Contrasting Origins of the Mixed (NYF + LCT) Signature in Granitic Pegmatites, with Examples from the Moldanubian Zone, Czech Republic

Abstract

Two granitic pegmatites with mixed (NYF + LCT) signatures from the Moldanubian Zone, Czech Republic, were examined to determine their origin. On the basis of geological setting, paragenesis, geochemical modeling and chemical composition of rocks and minerals, two contrasting modes of origin were discerned. The Li-bearing (lithium micas + elbaite) Kracovice pegmatite, with "amazonite", samarskite and fergusonite, is the product of the strong fractionation of pegmatite-forming melt derived from the orogenic I-type Třebíč pluton. Geological, petrological, compositional and isotopic data from this ultrapotassic melasyenite-melagranite point to mixing of a mantle-derived magma with (leuco)granitic melt from crustal rocks. The granitepegmatite system of the Třebíč pluton and the mainly Li-bearing Kracovice pegmatite are typical examples of the mixed (NYF + LCT) petrogenetic family. The contaminated elbaite-subtype pegmatite Bližná I, Český Krumlov Unit, with lithium tourmalines (elbaite, liddicoatite, uvite), primary REE minerals [bastnäsite-(Ce), parisite-(Ce), allanite-(Ce)] and Ca(Mg)-rich minerals (diopside, uvite, titanite), gained its mixed signature through a unique pre-emplacement process involving the external contamination with Ca, Mg and REE of an evolved (LCT) pegmatite-forming melt by distal carbonatite-like marbles with an NYF signature. Despite the clearly mixed-signature mineralogy and geochemistry, the Bližná I pegmatite represents a special type of contaminated LCT pegmatite rather than a member of the mixed family.

Origin of the pegmatite veins within the skarn body at Vevčice near Znojmo (Gföhl Unit, Moldanubian Zone)

“…Pattison 1991), (b) in-situ dehydration melting of basic igneous rocks (e.g. Johnson et al 2012 and the references therein), (c) injection by acidic melt from a more distal source and its contamination by the host skarn (e.g., Novák et al 2012Novák et al , 2013Gadas et al 2014).…”

(Fe,Mn)-(Ti,Sn)-(Nb,Ta) oxide assemblage in a little fractionated portion of a mixed (NYF + LCT) pegmatite from Piława Górna, the Sowie Mts. block, SW Poland

“…10 Ternary plots of tourmaline compositions from the Julianna-type dykes: a -X-site vacancy-Ca 2+ -(Na + K) rounding host rocks (e.g. Galliski et al 2008;Novák et al 2012), (2) pre-emplacement contamination of the pegmatite-forming melt, (3) influx of a more primitive pegmatitic melt, (4) competition for the elements among co-crystallizing minerals, and (5) local changes in melt composition.…”

“…However, the pegmatites only exceptionally attain a very high degree of geochemical evolution, and also contain typical LCT-signature minerals. Recently, Novák et al (2012) described pegmatites from Bližná and Kracovice, Czech Republic, as representatives of such a mixed NYF + LCT signature in the Moldanubian Zone.…”

The Julianna pegmatite vein system at the Piława Górna Mine, Góry Sowie Block, SW Poland – preliminary data on geology and descriptive mineralogy

“…There fore the en tire peg ma titic sys tem is as signed to the mixed NYF + LCT fam ily. Ge netic sce nar ios pro posed to ex plain the mode of or i gin of hy brid NYF + LCT pegmatites in clude par tial melt ing of an only par tially de pleted crustal source, anatexis of mixed de pleted and undepleted lithologies as well as var i ous pos si bil i ties of ex ter nal con tam i na tion (Èerný, 1991;Èerný and Ercit, 2005;Mar tin and De Vito, 2005;Novák et al, 2012).…”