Geochemistry and petrogenetic features of the Early Cambrian volcanism inTelbesmi Formation, Mardin-Derik, SE Turkey

The Late Neoproterozoic/Early Palaeozoic successions in Southeast Anatolian Autochthon Belt, representing the northern edge of Arabian Plate in SE Turkey, occur in Derik (Mardin), Tut-Penbegli (Adıyaman), Samur Dag (Hakkari) and Amanos (Hatay) areas. In the Mardin-Derik area the Early Paleozoic rock-units are composed from bottom to top of Telbesmi, Sadan, Koruk, Sosink and Bedinan formations, respectively. The Telbesmi Formation is made up of slightly metamorphosed fluvial sandstone/ mudstones alternating mainly with andesitic and rarely spilitic lava flows and pyroclastic rocks. The base of the formation includes andesitic/spilitic lavas, tuffs and aglomerates with rarely rhyolitic lavas, interlayered with mudstones. The upper part of the formation includes very thin-layered cherty recrystallized limestones and red, violet meta-sandstones/meta-siltstone alternations. The ichno-fossils (?Teichnus isp., Treptichnus rectangularis, Cocchlichnus isp.) near the transition to the Sadan formation indicates to the Early Cambrian. Upwards, the formation is transitional to Early Cambrian siliciclastic rock of Sadan Formation. The discontinuous conglomeratic band near the transitional between the Telbesmi and Sadan Formations is a channel-fill and does not correspond to an unconformity, as previously suggested. The succession is conformably overlain by Middle Cambrian Koruk Formation, and Upper Cambrian Sosink Formation. Trilobite bearing Late Ordovician Bedinan Formation unconformably overlies older units.
   The Derik volcanics are geochemically grouped as basalts, andesites and rhyolites and display a continuous evolutionary trend from transitional to calc-alkaline affinity, which are related to magmatic differentiation. N-MORB normalized multi-element and REE diagrams reveal that Derik volcanics show clear negative anomalies for Nb, Ti and Eu with enrichment in Th, La, Ce and LREE and have similarities with arc-related Late Neoproteroic lower crustal rocks of the Arabian Plate. The negative Nb and Ti anomalies imply the involvement of a subduction-modified mantle source, whereas the Eu anomaly clearly indicates the fractional crystallization of feldspar minerals. Derik volcanics have LREE >85 time chondrite (85-120), whereas HREE is <25 times chondrite (20-24) times, probably generated outside of the garnet stability field. The (La/Yb)N, (La/Sm)N and (Gd/Yb)N ratios vary from 3.29-3.92, 1.91-2.96, 0.85-1.51 for basalts, 3.88-6.53, 2.59-4.14, 1.00-1.53 for andesites and 5.68-5.92, 3.03-4.17, 1.00-1.07 for rhyolites, respectively. The negative Eu anomaly (Eu/Eu)N in basalts (0.54-0.84), in andesites (0.51-0.72) and in rhyolites (0.57-0.59) probably reflects the result of crystallization of the feldspar minerals from the melts at the source following the partial melting. The LREE/HREE, Nb/Zr and Nb/Y ratios of the studied rocks imply that Derik Volcanics may be differentiated from mafic lower crustal Arabian source. Petrogenetic modelling of Derik volcanics indicate that the initial stages of partial melting of the Arabian LC-source about 8-12 % produced a rhyolitic melts and followed by the partial melting of LC of about 9-18 % and 16-21 % to generate andesitic and basaltic melts.
   The volcanics have isotopically lower ⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd values than MORB and can be correlated well with the Early Proterozoic mafic granulites (lower crustal) of Tanzania. Four point whole-rock Rb-Sr isochron data of the volcanics reveal 533±25 Ma with an initial ⁸⁷Sr/⁸⁶Sr value of 0.7057.
   Based on the geology, geochemistry and petrogenesis, Derik volcanics may be related to an extension and subsequent rifting in the northern edge of Gondwanan Arabia, which resulted in opening of a new oceanic branch to the north of the Gondwana.