Growth and textural evolution during crystallization of quartz and feldspar in hydrous, rhyolitic melt

The growth rates of alkali feldspar and quartz were measured in a felsic melt to better understand the differences in textures seen in different types of rocks forming from melts of similar composition (e.g., rhyolites, granites and granitic pegmatites). We discuss the effects of undercooling (deviation from equilibrium) on the transition between different types of feldspar-quartz intergrowths and the effects of the physicochemical conditions on the growth rates of quartz and feldspar. These rates were measured in experimental run products on a hydrous (similar to 4 wt. % water), metaluminous granitic composition from experiments conducted in a piston cylinder apparatus at 600 MPa and temperatures from 500 to 800 degrees C. We also quantified the textural evolution of quartz and feldspar and their intergrowths in these experiments using fractal analysis. The growth rate measurements were compared against previously published growth rates of quartz and feldspar from experiments on felsic melts, some of which contained B and Li. The results of fractal analysis of the experimental samples are compared to previously published measurements of natural graphic granite (Baker et al. in Can Mineral 56:625-643, 2018). We demonstrate the implications of the results of this study and possible applications to granites and granitic pegmatites. We show how the differences in growth rates can influence the formation of porphyritic granite and the sequential crystallization in zoned granitic pegmatites. We also suggest the possibility of using fractal analysis of quartz and feldspar and their intergrowths as an indicator of the deviation from equilibrium because the degree of undercooling appears correlated with the parameters measured using fractal analysis.

Automated summary

This study measured the growth rates of alkali feldspar and quartz in a felsic melt under high temperature and pressure conditions, investigating the effects of physicochemical conditions and undercooling on their growth modes. Fractal analysis was used to quantify the textural evolution of quartz and feldspar, showing how differences in growth rates may influence the formation of different types of granites.