Exhumation of a migmatite complex along a transpressive shear zone: inferences from the Variscan Juzbado-Penalva do Castelo Shear Zone (Central Iberian Zone)

Abstract

High-grade metamorphic rocks associated with S-type granites are recorded in the Central Iberian Zone, Iberian Variscides. Though most of these occur as inferred metamorphic core complexes affiliated with detachment faults, others, such as the Figueira de Castelo Rodrigo–Lumbrales Anatectic Complex, crop out between low-grade metamorphic rocks separated by steeply-dipping strike-slip shear zones, such as the Juzbado–Penalva do Castelo Shear Zone. Our structural analysis has been able to constrain two major deformation stages during the Variscan D3: (a) D3a ductile deformation event, with clear sinistral kinematic criteria; and (b) D3b thrusting ductile–brittle deformation event. The petrological investigation confirmed the jump in metamorphic grade between the host rocks and the anatectic complex. P–T estimates on calc-silicate rocks interlayered with the metapelites of the anatectic complex provided minimum metamorphic peak conditions of T = 761 ± 50°C and P = 5.0 ± 1.0 kbar. However, petrological modelling results show that P–T conditions must have exceeded T > 800°C. Both structural and geothermobarometric data support a two-step model for the exhumation of the Anatectic Complex, including a 5 – 8 km vertical exhumation along a 65 – 100 km horizontal displacement due to a simple shear-dominated transpression mechanism during the Variscan D3 events.