Early Miocene high-pressure metamorphism in the Nevado-Filabride Complex of the Betic Cordillera, Spain: implications for subduction in the Western Mediterranean

Date
2014-08
Authors
Kirchner, Kory Lee
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Abstract

The Betic Cordillera of southern Spain is an orogen formed in response to convergence between Africa and Iberia, from the late Mesozoic to the present. The orogen consists of three main tectonic complexes, two of which have been subducted to depth, then exhumed back to the surface over short timescales. Subduction in the structurally higher of these complexes is relatively well constrained to the Eocene, but the timing of high-pressure metamorphism in the structurally lower complex, known as the Nevado-Filabride Complex, has been a topic of debate for several years due to conflicting geochronological data. Several proposed tectonic models for the Nevado-Filabride Complex are based on ages of single mineral phases. For example, models based primarily on 40Ar/ 39Ar dating on white mica in high-pressure schists require that the Nevado-Filabride and the overlying tectonic unit, the Alpujarride Complex, were coevally subjected to high-pressure metamorphism in the Eocene, and subsequently exhumed at different rates. More recent models, based on Lu-Hf dating on prograde garnets in eclogites, separate the timing of high-pressure metamorphism of the Nevado-Filabride Complex from the Alpujarride Complex by at least 10 m.y. We examine the viability of these models using multimineral Rb-Sr dating of blueschist and eclogite facies rocks in the Nevado-Filabride Complex. The multimineral isochron method uses the whole high-pressure mineral assemblage rather than a single phase, which allows testing for isotopic disequilibrium. Statistically valid Rb-Sr ages of two schists and one eclogite from the Nevado-Filabride Complex yield ages of 15.78+/-0.47, 15.8+/-1.1, and 17.6+/-1.1 Ma, respectively. The early Miocene Rb-Sr ages are in agreement with garnet Lu-Hf ages and zircon U-Pb ages for high-pressure conditions in the Nevado-Filabride Complex. The new ages imply that two episodes of subduction, punctuated by a period of extension and exhumation, occurred in the Western Mediterranean.

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