THE MISSING PART OF NW ANATOLIA OLIGO-MIOCENE VOLCANIC BELT: ALMACIK MOUNTAIN, NEW K-AR AGE AND ISOTOPIC AND GEOCHEMICAL DATA

The new radiometric age data reveal that the presence of Oligo-Miocene volcanic rocks in Almacýk Mountain similar with Armutlu Peninsula volcanic associations were first reported by Genc et al (2004) and Kürkçüoglu et al (2008). We present new Sr, Nd and Pb isotope data together with the geochemical data and K-Ar age (23.8±1.1) data for Oligo-Miocene volcanic rocks from the Almacik Mountain.

The Oligo-Miocene aged basaltic andesitic lavas comprised mainly of plg (An30-45) + cpx ± opx ± opaque minerals, display calc-alkaline affinity and Medium-K character. Evaluation of Mg # (41.6-45.2), Cr (20.5-34.2 ppm), Ni (0.9-30.6 ppm) and MgO contents (2.02-3.14 %) together, it may be claimed that these lavas are the products of evolved melts. Significant enrichments in LILE coupled with Nb-Ta and Ti depletions on N-MORB normalized diagrams and LREE enrichments (La/Sm: 2.9-4.3) with slightly HREE depletions (Gd/Yb: 1.5-1.7) with respect to the MREE’s on Chondrite-normalized multi-element diagrams may also imply that the subduction component and crustal contamination processes had played an important role in the evolution of magma.

Although ⁸⁷Sr/⁸⁶Sr_(i) (0.704918 - 0.705561) and ¹⁴³Nd/¹⁴⁴ Nd_(i) (0.512567- 0.512706) contents of the Oligo-Miocene lavas are comparable to the Bulk Earth and Mantle Array compositions, their Pb ( ²⁰⁶Pb/²⁰⁴Pb: 18.701, ²⁰⁷Pb/²⁰⁴Pb: 15.621, ²⁰⁸Pb/²⁰⁴Pb: 38.722), O (δ¹⁸O SMOW: 10.9) isotope values and eNd_(i) contents (-0.89 in a sample and range from 0.96 to 1.84 in the other samples), implying that the Oligo-Miocene volcanics were derived from a heterogeneous source which was generated by interaction of continental crust and depleted mantle. Subduction signatures of the Oligo-Miocene volcanics may be inherited from the previous subduction event(s).

Based on our evaluation for the geochemical and isotopic data collectively, we argue that partial melting of the subcontinental lithospheric mantle and variable amount of crustal materials contamination were the main processes for the evolution of the magma that produced the volcanic association of Almacik Mountain, during Oligo-Miocene time.

Considering the geochemical features and isotopic contents of Armutlu-Almacýk region Oligo-Miocene volcanism, we have concluded that it may be the central part of the coeval volcanic belt extend from the Thrace basin and Biga peninsula in the west to Galatian complex in the east.