Mineralogy and Geochemistry of Melilite Leucitites, Balçıkhisar, Afyon (Turkey)

Authors: CÜNEYT AKAL

Abstract: The Middle Miocene Afyon volcanic complex crops out in west-central Anatolia and comprises products of extensive potassic-ultrapotassic volcanic activity that intrude and/or cover the sedimentary formations of the western Taurides. Based on their stratigraphic setting, three stages of potassic and ultrapotassic volcanism are distinguishable. Melilite-leucitite lavas are the products of the first stage and followed by lamproites that represent the second stage of volcanism. The leucitite block- and fragment-rich Balçıkhisar volcaniclastic succession overlies the products of the first and second stage volcanism. Lacustrine sedimentary rocks cover the products of first and second stage volcanism. Lacustrine sedimentary rocks are gradationally overlain by the Tokluk volcano-sedimentary succession. Phonotephritic lava domes, dykes, and flows represent the third stage of volcanic activity. Melilite leucitites from two occurrences in the Afyon volcanic province were studied. The melilite leucitites crop out as small lava domes and spatially limited lava flows. The melilite-leucitite lavas typically have porphyritic texture and are composed of leucite, nepheline, barium feldspar, melilite, diopside, melanite, calcite, apatite, and opaque minerals. Melilite crystals are relatively gehlenite- to akermanite-rich. The diopsidic clinopyroxene phenocrysts are partially and/or completely transformed to melilite aggregates. Geochemically, the SiO2 content of the melilite leucitites varies from 44% to 47% and they are ultrapotassic with K2O contents of 9.5 to 11.8% and average Na2O/K2O of about 2. The Al2O3 and CaO contents are rather high (18%), whereas TiO2 contents are low (1.22%). Melilite leucitites have highly fractionated REE and incompatible element compositions. The Ba (8745 ppm), Rb (670 ppm) and Sr (4802 ppm) contents are enriched relative to primitive mantle and chondrites. The melilite leucitites were generated by partial melting of a primitive mantle source or an enriched source near a subduction zone and collision processes related to an active plate margin.

Keywords: leucite, melilite, leucicite, ultrapotassic, Afyon volcanic region, west-central Anatolia

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