back-arc

The Sanandaj-Sirjan Zone formed a piece of the northeastern section of the Gondwana continent during the Paleozoic era and separated it from the Asian plate by the Paleo-Tethys Ocean (Golonka., 2004). During the Middle and Late Triassic, coinciding the closure of the Paleo-Tethys, a rift called the Neo-Tethys Sea developed along the Zagros region, separating the Sanandaj-Sirjan zone from the Arabian plate (part of Gondwana). Crustal subduction is thought to have initiated in the Late Triassic to Early Jurassic (Berberian, 1981). This subduction led to the deformation of rocks and emplacement of intrusive masses during the Late Triassic in the southern part of the Sanandaj-Sirjan zone, and the emplacement of various masses from gabbroic to granitic rocks during the earlier times in the northern part of the zone from the Middle Jurassic (Shahbazi et al., 2010; Mahmoudi et al., 2011) to the Middle Cretaceous (Ghalamghash et al., 2009) and Early Eocene (Ahadnejad et al., 2010). The closure of the Neo-Tethys Sea and the subduction of the oceanic crust are often been associated with the emplacement of ophiolites along the Zagros Zone during the Late Cretaceous to Paleocene (Agard et al., 2005), in addition to some granitoid bodies of the Upper Eocene to Oligocene. According to (Berberian 1981 and Azizi and Moinevaziri, 2009), these bodies and their ages indicate the continuation of the oceanic crustal termination and the collision between the Arabian plate and Central Iran during the Neogene.Several valuable studies have been conducted on the magmatism of intrusive masses within the Sanandaj-Sirjan Zone, including the southern Dehgalan (Sarjoughian et al., 2015), northwest of Azna (Shabanian et al., 2009), Cheshme Sefid (Davoudian et al., 2007), Qorveh-Mobarakabad (Azizi and Asahara., 2013), southeastern Saqqez, and Almoghlaq batholith.

Regional Geology:

The study area is located in the northwestern part of the Sanandaj-Sirjan Zone, southeast of the city of Sanandaj in Kermanshah Province. It lies within the geographical longitudes of '35°47 to '30°47 east and latitudes of '35°34 to '38°34 north, within the southwestern portion of the 1:100,000 Sanandaj geological sheet. The upper Cretaceous phyllitic units are intruded by syenitic-gabbroic intrusive masses and have undergone metamorphism and transformation into hornfels due to neighboring intrusive masses and associated metamorphism. The geological map at a scale of 1:25,000 (Mokhtari and Kohestani, 2018) has delineated the rock units of the area (Figure 1).

Following the field studies and mapping with satellite images, field observations were conducted. Microscopic studies were carried out on the collected samples. Following the examination of thin sections of the rocks, 12 samples exhibiting the least evidence of secondary processes, including mineral alteration, cavity filling, and the presence of secondary veins and fractures, were selected. Subsequently, the samples were dispatchedare sent to the Zara-Azma Laboratory for chemical analysis. This entailed theThrough this analysis determination of the abundance of trace and rare earth elements using are determined by applying the inductively coupled plasma mass spectrometry (ICP-MS) and major oxides using the fusion method.Petrography:The study area consists of gabbro, syenite, alkali syenite, and quartz syenite. The dominant textures are hypidiomorphic granular, poikilitic, perthitic, and granophyric in the syenites, as well as sieve-textured fabric. All rock types trending northeast-southwest.Gabbroic Intrusions:Gabbroic intrusions are present in the southern part of the Baetar intrusive complex and at the margin of the syenitic intrusion. In the northern margin of the syenitic intrusion, two small outcrops of gabbroic intrusion are observed, where one is in contact with the syenitic intrusion, and the other is within the units of hornfelsed shale, siltstone, and metamorphos sandstone of Upper Cretaceous era.Syenitic Intrusion: In the central region of the Bakter intrusive complex, the outcrops of syenitic intrusion are widespead. The syenitic intrusion is in close proximity to the gabbroic intrusion, particularly in the southern region.Geochemistry Based on whole rock geochemical data, especially the La/Yb vs. La ratio, these rocks share the same genesis. Geochemical data, including LREE enrichment compared to HREE, positive Pb anomaly, and depletion of Nb and Ti, reveal the divergent regimes in a back-arc zone, through which the primary magmatic melts are generated in the Bakter intrusive complex. The presented geochemical patterns indicate that the parental magma of the studied rock group is derived from an enriched or metasomatized garnet lherzolite with a degree of partial melting within 5% to 15% range..

Field studies, petrography, and geochemical analysis run on the subject region reveal that it is composed of mafic and felsic rocks, with the felsic rocks mainly consisting of syenite, alkaline syenite, and quartz syenite, while the mafic rocks are of a gabbroic composition. Spider diagrams normalized to primitive mantle compositions reveal the enrichment in large-ion lithophile elements (LILE) and light rare earth elements (LREE) compared to high field strength elements (HFSE), with negative anomalies observed in niobium (Nb) and titanium (Ti). The parental magma of the studied plutonic rocks is derived from low degrees of partial melting (5-15%) of a garnet-bearing lherzolite source at depths of 95-110 kilometers. Considering that the intrusive rocks of the Baetar region are associated with post-collisional activities and the presence of a thin and stretched lithosphere is appropriate for their formation, it is assumed that a secondary subduction with a slab break-off mechanism occurred in the Sanandaj-Sirjan Zone for a relatively long period after the initial collision.

Takhte-Soleyman ortho-amphibolites as a part of Takab metamorphic complex are located in Northeast Takab. These rocks consist of amphibolite, Garnet-amphibolite, Kyanite-Garnet-amphibolite, Hornblendite and Epidote-amphibolite. Petrography and whole- rock geochemistry show that basalt, andesite and/or their intrusive equivalents with calc- alkaline to tholeiitic affinity and even a peridotite could be their protoliths. Trace element and Sr-Nd ratios imply that these rocks were from mantle melt sources. In chondrite normalized plots, these amphibolites can be classified into at least two groups. The first group is characterized by LREE depletion relative to HREEs and some with flat patterns. The second has an enrichment of LREEs relative to HREEs. These two different patterns and some other geochemical characters suggest MORB or MORB-like and arc affinities of the parental magmas. This can be related to the time progressive evolution of magmatism either from MORB or Back-arcto Arc or from Arc to Back-arc setting.

On the basis of field studies, mafic-intermediate bodies of Kashmar are subdivided into the old series (often stocks of gabbro, diorite, quartz diorite/monzodiorite) and the new series (quartz monzodiorite swarm dykes). In terms of crosscutting relationships, old series has the age between oldest volcanic units (57 Ma) and Eocene granitoids (40 Ma), but the swarm dykes are attributed to the post Eocene (Oligocene?) in age. Mafic-intermediate bodies with the high-K calc-alkaline to shoshonitic, metaluminous to low peraluminous, LILE/HFSE and LREE enrichment [(La/Yb)N=5.3-6.8] and depletion of HREE characteristics are reminiscent of the arcs in subduction zone. These features accompany with negative anomalies of Eu indicating magma generating at the depth of plagioclase stability and contaminiation of magma to continental crust, which remains during the melting of the garnet rock. Linear trend of the major oxides and trace elements in Harker diagrams indicate in importance of fractional crystallization of magma evolution. Average of initial isotope ratios of 87Sr /86Sr and 143Nd /144Nd (in age 50 Ma) for the old series samples are between 0.7054 /0.7062 and 51262-0.51264 respectively, and the εNdi has a range of 1.08 to 1.42. Average of initial isotope ratios of 87Sr /86Sr and 143Nd /144Nd (in age 30 Ma) for swarm dykes are 0.7056 and 512623 respectively, and the εNdi amount is .059. εNdi positive values and low ISr of all rocks with their TDM (0.6-0.8) implying that they form from partial melting of lithospheric mantle source, which modified to earlier subduction processes melts. Based on the Th/Ta versus Nb/Ta and Nb/Y versus Zr/Y charts, both subduction and rift forming processes were involved in the formation of Kashmar rocks. This feature is compatible with subduction of Sabzevar oceanic crust to the Lut block.