نوع مقاله : مقاله پژوهشی

نویسنده

پژوهشکده علوم زمین

چکیده

گدازه‌های بازیک کواترنری گریک (منتهی‌الیه بخش شمالی آذربایجان غربی)، در گستره‌ای با ضخامتی بیش از 30 متر، حاصلی از جوان‌ترین تکاپوهای آتشفشانی آرارات کوچک در خاور ترکیه است. این آتشفشان در فلات ایرانی‌ـ ترکی و در نزدیکی مرز ایران و ترکیه و ارمنستان قرار دارد که متأثر از برخورد قاره‌ـ قاره عربی اورازیایی است. شیمی همه نمونه‌ها با ویژگی‌هایی مانند نسبت‌های La/Nb بالا و LILE افزایش‌یافته تأثیر مؤلفه فرورانش را در ماگمای سازنده گدازه‌های کواترنری منطقه نشان می‌دهد. کلینوپیروکسن، الیوین، و پلاژیوکلاز کانی‌های تشکیل‌دهنده این گدازه‌ها هستند. با مطالعه شیمی‌کانی، مشخص شد کلینوپیروکسن (میانگین ترکیبی (Wo=0.38.5,En=0.51, Fs=12.9 ترکیب دیوپسید دارد. منطقه‌بندی شیمیایی عادی و نوسانی بلورهای کلینوپیروکسن بر تأثیر تحولات ماگمایی منطقه و شرایط متفاوت اکسیداسیون و فوگاسیته اکسیژن دلالت دارد. با بررسی‌ ترموبارومتری این کانی‌ها، تبلور پیروکسن در گستره دمایی 1200 درجه سانتی‌گراد (مقدار < AlVI 02/0 اتم در ساختار کلینوپیروکسن) و فشار 8 تا 11 کیلوبار مشخص شده است. درصد اکسید آهن فریک (1/0 تا 9/4) گریزندگی اکسیژن در منشأ ماگمایی گوشته لیتوسفری با سرشت نیمه‌آلکالن را نشان می‌دهد که در محیط فشارشی کلی با کشش‌های موضعی فوران یافته است.

کلیدواژه‌ها

موضوعات

عنوان مقاله [English]

Physico-chemical chaterctrestics of the Ararat basaltic lavas from the North Gerik (NW Azerbaijan) area, based on mineral chemistry of clinopyroxene: implications for magma evolution in post collision origin

نویسنده [English]

  • Munira Khairkhah

Research Institute of Earth Sciences

چکیده [English]

1-Introduction
North western Iran are neighboring of the largest Turkey volcanic centers (e.g. Ararat, Nemrut, Tendürek and Süphan volcanoes), within the Turkish-Iranian orogenic plateau. Quaternary basaltic lavas from NW Azerbaijan are related to the on-going Arabia-Eurasia collision zone, and inherited by subduction of the Neo-Tethyan Ocean at a Late Eocene age (~ 35 Ma). The Quaternary lava flows in north of Gerik are originated from the Little Ararat volcano (Agri Dag) and covered a large area in the end member of north western Azerbaijan. This youngest lavas(0.4Ma) are derived from little Ararat and cropped in northern part (NNW) of Iran.The Quaternary basic lavas show alkali to subalcaline trend. The volcanic rocks have basic composition( olivine basalts, hawaiites and mugearites) and show mainly hyaloporphyritic,intergranular, microlitic and hyalin textures . These alkali basaltes show Hawaian and Strombolian type eruptions. According to geochemical and mineralogical composition, these basaltic rocks are relative comparable with Ararat lava flow in Turkey. By geochemical studies, it is suggested that the volcanites evolved by fractional crystallization, low degree of partial melting and crustal contaminations. Geodynamic reaserch and tectonic setting  show, that these rocks  are related to Quaternary extensional tectonic regime and have erupted along the shear zone in pull-apart local  basin zone.
2-Materials and methods
Samples were collected from basic lava flows which have travelled much further than 100 km from Little Ararat, and producing narrow ribbons of lava to the valley floors in north of Gerik with typically thickness of 5 – 30 m. plagioclase+ clinopyroxene+ Olivine forms the essential mineral assemblage of these basalts. Accessory minerals include magnetite and apatite. Small biotite grains are occasionally present. Textures are hyaloporphyritic, with plagioclase, pyroxene and olivine phenocrysts. This section focused on the mineral chemistry of clinopyroxenes present in the basic lavas.
Approximately 5 representative crystals (comprising pyroxenes) were selected for electron microprobe analysis in Hubland Institute at Wuerzburg University in Germany. The analyses were performed with a JEOL JXA 8200 microprobe analyzer, with peak counting times of 20 s (except 40 s for Na) and a background time of 10 s (20 s for Na). Acceleration voltage was set to 15 kV and a beam current of 20 nA was used for all analyses.
3-Results and discussion
This paper presents basaltic lava flows by focused on mineral chemichemistry of pyroxenes in the northwest Iran, within the Turkish-Iranian plateau (eastern Anatolia); These lavas were generated from Ararat and were covered a big amounts of basaltic materials in eastern Turkey to N-Gerik) during a short time in the Quaternary. By Microprobe analyses, the compositional range of clinopyroxenes is (WO=0.38.5, En=0.51, Fs=12.9) and show that clinopyroxenes are diopside. According to geotermobarometrical studies, clinopyroxenes in the studied basalts were formed about 1205-1275°C temperature with 7.9- 11.2 Kbar pressure and the clinopyroxenes are situated above the line of Fe3+ show high oxygen fugacity
4- Conclusion, Keywords
Basaltic Quaternary lavas of N Gerik in NW Iran are generated from the Little Ararat volcano in eastern Turkey and these lava flows have travelled further than about 100 km to NW Iran. The main minerals in these these rocks, are Olivine + plagioclase ± clinopyroxene. Clinopyroxenes crystals represent normal and reverse zoning with sieve texture and diopside in composition. By chemistry studies of this mineral, it is indicated that, the clinopyroxenes were crystallized from the primary magmas at high temperature1205-1275°C, pressure of 7.9- 11.2 Kbar and depth average 35Km with high oxygen fugacity in their magmatic arc setting.

کلیدواژه‌ها [English]

  • Ararat
  • Basaltic lavas
  • Clinoppyroxene
  • Gerik
  • Termobarometry