Muhsin EREN, Yeşilot Meryem KAPLAN, Selahattin KADİR

Petrography, geochemistry and origin of Lower Liassic dolomites in the Aydıncık area, Mersin, Southern Turkey

Aydıncık yöresinde Alt Liyas karbonatları hakim olarak kireçtaşı ve dolomitik kireçtaşı ara seviyeleri içeren dolomitlerden oluşur. Bu karbonatlar gel-git çevresi ortamında çökelmiş ve daha sonra erken ve geç evre dolomitleşmesine uğramıştır. Petrografik olarak üç tip dolomit belirlenmiştir: (1) 13-65 $mu m$ kristal boyutlu ve iyi doku korunmalı çok ince ile ince kristalli dolomit (Tl); (2) 65-270 $mu m$ kristal boyutlu ve doku korunmasız iri kristalli şeker dokulu dolomit (T2); ve (3) bulanık iri dolomit kristalleri (T2) çevresinde açık dış kenar veya fenestral gözenekleri çevreleyen ve çimento dolgusu olarak görülen dolomit çimento (T3). T1-tip dolomitler gel-git çevresi çökellerin çökelmeyle yaşıt ornatımıyla deniz suyundan oluşmuş erken evreyi karakterize eder. Daha sonra T2-tip dolomitler yaklaşık 50 °C gömülme sıcaklığında T1-tip dolomitlerin yeniden kristallenmesiyle oluşmuştur ve T3-dolomitler aynı dolomitleşme sıvısından çimento olarak çökelmiştir. Yeniden kristallenme erken evre dolomitlerinin dokusunda, kristallenme derecesinde, izotop kimyasında ve iz element içeriğinde değişimlere neden olmuştur. X-ışını difraksiyonu verileri yeniden kristallenme dolomitlerinin (T2) erken dolomitlere (T1) göre hafifçe daha iyi kristallenmeli ve daha az kalsiyum içerikli olduğunu gösterir. İri kristalli dolomitlerin (T2) daha negatif $delta^{18}O$ değerleri ve düşük Sr içerikleri yeniden kristallenme sırasında diyajenetik sıvı ile dengeyi yansıtır. T2-dolomit örneklerinin $delta^{18}O$ ve Sr değerlerinin birlikte değişim eğilimi ilerleyici yeniden kristallenmeyi destekler şekilde kristal boyutuyla ters bir ilişkiyi gösterir. Dolomitlerin $delta^{13}C$ değerleri hemen hemen aynıdır ve dolomitleşme sırasında çok az değişim gösteren tipik denizel değerleri gösterir. T2-dolomitlerinin güneye doğru kapanan geometrisi, dolomitleşme sıvısının erken sıkışma sonucu olarak kuzeyden güneye platform karbonatlarının içine ilerlediğini gösterir.

Aydıncık yöresinde (Mersin, Güney Türkiye) Alt Liyas dolomitlerinin petrografisi, jeokimyası ve kökeni

In the Aydıncık area, the Lower Liassic carbonates consist predominantly of dolomites, including limestone and dolomitic limestone intervals. These carbonates were deposited in peritidal environments, and later underwent early and late stage dolomitization. Petrographically, three dolomite-types are determined: (1) very fine to fine crystalline dolomite (T1) with crystal size of 13-65 $mu m$ and a good fabric preservation, (2) coarse crystalline sucrosic dolomite (T2) with size of 65 to 270 $mu m$ and fabric destruction, and (3) dolomite cement (T3) that occurs as a clear outer rim to cloudy coarse dolomite crystals (T2) or as a pore-lining and cement-fill of fenestral pores. The T1-type dolomites characterize the early stage of dolomitization formed from seawater by syn-sedimentary replacement of peritidal sediments. T2-type dolomites are derived from T1-type dolomites by recrystallization at increased burial temperature of ~50 °C. T3-type dolomites are precipitated as a cement from the same dolomitizing fluid. The recrystallization caused changes in texture, crystal ordering, isotope compositions and trace element contents. X-ray diffraction data indicates that the recrystallized dolomite (T2) is slightly better ordered and less calcium-rich than early dolomites (T1). More negative $delta^{18}O$ values and lower Sr contents of the coarse crystalline dolomites (T2) reflect an equilibration with late diagenetic fluid during the recrystallization. The covariant trend between $delta^{18}O$ and Sr values of T2-type dolomites shows an inverse relationship with their crystal size, suggesting progressive recrystallization. The $delta^{13}C$ values of both dolomites are almost in the same range indicating the typical marine values that suggest little modification during late dolomitization. The T2-type dolomite geometry pinching out to the southward indicates an invasion of dolomitizing fluid from the north to the seaward into the platform carbonates, as a result of early compaction.

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