Tectonic setting of the Monte Castelo gabbro (Ordenes Complex, northwestern Iberian Massif): Evidence for an arc-related terrane in the hanging wall to the Variscan suture

Abstract

The Ordenes Complex is the largest of the allochthonous assemblages containing the Variscan suture in the northwestern Iberian Massif. Its uppermost tectonostrati-graphic unit overlies the ophiolitic units, and consists of a thick metasedimentary sequence, the Ordenes Series, intruded by orthogneisses and gabbros. In the lower part of the Ordenes Series, the large Monte Castelo gabbro (∼150 km 2 ) is surrounded by high-grade migmatitic paragneisses. Several shear zones cutting across the gabbro massif depict intermediate-pressure granulite facies, indicating a metamorphic evolution common with the surrounding paragneisses. Recent U-Pb geochronological data prove that the main tectonothermal evolution of the Ordenes Series took place in Late Cambrian-Early Ordovician time. These data suggest that the intrusion of the Monte Castelo gabbro (499 ± 2 Ma; U-Pb in zircons) was immediately followed by a Barrov-ian metamorphic episode that reached the granulite facies (493-498 Ma; U-Pb in monazites). A later Variscan overprint is indicated by U-Pb rutile ages of 380-390 Ma. Considering the geochronological evidence for almost coeval magmatism and metamorphism during the Early Ordovician, together with the geochemical characteristics of the Monte Castelo gabbro, an accretionary complex related to a Late Cambrian-Early Ordovician island arc appears as the more probable setting for the uppermost allochthonous terrane in the northwest of the Iberian Massif. This implies the presence of a convergent plate boundary in the oceanic realm between Laurentia and Gondwana, or close to it, during the early Paleozoic. The island arc later became involved in the Variscan convergence and accretion.