Mid-crustal Origin of Alkaline Magmas in the Arosa Zone: Evidence from Primary Analcime and Xenolith Interaction at the Rothplattenbach Complex (Germany)

Alfred Wassermann, Matthias Hanke

The petrogenetic evolution of the alkaline magmatic rocks of the Rothplattenbach Complex was investigated by X-ray diffraction phase analysis and crystallographic lattice-parameter determination. The study focuses on the identification of primary analcime occurring in paragenesis with Ca-rich plagioclase (An₇₇) and diopsidic clinopyroxene. Phase-equilibrium modelling indicates that magma evolution in a closed system through Rayleigh fractional crystallization resulted in
chemical convergence toward an analcime-saturated melt composition. Thermodynamic stability constraints for analcime (P > 5 kbar, T < 660 °C) imply a subvolcanic origin from deep-crustal levels at 15–20 km depth, contradicting the previous interpretation of these rocks as near-surface pillow lavas.
Jurassic Rotkalk xenoliths record a zoned interaction ranging from isochemical recrystallization to marble (lattice expansion in calcite) to allochemical infiltration metasomatism. The formation of glauconite through reduction of iron oxides in the Rotkalk and the occurrence of thomsonite provide evidence for intense fluid-mediated magma–carbonate hybridization. Preservation of these highpressure parageneses together with high amorphous contents indicates rapid release of the complex.
These results demonstrate that water and alkali saturation are primary characteristics of Cenozoic rift-related magmatism.