PETROLOGY
Year:2019 | Volume:10 | Issue:37|Papers Count:6

The study area is located 10 km east of Khousf (southwest of Birjand), in the northwestern margin of Lut block and it’ s boundary with Sistan suture zone. Eocene-Oligocene volcanics (dacite, rhyodacite, andesite, trachy andesite, and basaltic andesite), subvolcanic rocks (microdiorite) and pyroclastics (tuff, bereccia and agglomerate) are observed in this area crosscutting the upper Cretaceous ophiolitic melange and Eocene flysch. Mineralogically, the andesite is dominated by the presence of poikilitic plagioclase, hornblende, biotite and pyroxene with porphyritic and microgranular groundmass, glomerophorphyritic and poikilitic textures. Quartz, plagioclase, hornblende, biotite and sometimes pyroxene are the essential minerals in the dacites and fine crystals of sanidine in the trachy andesites and the rhyodacite. Amphibolitic and pelitic xenolites are common in the investigated rocks. Disequilibium evidences such as sieve texture, zoning, resorption rim, rounding, corrosion and dusty margins are observed in these units. The studied rocks belong to the medium to high potassium calc-alkaline series. The trace and REE patterns normalized to primitive mantle and chondrite show these volcanics are co-magmatic and have high LREE / HREE that represents a subduction-related environment and active continental margin. Geochemical evidences demonstrate that these volanics have high SiO2 adakitic nature. Based on the petrogenetic diagrams the rocks under study generated from partial melting of metasomatized mantle wedge and or garnet amphibolitic source resulting of thickened lower crust.

The Alborz-Azerbaijan magmatic zone in northwest of Iran, particularly in the Talysh zone is the important volcanic zone during the Cenozoic period. The south Germi basaltic rocks (Ardabil province), trending northwest-southweast, exposed as several outcrops. The studied basaltic rocks in Barzand area (the continuation of Kousmalyn basaltic rocks in Iran) are in parallel trend with Poshtasar basalts, but they, according to stratigraphic study, are older. The studied basalts with prismatic structure mostly have mugearitic composition and show hyalomicroporphyritic and glomeroporphyritic textures. The main minerals are Ca-rich plagioclase and diopsidic clinopyroxenes. A lesser amount of opaque minerals accompanied by microlitic sanidine, pargasitic amphibole, biotite and leucite can be seen in these rocks. Leucites generally have been altered to analcime. The parent magma of these rocks has a potassic and shoshonitic nature. On multi-element diagrams Ta, Ti and Nb show depletion and LILE, LREE are enriched suggesting their primary magma have been originated from a subduction related mantle source. According to radiogenic isotopic ratios, the mantle source of the Barzand basaltic rocks have been derived 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), it is estimated that the age of the studied rocks varies from 39± 1. 7 to 41± 1. 5 million years. Geochemical evidences show the primary magma of the Barzand basaltic rocks was originated from 5% partial melting of a phlogopite-beaing spinel garnet lherzolite mantle while the Poshtasar basaltic rocks from more than 5% 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 generated from turbiditic sediments. The Barzand basaltic rocks display common features with those of the younger Poshtasar basaltic rocks, indicating they all have been erupted in a Post collisional arc 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 accompanied by pyroclastic rocks such as tuff. The main minerals in the basaltic andesite include of plagioclase, pyroxene and olivine, in the andesite consists of plagioclase, hornblende and pyroxene and in the dacites composed of plagioclase, 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 are enriched in LREE relative to HREE. The high value of LREE and large ion lithophile elements (LILE) relative to that of HFSE and negative Nb and Ti, anomaly 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.

Magmatic evolution, partial melting and petrogenesis of the basaltic rocks from Marivan-Palangan ophiolite suite, Zagros suture zone, western Iran have been studied. Basaltic rocks mostly exhibit pillow structures and have remarkable outcrops in the crustal succession of the Marivan-Palangan ophiolite. Based on petrographic and geochemical criteria, two distinct rock types are recognized. Regarding the chemical classifications, both types are basalt. The results of CIPW norm calculations reveal their olivine tholeiitic (abyssal tholeiitic) nature. They also belong to the tholeiitic magma series close to the transitional type. The high Ti contents of the basalts are indicative of the High-Ti affinity of the ophiolites. Fractionation of variable amounts of olivine, plagioclase and clinopyroxene is inferred from variation diagrams, particularly for the type two rocks. Although both rock types resemble the E-MORB on the multi-element diagrams, they are remarkably distinct regarding the HREE and MREE contents. Partial melting models suggest that the basalts have been originated after 5-25% (type 1) and 15-25% (type 2) partial melting of a spinel lherzolite mantle source. Besides, Sm/Nd, Ce/Sm, Sm/Yb and La/Yb ratios of the samples are close to an EMII-type enriched mantle; that may be indicative of contribution of EMII-type components to the petrogenesis of the magmas. It is concluded in this research that variable degrees of partial melting of a spinel peridotite mantle source, combined with the contribution of EMII-type components generated the enriched nature of the ophiolite-related basalts in 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 occurred after Neo-Tethys closure at the Quaternary.

Sabalan volcano is located in the northwest part of Iran, 25km south of Meshkinshahr in the west Alborz-Azerbaijan structural zone. The eastern and the northern slopes of Sabalan are predominantly covered with significant amounts of white-gray pyroclastic deposits 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 is significant in dispersing these deposits from the west to the east. The overall geochemical features of the Sabalan’ s ignimbrite including the rhyodacitic nature with calc-alkaline signatures as well as the high ratios of Ba/Nb>28 and Ba/Ta>450 are consistent with subduction related zone.