ABDURRAHMAN DOKUZ & ERKAN TANYOLU, ERKAN TANYOLU

Doğu Pontidler’deki Erken Jura ve Geç Kretase Yaşlı Kırıntılı Kayaçların Kaynak Alanı, Mineral Boylanması ve Yüzeysel Ayrışması Üzerine Jeokimyasal Sınırlamalar, Yusufeli (Artvin), KD Türkiye

Yusufeli (Artvin) yöresindeki Erken Jura ve Geç Kretase yafll› fleyl ve kumtafllar›; kaynak alanlar›, sedimantolojik geliflimleri ve tektonik ortamlar› hakk›nda daha fazla bilgi edinmek, ayr›flma, hidrolik boylanma ve tortul-döngü olaylar›n›n kaynak alan karakteristikleri üzerindeki etkisini de¤erlendirmek amac› ile petrografik ve jeokimyasal aç›dan incelenmifltir. Hamur (matriks) ve mineralojik içeriklerine göre, Erken Jura yafll› kumtafllar› arkozik arenit ve vake, Geç Kretase kumtafllar› ise litik arenit ve vake olarak tan›mlanm›fllard›r. La/Sc, Cr/Ni, Co/Th, Th/Sc ve Cr/Th gibi element oranlar› her iki istif için de mafik bir kaynak alana iflaret etmektedir. Bununla birlikte, bir çok örnekteki yüksek Th/Sc ve düflük Cr/Th oranlar› da felsik bir kayna¤›n katk›s›n›öngörmektedir. Örnekler, kullan›lan oransal diyagramlarda, mafik ve felsik kaynaklar›n katk›s› ile oluflan iki bileflenli bir kar›fl›m e¤risi boyunca da¤›lmaktad›rlar. Göreli olarak düflük Cr/Ni oranlar›, kaynak alanda ultramafik litolojilerin olmad›¤›na iflaret etmektedir. Kumtafllar›n›n, üst k›tasal kabuk bileflimini temsil etti¤i düflünülen postArkeen Avusturalya fleyllerine (PAAS) göre daha az bollukta iz element içerikleri, kaynak alanlar›ndaki tortul-döngü olaylar›n›n olas›l›kla daha az yo¤unlukta oldu¤una iflaret etmektedir. fieylerin kumtafllar›na göre daha fazlatüketilmifl hafif nadir toprak element da¤›l›mlar› da, kaynak alandaki kayaçlar›n bileflimlerindeki de¤iflimden ziyade mineral boylanmas›, kimyasal ayr›flma ve/veya tortul-döngüdeki de¤iflimleri yans›tmaktad›r. Alt Jura ve Üst Kretase tortullar›nda gözlenen kimyasal ayr›flma indeksi (chemical index of alteration = CIA) (s›ras› ile ortalama 53 ve 46) kaynak alanlar›n›n orta derecede kimyasal ayr›flma olaylar›na u¤rad›¤›n› öngörmektedir. Bununla birlikte, CIA indeksinin Alt Jura istiflerinde yukar›ya do¤ru artmas›; kaynak alan›n, olas›l›kla iklim ve tektonik de¤iflimler nedeniyle, giderek daha fazla kimyasal ayr›flmaya u¤rad›¤›n› gösterebilir. Di¤er taraftan, Üst Kretase istiflerinde CIA indeksinin yukar› do¤ru genel bir azal›m›; tortullar›n göreli olarak daha az ayr›flan bir kaynaktan kaynakland›¤›n› göstermekte ve olas›l›kla da artan tektonik etkinlik nedeni ile artan erozyon oran›n› yans›tmaktad›r. Analiz edilen kumtafllar›n›n bileflimsel olarak olgunlaflmam›fl olmas› yitimle iliflkili ortamlar›n tipik özelli¤idir. SiO2/Al2O3, K2O/Na2O oranlar› ve Co, Sc, Th ve Zr içerikleri okyanussal ada yay› ve k›tasal yay ortamlar›n› göstermektedir.

Anahtar Kelimeler:

fleyl ve kumtafl›, modal analiz, jeokimya, kaynak alan, hidrolik boylanma, kimyasal ayr›flma indeksi, Yusufeli-Artvin, Kuzeydo¤u Türkiye

Geochemical Constraints on the Provenance, Mineral Sorting and Subaerial Weathering of Lower Jurassic and Upper Cretaceous Clastic Rocks of the Eastern Pontides, Yusufeli (Artvin), NE Turkey

An integrated petrographic and geochemical study of the shales and sandstones of Early Jurassic and Late Cretaceous age in the Yusufeli area (Turkey) was carried out to obtain more information on their provenance, sedimentological history and tectonic setting, as well as to evaluate the influence of weathering, hydraulic sorting and recycling processes upon source-rock signature. Depending on their matrix and mineralogical content, the Lower Jurassic sandstones are identified as arkosic arenite and wacke, while the Upper Cretaceous sandstones are defined as lithic arenite and wacke. Elemental ratios such as La/Sc, Cr/Ni, Co/Th, Th/Sc and Cr/Th indicate a mafic source for the majority of each of the sandstone sequences. However, in several samples, high Th/Sc and low Cr/Th ratios suggest a contribution from a felsic source. Using element-ratio diagrams, all of the samples plot along a curve consistent with a two-component mixing model, consisting of a dominant mafic and a subordinate felsic source. The relatively low Cr/Ni ratios provide no support for significant amounts of ultramafic lithologies in the source area. The lower trace-element contents of the studied sandstones in relation to post-Archean Australian shales (PAAS), which are representative of the composition of the upper continental crust, indicate that recycling processes in their source areas were probably less intense than those of the PAAS. The more LREE-depleted patterns of the shales relative to sandstones likely do not reflect changes in source-area composition but, rather, variations in mineral sorting, chemical weathering and/or sediment recycling. Chemical index of alteration (CIA) indices observed in the Lower Jurassic and Upper Cretaceous sediments (average of 53 & 46, respectively) suggest that their source area underwent moderate degrees of chemical weathering processes. However, an upward increase in the values of the CIA indices in the Lower Jurassic profiles may indicate that the source area gradually underwent more intense chemical weathering, possibly due to climatic and/or tectonic variations. On the other hand, an upward decrease in the CIA indices of the Upper Cretaceous profiles generally demonstrate that the sediments were derived from a relatively less-weathered source terrain, reflecting an increased erosion rate likely due to increasing tectonic activity. The compositional immaturity of the analysed sandstone samples is typical of subduction-related environments, and their SiO2/Al2O3 and K2O/Na2O ratios and Co, Sc, Th and Zr contents reflect their oceanic and continental-arc settings.

Keywords:

shale and sandstone, geochemistry, provenance, hydraulic sorting, alteration, Yusufeli-Artvin, northeastern Turkey,

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