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The analcime problem and its impact on the geochemistry of ultrapotassic rocks from Serbia

D. Prelevi^1,*, S. F. Foley², V. Cvetkovi¹ and R. L. Romer³

¹ Faculty of Mining and Geology, University of Belgrade, uina 7, 11000 Belgrade, Yugoslavia
² Institute of Geological Sciences, University of Mainz, Becherweg 21, 55099 Mainz, Germany
³ GeoForschungsZentrum Potsdam, Telegrafenberg, D-14473 Potsdam, Germany

^* E-mail: prelev{at}sanu.ac.yu

Tertiary ultrapotassic volcanic rocks from Serbia occasionally display low levels of K₂O and K₂O/Na₂O. In these rocks, analcime regularly appears as pseudomorphs after pre-existing leucite microphenocrysts. The process of leucite transformation in Serbian ultrapotassic rocks is very thorough: fresh leucite survives only in ugandites from the Koritnik lava flows as well as in rare inclusions in Cpx. This paper focuses on the impact of ‘analcimization’ on the mineralogy and geochemistry of the Serbian ultrapotassic rocks, using the samples where leucite survived as a monitor for the process.

Analcimization has had a great impact on the geochemistry of the rocks, but affects only a restricted number of chemical parameters. These are the falsification of the original K₂O/Na₂O ratio, the decoupling of large-ion lithophile elements resulting in considerable depletion of Rb and K₂O, but not of Ba, and sporadic, but extreme enrichment of Cs in some analcime-bearing samples (up to 900 ppm). Analcimization is also recognized by an increase in whole-rock ¹⁸O values of ~3 compared to fresh rocks, which correlates with the level of whole-rock hydration. Finally, the ⁸⁷Sr/⁸⁶Sr enrichment at nearly constant ¹⁴³Nd/¹⁴⁴Nd demonstrated by some rocks can also be explained by the analcimization of leucite. For samples with variable ⁸⁷Sr/⁸⁶Sr from the same lava flow, ⁸⁷Sr/⁸⁶Sr values correlate with modal analcime abundance (ex-leucite), loss on ignition of whole-rock and whole-rock ¹⁸O values. The extreme depletion in K and enrichment in Na, together with modification of other geochemical parameters, may have led to the misinterpretation of the origin and geodynamic affiliations of the Serbian ultrapotassic rocks, had the effects of analcimization not been taken into account.

KEYWORDS: analcimization, leucite, lamproites, kamafugites, alteration, oxygen isotope

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