Provenance of the HP–HT subducted margin in the Variscan belt (Cabo Ortegal Complex, NW Iberian Massif)

Abstract

The Variscan Upper Allochthon is a continental-affinity terrane that recorded a Cambrian-Ediacaran magmatic arc generation, a subsequent transition to a passive margin, and a collision-related high-P metamorphism during the Devonian-Carboniferous amalgamation of Pangea. The objective of this article is to decipher which continental margin subducted in the Devonian high-P-high-T (HP-HT) event. To do so, a provenance study is presented using combined U-Pb (n = 613) and Lu-Hf (n = 463) isotopic LA-ICP-MS zircon analyses and Sm-Nd whole-rock (n = 5) determinations. These analyses have been performed on five samples of the Banded Gneisses (Cabo Ortegal Complex, NW Iberia), which forms a part of the HP-HT bottom member of the Upper Allochthon. PalaeozoicNeoproterozoic zircon ages (34.7%) have a maximum abundance at 522-512 Ma, peaks at 575, 561, 545 Ma and minor abundance peaks between 780 and 590 Ma, and show from their Lu-Hf compositions a volcanic arc mixing pattern. This arc was probably related to the Cadomian arc system. The Mesoproterozoic population is scarce and scattered (2.8%), and due to its Lu-Hf pattern, it is proposed that this population is also West Africa Craton derived. The Paleoproterozoic population (39.6%) is concentrated at 2.07 Ga and it is linked to the Eburnean Orogeny, where depleted mantle derived magmas intruded an Archean craton margin. This craton is represented by the Archean population (22.8%), which is grouped at 3.0, 2.68-2.61 and 2.52-2.48 Ga, and shows long-term reworking processes and at least two juvenile magma intrusions. These data show that the Variscan Upper Allochthon has a West African provenance and therefore, it strongly suggests that the NW Iberian allochthonous complexes and their correlative European terranes are also West Africa derived. These results allow us to finally clarify that the first high-P event, recorded during the eo-Variscan amalgamation of Pangea, was attained by the subduction of the margin of Gondwana under Laurussia.

Post-collisional magmatism: Crustal growth not identified by zircon Hf–O isotopes

“…Moreover, the pre-Variscan crust of the FMC is dominated by paragneisses (and meta-granitoids derived from melting of the latter) corresponding to former sediments of the north Gondwana margin, which are characterized by Archean (2.5-3.3 Ga), Paleoproterozoic (1.8-2.2 Ga) and Neoproterozoic (0.5-0.7 Ga) detritus, as indicated by age patterns of detrital zircon grains (Albert et al, 2015;Linnemann et al, 2014). Fig.…”

Contrasting isotopic sources (Sm-Nd) of Late Ediacaran series in the Iberian Massif: Implications for the Central Iberian-Ossa Morena boundary

“…This interpretation is clearly confirmed at present by the existence in those complexes of a group of different ophiolitic units (Arenas and Sánchez Martínez, 2015;Arenas et al, 2018) and a pair of successive high-P metamorphic events dated at c. 400 and 370 Ma (Ordóñez Casado et al, 2001;Fernández-Suárez et al, 2007;Abati et al, 2010;Arenas et al, 2014;Díez Fernández et al, 2016). Several works have confirmed that the Ava-lonian-African margin of Gondwana has been an active margin for a long time, at least between 750 and 500 Ma, the Avalonian-Cadomian arc (Rodríguez- Pereira et al, 2006;Linnemann et al, 2008Linnemann et al, , 2014von Raumer and Stampfli, 2008;Fuenlabrada et al, 2010;Albert et al, 2015a, 2015b, von Raumer et al, 2015Andonaegui et al, 2016). According to this interpretation, the Central Iberian Zone very likely represents a back-arc basin located behind the most active part of the margin (Fernández-Suárez et al, 2014;Albert et al, 2015a;Orejana et al, 2015;.…”

A tectonic carpet of Variscan flysch at the base of a rootless accretionary prism in northwestern Iberia: U–Pb zircon age constrains from sediments and volcanic olistoliths

“…The structurally lower imbricated thrust system hosts the Almendra Formation (Vacas and Martínez Catalán, 1987) which is a calciturbidite made of phyllite and calcarenite rhythms up to several meters thick . Local lenses of polygenic conglomerates and microconglomerates holding exotic clasts and pebbles, and lithified intraclasts were firstly described by Aldaya et al (1976). Their lithologies include phyllite, sandstone, litharenite, quartzite, limestone, orthogneiss, rhyolite and felsic volcanic tuff (Fig.…”

“…6d and e). The most consolidated sedimentary fragments suggest that they were recycled within the basin, as a wild flysch (Ribeiro and Ribeiro, 1974;Aldaya et al, 1976;González Clavijo and Martínez Catalán, 2002;. These fragments were considered a proof of its synorogenic character, and also evidence that the basin was fed from areas of the Variscan belt already deformed and metamorphosed (Antona and Martínez Catalán, 1990;González Clavijo and Martínez Catalán, 2002;Martínez Catalán et al, 2004, 2008.…”