نشریه پترولوژی
پیاپی 37 (بهار 1398)

Abstract Abstract The study area is located in 10 km east of Khousf (southwest of Birjand).This area is situated in the northwest margin of suture zone and it’s boundary with Lut block. The area comprises outcrops of Eocene- Oligocene volcanics with dacite, rhyodacite, andesite, trachy andesite, microdiorite, basaltic andesite composition and pyroclastic rocks (tuff, bereccia and agglomerate) which cross- cut of the upper Cretaceous ophiolitic melange and Eocene flysch. The common texture in these rocks is porphyritic with microgranular matrix, glomerophorphyritic and poikilitic Plagioclase, amphibole, biotite and pyroxene are the main phenocrysts in the andesites. Plagioclase, biotite, amphibole and quartz are the main minerals in dacitic rocks. There is also fine crystals of sanidine in rhyodacite and trachy andesites.. Presence of amphibolittic and pelitic xenolites are very common in these rocks. Signs of magma contamination and mixing, such as sieve texture, zoning in minerals, marginization, compaction, corrosion and dusty margins are observed in these units. These rocks belong to the medium to high potassium calc-alkaline series. The trace and REE patterns normalized to primary mantle and chondrite show these volcanics are co- magmatic and high proportion of LREE / HREE in the calc-alkaline rocks represents a subduction-related environment and active continental margin. Geochemical evidence show that these volanics have adakitic nature. Based on petrogenetic diagrams the mentioned adakites resulted from crystallization of melts originated from partial melting metasomatized mantle wedge above subduction zone and or garnet amphibolitic source resulting of metamorphism of thicked lower crust.

basaltic rocks in Barzand area (Continue to Kousmalyn basaltic rocks in Iran) show parallel trend with Poshtasar basalt and they have older age according to stratigraphy in comparison with Poshtasar basalts. These basalts whit prismatic structure mostly have mugearitic composition and show hyalomicroporphyritic and glomeroporphyritic textures. The main minerals are Ca-rich plagioclase and diopsidic clinopyroxenes. Leucites generally have been altered to analcime. The parent magma of these rocks has a potassic and shoshonitic nature. In multi- element diagrams Ta, Ti and Nb show depletion and LILE, LREE have enrichment that suggests their primary magma have been originated from a subduction related mantle source. According to radiogenic isotopic ratios, mantle source of Barzand basaltic rocks have been drived from a fertilized mantle source during 0.8 to 0.9 billion years ago. According to initial 87Sr/86Sr (0.70499 ± 0.0004) and initial 143Nd/144Nd ratios (0.5127 ± 0.0004) estimated an age for studied rocks varies between 39±1.7 to 41±1.5 million years ago. Geochemical evidence show the Barzand basaltic rocks primary magma originate from 5% partial melting of a phlogopite-beaing spinel garnet lherzolite mantle while the Poshtasar basaltic rocks from more than 5% -10% partial melting of a phlogopite-beaing spinel garnet lherzolite mantle. These rocks are related to a subduction zone mantle source that metasomatized by subduction related agents that mainly originated from turbditic sediments.The Barzand basaltic rocks Has geochemical characteristics common features with younger Poshtasar basaltic rocks and they have been erupted in a Post collisional arcs tectonic setting.

Volcanic rocks are observed in Ebrahim abad area, southwest of Gazik and in southern Khorasan province that located in northern part of the Sistan suture zone. The studied lavas consist of basaltic andesite, andesite and dacite with Eocene to Miocene age accompain pyroclastic rocks such as tuff. Main minerals in basaltic andesite include of plagioclase, pyroxene and olivine, in andesite consists of plagioclase, hornblende and pyroxene and in dacites composed of plagioclas, quartz and hornblende. Intermediate volcanic rocks have porphyritic with microlitic groundmass, megaporphyritic, glomeroporphyritic and poikilitic textures and common texture in dacite is porphyritic with microgranular groundmass. These rocks show disequilibrium evidences such as zoning in plagioclases, sieve texture and embayment in some minerals. Geochemical studies indicate that the lavas of Ebrahim abad area belong to calc alkaline series. These rocks have enrichment in LREE relative to HREE. The high value of LREE and large ion lithophile elements (LILE) relative to HFSE and negative anomaly in Nb and Ti, represent a subduction tectonic environment and active continental margin. The original magma of these lavas probably derived from partial melting of spinel- garnet lherzolite mantle that enriched by subducted slab fluids.

Basaltic rocks from Marivan-Palangan ophiolite suite (MPO), Zagros suture zone, western Iran, are studied for magmatic evolution, and source rock partial melting processes. The basaltic rocks have remarkable outcrops in the crustal sequence of the MPO and show pillow structure. In terms of microscopic and chemical characteristics, two distinct rock groups are characterized. Based on chemical classification, both groups are basalt. CIPW norm calculations indicate that all basalts are olivine tholeiite (abyssal tholeiite). In terms of the magma series, they also show tholeiitic affinity, somehow close to transition series. Besides, based on the TiO2 content, they belong to high Ti-type ophiolite. Fractionation of varying amounts of olivine, plagioclase, and clinopyroxene has been inferred from Fenner's diagram, especially for group II samples. Although the samples from both groups are similar to E-MORB in multi-elemental diagrams, the show remarkable distinction in HREE and MREE. Melting models show the various degrees of partial melting from a spinel lherzolite source rock (5-25% for the group I and 15-25% for group II). Due to similarity of Sm/Nd, Ce/Sm, Sm/Yb and La/Yb ratios of both groups to enriched mantle type II (EMII) ratios, contribution of EMII-type components to the source of magma is thought to be responsible for the enriched nature of basalts. It is proposed here that variable degrees of partial melting of a spinel-facies peridotite source and contribution of EMII-type components to the source region controlled enriched nature of the ophiolitic basalts of this part of the Neo-Tethyan realm.

In the Mahabad region accompaniment of trachytes and trachy-andesites with OIB-like alkali-basalts are shown. Trachytes at the Soltan Mountain heights, trachy-andesites as darker enclaves in it and alkali-basalts in the three sites, Khatton Bagh, Bardarashan and Badam are outcropped. Alkali-basalts have shoshonitic, met-aluminous and OIB-like characteristics with LREE/HREE enrichment and generate at deep garnet peridotitic source. Trachytes and its intermediate trachy-andesite enclaves at the Sltan Mountain with shoshonitic, per-aluminous affinity have high LREE/HREE ratios with HFSE and Ti depletion and continental arc relevant signatures. The Soltan Mountain trachytes have adakitic nature which accompany with the High niobium basaltic (HNB) affinity of Badam, Barderashan and Khatoon Bagh basalts of Mahabad and their similarities to Ghorveh HNB could clarify the relationship between adakites and basalts along the Neo-Tethys post collisional activities. Computer modelling clarify the fractional crystallization opportunity for the intermediate to acidic terms production from mantle basaltic liquid. In addition, very high contents of incompatible elements in the most evolved rocks suggest the occurrence of open-system magmatic processes. Continuous fractional crystallization from a primitive basaltic source, similar to post-plateau coeval basalts, towards acidic terms combined with small rates of assimilation (AFC) in a shallow magmatic reservoir, is best explains for the young volcanic association in the Mahabad. These phenomena are occurred after Neo-Tethys closure at the Quaternary.

Sabalan volcano is located in the northwest part of Iran and 25km south of Meshkinshahr in the west Alborz-Azerbaijan structural zone. The rocks forming the Sabalan and its eastern and northern slopes are covered with significant amounts of white-gray pyroclastic deposits, which is indicating a very intense explosive activity at the last stage of this huge volcano activity. After conducting field studies and analyzing the results obtained using the mathematical calculations, a Plinian eruption was detected, and the extrapolated volume of the latest pyroclastic fall eruption was estimated 3.24 km3, and the eruption of Sabalan rated a VEI of 5. Also, due to the shape of the isopach curve, the role of wind in dispersing these deposits is from the west to the east. Also based on geochemical data, the Sabalan’s ignimbrite was rhyodacitic with calc-alkaline signatures and the high ratios of Ba/Nb>28 and Ba/Ta>450 are consistent with subduction related zone.