The genetic significance of snowball quartz in highly fractionated tin granites of the Krusne Hory/Erzgebirge

摘要

Snowball-textured quartz phenoblasts are magmatic-hydrothermal fluid saturation textures and occur widespread in highly evolved alkali feldspar and topaz-bearing granites. They are characterized by zonal arrangement of entrapped matrix minerals (albite, K-feldspar, mica) alternating with inclusion-free zones. Case studies from the Schellerhau and Podlesi granites (Erzgebirge/Krusne Hory) showed a crystallization sequence albite-K-feldspar-Li-mica-snowball quartz. If the growth velocity increased rapidly, beside silicate melt drops also predating and simulatanously formed matrix minerals were overgrown and trapped in the quartz. CL and growth zoning studies indicate a kinetically caused increase of the growth velocity with crystal size in a nearly non-convecting crystal mush. Wavy zones of snowball quartz are interpreted as products of small scale diffusion fronts which lead to the change of CL properties developed during the crystal growth and caused by rapid variation of the melt conditions.