Original paper
Depth and timing of magma evolution underneath the Emmelberg scoria cone in the West Eifel Volcanic Field
Schmitt, Axel K.; Klügel, Andreas; Böddeker, Sandra; Gothieu, Matthias; Gerdes, Axel
Neues Jahrbuch für Mineralogie - Abhandlungen Band 198 Heft 2 (2023), p. 101 - 118
44 references
published: Jun 27, 2023
published online: Jan 27, 2023
manuscript accepted: Nov 8, 2022
final revised version received: Nov 7, 2022
manuscript revision requested: Sep 29, 2022
manuscript received: Aug 17, 2022
Open Access (paper may be downloaded free of charge)
Abstract
The Emmelberg scoria cone in the West Eifel Volcanic Field, Germany, contains crystal-rich enclaves of ultramafic and evolved composition that were ejected during the terminal phase of the eruption. Chemical compositions of clinopyroxene in ultramafic enclaves (clinopyroxenites ± phlogopite and amphibole) partly overlap with those in the olivine-nephelinite host lava, suggesting crystallization from consanguineous magmas that occurred near the crust-mantle boundary (29 ± 3 km depth) as constrained by clinopyroxene-melt barometry. Fluid inclusion barometry of ultramafic enclaves, by contrast, indicates temporary magma ponding at shallower crustal levels (11 ± 2 km). Evolved enclaves are syenites with a paragenesis of alkali feldspar (dominantly anorthoclase), nosean, magnetite, and biotite with comparatively low Mg-numbers (Mg# = 41–51). Although no direct barometric constraints are available for the syenite enclaves, evolved melt generation in upper-crustal magma reservoirs is inferred from published experimental phase relations at depths that are similar to those indicated by fluid inclusion barometry for ultramafic enclaves. Accessory zircon in syenite enclaves reveals multiple differentiation and zircon crystallization episodes between c. 235 and 85 ka, predating the published eruption age of c. 49 ka. In some cases, early formed zircon subsequently recrystallized, leading to resetting of U-Th ages, trace elemental and O isotopic compositions, whereas Hf isotopic compositions remained unchanged. This suggests that evolved upper crustal intrusions were affected by hydrothermal alteration, where trace element enriched magmatic zircon was replaced presumably at subsolidus conditions via dissolution-reprecipitation processes. These results indicate a complex, polybaric, and multi-episode magma plumbing system underneath the Emmelberg scoria cone, where magma intrusion and differentiation occurred in the subsurface without contemporaneous eruptions.
Keywords
clinopyroxene • zircon • geothermobarometry • geochronology • continental intraplate volcanism