Geochemistry and tectonic significance of the Fannuj-Maskutan SSZ-type ophiolite (Inner Makran, SE Iran) (original) (raw)
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Ofioliti, 2016
One of the largest worldwide accretionary wedges is exposed in the Makran region (SE Iran). The backstop of this accretionary wedge consists of an imbricate stack of continental and oceanic units, referred as North Makran domain. This domain is characterized by a km-thick shear zone, along which the metamorphic Bajgan Complex is thrust onto the Coloured Melange Complex. Along this shear zone two slices of ophiolites have been identified in the Sorkhband area. The upper tectonic slice consists of gabbros, whereas the lower tectonic one consists of mantle peridotites associated with dunites and chromitite ore deposits. Petrography and geochemistry of gabbros clearly indicate an N-MORB-type affinity, suggesting that they were generated at mid-ocean ridge setting. In contrast, mantle peridotites consist of harzburgites and depleted harzburgites, both showing geochemical features suggesting their genesis in a SSZ setting. The new data presented in this paper indicate that the slices of o...
The nature, magmatic evolution, and geodynamic setting of both inner and outer Makran ophiolites, in SE Iran, are enigmatic. Here, we report mineral chemistry, whole-rock geochemistry, and Sr–Nd–Pb isotope composition of mantle peridotites and igneous rocks from the Eastern Makran Ophiolite (EMO) to assess the origin and tectono-magmatic evolution of the Makran oceanic realm. The EMO includes mantle peridotites (both harzburgites and impregnated lherzolites), isotropic gabbros, diabase dikes, and basaltic to andesitic pillow and massive lava flows. The Late Cretaceous pelagic limestones are found as covers of lava flows and/or interlayers between them. All ophiolite components are somehow sheared and fragmented, probably in Cenozoic time, during the emplacement of ophiolite. This event has produced a considerable extent of tectonic melange. Tectonic slices of trachy-basaltic lavas with oceanic island basalt (OIB)-like signature seal the tectonic melange. Our new geochemical data indicate a magmatic evolution from fore-arc basalt (FAB) to island-arc tholeiite (IAT)-like signatures for the Late Cretaceous EMO lavas. EMO extrusive rocks have high εNd(t) (+8 to +8.9) and isotopically are similar to the Oman lavas. This isotopic signature indicates a depleted mid-ocean ridge basalt (MORB) mantle source for the genesis of these rocks, except isotopic gabbros containing lower εNd(t) (+5.1 to +5.7) and thus show higher contribution of subducted slab components in their mantle source. High 207Pb/204Pb and 208Pb/204Pb isotopic ratios for the EMO igneous rocks also suggest considerable involvement of slab-derived components into the mantle source of these rocks. The variable geochemical signatures of the EMO lavas are mostly similar to Zagros and Oman ophiolite magmatic rocks, although the Pb isotopic composition shows similarity to the isotopic characteristic of inner Zagros ophiolite belt. This study postulates that the EMO formed during the early stages of Neo-Tethyan subduction initiation beneath the Lut block in a proto-forearc basin. We suggest subduction initiation caused asthenospheric upwelling and thereafter melting to generate the MORB-like melts. This event left the harzburgitic residues and the MORB-like melts interacted with the surrounding peridotites to generate the impregnated lherzolites, which are quite abundant in the EMO. Therefore, these lherzolites formed due to the refertilization of mantle rocks through porous flows of MORB-like melts. The inception of subduction caused mantle wedge to be enriched slightly by the slab components. Melting of these metasomatized mantle generated isotropic gabbros and basaltic to andesitic lavas with FAB-like signature. At the later stage, higher contribution of the slab derived components into the overlying mantle wedge causes formation of diabase dikes with suprasubduction zone – or IAT-like signatures. Trachy-basalts were probably the result of late-stage magmatism fed by the melts originated from an OIB source asthenospheric mantle due to slab break-off. This occurred after emplacement of EMO and the formation of tectonic melange
journal of sciences islamic republic of iran, 2014
The Kuhshah ultramafic complex is located in the south-east of Sanandaj-Sirjan metamorphic zone, near the probable remnants of Neotethys plate in Iran. It consists of highly depleted harzburgites, dunites, chromitite bands and altered gabbros. The ultramafic parts have been intruded by numerous clinopyroxenite dykes and veins. In the harzburgites, there are different generations of olivine, orthopyroxene and spinel. The chemical composition of the first generation minerals indicates that the harzburgites are depleted in incompatible elements, but the other generations show melt/rock interaction features which can be found in a suprasubduction zone setting. During subduction of the Neotethys plate beneath the Sanandaj-Sirjan zone, a back arc basin developed between the Sanandaj-Sirjan arc and the central Iranian microcontinent. Whole rock and mineral chemistry, specially, cromespinels, show that the harzburgites formed underneath this basin. After depletion, ascending melts with boni...
Tectonomagmatic evolution of the South Dehshir Ophiolite, Central Iran
Geological Magazine, 2015
The South Dehshir Ophiolite, exposed along the southwestern margin of the Central Ira-nian microcontinent, comprises a mantle and a crustal sequence. This massif contains serpentinized peridotites, gabbros and diabase sheeted dykes cutting across wherlite, pegmatitic gabbro and plagio-granite, and is tectonically covered by radiolarites, pelagic limestones with Late Cretaceous microfauna and lavas. Under a microscope, the gabbros exhibit mesocumulate, adcumulate and hetradcumulate textures presumably formed in open magma chambers. Hypabyssal and lava samples show tholeiitic to calc-alkaline affinities, and their compositions cluster in the fields of arc or supra-subduction zone magmatism. Binary diagrams display compositional trends of magmatic differentiation, plotting away from the mid-ocean-ridge basalt (MORB) (tholeiitic) trend and following the arc (calc-alkaline) trend. Rare Earth element (REE) and spider diagrams display various enrichments at different levels. Moderate enrichment of the large-ion lithophile elements (LILE; Rb, Ba, Th, U, La, Sr) relative to the high-field-strength elements (HFSE; Nb, Ta) and slight depletion of Ti and Zr in some patterns may be attributed to variable influences of subduction components over the depleted mantle wedge. Elemental ratios display evidence for the contribution of both fluids and melt released from the subducted slab. Few patterns display ocean-island basalt (OIB) characteristics, possibly attributed to the involvement of local mantle plumes on the partial melting of the mantle wedge. This evidence indicates that the crustal rocks of the South Dehshir Ophiolite formed in a supra-subduction zone.
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 Al 2 O 3 , 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 87 Sr/ 86 Sr ranges from 0.7033 to 0.7044, suggesting modification due to seafloor alteration. Most Neyriz magmatic rocks are characterized by less radiogenic 207 Pb/ 204 Pb (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.