A COMBINED ROCK MAGNETIC AND GEOCHEMICAL INVESTIGATION OF UPPER CRETACEOUS VOLCANIC ROCKS IN THE PONTIDES, TURKEY

Abstract

Upper Cretaceous volcanic rocks were collected at 24 sites along the Pontides, N-NE Turkey, for rock magnetic and geochemical studies. Rock magnetic and petrographic methods showed that the lavas are characterized predominantly by titanomagnetites with a mixture of pseudo-single and multi-domain grains, whereas in tephrite single domain titanohematite was dominant. Measurements of magnetic susceptibility and the geochemical properties on different volcanic rock types provide important knowledge about the magnetic stability of the rocks. The magnetic properties are interpreted in terms of the composition, concentration, magma generation. Tephrite and phonotephrites with the highest intensities (5200 mA/m) and high magnetic susceptibility values (2585 x 10(-5)), largest grain sizes and Fe/Ti values, showing minor or no alteration are the most magnetic stable samples in contrast to dacites with the lowest intensity-magnetic susceptibility (520 mA/m - 573 x 10(-5)) and high alteration degree. The basanite samples show very low NRM (48-165 mA/m) but very high magnetic susceptibility (2906-3100 x 10(-5)) values suggesting the alteration of Fe-Ti minerals. It is shown that the magnetic properties of the basic to acidic rocks show a systematic variation with magma differentiation and could be related to fractional crystallization. Major and trace elements revealed that the lavas are compatible with complex magma evolution, with mineral phases of olivine+magnetite+clinopyroxene in basic series, amphibole++clinopyroxene in intermediate rocks and plagioclase+clinopyroxene+biotite in acidic series.