نشریه پترولوژی
پیاپی 50 (تابستان 1401)

In the study area, in Guilan and Mazandaran provinces and the northern heights of the Alborz mountain range in northern Iran, several gabbroic and ultramafic assemblages of different ages have been reported. In the Western Mazandaran provinces, in the south of Ramsar city (in the north of Iran), several gabbroic plutons with tholeiitic nature are found as a part of the Alborz Magmatic Zone. Alborz Magmatic Zone, which is a region with active deformation, is located in the collision zone of two Eurasian and Arabian plates, where a Neo-Tethyan oceanic lithosphere (Southern Caspian Sea Ocean) has subducted beneath the Central Iran continental lithosphere (Salavati et al., 2013). In this paper, the authors try to discuss the origin and tectonic situation of these gabbroic plutons in the northern part of the western Alborz orogenic belt based mainly on geochemical data.

Regional Geology:

Geologically, the study area is a part of the Ramsar 1:100,000 geological map (Baharfiruzi et al., 2002) is a part of the Gorgan-Rasht zone and the subsidence of the Caspian Sea. It is composed of the Quaternary Caspian deposits and mainly the Jurassic and Cretaceous volcanic rocks. The studied gabbroic plutons in the west of Mazandaran province in the south of Ramsar city, are exposed within the Jurassic shale and sandstone (TR3j2s,sh) around Javaherdeh (Javaherdeh F.).

In this study, 55 rock samples were collected based on the field relations. According to the petrographic studies, 13 samples have been analyzed by ICP-MS and ICP-AES methods in SGS Lab in Canada. Petrography and Whole rocks chemistry Based on microscopic studies, the studied gabbroic bodies consist of plagioclase, clinopyroxene, and olivine with intergranular texture. Iron oxides (titanomagnetite), titanite, and apatite are common accessory minerals. In addition, biotite, epidote, tremolite-actinolite, chlorite, serpentine and opaque minerals are the secondary minerals. Euhedral grains of plagioclases (50 to 70 vol.%) have been altered to chlorite and epidote (Propylitic alteration). The subhedral clinopyroxene crystals (15-30 % vol.) with diopside composition are the main mafic mineral observed in almost all studied rocks. Some clinopyroxenes show poikilitic texture. Chloritization and uralitization are also observed in the clinopyroxene. Olivine is usually serpentinized. Based on the geochemical data, these studied gabbroic bodies have SiO2 of 48.2-52.4 wt%, TiO2 of 1.25- 2.5 wt%, MgO of 4.25-8.51%, and Fe2O3tot of 11.2-14.1 wt%. On the bivariate rock type discrimination diagrams, the studied rocks fall in the subalkaline basalt field. Their chondrite, N-MORB (Normal Mid-Oceanic Ridge Basalt), OIB (Oceanic Island Basalt), and primitive mantle-normalized REE (Rare Earth Elements) and trace elements’ patterns are sub-parallel and show linear and homogeneous profiles with a moderate positive slope from HREE (Heavy Rare Earth Elements) to low field strength elements (LFSE). There are clear similarities between all samples and the typical OIB pattern. On the tectonic discrimination diagrams, all rocks fall in the within-plate basalt field and the plume sources as their tectonic environment. Based on all geochemical data, the studied rocks indicate oceanic ısland tholeiite (OIT) characteristics.

The geochemical data suggest that the studied rocks have chemical characteristics similar to HIMU-OIB and provide an additional argument for their derivation from an asthenospheric mantle source. They are distinct from the lower and upper crust and show OIT (Oceanic Island Alkaline or OIA) gabbroic signature. All geochemical signatures of investigated gabbros imply that crustal contamination did not play a significant role in the magma evolution. Based on field observation, the studied mafic plutons are similar to the alkaline gabbros of the eastern Guilan region; while based on the geochemical and lithological signature, there are a lot of similarity between these two groups. For example, both of them show oceanic island basalt characteristics (OIB). Their difference is that the rocks of eastern Guilan are alkaline type (OIA), while the investigated rocks in this research show oceanic ısland tholeiite (OIT) affinity. Based on the proposed tectonomagmatic model, the studied gabbroic samples are a part of the oceanic plume, that formed in a suprasubduction setting caused by the action of the slab window. Therefore, in the center of the plume, the investigated OIT gabbroic rocks (with the tholeiitic nature), and in the margin, the gabbros and basalts with the alkaline nature of OIB (in the west of the studied area, in the eastern part of Gilan province) were formed.

All data on the petrological and geochemical features of the studied rocks draw the following conclusions Studied gabbroic plutons are outcropped as several small bodies within the Jurassic rocks in the south of Ramsar city, along the Javaherdeh road.Based on petrographic studies, the rock specimens are dark green to black, and are including plagioclase and clinopyroxene±Olivine as the main mineral with intergranular and sub-ophitic textures. Geochemically, southern Ramsar gabbroic bodies show tholeiitic nature. In the chondrite, primitive mantle, and N-MORB normalized diagrams, they indicate enrichment of light rare earth elements and show a trend similar to OIB (oceanic island basalt) trend. In the primitive mantle-normalized pattern, they show Th, Nb, Zr, Rb depletion. Studied samples are formed from 10-20% of partial melting of garnet-spinel lherzolite at 80-100 Km Depth. Based on the proposed tectonomagmatic model, the subduction of the active spreading center of the Neo-Tethys oceanic crust (Southern Caspian Sea Ocean, to the south) produced a slab window in the subducted oceanic lithosphere, allowing infiltration of asthenospheric hot melt as an oceanic plume. The studied gabbros were formed in the center of this oceanic plume.

Magmatic fluids released at the end of magmatic crystallization interact with carbonate country rocks through metasomatism and form the skarns (Meinert et al., 2005). In general, the magmatic rocks are associated with skarnization range from diorite-gabbro to granite in composition (Kuşcu et al., 2019). The ore deposits types (e.g., Cu, Au, Fe, Zn) are controlled by the nature of the magmatic source region (Wu et al., 2017). In this study, we first synthesized the spatial distribution, and geological and geochemical data of Late Mesozoic (Middle Jurassic–Early Cretaceous) granitoids and associated mineral deposits of Songhor plutonic in the northwest of Sanandaj-Sirjan zone and compared with worldwide skarn associated granitoids. Finally we try to reach a better understanding evolution of granitoid and correlations between skarns and the related mineralization of the Songhor plutonic assembleg during Late Mesozoic. 

Redgional geology:

The Galali Fe deposit is typical of skarn deposits in the Sanandaj-Sirjan metallogenic zone (SSZ), in the northeast Songhor. Mineralization in the Galali deposit is related to Songhor intrusive rocks assembleg (Eocene-Oligocene) and orebodies are predominantly hosted by Songhor Formation (Triassic-Jurassic). Petrography and geochemistry of Songhor granitoid complex (SGC) intrusive rocks have been investigated by some workers (i.e. Aliani, 2014).

Major oxides were determined using an X-ray fluorescence spectrometer, whereas the trace and rare-earth elements were analyzed by Inductively Coupled Plasma Atomic Emission Spectrometry (ICP-AES) at Zarazma laboratory company, Tehran Iran.Petrography, mineral chemistry, whole rock chemistry Based on mineralogy and geochemistry, three main facies have been recognized in the Songhor plutonic assemblage: (1) the granite (leucocratic), (2) monzonite to diorite (mesocratics) and (3) gabbro. In the Galali district both calcic exoskarn and endoskarn occur along the contacts between granitoid and host carbonated rocks. The field observations and the whole rock geochemical data obtained from the intrusive rocks in the northeast of Sonqor indicate the relationship between the region's deposits (Iron Skarn) and their tectonic and petrogenic nature.In general, Songhor granitoid complex mineralogically contain Ca-plagioclase and magnesio-hornblende, and it has a metaluminous (ASI < 1) composition, with LREE, LILE enrichment and HREE depletion. The SGC samples can be divided into two groups, based on their SiO2 contents, Na2O/K2O ratios, and incompatible elements compositions. The samples have a relatively wide range of SiO2 content and Mg# values, indicating that these units formed from magmas with variable amounts of fractional crystallization. In the Harker diagrams, there is a negative correlation between SiO2 and FeO, MgO, and CaO. Thus, these rocks record the hornblende and biotite fractionation. Similar to studies of Wu et al. (2017), the strong depletion in Ba, Sr, Eu, and Ti points to an extensive fractional crystallization during the emplacement.

Fractional crystallization would significantly increase the concentrations of W or the other incompatible elements in the magmas, which is necessary for the occurrence of related mineralization (Romer and Kroner, 2016). The results of this research indicate that the Songhor A-type alkali granite formed from the depleted mantle-derived magmas which have undergone assimilation and fractional crystallization processes. The progression from I-type to A-type magmatism appears to mark a significant change from a collisional to an extensional setting in this region, in the Late Jurassic. Geochemical characteristics in Rb versus Sc and Rb/Sr versus Zr diagrams show that mesocratic phase 1 unit, highly-oxidized and less-evolved granitoids generated by a mixture of mantle-derived and mature crust-derived components, related to subduction of Neo-Tethyan oceanic crust beneath the Sanandaj-Sirjan zone. These intrusions are syn-collisional and have a high potential for skarn Fe- Cu-Au deposits.The A-type leucocratic granite shows an alkaline nature, high potassium, high ratio Ga/Al, enrichment of HFSE (i.e. Ta, Nb, Ga, Zr, and Pb), Eu, and compatible elements (i.e. Cr) depletion in the pattern. It has formed in a magmatic arc or post-collisional setting from a hybrid source, with crustal and mantle components, contaminated by interaction with the upper crust. It is a highly-evolved granitoid that can be highly prospective for Mo, Sn, and W mineralization.

One of the main products of caldera collapses is ignimbrite, and there is a direct relationship between the volume of ignimbrite and caldera dimensions. During caldera collapse, a large volume of magma is explosively discharged as products and pyroclastic flows (Druitt and Sparks, 1984). Lava domes and cones are placed along or inside the caldera rim structures after the collapse (Aguirre-Díaz, 1996). The studied area, in the northwest of Takestan, is a part of the Alborz and Western Alborz magmatic arcs, which consisting of some granitoid rocks, volcanic units, dacitic and rhyolitic domes as well as pyroclastic units. The presence of ignimbrite and dacitic and rhyolitic domes along with mafic volcanic rocks and intrusive rocks show the occurrence of the caldera in the region.In the Alborz and Urumieh-Dokhtar belts, extensive volcanic eruptions occurred in the Eocene. The studied area lies in the western Alborz zone. The first studies of stratigraphy and classification of the Paleogene volcanic and intrusive rocks in western Alborz are related to Taleghan and Qazvin regions (Annells et al., 1975). The dominant feature in the northwest of Takestan is pyroclastic deposits, lava flows, and intrusive and sub-volcanic rocks. Volcanic rocks and lava flows are placed on the tuffs with a more limited expansion. Sub-volcanic rocks are in the form of dacitic and rhyolitic domes and intrusive rocks intruded the tuffs.

Method of study:

In this research, about 120 samples of different volcanic, intrusive, and pyroclastic rocks are gathered, then 56 thin sections were prepared and studied. To measure the main and rare elements, 29 suitable samples including 16 fresh samples of intrusive rocks and 13 volcanic samples were selected and analyzed using ICP-OES and ICP-MS at Zarazma Minerals Studies Laboratory in Tehran.

Volcanic rocks are rhyolite, dacite, andesite, trachyandesite, basaltic andesite, and basalt, with porphyry texture and a glassy or microlitic groundmass. Evidences such as the presence of ocelli quartz, plagioclases with sieve texture and the formation of a thin layer of fine pyroxenes at the edge of some pyroxenes show that the basaltic and basaltic andesitic rocks are contaminated with crustal compounds or mixed with crustal melts. The intrusive rocks are alkaline granite, granite, quartz monzonite, quartz monzodiorite, and monzodiorite, which often have a granular texture. The absence of muscovite and hypersolvus feldspars in alkaline granites and quartz monzonites confirms that these granites are anorogenic (Kleemann and Twist, 1989).

Whole rocks chemistry:

Basic to felsic members are seen in both intrusive and volcanic groups. The range of SiO2 changes from 53.52-75.23% for intrusive rocks and 51.11-74.8% for volcanic rocks. MgO is 0.19-4.66% for intrusive rocks and 0.24-4.18% for volcanic rocks, and Al2O3 is 10.22 and 17.2% for intrusive rocks and 10.24 and 16.66% for volcanic rocks. According to Irvine and Baragar (1971) diagram, volcanic samples and most of the intrusive samples are sub-alkaline. In AFM ternary diagram, volcanic and intrusive samples have a calc-alkaline trend.

 Petrogenesis Trace elements variation diagrams show two groups with two separate trends in the volcanic rocks. The first group (rhyolites and some dacites) show the fractional crystallization trend, but the second group (andesite basalts and andesites and some dacites) demonstrate the partial melting trend. This phenomenon can also indicate the different origins of these rocks. Intrusive rocks are also divided into two groups with a parallel trend, which have a dominant trend of fractional crystallization trend, and probably these two groups also have different origins. The spider diagrams show that all the samples are enriched concerning the primary mantle. Some of the anomalies observed in these diagrams indicate that these rocks originated in subduction zone. Crustal contamination or extensive melting of crustal materials occurred in the region. Genetic classification of granitoids and tectonic setting Discrimination diagrams show that the samples of monzodiorite and quartz monzonites are I-type and alkaline granites are A-type (subgroup A2). According to the tectonic setting diagrams, except for some alkaline granites, the other samples are in the field of volcanic arc setting rocks. Alkaline granites and rhyolites are related to the post-orogenic setting, quartz monzonitic samples are related to the late orogenic setting, and granites are located in the range between these two groups. The dacitic samples are located in the syn-collision to late orogeny environments. Acidic volcanic and intrusive samples are the melting results of amphibolites (some dacites and quartz monzonites) and some others are probably melting result of the metagraywakes (rhyolites, some dacites, and alkali granites). Basalt, basaltic andesite, and monzodiorite to monzogabbro, are product of mantle melting in the volcanic arc and rhyolite, dacite, alkali granite, and quartz monzonite, are the result of crustal melting.

Caldera formation:

The presence of ignimbrite units, dacitic and rhyolitic domes with a semi-circular arrangement in the east of the intrusive rocks, and co-ignimbrite breccias, reveal the possibility of caldera formation in the region. Considering the relatively less amount of ignimbrites, the dimensions of the formed caldera were probably small. The presence of dacitic and rhyolitic domes on one side of the intrusive rocks shows that the type of caldera is trap-door type.

The volcanic outputs of the Paleogene are the most conspicuous magmatic products in Iran and the Urmieh-Dokhtar magmatic belt (UDMB) and as the main manifestations of this magmatism (Verdel et al., 2011). They are  part of the southern Eurasian active continental margin formed by the Neotethyan subduction beneath the Central Iranian microcontinent. Geochemically, the magmatism of UDMB is mostly calc-alkaline (especially in the Eocene), although, towards the Oligocene and younger times, alkaline magmatism is also observed (Verdel et al., 2011). The magma origin, partial melting conditions, homogeneity/heterogeneity of mantle sources, evolutionary processes, and tectonic setting of the magma formation are some debatable issues. The purpose of the present study is to investigate field relations, geochemical diversity, and mantle source characteristics of the volcanic units exposed in the south of Qom (Kahak area).

The bulk rock major and trace elements contents were obtained by inductively coupled plasma-optical emission spectrometry (ICP-OES) and inductively coupled plasma-mass spectrometry (ICP-MS), respectively, at Zarazma laboratory in Tehran, Iran. The sample powders were melted using lithium metaborate, dissolved and the final solution were analyzed by ICP-OES. To obtain rare earth and trace element contents, the sample powders were dissolved using multi-acid procedure and then the solution has been analyzed by ICP-MS. The detection limit for rare earth and trace elements is between 0.01 to 1 ppm.

Field Evidence and Petrography:

The Eocene eruptive products in Kahak region are shown by broadly exposed lava flows and pyroclastics alternated with volcaniclastic and carbonaceous sediments. They are somewhere overlain by clastic sediments of the Lower Red Formation (Early Oligocene) allowing us to deduce their relative age. The thickness of lava units varies from <10 to several tens of meters. The basic lavas, the subject of this study, are dark gray to brownish-colored and aphyric to porphyritic. They show various textures of hyalopilitic, hypocrystalline, intersertal to intergranular, and ophitic. Some of the samples contain plagioclase- to clinopyroxene-phyric, in which the size of phenocrysts may reach up to 1 cm. Plagioclase, clinopyroxene, and Fe-Ti oxides are the common phenocrystic to microphenocrystic phases. Plagioclase is the most abundant phase (up to 40 vol.%). Clinopyroxene (5-10 vol.%) is the common ferromagnesian phase in most of the samples occurring as phenocrystic or interstitial phase. Olivine is rarely observed (<5 vol.%), and when present, it is almost altered to secondary products such as chlorite, serpentine, and iddingsite. Fe-Ti oxides are <0.5 to 0.2 mm-sized and form <5% of the mode.

Geochemistry:

The SiO2 content in the study samples ranges from 50.5 to 53 wt.% and in Zr/TiO2 versus Nb/Y diagram, they fall in subalkaline basalt field. The TiO2 amount varies from 0.42 to 1.92 wt.%. Also, the CaO, FeOt, and Na2O+K2O show relatively wide variation from 2.3 to 13.8, 4.9 to 12.7, and 3.2 to 9.3 wt.%, respectively.Mg# [(MgO/MgO+FeOT)*100] ranges from 34 to 52.6. Based on variation diagrams, normalized rare earth elements (REE), and multi-element patterns, the samples can be divided into four distinct groups (Fig. 1A). Group 1 rocks have a lower LREE/HREE ratio than the others and are characterized by (La/Yb)N values of 2.5-4.7. Group 2 rocks are Eu-depleted in which the (La/Yb)N ratio ranges from 3.7 to 4.6. Group 3 rocks display steeper REE patterns with (La/Yb)N ratio of 8.7-9.7. In Group 4 rocks, HREE display lower concentration, and the patterns are relatively steeper [(La/Yb)N= 8.4-10.4]. In the normalized multi-element diagrams, all the samples display relative depletion of high field strength elements (HFSE) such as Nb, Ta, Ti, Zr, Y, and Hf with respect to large ion lithophile elements (LILE) (i.e. Rb, K, Sr, and Ba). Despite the overall similarity of multi-element diagrams, there are some geochemical distinctions between them, for example, Group 1 and Group 4 rocks show Sr enrichment or a more pronounced P negative anomaly than the others.

Variations in the major and trace element contents and the different REE and multi-element patterns are all indicating of geochemical distinction between the studied volcanic rocks (Groups 1 to 4) of the Kahak area. In the major and trace element versus MgO variation diagrams, the scattered plots are also inconsistent with cogenetic relationships among different rock groups. The varied REE values or variously-sloped REE patterns can be attributed to heterogeneous mantle sources or different partial melting conditions. In the studied samples, the Nb/La ratio of 0.20 to 0.58 suggests a lithospheric mantle origin. To model the mantle source and partial melting condition, Sm/Yb versus Sm plot has been used, by which, it is inferred that:All the samples of the Kahak region were derived from a spinel-garnet-lherzolite mantle source with spinel: garnet ratio of 50:50. Geochemical differences between the samples under study are more probably the result of different degrees of partial melting. Accordingly, the primitive magma of Group 3 rocks was derived from the lower degree of partial melting (<10%), while those of the Group 1 rocks resulted from a higher degree of partial melting (10 to 20%). The Groups 2 and 4 rocks fall between these two ranges.Geochemical differences of the Kahak volcanic rocks is most likely the result of partial melting conditions rather than distinct mantle sources. Fig. 1. A) Chondrite-normalized REE patterns of the Kahak volcanic rocks; B) The proposed tectonic model for the Eocene magmatism of UDMB. Relative depletion of HFSE (such as Zr, Ti, Nb, and Ta) in normalized multi-element diagrams is commonly attributed to subduction zone geochemical signature. Also, in the tectonic discrimination diagrams (e.g. Th-Hf/3-Ta diagram), all samples plot in the field of calc-alkaline basalts of arc environment. Therefore, based on our geochemical data, the volcanic rocks of the Kahak region related to arc magmatism. The structural evidences like normal faulting and sedimentary basins with thickened Eocene volcanic-volcanoclastic associations point to an extensional environment. It is probable that the Neotethyan slab-rollback was responsible for the intra-arc (or back-arc) extensional environment in which asthenospheric upwelling led to partial melting of the metasomatized lithospheric mantle and voluminous Eocene magmatism of UDMB (Fig. 1B).

The NW-trending Urumieh-Dokhtar Magmatic Belt (UDMA) in the central-western Iran is a part of the Eastern Tethyan orogenic belt (Deng et al., 2018) and over past decades, it has become one of the most significant polymetallic (Au-Cu-Fe-Pb-Zn) provinces in Iran (Rabayrol et al., 2019; Ismayıl et al., 2021). While the Cenozoic magmatism and metallogeny of the UDMA are well known, its ore-forming potential during this period is still poorly understood (Alipour-Asll, 2019; Tale Fazel et al., 2019; Zamanian et al., 2020). Despite well-known Eocene to Miocene hydrothermal systems associated with extension-related and/or arc magmatism from the UDMA, the temporal and spatial association between continental-arc setting, Middle-Eocene magmatism and epithermal Au mineralization in the Buin-Zahra Range is not well understood. The Atash-Anbar polymetallic deposit (35°44′ N and 49°35′ E) is located ca. 70 km south of Qazvin city, central-northern Iran. A drilling program ~1000 m (including ten drill-holes) identified about 2 Mt of proven reserves grading at 2.13 g/t Au (locally up to 14 g/t) and 4.11% Pb + Zn + Cu (Pirooz, 2015). Herein, we focus on the textural, paragenetic relationships, and mineral chemistry of the Atash-Anbar polymetallic deposit. We focus on: (1) documenting the chemical composition of the different sulfides, (2) determining the chemical state of gold in iron sulfides, and (3) determining the sulfur activity

About 70 rock samples were collected from various parts of the deposit to determine the mineralogy, mineral textures, and mineral chemistry. The chemical composition of ore minerals was analyzed at the Analytical Center for multi-elemental and isotope research of SB RAS in Novosibirsk, Russia using a JEOL JXA-8100 electron microprobe (Japan) with five wavelength dispersive spectrometers and an energy dispersive spectrometer. Operating conditions were: 15-20 kV accelerating voltage, 30 nA beam current, and 20-30 s counting time. Sulfides and sulfosalts were analyzed for Fe, S, As, Cd, Sb, Te, Ag, Au, W, Zn, Pb, Bi, Hg, and Cu. The detection limits for major and minor elements are approximately 0.05 and 0.01 wt.%, respectively. Ten rock powdered samples were also analyzed using the X-ray diffraction (XRD) spectrometry (X′ pert Philips) in order to identify the mineralogy of clay minerals at the Iranian Mineral Processing Research Center (IMPRC), Karaj.

Au-polymetallic veins in the Atash-Anbar deposit with an epigenetic nature and an approximate length of 5 to 80 meters and a maximum depth of 45 meters have been formed in the Middle Eocene andesite, dacite, and rhyodacite host rock (Eord unit). Mineralization with vein and veinlet, crustiform, colloform, breccia, and disseminated textures were occurred in three stages: pre-mineralization (disseminated pyrites), main mineralization (chalcopyrite vein (II-A), quartz-sulfide breccia veins (II-B) and barite-sulfide vein (II-C)), and post mineralization (late carbonate vein (III-A) and supergene (III-B)). Argillic alterations (kaolinite-illite±dickite mineral assemblage) together with silicification alteration (quartz± jasper assemblage) are the main alterations in the area. At Atash-Anbar deposit, gold is present both as native gold (Au0) and invisible solid-solution (Au+) in sphalerite and pyrite compositions. According to Co/Ni ratio (1.2 to 45), pre-mineralized disseminated pyrites have volcanic affinity and zoned pyrites have a hydrothermal origin. As the result of changes in Cd concentration (4657 ppm on average) and Zn/Cd ratio (18.68 on average) in the sphalerites, mineralization considered as high-medium temperature hydrothermal system (200 to 250 °C). Also, based on FeS mol% content of sphalerites (which varies from 0.11 to 0.4), the LogfS2 changes are between -10 and -14, which are consistent with the high to intermediate sulfidation deposit. Due to the paragenetic association of gold with pyrite and sphalerite ores, the absence of oxide minerals, the medium temperature of ore-forming fluid, near-neutral to acidic nature of pH (presence of argillic±sericitic alteration), and the high activity of sulfur show that the gold solubility and its transfer were conducted by Au(HS)2– bisulfide complex. Evidence suggests that the increased logfO2 is caused the decreased S2– activity and it has resulted in instability of bisulfide complex and gold deposition in quartz-sulfide breccia vein.

Granites are the most important components of the continental crusts. As an important part of the Alpine-Himalayan global belt and the result of the Tethys evolutionary cycle, the Urmia-Dokhtar Magmatic Arc (UDMA) has formed during different magmatic periods. The most important magmatic episode of UDMA igneous rocks, which is the result of lithospheric extenssion and extensive magmatism, occurred during 55 to 37 Ma (Moghadam et al., 2015). In order to enhance our understanding of tectonomagmic evolution of the continental crust during this period, in this research, the intrusive masses of Aftabru and Qlichkandi will be investigated using geochemical data and the isotopic composition of neodymium and strontium. The mentioned intrusive masses are located in the southwest region of Buin Zahra in Central Iran zone.

Urmia-Dokhtar magmatic arc with Cenozoic intrusive and Eocene-Quaternary extrusive rocks shows different levels and rock outcrops in terms of time of origin and erosion rate, same what is seen in the subduction arc of the Andean continental margin. The lithospheric stresses caused by the interaction of the African-Eurasian-Indian lithosphere led to the emergence of Paleogene extensive magmatic activity and a magmatic flare-up lasting 30 Myrs during Eocene and Oligocene. As a result, more than 4 km of Paleogene igneous rocks formed in Saveh, Zarandiyeh, and Tafresh regions. In the south of Bouin Zahra region, pyroclastic outcrops and Eocene lava with a width of about 5km2 and 10 km2 are found in Aftabru and Qlichkandi areas, respectively.

After field observations and detailed textural and petrographic studies, 12 suitable samples with minimal weathering and alteration were selected from intrusive rocks and analyzed by XRF and ICP-MS methods for major, trace and rare earth elements. 6 whole rock samples were analyzed for Sr-Nd isotopes.

Petrography:

In Aftabru and Qlichkandi areas, quartz monzonites intruded the lower-middle Eocene volcanic and pyroclastic rocks.

Petrological observations show that the Aftabru intrusion contains 7-16 Vol% quartz, 25-30 Vol% K-feldspar, 39-54 Vol% plagioclase, 5-10 Vol% pyroxene, 8-15 Vol% amphibole, as well as 1 Vol% accessory minerals.

Qlichkandi:

The medium-grained Qlichkandi intrusive rocks with granular texture, composed of quartz 9-15 Vol% quartz, 25-28 Vol% K-feldspar, 35-45 Vol% plagioclase, 1-5 Vol% pyroxene, 5-10 Vol% of the common mafic mineral of amphibole, 5 Vol% biotite, and less than 1 Vol% accessory minerals.

Based on new geochemical and isotopic data, we will investigate the tectonic location, genesis and magmatic processes affecting the parental magma and the possible source rock of the intrusive masses in the south of Bouin Zahra region.Tectonic-magmatic zone:As  the pattern of rare earth elements and the spider diagrams of enrichment in LILE and LREE elements and depletion of HFSE and HREE elements display, the most important characteristic of intrusive rocks in the studied area is their similarity to continental margin arc rocks.   Generation and magmatic processes:Some incompatible trace elements ratios, such as Y/Nb, Nb/Ta and Nb/La that are less affected by diffrentiation are good indicators for investigation of the magma origin and the crustal contamination effect on the magma. The higher amounts of Y belong to crustal melts or impregnation with crustal materials, and the higher amounts of Nb belong to melts derived from the mantle. In the studied intrusive rocks, Y/Nb ratio is 1.6 on average with a range of 0.6-3.6, which probably indicates mantle with crustal mixing in the magama origin. The Nb/La value is 0.1 in primary mantle and 0.46 in the crustal rocks (Morata et al., 2005), it is about 0.86 on average and equals to the range of 0.1-0.52 in the intrusive rocks of the southern region of Buin Zahra (Qlichkandi, Aftabru). This value indicates a mantle origin for the studied rocks. The Nb/Ta value in mantle rocks is 17.5 and in crustal rocks it is equal to 11-12 (Green, 1995). This ratio is 15 on average (ranging from 8 to 8. 26), supporting the mantle origin as well.Neodymium model age (460-550 Ma) and positive ԑNd(t) indicate the Cadomian origin of lithospheric rocks of the Aftbaru region, while the model age of the samples from Qlichkandi region is 0.9. It shows ԑNd(t) less than zero. This difference is probably due to the high magma mixing with crustal materials in Qlichkandi region, which is confirmed by the diagram of ԑNd versus Sr isotope. 143Nd/144Nd ratio for the Aftabru samples is 0.51270-0.51280. But in the Qlichkandi samples, it is 0.51252-0.51242, which is a sign of contamination with the lower continental crust materials and a tendency towards lower crust. 87Sr/86Sr ratio is 0.70472-0.70510 in Aftabru and 0.70631-0.70607 in Qlichkandi samples. Therefore, according to the intrusive rock petrologic diagrams, it shows signs of contamination with the underlying crustal materials.