Chemistry of mafic minerals and thermobarometry of Bazman Quaternary volcanic rocks

  Abstract 1-Introduction Bazman volcano is located on Chgay - Makran magmatic arc. This magmatic arc with east-west trend is 500 km long and 150 km wide and extends from southeastern Iran to southwestern Pakistan. Early evolution of the Makran zone from the upper Oligocene to the upper Miocene is characterized by turbidite sediments deposited on the oceanic crust. There are several quaternary volcanoes such as Bazman, Taftan, Shahsavaran and Soltan (southern Pakistan) which situated along northward of Makran subduction zone as continental arc magmatism. Bazman volcano as an astratovolcano have been explosive and non-explosive eruptions in Quaternary. The purpose of this paper is to investigation of mafic minerals chemistry and thermobarometry of Bazman Quaternary volcanic rocks.   2-Materials and Methods Based on field studies, its volcanic rocks are classified into two groups Quaternary and Neogene volcanic rocks. Petrographic studies were conducted with the preparation of thin sections of rock and were named rocks. Three samples Quaternary units (Qa1, Qa2, Qa3) were selected for microprobe analysis.  Mineral analyses were conducted at Iran Minerals Processing Research Center Electron Microprobe Laboratory using a Cameca SX100 electron probe microanalyzer outfitted with combined WDS and EDS systems. The analyses were conducted at an accelerating voltage of 15 kV and a beam current of 10 nA.   3-Results and discussion  According to Petrographic studies, unit Qa1 was formed of  andesite, andesitic basalt, basaltic andesite and andesite lithic crystal tuff, unit Qa2, andesite to trachyte andesite and basaltic andesite, unit Qa3,andesitic basalt, olivine basalt and basaltic andesite. 3 sample basaltic andesite and olivine basalt Quaternary units were selected for mafic mineral microprobe analysis. Olivine phenocrysts of olivine basalt are chrysolite mineral. Clinopyroxenes are augite and orthopryoxene are clinoenstatite. Amphibole minerals are hornblende, magnesio-hornblende, hastingsite hornblende, tschermakite hornblende and tschermakite. Pyroxene composition in igneous rock depends on the chemical composition and tectonic setting of the host lava which can be used widely to determine the series of magmatic and physicochemical conditions such as pressure, temperature, oxygen fugacity. Ca+Na vs Ti diagram define that the pyroxenes occur in the range of tholeiitic calc-alkaline. Al2O3 vs TiO2 diagram was drawn to identify the nature of magma. This diagram  shows orthopyroxene of both the Quaternary units and clinopyroxene unit Qa1 calc-alkaline ranges and clinopyroxene unit Qa2 tholeiitic tendency. The chemical composition of the samples pyroxenes evaluates the physicochemical conditions of magma such as pressure, temperature, oxygen fugacity.Using Al IV + Na vs Al IV + 2Ti + Cr diagram which depend on the amount of 3-valent iron in pyroxenes, we can get oxygen fugacity. The diagram is set based on the aluminum balance in the tetrahedral position and Cr3+in the octahedral position. The Fe3+ in pyroxenes can be displaced 3-valence elements such as AlVI, Ti and Cr in the octahedral position. In the other hand, Fe3+in pyroxenes depends on the amount of AlVI which means that it depends on the aluminum balance in tetrahedral and octahedral position. The pyroxenes which crystallized at high oxygen fugacity, has been situated above the line of Fe3+. Furthermore, Papike and Cameron (1976) have mentioned the distances of the samples from the Fe3+ line and noted that further distances of the samples from this line were indicating more oxygen frugalities in their geological setting. In this diagram samples are located above the line of Fe3+. In order to study thermodynamic conditions pyroxene crystallization is used methods Soesoo (1997) and Patrikia (2008). By Soesoo (1997)  clinopyroxene samples operating temperature range 1200c°-1300c° and comprised approximately at 1250 c°. Orthopyroxene samples are composed at temperatures from 1150c°-1200 c°. Accordingly, the pressure range of clinopyroxenes is 6-10 kbar and Orthopyroxenes 2-5 kbar. By Patrkia (2008) orthopyroxenes crystallization temperature  Qa1 unit is 1124c° and clinopyroxene and  1145c°  and the setting pressure at the orthopyroxene is 5 Kbar and clinopyroxene 8 Kbar. Orthopyroxenes crystallization temperature  Qa2 unit is 1068 c° and clinopyroxene and  1100 c°  and the setting pressure at the orthopyroxene is 2.2 Kbar and clinopyroxene 4.3 Kbar. According to both methods clinopyroxene crystallization, temperature is higher than orthopyroxene in  Quaternary units. Temperature and pressure of the magmatic Qa1 little more than Qa2. Reduce pressure and temperature conditions of crystallization orthopyroxene can be attributed to an increase in the amount of iron in the crystal lattice orthopyroxene. The temperature and pressure determined on the basis of the mineral olivine  in olivine basalt rock (Qa3 unit) are about 1100 °C and 5.5 Kbar.   4-Conclusion 3 sample Quaternary units were selected for microprobe analysis. According to the pyroxenes chemical composition in basaltic andesite rock, orthopyroxene of both the Quaternary units and clinopyroxene unit Qa1 are calc-alkaline ranges and clinopyroxene unit Qa2 is tholeiitic tendency. The pyroxenes which crystallized at high oxygen fugacity. Clinopyroxene crystallization temperature is higher than orthopyroxene in  Quaternary units. Temperature and pressure of the magmatic Qa1 little more than Qa2. Reduce pressure and temperature conditions of crystallization orthopyroxene can be attributed to an increase in the amount of iron in the crystal lattice orthopyroxene. It could be related to tholeiite conditions magma before crustal contamination. The average pressure and temperature of the Quaternary magma chamber based on mafic minerals can be estimated at 1141 °C and 5.5 Kbar.