Magnesian tourmalines from plagioclase–muscovite–scapolite metaevaporite layers in dolomite marble near Prosetín (Olešnice Unit, Moravicum, Czech Republic)

Abstract

Radial aggregates of blue-grey tourmaline were found in plagioclase-muscovite-scapolite metaevaporite layers in dolomite marble near Prosetín (Olešnice Unit, Moravicum, Czech Republic). It occurs in association with plagioclase (An15-37), muscovite, scapolite, phlogopite, vermiculite, pumpellyite-(Al), and clinozoisite. Electron-microprobe analyses of tourmaline show dravitic composition with very high content of Mg (1.92 to 2.77 apfu), Al (up to 6.71 apfu), low content of Fe (up to 0.39 apfu) and variable amounts of vacancies (0.09 to 0.47) and Ca (0.03 to 0.29 apfu) in the X-site. Some analyses correspond to "oxy-dravite" and some others almost attain magnesio-foitite compositions. The proportion of X-site vacancy decreases from the crystal cores to their rims while Ca content increases. Generally, charge excess due to the high Al-contents is balanced either by an increasing X-site vacancy or by deprotonization ofWOH; theWO2-content calculated from charge-balanced formula attains 0.71 apfu. Lattice parameters [a = 15.9116(6) Å; c = 7.1987(4) Å] and deduced average bond lengths ( = 1.995 Å; = 1.929 Å) indicate a relatively high Al-Mg disorder. Three main substitution mechanisms are inferred to operate in the studied magnesian tourmalines: (1) CaMg(NaAl)-1, mainly in Ca-enriched dravite, (2)X?Al(NaMg)-1in nearly magnesio-foititic compositions, and (3) AlO(MgOH)-1in "oxydravitic" members. The tourmaline is relatively poor in trace elements; only Ti, Sr, and Ga exceed 100 ppm according to LA-ICP-MS study. There is a pronounced positive correlation between Sr and Ca (r2= 0.77), which suggests that Sr accumulated in Ca-enriched zones of dravite. The pale blue-grey color of the studied tourmalines is most likely a result of Fe2+crystal feld transitions along with the absence of signifcant amounts of other chromophores. Trace-element contents, mineral assemblage and compositional zoning of tourmalines as well as host-rock mineral association suggest prograde metamorphic growth and support metaevaporitic origin of the plagioclase- muscovite-scapolite rocks.