Relationships between very low-grade metamorphism and tectonic deformation: examples from the southern Central Iberian Zone (Iberian Massif, Variscan Belt)

Abstract

We have studied the syn-kinematic very low-grade metamorphism in a polyphase Variscan deformed region using X-ray diffraction techniques. Two phases of regional metamorphism are related to their respective episodes of penetrative deformation in the southern Central Iberian Zone. The data obtained suggest that the rocks did not reach metamorphic equilibrium, but strain favoured the progress of mineral reactions in the more deformed parts. The first deformation is Devonian in age and consists in a heterogeneous ductile shearing coeval with large-scale recumbent folding that were produced under high-anchizone to epizone metamorphic conditions. The heterogeneity of the shearing originated strain gradients that can be said to enhance the growth of new minerals and the illite polytype transformation in the highly strained overturned limb of the preserved pile of recumbent folds, but illite crystallinity remained constant throughout the structure. The second deformation is Mid-Carboniferous in age and consists in an upright folding that took place under late diagenesis to low-anchizone metamorphic conditions. The distribution of mineral parageneses and illite crystallinity across one of the upright folds suggests that strain gradients favoured the metamorphic reaction progress from the hinge (low strain) towards the limbs (high strain). Other characteristics of the region such as a metamorphic gap associated with an unconformity at the base of the Lower Carboniferous rocks, or cryptic contacts aureoles surrounding volcanic intercalations and a large granitic batholith, are also studied.