Development of the Late Oligocene Zákupy Diatreme, Czech Republic: New insights into protracted diatreme emplacement

The Zákupy diatreme is well exposed by an abandoned quarry on Kamenický Vrch Hill (Czech Republic) located in the Lusatian (Lausitz) Volcanic Field in the eastern part of the NE-SW trending Ohře Rift. The abandoned quarry's walls extend upward over 50 m exposing the lower and upper diatreme facies. Petrographic analyses revealed two main compositionally distinct suites of intrusive and extrusive bodies: dikes, sills, and lava-lakes with relics of fine-grained basanite (to limburgite) and a set of rhönite-bearing picrobasalt dikes and sills. The parallel, almost linear, and relatively fractionated (La_N/Yb_N 21.98–26.21) chondrite-normalized REE patterns suggest the erupted magmas originated through low-degree partial melting and did not undergo any significant fractionation. Close relationships between basanites and picrobasalts is confirmed also with mineral chemistry with almost identical compositional trends of clinopyroxene and spinel-group phases. The negligible difference between bulk-rock Mg# (0.71–0.72 for basanites and 0.74 for picrobasalts) and melt Mg# calculated from olivine composition (0.70 for basanites and 0.68–0.70 for picrobasalts) reflects rather a load of mantle-derived (high Mg#) xenoliths and xenocrysts rather than the presence of cummulitic olivine. Anisotropy of magnetic susceptibility data reveals magnetic fabrics of variable orientations that reflect upward, downward, and subhorizontal magma flow within the diatreme complex. Paleomagnetic data fall into three general groupings that we interpret to reflect magma emplacement during at least three phases. These data also support tilting, rotation, and slumping of diatreme host rocks causing physical displacement of the intrusions following remanence acquisition. Four new ⁴⁰Ar/³⁹Ar age determinations from the picrite (n = 1) and basinite (n = 3) yield relatively undisturbed age spectra that constrain the age of emplacement between 31.24 and 32.40 Ma, indicating that magmatic evolution of the Zákupy diatreme spanned 1.2 million years. This multidisciplinary study reveals that the Zákupy diatreme evolved from several similarly sourced magmatic pulses. Repeated molten fuel coolant interactions produced a complex diatreme setting consisting of multiple eruptive events leading to coherent intrusions, various pyroclastic and hyaloclastite materials, local contact metamorphism, and other evidence of explosive hot rock-fluid interactions. The complex evolution and eruptive behavior, together with a protracted time of the diatreme activity, illuminate that small, seemingly simple volcanoes experience a very dynamic evolution involving multiple intrusive events, changing eruptive styles, and surface morphologies.