Origin of analcimes in the Tertiary volcanic rocks from the Eastern Pontides (NE Turkey): a textural, mineralogical and geochemical approach

摘要

Silica-undersaturated mafic volcanic rocks of Tertiary age in form of lava flows, dikes and sills of tephritic compositions extend along the northern part of the Eastern Pontides. These rocks typically contain Ca-clinopyroxene (Wo_(32-53)En_(26-54)Fs_(8-24)), analcime, Fe-Ti oxides, apatite and minor olivine, plagioclase (An_(49-94)), K-feldspar (Or_(29-66)), hornblende and phlogopite in mi-crogranular porphyritic, hyalo-microlitic porphyritic and glomeroporphyritic textures. Petrochemical features of the studied rocks, such as enrichment in LILE, depletion in Nb, Ta and Ti and moderate or high LREE/HREE ratios, provide strong evidence for melt production from a subduction-metasomatized lithospheric mantle. Besides, the rocks are characterized by their low K_2O contents and low K_2O/Na_2O ratios as a result of analcimization. Analcime detected in these rocks appears to have formed by pseudomor-phic mineral replacement reactions. Euhedral or anhedral analcime crystals, 100-600 μm in diameter, are set in groundmass and display six-sided polygon or rounded shape and irregular cracks. The analcime pseudomorphs are colourless, milky white to grey and occur in clinopyroxene or in the groundmass. Based on the electron microprobe results, analcimes contain a minor amount of cations other than Na and probably represent the remnants of leucite as marked by K-zoning. X-ray diffraction studies of the analcime-bearing samples confirm the presence of cubic and tetragonal analcime crystals in addition to tetragonal leucite. The presence of Ca-clinopyroxene and plagioclase rather than their sodic equivalents, presence of anhydrous mineral phases and lack of Na differentiation trend as well as partial alteration of the rocks likely supports the secondary growth of analcime either during cooling or after magma solidification.