Evidences for Long Residence Time for the Miocene Silicic Ignimbrites of the Pannonian Basin, Eastern-Central Europe: Constrains from In-Situ U-Pb Zircon Dating

Abstract:

There is an on-going debate about the length of residence time in silicic magmatic systems. Here, we present an example of a silicic volcanism characterized by several Ma long crystallization history, which is comparable with that of large silicic batholiths. In the Carpathian-Pannonian region, an ignimbrite flare-up episode occurred during the Miocene. This volcanism was fed by large volume silicic magmas and was coeval with the formation of the Pannonian basin by major lithospheric stretching. Excellent outcrops of these volcanic products are found in the Bükkalja Volcanic Field, northern part of the region. We performed high-precision in-situ zircon U-Pb dating by LA-ICP-MS using selected samples collected from continuous stratigraphic sections, comprising pyroclastic flow and fall deposits of the entire volcanism. We conducted at least 50 spot analyses for each samples and applied strict selection procedure to use only concordant data. The youngest age population of the Isoplot Unmix ages (in case of >5 MSWD; most of the samples) or the weighted mean ages are considered to be the closest to the eruption ages. According to these age data, the volcanism lasted from 18.2 Ma to 14.0 Ma, and the volcanic deposits can be well correlated. Six eruption events are distinguished separated by a few 100’s ka up to 1.6 Ma repose times. In case of most samples, the large MSWD values and the wide range of ²⁰⁶Pb/²³⁸U isotope ratios of the individual spot analyses suggest prolonged (even more than 1.5-2 Ma) zircon crystallization. Based on the evaluation of the whole data set, 10 zircon crystallization peak-periods started at 19.6 Ma can be clearly distinguished. This is consistent with a model involving a long-lasting silicic crystal mush system with periodic rejuvenation of zircon crystallization and withdrawal of medium to large volume eruptible magmas. This requires elevated heat-flux and thermally prepared upper crust during the Middle Miocene continental rifting period.