جایگاه زمین ساختی ماگمایی سنگ‌های آتشفشانی پالئوژن رودبار (شمال ایران)

سنگ‌های آتشفشانی پالئوژن رودبار (شمال ایران) دارای تنوع ترکیبی بین اولیوین بازالت، آندزیت بازالتی، هورنبلند آندزیت پیروکسن دار و آندزیت هستند. از نظر ژئوشیمیایی آنومالی‌های منفی  ti، nb ، ta ، zr، ba، p و آنومالی مثبت rb، pb،th ، k از شاخصه‌های آلودگی پوسته‌ای این سنگ‌ها هستند. مطالعات ژئوشیمیایی نشان می‌دهد که سنگ‌های بازیک منطقه با پوسته تحتانی و سنگ‌های حدواسط با پوسته فوقانی آلوده شده‌اند. تغییرات y/nb در برابر zr/nb و نیز rb/y در برابر nb/y و ba/la در برابر la/sm بیانگر آلودگی پوسته‌ای ماگماهای منشاً گرفته از منبع مورب می‌باشد. در نمودارتشخیص جایگاه‌های تکتونیکی تمامی نمونه‌ها در محدوده بازالت‌های ریفت درون قاره‌ای قرار گرفته‌اند. بررسی‌های ژئوشیمیایی نشان می‌دهد که سنگ‌های منطقه از یک منبع گوشته‌ای تیپ مورب با رخساره اسپینل در یک محیط ریفتی درون قاره‌ای حاصل شده که با سنگ‌های پوسته قاره‌ای آلوده شده‌اند.

Tectonic-magmatic position of Paleogene volcanic rocks of Roudbar (north of Iran)

IntroductionThe 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.MethodologyAfter 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 ICPMS method at SGS laboratory in Toronto. In order to analyze data, the software Igpet 2007 and GCDkit are used.DiscussionThe 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.ConclusionGeochemical 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 mantlederived magmas with continental crust. All geochemical evidence indicates thatthe 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 withinplate continental rift magmatisms.