C2/c pyroxene phenocrysts from three potassic series in the Neogene alkaline volcanics, NE Turkey: their crystal chemistry with petrogenetic significance as an indicator of P-T conditions
Thompson, Richard M.
Burt, Jason B.
Downs, Robert T.
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Chemical and structural data are reported for C2/c pyroxene phenocrysts collected from three potassic series (Group A: basanite-tephrite, Group B: tephrite-phonolitic tephrite, Group C: alkaline basalt-trachybasalt) of the Neogene alkaline volcanics (NAVs) in northeastern Turkey, in order to investigate the evolution of the magmatic plumbing system and the location of magma chamber(s) with crystallization conditions. The rock series hosting the clinopyroxene phenocrysts show generally porphyritic texture and have a variable phenocryst-rich nature (20-58%), with phenocryst assemblages characterized by cpx +/- A ol +/- A plag +/- A foid +/- A amp +/- A bio. The clinopyroxene phenocrysts can be chemically classified as Ti- and Fe3+-rich Al-diopsides for Groups A and B (AB-cpxs) and Ti- and Fe3+-poor Al-diopsides for Group C (C-cpxs). They have poorly variable composition, clustering in the diopside field. Structurally, the diopside groups have nearly similar a (ranging from 9.73 to 9.75 A...), V (cell) (437.2-440.9 A...(3)), and a OE (c) beta > angle values (106.01A degrees aEuro106.23A degrees), but some differences in polyhedral parameters and geometries of the AB-cpxs and C-cpxs have been observed. For example, the AB-cpxs are characterized by larger c (5.27-5.30 vs. 5.25-5.28 A...), V (T) (2.27-2.30 vs. 2.23-2.28 A...(3)), and V (M2) (25.53-25.72 vs. 25.41-25.59 A...(3)) values and smaller b (8.87-8.88 vs. 8.88-8.91 A...) and V (M1) (11.49-11.63 vs. 11.64-11.83 A...(3)) values with respect to the C-cpxs. In addition, the AB-cpxs show higher values of V (M2)/V (M1) (2.20-2.23) due to large V (M2) and small V (M1) compared to the V (M2)/V (M1) ratios of the C-cpxs (< 2.19). Such differences in the crystal structure of the AB-cpxs and C-cpxs from the NAVs are partly related to different crystallization pressures, but mostly related to variation in melt composition and, possibly, the influence of other crystallizing mineral phases. In particular, R(M2-O1) and R(M1-O2) (i.e. bond lengths) differences in the clinopyroxenes of different groups support the presence of evolved host rocks with different alkaline character (i.e. silica-undersaturated Groups A-B and silica-saturated Group C). Based on the cpx-geothermobarometry, the crystallization pressures for the C-cpxs are lower than 4.5 kbars, but the AB-cpxs have relatively high-pressure values (5.6-10.6 kbars), suggesting that the AB-cpxs crystallized in higher pressure environments. The relatively higher crystallization temperatures of the AB-cpxs also indicate higher cooling rates. The P-T estimates suggest that the source regions of the clinopyroxene phenocrysts from the NAVs were crustal magma chambers in a closed plumbing system at a moderate- to low-pressure regime.