AbstractThe evolution of the trace-element patterns of quartz during crystallization
of pegmatite melt was investigated using laser ablation inductively coupled
plasma mass spectrometry. The contents of Al, B, Ba, Be, Cr, Fe, Ge, Li, Mn,
P, Rb, Sn, Sr and Ti were analysed in quartz from the border, intermediate
and core zones of four granitic pegmatites differing in degree of
fractionation and origin. The material investigated originates from the
pegmatite district of the Strážek Unit, Moldanubian Zone, Bohemian Massif,
Czech Republic and includes: lepidolite LCT (Li-Cs-Ta) pegmatite from Rožná;
berylcolumbite LCT pegmatite from Věžná; anatectic pegmatite from Znětínek;
and intragranitic NYF (Nb-Y-F) pegmatite Vladislav from the Třebíč Pluton.
The abundances of the elements analysed varied over wide intervals: <1 to
32 ppm Li, 0.5 to 6 ppm B, <1 to 10 ppm Ge, 1 to 10 ppm P, 10 to 450 ppm
Al, 1 to 45 ppm Ti and <1 to 40 ppm Fe (average sample contents).
Concentrations of Be, Rb, Sr, Sn, Ba, Cr and Mn are usually <1 ppm.
Quartz from LCT pegmatites exhibits a distinct evolutionary trend with a
decrease in Ti and an increase in Al, Li and Ge from the pegmatite border to
the core. In comparison with the most fractionated rare-metal granites,
pegmatitic quartz is relatively depleted in Al and Li, but strongly enriched
in Ge. Quartz from simple anatectic and NYF pegmatites is poor in all trace
elements with their evolution marked by a decrease in Ti and a small
increase in Ge. There is little Al or Li and neither shows any systematic
change with pegmatite evolution. Using the Ti-in-quartz thermobarometer, the
outer zones of the Znětínek and Vladislav pegmatites crystallized at ∼670°C,
whereas the border zone in the Rožná pegmatite yields a temperature near
610°C.
The Saxo-Danubian Granite Belt: magmatic response to post-collisional delamination of mantle lithosphere below the southwestern sector of the Bohemian Massif (Variscan orogen)