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 Rothplattenbach Complex (Arosa Zone, Germany) hosts alkaline magmatic rocks that provide unique insights into the mid-crustal evolution of water-saturated melts. This study presents a detailed phase-analytical investigation using XRD and SEM/EDX to characterize the primary magmatic mineralogy and its subsequent autometasomatic alteration. A key discovery is the occurrence of primary magmatic analcime, which, in equilibrium with anorthite-rich plagioclase (An77) and diopsidic clinopyroxene, serves as a precise geobarometer. Thermodynamic constraints require pressures of 5–6 kbar and temperatures below 660 °C for the crystallization of primary analcime, indicating that the magma solidified at mid-crustal depths of 15–20 km.

The petrogenetic evolution occurred within a closed system, where internally derived magmatic fluids triggered pervasive deuteric alteration. Depending on the local CO₂ fugacity, the system followed two distinct pathways: a zeolitic pathway (XCO₂ < 0.02) leading to the formation of thomsonite and natrolite, and a carbonate pathway (XCO₂ > 0.02) resulting in a calcite–natrolite–montmorillonite assemblage. The latter was locally triggered by the interaction with entrained Upper Jurassic red limestone xenoliths.

The coexistence of high-pressure magmatic phases and sedimentary xenoliths suggests a geodynamic setting within a subduction channel or tectonic mélange. In this environment, water-rich, unlithified sediments were transported to mid-crustal levels, where they encountered rising alkaline magmas. The resulting "intrusive pillows" and metasomatic reaction zones (including the formation of celadonite and monomineralic platy analcime) demonstrate that the characteristic pillow morphology was formed by expansion into a ductile, hydrous medium under high lithostatic pressure rather than by shallow-marine extrusion. These findings challenge traditional interpretations of the Arosa Zone magmatites and highlight the significance of primary analcime as a marker for deep-seated, water-saturated magmatism.