نشریه پترولوژی
پیاپی 49 (بهار 1401)

Numerous intrusive rocks of various sizes, dominated by granodiorite-granite, intrude the Sanandaj-Sirjan Zone (SSZ) (Torkian et al., 2008; Mollaei Yeganeh, 2018). They are S-I-and A-types in composition and ideal for studying the linkage between tectonic settings and the magmatic evolution of igneous rocks. Diverse scenarios have been proposed to explain the origin of A- type granitoids: (i) extreme fractional crystallization of parental mafic magmas derived from the mantle (with/ without crustal contaminations, (ii) high temperature partial melting of crustal granitic rocks (iii) partial melting of granulites OR charnockite, (iv) a metasomatic origin (Eby, 1992). These Atype granites are generally thought to have formed in an extensional setting, but their source regions and tectonic attributes are still a topic of debate. The present paper describes the field relationships, petrography and whole-rock geochemistry of A-type granitoid; these data are then used for the interpretation of the magma sources, tectonic setting and magma-generation processes for A2-type granitoid in NE-Sonqor.

Ahmadabad deposit is located 30 km northeast of the Semnan province, between the Alborz and the Central Iran sedimentary-tectonic structural zones. The Extensive magmatism of the Cenozoic age, with some irregularities, distributed throughout Iran (Emami et al., 1993). Magmatism in two dominant volcanic zones of Iran; the Urmia-Dokhtar trending northwest-southeast and the Alborz with east-west trend in the north of Iran started at the Cretaceous; while the most widespread magmatism occurred in the Eocene (Alavi, 1994). The youngest volcanism occurrence in the Alborz basin shows a close chemical relationship with continental arc rocks formed in the continental collision environment (Asiabanha et al., 2012). The exposed rocks in Ahmadabad district include the Cenozoic dacite, andesite, trachyandesite, basaltic andesite and basalt, associated with pyroclastic rocks comprises green to gray acidic tuffs with intercalations of shale, sandstone and conglomerate. Monzonite, monzodiorite and infrequent granodiorite and granite intruded the volcanic rocks sequence.

During field work, 54 samples were collected from the host rocks, alteration and mineralized zones. For petrographic study, 14 thin sections and 21 thin-polished sections were prepared and studied by ZEISS Axioplan2 research type polarized microscope at Kharazmi University, Tehran branch. After careful petrographic study, 41 samples of igneous units prepared at Kharazmi University for whole rock geochemical analysis and analyzed at Zarazma Co. using XRF and Iran Mineral Processing Research Center using ICP-MS methods. To complete mineralogical studies, 3 samples of mineralization zones were analyzed by XRD method. After preparation and separation of zircon from selected samples at Kharazmi University of Tehran, zircon of monzodiorite intrusion was dated by U-Pb method using ID-TIMS spectroscopy at Faculty of Earth Sciences, University of Oslo, Norway with Finnigan MAT-262 instrument.

Whole-rock geochemical analysis indicates that the intrusions have calc-alkaline, I-type and metaluminous nature which is consistent with the typical features of volcanic arcs granitoids in subduction zone of active continental margin. LREEs enrichment relative to HREEs and enrichment in LILE (e.g., Ba, K, Sr and Th) relative to HFSE (e.g., Ti) imply that magmatism formed in an active subduction zone. The REE pattern and high (La/Yb)n ratio (1.59 to 60.05) in all samples, verify the high amount of garnet in the source region. Rare earth elements such as La and Sm do not change with mineralogical changes in the source rock; thus, these elements represent the total composition of the source rock. It seems that LREEs enriched mantle of garnet-lherzolite composition with 1 to 10% partial melting (Aldanmaz et al., 2000) are the main process in magma development that generated igneous rocks in the region. Partial melting and crustal contamination are the main processes in magma development that generated igneous rocks in the Ahmadabad region. Based on petrographic, geochemistry and geochronology data of the current research and the Verdel et al. (2011) studies, the proposed tectonomagmatic pattern is as follows:Simultaneous with the last subduction of the oceanic plate of Neotethys below Central Iran in the Upper Cretaceous-Paleogene, the Alborz back-arc basin was opened. The subducted slab and the accompanying sediments under high temperature and pressure conditions were dehydrated and caused melting process. Melt and fluids enriched in LREEs and depleted in HREE and HFSE ascending in the mantle wedge causes the crust to melt and an extensive magmatism developed. The magmatic phase in the Ahmadabad region is simultaneous with the beginning of flare-up, especially in the Urmia-Dokhtar zone and the northern zone of Central Iran-South Alborz in the Upper Cretaceous-Paleogene.

Based on field and petrographic studies of the Ahmadabad volcanic rocks, a range of intermediate to acidic rocks including andesite, trachyandesite and dacite formed followed by subvolcanic shallow intrusive bodies. Extensive argillic and silicic alterations along with signs of mineralization occurred in both volcanic and intrusive rocks. Plagioclase, alkali feldspar, quartz and amphibole are the main and the abundant minerals of the rocks under study Geochemical properties of the major and trace elements show that the igneous rocks of Ahmadabad are calc-alkaline, metaluminous and I-type. Enrichment in LREEs and LIL elements and depletion in HRE and HFS elements are indicative of a subduction related magmatism of the continental active margin associated with the crustal contamination process. Zircon dating of the monzodiorite intrusion yielded U-Pb age of 51.76 ± 0.10 Ma indicating the Early Eocene magmatism, which coincides with the beginning of the extensive Eocene magmatism (flare-up) in Iran. Based on petrography and geochemical studies, the granitoids were formed by melting of metasomatic mantle wedge due to released fluids from the subducted slab.

Tourmaline is an indicator mineral of pegmatite, granites and pneumatolytic veins. In metamorphic rocks, it is formed by the process of boron metasomatism and in sediments by the recrystallization of destructive particles. This mineral crystallizes in various temperatures, pressures and geological environments so can be widely used in lithological studies.The study areas are part of the geological map of Kashan located in the west of the volcanic belt of Central Iran (Urmia-Dokhtar belt). A number of studies have been carried out regarding the geological and petrological of the areas of study: Among those, petrology and geological studies of metamorphic aureole in Qohroud granitoid (Ahankoub, 2003), petrography and petrology of Qohroud plutonic rocks (Jafari, 2001), the type of garnet zoning in skarns of Qohroud Intrusion (Farazdel et al., 2005) and geochemistry and petrology of mafic-intermediate intrusions in mineralized region of Qamsar, Kashan Yar-Ali (2016) are more notable. The purposes of the present study are to determine the origin of tourmalines from the study areas on the base of their petrographic and geochemical characteristics as well as their structural formula and to compare these minerals as well.

Following field observations, 25 thin sections were prepared for mineralogical and textural studies using Olympus polarizing microscope (BH2). 10 samples of tourmalines were analzyed by XPM analysis at Binalood Company. The results of microprobe analyses of tourmalines from hornfelses (Ahankoub, 2003) carried out at the Oklahoma University (USA) were also used for the present study. All these data are given in Tables 1 and 2.

The areas under investigation are predominantly compose of sedimentary, metamorphic (hornfels, marble, skarn) and intrusive (granodiorite, monzogranite, tonalite) rocks. Plagioclase, orthoclase, quartz, hornblende, and biotite as major with sphene, zircon, tourmaline and opaque as minor minerals are dominant. Tourmalines in these areas are metasomatism in nature and as needles and small crystals. are mainly found in fractures. Tourmaline crystals with zoning, formed in post-crystallization and hydrothermal stages, were subjected to influence of boron-rich solutions presumably point to mixing of fluids in open systems. Green tourmalines replaced feldspars possibly caused by tourmalinization in the pneumatolitic stage by reactions of boron with wall rocks.Based on X position and Ca, Na, K contents, most of tourmalines are alkaline. The chemical composition of tourmaline in hornfels is mainly dravite and in granitoids is dravite with a tendency towards schorlite. The formation of tourmaline instead of biotite and chlorite in metamorphic rocks shows the its formation as a result of reactions of boron fluids with aluminosilicates. The studied tourmalines lie below the line  and therefore, have Al substitution in the Y position. As the Fe-Mg-Ca and Al-Mg-Fe diagrams display these tourmalines are mainly plotted within the metapelites and metapesamites in calcium poor quartz- tourmaline rocks. The ratio of FeO / FeO + MgO tourmalines from the hornfels is between 0.58 to 0.6 (ave. 0.59), which confirms they have crystallized in an open magmatic system with the presence of boron with an external origin and indicates the involvement of atmospheric water during their formation in the hydrothermal system. This ratio in tourmalines from the granitoids are between 0.56 to 0.69 (ave. 0.63), which can indicate the formation of these tourmalines during the mixing of magmatic and hydrothermal fluids.

The chemical composition of the studied tourmalines based on cationic substitutions is in the range of alkaline tourmalines, which indicates a higher amount of K and Na at position X compared to Ca and confirm their formation in acidic and low temperature conditions. The chemical composition of tourmalines from granitoids is in the range of dravite and schorlite and the chemical composition of tourmalines from hornfels are mainly dravite, which represents the cation exchanges of Fe, Mg in constant values of Ca and Al. The studied tourmalines are located in the range of calcium-poor metapelites and metapsamites and calcium-poor quartz-tourmaline rocks and coexist with the aluminum-saturated phase. Based on the amount of FeO / FeO + MgO in tourmalines, the openness of the system and the presence of boron from an external source is confirmed.

Mesozoic-Paleogene ophiolitic sequences in Iran are in association with opening and closing of Neo-Tethyan oceanic crust. Temporally and specially, these ophiolitic sequences subdivided into five different groups (or belts) (Shafaii Moghadam and Stern, 2014):Zagros outer belt (late Cretaceous-Paleogene); Zagros inner belt (late Cretaceous); Sabzevar- Torbat e Heydarieh belt (late Cretaceous-early Paleocene); Birjand-Nehbandan belt (early to late Cretaceous);
Makran belt (late Jurassic-Cretaceous). The Zagros outer belt’s ophiolitic sequences cropped out along the NW-SE trending Zagros main thrust and including Maku-Khoy-Salmas, Kermanshah (-Kurdistan), Neyriz and Esfandagheh (or Haji-Abad) ophiolites. The Kermanshah ophiolite (KO) is an ophiolitic tectonic complex, which comprises imbricated dismembered fragments of mantle peridotites, gabbros, sheet dykes and basaltic rocks which generally occur as thrust-bound tectonic slices. The Sahneh-Harsin ophiolitic complex(SHOC) is the southeastern part of KO. Detail petrological study has not been carried out on the basaltic rocks from Ali-Abad Garos area(AGA) as a part of the SHOC. Thus, the present study focuses on the field observations, petrographic study as well as whole-rock geochemical analyses (XRF and ICP-MS) of the basaltic rocks widespread in AGA.

Regional geology:

KO cropped out in the northwest of Zagros main thrust (ZMT) where Zagros sedimentary belt join the Sanandaj-Sirjan magmatic-metamorphic belt. In SHOC as the southeastern part of Kermanshah ophiolites Triassic-Cretaceous limestone and radiolarites as well as Mesozoic metamorphic-sedimentary rocks are the oldest lithological units. In the following, ophiolitic sequence related lithological bodies such as peridotites, meta-peridotites, gabbros, meta-gabbros, diabasic dikes and volcanic rocks including pillow lava-massive basalts cropped out which are in accompanied by Eocene extrusive and intrusive rocks. Pliocene to Quaternary sedimentary rocks are the youngest lithological units in Sahneh-Harsin region. In Sahneh-Harsin region ophiolitic sequence related basaltic rocks cropped out in Ali-Abad Garos, Tamark, Gashor areas.

We collected ~70 samples of both pillow lava and massive basaltic rocks in-AGA. Based on petrographic characteristics 10 of the freshest samples were analyzed for whole-rock major and trace elements by X-ray Fluorescence (XRF) and Inductively Coupled Plasma-Mass Spectrometry (ICP-MS) in ALS Chemex lab.

Field observations:

Ali-Abad Garos ophiolitic sequence related basaltic rocks mostly show pillow lava structure. However, minor massive basaltic rocks are exist. In the northern part of the study area basaltic rocks are set atop of gabbroic units. However, gabbro-basalt boundary covered by Quaternary sediments. In some parts, basaltic bodies are in association with plagiogranites. Via a fault boundary upper Triassic-Cretaceous limestone units (Biston Formation) thrusted over basaltic rocks.

Petrography:

The rocks under study are dominated by plagioclase (up to 20%), clinopyroxene (up to 10%) and opaque minerals (up to 2%) phenocrysts set in a fine-grained to glassy ground mass. Chlorite, calcite, epidote, quartz, zeolite and opaque minerals as secondary minerals formed by altered phenocrysts and groundmass. Massive basaltic rocks mostly show porphyritic texture and to lesser extent glomeroporphyritic, intergranular and amygdaloidal textures are present. The rocks studied are characterized by plagioclase (up to 10%), clinopyroxene (up to 20%) and opaque minerals (up to 5%) phenocrysts set in a fine grain to glassy groundmass. Secondary minerals in massive basaltic rocks are similar to those presented for pillow lava basaltic rocks. Whole-rock geochemistry, tectonic setting and source characteristics The studied basaltic rocks show tholeiitic signature. Major element oxides values against SiO2 values, as fractionation index, indicate fractional crystallization as an effective parameter in generation of these rocks. In chondrite-normalized REEs diagram, the basaltic rocks exhibit slight LREE enrichment relative to HREEs (LaN/YbN values ranging from 2.91 to 3.35 and Eu/Eu* values vary from 0.91 to 1.01). In primitive mantle-normalized multi-elements diagram, the patterns of basaltic rocks are consistent with plume related mid-oceanic ridge basalts (P-MORB), against within plate basalt (WPB or OIB), normal mid-oceanic ridge basalts (N-MORB) and enriched mid-oceanic ridge basalts (E-MORB). This case is in agreement with situation of the study samples in basaltic rocks with different geochemical signatures on discriminative diagrams. Parallel trends of the studying basaltic rocks on spider diagrams indicate that they are co-genetic. Also, different elemental ratios as well as the situation of the rocks under study in different geochemical diagrams reveal that parental magma (or magmas) could be formed by low degree partial melting (<3%) of a spinel peridotite as a source rock. Integration of our new presented geochemical data from the study basaltic rocks with other previously presented geochemical data from other ophiolitic sequence related basaltic outcrops within (SHOC) (Tamark and Gashor basaltic rocks) reveal that in this ophiolitic complex basaltic rocks are heterogeneous. Briefly, three different groups of basaltic rocks with P-MORB, E-MORB and OIB geochemical features exist in (SHOC). Comprehensive understanding of relation of these three different basaltic groups are needed to further elemental-isotopic geochemical data coupled with geochronological data.

The Bostanabad-Hashtrood gabbro-diorite intrusive body in the south of Tikmehdash has intruded the Eocene volcanic-sedimentary rocks. The chemistry of the constituent minerals in this intrusive body shows that the plagioclases and clinopyroxenes have labradoritic and diopsidic composition, respectively, and are accompanied by the subordinate amounts of Mg-rich biotite and gangue minerals. The chemistry of the existing diopsides in these rocks indicates that their parent magma belongs to subalkaline magmatic series with a continental arc tectonic setting. The thermometric considerations showed that the clinopyroxene was crystallized at temperatures around 1200°C. Based on AlVI content (0.048-0.102) in the crystal lattice of the clinopyroxenes, it was ascertained that this mineral was formed under the pressure range of 6 to 9 kbars equivalent to depths between 19 and 28 km. Additionally, this study revealed that the clinopyroxenes in these rocks were formed from a magma which contained 10% water. The presence of some ferric iron in these clinopyroxenes with titanomagnetite is indicative of high oxygen fugacity in the parent magma.

Mineralization in the Marshoun 2 occurrence occurs as quartz-sulfide veins hosted by intermediate tuff units, and is divided into four stages. Stage 1 is represented by silicification of host rocks along with minor disseminated pyrite. Stage 2 is characterized by quartz-sulfide (chalcopyrite, pyrite) veins and breccia cements. Stage 3 is characterized by quartz (calcite)-sphalerite-galena ± chalcopyrite ± pyrite veins and breccia cements. Stage 4 is barren post-ore quartz-calcite vein-veinlets. The hydrothermal alteration includes of silicification, intermediate argillic, carbonate, and propylitic alteration. The ore minerals are pyrite, chalcopyrite, galena, and sphalerite; quartz, sericite, chlorite and calcite are present as gangue minerals. Ore minerals display vein-veinlet, brecciated, comb, crustiform, colloform, cockade, plumose, bladed, skeletal, and vug infill textures. Similar Chondrite–normalized rare elements and REE patterns of mineralized samples indicate that hey may have fomed by same hydrothermal process. Microthermometric data reveal that ore-forming fluids at the Marshoun deposit belong to the moderate-temperature and low-salinity H2O–NaCl system. Fluid boiling and mixing were important processes in the evolution of the ore-forming fluids. Marshoun 2 is an intermediate-sulfidation epithermal deposit.