GENERATION OF ALKALI MAGMAS OF THE KULA VOLCANIC PROVINCE, WESTERN TURKEY BY DECOMPRESSIONAL MELTING OF SPINEL PERIDOTITE SOURCE

Alkaline volcanism in the Kula Volcanic Province (KVP) in Western Turkey exhibits a suite of basanites, phonotephrites, tephriphonolites and trachybasalts. When projected on the normative pseudoternary olivine-diopside-nepheline diagram of Sack et al. (1987), the bulk compositions of the KVP rocks display a path parallel to and just below of the experimentally derived 1 atm cotectic suggesting differentiation of their parental magmas at low pressures.

Trace element analysis of the KVP alkali rocks constrains the composition of their source rocks to spinel peridotite zone in the mantle. Using the Melts algorithm (Ghiorso and Sack, 1994) we explored the question whether alkaline magmas of the KVP can be generated by decompressional melting of spinel peridotite. An average spinel peridotite xenolith from alkaline rocks was melted under isentropic conditions from 23 to 8 kbar and the generated melt compositions were projected on the normative pseudoternary olivine-diopside-nepheline diagram of Sack et al. (1987). Olivine+plagioclase fractionation path projected from the low-pressure melt generated by decompression coincides with the most primitive rock compositions of the KVP. The diversity of rock compositions have been then modeled using isenthalpic magma mixing at low pressure.

These results confirm that degree of decompressional melting less than 3% of spinel peridotite can produce the magmas of the KVP alkaline rocks.