ROCK-MAGNETIC INVESTIGATION OF THE KIZILKAYA IGNIMBRITE (CENTRAL ANATOLIAN VOLCANIC PROVINCE)

The 5 Ma-old Kizilkaya ignimbrite is the youngest large-volume unit of the ignimbritic sequence exposed in the central Anatolian Volcanic Province. A rock magnetic study was carried out in order to evaluate the homogeneity of the ignimbrite through the entire deposit unit, by means of anisotropy of magnetic susceptibility (AMS), magnetic mineralogy and magnetic remanence determinations. Six areal distributed localities were sampled at different stratigraphic height within the devitrified portion of the ignimbrite. A total of 33 sites were sampled and 444 oriented specimens recovered and analyzed.

Both the isothermal remanent magnetization (IRM) acquisition curves and the Lowrie method point to a low-Ti magnetite as the main magnetic carrier; locally, the occurrence of a minor high-coercivity mineral (oxidized magnetite and/or hematite) is suggested by the high field value (B > 0.8 T) needed to reach IRM saturation.

Stepwise thermal and AF demagnetization were systematically performed. The characteristic remanent magnetization (ChRM) was clearly isolated by Zijderveld diagram analyses at some sites. Here, the mean value from 20 sites (D = 175°, I = -42°) is fully consistent with previously published paleomagnetic data (Piper et al., 2002). In other sites, remanence appears more complex. During both AF and thermal demagnetization, the remanence direction moves along a great circle and no stable end-point direction is found. Due to the occurrence of two remanence components with overlapping coercivity and blocking temperature spectra, the mean site ChRM was estimated combining best-fit great circles and lines (McFadden & McElhinny,1998). Here, paleomagnetic directions vary significantly along the stratigraphic section; the magnetic declination ranges from 170° to 210°, the inclination from -34° to -55°. Large deflections from the mean direction occur at sites characterized by the occurrence of oxidized magnetite and or hematite. Besides, at these sites the magnetic susceptibility is one order of magnitude lower than usual. These results suggest that at some sites the Kizilkaya ignimbrite acquired first a thermal remanent magnetization and then, during the final cooling or a short time after, a chemical remanent magnetization component.

AMS measurements detected a well developed magnetic fabric. Magnetic foliation is gently dipping at most sites and its plunge is regarded as a proxy of the flow direction. At four out of the six studied localities, the mean flow direction agrees with the vent location in an area between Derinkuyu Basin and Gollu Dag rhyolitic massif proposed in the literature (Le Pennec, 2000). However, the magnetic fabric changes along the stratigraphic section: the magnetic foliation is nearly horizontal in the central part of the deposit, independently of its thickness.

Notwithstanding Kizilkaya ignimbrite is a single cooling unit, its magnetic properties suffer substantial variations through the deposit and the remanent magnetization characteristics show that the acquisition of remanence results from the overlapping of thermal and chemical processes.

The Kizilkaya case shows that thick pyroclastic deposits should be sampled according a stratigraphical approach, with different sites at different stratigraphic heights at each individual outcrop/location. Otherwise, undersampling may severely affect the paleomagnetic results.