Li-bearing tourmalines in Variscan granitic pegmatites from the Moldanubian nappes, Lower Austria

Abstract

Crystal structures, chemical (including light elements) and spectral data (optical and Mossbauer spectroscopies) were used to characterize coloured (brown, pink, green) tourmalines from three granitic pegmatites from the Moldanubian nappes (Konigsalm, Maigen and Blocherleitengraben; Lower Austria). The tourmalines can be classified as fluor-schorl, schorl, foitite, magnesiofoitite, olenite and fluor-elbaite with varying Li contents, up to ~ 1.2 wt% Li_2O. Coexisting minerals are quartz, plagioclase (up to An_7), microcline, garnet (spessartine-almandine), muscovite, biotite (annite), very rare lepidolite, apatite, monazite-(Ce), xenotime-(Y), allanite-(Ce) and zircon. The chemical composition of the Fe^(2+)-rich tourmaline samples (up to ~ 1.0 wt% TiO_2) varies from fluorschorl, with α = 15.987(2), c = 7.163(2) A to ^X(_(0.63)Na_(0.37)) ^Y(Fe^(2+)_(1.12)Al_(1.09)Mg_(0.56)Mn^(2+)_(0.08)Fe^(3+) _(0.07)Li_(0.02)Ti^(4+)_(0.01)Zn_(0.01)_(0.04)) ^Z(Al_(5.74)Mg_(0.26)) (BO_3)_3 [Si_(5.96)Al_(0.04)O_(18)] ^V(OH)_3 ^W[(OH)_(0.95)F_(0.05)], strongly dichroic (pink and blue) foitite, with α 15.9537(2), c = 7.1448(4) A, to ^X(_(0.51)Na_(0.49)) ^Y(Fe^(2+)_(0.97)Al_(0.93)Mg_(0.75)Fe^(3+)_(0.23)Mn^(2+) _(0.04)Li_(0.01)Ti^(4+)_(0.01)_(0.06))^Z(Al_(5.72)Mg_(0.28)) (BO_3)_3 [Si_(5.95)Al_(0.05)O_(18)]^V(OH)_3^W[(OH)_(0.91)O_(0.06)F_(0.03)], magnesiofoitite, with α = 15.9476(4), c = 7.1578(4) A. The chemical composition of the Al- and Lirich and Mn^(2+)-bearing (up to ~ 5.7 wt% MnO) samples varies from ^X(Na_(0.84)Ca_(0.02)_(0.14))^Y(Al_(1.35)Li_(0.78)Mn^(2+) _(0.65)Ti^(4+)_(0.01)_(0.21))^ZAl_6(BO_3)_3[Si_(5.92)Al_(0.04)B_(0.04)O_(18)]^V(OH)_3^W[F_(0.81)(OH)_(0.19)], fluor-elbaite with α = 15.8887(3), c = 7.1202(3) A, to ^X(Na_(0.76)Ca_(0.12)_(0.12))^Y(Al_(1.52)Li_(0.69)Mn^(2+) _(0.43)Fe^(2+)_(0.09)_(0.27))^ZAl_6(BO_3)_3 [Si_(5.71)B_(0.29)O_(18)]^V(OH)_3^W[F_(0.69)(OH)_(0.31)], B-rich fluorelbaite, with α = 15.8430(3), c = 7.1051(3)A. A positive correlation between the and bond lengths in tourmalines where the Z site is only occupied by Al (R^2 = 0.617) is useful to correct the bond length for the inductive effect of the varying bond length. This is important for producing accurate assignments for the different 6-coordinated sites in tourmaline. On the basis of Sm-Nd (garnet, monazite), U-Th-Pb, and U-Pb ages (monazite), the pegmatites crystallised during the Variscan tectonometamorphic event in the Visean (339 4 Ma Maigen, 332 3 Ma Konigsalm). These ages are in the range of the earliest intrusions of the South Bohemian pluton (Rastenberg type durbachites). However, on the basis of the spatial relationship of the pegmatites and the Rastenberg type intrusions, a linkage of the intrusive body and the pegmatites is unlikely. Alternatively, the pegmatites may have evolved as granitic pegmatitic melts during decompression from the surrounding country rocks in the frame of exhumation of the Moldanubian nappes after the peak of the Variscan metamorphism.