جايگاه زمين ساختي ـ ماگمايي سنگ‌هاي آتشفشاني پالئوژن رودبار (شمال ايران)

سنگ‌هاي آتشفشاني پالئوژن رودبار (شمال ايران) داراي تنوع تركيبي بين اوليوين بازالت، آندزيت بازالتي، هورنبلند آندزيت پيروكسن دار و آندزيت هستند. از نظر ژئوشيميايي آنومالي‌هاي منفي Ti، Nb ، Ta ، Zr، Ba، P و آنومالي مثبت Rb، Pb،Th ، K از شاخصه‌هاي آلودگي پوسته‌اي اين سنگ‌ها هستند. مطالعات ژئوشيميايي نشان مي‌دهد كه سنگ‌هاي بازيك منطقه با پوسته تحتاني و سنگ‌هاي حدواسط با پوسته فوقاني آلوده شده‌اند. تغييرات Y/Nb در برابر Zr/Nb و نيز Rb/Y در برابر Nb/Y و Ba/La در برابر La/Sm بيانگر آلودگي پوسته‌اي ماگماهاي منشاً گرفته از منبع مورب مي‌باشد. در نمودارتشخيص جايگاه‌هاي تكتونيكي تمامي نمونه‌ها در محدوده بازالت‌هاي ريفت درون قاره‌اي قرار گرفته‌اند. بررسي‌هاي ژئوشيميايي نشان مي‌دهد كه سنگ‌هاي منطقه از يك منبع گوشته‌اي تيپ مورب با رخساره اسپينل در يك محيط ريفتي درون قاره‌اي حاصل شده كه با سنگ‌هاي پوسته قاره‌اي آلوده شده‌اند.

Introduction The study area in north of Iran is located between 49 °, 30' and 50 °, 00' longitudes and 36 °, 30' to 37 °, 00' latitudes and in the 1.100000 sheet of Jirandeh. This area is considered as part of the structural zone of Iran in Alborz zone and is part of Tertiary zone. In this paper, with the help of geochemical evidence in Rudbar region, as part of Paleogene magmatism in Alborz, an attempt has been made to comment on the tectonic petrogenetic of the region. Methodology After field studies, considering lithological varieties of the volcanic units in the region, 100 samples were collected and thin sections were prepared and studied in terms of petrography using polarizing microscope. Then, among the mentioned samples, 22 samples with the lowest weathering and most lithological variety were selected to analyze the major elements using XRF method and the trace and REE elements by ICP-MS method at SGS laboratory in Toronto. In order to analyze data, the software Igpet 2007 and GCDkit are used. Discussion The studied area is located in the sheet of Jirande at a scale of 1:100000 ,that are outcrops of volcanic and pyroclastic rocks of Paleogene age. Based on petrographic studies carried out on the lavas’ units, three rock units were distinguished: a) olivine basalts, b) andesitic basaltic and basaltic andesite, c) hornblende pyroxene andesites and andesites. In most investigated rocks, there are different types of xenoliths and xenocrysts. Xenoliths are composed of gabbro, diorite and sometimes basalt. These xenoliths and xenocrysts are petrographic evidence for magmatic contamination. The positive correlation of Na2O and K2O and the negative correlation of Fe2O3, MgO, CaO oxides with the increasing of SiO2 evidence, indicates fractional crystallization in the magma evolution trend of rocks in the area. The constant trends are also maintained through series, which were exposed to the AFC process and assimilation with fractional crystallization. Comparing the pattern process of incompatible rare elements to crust values in mafic and intermediate terms indicates crustal contamination of mafic rocks to the lower crust and contamination of intermediate rocks towards upper crust. Linear correlation between the ratio of Y/Nb compared to Zr/Nb indicates the origination of magmas from MORB source mantle which were somewhat contaminated with the continental crust rocks. Conclusion Geochemical studies represent original relationship between all the studied rocks. This relationship indicates the crystal fractionation in the magma that forms these rocks. Investigations of the ratios of incompatible trace elements, suggest that the mafic samples of the region are close to MORB asthenosphereic mantle source. Also, the trends between primary and evolved samples indicate a linear arrangement between the MORB source mantle and the continental crust, representing an interaction of the MORB mantle-derived magmas with continental crust. All geochemical evidence indicates that the volcanic rocks in the area were originated from melting of a MORB asthenosphere mantle source with spinel facies, which was contaminated with the continental crust rocks to some degree. The crustal contamination of these basalts has caused, firstly, these rocks to follow exactly the elemental processes of the crust, and secondly misleadingly show the geochemical characteristics of rocks in subduction zones. This means generating magmas from mantle MORB source with crustal contamination are commonly seen in within-plate continental rift magmatisms.