MINERAL-CHEMISTRY OF ILICAKSU LAMPROITIC DYKES FROM GEDIZ REGION, WESTERN ANATOLIA

Whole rock and mineral compositions of lamproitic dykes with alkaline character from Ilicasu-Gediz, located at the further east side of E–W trending Miocene Simav Graben in western Anatolia have been investigated. Ilicaksu lamproites are similar “Mediterranean type SiO₂-rich lamproites” (52.7 - 57.8 wt. % SiO₂). They are characterized by relatively low CaO, Al₂O₃ and high K₂O/Na₂O, Mg#, Cr and Ni contents. They are enriched in LILE relative to HFSE, and show depletion in Ti, Nb and Th.

Ilicaksu lamproites consist of olivine (with Cr-spinel), mica (phlogopite), clinopyroxene (diopsite-augite), K-feldspar (sanidine) and apatite, with accessory magnetite and ilmenite. One sample also includes negligible nickeline (NiAs), which is unusual mineral in lamproites. They are olivine-phyric, which have coarse olivine xenocrysts that host Cr spinels. The olivine xenocrysts can be considered as mantle olivines (CaO<0.18 wt. %). They are zoned, forsterite-rich (up to Fo 92) and NiO contents less than 0.8 wt. % in the core, and show low Fo (up to 82) and NiO (<0.25 wt. %) in the rim. The some olivine xenocrysts are completely resorbed calcite and iddingsite. The phlogopites have low Mg# and TiO₂ (up to 4 wt. %) and opacite rims with high TiO₂ (up to 9 wt. %) some of them. The diopside and augites have typical lamproitic clinopyroxene character, and a high Mg# and low TiO₂ and Na₂O. Apatite occurs as prismatic shapes, needle-like microphenocrysts as well as inclusions in phlogopites.

According to petrographic and mineral chemical characteristics, there are two discrete crystallization phase: (1) olivine (Cr spinel-bearing) ± apatite (2) phlogopite + clinopyroxene + sanidine + Fe oxides. Calculated crystallization temperatures range from 1239 to 1375°C for olivine-spinel and 744 to 1090 °C for phlogopite, clinopyroxene and magnetite-ilmenite. Pressure estimates have yielded in the range 1.6 to 1.9 GPa (50 to 58 km depth) for Cr-spinel and 0.03 to 1.0 GPa (1 to 31 km depth) for clinopyroxene. The results suggest that Ilicaksu lamproites were probably generated from phlogopite-bearing lithospheric source, and the extraction depth from mantle source is less than 58 km (like Mediterranean lamproites). They had occurred at collision-related extensional tectonics (or post orogenic) areas controlled by low-pressure crystallization.