Evidencias de la temprana exhumación Varisca del manto de Cabo Ortegal (noroeste de España)

Resum

We have studied the structural relationships at the eastern border of the Cabo Ortegal Complex in a rock sequence formed by normal-mid ocean ridge basalt (N-MORB) eclogites, high-pressure and high-temperature (HP-HT) migmatitic gneisses and metasedimentary gneisses of intermediate-pressure, both derived from greywackes. The migmatitic gneisses include eclogite blocks-in-matrix and were intruded by leucodioritic dykes. Moreover, the metasedimentary gneisses are mechanically overlying the migmatitic gneisses by a subtractive shear zone. The whole sequence was heterogeneously deformed and is characterised by a retrograde composite foliation (S1). U-Pb -ID-TIMS in monazite dated the intrusion of the leucodioritic dykes at 480±5 Ma. U-Pb SHRIMP-RG zircon ages from an eclogite block-in-matrix dated the eclogitization at 390.4±1.2 Ma Theses ages suggest the occurrence of two high-grade metamorphic events: partial melting and granulitization at 480-485 Ma; and high temperature eclogitization at 390 Ma. The main foliation S1 transects the isoclinal folds formed in the leucodioritic dykes, and concordantly surrounds the eclogite blocks. We conclude that the progressive deformation imprinted on the whole sequence represents the structural evidence for early Variscan exhumation (ca. 390 Ma) of a subducted crustal slice.