Vanadium-bearing tourmaline in metacherts from Chvojnica, Slovak Republic: crystal chemistry and multistage evolution

Abstract

Vanadium-bearing tourmaline was found in Paleozoic metacherts near Chvojnica in the Strážovské vrchy Mountains, Slovakia. The geochemistry of metacherts suggests that the protolith was deposited under anoxic conditions. Tourmaline from Chvojnica displays strong chemical zoning, with high Mg and a very low Fe content. The core has the composition of magnesiofoitite with a low V content, and is mainly controlled by alkali-deficient substitution [ X □(Al,V)(NaMg) -1 ]. The other zones have a dravitic composition considering ordering of all Mg to the Y sites, but they may reach the composition of olenite if one proposes strong Al-Mg disorder (not proven by structural refinement). The intermediate zone is mostly enriched in V (up to 3.3 wt.% V 2 O 3 , 0.42 apfu V), and V is considered to be incorporated into the structure by VAl -1 substitution. Two rim zones were observed. Rim 1 is depleted in V but is enriched in Al. Rim 2 is similar in composition to the intermediate zone, with increased V. The increased content of trivalent cations, including Al, V and Cr in comparison to divalent Mg and Fe, suggests a deprotonation of (OH)by the proton-deficient substitution Y (Al,V)O Y Mg -1 (OH) -1 . The increase of Ca, Ti and Mg content from core to rim may be the result of several substitutions, including TiMg(Al,V) -2 , CaMgO X □ -1 (Al,V) -1 (OH) -1 . The V-bearing tourmaline from Chvojnica is the result of multistage Pre-Alpine regional metamorphism. The core of the grains is a product of the lower-grade M1 metamorphic event, whereas the intermediate zone corresponds to the second metamorphic event, M2. Medium-grade metamorphism (580 < T < 600°C) remobilized V from the organic matter present in the protolith of the metacherts. Rim 1 may be the result of reheating of the host rock due to the intrusion of the Malá Magura granitic massif (M3 event). The chemical composition of rim 2 was most likely influenced by the remobilization of V from older V-bearing silicates during the M3 (M3b stage?) metamorphic event. SoMMAireNous décrivons la tourmaline vanadifère découverte dans des métacherts paléozoïques près de Chvojnica, montagnes Strážovské vrchy, Slovaquie. D'après la géochimie de ces roches, le protolithe aurait été déposé en conditions anoxiques. La tourmaline de Chvojnica fait preuve d'une zonation chimique importante, avec des compositions fortement enrichies en Mg et une teneur en Fe très faible. Le noyau a la composition de magnésio-foïtite à faible teneur en V, et sa composition répond à la substitution impliquant une déficience en alcalins [ X □(Al,V)(NaMg) -1 ]. Les autres zones possèdent une composition dravitique si tout le Mg est placé au site Y, mais pourraient atteindre le champ de l'olénite si on propose un fort désordre dans la distribution des atomes Al-Mg, ce qui n'est pas encore documenté avec un affinement de la structure. La zone intermédiaire est enrichie en V (jusqu'à 3.3% V 2 O 3 , poids, 0.42 apfu V), et le V serait incorporé dans la structure selon la substitution VAl -1 . Il y a deux zones distinct...