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

Abstract

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. % P 2 O 5 ) 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 precipitation 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.

Sc- and REE-rich tourmaline replaced by Sc-rich REE-bearing epidote-group mineral from the mixed (NYF+LCT) Kracovice pegmatite (Moldanubian Zone, Czech Republic)

“…300 m W of the edge of the Třebíč Pluton. It represents the most evolved pegmatite body from the population of dominantly NYF pegmatites related to the Třebíč Pluton (Němec 1990;Novák et al 1999Novák et al , 2012Škoda et al 2006;Čopjaková et al 2013a). From the contact inward, the pegmatite consists of: a narrow zone of a coarse-grained granitic unit (Kfs+Pl+Qz+Bt+Ms+Ttn), a wide graphic unit (Kfs+Qz+Tu±Bt), which evolves to minor blocky K-feldspar, and an albite unit situated close to a small quartz core in the most differentiated part of the dike.…”

“…Tourmaline is present in several morphological, paragenetic, and compositional types (Novák 2000;Čopjaková et al 2013a). Black to brown prismatic crystals and their aggregates, up to several centimeters in size, occur in the graphic unit, blocky K-feldspar, albite unit, and massive quartz; their composition correspond to Al-rich schorl (Al ≥ 6.57 apfu).…”

“…The pegmatites range from barren to highly fractionated dikes with LCT (enriched in Li, Cs, Ta; dated at 340-325 Ma, Novák et al 1998a;Melleton et al 2012), NYF (enriched in Nb, Y, F; dated at ~340-335 Ma) and mixed NYF+LCT geochemical signatures using the current classification schemes (Černý and Ercit 2005;Černý et al 2012). The Třebíč syenite (durbachite) Pluton (for more details see, e.g., Janoušek and Holub 2007) hosts a population of intragranitic NYF pegmatites ranging in textural-paragenetic and geochemical features from primitive metaluminous allanite-subtype pegmatites with allanite-(Ce), titanite, ilmenite, and tourmaline (dravite > schorl), to more evolved, metaluminous to slightly peraluminous euxenitesubtype pegmatites containing tourmaline (schorl), ilmenite, titanite, aeschynite-, and euxenite-group minerals, beryl, zircon, tinzenite, and herzenbergite (Škoda and Novák 2007;Novák et al 2011aČopjaková et al 2013a).…”

“…From the contact inward, the pegmatite consists of: a narrow zone of a coarse-grained granitic unit (Kfs+Pl+Qz+Bt+Ms+Ttn), a wide graphic unit (Kfs+Qz+Tu±Bt), which evolves to minor blocky K-feldspar, and an albite unit situated close to a small quartz core in the most differentiated part of the dike. Typical minor-to-accessory minerals include tourmaline, Y,Sc-enriched spessartine, topaz, Li micas (Mn-rich polylithionite, masutomilite), beryl, cassiterite, zircon, niobian rutile, minerals of the columbite-, fergusonite-, and samarskite-groups, wolframoixiolite, F-rich hambergite, monazite-(Ce), xenotime-(Y), and an REE-bearing epidote-group mineral (Němec 1990;Novák et al 1998bČopjaková et al 2013a). Yttrium-and REEbearing oxides are almost exclusively associated with garnet in the albite unit.…”

“…Secondary allanite sometimes occurs as a breakdown product of feldspars, biotite, and eudialyte during interaction of hydrothermal fluids with granitoid rocks (Pantó 1975;Ward et al 1992;Coulson 1997). Nevertheless, the mineral assemblage tourmaline + allanite is very rare in granitic pegmatites and other granitic rocks (Novák et al 2011aČopjaková et al 2013a). …”