Carnian (Upper Triassic) Lavas and Tuffites from the Mersin Mélange: Evidence for Intraoceanic Arc Rifting in the Northern Neotethys

Abstract

In Anatolia, the northern branch of Neotethys is represented by Izmir-Ankara-Erzincan (IAE) Ocean, whose fragments and associated continental entities are preserved in the IAE Suture. However, the traces of this northerly located ocean are not solely bound to the IAE Suture, but can also be found further to the south. The Mersin Mélange, situated in southern Turkey, is such a place that preserves the southerly transported remnants of the northern Neotethys. The Mersin Mélange essentially displays block-in-matrix features, with blocks/slices of different age and lithological characteristics embedded in a clastic matrix. Within the mélange, the Degirmenocagi region consists of a volcano-sedimentary sequence of middle Carnian age, which is characterized by lavas and tuffites interbedded with pelagic sedimentary lithologies. The lavas are chemically of basaltic/basaltic-andesitic composition, whereas the tuffites are of rhyolitic composition. Both lavas and tuffites display negative Nb anomalies, suggesting the involvement of subduction-related components. The high Zr/Nb and low Nb/Yb ratios (compared with normal mid-ocean ridge basalt [N-MORB]) imply a variably depleted mantle source, which has experienced previous melt extraction. The ratio-based melting systematics reflects melt generation largely dominated by spinel-facies melts. The overall geological and geochemical characteristics of the Degirmenocagi sequence suggest an oceanic backarc setting during the Carnian, which may have developed by the rifting of the Sorgun Arc. Although this new finding shows that the Triassic intraoceanic subduction is not uncommon in the Neotethys, it also strengthens a pre-Liassic opening of the northern domain (i.e., the IAE Ocean) and may further explain the longevity of the Middle–Late Triassic intraoceanic subduction in the northern Neotethys.