زمین‌دمافشارسنجی و سرشت گدازه‌های حد واسط جنوب آتشفشان دماوند

دماوند آتشفشان چینه‌ای خاموشی در کمربند ماگمایی البرز در شمال ایران است که با مخروطی مرکب (بیش از 400 کیلومترمربع) روی پوستة نسبتاً ستبر رشته کوه البرز (58-67 کیلومتر) در فلات ایران جای ‏‌گرفته است و مرتفع‌ترین کوه (5671 متر) در خاورمیانه و جنوب آسیا به‌شمار می‌رود. تکاپوی‌های آتشفشان دماوند با خروج حجم‌های کوچک و مجزایی از روانه‌های مافیک (تفریت، بازانیت، تراکی بازالت و آلکالی‌الیوین‌بازالت‌) نزدیک به 1.8 میلیون سال پیش آغاز‏‌شده است و تا 600 هزار سال پیش با فوران‌ گدازه‌های حد واسط تا اسیدی (تراکی‌آندزیت، تراکی‌داسیت و تراکیت) ادامه یافته است. بر پایة تجزیه نقطه‌ای کانی‌های سازندة این گدازه‌ها، ترکیب پلاژیوکلاز (an31-58)، آندزین تا لابرادوریت و فلدسپار (or32-65) سانیدین و انورتوکلاز است، میکای فلوگوپیت (fe2+/(fe2++mg)<0.3)، کلینوپیروکسنِ اوژیت (wo42-45 en42-47 fs10-13) و دیوپسید (wo46-48en43-46fs8-10) و کانی‏‌های تیره تیتانومگنتیت است. با محاسبات زمین‌دمافشارسنجی، تبلور فلوگوپیت‌ها در دمای 843 تا 819 درجة سانتیگراد و فشار 6 تا 0.63 کیلوبار و کلینوپیروکسن‌ها در دمای نزدیک به 1200 درجة سانتیگراد و فشار 10 تا 6 کیلو بار روی داده است‏‌.

geothermobarometry and nature of intermediate lavas from the south of damavand volcano

damavand stratovolcano is in alborz magmatic belt (amb), in the north of iran, and the iranian plateau. this large dormant volcano was constructed with a composite large cone (more than 400 km2). damavand is known as the highest mountain (elevation ~5671 m) in the middle east and south asia. the youngest known eruptions of damavand volcano (7.3 ka), which mostly erupted on the western side of the summit, are composed of trachyandesite, and trachyte lavas with pyroclastic while the older eruptions (1.8-0.6 ma) consisting of alkali olivine basalts, tephrite basanite, and trachyandesite, emplaced at the base of the volcano. the current damavand cone is located above the old and eroded building and periodically includes trachyandesite- trachyte lavas with small eruptions of mafic lavas and pyroclastic rock. these lavas formed in the youngest geological period (quaternary) covered mesozoic deposits (shemshak and lar formations). the intensity of alborz volcanic activities extended into the tertiary, but was not uniform, as its maximum occurred in the late eocene and oligocene, and after a period, volcanic activities re-intensified in the pliocene.regional geologydamavand volcano is in the center of alborz mountain belt, in the iranian plateau. the stratigraphic evidence indicates that the volcanism occurred in the two phases the older (old damavand) and the younger sequences (younger damavand)analytical methodsafter initial petrographic studies, suitable samples were analyzed for whole-rock composition determined by xrf and icp-ms analyses at the geological survey of iran.mineral composition determined by epma (electron probe micro-analyser) at iran mineral processing research center. the analysis was performed at this center by the electronic microprocessor model cameca-sx 100 made by the french company cameca. this device is equipped with a spectrometer with an electron diode receiver and works automatically based on a high accuracy of 1% and the simultaneous operation of several diode detectors and electron beam stability with a carbon coating.petrography, minerals, and whole rocks chemistrythe analyzed samples roughly cluster in the two zones, mafic alkaline rocks (tephrite-basanite) and intermediate-felsic rocks (trachyandesite- trachyte). the main purpose of this study is the mineral chemistry of the ziar-lasen trachyandesites, with a sio2 content of 57wt%. the predominant mineral assemblages of these rocks are plagioclases (andesine-labradorite), k-feldspar (sanidine-anorthoclase), clinopyroxene (augite-diopside), mica (phlogopite, biotite), apatite, and ti-magnetite and the dominant textures are porphyry and microlithic porphyry textures with main phenocrysts. feldspars show signs of disequilibrium and sieve textures in their cores and rim and desorption and skeletal. these phenocrysts, which are sometimes as glomerocrysts, show zoning some mafic enclaves with variable textures and mineralogy are seen in the matrix of the trachyandesitic rocks.result and discussion volcanic activities in the middle part of central alborz mountain initiated about 2 million years ago with the eruption of mafic to intermediate-acidic lavas and pyroclastic rocks. the most common minerals of the trachyandesits from the south of damavand (polour to ziar-lasem) are felspar, pyroxene, mica, apatite, and opaque minerals., whereas the intermediate lavas are characterized by the presence of plagioclases (an31-58, andesine), alkali feldspars (or32-65, sanidine to anorthoclase), pyroxene, mostly augite (wo42-45 en42-47fs10-13) rare diopside (wo46-48 en43-46 fs8-10), mica phlogopite (fe2+/(fe2++mg) <0.3) as well as high mg # (67-76) and ti-magnetite.chemically, the clinopyroxenes are characterized by high mg # (97-76) and phlogopites by fe # <0.33, high ti (70-79), and high mg # (76-67) and so, the nature of host magma is sub-alkaline.on the base of thermobarometer data, the clinopyroxene and phlogopite phenocrysts from ziar-lasem trachyandesites crystallized at a wide range of temperature and pressure.the crystallization temperature for clinopyroxenes, ranges from 1180 to 1250 °c, at 6-10 kbars pressure (equi. to 22-36.5 km depth), and for mica varies from 819 to 843°c, at 0.1-1.14 kbars pressure (equal to 4-0.65 km depth.). base on clinopyroxenes compositions (mg#>80) the crystallization of these minerals was re-equilibrated with high mg# melts (50–54) (eskandari et al., 2018). the similarity of these values with those of basalts indicate their deep crystallization from more mafic magma (lanzafame et al., 2013). it is suggested that the clinopyroxenes of the intermediate volcanic products of ziar-lasen were crystallized at 22-36.5 km depth, approximately equivalent to lower crust (28–33 km), and evolved by assimilation, fractionation, and contamination when their parent magma erupted from deep depth to the surface or crystallized at shallow chambers within a thick crust.acknowledgmentthe author is grateful to the editor-in-chief and esteemed referees of the petrological journal for their useful constructive comments, to the research institute for earth sciences and to dr. karimi.