Distributions of Y + REE and Sc in tourmaline and their implications for the melt evolution; examples from NYF pegmatites of the Třebíč Pluton, Moldanubian Zone, Czech Republic
Tourmalines from NYF and mixed (NYF + LCT) pegmatites of the Třebíč Pluton (Moldanubian Zone, Czech Republic) are generally characterized by variable and relatively high concentrations of Y + REE (4-457 ppm) and Sc (18-765 ppm). They show enrichment in LREE (Ce > La); the chondrite-normalized REE patterns decrease uniformly from La to Tb; Tb to Tm are mostly below the detection limits (< 0.01-0.08 ppm) and are followed by an upturn to Yb and Lu. Our textural and paragenetic observations indicate that growth of magmatic tourmaline reflects Y + REE contents in the melt. Tourmaline is highly compatible with Sc and shows exceptionally high contents of Sc compared to the analyzed host pegmatite units (less than 1 ppm Sc). The contents of Sc positively correlate with ∑Y + REE. Two distinct evolutionary trends of trace-elements in tourmaline were distinguished. (i) A conspicuous increase in Y + REE and Sc in tourmaline (dravite, Ca-rich dravite) from primitive euxenite-type pegmatites reflects their enrichment in the melt with the progressive crystallization and fractionation. It also was enhanced by low P (< 0.02 wt. % P2O5) and Ca contents (< 0.8 wt. % CaO) in host pegmatite unit preventing precipitation of Y + REE phosphates and allanite, respectively. (ii) Tourmaline (schorl, fluor-schorl, fluor-elbaite) from more evolved pegmatites (evolved euxenite-type and mixed (NYF + LCT) pegmatite) show a depletion in REE and Sc with progressive crystallization reflecting most likely the coeval crystallization of REE-rich minerals, especially monazite-(Ce), xenotime-(Y), samarskite- and aeschynite-group minerals. The Eu anomalies in tourmaline vary significantly from positive (up to Eu/Eu* ~ 16) to substantially negative ones and are related to the Eu contents in the melt and degree of its fractionation.
SNIP (Scopus, 2015): 0.700
IF (ISI, 2015): 1.326
Policy: Open Access