Geochemistry and Thermobarometry of Bazman volcanic rocks, An example of active continental margin volcano

  Bazman volcano is located in the Makran-Chaghi magmatic arc in southeastern Iran. Many studies have been done on this volcano, especially of petrology such as: Biabangard and Mollazadehbarvati, 2017; Berberian and Berberian, 1982; Mohammadi, 2005; Sahandi et al., 2010; Schmidt and Dabiri, 2019; Firoozkoohi, et al., 2017; Ghodsi et al., 2006; Glamghash et al., 2015; Karimpour et al., 2012; Stocklin, 1977; Richards et al., 2012. In this article we study the origin and tectonomagmatic environment of Bazman volcano with by geochemical data of whole rock and mineral chemistry.   MATERIALS AND METHODS We were selected 150 samples from of Bazman volcano. Then prepared 100 thin section and selected 15 samples with the least alteration for analyses. Samples were analyzed by XRF and ICP-MS methods at Yamagatai University in Japan. In order to study the chemistry of minerals in Bazman volcanic rocks, 18 points of plagioclase minerals, 12 points of pyroxene minerals and 7 points of hornblende minerals were analyzed by electron microprobe.   RESEARCH FINDINGS Bazman volcano is composed of andesite, basalte, dacite and rhyodacite lavas and similar pyroclastic flows. These rocks have mainly plagioclase, pyroxene, olivine, hornblende, biotite, and quartz minerals These rocks have disequilibrium textures which indicates an unstable state in the magmatic chamber.   Lavas in Bazman volcano show calc-alkaline to slightly tholeiitic trend. All rock samples of Bazman volcano have a weak negative anomaly in Eu, which is probably due to plagioclase differentiation or high oxygen fugacity. Based on Electron microprobe analyses, pyroxene, hornblende and plagioclase minerals have diopside, augite, hornbelend and Labradorite to Bytonite. Temperature-barometry based on these minerals showed that Bazman volcanic rocks were formed at 800 to 1200 ° C and a least 2 Kbar pressure and high oxygen fugacity.   DISCUSSION Andesite rocks are one of the most common rocks in the continental margin, they are composed of plagioclase, biotite and hornbelend minerals. Unequilibrium textures such as Zoning, resorption, corrosion gulf, and sieve texture are common in these minerals. These evidences are found in the Bazman volcanic. Orogenic andesite shows enrichment of LIL elements relative to HFS elements. The pattern of normalized multi-element diagrams of Bazman rocks compared to the primary mantle has a relative enrichment of LIL elements relative to HFS elements and a negative anomaly in Ti and P elements. Also, pattern of rare earth elements normalized to Chondrite shows a relative enrichment of LRE elements relative to HRE elements with negative Eu anomaly. Thermo-barometry on minerals in Bazman andesite's indicates that lava formed between 800 to 1200°C temperature and high fugacity. Minerals and whole rock geochemical evidences show that Bazman volcano is dependent to subduction environments and it seems formed by subduction of the Indian oceanic crust under the Iranian plate.   CONCLUSION Bazman volcano is Stratovolcano that composed of andesite, basalt, andesite-basalt, dacite and rhyolite lavas with their pyroclastic equivalents, ignimbrite and tuff flows. The rocks of this volcano are calc-alkaline to slightly tholeiitic magmatic series and belonging to the active continental margin. These rocks have relative enrichment and negative anomalies in the elements, Nb and Ti, relative to the HFS elements. They are enriched in LRE elements relative to HRE elements and all rock samples show weak negative anomalies in Eu. Electron microprobe data on the plagioclase, pyroxene and hornblende minerals show that are labradorite to bitonite hyperstene and augite, and magnesium-chandelier, respectively. Thermo-barometry based on these minerals shows that Bazman volcanic rocks were formed between 800 and 1200 °C and least 2Kbar pressure and high oxygen fugacity. Bazman volcano was formed by subduction of the Oman oceanic crust under the central Iran.