The Variscan gabbros from the Spanish Central System: A case for crustal recycling in the sub-continental lithospheric mantle?

Abstract

Keywords: Gabbroic rocks Crustal recycling Variscan orogeny Uthospheric mantle Spanish Central SystemThe gabbroic intrusions that crop out along the Spanish Central System (SCS) are geochemically heterogeneous, including primitive and evolved rocks. Differentiation is mainly related to fractionation of Cr-spinel and olivine, but mixing with coeval granitic magmas or crustal assimilation may have also played a role in the evolution of the most differentiated rocks. The most primitive uncontaminated gabbros show arc like trace element chondrite and primitive-mantle normalised patterns, characterised by large ion Iithophile elements (ULE)-Iigh t rare earth elements (tREE) enrichment, Sr and Pb positive and Nb-Ta-Ti negative anomalies. However, paleogeographic constraints suggest that the SCS was located far from subduction zones, so these geochemical signatures could be better explained by a recycling of continental crustal components within the mantle. The most primitive SCS gabbros expand the Sr-Nd isotopic compositional range of the Variscan basic magmatism in the Central Iberian Zone to more depleted values. This reflects a heterogeneous sub-continental Iithospheric mantle under central Spain ranging from a depleted mantle (ENd = +3.1. 87Sr/86Sr= 0.704) towards an isotopically enriched component (ENd = -1.6, 87Sr/ 86Sr = 0.706). Geochemical modelling suggests that mantle enrichment could be explained by minor lower crustal metapelitic granulite contamination (-2%). Additionally. the Sr-Nd-Pb isotopic ratios of the most primitive gabbros match the composition of the European subcontinental lithospheric mantle recorded in ultramafic xenoliths from western and central Europe.

Late-Variscan emplacement and genesis of the Vieira do Minho composite pluton, Central Iberian Zone: Constraints from U–Pb zircon geochronology, AMS data and Sr–Nd–O isotope geochemistry

“…Furthermore U-Pb geochronological and petrological data from Variscan gabbros support the argument that these rocks are not mafic precursors of the associated granite magmatism, but mostly coeval (e.g. Orejana et al, 2009;Villaseca et al, 2011). All these arguments preclude the possibility that the studied peraluminous granites represent an evolved fraction from a more basic magma.…”

“…These authors propose that the gabbro-norite rocks, although having a primitive character, show Sr-Nd isotope composition (Sr i =0.7049-0.7053, ε Nd =−2.1 to −2.5) suggesting the existence of a sub-Iberian enriched mantle during the Variscan event. The enriched nature of subcontinental mantle domains is also present in other sectors within the Variscan Belt (Fernández-Suárez et al, 2011;Janousek et al, 2000;Moreno-Ventas et al, 1995;Orejana et al, 2009;Pin and Duthou, 1990;Villaseca et al, 2011). Further constraints about the hybridisation process in the genesis of granitoids are given by the presence of mafic microgranular enclaves associated with granitic rocks, as they are generally considered as representing a mantle-derived component (Barbarin and Didier, 1992;Bea et al, 1999;Bonin, 2004;Cocherie et al, 1994;Collins et al, 2000;Dias and Leterrier, 1994;Dias et al, 2002;Didier, 1987;Janousek et al, 2004;Moreno-Ventas et al, 1995;Orsini et al, 1991;Renna et al, 2006;Silva and Neiva, 2004;Slaby and Martin, 2008;Vernon, 1984).…”

Paleozoic–Mesozoic Porphyry Cu(Mo) and Mo(Cu) Deposits within the Southern Margin of the Siberian Craton: Geochemistry, Geochronology, and Petrogenesis (a Review)

“…The last model (crustal recycling) is typical of collisional zones [117][118][119]. The magmatic events at Zhireken deposit occurred during continental collision, which is favorable for crustal thickening and the subsequent mantle interaction with the foundered continental crust.…”

“…Two possible mechanisms are generally assumed to account for the petrogenesis of mafic rocks with low εNd and high 87 Sr/ 86 Sr values: (1) they result from the mixing of mantle-derived and crustal melts during magma ascent [1], or (2) they are derived from the upper mantle metasomatized by melts from deeply recycled subducted continental crust and/or a delaminated thickened lithosphere. The last model (crustal recycling) is typical of collisional zones [117][118][119]. The magmatic events at Zhireken deposit occurred during continental collision, which is favorable for crustal thickening and the subsequent mantle interaction with the foundered continental crust.…”

Strength of the Iberian intraplate lithosphere: Cenozoic deformations and seismicity

“…The integrated strength of the strong mantle is 2-3 times the crustal one, whereas this range decreases to 0.2-0.8 with the weak mantle. There is no solid evidence for the mantle composition in the peninsular centre (Villaseca and Orejana, 2008;Orejana et al, 2009), but taking into account that only strong mantles support the main geodynamic features and processes (Burov and Watts, 2006;Burov, 2009Burov, , 2011, the estimates obtained with a compressive regime or a wet peridotitic mantle must be dismissed. The weakening of the lithosphere under these conditions would hamper the Alpine deformations that shaped the Iberia interior.…”