AMASYA CİVARI GEÇ KRETASE YAŞLI ULTRAPOTASİK VOLKANİKLERİNDE FARKLILAŞMA SÜREÇLERİ

Amasya dolaylarında Geç Kretase yaşlı birimler arasında Pontid yayönü havza çökelleri vebuna eşlik eden volkanik kayalar ile temsil edilen volkanoklastik bir istif yer alır. Bu birimiçerisindeki volkanik kayalar, Pontid kuşağı aktif kıta kenarı volkanik kuşağının ürünleriolan kalkalkalin kayalar ve bunlara eşlik eden alkalin-potasik/ultrapotasik kayalardan oluşur. Bu çalışmada, benzer jeokimyasal özellikler sergileyen alkali nitelikli kayalar arasında meydana gelmiş olan farklılaşma süreçleri petrolojik modeller bazında ele alınmaktadır.Petrografik olarak lösitit, minet ve trakit olarak tanımlanan alkali ultrapotasik kayalar volkanoklastik istif içerisinde stok, dayk ve lav akıntıları şeklinde gözlenirler. Çarpışmalı tektonizmanın kontrolüyle gelişen orojenik kuşaklarda gözlenen ultrapotasik ve yüksek potasyumlu kayaların farklılaşmasında düşük basınç kristallenme süreçlerine eşlik eden asimilasyon ve kabuk kirlenmesi (AFC) olayları etkindir. Amasya yöresi Geç Kretase ultrapotasik magmatizmasının en genç volkanik ürünleri trakitik kayalarla temsil edilir ve bunlarjeokimyasal özellikleri bakımından ultrapotasik kayaların en evrimleşmiş olanlarıdır. Buradan hareketle, volkanoklastik istif içerisindeki en ilksel örnek başlangıç bileşimi, OrtaPontidler'in Triyas yaşlı metapelitik kayaları da kirletici kabul edilerek, kristallenmeye eşlik eden kabuk katkısı süreçlerinin (AFC) ultrapotasik ergiyiklerden lösitit, minet ve trakitik litolojilerin farklılaşmasında etkin olup olmadığı belirlenmeye çalışılmıştır. AFC modelleme sonuçları trakitik kayaların ilksel ultrapotasik ergiyiklerin kristallenmeleri sürecinde, en fazla % 5 civarında Orta Pontid temel kayalarına ait birimleri özümsemesiyle oluştuğunu ortaya koymuştur. Bununla birlikte lösitit ve minet farklılaşması fraksiyonel kristallenme ya da eşlik eden kabuk kirlenmesi süreçleri ile mümkün görünmemektedir

TDIFFERENTIATION PROCESSES in LATE CRETACEOUS ULTRAPOTASSIC VOLCANICS AROUND AMASYA

Late Cretaceous lithologies around Amasya region are represented by Pontide fore-arcbasin units which corresponds a volcanoclastic sequence. This sequence has the productsof alkaline ultrapotassic magmatism accompanying calcalkaline lavas which are abundantalong Pontide arc. The ultrapotassic rocks which are classified as leucitite, minette andtrachyte based on their mineralogical composition, occur as dikes, stocks and rarely lavaflows as to be comprised by the Late Cretaceous Volcanoclastic Succession (LCVS). Fractional crytallization accompanied by assimilation (AFC) is a low pressure processes ableto differentiate ultrapotassic parental melts to various compositions in a continental margin tectonic setting.The trachytes are the youngest and the most evolved members of LCVS.Therefore we performed AFC modelling using the most primitive minette sample as starting composition and calculated the fractionation trends based on the theoretical mineralogical compositions. We also used the Triassic metapelitic basement rocks of Central

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