NEW GEOCHEMICAL AND PETROGRAPHIC DATA REGARDING MAGMA EVOLUTION AND EMPLACEMENT PROCESSES OF THE OLIGOCENE ZAKUPY DIATREME (CZECH REPUBLIC)

The formation of diatreme volcanoes in continental rift environments remains an important but poorly understood subject as the roots of these systems are not often accessible. The Oligocene Zákupy diatreme, Czech Republic, located on the SE flank of the NE-SW trending Eger Rift, near the intersect with the NW-SE dextral Labe-Odra fault system, is a quarried diatreme that exposes up to 50 m of the lower and upper diatreme facies. The Zákupy quarry consists of several protodiatreme-like intrusions that were injected into unconsolidated, lithic-rich, diatreme fill and then changed geometry, forming sills along bedding planes in water saturated, non-welded lapilli tuffs of upper diatreme facies. These sills are often highly fragmented with angular peperite horizons. Twenty-four samples from the late stage intrusions and varying diatreme facies were studied petrographically to distinguish mineral and textural variations among individual intrusions. Petrographic observations and field observations were supported by quantitative chemical analyses of individual minerals, conducted on Tescan MIRA 3GMU fitted with SDD X-Max 80mm2 EDS detector and AZtecEnergy software. Preliminary petrographic analyses reveal several distinct compositions, olivine analcimite, olivine lamproite, limburgite, and altered olivine nephelinite. Geochemical analyses, however, classify all these samples as ultrabasic (MgO : 14.5-16.5 ; SiO2 40.0-42.0) alkali basalts (picrobasalts and basanites) based on TAS diagram. All samples show parallel and almost linear chondrite normalized REE patterns with high degrees of LREE enrichment (La_N/Yb_N from 21.98-26.21). Likewise, all samples show congruent mantle-incompatible trace element variations with negative Rb, Th, T, and K anomalies. Preliminary results from paleomagnetic data show secular variation suggesting that the Zákupy diatreme was a long lived system in a syn-tectonic extensional stress regime that evolved from several similarly sourced magmatic pulses and molten fool coolant interactions that did not result in eruption.