Disclosing the magma genesis and storage beneath the Cretaceous Ebersbrunn diatreme, Saxony: Evidence from mineral chemistry and juvenile fragments

The Mesozoic to Cenozoic intraplate to rift-related magmatic activity in central Europe spawned both, intrusive and extrusive carbonatites. Located within the intersection of the Regensburg-Leipzig and Gera-Jachýmov fault zones, the eroded diatreme of Ebersbrunn provides a unique opportunity for mineralogical and petrographic investigations to explore the magma genesis and its lithospheric evolution. Electron microprobe analyses on several mineral phases were acquired to obtain thermobarometric estimations and unravel the internal textures. Moreover, detailed microscopic studies on juvenile fragments are used for the characterization of volcanic facies. Here, the compositional variation in amphibole (with Mg# from 0.804 to 0.932) and mica (Al-rich phlogopite and eastonite) can be linked to the presence of lamproite components in the generation of supplying magma system and show a multi-episodic crystallization history. Thermometric estimations for the carbonatitic magma showed a wide range from 450 to 672°C (obtained on low- and high-Mg calcite and dolomite). The joint presence on high-T amphibole (872 to 965 °C) and mica ( ca. 950 °C) suggests the contemporaneous crystallization from a single magma storage and the presence of mid-crustal magma storage at 15 to 21 km.

The petrographical studies on circular/ ellipsoidal juvenile fragments revealed different populations of peletal lapillis, with either dry or wet silicate, Fe-Ti-oxide, and/or carbonate phases in the kernel and a carbonatitic and/or silicic shell composition. This indicates the influence of hybrid magma and emphasizes the multi-stage layering process. The oriented prolate-shaped apatite crystals at the rim of pelletal lapillies are addressed to turbulent motion during magma ascent.