Trace element composition of quartz from the Variscan Altenberg–Teplice caldera (Krušné hory/Erzgebirge Mts, Czech Republic/Germany): Insights into the volcano-plutonic complex evolution

In this study, the internal structures and appropriate trace-element distributions within quartz grains were investigated using a laser ablation ICP-MS technique combined with a hot-cathode and scanning cathodoluminescence analysis. The studied Variscan magmatic suite is located in the Altenberg–Teplice Caldera (eastern Krušné hory/Erzebirge, along the Czech Republic/Germany border) and composed of co-magmatic volcanics (peraluminous basal rhyolite and associated dacite, overlying three units of subaluminous Teplice rhyolite and granite porphyry) and plutonic granites (two intrusions of A-type biotite and zinnwaldite granites). The trace-element distribution in the granite porphyry and rhyolite quartz demonstrates a distinct zoning in the cathodoluminescence images and chemical compositions: the core is poor in Ti, the outer-rim zones are enriched in Ti, and the trends recorded for Al are opposite to those of Ti. The quartz from granites is either very weakly zoned or homogeneous. Based on the chemical composition of quartz and its zoning, we assume that the quartz crystals began to crystallize gradually in the primary deep-seated magma chamber with a P–T condition of 10 kbar and 750 °C and in an environment with an increasing concentration of Al and decreasing concentration of Ti. The early stage was followed by the subsequent adiabatic ascent into a shallow magma chamber (estimated by the Ti-in-quartz thermobarometry at approximately 700 °C, 2 kbar), during which crystal rims rich in Ti formed and then extruded. After the caldera collapse, part of the magma intruded as granite, and in the environment of the residual melt rich in water and flux agents, late quartz grew in the form of snowball quartz with an increased Al content and decreased Ti content.