Mélanges are abundant in both accretionary and collisional orogenic belts. Their chaotic, block-in-matrix structure can have different origins: sedimentary mélanges can be overprinted by later metamorphic and deformative events or, conversely, tectonic mélanges can form directly at the plate interface, at different tectonic levels and either in prograde (i.e. during underplating) or retrograde (i.e. during exhumation) conditions.

The HP-metaophiolitic Voltri Massif (W Alps, Italy), considered as an exhumed piece of the plate interface of the Alpine orogen, includes various, well-preserved examples of tectonic mélanges at different scales (from m- to km-scale). Here, we investigate a 100 meters-thick tectonic mélange, where blocks of various metamorphic lithologies (e.g. metagabbro, eclogite, serpentinite, calchschist and qtz-micaschist) and sizes (0,1-m- to 10-m scale) are dispersed within an intensely foliated, lithologically heterogeneous matrix made of a mixture among serpentinite-schist, chlorite-actinolite schist and graphitic schist, predominantly equilibrated at grenschist facies conditions.

Preliminary field investigations reveal a pronounced strain and metamorphic partitioning between the matrix and the blocks. These latter show internal metamorphic layering, shear zones and extensional veins discordant to the pervasive s-c-fabric and folding that characterize the enclosing matrix. Locally, eclogitic blocks show progressive internal fragmentation (e.g., fracturing/veining) up to pervasive brecciation. Petrographic/microanalytical investigations on the most preserved (Fe-Ti-bearing) metagabbro and metabasalt blocks indicate prograde peak metamorphism either in eclogite (grt + omp + rt ± Na-amp ± ph assemblage) or blueschist-facies (Na-amp + ttn + chl ± ep ± ph assemblage); some eclogites show either a retrograde syn-tectonic stage in blueschist facies or a static greenschist overprint. PT estimates on eclogitic blocks indicate a peak stage at P = 18,6 ± 1,0 Kbar (gnt-ph-cpx geobarometer) and T = 530 ± 10°C. The block-matrix transition is characterized by dm- to cm-thick metasomatic rinds rich in hydrous minerals, such as tremolitic amphiboles, biotite, chlorite and minor titanite, tourmaline, adularia and sulphides. Locally, tensile fractures filled by a polymineralic gouge material with the same mineral composition (±biotite) and syntectonic extensional veins with fibrous amphibole depart from the rinds and intrude the prisitne blocks. Abundant hydrothermal fluid circulation is suggested, among other, by peculiar microstructures, i.e. the growth of chlorite in vermicular form.

The block-in-matrix structures and microstructures (shear zones and extensional cracks repeatedly crosscuting eachother) point to the occurrence of a cyclic deformation characterised by episodic switch between brittle and ductile regimes and changes in the rehological properties of blocks and matrix. The occurrence of (i) abundant mélange matrix, (ii) metasomatic rinds digesting blocks with (iii) sets of veins/cracks irradiating inside the intact rocks suggest the key-role played by fluids in the evolution of the Piota River mélange.

The evidence recorded in the studied lithologies, such as episodic switch between deformation regimes assisted by transient exceed of the rock tensile strenght by pore fluids overpressure, would permit to better understand the mechanisms controlling slow earthquake generation at shallow plate interface. Morover, this study, combined with studies of other melange occurrences of the Voltri Massif, will help to better understand the complex geodynamic phenomena acting on collisional orogens.

How to cite: Locatelli, M., Federico, L., Cianfarra, P., Morelli, D., and Crispini, L.: Study of tectonic mélanges from a fossil plate interface: probing geodynamic phenomena, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12006, https://doi.org/10.5194/egusphere-egu23-12006, 2023.

Seamounts are topographic highs of the oceanic plates, and they are passively carried toward convergent margins where they may interact with the frontal part of the subduction complexes, modifying their shape and influencing the operating tectonic processes. In this tectonic setting, seamount fragments can be transferred from the subducting plate to the accretionary prism with different mechanisms, including deformation within the subduction channel, accretion via decapitation of the seamount summit by the basal décollement of the prism, and offscraping and underplating of thrust-bounded assemblages at both shallow (4-8 km) and deep (20-30 km) structural levels of the prism. In this complex tectonic scenario, it is not completely clear which are the factors controlling deformation mechanisms and localization of the basal décollement below, inside, or above the subducting seamount. Detailed geological mapping, stratigraphic-structural analysis and petrological studies are promising tools to better understand the mechanism of seamount materials accretion, providing data to recognize the role of subducting seamounts for the geodynamic evolution of exhumed accretionary and collisional orogenic belts.

We present here new structural and thermobarometric data on the Durkan Complex to discuss how Late Cretaceous seamount materials has been accreted into the Makran accretionary prism (SE Iran) during the Late Cretaceous – Paleocene subduction-accretionary stages. Throughout a map- to micro-scale structural studies of the western part of this Complex, we describe its structural and tectono-metamorphic evolution using crosscutting relationships between structural elements and stratigraphic unconformities.

Our results indicated that seamounts material has been incorporated in the prism as imbricated tectonic units separated by NNW-striking thrust zones. During the accretion, seamounts successions are folded by sub-isoclinal folds, associated with a blueschist facies axial plane foliation and shear zones along the limbs. These shear zones show block-in-matrix fabric and are mainly composed of volcaniclastic material from the seamount slope successions indicating that the seamount stratigraphy play a key role in controlling the position of the basal décollement of the prism during underplating. Thermobarometric estimates indicate that the accretion took place at T = 160-300 °C and P = 0.6-1.2 GPa, corresponding to a depth of 25–40 km. This data indicates the incorporation of seamount materials via underplating at blueschist facies conditions within the Makran subduction complex. The folds and shear zones formed during the accretionary stage are later deformed by open to close folds associated with normal faults, recording the progressive exhumation of the accreted seamount materials at shallower levels of the Makran Accretionary Prism. The unconformable deposition of upper Paleocene – Eocene turbiditic successions onto the exhumed seamount materials of the Durkan Complex constrain the accretionary stages during the Late Cretaceous – early Paleocene evolution of the Makran Accretionary Prism.

How to cite: Barbero, E., Di Rosa, M., Pandolfi, L., Delavari, M., Dolati, A., Zaccarini, F., Saccani, E., and Marroni, M.: Processes of seamount materials accretion in subduction complexes: The example of the Durkan Complex (Makran Accretionary Prism, SE Iran), EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4276, https://doi.org/10.5194/egusphere-egu23-4276, 2023.

Ophiolites may originate in a variety of oceanic settings such as mid-ocean ridges, intra-oceanic and continental margin volcanic arc, marginal basins, and seamounts. Ophiolites from different settings show distinctive lithological features and geochemical fingerprinting, so that they can conversely be used to identify their geodynamic setting of formation. Therefore, ophiolite geochemistry coupled with geochronological data represents an effective tool for tracking the magmatic events occurring during the life of an oceanic basin and surrounding continental margins. The northern part of the Makran Accretionary Prism in south Iran is characterized by extensive occurrence of tectonically imbricated ophiolitic, metaophiolitic, and ophiolitic mélange units, which represent or incorporate remnants of the Neotethys Ocean located between the Lut block and the Arabian Plate and of its northern continental margin. In this contribution we present a review of geochemistry and age data of volcanic rocks from these units with the aim of defining the nature and tectono-magmatic evolution of the Middle East sector of the Neotethys.

The North Makran ophiolitic units are from north to south (from the structural top to bottom): 1) the Ganj Complex; 2) the Northern Ophiolites including Band-e-Zeyarat/Dar Anar, Remeshk-Mokhtarabad, and Fannuj-Maskutan units: 3) the Deyader Complex; 4) the Bajgan Complex; 5) the Durkan Complex; 6) the Sorkhband-Rudan ophiolites; 7) the Coloured Mélange. The Deyader, Bajgan, and Durkan Complexes show variable extents of HP-LT metamorphic imprint.

The Ganj Complex consists of island arc tholeiitic (IAT) and calc-alkaline (CAB) volcanic sequences showing Turonian-Coniacian age (biostratigraphic data). This unit represents a Late Cretaceous volcanic arc that was likely forming at the southern margin of the Lut Block. Units of the Northern Ophiolites and the Bajgan metaophiolites show similar geochemistry and age. They are largely represented by mid-ocean ridge basalts (MORB) showing either normal (N-) and enriched (E-) compositions. Biostratigraphic and zircon U/Pb radiometric datings suggest Early Cretaceous and Late Jurassic-Early Cretaceous ages for the Northern ophiolites and the Bajgan Complex, respectively. The Durkan and Deyader Complexes are both Late Cretaceous in age. The Deyader metaophiolites range in composition from N-MORB to E-MORB and comparatively more enriched plume-type MORB (P-), whereas the Durkan metaophiolites show P-MORB and very enriched alkaline affinities and have been interpreted as remnants of a seamount chain. The Coloured Mélange includes volcanic arc basalt of both Early and Late Cretaceous age, as well as Late Cretaceous enriched oceanic plateau basalts and alkaline basalts (all ages based on biostratigraphic data).

This study indicates that the North Makran ophiolites and metaophiolites represent fragments of a unique Late Jurassic – Cretaceous oceanic basin, which was increasingly affected by mantle plume activity from Early to Late Cretaceous and experienced different extents of plume-ridge interaction in different times and areas. The different ophiolitic units represent distinct portions of the oceanic basin including plume proximal and plume distal mid-ocean ridges, seamounts. From Late Cretaceous, this basin subducted below the Lut Block forming the Ganj volcanic arc. 

How to cite: Saccani, E., Barbero, E., Pandolfi, L., Delavari, M., Dolati, A., Marroni, M., Catanzariti, R., and Chiari, M.: Nature and evolution of the Middle East Neotethys: New constraints from geochemistry and age of ophiolites and metaophiolites from the Makran Accretionary Prism (SE Iran), EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5270, https://doi.org/10.5194/egusphere-egu23-5270, 2023.

Studies of exposed high pressure-low temperature (HP-LT) metamorphic complexes are critical for advancing our understanding of subduction processes, such as underplating, metamorphism, and exhumation. Exhumed blueschist-facies metasedimentary and volcanic rocks exposed on the Pelion peninsula (eastern Thessaly, Greece) represent one of the largest coherent exposures of subduction-complex rocks in the eastern Mediterranean and are key for understanding early Cenozoic Hellenic subduction processes. In this study, we present new detrital zircon and apatite U-Pb data to reconstruct the stratigraphic anatomy and provenance of these rocks and to understand their correlation with other Aegean (Cycladic) HP-LT rocks and the Pelagonian Zone of mainland Greece.

Detailed new U-Pb zircon and apatite data show two distinct, coherent, and stratigraphically upright structural slices, with (1) the South Pelion slice consisting of Permian-Late Cretaceous strata overlying Carboniferous basement and (2) the North Pelion slice comprising Triassic-Late Cretaceous strata overlying Neoproterozoic basement. Both slices exhibit Late Cretaceous strata at the top of the section characterized by cosmopolitan detrital zircon (DZ) signatures. Zircon U-Pb data of rim overgrowths suggest subduction-metamorphism occurred during the early Cenozoic with temperatures not reaching >450°C, as indicated by non-reset or -recrystallized apatite U-Pb ages and the absence of garnet.

Comparison of compiled DZ data from the CBU and our data from the Pelion blueschists supports a correlation in the pre-subduction paleogeography, with protolith deposition during Permo-Carboniferous intra-arc extension and early Mesozoic Adria-Pindos rifting. The data show that the Pelion blueschists, representing lateral equivalents of the CBU, are comprised of two coherently underplated upper-crustal slivers, separated by Late Cretaceous flysch, and metamorphosed during Cenozoic Hellenic subduction beneath the Pelagonian convergent margin.

How to cite: Hinshaw, E. R., Stockli, D. F., and Soukis, K.: Coherent subduction underplating of CBU-correlative blueschist-facies metasedimentary slices, Pelion, Greece, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12802, https://doi.org/10.5194/egusphere-egu23-12802, 2023.

The Nan River mafic-ultramafic belt was identified when detailed geological mapping of NE Thailand began in the 1970s, and suspected to represent a suture zone. However, in the absence of an obvious ophiolite, its tectonic status was not confirmed until two short papers (Barr et al., 1985 and Barr & Macdonald, 1987) reported the discovery of associated  blueschists. Unfortunately military restrictions on access to detailed topographic maps meant they that they did not state an exact location and the outcrops were “lost” to Thai geologists and no further research was conducted. The two “lost” blueschist localities (south of Nan and west of Uttaradit) were recently re-discovered and related winchite – barroisite schist units identified. Additionally, garnet – glaucophane/riebeckite – white mica – quartz – magnetite – titanite – rutile ± albite ± stilpnomelane bearing gneisses were found among the bedload of a stream cutting through these schists. These gneisses are believed to be derived from “exotic” blocks in a mapped, but poorly exposed thrust sheet of tectonic melange, but to date no in place examples have been found. Similar blueschists/greenschists, gneisses and related garnet – white mica schists have been found further north as cobbles on point bars of the Wa river (west Nan), which cuts through a different section of the mafic – ultramafic unit in a mountainous and inaccessible national park.

At both in-place blueschist locations the schists have undergone two episodes of deformation, producing well developed schistosities and tight folding. The blue amphiboles are crossitic in composition. They do not contain garnet nor lawsonite, but abundant epidote and white mica with elevated phengite content. They are interbanded with winchite – barroisite bearing schists. The observed mineral assemblages are poorly suited to apply well established geothermobarometers. However, a PT window of the metramorphic overprint could be established with ca. 450 to 550 °C and 0.6 to 1.0 GPa. Geothermobarometry of the blue amphibole and garnet bearing exotic gneisses from the first blueschist locality (south Nan) indicates peak T conditions of ca. 550°C and a max. P of ca. 1 GPa. Comparable blue amphibole and garnet bearing gneisses from the second locality (Wa river) indicate similar peak PT conditions.

In-situ U-Pb zircon analyses from 6 blue amphibole – phengite bearing gneiss samples gave weighted mean ²⁰⁶Pb/²³⁸U dates ranging from 312 to 326 Ma, which is interpreted as the age of the protolith. Accessory phases within the blueschists and gneisses include variously zircon, titanite, rutile, allanite and monazite. Planned analysis of these phases should provide the age of HP/LT metamorphism.

How to cite: Sawasdee, P., Booth, J. E., Skrzypek, E., and Hauzenberger, C. A.: Blueschists and blue amphibole gneisses in the Nan suture zone, NE Thailand, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8023, https://doi.org/10.5194/egusphere-egu23-8023, 2023.

Most ophiolitic mélanges and chaotic rock units in exhumed subduction zone complexes and orogenic belts are commonly interpreted as the products of tectonic processes (e.g., underplating and return flow) acting at intermediate to great depths (depth > 10–15 km, T > 250 °C) at convergent margins. Conversely, observations from modern and ancient, non- to poorly metamorphosed subduction–accretion complexes (recognized as mélanges and chaotic rock units) around the world show that these rock associations: (1) likely formed at shallow structural levels first, and (2) were later subducted and became tectonically reworked. As such, they mainly consist of broken formations (> 21.5%), and sedimentary (c. 20%), polygenetic (> 13.7%) and/or diapiric (c. 6.7%) mélanges. Tectonic mélanges are limited to <3.0% (in surface distribution), suggesting that tectonic processes do not make efficient mixing mechanisms at shallow structural levels. Subduction of structural inheritances (e.g., ocean-continent transition zones, and lithological and structural heterogeneities in ocean plate stratigraphy – OPS – assemblages) plays a more significant role in forming mélanges and chaotic rock units at shallow depths; it can also control the origin and location of plate interface and the dynamics of the wedge front (i.e., tectonic accretion vs. erosion). However, not all chaotic rock units that formed at shallow structural levels may become subducted; but, if subducted, their fate might be different depending on whether they become part of the plate interface or if they become part of the lower plate. Our global field observations, suggesting that most mélanges and chaotic rock units form at shallow depths, have significant implications for the tectonic evolution of subduction zone complexes and orogenic belts.

How to cite: Festa, A., Barbero, E., Dilek, Y., Remitti, F., Ogata, K., and Pini, G. A.: Significance of ophiolitic mélanges and chaotic rock units in the evolution of subduction complexes and orogenic belts, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-9853, https://doi.org/10.5194/egusphere-egu23-9853, 2023.

Ophiolite is the remnants of ancient oceanic crust and mantle, which can reveal the tectonic evolution of paleo-oceanic basins. Podiform chromite deposit in ophiolites can retain the original information of petrogenesis and mineralization during the later deformation and metamorphism process, and it is a key object that can be used to decipher the origin and tectonic setting of ophiolites and the evolution of paleo-oceanic basins. Ophiolite suites are widely developed in Hegenshan and Solonker tectonic belts, the Inner Mongolia segment of the southern Central Asian Orogenic Belt. However, the genesis and tectonic environment of ophiolite and associated podiform chromitites remain debated, which restrict the understanding of the tectonic evolution and metallogenic background of the orogenic belt. Here, we conducted a detailed study of field, petrography, and mineral chemistry on the podiform chromitites in the Hegenshan and Solonker ophiolites in Inner Mongolia to explore their origin and tectonic environment. Petrographic results show that the Hegenshan chromites contain abundant high-pressure, hydrous mineral inclusions of sodic amphibole, white mica, and clinopyroxene, along with previously reported ultra-high pressure minerals (e.g., diamond); whereas the Solonker chromite contains minor white mica inclusions. Mineral chemical analysis shows that the Hegenshan ophiolite is dominated by high-Al type spinels with subordinate high-Cr type spinels; whereas the Solonker ophiolite mainly contains High-Cr type spinels. Accordingly, we suggest that the Hegenshan chromitites formed initially in a mid-ocean ridge (MOR) setting of a backarc ocean basin, then experienced modification in a suprasubduction zone (SSZ) setting, with deep mantle recycling and two stages of melt-peridotite interactions due to backarc subduction initiation; and the Solonker chromitites formed by boninitic melt-peridotite reaction in the SSZ forearc setting probably due to slab roll-back or subduction re-initiation following ridge subduction. These findings provide important constraints on the petrogenesis of chromites/ophiolites, regional tectonic evolution and mineralization background of chromitites in the Inner Mongolia segment of the Central Asian Orogenic belt.

How to cite: Qian, Q., Huang, B., Fu, D., Liu, M., Kusky, T., and Wang, L.: Genesis and tectonic setting of podiform chromitites in the Hegenshan and Solonker ophiolites, Inner Mongolia, southeastern Central Asian Orogenic Belt, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14650, https://doi.org/10.5194/egusphere-egu23-14650, 2023.