Mineral chemistry and petrology of highly magnesian ultramafic cumulates from the Sarve-Abad (Sawlava) ophiolites (Kurdistan, NW Iran): New evidence for boninitic magmatism in intra-oceanic fore-arc setting in the Neo-Tethys between Arabia and Iran

The Sarve-Abad (Sawlava) ophiolitic complex consists of several tectonically dismembered ophiolitic sequences. They are located along the Main Zagros Thrust Zone, which marks the ophiolitic suture between the Arabian and Sanandaj–Sirjan continental blocks. They represent a portion of the southern Neo-Tethyan oceanic lithosphere, which originally existed between the Arabian (to the south) and Eurasian (to the north) continental margins. The Sarve-Abad ophiolites include cumulitic lherzolites bearing minor dunite and chromitite lenses in places. The main rock-forming minerals in ultramafic cumulates are cumulus olivine and inter-cumulus clinopyroxene and orthopyroxene. Minor (<5%) chromian spinel occurs as both cumulus and inter-cumulus phases. Cr#, Mg# and TiO2, Cr2O3, and Al2O3 concentrations of chromian spinel from ultramafic cumulates and chromitites plot in the forearc and boninite spinel fields, respectively. Clinopyroxene has very high Mg# and low TiO2 contents. Calculated TiO2 and Al2O3 compositions and Mg# in the parental melt that was in equilibrium with chromian spinel and olivine are consistent with supra-subduction zone-type compositions. Whole-rock geochemistry of the ultramafic cumulates is characterized by very low incompatible element content and a general enrichment in Th with respect to Ta and Nb. Chondrite-normalized REE patterns show different trends with either (La/Sm)N 1 and (Sm/Yb)N < 1 (U-shaped pattern). Both these patterns are compatible with boninitic-type parental melts. Accordingly, petrogenetic modeling using REE composition indicates that Sarve-Abad ultramafic cumulates may have formed by small degrees (5–15%) of fractional crystallization from typical boninitic melts characterized by either light REE/medium REE depletion or enrichment. Mineral chemistry and whole-rock chemistry clearly indicate that the Sarve-Abad ultramafic cumulates and chromitites record an episode of boninitic magmatism that occurred within the southern Neo-Tethys Ocean during the Late Cretaceous. Boninitic melts in the Sarve-Abad ophiolites were formed by partial melting of depleted peridotite which made up the residual mantle after MORB-type melt extraction. This was subsequently enriched with light REE and large ion lithophile elements by subduction-derived fluids. It is therefore suggested that this boninitic magmatism was generated in the forearc sector of a short-lived intra-oceanic arc that was located southward with respect to the ‘‘Andean-type’’ subduction below the Sanandaj–Sirjan continental margin

New evidence for supra-subductionzone ophiolites in the Vardar Zone from the Vermion Massif (northern Greece): Implication for the tectono-magmatic evolution of the Vardar oceanic basin

The Vermion Massif (northern Greece) is located across the boundary between the Pelagonian and Vardar Zones and includes several tectonic units bearing ophiolitic rocks, which represent remnants of the oceanic lithosphere formed in the Neotethyan Vardar Ocean, between the Pelagonian and the Serbo-Macedonian continental realms. This massif consists of tectonic units belonging to the Pelagonian Domain, which are tectonically overlain by units associated with the Almopias sub-Zone (Vardar Zone). Ophiolitic rocks consist of mantle harzburgites and ophiolitic mélanges. The ophiolitic mélanges incorporate rocks exhibiting a wide range of composition, including various intrusive rocks and volcanic rocks ranging from basalts, basaltic andesites, andesites, dacites, to rhyolites. Incompatible elements and rare earth elements analyses indicate that a number of different rock-types formed in distinct tectonic settings can be identified. Mantle harzburgites have a very depleted nature and represent portions of the supra-subduction (SSZ) mantle developed in an intra-oceanic arc setting. The mélanges units include six rock types variably distributed in the Pelagonian and Almopias Units. They are: (1) calc-alkaline rocks with marked depletion in Nb, Ta Ti and enrichment in LREE, Th; (2) LREE-depleted N-MORB; (3) LREE-enriched E-MORB; (4) alkaline within-plate basalts showing marked enrichments in Th, Ta, Nb, LREE; (5) low-Ti island arc tholeiites featuring depletion in HFSE; and (6) very-low-Ti boninites characterized by strong depletion in HSFE and REE. Previous interpretations have referred the Vardar ophiolites to a MORB-type oceanic setting and to a MORB-type backarc setting; however, the widespread occurrence of SSZ ophiolites has never been documented in the Vardar Zone before and is particularly important as it testifies for the existence of an intra-oceanic arc basin in the Vardar oceanic domain. The results presented in this paper compared with literature data on other magmatic rocks within the Vardar Zone suggest that the opening and closure of the Vardar Ocean record several distinct accretion events in this basin, that is oceanic crust generation at mid-ocean ridge, alkaline seamounts in the oceanic domain and SSZ setting, as well as two accretion events in the western realm of the Serbo-Macedonian continent, that is volcanic arc and backarc settings. Based on the comparison between the modern west Pacific subduction system and the results from this study a new model for a multistage tectono-magmatic evolution of the Vardar Ocean is proposed

Petrogenesis and tectono-magmatic significance of basalts and mantle peridotites from the Albanian-Greek ophiolites and sub-ophiolitic melanges. New constraints for the Triassic-Jurassic evolution of the Neo-Tethys in the Dinaride sector

The Albanide–Hellenide ophiolites and related ophiolitic mélanges include eight different types of volcanic and subvolcanic rocks: 1) Triassic, within-plate alkaline rocks (WPB); 2) Triassic high-Ti mid-ocean ridge basalts showing enriched compositions (E-MORB); 3) Triassic and Jurassic high-Ti mid-ocean ridge basalts showing normal compositions (N-MORB); 4) Jurassic basalts with geochemical features between MORB and island arc tholeiites; hereafter defined as medium-Ti basalts (MTB); 5) Jurassic low-Ti, island arc tholeiitic (IAT) rocks; 6) Jurassic very low-Ti (boninitic) rocks; 7) Jurassic backarc basin basalts and basaltic andesites (BABB); 8) Triassic and Jurassic calc-alkaline rocks (CAB). The geochemical and petrogenetic features of these rock-types, as well as the results from REE modelling of mantle sources, primary melt generation, and mantle residua indicate that they have formed in distinct tectonic settings within an oceanic environment. Both Triassic and Jurassic N-MORBs primary magmas derived from ~ 10 to 20% partial melting of a primitive asthenosphere, whereas Triassic alkaline WPB basalts originated from low degrees of partial melting of an OIB-type mantle source and were most likely erupted in seamounts. Triassic E-MORBs originated from ~ 12% partial melting of a primitive asthenosphere influenced by the OIB-type component. The residual MORB mantle is represented by depleted lherzolites, which are commonly found in the Albanide–Hellenide ophiolites. Mid Jurassic MTB and IAT primary magmas derived from ~ 10% and 10–20% partial melting of the MORB residual mantle, respectively with the variable addition of subduction components and were erupted in an intra-oceanic, supra-subduction zone setting. The residual mantle associated with these magmatic events is represented by harzburgites. Mid Jurassic boninitic primary magmas may have originated either from 10 to 20% partial melting of the MTB and IAT residual mantle or from ~ 30% partial melting of the MORB residual mantle. In both cases, the depleted mantle sources were enriched in light rare earth elements (LREE) by subduction-derived fluids. The extremely depleted harzburgites, which are widespread in the Albanide–Hellenide ophiolites, are interpreted as the residual mantle associated with boninite formation. Mid-Late Jurassic CABs originated from ~ 15 to 20% partial melting of a depleted peridotite mantle significantly enriched in Th and LREE by subduction-derived fluids, whereas BABBs originated from 10 to 20% partial melting of a primitive asthenosphere somewhat enriched in Th and LREE by a nearby subduction. Both these rock-types were erupted in a continental arc-backarc setting. The different rock-types of the Albanide–Hellenide ophiolites record the fundamental stages of the Triassic–Jurassic evolution of the Neo-Tethys in the Dinaride sector: from sea-floor spreading, after continental break-up, to intra-oceanic subduction initiation and supra-subduction zone (SSZ) lithospheric accretion

CHAP. 28. TRIASSIC MAGMATISM AND JURASSIC OPHIOLITES AT THE MARGINS OF THE ADRIATIC MICROPLATE

Triassic magmatic rocks, spatially associated with Jurassic ophiolites, are distributed along the Apennine-Alpine-Dinaride-Hellenide orogenic belts around the Adria continental plate. They can be related to a rifting phase which is precursor of the Jurassic oceanization and show different affinities. Alkaline to transitional anorogenic magmatism charactetises at the western margin of the Adria plate, from Calabria to Tuscany, while rocks of the calc-alkaline/shoshonite orogenic series are widespread at the northern and eastern margins, from the Alps to Dinarides and Hellenides. The apparent discrepancy between the “orogenic” character of the latter and the general “anorogenic” setting of the former can be explained if mantle sources at the northern and eastern Adria margins inherited subduction-related geochemical components from the Hercynian orogenic cycle. The subsequent Jurassic oceanization produced multiple oceanic basins: the Western Tethyan (Ligure-Piemontese) basin at the western margin, and the Serbian, Mirdita-Pindos basins of the Subpelagonian Zone at the eastern Adria margin. The western ophiolites of the Alpine-Apennine orogenic belt display exclusively MORB affinities and structural features which indicate discontinuous oceanic crust generation along a “slow-spreading” system characterized by passive lithospheric extension and tectonic denudation of large sectors of subcontinental mantle peridotites. In contrast, the Subpelagonian Zone ophiolitic complexes are characterized by the juxtaposition of two subparallel belts, of MORB-type to the west, and of suprasubduction-type to the east, implying the development of intra-oceanic subduction processes within a pristine oceanic basin. The hypothesised original location of this basin between the Adria and Pelagonian continental blocks is to be preferred with respect to a possible location in the Vardar-Almopias oceanic basin

New evidence for supra-subduction zone ophiolites in the Vardar zone of Northern Greece: implications for the tectono-magmatic evolution of Vardar Oceanic Basin.

The Vermion Massif (northern Greece) is located across the boundary between the Pelagonian and Vardar Zones and includes several tectonic units bearing ophiolitic rocks, which represent remnants of the oceanic lithosphere formed in the Neotethyan Vardar Ocean, between the Pelagonian and the Serbo-Macedonian continental realms. This massif consists of tectonic units belonging to the Pelagonian Domain, which are tectonically overlain by units associated with the Almopias sub-Zone (Vardar Zone). Ophiolitic rocks consist of mantle harzburgites and ophiolitic mélanges. The ophiolitic mélanges incorporate rocks exhibiting a wide range of composition, including various intrusive rocks and volcanic rocks ranging from basalts, basaltic andesites, andesites, dacites, to rhyolites. Incompatible elements and rare earth elements analyses indicate that a number of different rock-types formed in distinct tectonic settings can be identified. Mantle harzburgites have a very depleted nature and represent portions of the supra-subduction (SSZ) mantle developed in an intra-oceanic arc setting. The mélanges units include six rock types variably distributed in the Pelagonian and Almopias Units. They are: (1) calc-alkaline rocks with marked depletion in Nb, Ta Ti and enrichment in LREE, Th; (2) LREE-depleted N-MORB; (3) LREE-enriched E-MORB; (4) alkaline within-plate basalts showing marked enrichments in Th, Ta, Nb, LREE; (5) low-Ti island arc tholeiites featuring depletion in HFSE; and (6) very-low-Ti boninites characterized by strong depletion in HSFE and REE. Previous interpretations have referred the Vardar ophiolites to a MORB-type oceanic setting and to a MORB-type backarc setting; however, the widespread occurrence of SSZ ophiolites has never been documented in the Vardar Zone before and is particularly important as it testifies for the existence of an intra-oceanic arc basin in the Vardar oceanic domain. The results presented in this paper compared with literature data on other magmatic rocks within the Vardar Zone suggest that the opening and closure of the Vardar Ocean record several distinct accretion events in this basin, that is oceanic crust generation at mid-ocean ridge, alkaline seamounts in the oceanic domain and SSZ setting, as well as two accretion events in the western realm of the Serbo-Macedonian continent, that is volcanic arc and backarc settings. Based on the comparison between the modern west Pacific subduction system and the results from this study a new model for a multistage tectono-magmatic evolution of the Vardar Ocean is proposed