Geochemistry of Pliocene/Pleistocene basalts along the Central Anatolian Fault Zone (CAFZ), Turkey

Pliocene-Pleistocene volcanism accompanied strike slip-related transtension al deformation along the Kizilirmak fault segment of the Central Anatolian fault zone (CAFZ) in the west of Sarkisla (Sivas-central Turkey). These vol canic rocks are represented by alkali olivine basalts. They can be divided into four different sub-groups on the basis of their Zr, Nb, TiO2 contents. A primitive mantle-normalized incompatible trace element diagram for four subgroups shows close similarity to typical OIB pattern. Some of the incomp atible trace element ratios (Ce/Y, Zr/Nb, La/Ba, La/Nb) are also akin to OI B values. Highly fractionated REE patterns (La/Yb-N=24.7-9.2) with no Eu an omaly are the main features of the alkali basalts and are comparable to alk aline volcanism in continental rift zones. On the basis of Al2O3/TiO2, Nb/Y , Zr/Y Zr/Nb ratios, the geochemical differences among four sub-groups can be explained by variable degrees of partial melting of compositionally simi lar mantle source. Th/Nb, Th/Y, Nb/Y ratios and the primitive mantle-normal ized trace element diagram suggests significant amount of crustal involveme nt for most of the alkali olivine basalts erupted along the CAFZ. Rupture o f the continental lithosphere by strike-slip-related transtensional deforma tion might have caused decompressional partial melting of the asthenospheri c mantle and generating alkali olivine basalts in this region. (C) 2001 Edi tions scientifiques et medicales Elsevier SAS.