Vanadian and chromian garnet- and epidote-supergroup minerals from Čierna Lehota, Strážovské vrchy Mts., Slovakia

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Marco E. Ciriotti
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Vanadian and chromian garnet- and epidote-supergroup minerals from Čierna Lehota, Strážovské vrchy Mts., Slovakia

Messaggio da Marco E. Ciriotti » sab 14 ott, 2017 11:16

Prossima pubblicazione.

Referenza:
▪ Bačík, P., Uher, P., Kozáková, P., Števko, M., Ozdin, D., Vaculovič, T. (2017): Vanadian and chromian garnet- and epidote-supergroup minerals in metamorphosed Paleozoic black shales from Čierna Lehota, Strážovské vrchy Mts., Slovakia: crystal chemistry and evolution. Mineralogical Magazine, 81, (in press).

Abstract:
Silicate minerals enriched in V, Cr and Mn including garnets and epidote-supergroup members, in association with amphiboles, albite, hyalophane, titanite, chamosite, sulphides and other minerals occur in Devonian black shales near Čierna Lehota in the Strážovské vrchy Mts., Slovakia. The garnets have high concentrations of V, Cr and Mn (up to 17 wt.% V2O3, 11 wt.% Cr2O3 and 21 wt.% MnO) and several compositional types. Vanadian-chromian grossular (Grs 1) usually preserves primary metamorphic oscillatory zoning, whereas solid solutions between goldmanite (Gld2A-B), V- and Cr-rich grossular and spessartine (Grs 2A B, Sps 2) form irregular domains or crystals with variable zoning. Dominant substitutions in the garnets include CaMn-1 and (V,Cr)Al-1, resulting in coupled Ca(V,Cr)Mn-1Al-1. Epidote supergroup minerals occur as abundant anhedral crystals with variable chemical zoning. Nearly all crystals have a complete zoning sequence beginning with REE-rich allanite-(La), followed by mukhinite and 27 by V- and Cr-rich clinozoisite to mukhinite and V- and Cr-poor linozoisite. Similar to the garnets, the epidote-supergroup minerals are enriched in V, Cr and Mn (<7 wt. % V2O3, <5 wt. % Cr2O3, and <3 wt. % MnO). Lanthanum is the dominant REE (up to 11.5 wt. % La2O3) in allanite-(La). The composition of epidote-supergroup minerals is controlled by REEFe2+(CaAl)-1, REEMg(CaAl)-1, REEMn2+(CaAl)-1, and REEFe2+(CaFe3+31 )-1 substitutions introducing REE, together with VAl-1 and CrAl-1 substitutions. The negative Ce and slightly positive Eu anomalies displayed in chondrite-normalized patterns and enrichment in V, Cr and Mn are ascribed to the geochemical properties of the protolith. The studied minerals exhibit multi-stage evolution: (1) presumed low-grade greenschist-facies metamorphism and (2) development of V- and Cr-rich zones in both garnet- and epidote supergroup minerals which result from late-Variscan contact metamorphism due to granitic intrusion of the Suchý Massif. Decreased temperature following the metamorphic peak likely resulted in formation of REE-, V- and Cr-poor clinozoisite and secondary garnet.
Marco E. Ciriotti

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