Ar-40/Ar-39 Age Constraints on HP/LT Metamorphism in Extensively Overprinted Units: The Example of the Alpujarride Subduction Complex (Betic Cordillera, Spain)

Widespread overprinting of early high-pressure/low-temperature (HP/LT) subduction stages due to subsequent collisional or late-orogenic tectono-metamorphic events is a common feature affecting the interpretation of geochronologic data from HP/LT orogens. The Betic-Rif orogen is exemplary in this connection as a great majority of published radiometric ages are found to cluster around 20 Ma. This clustering is commonly interpreted as reflecting a short, yet complex, succession of tectono-metamorphic events spanning only over a few Myr, including back-arc extension and overthrusting of the Internal Zones on the External Zones. An alternative explanation consists in the poor preservation of a much earlier HP/LT metamorphic event, presumably Eocene, coeval with subduction and crustal thickening in the Internal Zones, and particularly the Alpujarride Complex. However, this age is vividly debated due to widespread resetting by the Early Miocene HT/LP overprint. In this study, we provide new Ar-40/Ar-39 evidence from white micas selected along an E-W section of the Internal Betics, from the central to the eastern Alpujarride Complex. Our new data show (a) that exceptionally well-preserved HP/LT parageneses in this unit retain a well-defined Eocene age around 38 Ma, and (b) that widespread 20 Ma ages recorded all along the section correspond to a regional stage of exhumation, coeval with a major change in the kinematics of back-arc extension. Our study provides conclusive evidence that Ar-40/Ar-39 dating of carefully targeted HP/LT associations can overcome the problem of extensive late-orogenic overprinting, testifying for an Eocene HP event around 38 Ma in the Betic-Rif orogen.