Noble gases and rock geochemistry of alkaline intraplate volcanics from the Amik and Ceyhan-Osmaniye areas, SE Turkey

Abstract

The SE part of Turkey is characterized by a transtensional regime within the complex collision zone between the Anatolian, Arabian and African plates, which is bounded by two main faults, Dead Sea Fault and its splays on east and the Karasu Fault on west. In this tectonic and geodynamic context developed the Amik and further North, Erzin and Toprakkale districts, which are located onshore the Iskenderun Gulf, with the occurrence of a widespread and young alkaline volcanism dated from 1.57 to 0.05 Ma in Amik, and 2.25 to 0.61 Ma in Toprakkale. Here we present the results of a petrochemical and noble gases (HeeAr) study performed in basalts and basanites from the Basins in order to constrain the features of the mantle source. The major and trace elements composition indicate that the involved mafic melts could be the result of 0.8–2% partial melting of a predominantly spinel and garnet +spinel mantle, which has typical features of intra-plate OIB magmatism. The 4He/40Ar* ratios display two distinct ranges, roughly< 1 and> 1, for basalts and basanites respectively. The 3He/4He ratio of fluid inclusions in olivine crystals ranges from 7.29 to 8.03 Ra (being Ra the atmospheric 3He/4He ratio of 1.39 ×10−6), which implies a rather homogenous helium isotope signature of the mantle source. Such values are commonly recorded in MORB-like reservoirs (namely 8 ± 1 Ra), confirming that lavas erupted in SE Turkey did not suffer any important process of crustal contamination. Combining the main evidences from the geochemical data of both rocks and noble gases of fluids inclusions, we suggest that the melt generation of the alkaline magmas was triggered by a stretching lithosphere resulting from asthenosphere upwelling and decompression melting, analogous to geodynamic models of the coeval Syrian alkaline volcanic rocks and the older NW Anatolian rocks, rather than by subduction or plume-related processes. Finally, the combined results of noble gas-rock geochemistry point to a widespread mantle metasomatism as well as absence of crustal assimilation and negligible crustal contamination in contrast to former conclusions based only on the solid rock geochemistry.