Geochemistry and tectonic significance of the tectonomagmatic units in the helete (Kahramanmaraş) region

The well-exposed Helete (Kahramanmaraş) region of SE Anatolia is critical for understanding the timing and processes of Late Mesozoic-Early Cenozoic subduction/accretion of the S Neotethys. The main outcrop (c. 26 km long) is characterized by NE-SW-trending, northward-dipping imbricate thrust slices that are located low in the stack of Tauride thrust sheets that was finally emplaced onto the Arabian foreland during the Mid-Miocene. From the structural top downwards, the main units in the study area are: (1) Malatya Metamorphic Unit, a Mesozoic meta-carbonate platform, which is interpreted as part of the northern continental margin of the Southern Neotethys; (2) Late Cretaceous-Middle Eocene carbonates (Harami Formation); (3) Ophiolitic rocks (Meydan ophiolite); (4) Volcanogenic rocks (Helete unit); (5) Tectonic melange and (6) Arabian foreland. The above tectonic units are all intruded by granitoid rocks, except the Malatya Metamorphic Unit in the north and the Arabian foreland in the south. The present study focuses on the Meydan ophiolite, the Helete unit and the underlying melange unit. The Meydan ophiolite is composed of structurally distrupted oceanic crustal rocks including ultramafic-mafic cumulates, isotropic gabbros, sheeted dykes and volcanic-sedimentary rocks. Major and trace-element geochemistry, coupled with mineral chemistry, suggest a subduction-related tectonic setting for ophiolite genesis. The siliceous intrusive and extrusive rocks that cut the Meydan ophiolite (i.e. granite, granophyre, aplite and rhyolite) have yielded new U-Pb zircon ages of 83 to 88 Ma. The Helete unit is characterized by wide range of basic-silicic extrusive rocks (i.e. basalt, basaltic-andesite, andesite, rhyodacite and dacite). Geochemically, the volcanics of the Helete unit evolved from a tholeiitic parental magma. Mid-ocean ridge basalt-normalised spider diagrams of the basic extrusives display marked negative Nb anomalies, suggesting eruption in a subduction-related setting. The granitoid rocks that cut the Helete unit (i.e. granite, granodiorite, tonalite, gabbro, diorite and dacite) give U-Pb zircon ages of 86-93 Ma. The melange unit is dominated by blocks of neritic limestone, recrystallized limestones, altered volcanic rocks and rare amphibolite, with a locally variable matrix of serpentinite or volcanogenic material. Granitoid intrusions (e.g. granite-porphyre, granodiorite-porphyre and quartz-diorite) cutting the melange unit also gave a Late Cretaceous age (87 Ma; zircon U-Pb) . Field mapping shows that the U. Cretaceous granitoid rocks that cut the Helete unit come within several tens of metres, stratigraphically overlying Middle Eocene sedimentary cover (i.e. thin basal clastics, followed by nummulitic carbonates). The new isotopic ages, coupled with field mapping, structural and sedimentary evidence, therefore, seriously question the existing interpretation of the Helete volcanic unit as Eocene in age. All of the tectonomagmatic units studied appear to have formed in a supra-subduction zone setting, above a north-dipping subduction during Late Cretaceous.