Mehmet ARSLAN, Terry L. SPELL, Sevcan KÜRÜM, Durmuş BOZTUĞ, Ayten ÖNAL

Petrology and $^{40}Ar-^{39}Ar$ age of the bimodal orduzu volcanics (Malatya) from the western end of the eastern Anatolian Neogene volcanism, Turkey

Doğu Anadolu’da yaygın olarak yüzeylenen Neojen magmatizmasının bir ürünü olan Yamadağ volkanitlerine ait Orduzu volkanitleri (Malatya), riyolit, riyolitik dayk, trakiandezit ve bazaltik trakiandezit dayklarından oluşmaktadır. Bazaltik trakiandezitik dayklardaki mafik globüller, trakiandezitiklerdeki bazaltik trakiandezit anklavları, bazı mikroskopik dokusal özellikler ve jeokimyasal bulgular magma karışmasının verileri olarak değerlendirilmiştir. Orduzu volkaniti kayaçlarında aşamalı gazsızlaştırma tekniğiyle yürütülen $^{40}Ar-^{39}Ar$ radyometrik yaş tayini çalışmalarında, riyolitlerde, riyolitik dayklarda ve bazaltik trakiandezitik dayklarda Orta Miyosene karşılık gelen yaşlar (16 My) elde edilmiştir. Primordial manto’ya göre normalleştirilmiş örümcek diyagramları, riyolit ve riyolitik daykların, andezit ve bazaltik trakiandezitik dayklara göre bazı büyük iyon çaplı elementler (Cs, Rb, Ba, K, Th, U) bakımından zenginleşme; ancak Sr, Nb, Ti bakımından kuvvetli ve bazı kalıcılığı yüksek elementler (Hf, Zr) bakımından ise hafifçe fakirleşmeye uğradıklarını göstermektedir. Chondrit’e göre normalleştirilmiş nadir toprak elementleri örümcek diyagramları ise riyolit ve riyolitik dayklar için bağıl olarak nisbeten daha yüksek bir (La/Lu)CN (18–47) ve Eu/Eu* (0.57–0.92) oranları; trakiandezitler ve bazaltik trakiandezitik dayklar için ise daha düşük bir (La/Lu)CN (7–13) ve Eu/Eu* (0.86–1.05) oranları gösterirler. Jeokronolojik, arazi, petrografik ve jeokimyasal veriler, Orduzu volkanitlerinin oluşumunda, konumsal ve zamansal olarak üç farklı magmanın etkin olduğunu göstermektedir. Bunlar, (1) kabuksal kökenli, kalk-alkalen riyolitik magma; (2) litosferik manto kökenli, kalkalkalen magma; (3) litosferik manto kökenli orta derece alkalen magma kaynağıdır.

Doğu Anadolu Neojen volkanizmasının batısındaki bimodal orduzu (Malatya) volkanitlerinin petrolojisi ve $^{40}Ar-^{39}Ar$ yaşı

The Orduzu volcanics, which are part of the Yamadağ volcanics in the Malatya region, include rhyolite, rhyolitic dykes, trachyandesite and basaltic trachyandesitic dykes. Mafic globular occurrences within the basaltic trachyandesitic dykes, the existence of basaltic trachyandesite enclaves within the trachyandesite, and other textural and geochemical evidence all indicate magma mingling/mixing. Incremental $^{40}Ar-^{39}Ar$ dating on plagioclase from the rhyolite, from rhyolite dykes and basaltic trachyandesite yielded consistently 16 Ma (Middle Miocene). Primordial mantle-normalized spider diagrams of the rhyolite and rhyolitic dykes represent enrichments in some large ion lithophile elements (Cs, Rb, Ba, K, Th, U) but remarkably depletion in Sr, Nb, Ti, Eu and slight depletion in some high field strength elements (Hf, Zr) relative to the trachyandesites and basaltic trachyandesitic dykes. Chondritenormalized rare earth element spidergrams yield a high $(La/Lu)_{CN}$ (18–47) and Eu/Eu* (0.57–0.92) ratios for the rhyolites and rhyloitic dykes, but a low $(La/Lu)_{CN}$(7–13) and Eu/Eu* (0.86–1.05) values for the trachyandesites and basaltic trachyandesitic dykes. From the field relations and geochemical data, it is concluded that three distinct magma sources were spatially and temporally involved in the genesis of the Orduzu volcanics, that include a calcalkaline, crustal-derived rhyolitic melt, another calc-alkaline, lithospheric mantle-derived andesitic magma, and a mildly alkaline, lithospheric mantle-derived basaltic magma.

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