مینرال‌شیمی کانی‌ بیوتیت، رهیافتی بر پتروژنز آتشفشان سارای، شمال‌غرب ایران

IntroductionBiotite is one of the most abundant ferromagnesian hydrous minerals in igneous rocks and could be crystalized under a wide range of temperature-pressure situations (Spear 1984). Biotite mineral chemistry can reveal the nature of magma and tectonic setting of parent igneous rocks (Abdel-Fattah, 1994&1996). Saray volcanic-intrusive complex is located east cost of Urmia Lake and according to Aghanabati (2004) it belongs to central Iran structural zone. Previous studies indicate that Saray magmatism has shoshonitic to ultrapotassic affinities and according to Moine vaziri (1991) the lowest layer of Saray volcano is 7.8 million years old. Materials and methodsSaray volcano majorly consists of Leucitic lavas and related pyroclasts with a 20 degrees slope. This sequence can be seen all over the volcano. After a probable inactive period, the second stage of its activity began by the eruption of more evolved trachytic magmas and related pyroclasts, which had very less volume than leucititic eruption. Lamprophyric dykes mainly minette, monchiquite and spessartite can be seen in the volcano. Trachytic and lamprophyric occur periodically and intrude each other. Emplacement of a syenitic stock in central Saray valley and intrusion of some syenitic dykes, are probably the last magmatic activities of Saray volcano. Result and DiscussionIn this study, we carried out 17 microprobe analyses of biotite minerals in trachytic, syenitic, minette and monchiquite rocks. According to Reider et al. (1998), in Saray volcano micas of minette and monchiquite are phlogopite and micas of trachyte and syenite are from biotite and phlogopite types. According to Nachit et al. (2005) the majority of studied biotites are categorized as re-equilibrated primary biotites. In ternary diagram proposed by Wones & Eugster (1963), which graphically show the position of main buffer reaction in biotite compositions, the majority of analyzed biotite spots, scatter around the HM buffer reaction line. By depicting the buffer reaction curve of analyzed biotites, we can see that many biotites of Saray volcano crystalized when the oxygen fugacity of magma was about -9. By investigating the chemical composition of studied biotites, it can be deduced that the only index which could separate Saray biotite meaningfully is Mg#. The Mg# of lamprophyre biotites is more than 0.9, in syenite it is between 0.8-0.9 and in trachyte it is around 0.7. This finding is consistent with petrological facts because biotites of the most evolved rocks in Saray volcano (Trachyte) have the least Mg# and lamprophyre has the highest Mg#. However chemical composition of biotite of Saray volcano shows that majority of them were formed in primary lamprophyric, except biotite of trachyte sample G5A, which was formed as fractional crystallization went on. Besides, considering the differences between the chemical composition of biotites of trachyte and syenite, it can be deduced that in Saray volcano, two or more trachyte-syenite reservoirs existed. ConclusionMica types of Saray volcano are biotite and phlogopite. The Mg# of lamprophyre biotites is more than 0.9, in syenite it is between 0.8-0.9 and in trachyte it is around 0.7 which is completely consistent with petrological facts. By applying indirect methods, oxygen fugacity of magma during biotite crystallization was found to be about -9. Comparing the chemical composition of biotites in different rock types of Saray volcano indicate that trachytic magma forms as a result of magmatic differentiation of lamprophyric magma. Some biotites were formed in parent lamprophric magma and some others were formed in trachytic magma after magmatic differentiation. Furthermore, the presence of two or more trachyte-syenite reservoirs in Saray volcano can be acceptable or it can be assumed that syenite reservoirs are roots of trachytic dykes and domes which were formed through weight differentiation.