Early Miocene subduction in the western Mediterranean: Constraints from Rb-Sr multimineral isochron geochronology

Abstract

The Betic Cordillera of southern Spain is a complex orogen formed in the context of convergence between Africa and Iberia from the Mesozoic to the present. The internal zone of the orogen includes three tectonic complexes, two of which have been subducted to high-pressure conditions, then exhumed back to the surface during subsequent extension. Subduction in the structurally lower complex, known as the Nevado-Filabride Complex (NFC), has been a topic of debate for several years due to conflicting geochronological data. Here we use multimineral isochron 87 Rb/ 86 Sr dating on carefully selected mineral samples from high-pressure metamorphic rocks in the NFC to better constrain the timing of high-pressure metamorphism and subduction in the region. Out of five samples analyzed, statistically valid multimineral isochrons were obtained for one eclogite and two schists, yielding ages of 20.1 6 1.1, 16.0 6 0.3, and 13.3 6 1.3 Ma, respectively. Despite that the other two eclogite samples appeared to preserve prograde mineral assemblages, low 87 Rb/ 86 Sr ratios in white mica precluded precise age calculations. These new ages are in close agreement with previously published Lu-Hf ages on garnet and U-Pb ages on metamorphic zircon overgrowths for the same rocks, but are substantially younger than published data from the 40 Ar/ 39 Ar technique. Combined with recently published tomographic images of slab structure beneath the Alboran Sea, the new ages support a tectonic model in which subduction occurred both prior to the Miocene and during the early to mid-Miocene, but that it was punctuated in time by a pulse of extensional exhumation in the early Miocene associated with lithospheric delamination and/or slab tearing.

Two Tertiary metamorphic events recognized in high‐pressure metapelites of the Nevado‐Filábride Complex (Betic Cordillera, S Spain)

“…Other researchers believe that younger ages (<20 Ma) are related to the HP metamorphism in the NFC. These younger ages are supported by Lu‐Hf garnet dating (18–14 Ma: Platt, Anczkiewicz, Soto, Kelley, & Thirlwall, ), U‐Pb dating on zircon (17–15 Ma: Gómez‐Pugnaire et al., ; López Sánchez‐Vizcaíno, Rubatto, Gómez‐Pugnaire, Trommsdorff, & Müntener, ) and multimineral 87 Rb/ 86 Sr isochron dating (20–13 Ma) by Kirchner, Behr, Loewy, and Stockli (). Although Kirchner et al.…”

“…Under these circumstances, the various ages obtained by Kirchner et al. () applying multimineral 87 Rb/ 86 Sr isochron dating (20.1, 16.0, and 13.3 Ma) to rocks from the NFC (see section 2) become better understandable, as they are related to the metamorphism during the second P–T loop (Figure a) caused by nappe stacking and subsequent orogenic extension (Balanyá et al., ; García‐Dueñas et al., ; Platt et al., ). Also Gómez‐Pugnaire et al.…”

“…Booth‐Rea et al., ), because recent work showed that the NFC and AC yielded different metamorphic ages (e.g. Kirchner et al., : <20 Ma for the NFC and c . 50 Ma for the AC).…”

“…Younger HP metamorphism was also suggested from Lu‐Hf (18–14 Ma: Platt et al., ), U‐Pb ( c . 15 Ma: López Sánchez‐Vizcaíno et al., and 16.5–17 Ma: Gómez‐Pugnaire et al., , ), and multimineral 87 Rb/ 86 Sr isochron (20–13 Ma: Kirchner et al., ) dating methods.…”

“…Although Kirchner et al. () favoured the younger dates (<20 Ma), they questioned the correlation between metamorphism and Lu‐Hf dating of garnet and zircon because (1) the Lu‐Hf system can be problematic due to trace mineral inclusions, multiple stages of garnet growth or disequilibria due to incomplete isotopic homogenization, and (2) zircon may not grow during all metamorphic events.…”