Re-heating of rhyolitic leftovers in the Halle Volcanic Complex: an insight from zircon ages and composition.

The Halle Volcanic Complex is composed of rhyolites interpreted as intrusive-extrusive complexes that pierced host sedimentary cover during their vertical growth. Zircon ages from several units vary from 291.7 ± 1.8 Ma to 301 ± 3 Ma suggesting the prolonged evolution of this subvolcanic-volcanic system. In this study, we sampled the Landsberg (301 ± 3 Ma) and the Petersberg (292 ± 3 Ma) laccoliths to better identify the magmatic processes involved in silicic magma formation and their duration.  Altogether seven depths have been analyzed from these two laccoliths including electron microprobe analyses of zircon and apatite and U-Pb SHRIMP dating of zircon. At the first sight, zircon is chemically similar within and between laccoliths. Additionally, SHRIMP ages are scattered over 30 Ma for each sample in Landsberg. These ages overlap with two Concordia ages obtained for the uppermost horizon (289.7±2.8 Ma) and the lowermost horizon (297.1±1.7 Ma) in the Petersberg laccolith. The ages suggest that the volcanic system was active for at least 10 Ma and similar age range is recorded in both laccoliths. The scatter of ages seems to indicate the formation of the laccoliths over a prolonged period of time with periodic reactivation of the magma chamber, but the lead loss cannot be excluded. Also, prolonged formation may indicate either younger pulses reactivating previously formed parts of the magma chamber or multiple unrelated  magma injections amalgamated separately within the system.

The processes involved in the prolonged evolution of the magmatic system in Halle are evident from petrographic analyses of thin sections, where zircon can be imagined in association with other phases. Both zircon and apatite occur almost exclusively within complex glomerocrysts, an assemblage of major phases (variably altered biotite, feldspar, pyroxene). Such glomerocrysts were described in the literature and interpreted as remnants of crystal mush, probably re-mobilized at the final stage (heating episode) before laccoliths emplacement. The glomerocrysts in Petersberg and Landsberg laccoliths are similar leftovers of previous magmatic episodes, but they are special in that they contain abundant zircon and apatite. Such a picture is consistent with the evolution of magma in a long-lived magmatic system that underwent at least one reactivation. The major implication is that in some systems large proportion of zircon may represent the early stages of magma evolution, this context may be missed without detailed textural observations of zircon occurrence and associations.

Acknowledgements: Christoph Breitkreuz is thanked for his constant help with our rhyolitic research. The research has been funded by the NCN research project to AP no. UMO-2017/25/B/ST10/00180