New Mexico Geological Society Annual Spring Meeting — Abstracts

The emplacement of silica-undersaturated magma in continental rift volcanoes remains poorly understood because the roots of these systems are not often accessible. The Miocene Merunice and Oligocene Zákupy diatremes, Czech Republic, are located within or on the SE shoulder of the Eger Rift. These diatremes provide a unique opportunity to conduct a comparative emplacement study, in near 3-dimensions, of their sub-volcanic magma plumbing systems. Studies across the rift reveal that magma compositions show a temporal evolution trend that coincides with three rift phases, pre-rift (79-49 Ma), syn-rift (42-16 Ma), and late rift (16-0.3 Ma). Here we report data on how varying degrees of alkaline magma generation paired with a dynamic rift stress regime yield unique emplacement mechanisms of presumed monogenetic rift diatremes. Field observations and laboratory data at both diatremes indicate multiple emplacement and eruptive events, as shown by variation in composition, eruptive materials and cross cutting relationships between dikes and sills that reflect different emplacement dynamics. Anisotropy of magnetic susceptibility (AMS) data collected from 25 Zákupy diatreme sites reveal primarily oblate magnetic fabrics suggesting magma flowed up, down, and laterally away from the suspected main conduit but have a NE-W/SW flow trend from a probable NE magma source, that is in accordance with the syn-tectonic rift stress regime (NE-SW). Preliminary paleomagnetic data reveal that the intrusions are of both normal and reversed polarity and show scatter about their respective expected polarity reference directions that could be related to sub-volcanic deformation of the diatreme. Curie point estimates show that the magnetic mineral phases carrying the characteristic remanent magnetization are moderate to high titanomagnetite with ilmenite inclusions in clinopyroxene and diospside. AMS data from the main feeder dikes from the Merunice diatreme display a steep upward NW and NE emplacement trend, suggesting a SW magma source, that follows the same rift trend of NE-SW. Petrologic observations indicate that the rocks are olivine basanites with little to no compositional variation between intrusive and extrusive products. Paleomagnetic data from the Merunice diatreme are underway and should aid with defining subvolcanic deformation during the growth of the diatreme. The results from this multidisciplinary study suggest that these presumed monogenetic systems display characteristics that are better described by a polygenetic emplacement model with a strong structural control dictated by the Eger rift stress configuration.