Study of Petrography and petrogenesis of Monavvar area Spessartite dykes (East Azerbaijan Province)

Many lamprophyric dykes' outcrops are found in Azerbaijan (in the northwest of Iran). These dykes which were the subject of many studies are including camptonite dykes in Misho Mountain, kersantite dykes of Goye-Poshti Mountain of Maragheh, camptonite and sannaite dykes in Horand, minette dykes of Varzeghan, minette dykes of Marand, minette dykes of Khoy, and minette dykes of Saray volcano.For the first time, Amel (1994) reported the occurrence of lamprophyre in the Monavvar region. According to him, this lamprophyre is spessartite and has Calc-alkaline affinities. In this study, we performed a detailed petrographic study of this lamprophyre. Besides, by using clinopyroxene mineral chemistry and whole rock chemistry, we try to investigate the petrogenesis of these lamprophyres from different aspects.General geologyMonavvar region is located in the east Azerbaijan province of Iran. Monavvar region is a part of the Alborz-Azarbaijan zone. Field observations show two spessartite dykes intruded in the andesitic lavas of the studied region. The age of andesitic lava and spessartite dykes is Plio-Quaternary because the andesitic lava intruded in Pliocene pyroclastic lava. Spessartite has a blackish-brown color in the hand specimens.PetrographyThe main petrographic texture of these lamprophyres is the Porphyry texture. The major minerals are plagioclase microliths (10-15 volume %), orthoclase (5-10 volume %), hornblende phenocrysts with burnt rim (40-50 volume %), clinopyroxene (>20 volume %), and biotite (10-15 volume %). The accessory minerals include zircon, sphene, and apatite.The plagioclase has higher content than orthoclase and both of these minerals could be seen only as microlith. Regarding the nomenclature scheme of Le Maître (2002), these features indicate that these lamprophyres are spessartite.Mineral ChemistryThe mineral chemistry of amphibole shows a magnesio-hastingsite composition. However, biotite is phlogopite-eastonite and feldspars are orthoclase and oligoclase in composition.DiscussionMineral chemistry of clinopyroxene studiesThe clinopyroxenes are in the field of Quad in the Q-J diagram and diopside in the En-Fs-Wo diagram.According to the AlVI+2Ti+Cr-AlIV+Na diagram for clinopyroxenes, Monavvar spessartite has occurred in almost the stable and low oxygen fugacity status. Based on Soesoo (1997), the clinopyroxenes were crystalized under 1100-1200 ℃ and 2-6 kbar. The chemical composition of clinopyroxenes indicates subduction-related volcan