نشریه پترولوژی
پیاپی 51 (پاییز 1401)

Zardkooh is located in Western Alborz zone in the structural divisions of Iran. According to many published researches, most of the magmatic activities in the north and northwest of Iran are related to the Neotethys orogeny process, which are the result of the subduction of the Neotethys crust, the collision of the Arabian plate with Iran, and the tectonic regimes that occurred following the collision. For the purpose of the present study, the nature, the genesis and the tectonic setting of intermediate intrusive rocks were determined using whole rock and mineral chemistry. Incidentally, Microprobe analyses results were used to document the precise chemical composition of the rocks studied and the formation conditions of the minerals as well.

Geology of the area:

The exposed rock units in the studied area mostly include the Eocene (lava and pyroclastic rocks belonging to the Karaj Formation), but locally the Paleocene (the detrital sedimentary rocks of Fajan Formation), the Permian (the detrital sediments of Dorood and the limestones of Ruteh Formations) and the Carboniferous (the limestone of Mobarak Formation) outcrops and the Oligo-Miocene intrusive bodies are also present. Most of the intrusive masses are composed of gabbroic rocks, but some of them also have intermediate compositions, which are the goal of this research. These rocks are mostly seen as dykes and relatively small stocks.

In order to investigate the geochemical properties, five samples were sent to Zarazma company in Iran for whole rock chemical analysis. In the laboratory of Zarazma company, the amounts of oxides of the major elements were determined using the ICP-OES method. The abundance of rare earth elements and refractory elements were measured by ICP-MS method. A monzonite sample with freshest minerals was analyzed by EPMA at the Iran Mineral Processing Research Center in Karaj (I M P R C), in order to understand the chemical compositions of minerals and their application in petrogenesis.

Petrography:

According to petrographic studies, the intermediate intrusive bodies exposed in the study area are mostly microdiorite, monzonite and monzodiorite. The textures of monzonites are porphyroid and their minerals are zoned. The principal minerals are plagioclase (An17-An45) and orthoclase, and the minor minerals are mostly biotite (phlogopite to magnesium-rich biotite), amphibole (ferropargasitic hornblende to pargasitic hornblende), clinopyroxene (augite and salite) as well as opaque minerals set in a groundmass consisting of kaolinized orthoclase, some quartz and opaque minerals as well. Microdiorite has porphyroid texture and the phenocrystals include kaolinized plagioclase, biotite, hornblende and opaque minerals, and the groundmass is composed of altered plagioclase, few quartz and opaque minerals. The textures of monzodiorite are various in different samples, so that in some it is medium granular and in others is intergranular. All the samples are characterized by the presence of the abundant plagioclase (An31) and orthoclase, clinopyroxene, biotite, opaque minerals and a small amount of quartz, apatite and zircon are also occurred in some samples.

Mineral Chemistry:

The chemical compositions of biotites and clinopyroxenes indicate that the magmatic series of intrusive igneous rocks of the studied area is sub-alkaline and calc-alkaline type.Based on the composition of clinopyroxenes, the tectonomagmatic environment of the magma formation of the investigated rocks is volcanic arc and active continental margin type. Based on the microprobe data obtained from the biotites, the studied granitoids are classified as the ilmenite series and were derived from the mantle material or a mixture of mantle –crust.

Whole Rock Geochemistry:

The high potassium calc-alkaline and shoshonitic nature of the rocks under study are displayed by magmatic series determination diagrams, Moreover, the REE similar pattern point to their common origin. The strong negative anomaly of Nb, Ta, and Ti elements in the spider diagrams are of the prominent characteristics of subduction-related continental arc magmas. The positive anomaly of Pb, K and in general the enrichment of LILE elements are also attributed to crustal contamination of magmas. The geochemical data on the tectonomagmatic discrimination diagrams are placed in the realm of volcanic arc environment located on the continental crust. Their parent magma has a compositional similarity with melts derived from an enriched mantle, and according to various diagrams, it was derived from about 10-20% melting of a garnet-spinel peridotite source enriched by mantle metasomatism as a result of the addition of products derived from the subducting slab at depths of 100 to 110 km.

The geochemical data in combination with mineralogical study indicate that the investigated rocks belong to a subduction environment and were originated from an enriched source, but due to the great distance between the Cenozoic magmatic belt of Alborz and Urmia Dokhtar magmatic arc (and the subduction site of the Neotethys oceanic crust) a back-arc basin can be considered for the region, which was in the early stages of evolution but was completely affected by the characteristics of the arc magmas (Teimouri, 2011; Asiabanha and Foden, 2012). Like magmatic arcs, the formation of magma in these environments was also caused by the inflow of fluids resulting from the dewatering of the subducting oceanic slab and the melting of the metasomatized mantle wedge on it. During magma ascent through the continental lithosphere, the magma suffered contamination and some of its chemical properties have been dictated to the magma.

In the Goorid quarry rubble mine 5km west of Sarbisheh city (Southern Khorasan) outcrops of andesitic lavas with columnar structures exist and from the view of geological subdivisions, located in the eastern part of the Lut block. The magmatic activity in the Lut block began in the middle Jurassic (165-162 Ma) and reached its peak in the Tertiary. Volcanic and subvolcanic rocks of the Tertiary age cover over half of the Lut block with up to 2000 m thickness and formed due to subduction before the collision of the Arabian and Asian plates (Camp and Griffis, 1982). In the northern parts of the Lut block (eastern Iran), andesitic volcanic rocks along with dacite and rhyodacite have erupted in the interval of 50 million years from the late Cretaceous to the lower Neogene (Jung et al., 1983). In the northwestern and the western parts of Sarbisheh, the outcrops of Tertiary volcanic rocks (Eocene-Oligocene to Pliocene) with basic, intermediate, and acidic compositions along with pyroclastic deposits are observed that andesitic lavas are widespread displaying columnar structure around the village of the Goorid, shows. In this research, the geometrical characteristics, origin, and tectonic setting of the columnar lavas of the Goorid mine have been investigated.

Regional Geology:

The studied area is located in the southern part of Sarbisheh 1:100000 geological map. The major rock units in the studied area include Tertiary volcanic and pyroclastic rocks. The oldest rock unit of the area under study consists of turbidite deposits consisting of alternating dark green to gray shales and brown sandstone belonging to the Paleocene-Eocene, which are observed in the west and the southwest of Goorid-e-Paein village. Dark-colored units with basaltic andesite and pyroxene andesite compositions cover a large part of the studied area. These rocks are exposed in the form of single and connected hills around the village of Goorid, which shows a unique columnar structure on the western margin of the village.

This research is based on field studies, sampling of rock units, measurement of geometric features of columns, the study of thin sections, and the results of chemical analyses of 11 rock samples. Some 11 samples of volcanic rocks from the Goorid quarry rubble mine were selected and analyzed in Zarazma company, Tehran, Iran by alkaline melting method for major elements (code AF-01) and ICP-MS (for rare and rare earth elements) (code MMS-01).

Petrography and Geochemistry:

Based on petrographic studies, the volcanic rocks of Goorid area have andesitic (pyroxene andesite) composition. Lava-forming minerals including abundant plagioclase, pyroxene, and sometimes opaque minerals. The main texture of these rocks is porphyry with vitreous microlitic groundmass, glomeroporphyry and poikilitic. Zoning, sieve texture, and embayment in plagioclases are signs of disequilibrium conditions during magma crystallization. Investigation of geometric properties of columnar jointing in the Goorid mine shows that columns are a mixture of 4, 5, 6, and 7-side polygons and belong to the irregular group. Based on measurements in three separate blocks of Goorid mine andesitic columns (each block with an area of about one square meter), the average hexagonality index is 0.83. Geochemically, these rocks have an andesite composition with high-K calc-alkaline nature point to subduction zones. Total REE in the Goorid quarry rubble mine columnar lavas show a range from 185.36 to -204.23ppm and a uniform pattern with LREE enrichment relative to HREE similar to calc-alkaline rocks with (La/Yb)N=8.26-9.76, (Ce/Yb)N=6.76-7.85 and weak negative Eu anomalies (average: Eu/Eu*=0.89). Enrichment in LREE relative to HREE with enrichment in LILE (except Ba) and depletion in HFSE (Nb, Ti, P) in studied lavas suggests active continental margin volcanic arc magmatism.

A thorough understanding of the occurrence of volcanic rocks in continental orogenic belts, as well as of their origin and source material, is an important component of studies on continental dynamics. Such rocks provide a window into the mantle and aid our understanding of the formative mechanisms of heat–stress anomalies, crust-mantle interaction, material exchange, and geological processes in the deep earth (Liu et al., 2012). Based on the different tectonic environment discriminant diagrams, Goorid andesitic lavas are located in the active continental margin and mantle-enriched areas. Geochemical characteristics of Goorid volcanic rocks, such as LILE and LREE enrichment and depletion of HFSE (negative anomaly of Nb, Ti, and P) and HREE ((Tb/Yb)N=1.40-1.88), indicating the absence or the presence of a small amount of garnet at the origin. The low Dy/Yb ratio (1.78 to 2.32) in these rocks confirms the spinel lherzolitic source. Using the (La/Yb)N (LREE/HREE) and (Gd/Yb)N (MREE/HREE) ratios it was also found that the samples are plotted in the spinel stability field and adjacent to the spinel lherzolite melting curve with about 90% spinel (Açlan et al., 2020).

The Urumieh-Dokhtar Magmatic assemblage includes various volcanic and intrusive rocks from Eocene to the Pleistocene. The dominant theory in describing the tectonomagmatic environment of the Urumieh-Dokhtar Magmatic assemblage emphasizes the formation of calc-alkaline rocks from Eocene to Miocene as a result of subduction in an active continental margin tectonic environment. Volcanic activities after the Miocene are observed to a smaller extent in some parts of the Urumieh-Dokhtar Magmatic assemblage, including in Bijar, Shahrbabak, and Dehaj (Stern et al., 2021). These activities are characterized by the formation of Pliocene andesite-dacite rocks and Pleistocene andesite-basalt rocks in the Dehaj area. Post-collision magmatism in the Dehaj region is calc-alkaline and the subduction process influences its origin before the Miocene (Pang et al., 2016). In this research, the origin and petrogenetic evolution of Pliocene hyalo-andesitic rocks are studied using field investigation, geochemical data of rare elements, and isotopic ratios (87Sr/86Sr and 143Nd/144Nd) in the Dehaj area. This knowledge can help to understand the tectonic events of the Urumieh-Dukhtar Magmatic assemblage.

Geology of the area:

Hyallo-andesites of Dehaj, which are located in the northwest of Kerman province, SE part of the Urumieh-Dukhtar Magmatic assemblage. Tectonically, this area is very active and is located between Anar and Dehshir faults. The Pliocene volcanic phase is the most important magmatic activity has caused the formation of andesitic to dacite volcanic and semi-volcanic rocks in the region. Pliocene hyalo-andesites of Dehaj have also been exposed in the form of several outcrops in the vicinity of the Aj volcanic peaks. Pliocene hyalo-andesites of Dehaj are also exposed in the form of several outcrops near the Aj volcanic peaks. Hyallo-andesites are composed of a lava unit with a thickness of fewer than 10 meters on the underlying pyroclastic unit.

Forty rock samples of Dehaj hyalo-andesites were collected during the field visit. After petrographic studies, thirty samples were selected for XRF and ICP-MS chemical analysis. The Middlemost (1989) method was performed to separate total iron into FeO and Fe2O3. Five samples were used to calculate isotopic ratios of strontium and neodymium.

Petrography:

Pliocene hyalo-andesites of Dehaj are dark gray to black with a fine grain texture. Petrographically, the rocks under study, are formed from plagioclase microlites and very fine brown hornblende crystals in a cryptocrystalline and glassy matrix, and quartz crystal is not visible. The hyallo-andesitic rocks of Dehaj have disequilibrium textures, including sieve texture and oscillatory zoning in plagioclase microphenocrysts.

Geochemistry:

Pliocene hyallo-andesitic rocks of Dehaj are classified in the dacite range and the andesite-dacite boundary based on the TAS classification (Le Bas et al. 1986). Based on SiO2 versus Na2O+K2O, AFM, and K2O versus SiO2 diagrams, these rocks are calc-alkaline with moderate potassium nature. LILE elements such as Sr, K, Rb, Ba, and Th showed enrichment, and HFSE, especially Nb, Ta, and Ti presented depletion. The primitive mantle-normalized multi-element diagram showed the positive anomaly of Sr and the negative anomaly of Nb. Chondrite-normalized REE patterns showed enrichment of LREE compared to HREE with La/Yb>9 and Sm/Yb>1.8 ratios without any positive or negative Eu anomalies. The initial ratios calculated for 87Sr/86Sr varied from 0.704498 to 0.704967 and for 143Nd/144Nd from 0.512821 to 0.512842. The calculated values of ℇNd also vary between +3.55 and +3.98.

Geochemical features including high amounts of strontium (Sr>750 ppm), low amounts of yttrium (Y<8ppm) and HREE, high Sr/Y ratio, and the strongly fractionated pattern of REE without Eu anomalies, define the adakitic nature of Dehaj hyallo-andesites. Enrichment in LILE elements and depletion in HFSE elements (i.e., Nb, Ta, Ti) in Dehaj adakites show the connection of these rocks with the pre-Pliocene subduction environment. High values of Sr and Rb/Sr ratios between 0.02 and 0.04 and La/Yb>10 along with isotopic ratios of 87Sr/86Sr and 143Nd/144Nd as well as positive values of ℇNd (+3.55 to +3.98) demonstrate the hyalo-andesitic rocks of Dehaj were formed as a result of the melting of the oceanic slab. Several reasons indicate the presence of garnet in the origin of the Pliocene adakites of Dehaj and the remaining of this mineral in the slab resulting from partial melting, which also represents the eclogite composition of the source rock. These reasons include values of Y<8ppm and Yb<0.76ppm and the sloping pattern of REE elements with La/Yb>9 and Sm/Yb>1.8 ratios. The melting process of the eclogite oceanic plate occurred at the same time or following the tectonic collision between the Central Iranian and Arabian tectonic plates during the Miocene-Pliocene.

Pliocene hyallo-andesitic rocks of Dehaj were created as a monogenetic magmatic activity. Geochemical evidence indicates that the partial melting of subducting oceanic crust is the origin of silica-rich Dehaj adakites. The Pliocene Dehaj adakites formed after the cessation of Neotethys subduction and in a tectonic environment following the collision. Thinning and separation of the subducting oceanic crust and sinking into the asthenosphere gave rise to melting conditions. These events have occurred in an active tectonic environment and simultaneously with the formation of numerous faults. The occurrence of numerous outcrops of Pliocene volcanic rocks along with Pliocene hyallo-andesitic lavas and Pleistocene basaltic lavas in the study area compared to nearby areas indicates the presence of multiple fractures in the crust of this area. These fractures have caused the rapid ascent of adakite melt from the subducting oceanic crust. During the ascent, the adakite melt has been contaminated by the upper metasomatized mantle and crust to a small extent.

Granites are among the most abundant rocks associated with orogenic regions that have a wide range of compositions (Atherton and Tarney, 1979; Pitcher, 1993). Among the main environments related to orogeny are continental arcs and subduction zones (Brown et al., 1995). Knowing the composition and the nature of granites helps to understand the evolution stages of the continental crust (Barbarin, 1999).Among the methods of knowing the nature, the composition, and the petrogenetic origin of granites is the precise determination of the chemical composition of their constituent minerals. The chemical composition of minerals indicates the temperature, the pressure, and the nature of granite at the time of emplacement (Kaygusuz et al., 2008). Sometimes it may not be possible to accurately determine the nature and composition of the stone with regard to the main minerals due to the influence of alteration and some other processes that may have caused changes in their values, so it is possible to determine the conditions of stone formation by relying on the composition of some secondary minerals such as tourmaline Restored the original environment. Since tourmaline is formed in a wide range of geochemical conditions, it can contain petrogenetic information from two different metamorphic and magmatic conditions (Henry and Guidotti, 1985; Henry and Dutrow, 1996).

Regional Geology:

On the geological maps of the region, the distribution of rocks in the Deh-Salm metamorphic complex is limited to the lower and upper parts (Naderi Mighan and Akram, 2005; Akrami and Naderi Mighan, 2005; Hamzehpour, 2005), which is probably equivalent to the formations Nayband and Shemshak Formations belonging to the late Triassic-Jurassic (Stöcklin et al., 1972).New dating results have introduced the age of the Deh-sal-m metamorphic complex as 168-163 million years based on the U/Pb ratio on single grains of zircon, monazite, and xenotime (Mahmoodi et al., 2009). Such age is consistent with the published data of Shah-Kuh granite by the K-Ar method of 158-168 Ma (Esmaeily et al., 2005). The small age range related to the cooling of the Shah-Kuh granite and its marginal metamorphic complex has been attributed to rapid tectonic changes in a magmatic back-arc environment (Mahmoodi et al., 2009). From west to east, five tectonic and lithological units, from A to E, are recognized in the Deh-Salm metamorphic complex. Also, five metamorphic phases, from D1 to D5, have been identified in the region, subjected to the existing tectonic units with different intensities and quality (Arefnejad, 2010; Bahramnejad, 2015).

For mineralogical and petrological studies, 50 thin sections of the dykes were prepared and studied by a polarizing microscope. To determine the chemical composition of the tourmalines in the studied dykes, chemical analysis was carried out by electron microscanning on the main elements in the tourmalines in selected samples by the JOEL-JXA-8600M automatic superprobe with an accelerating voltage of 15KV took place at the Department of Earth and Environmental Sciences, Yamagata University, Japan.

Petrography and Mineral Chemistry:

Based on the microscopic observations, in the dykes, quartz with about 35 to 40 volume percent has direct extinction. Euhedral and subhedral orthoclase is 25 to 30 volume percent. Some samples contain coarse-grained and subhedral plagioclase (about 20 volume percent). Tourmaline and muscovite with 10 to 15 volume percent of the rock are secondary minerals. Muscovites are usually flaky minerals with a bunch of parallel faces. Zircon occurs as an inclusion in orthoclase. The main texture of the studied rock is granular.Tourmalines are euhedral. In the longitudinal thin section, they are elongated crystals, and in the transverse sections, they show tetra- and hexagonal shapes and inverse blue-to-pink pleochroism. The tourmalines of dykes with high Fe/(Fe+Mg) ratio are schorle in composition and they are classified as alkaline type because of their high Na2O content.

The deformation phases caused felsic dykes injection in the Deh-Salm metamorphic complex, especially in the migmatites which are widespread in the eastern parts of the Deh-Salm metamorphic complex. The dykes are characterized by the presence of quartz, plagioclase, orthoclase, muscovite as well as tourmaline. The high Fe/(Fe+Mg) ratio, the absence of chemical zoning, and the euhedral shapes are evidence of the magmatic origin of tourmalines.Based on tourmalines composition, the dykes in the Li-poor granites resulted from a late magmatic event concurrent with the extensional tectonics.Several deformation events of D1, D2, and D3 are recognized in this area. These dykes formed during an extensional deformation event of D3 that caused them to crosscut the major schistosity of the metamorphic rocks. Therefore, the dikes occurred after the migmatization and probably simultaneously with the uplift of the Deh Salam metamorphic complex.

At the northeastern end of the Sabzevar ophiolitic zone and the southern edge of the Binalud zone and south of Quchan, a 200 km long young magmatic arc consisting mostly of calc-alkaline to adakitic volcanic-intrusive rocks are exposed which extended to Esfarayen. In the present study, the Shotorsang hypabyssal granitoids in the northeast of Sabzevar and the Binalud structural zone are investigated. No dating data and comprehensive geochemical study of these units have been published, and the origin of hypabyssal units is unclear. Accordingly, new petrographic and geochemical data and U–Pb zircon ages of intermediate-acidic intrusives of Shotorsang in the northeast of Iran are reported.

Geology:

The hypabyssal Shotorsang granitoids, as a part of the magmatic arc in the north of the Sabzevar ophiolitic belt, are located 82 km northeast of Sabzevar and in the Binalud structural zone. Based on the field studies and 1:5000 geological map, the rock units of the Shotorsang area include early Cretaceous limestone, Eocene sedimentary and volcanic rocks, the Miocene hypabyssal stocks and dykes, and new Plio-Quaternary deposits. Intrusive units including dikes and hypabyssal stocks composed of dacite, granodiorite, and quartz monzonite. The dikes with acidic composition crosscut the Eocene sedimentary and volcanic succession.

During the field investigations in the Shotorsang area, 80 samples were taken from most of the geological units, and 40 thin sections were studied by a polarizing microscope for petrographic studies. Six representative and least altered samples from the Shotorsang hypabyssal granitoids were analyzed for major oxides and trace elements using XRF and ICP-MS in the Acme Laboratories (Canada). Two samples from dacite dike and granodiorite stock (samples Ab8 and Ab59) were selected for dating studies according to the suitable size and abundance of zircon crystals.U-Pb dating was conducted by laser ablation-multiple collector-inductively coupled plasma-mass spectrometry (LA-MC-ICP-MS) in the Laboratory of Isotope Geology at the Institute of Mineral Resources, Chinese Academy of Geological Sciences, Beijing, China.

Petrography:

Based on petrographic studies, the Shotorsang hypabyssal units can be divided into three groups including granodiorite and quartz monzonite stocks as well as dacite dikes. The granodiorite is composed mainly of quartz, plagioclase, orthoclase, hornblende, and biotite with accessory zircon, sphene, and apatite. This unit dominantly displays a porphyritic to glomeroporphyric texture.Dacite dikes have mineralogical and textural similarities with those of the granite stocks. Mineralogically, these units include quartz, plagioclase, orthoclase, biotite, and hornblende with a felsitic porphyry texture.Magmatic fluids released during the intrusion of quartz monzonite bodies are the main factor for skarnization in the area.

Geochemistry:

On the total alkali-silica diagram (Middlemost, 1985), the intrusive bodies and dikes are plotted in the fields of quartz monzonite, granodiorite, and granite, which is consistent with their petrographic observations. On the SiO2 versus K2O discrimination diagram (Peccerillo and Taylor, 1976), the Shotorsang hypabyssal granitoids fall mainly in the medium- to high-K calc-alkaline domains. On chondrite and primitive mantle-normalized diagrams, the investigated samples display enrichment in LILE and LREE and depletion in HFSE and HREE.On Rb versus Nb+Y tectonomagmatic discrimination diagram (Pearce et al., 1984), the Shotorsang hypabyssal granitoids are plotted in volcanic arc granite field.U-Pb datingThe two samples from the dacitic dike and granodioritic stock yielded zircon U-Pb ages of 22.52±0.38 and 22.56±0.35 Ma, respectively. The Th/U ratio for zircons from the dacitic dike and granodioritic stock is more than 0.1, which is different from metamorphic zircons and compatible with magmatic zircons.

The remarkable geochemical criteria of hypabyssal Shotorsang granitoids are Sr (382.5-607 ppm), high Sr/Y ratios (26.4-62.5), LaN/YbN (9.5-17.1). The low values of Y (9.7-16.3) and HREE are similar to those of adakites. The low values of Yb and Y, and high ratios of Sr/Y, as well as La/Yb, could be attributed to the presence of residual garnet and hornblende or as fractionated minerals.The LILE (large-ion lithophile elements) enrichment and HFSE (high-field strength elements) depletion are typical features of calc-alkaline magmas related to subduction zones, which originated from partial melting of a subducted oceanic slab or a supra-subduction mantle wedge.The lack of Eu anomaly in adakites, as well as the Shotorsang hypabyssal granitoids with abundant plagioclases, is related to magma ƒO2. Under oxidizing conditions, Eu occurs dominantly as Eu3+, leaving lesser Eu2+ to be incorporated into plagioclase.There are evident changes in Sr-Nd isotopic ratios of the magmatic rocks of the northern Sabzevar ophiolitic belt, and it seems that the (87Sr/86Sr)i ratios increase, however, the εNdi values decrease from the old to the young rocks, which likely indicates the effect of the crustal materials on the source magma.It can be inferred that the Cretaceous-Paleocene magmatism is subduction-related, and from Eocene onwards, the igneous rocks of this zone, including the Miocene Shotorsang adakites with an age of 22.5 million years, have a post-collisional nature.

The Shotorsang hypabyssal granitoids, as a part of the magmatic arc of the northern Sabzevar ophiolitic belt, formed as dike and stock with dominantly porphyritic textures and intruded the Cretaceous-Eocene volcano-sedimentary succession. The two samples from the dacitic dike and granodioritic stock yielded zircon U-Pb ages of 22.52±0.38 Ma and 22.56±0.35 Ma, respectively, indicating the importance of Miocene time in iron mineralization in this zone. The geochemical features of the Shotorsang hypabyssal granitoids correspond to the field of post-collisional volcanic arc rocks (VAG) and possess similarities with those of the adakitic rocks, originating from the subducting slab.These rocks probably have an eclogite or amphibolite garnet origin resulting from the metamorphism of the Sabzevar Neotethyan oceanic lithosphere, which was subducted under the Alborz zone.

The Precambrian-Early Cambrian magmatism in Iran includes magmatic rocks in the central, northwestern (e.g. Tekab-Zanjan zone), and northeastern parts of the Iranian plate. Although in the previous literature, the petrogenesis and tectonic setting of these rocks were attributed to the rifting and extensional tectonic regime but recent studies emphasized that they formed in a convergent subduction-related tectonic setting. In this study geology, geochemistry, petrogenesis, and dating of Doran and Shah Bolaghi (Moghanlu) intrusions as the best examples of the Precambrian-Early Cambrian magmatism in the NW Iran have been investigated.

Geology:

In NW Iran, there are intrusive and related volcanic rocks that have been outcropped in a special stratigraphic position. The intrusive rocks cut the Precambrian Kahar Formation rocks and are overlaid by the Bayandor Formation rocks. Stratigraphically, the equivalent volcanic rocks (Gara Dash rhyolitic series rocks) lie between over Kahar and lower Soltanieh Formation rocks. In the area, Doran–type intrusions including Doran, Moghanlu, Incheh, Sarveh Jahan, Mahneshan, Aghkand, and Alam Kandi, have been intruded as batholith (e.g. Shah Bolaghi) and as stocks (e.g. Doran and Sarveh Jahan) and mainly they affected by dynamo thermal metamorphism. The studied Doran and Moghanlu intrusions included different felsic pulses of the oldest mica granite, hololeucocratic porphyritic granite, and the youngest potassium feldspar-bearing pink granite associated with basic rocks.

Petrography:

The studied plutons including different pulses display various mineralogy. The Hololeucocratic mica granites show granular to porphyry textures including quartz, alkali feldspar, and plagioclase associated with biotite and rare muscovite. Some albite crystals are magmatic and chessboard albites formed during Na-metasomatism. Hololeucocratic porphyry granite show various porphyry and granular as well as cataclastic textures. Secondary chessboard and myrmekitic albite developed along with primary albite and antiperthite feldspars. Rare primary muscovite can be found. Pink granite includes quartz and alkali feldspar and plagioclase minerals with porphyry to granular textures and some hypersolvus potassium feldspar megacrysts developed. Rare muscovite and biotite crystals have been developed. In all granitic rocks, recrystallization and secondary minerals have been developed due to metamorphism, tectonic and metasomatism processes.

Geochemistry:

All granitic rocks show high silica content (79-82 wt.%) and in the R1-R2 diagram located in alkali granite to syenogranite fields and metaluminous to weak peraluminous. Basic rocks have 49-50 wt% SiO2 content and are olivine gabbro. The TiO2, Fe2O3, MgO, CaO, and P2O5 contents of studied granitic rocks are very low and porphyry granite and mica granites show very low K2O and high Na2O contents but mica granites show K2O/Na2O~1. In the spider diagrams, the porphyry granite samples show enrichment in the Zr, Th, Nb, Ta, U, and HREE (heavy rare earth elements) and trough in the LREE (light rare earth elements), and large ion lithophile elements (LILE: Cs, Rb, Ba, K), P, Ti, E, however, the mica granites are characterized by LILE (Cs, Rb, Ba), Sr, K, P, Ti, Eu, HREE, LREE, Zr, Th, and U enrichment, and the pink granites show a diverse pattern with enrichment in LILE and LREE and depletion in Sr, P, Nb, Ta, and Ti. All granitic samples show very clear troughs in Eu and Eu/Eu*.

U-Pb Dating:

Zircon crystals from Doran porphyry granite and the Shah Bolaghi pink granite have been dated by the U-Pb method. The porphyry granite apparent 206Pb/238U ages range from 515 to 1192 Ma. In the relative frequency histogram, the porphyry granite ages show two peaks in 550 and 1050 Ma, which older ages can be related to inherited crystals and or captured crystals during ascending of the magma. Pink granite apparent 206Pb/238U ages range from 478 to 657 Ma. The porphyry granite zircon crystals' younger ages yield a weight mean of 565 ± 28 Ma, 95% conf. n=14, MSWD=0.54 and the pink granite ages yield a weight mean 538 ± 12 Ma, 95% conf. n=23, MSWD=1.02. Dating results are compatible with field observation that the pink granite cut porphyry granite.

The studied granitic intrusions are inhomogeneous and different mica granite, granite porphyry, and pink granite associated with basic rocks could be distinguished. The investigated granitic rocks are marked by A/CNK<1.1 and Zr+Nb+Ce+Y <300 as well as 104*Ga/Al<2.6 that are different from S- and A-type granites and very high silica content and lower La, Y, Nb, Zr, Ga, Zn, Ce contents which led us to be considered as fractionated I-type granites. Different petrogenetic models have been proposed for fractionated I-type granites generation, including:(1) Partial melting of the crust which the melt composition may have been modified by fractional crystallization;(2) fractional crystallization of hornblende and clinopyroxene from initial intermediate to mafic melts;(3) involvement of a fluid residual or hydrothermal phase during late magmatic stages.