U–Th–Pb geochronology of zircon and monazite from syenite and pincinite xenoliths in Pliocene alkali basalts of the intra-Carpathian back-arc basin

Abstract

Phreatomagmatic eruptions in the Lučenec Basin in Slovakia ejected fragments of igneous rocks of unknown age and ranging in composition from gabbro to syenite and orthopyroxene granodiorite – pincinite. Two kinds of syenites with different Fe 3+ contents composed of anorthoclase and interstitial metaluminous trachyte-to-alkali rhyolite glass represent the final products of alkali basalt differentiation at depths of 10 and 15–18 km. Pincinite composed of plagioclase, quartz, orthopyroxene, ilmenite and interstitial peraluminous dacite glass is a quenched supersolidus anatectic melt formed at the depth of 17–20 km by high-temperature dehydration melting of biotite-bearing gneissic protolith. Zircon from pincinite and syenite, and monazite from syenite were dated using excimer laser ablation ICP-MS. The obtained ages cluster at 5.8 ± 0.1 and 5.4 ± 0.1 Ma, thus corroborating the genetic link between the two rock-types and constraining the residence time of at least 300 ± 100 ka of the parental basalt in the deep crustal magma storage zones. The inferred ages overlap the Upper Miocene–Pliocene boundary, and are considerably younger than spatially associated basaltic lava flows dated using the K/Ar method. Consequently, the fluviolacustrine Poltár Formation overlying the maar structure must have sedimented during the Pliocene and not during the Upper Miocene (Pontian) as previously assumed.