نوع مقاله : مقاله پژوهشی
نویسندگان
1 گروه زمین شناسی، دانشکده علوم زمین، دانشگاه شهید بهشتی، تهران، ایران
2 دانشگاه پیام نور
چکیده
کلیدواژهها
موضوعات
عنوان مقاله [English]
نویسندگان [English]
Introduction
Qezelje-Kand stratovolcano is located north of Sanandaj-Sirjan zone and in Tabriz-Hamadan volcanic belt. This volcano consists of a mixture of lava, bombs, scoria and ash, that several enclaves can be seen inside the lava and their ejecta. The felsic enclaves, can be seen in different sizes from a few millimeters to a few decimeters, and the mafic enclaves can be seen from a few millimeters to a few centimeters. Due to the mineralogical similarity with the host rocks, there is a debate about whether they are autoliths or xenoliths. In this research, mafic enclaves including phlogopite-pyroxenite and pyroxenite have been studied. The physical and chemical conditions of magma crystallization and magma origin were investigated using petrography, mineral chemistry and thermobarometry such as pyroxene, and phlogopite and comparing them with the host volcanic rock. This information has been used to determine whether the enclaves are autoliths or xenoliths.
Materials and Methods
After fieldwork and sampling, thin sections were prepared for petrographic studies. Then, two enclave samples (including phlogopite-pyroxenite and pyroxenite) and one host volcanic rock (alkali basalt) sample were selected for Electron microprobe analysis, and their pyroxene and phlogopite minerals were analyzed. Two enclave samples at Yamagata University in Japan have been analyzed by the WSD method by an automatic electronic processor (EPMA) model JEOL.JXA-8600 with a voltage of 20 kV and a current of 8-10 x 2 amperes. The diameter of the electron beam is 5 µm and the data concentration is calculated based on the ZAF computer program. Alkali basalt sample was analyzed in the laboratory of the Istituto di Geologia Ambientale e Geoingegneria (CNR, Roma). They used a Cameca SX50 instrument with five wavelength-dispersive spectrometers (WDS). with a voltage of 15 kV and using 1 μm beam diameter for pyroxenes and 10 µm beam for biotite.
Results and Discussion
The minerals of phlogopite-pyroxenites include clinopyroxene, phlogopite and apatite with cumulate texture. Pyroxenite enclaves contain clinopyroxene with a small amount of phlogopite with granular texture. Alkali basalts are composed of pyroxene, plagioclase, olivine, biotite, hornblende and Opaque minerals with porphyric texture. According to the results of the chemical analysis of the samples, the composition of pyroxenes of all three samples are calcium-magnesium-iron and Diopside type. Clinopyroxenes are placed in the tetrahedral range in the Al and Si distribution diagram. Mg# for the clinopyroxenites of all three samples is above 0.7, which indicates their crystallization from a primary magma. Al2O3 versus SiO2 and Ti versus Ca+Na graphs were used to determine the magmatic series, according to these two graphs, pyroxenes are in the alkaline to subalkaline area. According to the graph of Na+AlIV versus Cr+2Ti+AlVI, the samples were formed in the state of high oxygen fugacity. By using the value of AlIV versus AlVI, the amount of water pressure at the time of crystallization for clinopyroxene of the phlogopite-pyroxenite sample and the host volcanic rock is 10% and less than 10% for the pyroxenite sample. The graphs of TiO2 versus Alz and also TiO2 versus AlIV*100 were used to determine the tectonic environment of the host magma, and the samples show the same trend as the magmatic arc. The biotite of the phlogopite-pyroxenite sample and the host volcanic rock are of the phlogopite type and are re- equilibrate mica in terms of their origin. In various diagrams drawn based on the chemistry of minerals, the position of clinopyroxenes of the phlogopite-pyroxenite sample is the same as the host volcanic rock, but they are not the same as the clinopyroxenites of the pyroxenite enclave sample. Two methods of clinopyroxene-melt and single-clinopyroxene were used for thermobarometry. The phlogopite-pyroxenite and alkali basalt enclaves were in equilibrium with the host magma whole rock. The calculated temperature for the phlogopite-pyroxenite enclave is 1149-1245 and for the alkali basalt is 1190-1238ºC and the pressure calculated for them is 4.4-14.98 and 7-13.83 kb, respectively.
Conclusion
Based on the chemistry of the clinopyroxene crystals in the enclaves and the host rock, the composition of the magma is alkaline to sub-alkaline, it corresponds to the conditions of high oxygen fugacity and they came from a subduction-related tectonic origin. The results of thermobarometry show that their Magma Chamber is formed in the crust. According to the petrographic and geochemical evidence, it can be said that the phlogopite-pyroxenite sample is an autolith and is formed from the host magma, and the pyroxenite sample is probably a piece of mantle xenolith that was removed along with the eruption and reached the earth's surface.
کلیدواژهها [English]
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