Ofioliti最新文献

{"title":"P-T-T history of the Amasia and Stepanavan sub-ophiolitic metamorphic units (NW Armenia, Lesser Caucasus): implications for metamorphic sole development and for the obduction process","authors":"M. Hässig, Y. Rolland, R. Melis, M. Sosson, G. Galoyan, O. Bruguier","doi":"10.4454/OFIOLITI.V44I1.464","DOIUrl":"https://doi.org/10.4454/OFIOLITI.V44I1.464","url":null,"abstract":"The Sevan-Akera suture zone ophiolites are relics of a vast ophiolitic nappe which testifies a major obduction event, up to 300 km of horizontal transport, of the northern branch of Neotethys oceanic crust over the South Armenian/Taurides continental block. Near the locality of Amasia (NW Armenia), garnet-bearing amphibolites are preserved within a greenschist facies tectonic melange unit located below the non-metamorphic obducted oceanic unit. The garnet amphibolites show two parageneses: (1) garnet-amphibole-plagioclase granoblasts which crystallized along the S 1  foliation intensely folded and recrystallized into (2) epidote-chlorite-phengite during retrogression and S 2 deformation. S 1  and S 2  deformation stages feature top-to-the-South ductile shearing, interpreted as the motion of the ophiolite nappe during obduction. Thermobarometry reveals a metamorphic history with two P-T fields: (1) an amphibolite stage, T = 600 ± 20°C and 6 ≤P ≤7 kbar,followed by (2) a greenschiststage, T = 350 ± 30°C and 1.25 ≤P ≤4.5 kbar.  40 Ar/ 39 Ar dating on amphiboles and white micas yields similar within-error ages of 88-92 ± 2 Ma. U-Pb dating on rutile yields an age of 90.2 ± 5.2 Ma. These results are complemented by new and pre-existing characterizations of lithologies in a similar structural position 40 km east, in the locality of Stepanavan. There, newly identified eclogite yield metamorphic conditions of T = 575 ± 25 °C and 17.5 ≤P ≤20 kbar.The P-T-t history of these metamorphic units argues for a rapid tectonic process featuring intra-oceanic subduction below a relatively hot oceanic lithosphere, slicing of the overriding oceanic domain, underplating of this subducted material along the hanging wall of the subduction zone and formation of a metamorphic sole as part of an ‘obduction channel’.","PeriodicalId":54690,"journal":{"name":"Ofioliti","volume":null,"pages":null},"PeriodicalIF":1.3,"publicationDate":"2019-01-15","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"41319260","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Origin of the spinel-pyroxene symplectites in the harzburgites from the New Caledonia Peridotite","authors":"A. Secchiari, A. Montanini, D. Bosch, P. Macera, Dominique Cluzel","doi":"10.4454/OFIOLITI.V44I1.463","DOIUrl":"https://doi.org/10.4454/OFIOLITI.V44I1.463","url":null,"abstract":"The New Caledonia ophiolite (Peridotite Nappe) hosts one of the largest and best-exposed mantle section worldwide, providing an exceptional insight into upper mantle processes. The Peridotite Nappe is mostly dominated by harzburgites, locally overlain by mafic-ultramafic cumulates, but also includes minor spinel and plagioclase lherzolites, cropping out in the northern part of the island.  The New Caledonia harzburgites are low-strain tectonites, showing dominant porphyroclastic textures. The main mantle paragenesis is constituted by olivine (~ 75-85 vol%), orthopyroxene (~ 15-25 vol%) and spinel (< 1 vol%), while primary clinopyroxene is notably absent. An important textural feature of these mantle rocks is represented by the common occurrence of spinel-pyroxene symplectitic aggregates. In this work, we present a petrographical, textural and major element chemical characterization of the spinel-pyroxene symplectitic intergrowths occurring in the New Caledonia harzburgites (Kopeto, Central Massif, and Yate, Massif du Sud). Based on textures, size and relationships with the other mineral phases, these spinel-pyroxene clusters have been divided into two types, named type-A and type-B. Type-A symplectites occur in the Kopeto harzburgites and are composed of spinel-orthopyroxene (± clinopyroxene) intergrowths. In type-A symplectites, symplectitic spinel (Spl2) occurs as abundant vermicular shaped grains, ranging in size from ~ 0.5 to 2 mm. By contrast, spinel of the porphyroclastic assemblage (Spl1) shows smaller size (in the range of few m) and notably lower abundances (< 1%). Type-A symplectites develop exclusively on porphyroclastic olivine, which in turn displays evidence of chemical disequilibrium and corroded outlines.  Bulk major element composition reconstructed for type-A symplectites rule out a derivation from a pre-existing garnet phase, as the model garnet compositions do not satisfy garnet stoichiometry, being characterized by Si deficiency. By contrast, major element chemical variations of the symplectitic phases, coupled with the high abundance of Spl2 and olivine resorption, suggest an origin from reactive percolation of opx-saturated hydrous melts or slab-derived fluids in a subduction zone setting. Type-B symplectites are found in Yate sample and consist of spinel-orthopyroxene (± clinopyroxene). They are characterized by smaller size (few hundreds of m, i.e. “micro-symplectites”) and different shapes compared to type-A symplectites, growing as vermicular, “myrmekite-like” intergrowths at the rims of porphyroclastic orthopyroxene. Major element chemical compositions of type-B symplectites are consistent with an origin as “cooling textures”. These structures may derive from unmixing of a high-T, Al-Cr rich, orthopyroxene due to the decreased solubility of the Cr-Al component (CrMgTs) during post-melting lithospheric cooling at T < 900°C.","PeriodicalId":54690,"journal":{"name":"Ofioliti","volume":null,"pages":null},"PeriodicalIF":1.3,"publicationDate":"2019-01-15","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"42493753","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Petrogenesis of high-grade metamorphic soles from the Central Dinaric Ophiolite belt and their significance for the Neotethyan evolution in the Dinarides","authors":"B. Šegvić, Damir Slovenec, R. Altherr, E. Babajić, R. F. Mählmann, B. Lugović","doi":"10.4454/OFIOLITI.V44I1.462","DOIUrl":"https://doi.org/10.4454/OFIOLITI.V44I1.462","url":null,"abstract":"High-grade metamorphic soles in NE Bosnia and Herzegovina make part of the Krivaja-Konjuh ophiolite complex (KKOC), which is one of the most important constituents of the Jurassic ophiolite melange of the Central Dinarides. Several rock types were distinguished within the investigated metamorphic suite - sapphirine and corundum amphibolites, garnet-clinopyroxene±orthopyroxene amphibolites, clinopyroxene±garnet amphibolites, amphibolites per se and clinopyroxene+plagoclase±garnet gneisses.Peak temperature and pressure conditions calculated from different mineral pairs were estimated to range between 850 and 1100°C at 1.1 to 1.3 GPa. A prograde multi-stage metamorphic history of analyzed rocks was followed by the post-peak relaxation witnessed in decomposing porphyroblasts of garnet and ubiquitous formation of amphibole and orthopyroxene rims around clinopyroxene. The whole-rock chemical composition of magmatic protoliths largely defines it as cumulates from supra-subduction zones and scarcely as MORB-like tholeiitic mafic extrusives. Their origin is linked to near-ridge crust generation of a back-arc basin. Following the Middle Jurassic contraction of Neotethyan a portion of these rocks was likely entrained in an incipient subduction/thrusting system characteristic for the formation of metamorphic soles. According to geological evidences, and most notably, the age of radiolarians taken from KKOC basalts, the formation of metamorphic sole rocks must have followed a rapid transition in the Dinaridic Neotethys geotectonic setting before its final obduction onto the Adria margins in the next 15 Ma before the end of Jurassic.","PeriodicalId":54690,"journal":{"name":"Ofioliti","volume":null,"pages":null},"PeriodicalIF":1.3,"publicationDate":"2019-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"70970428","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Preliminary geochemical data of the mafic rocks from the Ovacik and Pülümür Ophiolite Zone (Eastern Anatolia, Turkey): implications for the geodynamic evolution of the Northern Neotethyan Ocean","authors":"Okay Çimen, A. Önal","doi":"10.4454/OFIOLITI.V43I2.458","DOIUrl":"https://doi.org/10.4454/OFIOLITI.V43I2.458","url":null,"abstract":"The Izmir-Ankara-Erzincan-Sevan-Akera Suture Belt (IAESASB) stretches from the Aegean Sea to the Lesser Caucasus and includes the remnants of the Northern Neotethys. The Ovacik and Pulumur Ophiolite Zone (OPOZ) to the north of Tunceli is less-known members of this belt. In this study, preliminary geochemical data from the basaltic and gabbroic rocks from this zone are presented to provide the first insights into their geochemistry. The studied mafic samples exhibit geochemical characteristics of an intra-oceanic subduction system. Three types of samples were differentiated on the basis of their major, trace and rare earth element (REE) geochemistry. The first one is boninitic and displays highly depleted elemental pattern (relative to N-MORB) and concave REE profile. The second type is akin to back-arc basin basalts (BABB) and generally reflects normal-MORB (NMORB)-like high field strength element (HFSE) patterns coupled with negative Nb anomalies. The last type is relatively similar to the BABB-type, but it lacks the marked negative anomaly in Nb and can be ascribed to the mid-ocean ridge basalt (MORB). The overall geochemical features suggest that these mafic rocks were predominantly generated in a supra-subduction setting from a subduction-modified mantle source. It is further suggested that the Ovacik and Pulumur Ophiolite Zone was formed in an oceanic arc-basin system within the northward subducting Northern Neotethys.","PeriodicalId":54690,"journal":{"name":"Ofioliti","volume":null,"pages":null},"PeriodicalIF":1.3,"publicationDate":"2018-07-24","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"49343264","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Geochemical and Sr-Nd-Pb isotopic characteristics of extrusive rocks within the Çetmi Ophiolitic Mélange, Biga Peninsula","authors":"Aykut Güçtekin","doi":"10.4454/OFIOLITI.V43I2.459","DOIUrl":"https://doi.org/10.4454/OFIOLITI.V43I2.459","url":null,"abstract":"The different geochemical characteristics of the extrusive rocks of the Cetmi Ophiolitic Melange (COM) a remnant of the Intra-Pontide Suture (IPS) in the Biga Peninsula, NW Turkey, evidence the episodic formation of oceanic crust. Geochemically, volcanic rocks are divided into three groups: ocean island basalts (OIB), enriched mid-ocean ridge basalts (E-MORB) and basalts with supra-subduction zone (SSZ) affinity. While the OIB type samples were produced by small degrees partial melting from an enriched source, the E-MORB type samples point to mixing of enriched and depleted sources. In the OIB-type samples, the depletion of Zr and Hf compared to Nd as well as enrichment of Nb-Ta are attributed to presence of the amphibole in the mantle source. Radiogenic Pb isotope compositions towards EMII support metasomatic processes related to fractionation in the mantle reservoir. The SSZ samples are characterized by enriched light rare earth elements (LREE) and depleted high field strength elements (HFSE). The trace element characteristics of the SSZ samples indicate that a depleted MORB mantle source interacted with subduction-derived fluids due to the slab dehydration processes. Besides, the Sr-Nd-Pb isotope characteristics suggest that these samples originated from a MORB-type mantle source metasomatized by fluids or melts from subducted sediments. The major, trace element and isotopic data of the volcanic rocks in the COM indicate that the oceanic lithosphere is generated along a SSZ-type spreading centre at an arc-basin system within the Intra-Pontide Ocean.","PeriodicalId":54690,"journal":{"name":"Ofioliti","volume":null,"pages":null},"PeriodicalIF":1.3,"publicationDate":"2018-07-24","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"43623173","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Petrology and geochemistry of sheeted dykes and pillow lavas from the Sabzevar Ophiolitic Mélange (Northeast Iran): new Constraints for the Late Cretaceous evolution of the Neo-Tethys Oceanic Basin between the Central Iranian Microcontinent and Eurasia","authors":"Zahra Rezaei, M. Noghreyan, E. Saccani","doi":"10.4454/OFIOLITI.V43I2.461","DOIUrl":"https://doi.org/10.4454/OFIOLITI.V43I2.461","url":null,"abstract":"TheSabzevarophiolite-bearingmelangemainlyincludestectonic slicesofophiolitesregardedas remnants of the Late Cretaceous oceanic basin located between the northern margin of the Central Iran Microcontinent and the southern margin of the Turan (Eurasia) plate, as well as remnants of its northern continental margin. We present here new mineral chemistry and whole rock chemical data on the sheeted dyke complex and pillow lavas series from this ophiolite. The sheeted dykes consist of basalts and basaltic andesites showing island arc tholeiitic (IAT) affinity (Group 1). They display low TiO 2 (0.37-0.81 wt%) contents and N-MORB normalized incompatible element patterns featuring Th enrichment and Ta and Nb depletion. The pillow lavas show different chemical features compared to the sheeted dykes and can be subdivided in two distinct geochemical groups. The first Group of pillow lavas (Group 2) mainly consists of andesites showing a clear calc-alkaline (CA) affinity with low TiO 2 (0.71-0.81 wt%) contents. Their N-MORB normalized incompatible element patterns display marked enrichments in Th, U, La, and Ce, and depletion in Ta, Nb, and Ti. The second Group of pillow lavas (Group 3) is mainly represented by alkaline basalts with high TiO 2 (1.54-3.45 wt%) contents. N-MORB normalized incompatible element patterns show significant Th, Ta, and Nb enrichments. Trace element modeling shows that CA primary melts were generated by 20% partial melting of a depleted lherzolite mantle source that had been enriched by continental crust chemical components. IAT basalts were generated from various degrees of partial melting of either depleted lherzolitic or harzburgitic mantle sources bearing no or negligible influence from continental crust components. Alkaline basalts were generated by polybaric partial melting of an enriched within-plate oceanic mantle source. A new tectono-magmatic model that can explain the different rock association cropping out in the Sabzevar ophiolite is proposed. This model implies the existence, on the southern margin of the Turan plate during the Late Cretaceous, of a continental arc-forearc system where CA and IAT rocks were erupted, respectively. Alkaline basalts were erupted in a subduction-unrelated oceanic basin and then tectonically accreted to the forearc as a consequence of the collision between the seamount and the arc-forearc system.","PeriodicalId":54690,"journal":{"name":"Ofioliti","volume":null,"pages":null},"PeriodicalIF":1.3,"publicationDate":"2018-07-24","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"47647560","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"First occurrences of Ni-V-Co phosphides in chromitite from the Agios Stefanos Mine, Othrys Ophiolite, Greece","authors":"E. Ifandi, F. Zaccarini, B. Tsikouras, T. Grammatikopoulos, G. Garuti, Sofia Karipi, K. Hatzipanagiotou","doi":"10.4454/OFIOLITI.V43I2.460","DOIUrl":"https://doi.org/10.4454/OFIOLITI.V43I2.460","url":null,"abstract":"Phosphide minerals, generally less than 20 μm in size, are mainly made of Ni-V-Co-Mo constituents, discovered in concentrates obtained from chromitite samples from the Agios Stefanos mine, Othrys ophiolite, Greece. Spinel from the chromitite is rich in Fe 2 O 3 and very similar to chromites from podiform chromitites. Its Cr/(Cr+Al) ratios are lower than those of chromites with a boninitic affinity. On the basis of their chemical compositions, the following phosphide minerals have been identified: melliniite, nickelphosphide and two phases that can be classified as (i) either Ni-allabogdanite or Ni-barringerite, and (ii) either V-allabogdanite or V-barringerite. Under reflected-light microscope, all the inspected phosphide minerals display similar reflectance and a creamy-yellowish color, despite their different compositions. Melliniite is isotropic, while nickelphosphide shows a weak anisotropism. The Ni-allabogdanite or Ni-barringerite and V-allabogdanite or V-barringerite display a strong anisotropism. They occur associated with awaruite, pentlandite, native vanadium, vanadium sulfides, Mo-Ni-V-Co alloy (probably hexamolybdenum), Hg selenide (possible tiemmanite) and several minerals composed of Ni, As and Sb. A few of them were in contact with chromite, chlorite, quartz and glass. This mineralogical assemblage suggests that the discovered minerals are natural in origin and they do not represent an artefact related to the sample preparation. Furthermore, it is suggested that the investigated minerals have been crystallized in a local reducing geochemical environment. During the serpentinization of peridotites, reducing fluids containing dissolved H 2  are released from the reduction of H 2 O. Therefore, these phosphides and the associated minerals may have precipitated during the serpentinization process at low temperature. Nickel and cobalt may have been released during alteration of olivine or they were originally hosted in magmatic sulfides that were altered during the serpentinization. Phosphorous may represent an alteration product of apatite and olivine. Vanadium was probably released during the alteration of the host chromite. We speculate that Mo was originally hosted in sulfides or oxides and after its remobilization it was incorporated into the crystal lattice of the Mo-rich phases. Alternatively, the high reducing minerals of the Othrys may have formed because of the interaction of their host rock with a lightning. The stoichiometry of the analyzed phosphide minerals indicates that some grains found in the Othrys chromitites may represent new mineral species, such as Ni-allabogdanite or Ni-barringerite, as well as V-allabogdanite or V-barringerite. However, their small size and complex micro-intergrowths with other minerals inhibit the elaboration of an X-ray diffraction study, which would unequivocally determine their nature. To the best of our knowledge, melliniite and nickelphosphide in the Othrys chromitite represent the first fi","PeriodicalId":54690,"journal":{"name":"Ofioliti","volume":null,"pages":null},"PeriodicalIF":1.3,"publicationDate":"2018-07-24","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"47692813","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"First occurrences of Ni-phosphides in chromitites from the ophiolite complexes of Alapaevsk, Russia and Gerakini-Ormylia, Greece","authors":"Alkiviadis Sideridis, F. Zaccarini, T. Grammatikopoulos, P. Tsitsanis, B. Tsikouras, E. Pushkarev, G. Garuti, K. Hatzipanagiotou","doi":"10.4454/OFIOLITI.V43I1.456","DOIUrl":"https://doi.org/10.4454/OFIOLITI.V43I1.456","url":null,"abstract":"Small grains, approximately 15 μm in size, of a Ni-phosphide, have been discovered in two chromite deposits associated with altered mantle peridotite: (i) in Bakanov Kluch, Alapaesk ophiolite, Russia and (ii) in Agios Dimitrios, Gerakini-Ormylia ophiolite, Greece. On the basis of the chromite composition, both the Bakanov Kluch and the Agios Dimitrios chromitites are classified as Cr-rich chromitites with a boninitic affinity. Under reflected-light optical microscope, the Ni-phosphides are yellow in colour and isotropic. Their calculated stoichiometry, based on the average of 20 electron microprobe analyses, approaches the ideal formula (Ni,Fe) 5 P. One essential condition for the formation of terrestrial minerals in the Ni-Fe-P system is the presence of a highly reducing local geochemical environment. Serpentinization of oceanic peridotites produces reducing fluids containing dissolved H 2 resulting from the reduction of H 2 O. Therefore, we suggest that the (Ni,Fe) 5 P from the Alapaevsk and Gerakini-Ormylia were very likely formed after the precipitation of the host chromitites during the serpentinization process at low temperatures. Nickel may have been released during the alteration of olivine and phosphorous may represent an alteration product of apatite. The (Ni,Fe) 5 P grains analyzed in the present work contain the highest value of Ni and they are characterized by the highest Ni to P ratio reported so far from similar minerals. Their stoichiometry indicates that the (Ni,Fe) 5 P grains found in the Alapaevsk and Gerakini-Ormylia chromitites represent a new mineral species. However, their small size prevents the acquisition of crystallographic data to support this assumption.","PeriodicalId":54690,"journal":{"name":"Ofioliti","volume":null,"pages":null},"PeriodicalIF":1.3,"publicationDate":"2018-01-20","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"41686719","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Investigation of silicate micro-inclusions from Orhaneli and Harmancik chromitites (NW Turkey): new ultrahigh-pressure evidence from Western Tethyan ophiolitic chromitites","authors":"Mehmet Akbulut","doi":"10.4454/OFIOLITI.V43I1.453","DOIUrl":"https://doi.org/10.4454/OFIOLITI.V43I1.453","url":null,"abstract":"In this study, mainly silicate and some sulfide micro-inclusions hosted in magnesiochromites of chromitites from Orhaneli Ophiolite Complex and Harmancik Peridotites in northwestern Turkey are studied. Existence of randomly distributed anhedral (globular), selectively oriented lamellar/needle-shaped silicate micro-inclusions and several other cubic/octahedral inclusions hosted in magnesiochromites are noted along with Ir-group platinum-group element (IPGE) bearing phases and base metal sulfides (BMS). The microscopic observations, Raman, electron dispersive and wavelength dispersive spectrometry data have shown that the randomly distributed anhedral (globular) silicate inclusions mainly consist of forsterite and diopside. Selectively oriented lamellar/needle-shaped silicate micro-inclusions are determined as diopsides. Some of the cubic/octahedral inclusions are identified as (cubic) Mg-silicates, while some other inclusions are identified as euhedral to subhedral clinopyroxene and Na-K-Ca and Mg bearing silicates. An important amount of the studied cubic/octahedral inclusions are identified as negative crystals of cubic/octahedral form (some in form of trigonal or macle crystals). The oriented lamellae/needles of diopside are found both in the Orhaneli and Harmancik chromitites. Cubic/octahedral Mg-silicates and negative crystals are only detected in a multi-textured chromitite sample (with dunitic-orbicular, semi-massive and massive textures), which lacks the oriented diopside lamellae/needles. Although no other ultrahigh-pressure (UHP) phases are found, the existence of oriented diopside lamellae/needles in the Orhaneli and Harmancik chromitites are interpreted as the first in-situ evidence of UHP conditions and possibly their deep mantle recycling history. It may be speculated that the occurrence of micro-lamellae/needles of diopside in the Orhaneli chromitites within the cumulate dunites result from entrapment of recycled relics of UHP chromitites within the upwelling primordial melts that replenished the evolved magmas ponding in the MTZ. A step-by-step textural interpretation of the multi-textured Harmancik chromitite sample also suggested at least two stages of chromite formation, the former represented with the tightly packed nodular chromite nuclei within the dunitic orbicules, and the latter with surrounding semi-massive to massive (tightly packed) chromitites. The former stage can clearly be traced to be finally incorporated within the latter. The cubic/octahedral Mg-silicates and negative crystals are both seen within the first stage chromitite nuclei of the dunitic orbicules and the second stage surrounding chromitite. Occurrences of these inclusions are also questioned for a possible link between the textural evolution of chromitite and the UHP processes, and possibilities for finding a field (macroscopic) evidence for discriminating UHP chromitites. Due to some conflicting findings, it is concluded that more data are needed for re","PeriodicalId":54690,"journal":{"name":"Ofioliti","volume":null,"pages":null},"PeriodicalIF":1.3,"publicationDate":"2018-01-20","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"48108148","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Structural setting and metamorphic evolution of a contact aureole: the example of the Mt. Capanne pluton (Elba Island, Tuscany, Italy)","authors":"E. Pandeli, R. Giusti, F. Elter, A. Orlando, L. Orti","doi":"10.4454/OFIOLITI.V43I1.455","DOIUrl":"https://doi.org/10.4454/OFIOLITI.V43I1.455","url":null,"abstract":"This paper shows an updated geological, petrographical-chemical and structural picture of the contact aureole of the 6.9 Ma Mt. Capanne monzogranitic pluton in western Elba Island (Tuscan Archipelago), that is one of the most known Tertiary intrusive bodies in Italy. Most of the foliated metamorphic rocks derived from a Mesozoic Ligurian-type ophiolitic succession (Punta Polveraia-Fetovaia Unit) and are in the horneblende to pyroxene hornfels facies with estimated Tpeak > 610°C (locally > 650°C). Local variations of the metamorphic zonation of the at least 150-200 m thick aureole are due to the different lithological, physical and structural (e.g., bedding, fracturing) nature of the original rocks, but also to hydrofracturing phenomena which occurred during the syn-metamorphic upflow of the metasomatic hot fluids. The history of the emplacement and uplift of the Mt. Capanne pluton was defined through the metamorphic-structural evolution of its contact metamorphic aureole. The growth of the thermometamorphic minerals took place in several stages being both syn-kinematic with respect to the main ductile D 2 folding event (connected to the plutonic intrusion) and static post-kinematic one. Ductile shear zones, characterized also by mylonites, were also active during recrystallization in a general shear-type regime (average value of W m = 0.7) related to the vertical uplift of the pluton (pure shear) and lateral ductile flow of the covers (simple shear). The exhumation of the cooled pluton continued producing tangential cascade-type folding event (D 3 ), detachment faults, and later high-angle faulting in the host rocks. The role of Western Elba in the geological frame of the whole island since Late Miocene times is also outlined in the paper.","PeriodicalId":54690,"journal":{"name":"Ofioliti","volume":null,"pages":null},"PeriodicalIF":1.3,"publicationDate":"2018-01-20","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"46811529","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}