Nature of the Early Cretaceous lamprophyre and high-Nb basaltic dykes, NE Turkey: Constraints on their linkage to subduction initiation of Neotethyan oceanic lithosphere

Abstract

Late Mesozoic evolution of the Neotethys Ocean is critical and still remains unclear along the entire Sakarya Zone (SZ), Turkey, because previous interpretations of the Middle to Late Jurassic rifting of the SZ have not been consensually accepted. We identified, for the first time, the Early Cretaceous dykes intruded into the Late Jurassic and Late Carboniferous bodies in the Camlkaya (Erzurum) area in the SZ. Here, we present a data set of new field observations, Ar-40/Ar-39 geochronology, whole-rock geochemical and Sr-Nd isotopes to find out the tectonic setting of previously undated basic to intermediate igneous rocks. Ar-40/Ar-39 technique on the groundmass separations yielded the plateau ages of 128.78 +/- 1.02 to 122.89 +/- 0.82 Ma. The investigated dykes are compositionally divided into two sub-groups: calc-alkaline lamprophyres (CAL) and high-Nb alkaline basalts (HNB). They show enrichment in large ion lithophile elements (LILE: e.g., Rb, Ba, K) and depletion in high field strength elements (HFSE: Nb and Ti) as well as moderate fractionation of light rare earth elements relative to heavy rare earth elements. The HNB have high Mg# (47-66), and relatively unradiogenic Sr [Sr-87/Sr-86(t) = 0.70385 to 0.70510] and Nd [epsilon(Nd)(t) = +0.5 to +5.3] isotopic composition, whereas the CAL possess moderate to high Mg# (32-72), and relatively high Sr [Sr-87/Sr-86(t) = 0.70586 to 0.71379] and Nd [epsilon(Nd)(t) = +1.5 to -9.9] isotopic composition. Early Cretaceous high-angle subduction triggered upwelling of the asthenosphere at 126 Ma, which likely generated a heat anomaly. This anomaly has probably induced melting of the oceanic slab and the metasomatized mantle to form the CAL and HNB. The HNB were generated by low degrees of melting of mantle domain that was previously reacted by oceanic slab-related adakitic melts at the depths of the garnet stability field (>= 70 km). However, the CAL were probably formed at shallower mantle levels within the garnet-amphibole field by a relatively higher degree of melting of the mantle, also modified by subducted slab sediments. A new field, age, and petrochemical data from the Early Cretaceous dykes, combined with the presence of Aptian deep-water sediments to the north and the Late Jurassic-Early Cretaceous platform carbonates deposited in the passive margin setting along the entire SZ, imply the initiation of active-margin magmatism in the SZ. We suggest that the coexistence of the Early Cretaceous CAL and HNB points to the formation of an Andean-type active margin above a north-dipping Neotethyan oceanic slab. Their emplacement ages are associated with the inception of the magmatic arc structure and refer to a significant geodynamic turnover from passive-margin to active-margin tectonics along the SZ. (c) 2020 Elsevier B.V. All rights reserved.