Fikret KOÇBULUT, Halil GÜRSOY, Orhan TATAR, Haluk TEMİZ, Jean Claude GUEZOU, Sema YURTMEN, B.Levent MESCİ

Intracontinental quaternary volcanism in the Niksar pul-apart basin, Nort Anatolian Fault Zone, Turkey

Kuzey Anadolu Fay Zonu’nun doğu segmentleri üzerinde yer alan Niksar Havzası 1939 ve 1942 depremleri ile ilişkili birbirine paralel olmayan iki ana fay ile sınırlı, genç sigmoidal bir çek-ayır havzadır. Fay geometrisi havzanın düzensiz DKD kenarında karmaşıktır. Burada genç Pliyo–Kuvaterner yaşlı volkanik kayaçlar havza sedimanlarını da kesen doğrultu-atımlı fay çiftleri boyunca yüzeye çıkmaktadır. Niksar havzası civarında yüzeyleyen volkanik kayaçlar yüksek hassasiyetli K-Ar yöntemiyle yaşlandırılmış olup, yaşlar 542 ± 9 ka ve 567 ± 9 ka arasındadır. Lavlar çoğunlukla alkalin (sodyumca zengin) özelliktedir ve bazaltik trakiandezit (Mujearit) and trakiandezit (benmorit) ile az oranda sub-alkalin bileşimli dasit, andezit, riyodasit ve riyolitten oluşmaktadır. Bazaltik ve felsik lavlar arasında büyük bir bileşimsel boşluk olmasına rağmen, ana ve iz element dağılımları hem bazaltik hem de felsik lavların eşkökenli olduğunu gösterir. Ana oksitlerin ile iz elementlerin bolluğu sistematik olarak bu bileşimsel dağılım içinde çeşitlilik sunmaktadır. Gözlenen fazlardaki fraksiyonel kristalleşme ortaç ve ileri derecede gelişmiş ürünlerdeki farklılığı ifade etmektedir. Derinde bazaltlardaki amfibol fraksiyonelleşmesi silikatca doygun bir yönelime neden olurken, alkali feldispat fraksiyonelleşmesi ise kristalleşmenin son evrelerini işaret etmektedir. Belirgin kabuksal kirlenme gelişmiş magma içerisinde oluşmuştur, fakat kirlenme genellikle bunların bazaltik ürünlerinde çok az yada hiç yoktur. Alkali bazaltik kayaçlar OIB (okyanus adası bazaltlar) benzeri, LILE, HFSE, LRE zenginleşmesi ve az miktarda ilksel manto değerlerine nazaran azalma gösteren HREE değerleriyle karakterize edilen iz element dağılımları sunmaktadır. Tüm jeokimyasal dağılımlar farklı derecelerdeki kısmi ergime olayının birleşik etkisini, fraksiyonel kristalleşmeyi ve değişik derecelerde kabuksal kirlenmeyi göstermektedir.

Kuzey Anadolu Fay Zonu (Türkiye) boyunca Niksar çek-ayır havzasında kıta içi kuvaterner yaşlı volkanizma

The Niksar Basin is sited along the eastern segment of the North Anatolian Fault Zone in Turkey. It is a young sigmoidal pull-apart basin bordered by two non-parallel master faults associated with earthquakes in 1939 and 1942. The fault geometry along the irregular ENE margin of the basin is complex where young Plio–Quaternary volcanic rocks reach the surface along pairs of steep strike-slip faults which cut the basin sediments. The volcanic rocks around the Niksar Basin have been dated by high precision K-Ar dating and the ages range between 542 ± 9 ka and 567 ± 9 ka. The lavas are mainly alkaline (sodium dominated) in nature and include basaltic trachandesite (mugearite) and trachyandesite (benmoreite) with minor sub-alkaline compositions of dacitic andesite, rhyodacite and rhyolite. Despite the large compositional gap between basaltic and felsic lavas, major and trace element distributions indicate that both the basaltic and felsic lavas are cogenetic. Abundances of major oxides and trace elements vary systematically through this compositional spectrum. Fractional crystallization of the observed phases accounts for the diversity of intermediate and evolved products. Amphibole fractionation in basalts at depth causes the trend towards silica saturation while alkali feldspar fractionation dominates the final stages of crystallization. Significant crustal contamination has occured in the evolved magmas but contamination is generally minimal or absent in their basaltic parents. Alkaline basaltic rocks have OIB (ocean island basalt) like trace element patterns characterized by enrichment in LILE, HFSE, LREE and slight depletion in HREE relative to primitive mantle values. Overall geochemical variations indicate the combined effects of different degrees of partial melting, fractional crystallization and variable degrees of crustal contamination.

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