A combined mineralogical and Rb-Sr isotopic investigation was conducted on fault gouges from two locations at eastern part of the North Anatolian Fault Zone (NAFZ). The fault gouge samples contain no 2 M mica and consist chiefly of cryptocrystalline material, which is extensively altered to illitic clays (mixed-layered iilite-smectite) and some carbonate minerals. The Rb-Sr isochrones of leachate, residue and untreated aliquots from various size fractions of samples yielded two different illite generations. The oldest illite authigenesis, which started at about 35.6 Ma might be inherited from pre-existing faults along the Tethyan suture zone. The authigenic illites from finer fractions correspond to an isochron age of 8.3 Ma, which is in agreement with the findings of previous works that may suggest a middle Miocene age for the initiation of the NAFZ. Our Sr isotope data indicate that the metamorphic fluids contain some mantle components and mobilised during fault movement. (C) 2009 Elsevier Ltd. All rights reserved.