Age differences between separate magma pulses in two laccoliths (Halle Volcanic Complex): insight into processes of silicic magma formation

The Halle Volcanic Complex includes several laccoliths emplaced during Carboniferous-Permian flare-up (Breitkreuz & Kennedy, 1999). Previous dating of the laccoliths yielded a range of ages from 301±3 Ma (Landsberg) to 292±3 Ma (Petersberg). Textural and whole rock chemical composition of samples from several depths from both laccoliths suggest formation of the laccoliths in several pulses and in this study we have checked if a complementary record is preserved in chemical composition of accessory phases. Altogether seven depths have been analyzed from two laccoliths including electron microprobe analyses of zircon and apatite and U-Pb SHRIMP dating. Additionally four depths were analysed for U-Pb zircon age by CA-ID-TIMS. Zircon is chemically homogenous within and between laccoliths and has similar sets of inclusions. On the other hand, apatite shows variable Nd contents decreasing with depth for both laccoliths and apatite from Petersberg has more Cl, when apatite from Landsberg has more Na. The implication is that apatite chemistry is better at recording magma evolution than zircon. SHRIMP ages are scattered over 30 Ma for a single sample, which suggests the presence of antecrysts and lead-loss, or alternatively indicates formation of the laccoliths over a prolonged period of time.

Acknowledgements: The research was funded by the National Science Centre (Poland) the OPUS project UMO-2017/25/B/ST10/00180 to APietranik.

Breitkreuz, C., Kennedy, A. (1999) Magmatic flare-up at the Carboniferous/Permian boundary in the NE German Basin revealed by SHRIMP zircon ages. Tectonophysics. 302, 307-326.