Petrology of weakly differentiated alkaline, high-level intrusive rocks in the Zahořany-Chotiněves Belt near Litoměřice (Czech Republic)

Abstract

Weakly differentiated rocks such as basaltic trachyandesites occur rather rarely in the Cenozoic České Středohoří Volcanic Complex (Central Europe, Czech Republic). We present mineralogical, petrological and geochemical data for a basaltic trachyandesite sill located at the southern margin of the České Středohoří Volcanic Complex, where several smaller hills form a quasi-continuous belt between villages of Zahořany and Chotiněves. Uniform petrography and chemical composition provide compelling evidence that all individual outcrops belong to a single large sill, probably with the exception of the westernmost occurrence at the Křemín Hill. The sill is almost 5 km long (SW-NE) and likely up to 3 km wide (NW-SE). The elongated shape of the sill and its position suggest that the basaltic trachyandesite magma ascended along the Litoměřice Fault forming the southeastern edge of the NE-SW trending Ohře (Eger) Rift. The studied rocks are basic and alkali-rich (49.5-50.3 wt. % SiO 2 , sum K 2 O + Na 2 O = 7.8-8.1 wt. %), but their silica contents (volatile-free) approach the boundary of the intermediate domain. This correlates with low concentrations of compatible trace elements such as Cr (15-23 ppm). The limited degree of differentiation is reflected by smooth chondrite-normalized REE patterns (La N /Yb N = 18.2-19.3) with the absence of any significant Eu and/or MREE anomaly. The incompatible trace-element contents (Sr = 920-1080 ppm, Ba = 840-950 ppm, ƩREE = 280-330 ppm) typify weakly differentiated alkaline volcanic rocks within the Ohře Rift. Based on the chemical composition we suggest that these basaltic trachyandesites belong to the České Středohoří Volcanic Complex rather than to MgO-poor foidites of the Central Bohemia Volcanic Field. The intrusive age of the sill has been determined using conventional whole-rock K-Ar method at 29.12 ± 0.63 Ma (1σ). The initial Sr-Nd isotope compositions (87 Sr/ 86 Sr i = 0.7048, 143 Nd/ 144 Nd i = 0.51270), compare well with existing data for the České Středohoří trachybasalts and attest to their common origin. The single large Zahořany-Chotiněves sill partly fills the apparent gap in the differentiation trend between basanites to trachybasalts and trachyandesites to phonolites reported for the alkaline volcanism of the České Středohoří Mts.

Sample preparation and chromatographic separation for Sr, Nd, and Pb isotope analysis in geological, environmental, and archaeological samples

“…The three-column separation procedure using BioRad Sr and Ln resin columns was successfully employed in the isotopic study of plutonic and volcanic rocks (Walther et al 2016;Mysliveček et al 2018;Janoušek et al 2020;Casas-García et al 2021;Mach et al 2021;Janoušek et al 2022) as well as loess samples (Waroszewski et al 2021).…”

A new Oligocene flora from Ludvíkovice near Děčín (České středohoří Mts., the Czech Republic)

“…A large intrusive body associated with the Děčín Fm. on the southern margin of the České středohoří Volcanic Complex was recently dated to 29.12 ± 0.63 Ma (K-Ar, see Mysliveček et al 2018).…”