Late-orogenic magmatism in the Central European Variscides: SHRIMP U-Pb zircon age constraints from the Żeleźniak intrusion, Kaczawa Mountains, West Sudetes

Abstract

We present new U-Pb isotope data obtained using the sensitive high mass-resolution ion microprobe (SHRIMP) technique on zircon crystals from the Żeleźniak subvolcanic intrusion in the Kaczawa Mountains, West Sudetes, SW Poland. The intrusion comprises shallow-level unmetamorphosed and undeformed fine-grained rhyolites, rhyodacites, and trachyandesites and deep-level medium-grained monzogranites and granodiorites. The surrounding country rocks, thought to be fragments of a Variscan accretionary prism, are blueschist- to subsequent greenschist facies metavolcanic and metasedimentary rocks of the Kaczawa Complex. The Żeleźniak intrusion has been correlated with other late- to post-tectonic Variscan volcanic and plutonic bodies in the region, such as the Karkonosze Granite, but the scarcity and often problematic quality of age constraints and of geochemical data have made such correlations speculative. Our new SHRIMP zircon ages of ~315-316 Ma from the Żeleźniak intrusion probably corresponds to the main magmatic stage. However, a younger age of ~269 Ma, derived from some zircon rims, is more difficult to interpret but might reflect either a much younger igneous event or a hydrothermal episode. The new date of ~315-316 Ma for the undeformed Żeleźniak intrusion also provides an upper age limit for deeper-level tectonic and metamorphic processes in the Kaczawa accretionary prism. Furthermore, the new SHRIMP zircon ages are among the oldest obtained from the volcanic rocks within the Variscan Belt in Central Europe and may correspond to the final stages of the exhumation of the blueschist facies rocks in this part of the orogen.