic phase in Anarak region and it has led to metamorphism in blueschist facies of Turkmeni rocks. Acknowledgments: The authors wish to thank the University of Isfahan University for financial supports.

More than 20 tonalitic to granodioritic plutonic domes, Oligo-Miocene in age, have intruded into the older volcanic rocks, in southeast of Urumieh- Dokhtar Magmatic Belt, northwest of Shahr-e-Babak. These rocks have granoporphyritic texture and consist of Phenocrysts of plagioclase, amphiboles and biotites. The geochemical data show calc-alkaline affinities of these rocks, which formed in volcanic arc (I type) of an active continental margin. These data also show LILE and LREE enriched normalized multi-element patterns, and depleted of HFSE (Nb, Ta and Ti). Primitive mantle-normalized REE patterns display a dramatic decrease from LREE to HREE without any Eu anomaly. They have higher SiO2, Al2O3 and Sr content and Sr/Y and La/Yb ratios and lower MgO, Y and Yb contents than the normal calc-alkaline rocks, which reveal their adakitic characteristics. The high content of Sr, LREE enrichments, absente of Eu anomaly, HREE depletion, Y and Yb depletion pattern suggest the existence of garnet, amphibole and absence of plagioclase in the source rocks. Our data suggest that the plutons produced by partial melting of amphibole-eclogite or garnet-amphibolite, due to the subduction of Neotethys oceanic slab under the Central Iran continent.

Distribution of fusulinid faunas in different regions of Iran indicates the presence of both warm-water Peri-Tethyan-related assemblages and cool/cold-water Peri Gondwananindicating ones. In Alborz, most parts of Central Iran, Abadeh and Dzhulfa regions, genera and species of verbeekinid and neoschwagerinid fusulinids are present that in term of fusulinid paleobiogeography, they are assignable to Peri-Tethyan Realm. The warm-water fusulinid diversity in Abadeh and Dzhulfa regions is partly higher compared with Alborz and Central Iran suggesting that these regions may have occupied a comparatively lower latitudinal position within Tethyan realm. Kalmard region in Central Iran having southern anti-tropical fusulinid faunas is charachteristic of cool/cold-water Peri-Gondwanan faunas. Starting from Kubergandian through lower Midian, Zagros was located within the temperate climatic belt and only by middle-late Midian it has been closer to Equator.

Mantle sequences exposed in west of Fannuj- Maskutan area comprise of lherzolite and porphyroclastic cpx-bearing harzburgite in the lower part and recrystallized fine-grained lherzolite with chromitite lenses in the upper parts of the sequence. Petrography studies and microprobe data show evidence of melt/peridotite interactions, post-melting processes and subsolidus interactions associated with the appearance of two generations of deformed primary pyroxene-olivine and fine-grained pyroxene-olivine amphibole neoblasts. Second generation of minerals formed as inclusion, interstitial and fine-grained. These two groups of minerals have different geochemical characteristics, i.e., the first group are comparable with abyssal peridotites and the second group are comparable to suprasubduction peridotites. Therefore, the chemical compositions of different generations of minerals show different petrogenesis for ultramafic rocks of the Fannuj- Maskutan ophiolitie complex. Interpretation of whole rock chemical data indicate that these rocks have a depleted MORB mantle source which underwent 5-15% partial melting. Rare earth element patterns normalized with chondrite standard values and compared with patterns of depleted MORB mantle (DMM), indicate enrichment in LREE/MREE ration and show U-shape patterns. Thus, the peridotites of the Fannuj-Maskutan ophiolitie have experienced multistage evolution and show characteristics of abyssal environment to suprasubduction zone. It might be said that transition from abyssal environment to suprasubduction has been affected by fluids derived from the subducted slab.

In the Kalaybar Hasht-Sar area، in the western part of secondary Palaeo-Tethys suture zone (NW Iran)، gabbro intrusion، which is older than Paleocene in age، has cropped out. This gabbro is shoshonitic in composition with has arc type geochemical composition. The rocks show high K2O concentration and enrichment of Rb، Ba، U، Th and Pb and depletion of Ta، Nb، Ti also Hf، Zr on primitive mantle-normalized spider diagrams. The parental magma of gabbro is formed by low partial melting rate (less than 10%) of spinel-garnet lherzolite. According to trace elemental ratios، the gabbro was derived from a source strongly modied by subducted slab uids. Then crustal contamination caused the LILE enrichment in the magma. Considering accompany of the rocks with Palaeo-Tethys suture zone suit، it seems that gabbro was formed due to subduction of this oceanic crust. Metasomatism above subduction zone is an important process that produced heterogeneous mantle and thus changes in geochemical ratios of the magma.

In the view of global plate tectonic reconstruction, Iran is a part of the Alpine-Himalayan folded range which is formed in relation to closure of the Tethys Ocean between two huge continents of Gondwana and Eurasia. There have been many studies concerning the formation and closure times of the Neo-Tethys, therefore different opinions have been presented in this regards. In the previous studies, the ages of closure of the Neotethys and collision of Arabian and Iranian continents have been considered to be from the Late Cretaceous to the Neogene. Based on the U/Pb zircon data from the leucosome of the mac migmatites, and the 206Pb/238U age for migmatization processes in the Takht-e-Soleyman metamorphic complex, it is determined to be about ca. 24-32 Ma (corrected for 204Pb) and 25-29 Ma (corrected for 207Pb), corresponding to the Neotethys continental collision. In this regards, it can probably consider the extensive magmatic activities in the area in relation to Neogene extensional phases corresponding to the crustal thinning followed by the continental collision. The isotopic K-Ar data on shists from the Zarehshoran area, U-Th/He data on apatites from the Mahneshan area and 4 0Ar –39Ar isotopic ages on muscovite from pelitic shists from the previous studies support uplifting of the Takht-e-Soleyman metamorphic basement ca. 20 Ma (Early Miocene). Shortening of the crust and deepening of the Zagros thrust fault (probably to Moho depth) are more evidences for Miocene continental collision.