K puvodu analcimu v mezozoickych a kenozoickych vulkanitech Ceske republiky a jeho uloze v nazvoslovi hornin

Magmatic origin of analcime has been discussed for decades (e. g., Karlsson and Clayton 1991, 1993; Pearce 1993). Despite the fact that analcime has been re-classified as zeolite and its secondary (post-magmatic) origin is globally accepted (e. g., Roux and Hamilton 1976, Giannetti and Masi 1989, Wilkinson and Hensel 1994), it is still commonly used in classification of alkaline rocks in the Czech Republic. In these rocks, analcime can be mostly found in the pseudomorphs after leucite (Fig. 1a; see Rapprich 2003), or as a homogeneous anhedral filling in the groundmass (Fig. 1b). In the second case, the boundaries of individual analcime crystals cannot be identified due to the isotropic optical properties of analcime. Therefore, origin and growth of analcime is difficult to reconstruct.

For this contribution, we have studied samples of Mesozoic and Cenozoic alkaline rocks from three localities across the Czech Republic, where partly analcimized glass in the groundmass was preserved, with an aim to better understand the origin of analcime in alkaline rocks. Mesozoic augitite from the southwestern slope of the Petřkovice Mt. near Nový Jičín (sample TG05) displays small isometric colourless, optically isotropic domains (analcime) enclosed in originally glassy groundmass (Fig. 1c–f ). This texture suggests the analcime is replacing original glass in the groundmass, enclosing also microcrysts of the stable minerals in groundmass. Very similar texture was observed also in Oligocene augitite from Mětikalov in Doupovské hory Mts. (sample DR338, Fig. 2). In this rock, the analcime domains either mantle larger phenocrysts, or are distributed within the groundmass. The growth of the analcime domains on the edges of larger phenocrysts resembles growth of spherulites, which represent products of silica-rich glass recrystallization (e. g., Breitkreuz 2013). To further investigate this possibility, additional samples were collected from Kamenický vrch near Zákupy (Fig. 3). In individual samples, number and size of analcime domains vary, which suggests, that these domains represent various stages of a continuous growth. The X-ray elemental mapping of Al and Na (Fig. 4) then shows, that the analcime domains are represented by single grains of cubic analcime.

As a result, we may conclude that analcime in groundmass of Mesozoic and Cenozoic alkaline rocks originates from devitrification of original glass. These rocks hence should not be classified as “analcimites” or “analcimic …” but rather as “analcimized …” according to Le Maitre et al. (2005).

References

Breitkreuz, C. (2013): Spherulites and lithophysae – 200 years of investigation on high-temperature crystallization domains in silica-rich volcanic rocks. – Bull. Volcanol. 75, 4, 1–16.

Deer, W. A. – Howie, R. A. – Zussman, J. (2013): An introduction to rock-forming minerals (3rd edition). – Mineralogical soc. London.

Giannetti, B. – Masi, U. (1989): Trace-element behaviour during weathering of leucite in potassic rocks from the Roccamonfina volcano (Campania, southern Italy) and environmental implications. – Lthos 22, 4, 317–324.

Holub, F. V. – Rapprich, V. – Erban, V. – Pécskay, Z. – Mlčoch, B. – Míková, J. (2010): Petrology and geochemistry of the Tertiary alkaline intrusive rocks at Doupov, Doupovské hory Volcanic Complex (NW Bohemian Massif). – J. Geosci. 55, 3, 251–278.

Hughes, CH. J. (1982): Igneous Petrology. – 551 str. Academia. Praha.

Karlsson, H. R. – Clayton, R. N. (1991): Analcime phenocrysts in igneous rocks: Primary or secondary? – Amer. Mineralogist 76, 1–2, 189–199.

Karlsson, H. R. – Clayton, R. N. (1993): Analcime phenocrysts in igneous rocks: Primary or secondary? Reply. – Amer. Mineralogist, 78, 1–2, 230–232.

Le Maitre, R. W. – Streckeisen, A. – Zanettin, B. – Le Bas, M. J. – Bonin, B. – Bateman, P., ed. (2005): Igneous rocks: a classification and glossary of terms: recommendations of the International Union of Geological Sciences Subcommission on the Systematics of Igneous Rocks. – Cambridge Univ. Press. Cambridge.

Pearce, T. H. (1993): Analcime phenocrysts in igneous rocks: Primary or secondary? Discussion. – Amer. Mineralogist, 78, 1–2, 225–229.

Prelević, D. – Foley, S. F. – Cvetković, V. – Romer , R. L. (2004): The analcime problem and its impact on the geochemistry of ultrapotassic rocks from Serbia. – Mineral. Mag. 68, 4, 633–648.

Rapprich, V. (2003): Analcimizované leucity a Ti-flogopit Doupovských hor. – Zpr. geol. Výzk. v Roce 2002, 180–181.

Roux, J. – Hamilton, D. L. (1976): Primary igneous analcite – an experimental study. – J. Petrol. 17, 2, 244–257.

Streckeisen, A. (1976): To each plutonic rock its proper name. – Earth Sci. Rev-s 12, 1, 1–33.

Watkins, J. – Manga , M. – Huber, C. – Martin, M. (2009): Diffusion-controlled spherulite growth in obsidian inferred from H2O concentration profiles. – Contr. Mineral. Petrology, 157, 2, 163–172.

Wilkinson, J. F. G. – Hensel, H.-D. (1994): Nephelines and analcimes in some alkaline igneous rocks. – Contr. Mineral. Petrology 118, 1, 79–91.