Zagros Ophiolites Research Articles

Supra-subduction zone magmatism of the Neyriz ophiolite, Iran: constraints from geochemistry and Sr-Nd-Pb isotopes

The Neyriz ophiolite along the northeast flank of the Zagros fold-thrust belt in southern Iran is an excellent example of a Late Cretaceous supra-subduction zone (SSZ)-related ophiolite on the north side of the Neotethys. The ophiolite comprises a mantle sequence including lherzolite, harzburgite, diabasic dikes, and cumulate to mylonitic gabbro lenses, and a crustal sequence comprising a sheeted dike complex and pillow lavas associated with pelagic limestone and radiolarite. Mantle harzburgites contain less CaO and Al2O3, are depleted in rare earth elements, and contain spinels that are more Cr-rich than lherzolites. Mineral compositions of peridotites are similar to those of both abyssal and SSZ- peridotites. Neyriz gabbroic rocks show boninitic (SSZ-related) affinities, while crustal rocks are similar to early arc tholeiites. Mineral compositions of gabbroic rocks resemble those of SSZ-related cumulates such as high forsterite olivine, anorthite-rich plagioclase, and high-Mg# clinopyroxene. Initial εNd(t) values range from +7.9 to +9.3 for the Neyriz magmatic rocks. Samples with radiogenic Nd overlap with least radiogenic mid-ocean ridge basalts and with Semail and other Late Cretaceous Tethyan ophiolitic rocks. Initial 87Sr/86Sr ranges from 0.7033 to 0.7044, suggesting modification due to seafloor alteration. Most Neyriz magmatic rocks are characterized by less radiogenic 207Pb/204Pb (near the northern hemisphere reference line), suggesting less involvement of sediments in their mantle source. Our results for Neyriz ophiolite and the similarity to other Iranian Zagros ophiolites support a subduction initiation setting for its generation.

Geochemistry and tectonic evolution of the Late Cretaceous Gogher–Baft ophiolite, central Iran

The Late Cretaceous Gogher–Baft ophiolite is one of the best preserved remnants of Neo-Tethyan oceanic lithospheric within the inner Zagros ophiolite belt. The ophiolite comprises from bottom to top, harzburgites, pegmatite and isotropic gabbroic lenses within the mantle sequence, pillowed to massive basalts to dacites and pyroclastic rocks associated with blocks of pelagic limestone and radiolarite. Basaltic to dacitic sills crosscut the pyroclastic rocks. The ophiolite sequence is overlain by Turonian–Maastrichtian pelagic limestones (93.5–65.5Ma). Mineral compositions of harzburgites are similar to those of fore-arc peridotites and overlap with abyssal peridotites. Most Gogher–Baft ophiolite magmatic rocks show supra-subduction zone affinities, except for some E-MORB type lavas. The geochemical characteristics suggest that Gogher–Baft ophiolite magmatic rocks were generated during subduction initiation. These show progressive source depletion leading to the formation of MORB to boninitic magmas. Early E-MORB-type pillow lavas may have originated by melting mantle that was not affected by subduction components as the Tethyan oceanic plate began to sink beneath Eurasia as subduction began in the Late Cretaceous. Initial εNd (t) values range from +2.6 to +9 for Gogher–Baft magmatic rocks. Samples with radiogenic Nd overlap with least radiogenic MORBs and with Oman and other Late Cretaceous Tethyan ophiolitic rocks. The initial 87Sr/86Sr ratios range from 0.7048 to 0.7057, indicating modification due to seafloor alteration. Radiogenic 207Pb/204Pb isotopic compositions (systematically above the NHRL) and less radiogenic Nd isotopic compositions suggest the involvement of sediments in the mantle source in some magmatic rocks. Our results for Gogher–Baft ophiolite and the similarity of these to other Iranian Zagros ophiolites suggest a subduction initiation setting for the generation of these magmatic rocks.

U–Pb zircon ages of Late Cretaceous Nain–Dehshir ophiolites, central Iran

Late Cretaceous Zagros ophiolites are part of the c . 3000 km long Late Cretaceous Ophiolite Belt of SW Asia including the Troodos (Cyprus), eastern Mediterranean (Turkey, Syria), Zagros (Iran) and Semail ophiolites (Oman). This ophiolite belt represents a magmatic forearc that formed when subduction of the Neotethys began along the SW margin of Eurasia. Geochronological data for Zagros ophiolites are limited to a few K–Ar and 40 Ar– 39 Ar ages. New thermal ionization mass spectrometry U–Pb zircon ages indicate that the Nain and Dehshir ophiolites of central Iran formed c . 101–103 Ma, with Nain (102.9 ± 0.3 Ma) being c . 1 Ma older than Dehshir (100.9 ± 0.2 Ma; 100.4 ± 0.1 Ma), and that these ophiolites were emplaced almost immediately after formation (Nain emplacement 101.2 ± 0.2 Ma; Dehshir emplacement 99.0 ± 1.1 Ma). These formation ages are significantly older than the 98–90 Ma U–Pb zircon ages of other Late Cretaceous ophiolites in this belt such as the Kizildag (Turkey), Semail (Oman) and Troodos ophiolites (Cyprus). If the subduction initiation model applies to this ophiolite belt, it suggests that subduction initiation began near the Zagros margin and propagated at c . 7 cm a −1 to the east (Semail) and c . 15 cm a −1 to the west (Troodos). Supplementary materials: Simplified geological maps of the Nain and Dehshir ophiolites, showing sample locations, are available at www.geolsoc.org.uk/SUP18554 .

Geochemistry and petrogenesis of the Late Cretaceous Haji‐Abad ophiolite (Outer Zagros Ophiolite Belt, Iran): implications for geodynamics of the Bitlis–Zagros suture zone

The Haji‐Abad ophiolite in SW Iran (Outer Zagros Ophiolite Belt) is a remnant of the Late Cretaceous supra‐subduction zone ophiolites along the Bitlis–Zagros suture zone of southern Tethys. These ophiolites are coeval in age with the Late Cretaceous peri‐Arabian ophiolite belt including the Troodos (Cyprus), Kizildag (Turkey), Baer‐Bassit (Syria) and Semail (Oman) in the eastern Mediterranean region, as well as other Late Cretaceous Zagros ophiolites. Mantle tectonites constitute the main lithology of the Haji‐Abad ophiolite and are mostly lherzolites, depleted harzburgite with widespread residual and foliated/discordant dunite lenses. Podiform chromitites are common and are typically enveloped by thin dunitic haloes. Harzburgitic spinels are geochemically characterized by low and/or high Cr number, showing tendency to plot both in depleted abyssal and fore‐arc peridotites fields. Lherzolites are less refractory with slightly higher bulk REE contents and characterized by 7–12% partial melting of a spinel lherzolitic source whereas depleted harzburgites have very low abundances of REE and represented by more than 17% partial melting. The Haji‐Abad ophiolite crustal sequences are characterized by ultramafic cumulates and volcanic rocks. The volcanic rocks comprise pillow lavas and massive lava flows with basaltic to more‐evolved dacitic composition. The geochemistry and petrology of the Haji‐Abad volcanic rocks show a magmatic progression from early‐erupted E‐MORB‐type pillow lavas to late‐stages boninitic lavas. The E‐MORB‐type lavas have LREE‐enriched patterns without (or with slight) depletion in Nb–Ta. Boninitic lavas are highly depleted in bulk REEs and are represented by strong LREE‐depleted patterns and Nb–Ta negative anomalies. Tonalitic and plagiogranitic intrusions of small size, with calc‐alkaline signature, are common in the ophiolite complex. The Late Cretaceous Tethyan ophiolites like those at the Troodos, eastern Mediterranean, Oman and Zagros show similar ages and geochemical signatures, suggesting widespread supra‐subduction zone magmatism in all Neotethyan ophiolites during the Late Cretaceous. The geochemical patterns of the Haji‐Abad ophiolites as well as those of other Late Cretaceous Tethyan ophiolites, reflect a fore‐arc tectonic setting for the generation of the magmatic rocks in the southern branch of Neotethys during the Late Cretaceous. Copyright © 2012 John Wiley & Sons, Ltd.

Mineralogy, geochemistry and geotectonic significance of mantle peridotites with high-Cr chromitites in the Neyriz ophiolite from the outer Zagros ophiolite belts, Iran

The Neyriz ophiolite containing chromitite pods from the outer Zagros ophiolite belt was studied in the Abadeh Tashk area. It appears as four detached massifs in an area with 125km2 in south of Iran and is comprised predominantly of peridotites. Harzburgites and dunites are the most ultramafics in the massifs with rare mafic and gabbroic rocks. Mineralogical composition of chromian spinel in chromitite and host peridotite of the Neyriz ophiolite in comparison with those of the Nain ophiolite, to tracing their geographical variations between outer and inner sectors of the Zagros ophiolite belt, shows that there are remarkable and striking compositional variations between these two ophiolites. Chromian spinels of the Neyriz mantle peridotite and chromitite pods are characterized by higher contents of Cr# (56–79; average, 71) in harzburgites and dunites, and higher Cr# (73–82), Mg# (62–71) and lower Al (9.1–13.9wt% Al2O3) and Ti (up to 0.08wt% TiO2) in chromitite pods with respect to available data on samples from the Nain ophiolite (Cr# 40–61 of spinels in mantle harzburgites; Cr#: 59–73, Mg#: 60–70, Al2O3: 13.6–22.37wt%, TiO2: 0.13–0.40wt% of spinels in chromitites). Based on geochemical affinities, we contend that the Neyriz mantle peridotites were largely affected by percolating hydrous boninitic melts, to produce high-Cr chromitites, whereas chromitites of the Nain mantle peridotites carry geochemical imprints of boninitic melts with MORB-like affinity, suggesting their distinct geotectonic setting. The presence of the high concentrations of wolfram (W: 275–1276ppm) in the Neyriz mantle peridotites in comparison with those of the Nain mantle peridotites (W<3ppm), which is the one of the most striking geochemical features of these rocks, provides an additional evidence for the significant role of recycling of continental material such as subducted pelagic sediments into the sources of subduction zone magmas. Combining with the lithological variations of the Central Iran ophiolites from NE (harzburgite dominate Nain ophiolite with limited and small-size chromitite pods) to SW (harzburgite–dunite dominate Neyriz ophiolite with large potential of high-Cr chromitites), discernible geochemical tendencies of chromitites reveal that the Neyriz chromitites from the Zagros southern sectors (outer Zagros) are considered to be the product of typical of fore-arc-related boninitic melts, whereas the Nain chromitites of the inner Zagros ophiolite belt are associated with the back-arc basin-related boninitic parentage with affinities to MORB-like melts.

Hf–Nd isotope constraints on the origin of Dehshir Ophiolite, Central Iran

AbstractThe peri‐Arabian ophiolite belt, from Cyprus in the west, eastward through Northwest Syria, Southeast Turkey, Northeast Iraq, Southwest Iran, and into Oman, marks a 3000 km‐long convergent margin that formed during a Late Cretaceous (ca 100 Ma) episode of subduction initiation on the north side of Neotethys. The Zagros ophiolites of Iran are part of this belt and are divided into Outer (OB) and Inner (IB) Ophiolitic Belts. We here report the first Nd–Hf isotopic study of this ophiolite belt, focusing on the Dehshir ophiolite (a part of IB). Our results confirm the Indian mid‐oceanic ridge basalt (MORB) mantle domain origin for the Dehshir mafic and felsic igneous rocks. All lavas have similar Hf isotopic compositions, but felsic dikes have significantly less‐radiogenic Nd isotopic compositions compared to mafic lavas. Elevated Th/Nb and Th/Yb in felsic samples accompany nonradiogenic Nd, suggesting the involvement of sediments or continental crust.

Geodynamic evolution of Upper Cretaceous Zagros ophiolites: formation of oceanic lithosphere above a nascent subduction zone

AbstractThe Zagros fold-and-thrust belt of SW Iran is a young continental convergence zone, extending NW–SE from eastern Turkey through northern Iraq and the length of Iran to the Strait of Hormuz and into northern Oman. This belt reflects the shortening and off-scraping of thick sediments from the northern margin of the Arabian platform, essentially behaving as the accretionary prism for the Iranian convergent margin. Distribution of Upper Cretaceous ophiolites in the Zagros orogenic belt defines the northern limit of the evolving suture between Arabia and Eurasia and comprises two parallel belts: (1) Outer Zagros Ophiolitic Belt (OB) and (2) Inner Zagros Ophiolitic Belt (IB). These belts contain complete (if disrupted) ophiolites with well-preserved mantle and crustal sequences. Mantle sequences include tectonized harzburgite and rare ultramafic–mafic cumulates as well as isotropic gabbro lenses and isolated dykes within the harzburgite. Crustal sequences include rare gabbros (mostly in IB ophiolites), sheeted dyke complexes, pillowed lavas and felsic rocks. All Zagros ophiolites are overlain by Upper Cretaceous pelagic limestone. Limited radiometric dating indicates that the OB and IB formed at the same time during Late Cretaceous time. IB and OB components show strong suprasubduction zone affinities, from mantle harzburgite to lavas. This is shown by low whole-rock Al2O3and CaO contents and spinel and orthopyroxene compositions of mantle peridotites as well as by the abundance of felsic rocks and the trace element characteristics of the lavas. Similarly ages, suprasubduction zone affinities and fore-arc setting suggest that the IB and OB once defined a single tract of fore-arc lithosphere that was disrupted by exhumation of subducted Sanandaj–Sirjan Zone metamorphic rocks. Our data for the OB and IB along with better-studied ophiolites in Cyprus, Turkey and Oman compel the conclusion that a broad and continuous tract of fore-arc lithosphere was created during Late Cretaceous time as the magmatic expression of a newly formed subduction zone developed along the SW margin of Eurasia.

The Dehshir ophiolite (central Iran): Geochemical constraints on the origin and evolution of the Inner Zagros ophiolite belt

The Late Cretaceous Dehshir ophiolite is an important element within the Inner Zagros (Nain-Baft) ophiolite belt and contains all components of a complete “Penrose ophiolite,” including tectonized harzburgites, gabbros, sheeted dike complexes, pillowed basalts, and rare ultramafic-mafic cumulates. The cumulate rocks of this ophiolite are composed of plagioclase lherzolite, clinopyroxenite, leucogabbro, and pegmatite gabbro. All the massifs in the Inner Zagros ophiolite belt are overlain by Turonian-Maastrichtian pelagic limestones (93.5–65.5 Ma). Clinopyroxene compositions of Dehshir mafic rocks are similar to those of both boninites and island-arc tholeiites. Nearly all spinels from the inner ophiolite belt are similar to those of highly depleted harzburgites from intra-oceanic forearcs, although some Dehshir harzburgite spinels plot within the field for abyssal (mid-ocean-ridge basalt) peridotites. All components of the Dehshir and other ophiolites of this belt show strong suprasubduction-zone affinities, from harzburgitic mantle to ophiolitic lavas. Volcanic rocks have a mixture of dominantly arc-like (island-arc tholeiite, boninite, and calc-alkaline) and subordinate mid-ocean-ridge basalt–like compositional features, usually with mid-ocean-ridge basalt–like rocks at the base and arc-like rocks at the top. Our data for the Dehshir ophiolite and the similarity of these results to those for Iranian inner and outer belt ophiolites compel the conclusion that a geographically long, broad, and continuous tract of forearc lithosphere was created at about the same time during the earliest stages of subduction along the southern margin of Eurasia in Late Cretaceous time.

Review of the tectonic setting of Cretaceous to Quaternary volcanism in northwestern Iran

There are three parallel magmatic arcs in the northwest of Iran, of Cretaceous and Eocene–Miocene to Quaternary ages, trending in a NW–SE direction between the Main Zagros Thrust (MZT) in the southwest and the Tabriz Fault in the northeast. In this study, these volcanic belts are referred to as the Sanandaj Cretaceous volcanic (SCV), Sonqor–Baneh volcanic (SBV), and Hamedan–Tabriz volcanic (HTV) belts, respectively. The SCV belt consists mainly of mafic to intermediate submarine rocks with calc-alkaline affinity, and the SBV belt is composed of basalt, gabbro to dioritic bodies, with extrusive to subvolcanic magmatic textures and tholeiitic to alkaline affinity. These extend along the MZT between the Zagros ophiolite in the west and the SCV belt in the east. The HTV belt is part of the Urmieh–Dokhtar Magmatic Arc belt that extends across the Hamedan to Tabriz, and was active in the Miocene to Quaternary. The petrology and geochemistry of the northwestern Iranian volcanic zones indicate that they were generated at an active continental margin. In addition to the volcanic belts, there is a dismembered ophiolite along the MZT from Kermanshah to Turkey, in a NW–SE direction. These ophiolites are remnants of Neo-Tethyan oceanic crust which was obducted over the Arabian passive margin in the late Cretaceous. In this study, we propose that a collision between the Arabian and Iranian plates may have occurred in the middle to late Miocene, and that the Neo-Tethyan oceanic subduction beneath northwestern Iran ceased for a while. As a result, a gap in volcanic activity occurred between the Cretaceous and the Middle Miocene–Quaternary volcanism events. This gap in activity is not observed in southwestern Iran.

Early development of Tethys and Jurassic ophiolite displacement

Previous tectonic reconstructions of the Iranian segment1–4 of eastern Tethys have assumed that the stratigraphic positions of ophiolites from Zagros (late Cretaceous) and Makran (early Tertiary) represent the time of initial oceanic plate displacement. We present 40Ar–39Ar age spectra for mineral separates from metamorphic assemblages associated with the ophiolites of the eastern Zagros and the western Makran, Iran. One of the Zagros samples is from an amphibolite associated with metamorphosed basic and ultrabasic rocks while the other is from metamorphic olistolith within radiolarites, south of the main Zagros ophiolite body. The Makran sample is from a block of amphibolite contained within metamorphosed harzburgites. The apparent ages suggest that initial oceanic plate disruption and tectonic emplacement of the Zagros and Makran ophiolites occurred in the middle Jurassic (∼170 Myr ago) with further displacements in the late Cretaceous (∼97 Myr and ∼89 Myr ago). The similarity of the Jurassic age with ophiolite emplacement ages in Yugoslavia5 and Greece6 is possible evidence of a middle Jurassic coUision of an early Mesozoic Tethyan ridge system with both eastern Europe and central Asia.