40Ar/39Ar dating of poly-orogenic rocks: an example from the Cap de Creus peninsula (Eastern Pyrenees, Spain)

Abstract

A new ⁴⁰Ar/³⁹Ar study has been conducted on variably deformed rocks from the Cap de Creus massif in the Eastern Pyrenees with a focus on NW-SE crosscutting low-grade shear zones. Outside the shear zones, in the unsheared low pressure - high temperature Variscan metamorphic rocks (P∼3,5 kbar, T∼635 °C), mica porphyroblasts yield partially reset ⁴⁰Ar/³⁹Ar ages of 240 Ma to 150 Ma. In shear zones (P∼5-5,3 kbar, T∼435–505 °C) from the Northern Shear Belt, synkinematic fine-grained muscovite provides a crystallization age of 44-42 Ma while inherited clastic muscovite has ages between 175 and 100 Ma depending on grain size. In the Southern Shear Belt, magmatic biotite from granodioritic protomylonites preserves small amount of inherited argon while synkinematic newly crystallized metamorphic micas from mylonites and ultramylonites have crystallization ages between 44 and 38 Ma. Therefore, this step-heating and in situ laser probe ⁴⁰Ar/³⁹Ar study of micas from unsheared Variscan rocks and mylonitic to ultramylonitic rocks show a conspicuous relationship between the deformation gradient, the amount of syntectonic (re)crystallization, the change in mineral composition, the resetting of Variscan mica chronometers and the record of Eocene crystallization ages by newly formed synkinematic micas. It is suggested that low-grade shear zones in the poly-orogenic Cap de Creus massif were active during the main period of Eocene convergence of the Iberian and European plates and emplacement of the south-verging crustal nappes. We propose that such NW-SE mylonitic structures developed as R′-Riedel faults within a huge system of NE-SW trending crustal faults and that they may represent onshore precursors of offshore transfer faults related to the early opening of the Gulf of Lion.