Spinel and plagioclase peridotites of the Nain ophiolite (Central Iran): Evidence for the incipient stage of oceanic basin formation

The Nain ophiolites crop out along the western border of the central East Iran Microcontinent (CEIM) and consist of an ophiolitic mélange in which pargasite-bearing spinel and plagioclase mantle lherzolites are largely represented.Whole-rock and mineral chemistry data suggest that these rocks record the complex history of the asthenospheric and lithospheric mantle evolution. The spinel lherzolites have experienced low-degree (~5%) partial melting and contain clinopyroxenes with positive Eu anomalies (Eu/Eu⁎=1.10–1.48) suggesting that the partial melting occurred under oxidized conditions (fayalite–magnetite–quartz−0.8 to+1.3). The pargasite and coexisting clinopyroxene in these rocks are depleted in light rare earth elements (LREE) (mean chondrite-normalized CeN/SmN=0.045). The depleted chemistry of this amphibole reflects metasomatismduring interaction with H2O-rich subalkalinemaficmelts,most likely concurrentlywith or after the partial melting of the spinel lherzolites. The plagioclase lherzolites were subsequently formed by the subsolidus recrystallization of spinel lherzolites under plagioclase facies conditions as a result of mantle uprising, as evidenced by: (1) the development of plagioclase rims around the spinels; (2) plagioclase+orthopyroxene exsolution textures within some clinopyroxene grains; (3) an increase in plagioclase modal content coupled with an increase in modal olivine and a decrease in modal pyroxene and pargasite; (4) coincident decreases in Al, Mg, and Ni, and increases in Cr, Ti, and Fe in spinel, as well as decreases in Al and Ca, and increases in Cr and Ti in pyroxene and pargasite; and (5) the identical whole rock compositions of the spinel and plagioclase lherzolites, which rules out a magmatic origin for the plagioclase in these units. The Nain lherzolites have similar whole-rock and mineral geochemical compositions to subcontinental peridotites that are typically representative of Iberia-type rifted continental margins and ocean–continent transition zones (OCTZ), suggesting that they formed during the early stages of the evolution of the Nain oceanic basin. This means that the Nain lherzolites represent the Triassic–Jurassic western border of the CEIM or alternatively an associated OCTZ

Time-progressive mantle-melt evolution and magma production in a Tethyan marginal sea: A case study of the Albanide-Hellenide ophiolites

n/

Redefinition of the Ligurian Units at the Alps–Apennines junction (NW Italy) and their role in the evolution of the Ligurian accretionary wedge: constraints from mélanges and broken formations

We document that the undifferentiated chaotic Ligurian Units of the Monferrato–Torino Hill sector (MO-TH) at the Alps–Apennines junction consist of three different units that are comparable with the Cassio, Caio and Sporno Units of the External Ligurian Units of the Northern Apennines. Their internal stratigraphy reflects the character of units deposited in an ocean–continent transition (OCT) zone between the northwestern termination of the Ligurian–Piedmont oceanic basin and the thinned passive margin of Adria microcontinent. The inherited wedge-shaped architecture of this OCT, which gradually closed toward the north in the present-day Canavese Zone, controlled the Late Cretaceous–early Eocene flysch deposition at the trench of the External Ligurian accretionary wedge during the oblique subduction. This favoured the formation of an accretionary wedge increasing in thickness and elevation toward the SE, from the MO-TH to the Emilia Northern Apennines. Our results therefore provide significant information on both the palaeogeographical reconstruction of the northwestern termination of the Ligurian–Piedmont oceanic basin and the role played by inherited along-strike variations (stratigraphy, structural architecture and morphology) of OCT zones in controlling subduction–accretionary processes

Subduction signature of the Vardar ophiolite of North Macedonia: new constraints from geochemical and stable isotope data

Volatiles such as carbon (C) and sulphur (S) are commonly transferred into the mantle from subduction of oceanic lithosphere and overlying sediments. C and S isotopic signatures of magmatic rocks could be used as proxies of the slab components involved in the petrogenesis of subduction-related ophiolites. Therefore, in this work we investigated the major and trace element composition, as well as the C and S elemental contents and isotopic ratios (13C/12C and 34S/32S) of subvolcanic and volcanic rocks of the Vardar ophiolites of North Macedonia, which represent the remnants of the Mesozoic Tethyan oceanic lithosphere formed in supra-subduction zone tectonic settings. The ophiolites were sampled at Lipkovo and Demir Kapija localities, in the northern and southern part of North Macedonia, respectively. Based on whole-rock major and trace element composition, three groups of rocks can be distinguished: i) Group 1 rocks, which are subalkaline basalts having backarc affinity, ii) Group 2a and iii) Group 2b rocks, which are calc-alkaline basalts having arc affinity, with and without adakitic signatures, respectively. The qualitative petrogenetic models indicate that studied rocks formed by partial melting of mantle sources variably metasomatized by subduction-related components, such as aqueous fluids, sediment melts, and adakitic melts. Accordingly, all the North Macedonia ophiolites are characterized by C and S signatures which deviate from those typical for mantle and Mid Ocean Ridge melts. The variably low δ13C values recorded by Group 1 and 2 rocks could be related to the different contributions of melts released by subducting sediments rich in organic matter. However, we cannot exclude that such C-enriched signature is the result of isotopic fractionation during degassing process. In contrast, the enriched S isotopic signatures of the North Macedonia ophiolites suggest a major involvement of melts derived from the subducting sediments rich in sulphate phases. In particular, the calc-alkaline basalts of Group 2 rocks record more positive δ34S values than the subalkaline basalts of Group 1 formed in backarc basin suggesting that the subarc mantle sources were more affected by slab-released fluids than those of the backarc basin, which were more distal from the trenc

Petrology of the basaltic rocks of the Nankay Trough Basement

Major- and trace-element analyses, mineral chemistry, and Sr-Nd isotopic determinations were obtained on representative igneous rocks drilled from the Nankai accretionary complex (Site 808) during Ocean Drilling Program Leg 131. For the first time, the oceanic basement of the subducting plate below an accretionary prism has been reached. The Nankai Trough basement was encountered at a depth of 1289.9 mbsf and a total of 37.1 m of igneous rocks, middle Miocene (15.6 Ma) in age, was penetrated. Two main lithological units have been distinguished from the top downward; sill-like rocks (Unit I: Cores 105, 106, 107) and pillow lavas (Unit II: Core 108). Basalts are predominantly nonvesicular, hypocrystalline, aphyric to slightly phyric with intersertal to intergranular textures. Alteration is generally slight to moderate. All the basaltic rocks are cut by ramifying veins of varying widths. Secondary mineral assemblages (including vein fillings) are typical of submarine alteration and zeolite to low greenschist facies metamorphism. The order of crystallization of primary minerals is: olivine, Plagioclase, clinopyroxene. This, together with mineral chemistry, characterized by forsteritic olivine (Fo 84-85), highly anorthitic Plagioclase (up to An 90), and in particular the composition of clinopyroxene, are typical of normal mid-ocean ridge basalts (MORB). In terms of Zr/Y (2.9-3.8) and Zr/Nb (21-58), all the analyzed samples plot in the normal MORB field. The chondrite-normalized REE patterns confirm the close affinity with normal MORB type (LaN/SmN: 0.6-0.8). Note that such magmatism does not reveal any evidence of subduction-related geochemical components. The 87Sr/86Sr isotopic ratios range from 0.70339 in pillow lavas to 0.70317 in the least-altered basalts of sill units (ratios reduced to 0.70265-0.70271 by HC1 2.5 N hot leaching), whereas 143Nd/144Nd ratios are 0.51314-0.51326. These values conform with those of normal MORB. Stratigraphy, petrography, and geochemistry of the basaltic rocks recovered at Site 808 appear very similar to those from the Shikoku Basin basement (particularly Sites 442 and 443, DSDP Leg 58), analogously identified as normal MORB

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

Petrogenesis and tectono-magmatic significance of volcanic and subvolcanic rocks in the Albanide-Hellenide ophiolitic mélanges

Ophiolitic mélanges associated with ophiolitic sequences are widespread in the Mirdita-Subpelagonian zone (Albanide-Hellenide orogenic belt) and consist of tectono-sedimentary “block in matrix-type” mélanges. Volcanic and subvolcanic basaltic rocks included in the main mélange units are studied in this paper with the aim of assessing their chemistry and petrogenesis, as well as their original tectonic setting of formation. Basaltic rocks incorporated in these mélanges include: (1) Triassic transitional to alkaline within-plate basalts (WPBs); (2) Triassic normal (N-MORB) and enriched (E-MORB) mid-ocean ridge basalts; (3) Jurassic N-MORBs; (4) Jurassic basalts with geochemical characteristics intermediate between MORB and island arc tholeiites (MORB/IAT); (5) Jurassic boninitic rocks. These rocks record different igneous activities, which are related to the geodynamic and mantle evolution through time in the Mirdita-Subpelagonian sector of the Tethys. Mélange units formed mainly through sedimentary processes are characterised by the prevalence of materials derived from the SSZ environments, whereas in mélange units where tectonic processes prevail, the oceanic materials predominate. By contrast, no compositional distinction between structurally similar mélange units is observed, suggesting that they may be regarded as a unique mélange belt extending from the Hellenides to the Albanides, whose formation was largely dominated by the mechanisms of incorporation of the different materials. Most of the basaltic rocks surfacing in the MOR and SSZ Albanide-Hellenide ophiolites are incorporated in mélanges. However, basalts with island arc tholeiitic affinity, although they are volumetrically the most abundant ophiolitic rock types, have not been found in mélanges so far. This implies that the rocks forming the main part of the intra-oceanic arc do not seem to have contributed to the mélange formation, whereas rocks presumably formed in the forearc region are largely represented in sedimentary-dominated mélanges. In addition, Triassic E-MORBs, N-MORBs, and WPBs included in many mélanges are not presently found in the ophiolitic sequences. Nonetheless, they testify the existence throughout the Albanide-Hellenide belt of an oceanic basin since the Middle Triassic

Mid-ocean ridge and supra-subduction affinities in the Pindos Massif ophiolites (Greece): Implications for magma genesis in a proto-forearc setting

The Pindos ophiolitic massif is considered an important key area within the Albanide-Hellenide ophiolitic belt and is represented by two tectonically distinct ophiolitic units: 1) a lower unit, including an intrusive and a volcanic section; and 2) an upper ophiolitic unit, mainly including mantle harzburgites. Both units share similar metamorphic soles and tectono-sedimentary mélanges at their bases. The intrusive section of the lower unit is composed by an alternation of troctolites with various ultramafic rock-types, including dunites, lherzolites, olivine-websterites, olivine-gabbros, anorthositic gabbros, gabbros and rare gabbronorites. The volcanic and subvolcanic sequence of the lower unit can geochemically be subdivided into three groups of rocks: 1) basalts and basaltic andesites of the lower pillow section showing a clear high-Ti affinity; 2) basaltic andesites of the upper pillow section with high-Ti affinity, but showing many geochemical differences with respect to the first group; 3) very low-Ti (boninitic) basaltic and basaltic andesitic lava flows separating the lower and upper pillow sections, and dykes widespread throughout the Pindos ophiolites. These different magmatic groups originated from fractional crystallization from different primary magmas, which were generated, in turn, from partial melting of mantle sources progressively depleted by previous melt extractions. Group 1 volcanics may have derived from partial melting (ca. 20%) of an undepleted lherzolitic source, while group 2 basaltic rocks may have derived from partial melting (ca. 10%) of a mantle that had previously experienced MORB extraction. Finally, the Group 3 boninites may have derived from partial melting (ca 12-17%) of a mantle peridotite previously depleted by primary melt extraction of Group 1 and 2 primary melts. In order to explain the coexistence of these geochemically different magma groups, two petrogenetic models formerly proposed for the Albanian ophiolites are discussed

Tectono-magmatic significance of volcanic rocks from the Albanide-Hellenide ophiolitic mélanges

Ophiolites of the Albanide-Hellenide Subpelagonian Zone are widely associated with mélange units, which consist of polygenetic thrust sheets formed by various combination of tectonic and sedimentary processes operating at subduction zones during tectonic emplacement of ophiolites. Subpelagonian mélanges incorporate tectonically eroded and/or fragmented blocks of the subducting oceanic plate, as well as materials derived from the forearc region in the upper plate plate. Composition and petrology of volcanics included in these mélanges record the magmatic activities developed from the early stages of generation of oceanic lithosphere (including seamounts) up to its consumption in a converging margin setting and also provide information on the mélange formation and evolution. The Rubik Complex (Albanides) and the Avdella Mélange (Pindos, Hellenides) are typical tectono-sedimentary mélanges, which include Triassic-Jurassic basalts with both mid-ocean ridge (MORB) and ocean island (OIB) affinities. This suggests that they formed by incorporation of oceanic materials from the subducting plate. The tectono-sedimentary Koziakas Mélange (Hellenides) consists of a variety of different volcanic rocks: (1) transitional to alkaline basalts, trachyandesites, and trachytes; (2) both normal (N-) and transitional (T-) MORBs; (3) boninitic volcanics. No datings are available for these litho-types, although blocks of associated radiolarian chert indicate both Triassic and Jurassic ages. The Agoriani Mélange (Othrys, Hellenides), originated mainly through sedimentary processes, includes N-MORBs, alkaline OIBs, boninitic clasts and blocks, and MORB/IAT intermediate basalts. The mélange from the Argolis Peninsula (Hellenides) can be subdivided into four different mappable units. In the Lower and Upper Units sedimentary processes prevail with volcanics largely predominated by boninitic rocks, whereas N- and T-MORBs and alkaline OIBs are rare. By contrast, tectonic processes mainly characterize the Middle and Wedged Units. Volcanics in the Middle Unit are alkaline OIBs and Triassic and Jurassic N- and T-MORBs, whereas the Wedged Unit is exclusively composed of OIBs, thus representing a seamount fragment. In the Rubik and Koziakas Mélanges some boninitic dykes crosscut different tectonic slices, referred to both MORB and OIB sequences. This implies that tectonic incorporation of MORB-type and OIB-type materials occurred prior to the development of boninitic magmatism