Original paper

Distribution of zinc and its role in the stabilization of spinel in high-grade felsic rocks of the Moldanubian domain (Bohemian Massif)

Tajčmanová, Lucie; Konopásek, Jiří; Košler, Jan

Abstract

(Fe—Zn—Mg)-spinel-bearing assemblages in the high-grade rocks from three localities in the Moldanubian domain of the Bohemian Massif (Variscan belt of Central Europe) have been studied to monitor the partition behaviour of zinc along the pressure-temperature path followed by the host rock and to discuss the role of Zn in spinel stabilization. Spinels in two studied samples are characterized by low gahnite content (max. 0.09 Zn p. f. u), they crystallized only under silica-undersaturated conditions and the presence of zinc is thus assumed to be insignificant for their stabilization. One sample contains spinels that are gahnite-rich (up to 0.58 Zn p.f.u) leading to their stabilization within a quartz-bearing matrix. All spinel-bearing domains reflect late re-equilibration corresponding to decompression. However, in samples with preserved relics of a high-pressure mineral assemblage, changes in the zinc content attributed to prograde garnet growth were observed in the garnet cores. In order to monitor the distribution of zinc along the pressure-temperature path, textural observations have been combined with electron-microprobe and laser ablation ICP-MS data. The results point to preferential partitioning of zinc into biotite during the spinel-absent early stages of decompression and suggest that biotite was the main source of zinc for spinel crystallizing at low pressures.