Geochemical variations in phonolite by interaction with sedimentary country rock (Kaiserstuhl Volcanic Complex, Germany)

The contact zones of two Miocene subvolcanic phonolite intrusions towards underlying sedimentary country rock within the Kaiserstuhl Volcanic Complex, SW Germany, are exposed in drill cores. The phonolites form fine- to medium grained and homogeneous intrusive bodies. The dominant primary minerals are foids, sanidine, aegirine-augite, wollastonite; common accessories are andradite, fluorapatite, titanite and götzenite. Postmagmatic alteration transformed all foids to zeolites (Spürgin et al. 2019). The country rock consists of finely laminated, dark claystones with variable carbonate content and intercalated, thin sulphate beds. It belongs to the Oligocene Wittelsheim Formation, which was deposited in shallow lakes during the subsidence of the Upper Rhine Graben. At the contact to phonolite, brittle and ductile deformation structures developed on the cm- to dm-scale during emplacement. Furthermore, centimeter-wide magmatic veins were injected close to the contact, and crosscutting veins containing zeolites, calcite and fluorite penetrate phonolite and claystone.

Phonolites become obviously porphyric close to the country rock. Several major and trace elements show depletion in proximity to the contact (e.g., Na₂O, Al₂O₃, SiO₂, Zr, Ba), whereas others are enriched compared to the general phonolite chemistry (e.g., CO₂, MgO, Sr). Transition zones extend several meters from the contact into the phonolites.

Wall rock interaction, manifested by mineralogy and texture, is capable to alter the chemical composition of phonolitic magmas. However, exchange processes during magma emplacement work only in a zone of a few meters, which is thin compared to the intrusion sizes of several hundred meters.

Spürgin, S., Weisenberger, T.B. & Marković, M., 2019, American Mineralogist 104, 659-670.