زمین‌ شیمی مجموعه ماگمایی شمال‌ غرب ساوه (استان مرکزی)

مجموعه ماگمایی شمال‌غرب ساوه، واقع در قسمت مرکزی کمربند ماگمایی ارومیه دختر، شامل انبوهی از سنگ های آتشفشانی است که توسط گرانیتوئیدهای اواخر ائوسن الیگوسن پیشین قطع شده اند. از نظر پترولوژیکی، گرانیتوئیدها دارای ترکیب گابرودیوریت دیوریت، کوارتز مونزونیت،گرانودیوریت وگرانیت هستند؛ در حالی‌که سنگ های آتشفشانی به آندزیت بازالت، آندزیت و داسیت‌ تراکی‌داسیت طبقه بندی می شوند. بررسی‌های زمین‌شیمی سنگ کل در سنگ های نفوذی و آتشفشانی مورد بررسی نشان می دهد که این سنگ ها دارای تهی شدگی شدید از عناصرhfse  ( nb , zr و ti) با (la/sm) n = 0.44–1.18 و 0.91-1.35 = (dy/yb)n و غنی شدگی در lree و lile هستند که بیانگر تشکیل در طول ماگماتیسم کمان نرمال و محیط قبل از برخورد است. ویژگی های زمین‌شیمیایی سنگ های آذرین مورد بررسی (نسبت کم sr/y (کمتر از 30)، ناهنجاری های منفی eu (eu/eu* = 0.62–1.05)، تولید در یک منطقه فرورانش را نشان می دهد. ویژگی‌های این مجموعه ماگمایی با دیگر ماگماهای نوع عقیم کمربند ارومیه دختر مطابقت دارد. یک مدل زمین‌شیمیایی، پترولوژیکی و زمین ساختی یکپارچه نشان می دهد که ماگماتیسم مجموعه آذرین شمال‌غرب ساوه احتمالاً با ذوب‌بخشی پوسته زیرین جوان (آمفیبولیت) ناشی از فرورانش لیتوسفر اقیانوسی نئوتتیس همراه است؛ در حالی‌که سنگ های آداکیتی مولد درکمربند ارومیه دختر از ذوب‌بخشی پوسته زیرین ضخیم (گارنت-آمفیبولیت) تشکیل شده‌اند.

Geochemistry of northwestern Saveh magmatic complex (Markazi province)

IntroductionConsidering the wide extent of the UrumiehDokhtar magmatic arc and the presence of many intrusive and volcanic rocks in this belt, an important question from scientific and exploration points of view is “Why are some plutons productive whereas others are subproductive and/or barren?” Barren and productive magmatic systems related to calcalkaline arc magmatism are identified as normal or nonadakitic (low Sr/Y<20) and adakitic (high Sr/Y>20) magmas, respectively. Barren magmas are nonmineralized and have a low Sr/Y ratio, while high Sr/Y magmas are responsible for Cumineralization and are known as productive magmas that occur in all major orogenic belts worldwide (Cooke et al., 2005). Understanding their origin and petrogenesis is of critical importance to decipher their longterm growth and stabilization of the continental crust, and formation of economically valuable ore deposits (Monecke et al., 2018). Barren granitoid magmas typically form in precollisional subduction zone environments (Shahabpour, 1992), which is confirmed by the results obtained in this study. Magmatism in this region began in the Early Eocene and continued until the Pliocene. The volcanic and intrusive barrentype rocks (Eocene) that formed in a subductionrelated tectonic setting are characterized by calcalkaline and tholeiitic geochemical signature (Shahabpour, 2005). The northwest of Saveh magmatic complex is situated at the central part of the UrumiehDokhtar magmatic belt. The volcanic rocks of northwest of Saveh are crosscut by Late EoceneEarly Oligocene granitoids that are exposed over an area of about 100 km2. Materials and methodsApproximately 70 samples were intrusively collected from Mount Shahpasand and Neivesht volcanic rocks. Subsequently, 9 granitoid rocks and 7 volcanic rocks that showed the least amount of alteration were selected for wholerock geochemical analysis. The main elements analysis was performed by Xray fluorescence method using Optima 100V device and the analysis of rare earth elements was performed using Inductively Coupled Plasma Mass Spectrometry method and with ICP NeXION 300 device in the Lab West Laboratory of Australia. ResultsThe northwest of the Saveh magmatic complex is situated at the central part of the UrumiehDokhtar magmatic belt. The volcanic rocks of this area are crosscut by the Late EoceneOligocene granitoids. Wholerock geochemistry shows that the studied igneous rocks with low to medium potassium calcalkaline geochemical signatures have strong depletion in Nb and Ti and enrichment in LREE and LILE, which imply formation during normal arc magmatism. Sr/La and La/Yb trace element ratios show that all samples have evidence for slab fluid metasomatism and a mantle source affected by metasomatism. La/Nb and La/Ba ratios also confirms a subductionmodified lithosphere mantle source for the magmatic rocks in the northwest of Saveh. Geochemical evidence shows that these rocks are barrentype igneous rocks that have the same origin and differential crystallization is the dominant process in their petrogenesis. Barren magmatism in the northwest of Saveh is likely a result of partial melting of juvenile lower crust caused by subduction of the NeoTethys oceanic lithosphere, whereas productive adakitic rocks within the UrumiehDokhtar magmatic belt have formed by partial melting of thickened lower crust. DiscussionGranitoids have gabbrodiortitediorite, Quartz monzonite, granodiorite and granite composition, while volcanic rocks are petrologically classified as basaltic andesite, andesite and dacitetrachydacite. Geochemical studies of whole rocks indicate that they have strong depletions in HFSE (Nb, Ti, Zr) and enrichments in light rare earth elements and large ion lithophile elements compared to NMORB. Geochemical signature of the igneous rocks in northwest of Saveh (low Sr/Y ratio of almost <30) and their negative Eu anomalies (Eu/Eu∗ = 0.62–1.05) suggest generation in a subduction zone and precollisional setting. However, productive rocks elsewhere within the UrumiehDokhtar magmatic belt exhibit adakitelike calcalkaline magmatic characteristics (high Sr and Sr/Y, but low Y). Signature of this magmatic complex is consistent with other barrentype magmas through the UrumiehDokhtar magmatic belt. The low ratios of (La/Sm)N and (Dy/Yb)N (0.50–1.18 and 0.91–1.35, respectively) are similar to those from barrentype of granitoids. Examination of the studied samples on a Y versus MnO diagram (Baldwin and Pearce, 1982) shows that the samples have characteristics of barrentype igneous rocks. Haschke and Pearce (2006) suggested that a high Y content in barren magmas may record the participation of anhydrous phases during the early stages of magma genesis and so account for lack of associated mineralization. However, it may be possible that partial melting of the source is superficial, in agreement with a moderate precollisional crustal thickness of 35–45 km. Low Sr/Y (< 30) ratios measured in the Eocene–Oligocene northwest of Saveh igneous rocks suggest generation via islandarc magmatism, while a Sr/Y ratio of > 56 for productive rocks implies garnet, hornblende, and clinopyroxene minerals in the source, leading to enrichment of LREE/HREE (Castillo, 2012).