Interpretation of wide zircon U–Pb age distributions in durbachite-type Variscan granitoid in the Mórágy Hills

Abstract

In situ U–Pb analyses were performed on SEM-BSE, SEM-CL and Raman mapped zircons from the Variscan granitoids exposed in the Mórágy pluton, Hungary. However, the routinely used LA-ICP-MS could result only in reliable age constraints if the system was not overprinted by multiple geological processes that affect the isotope system of zircons. To overcome the ambiguities the new zircon U–Pb age data were evaluated carefully, first using simple statistical models, then a zircon internal texture related complex approach was applied. This method demonstrates that the U–Pb age in overprinted systems correlates with the structural state; the worse structural state zones showing younger, but still concordant ages. Individual zircon internal texture and structural state based evaluation made it possible to select the least overprinted age components of the system and identify five steps in the evolution of the studied intrusive rock. The two melts (granitoid and mafic) passed the zircon U–Pb isotope closure temperature ~ 355 ± 3 Ma ago during their cooling. Crystallization of the two mingled magmas overarched the 350–340 Ma period, including two intense zircon crystallization peaks (~ 347 Ma, ~ 333 Ma). The cessation of melt crystallization (~ 650 °C) happened ~ 334 ± 4 Ma ago, as indicated by the age of the “normal and long prismatic” zircons. Further confirming this statement, they are embedding in their rims the eutectic mineral assemblage. A Cretaceous post-magmatic event was identified according to slightly discordant U–Pb ages for the Mórágy pluton.