Age of anatexis in the crustal footwall of the Ronda peridotites, S Spain

Abstract

This study investigates the age of anatexis of a crustal sequence constituting the footwall of the Ronda peridotite slab, in the hinterland of the Betic Cordillera (S Spain, region of Istán). These rocks represent a polymetamorphic basement involved in the Alpine orogeny and show an increase in the proportion of melt towards the peridotites. Metamorphic conditions in the migmatites vary between T ≈ 675–750 °C at P ≈ 0.30–0.35 GPa. The timing of metamorphism and deformation of the migmatites around the Ronda peridotites is controversial and has been previously ascribed to either the Alpine or Variscan orogenies. We present U–Pb SHRIMP dating of zircons from six samples collected across the migmatitic sequence that provide a tighter age constraint on the metamorphism. Zircon ages are related to conditions of metamorphism on the basis of the relationships between zircon microstructures and degree of melting recorded by the host rocks. Anatexis occurred during the late stages of the Variscan orogeny (≈ 280–290 Ma), as indicated by ages of euhedral, oscillatory-zoned domains or new crystals in metatexites and diatexites. Thin, U-rich zircon rims that are affected by radiation damage yield discordant scattered dates between ≈ 260 and 30 Ma, which are interpreted as reflecting a thermal and fluid overprint during the Alpine orogeny that produced recrystallization and Pb loss in Permian zircons. This study identifies a previously unknown Variscan domain within the Betic Cordillera, and indicates, in accordance with previous studies, that Variscan basements recycled during the Alpine orogeny that formed the Betic Cordillera preserve pre-Alpine mineral associations and tectonic fabrics

Fast switch from extensional exhumation to thrusting of the Ronda Peridotites (South Spain)

“…Migmatites in the footwall and granitic dikes intruding the mantle and the crustal hangingwall yield a cluster of crystallization ages between 23 and 19 Ma (Esteban et al, ; Monié et al, ; Rossetti et al., ; Sánchez‐Rodríguez & Gebauer, ; Sosson et al., ). A thermal overprint on Hercynian migmatites in the thrust footwall during the Tertiary has been also reported (Acosta‐Vigil et al., ). A major shortening event during the Miocene therefore triggered the final crustal emplacement of the Ronda Peridotites (Précigout et al., ; Tubía et al, ).…”

Implication of the geology of the Guadaiza and Verde valleys (Malaga Province, Betic Cordillera) on the position of the Ronda peridotites and the structure of the Alpujárride Complex

“…Therefore, they do not correspond to the proposed dynamothermal aureole, forming migmatitesdiatexites, situated in the bottom of the peridotites and dated to ~22 Ma ago (Esteban et al, 2008 and. Equivalent rocks were dated as ~270 Ma by Sanz de Galdeano and Ruiz Cruz (2016a) whereas the white gneisses were dated as 280-290 Ma by Acosta-Vigil et al (2014). Probably the age of ~22 Ma corresponds to an Alpine tectonic rejuvenation of rocks where protoliths were previously formed during the Permian.…”

“…En el río Verde (sin incluir el sector del Hoyo del Bote) existen gneises blancos (Fig. 4, término 5 y columna C) considerados como migmatitas (migmatitas de Istán) datadas del Carbonífero terminal o del Pérmico inferior (Acosta-Vigil et al, 2014). Estas rocas pueden alcanzar un espesor superior a 200 m.…”

“…In the Verde valley, some white gneisses (Fig. 3, column C, number 5) have been considered migmatites (the Istán migmatites) with an estimated age of ~280-290 Ma (Acosta-Vigil et al, 2014). The thickness can be greater than 200 m.…”

Late Palaeozoic to Triassic formations unconformably deposited over the Ronda peridotites (Betic Cordilleras): Evidence for their Variscan time of crustal emplacement

“…3). Recently, Acosta-Vigil et al (2014) indicates a Variscan age (≈280-290 Ma) for the previous dated rocks, without mention the Alpine stage. Platt et al (2013) wrote that "the peridotites rest on a sequence of high-grade schist, marble, and granitic gneiss, with a layer several tens to 100 m thick of migmatitic breccia along the contact", their "Subperidotite complex", interpreted as formed by the hot overthrusting of the peridotites over the underlying Guadaiza unit.…”