Abstract

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 δ18O 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 δ18O 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.

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