Crystal chemistry and evolution of tourmaline in tourmalinites from Zlatá Idka, Slovakia

[摘要] Tourmalinites occur in early-Paleozoic metamorphic rocks of the Gemeric Unit near Zlatá Idka village, Western Carpathians, eastern Slovakia. Tourmaline compositions, analyzed with the electron microprobe, include a wide range oftourmaline species. Tourmaline in tourmalinites from Zlatá Idka is compositionally variable, with the dominant substitutionMg–Fe2+ consistent with prevalent schorl–dravite compositions and their fluor- and oxy-dominant counterparts – fluorschorl, fluor-dravite, oxy-schorl and oxy-dravite. Portions of tourmaline are enriched in Ca in the form of the fluor-uviteand magnesio-lucchesiite components. A subset of the compositions has Ti > 0.25 atoms per formula unit (apfu) andcorresponds to the hypothetical “magnesio-dutrowite”, Mg-dominant analogue of dutrowite. In addition, some of thetourmalines are X-site vacant and classified as foitite. The crystal chemistry of tourmaline is complex and influencedby several exchange mechanisms, including Mg(Fe)–1, Al□(Mg,Fe)–1Na–1, AlO(Mg,Fe)–1(OH)–1 (Mg,Fe)CaAl–1Na–1,MgCaOAl–1□–1(OH)–1, Ti0.5O(Fe,Mg)–0.5(OH)–1 and TiMg(Al)–2 substitutions. In general, tourmalines in all samples usuallyhave oscillatory-zoned dravitic cores and schorlitic rims (Tur I). However, in ZLT-4 and ZLT-6 samples, some crystalshave secondary Mg-dominant and Ca-enriched overgrowths (Tur II), partially replacing Tur I. Tourmalinites were mostlikely produced by regional or contact metasomatic processes, likely due to the intrusion of the Permian Poproč graniticmassif. Origin of tourmalinites likely results from the flow of late-magmatic to early post-magmatic B,F-rich fluidsfrom the granite intrusion into adjacent metamorphic rocks. The tourmaline crystallization and its resulting chemicalcomposition were controlled by both the metapelitic host rock and the granitic intrusion; the Mg-rich cores of the Tur Iare most likely compositionally related to the metapelitic host rock, whereas later schorlitic to foititic compositions inrims suggest origin due to the intrusion-triggered fluid flow. The significant changes and oscillations of tourmaline zoning imply a dynamic, unstable fluid regime. The late Ca-rich Tur II could result from subsequent metasomatic processesassociated with the alteration of host-rock minerals.

[学科分类] 自动化工程

[关键词] tourmaline supergroup;tourmalinites;magnesio-lucchesiite;titanium;metasomatism