Vanadian and chromian garnet- and epidote-supergroup minerals in metamorphosed Paleozoic black shales from Cierna Lehota, Strazovske vrchy Mountains, Slovakia: crystal chemistry and evolution

摘要

Silicate minerals enriched in V, Cr and Mn including garnets and epidote-supergroup members, in association with amphiboles, albite, hyalophane, titanite, chamosite, sulfides and other minerals occur in Devonian black shales near Cierna Lehota in the Straovske vrchy Mountains, 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 (Gld 2A,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 CaMn1 and (V,Cr)Al-1, resulting in coupled Ca(V,Cr)Mn1Al1. Epidote-supergroup minerals occur as abundant anhedral crystals with variable compositional zoning. Nearly all crystals have a complete zoning sequence beginning with REE-rich allanite-(La), followed by mukhinite and by V- and Cr-rich clinozoisite to mukhinite and V- and Cr-poor clinozoisite. In common with 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+) substitutions introducing REE, together with VAl1 and CrAl1 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 minerals investigated 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 Suchy Massif. Decreased temperature following the metamorphic peak probably resulted in the formation of REE-, V- and Cr-poor clinozoisite and secondary garnet.