پديد آورندگان :
مهوري، رضوان نويسنده دانشكده علوم,گروه زمين شناسي,دانشگاه اصفهان,ايران Mehvari, Rezvan , نقره ئيان، موسي نويسنده دانشكده علوم,گروه زمين شناسي,دانشگاه اصفهان,ايران Noghreyan, Moussa , شريفي، مرتضي نويسنده دانشكده علوم,گروه زمين شناسي,دانشگاه اصفهان,ايران Sharifi, Mortaza , مكي زاده، محمد علي نويسنده دانشكده علوم,گروه زمين شناسي,دانشگاه اصفهان,ايران Mackizadeh, Mohammad Ali , طباطبايي، سيدحسن نويسنده دانشكده مهندسي معدن,دانشگاه صنعتي اصفهان,ايران Tabatabaei, Seyed Hassan , ترابي، قدرت نويسنده دانشكده علوم,گروه زمين شناسي,دانشگاه اصفهان,ايران Torabi, Ghodrat
چكيده لاتين :
Introduction
The Nabar area that is a part of the Urumieh Dokhtar volcano plutonic belt is located in the south of Kashan. Research works such as Emami (Emami, 1993) and Abbasi (Abbasi, 2012) have been done about the geology of this area.
Rock units in the study area contain middle upper Eocene intermediate to acidic lavas and pyroclastic rocks, green marl, shale and sandy marls of Oligo Miocene, limestones of Qom formation, intrusive granitoids with Oligo Miocene age and quaternary travertine and recent alluvium (Emami, 1993). The volcanic and sub volcanic rocks of this area are composed of andesite, trachyandesite, dacite, rhyolite and porphyric pyroxene diorite along with pyroclastic rocks.
Materials and methods
In order to achieve the aims of this work, at first field surveying and sampling were done. Then, thin and polished thin sections were prepared. Some of the samples were selected for microprobe analysis and clinopyroxene minerals were analyzed by using JEOL JXA8800 analyzer with a voltage of 20 Kv and a current of 12 nA in the Kanazava University of Japan and CamecaSx100 analyzer with a voltage of 15 Kv and a current of 15 nA in the Iranian mineral processing research center, Karaj.
Discussion
On the basis of petrographic investigations, porphyritic, porphyroid, fluidal, amygdaloidal and porphyry with microlitic groundmass are common textures of these rocks. Also plagioclase, clinopyroxene, amphibole, biotite, sanidine and quartz are essential minerals, opaque, zircon and apatite as accessory minerals are observed in the studied rocks. Clinopyroxenes are observed with corona texture that resulted during the uralitization process. On the basis of minerals’ chemistry, pyroxenes are Fe Mg Ca type in composition (Morimoto et al., 1988). These clinopyroxenes are augite. Investigations indicate that mineral composition of clinopyroxene can be effectively used to evaluation the PT conditions during crystallization. Previous research works have proposed several methods such as Soesoo (Soesoo, 1997) and Putrika (Putrika, 2008). Thermobarometric studies of clinopyroxenes reveal that Nabar rocks were formed at temperatures of 900 1200 ˚C and the pressure of 25 kbar. According to the Aoki and Shiba (Aoki and Shiba, 1973) and Helz (Helz, 1973) approaches, the pyroxenes of the studied rocks are in a range of low to medium pressure that shows crystallization of clinopyroxenes during ascending of magma in different depths. Also according to Helz (Helz, 1973), the water vapor content in the crystallization of clinopyroxenes is more than 10 percent. Using AlIV +Na versus AlIV + 2Ti + Cr diagram which depends on the amount of ferric iron in pyroxenes, we can get oxygen fugacity. Based on this diagram, the pyroxenes which crystalized at high oxygen fugacity, has been situated above the line of Fe3+. Furthermore, Cameron and Papike (Cameron and Papike, 1981) have mentioned to the distances of the samples from the Fe3+ line and noted that further distances of the samples from this line are indicating more oxygen fugacities in their geological setting. On the basis of this diagram samples were located above the line of Fe3+and these rocks are formed in high oxygen fugacities. Pyroxene composition depends on the chemical composition and tectonic setting of the host lava which can be used widely to determine tectonic setting of the rocks. On the basis of approaches of Le Bas (Le Bas, 1962) and Sun and Bertrand (Sun and Bertrand, 1991), the chemical composition of clinopyroxenes shows that the studied rocks are related to calcalkaline series and orogenic settings.
Results
On the basis of mineral chemistry, pyroxenes are FeMgCa type in composition. These clinopyroxenes are augite. Thermometric studies of clinopyroxenes reveal that Nabar rocks are formed at temperatures of 9001200 ˚C. According to the distribution of aluminum in clinopyroxenes, these minerals were formed at 25 k bar pressure and water vapor content of more than 10 percent. Therefore, pyroxenes of the Nabar rocks are in a range of low to medium pressure that shows crystallization of clinopyroxenes during ascending of magma in different depths. Moreover, the volcanic rocks in Nabar were formed in high oxygen fugacity. The chemical composition of clinopyroxenes reveals that these rocks are related to calcalkaline series and orogenic settings.
Acknowledgments
The authors wish to thank the University of Isfahan for their financial supports.
References
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