Repeated age resetting in zircons from Hercynian–Alpine polymetamorphic schists (Betic–Rif tectonic belt, S. Spain)—a U–Th–Pb ion microprobe study

Abstract

Quartzitic plagioclase–biotite schists from the basement section of the Alpujárride nappe complex in the central Betic Cordilleras contain zircon of variable age. U–Pb ion microprobe zircon dating defines three major groups of primary ages of ∼1000, ∼600 and ∼300 Ma. Apart from these primary zircon ages, discordant ages are widespread, reflecting isotopic disturbance after these events. In addition, many concordant data between the major age clusters are interpreted, mainly on the basis of CL images, as partially reset older ages. Degree of isotopic resetting is shown to be highly variable, even in zircons of similar primary age record. This is argued to be mainly due to different histories of fluid access for individual zircon crystals in a (poly)metamorphic setting. The two older primary age clusters comprise zircon crystal cores showing anhedral, rounded outlines typical for sedimentary attrition, indicating Grenvillian and Pan-African events in the metamorphic/magmatic rock complexes contributing to the clastic sedimentary parent material of the schists. The youngest zircon age group comprises crystals whose morphology and CL appearance suggest metamorphic growth. Zirconium for this neoformation of zircon is thought to be derived from other crystals/minerals during the metamorphic recrystallization, simultaneously with the zircon growth. The concordia intersection age of 313±7 (2 σ) Ma for these crystals is interpreted as the age of the Hercynian metamorphism which affected these basement schists. This conclusion contrasts with current, exclusively Alpine orogenic models for this tectonic belt which question the occurrence of a Hercynian event. The discrepancy arises because in polymetamorphic rock complexes, conventional petrographic and structural investigations, on which these models are based, may be difficult to interpret without geochronological constraints. The present study does not offer age information on the Alpine history. Zircon rims which are thought to be of Alpine age were too thin to be measured.