نشریه پترولوژی
پیاپی 18 (تابستان 1393)

The volcano - sedimentary complex in the north of Shahrekord as a belt with NW-SW trendand 396 km2 area is parallel to the Zagros structural zone within the central part of the Sanandaj- Sirjan Zone. The volcanic rocks have mafic and intermediate affinity. The subvolcanic microdiorite rocks were exposed as small stocks. The pyroclastic rocks are composed of tuffs، lapilli tuffs، agglomerates، volcanic breccias and tuffites. To determine the age of the aforementioned volcanic rocks، due to the presence of fresh amphibole crystals in the andesitic and microdioritic rocks، 40Ar/39Ar dating method was applied. On the base of 40Ar/39Ar data، the studied volcanic and subvolcanic rocks have formed between 145 to 169 Ma corresponding to Callovian to Titonian stages (upper Dogger to upper Malm of Jurassic period). The dating results are well consistent with the other geochronological studies performed on some of the intrusive bodies in the Sanandaj-Sirjan Zone (i. e. Aligoodarz and Alvand intrusives) which are related to the subduction of Neothetyan Oceanic Lithosphere below the Central Iran Plate. Nevertheless، the aforesaid volcanism، belonging to middle Kimmerian orogenic phases in this part of the Sanandaj-Sirjan Zone، is reported for the first time.

The Shurchah antimony-bearing area is located in the Eastern Iranian flysch zone. The Cretaceous to Eocene schists and phyllites are the oldest geological units in the area. These rocks have been intruded by granitic bodies، felsic dikes، and quartz and stibnite veins. The granitic rocks in this area are composed of granite، granodiorite and aplite dikes. The granitoid rocks are generally massive، gray and coarse-grained with granular، porphyritic and cataclastic textures and contain mainly plagioclase، K-feldspar، quartz، biotite، muscovite and opaques. These rocks are commonly peraluminous to metaluminous، calc-alkaline and related to a magmatic arc. The SiO2 content of the intrusions varies from 61 to 75 wt. %. Variation diagrams show linear trends، with increasing SiO2 content Al2O3، CaO، FeOt، MgO، and TiO2 amounts decrease whereas، K2O and Na2O contents increase. On the base of MORB and ORG- normalized diagrams، the negative anomalies of Nb، Ti and positive anomalies of K، Rb، Th، U and especially Pb revealed an igneous mantle origin، especially the role of Sistan subduction oceanic crust and overlaying mantle wedge in the genesis of these rocks and also، the role of continental crust in magmatic contamination.

The Sungun Cu-porphyry deposit is located in the East Azarbaiyjan province، 75 Km NE of Ahar city. The porphyry type mineralization is related to the Miocene monzonite-quartz monzonite and diorite-granodiorite intrusive rocks. There are four distinguished alteration zones in the Sungun deposit which nearly envelope the ore body concentrically. The present research is concerned on the behaviour of REE in potassic، phyllic and argillic zones regarding their geochemical and economic potentials. Geochemical evidences of trace elements share many similarities between alteration zones. LREE enrichment and HREE depletion are well clear in three alteration zones. Eu anomaly in potassic zone is weakly negative، in phyllic is nearly positive but the argillic zone doesn’t show any Eu anomaly. Lower ratio of Ce/Ce* (0. 95) in potassic and the higher ratio in phyllic and argillic zones reveal prevailing reduced and oxidized conditions respectively. Similar physicochemical conditions such as pH، the availability of suitable complexes in fluids، and the occurrence of secondary minerals for REE uptake، have been prevailed on the evolution of these three alteration zones.

The metamorphic rocks of the Surian complex occurred along the Bavanat Valley، northeast of Fars province at the eastern edge of the Sanandaj-Sirjan zone. The complex is located between the Surian fault in the northeast، and Jian fault in the southeast، and consists of chlorite epidote schist، chlorite actinolite schist، quartz chlorite schist، mica schist and andalusite muscovite schist. Geochemical data (Al2O3، CaO، MgO، Ni، P2O5، TiO2 and REE) contents reveal that the primary rocks of andalusite muscovite schist، quartz chlorite schist and mica schist were formed in sedimentary environments، whereas those of the chlorite actinolite schist and chlorite epidote schist have igneous origin. The concentrations of major immobile elements (e. g. Al، Ti)، REE and Highs Field Strength Elements (HFSE) (e. g. Nb، Y، Zr)، indicate that the rocks of sedimentary origin were formed in active continental margin setting in association with turbidites of deep sea floor and those of igneous origin show tholeiitic basalt nature of rift basins.

The Takht- e baz granitoid in the northwest of Birjand is cropped out in the vicinity of pyroclastic units composing of tuff، marly tuff and breccia. Petrography and major elements analysis show that this intrusion is alkali granite to granite in nature. Common textures in these rocks are granular، graphic، myrmekite، granophyric and perthitic. Potasium feldspars (perthitic orthoclase and microcline)، quartz and sodic plagioclases are essential minerals. Biotite as the only mafic mineral and zircon، titanite and Fe-oxide are the accessory minerals of these rocks. Biotite is altered to chlorite، and feldspars to sericite، calcite and clay minerals. Age dating with zircon uranium-lead method indicated that the alkali granite is 55. 7 ± 0. 6 Ma (early Eocene) old. Chemically، the rock suite is characterized by high total alkali، Fe/Mg، Ga/Al، Hf، REE (except for Eu)، SiO2 and Zr and low contents of Ba، CaO، MgO، P2O5 and Sr. These features along with various geochemical discrimination diagrams suggest that the Takht-e baz granitoid is post-collisional A-type (A2-type). This suite may be derived from melting of continental crust in an extensional setting after collision of the Lut and the Afgan continental blocks in the east of Iran.

The Malayer plutonic complex is located in the northern Sanandaj-Sirjan zone of the western Iran. An intrusive body with granodioritic component known as the Samen granodiorite is in the center of this complex، which is accompanied by other granitic rocks and small mafic bodies. The major part of this intrusive body composed of tonalitic-granodioritic rocks. The monzogranitic rocks، unlike the other plutonic complexes of the Sanandaj-Sirjan zone such as the Alvand pluton constitute a small part of this complex، and probably formed and emplaced after tonalite-granodiorite body. In terms of geochemical characteristics these rocks are calc-alkaline، peraluminous and S-type، while tonalitic-granodioritic rocks are metaluminous and I-type in nature. These granitoids on the spider diagrams show enrichment in LILE (K، Rb and Th) and depletion in HFSE (Nb، P and Ti). Also they are enriched in LREE rather than HREE، the characteristics which are usually observed in the rocks from subduction environments and active continental margins. The biotites in the granodiorites are siderophyllite، amphiboles and plagioclases are hornblende and andesine type. Barometric calculations yield pressures of 1-1. 2 kbar (equal to 3-4 km of depth) for intrusive rocks. Crystallization temperature of the granodioritic rocks in 1Kbar pressure is estimated to be 592. 2 to 756. 9 °C and in 1. 2 Kbar pressure around 591. 2 to 757. 6 °C.

The studied volcanic rocks، in the middle part of the Central Iranian volcanic belt (Urumieh-Dokhtar magmatic arc) belong to a vast magmatic province located in the north of the Bitlis-Zagros suture zone. Petrographically، these rocks show dacite to andesite composition and include phenocrysts of plagioclase، amphibole and biotite with a prevailing porphyritic texture in a matrix composing of feldspar، quartz، microlite and glass. Having a relatively invariable composition، plagioclases are all andesine. Amphiboles are calcic and tschermakite type. Estimation of temperature and pressure of crystallization resulted from graphically integrating amphibole and amphibole-plagioclase geothermo-barometry equations indicates that the amphibole-plagioclase phenocrysts were crystallized in the temperature range of 763-823 °C with average of 797 °C and in the pressure range of 2. 2-4. 5 kbar with the average of 3. 4 kbar. The maximum depth of crystallization is evaluated at 17 km in continental crust.

One of the characteristics of the Ordovician to the Devonian rock units in the north of Shahrood is the presence of high volumes of igneous rocks with basaltic composition as dyke and sill. The greatest volume of these rocks are widespread in Soltan Meydan Basaltic Complex with a thickness of about 1000 m as numerous basaltic lava flows، basaltic agglomerates، tuffs and some sedimentary intercalations which have been formed during the upper Ordovician to the early late Silurian. Active extensional tectonic prevailing on the area and violent detaching in continental lithosphere in this interval time led to sudden decompression on the rising mantle plumes، providing conditions for extensive partial melting and generation of high volumes of basaltic magma. Geochemical evidences show that the produced magmas have transitional to alkaline nature and generated from a rising enriched mantle plume source in an extensional intracontinental rift setting. The nature and tectonic setting of this basaltic magmatism during the lower-middle Paleozoic indicate continuation of long lived extensional movements related to the earlier stages of Paleotethys rifting in the north of Iran.

The Sahand volcano is located in the NW of Iran. The volcano is stratovolcano and is dominated by pyroclastic materials and lava flows in the Miocene-Quaternary. The last eruptions included subvolcanic and volcanic domes with dacitic to rhyolitic composition، which are studied in this paper as the young Sahand. These rocks have petrological features including sieve texture and zoning in plagioclases. Common features in the young subvolcanic and volcanic rocks of the Sahand are abundance of plagioclase and amphibole، enrichment of Ba، Rb and Sr and negative Nb and Y anomalies in primitive mantle normalized multi element diagrams and enrichment of LREE relative to HREE in chondrite-normalized REE patterns. These characteristics and the location of the rocks studied in the Y-Sr/Y diagram implying adakitic composition for these rocks. Adakitic rocks of the young Sahand are high SiO2 Adakitic rocks. These rocks have been derived from partial melts of garnet-bearing crustal sources (thickened lower continental crust or oceanic crust) and underwent magmatic evolution during their ascent. The formation of the studied rocks have been related to the Arabian-Eurasia collision zone.