hadi shafaii moghadam

The mantle sequence of the Kuh-Siah ophiolitic suite is located in N Sabzevar, Central Iran structural zone and consists mainly of harzburgite, dunite, lherzolite, pyroxenite, and chromitite lenses.  The Fo% of olivines in peridotites ranges between 91-96%, and plotting them in the Ezubonin-Mariana peridotites. The Al2O3 content of orthopyroxene in harzburgites and lherzolites is relatively low, showing a tendency towards orthopyroxenes of forearc-abyssal peridotites. The clinopyroxene in the pyroxenites also has low Al2O3 and TiO2 content and placing it in the forearc peridotite field. While this mineral in harzburgite and lherzolites have a higher content of Al2O3 and TiO2 and plots in the abyssal peridotites field. The spinel in lherzolites resemble oceanic peridotites, while in harzburgites, it shows a tendency towards forearc and oceanic peridotites. The spinels in dunites exhibit high Cr# and appear to be derived from a boninitic melt in high degrees of partial melting from a depleted mantle source under hydrous conditions. Based on minerals chemistry characteristics (indicating the dual nature of the MORB_IAT_BONINITIC), the evolution of this mantle sequence has been involved in a supra-subduction zone, resulting from the northward subduction of the sub-branch of Neotethys Ocean (Sabzevar suture) beneath the Alborz during the Cretaceous.

Andesitic-dacitic subvolcanic domes of Nudeh Enghelab and Kuh Kamartang are located in the northern domains of the Sabzevar ophiolitic belt, and in the northeast part of the Central Iran structural zone. Geochemically, the studied rocks exhibit a metaluminous, calc-alkaline to high k-calc-alkaline nature, and are enriched in LILE and LREE and depleted in HFSE, HREE and negative anomaly in TNT elements, and have formed in an environment related to subduction zone. With attention to their other geochemical characteristics, such as a silica content (SiO2>61wt%), Al2O3>15wt%, MgO<2.2wt%, Na2O>3.3wt%, Sr/Y>24, La/Yb>8, can be classified these rocks as high silica adakites. The petrographical, geochemical and isotopic ((87Sr/86Sr)i=0.7047-0.7045, ƐNdi=6.02-6.10) characteristics display that the studied high silica adakites have been originated from partial melting of subducted oceanic slab of Neo-Tethys (Sabzevar sea/ocean sub-branch) under the Turan plate in amphibolite to garnet amphibolite facies and during the ascent to high levels, they show very little assimilation and contamination with continental crust.

The studied area is located in the southeast of Shahrood, north Torud, and northeast of the Central Iran structural zone. The area is one of the Iranian basement terrains which include a wide variety of igneous and metamorphic rocks. The study gabbroic bodies form a small part of this complex and their essential minerals are Plagioclase (labradorite, bytownite), pyroxene (calcic group and augite and clinoenstatite composition), olivine, biotite, and amphibole (calcic group and pargasite and pargasite hornblende composition). Based on the chemical composition of minerals, the magma forming of these rocks has calc-alkaline nature and during their crystallization, the fugacity of oxygen has been high. Geothermometry of pyroxene and amphibole suggest crystallization equilibrium temperatures at gabbroic rocks, for pyro,xene are  900-1200°C and for amphiboles are 820-990°C‑ respectively. Geobarometry calculation of these rocks, using an amount of total amphibole aluminum (Alt is between 2-2.5), estimated between 6.52 to 8.9 kbar and for pyroxene between 5 to 7 Kbar. These conditions are equivalent to the formation and final equilibrium at depth of 23 to 27 km of the lower crust.

Neybaz igneous and metamorphic complex (west of Saghand, North Yazd) is one of the Late Neoproterozoic Iranian basement terrains which include a wide variety of igneous and metamorphic rocks. Some parts of metapelitic outcrops of this complex can be considered typical evidence of partial melting, migmatitization and mylonitization. The obtained ages of the complex and the adjacent complexes (such as Chapedony, Tashk and Zaman Abad), based on K-Ar, Rb – Sr, Sm-Nd and U-Pb methods, show several ages  ranging from Late Neoproterozoic to Miocene. But the presence  granitic and gneissic pebbles bearing conglomerate which originated from basement of the Lower Cretaceous sedimentary sequence of the southwest of Saghand indicate that age younger than Cretaceous which attributed to Neybaz complex are not correct. Some of the reported ages are only reflected the tectonomagmatic or tectonometamorphism events which affected these crystalline basement  complexes during different times, such as Middle Jurasic metamorphism and Eocene to Miocene magmatism activities. Based on the regional evidence and field relationships, the range of 570 to 530 Ma is more consistency with actual regional geology. The events happened during this time, most likely related to Cadomian orogenic evolutions. Hafni isotopic investigations on granitic rocks of this complex indicate that the eHf of the studied rocks show negative values and varies from -1.02 to -18.87. The negative eHf values correspound with originating of granitic melt from crustal rocks or the same gneissic rocks of Neybaz complex (west of Saghand).

The southwest Salmas metamorphosed mafic-intermediate intrusive rocks are located in Sanandaj-Sirjan zone in NW of Iran. The metamorphosed intrusives rocks comprise metagabbro and metadiorite that intruded the Precambrian metamorphosed volcanic-sedimentary rocks. U-Pb zircon dating of two samples of metagabbro rocks yielded age of Ca. 548/7±2/5 Ma to 553/1±7/0 Ma and one sample of metadiorite rock yielded age of 542±6/5 Ma (late Neoproterozoic-Cambrian). Based on the obtained geochemical results, these rocks were crystallized from subduction-related melts.The ɛHf(t) values of these rocks are variable from  to 힏. Hf model ages of mafic-intermediate rocks from the SW Salmas show values of 593-1000 Ma (Neoproterozoic). U-Pb zircon dating and Hf isotope results indicate that late Neoproterozoic-Cambrian metamorphosed mafic-intermediate intrusive rocks in the northwest of Iran were related to widespread magmatism (Cadomian magmatism) along the northern margin of Gondwana.

The Robat ophiolite complex is located along the Nain- Baft fault and at the western margin of the Lut block. The volcanic rocks in the Robat ophiolites consisting of pillow lavas, lava flows and massive units that are classified as andesi-basalts, andesites, trachy-andesites, dacites and rhyolites. Geochemical results show that these rocks have tholeitic and calc -alkaline affinities and reside in supra-subduction related arc settings in tectonic discrimination diagrams. Studying the geochemical behavior of elements by using various petrogenetic diagrams suggest the occurrence of two different mantle sources for Robat volcanic rocks: 1) a mantle source which was affected slightly to moderately by slab fluids and/or melts and is responsible for arc island tholeitic series, 2) a mantle source which is highly effected by melts derived from melting of sediments associated with subducting slab, and is responsible for calc -alkaline series. It seems that the emplacement of the volcanic rocks in the Robat ophiolitic sequence was probably the result of processes related to the initial subduction process, upwelling and partial melting of subducted sediment and asthenospheric fluids in the source, along with differentiation and melting of portions of the oceanic crust.

Basalts, andesites, trachyandesites and latites with shoshonitic affinity are the main volcanic rocks of Eocene age in the Lahrud region (Ardabil). Plagioclase, K-feldspar, biotite and amphibole associated with clinopyroxene are the main constituents of trachyandesites (shoshonites) while clinopyroxene, plagioclase and biotite are the rock-forming minerals of basalts (absarokites). The Lahrud volcanic rocks show enrichment in LREE and are characterized by enrichment in LILE and depletion in HFSE. Petrographical observations along with geochemistry of rare earth and trace elements of these lavas suggest shoshonitic affinity and derivation from a subduction zone. The geochemical behavior of the Lahrud lavas reveals the role of the trench sediments in the source region and genesis of these volcanic rocks. These lavas exhibit low degree of partial melting from a garnet-spinel lherzolite source. The comparison between the Lahrud volcanic rocks with Plio-Quaternary lavas of Sabalan and Eocene lavas of Hashtjin region highlights the different mantle source and degree of partial melting for the genesis of these volcanic rocks. The formation of these lavas is linked to slab steepening and breakoff in a post subduction collision zone.