Diachronous post‐orogenic magmatism within a developing orocline in Iberia, European Variscides

Abstract

U‐Pb (zircon) crystallization ages of 52 late‐Variscan granitoid intrusions from NW Iberia (19 from new data, 33 from previous studies) constrain the lithospheric evolution of this realm of the Variscan belt of Western Europe and allow assessment of the relationship between oroclinal development and magmatism in late‐Carboniferous‐early Permian times. The U‐Pb ages, in conjunction with a range of geological observations, are consistent with the following sequence of events: (i) oroclinal bending starts at 310–305 Ma producing lithospheric thinning and asthenospheric upwelling in the outer arc of the orocline accompanied by production of mantle and lower crustal melts; (ii) between 305 and 300 Ma, melting continues under the outer arc of the orocline (Central Iberian Zone of the Iberian Variscan belt) and mid‐crustal melting is initiated. Coevally, the lithospheric root beneath the inner arc of the orocline thickened due to progressive arc closure; (iii) between 300 and 292 Ma, foundering of the lithospheric root followed by melting in the lithospheric mantle and the lower crust beneath the inner arc due to upwelling of asthenospheric mantle; (iv) cooling of the lithosphere between 292 and 286 Ma resulting in a drastic attenuation of lower crustal high‐temperature melting. By 285 Ma, the thermal engine generated by orocline‐driven lithospheric thinning/delamination had cooled down beyond its capability to produce significant amounts of mantle or crustal melts. The model proposed explains the genesis of voluminous amounts of granitoid magmas in post‐orogenic conditions and suggests that oroclines and similar post‐orogenic granitoids, common constituents of numerous orogenic belts, may be similarly related elsewhere.