Evidence of diverse depletion and metasomatic events in harzburgite–lherzolite mantle xenoliths from the Iberian plate (Olot, NE Spain): Implications for lithosphere accretionary processes

Abstract

Mantle xenoliths from the Olot volcanic district (NE Spain) comprise a bi-modal suite consisting of protogranular spinel lherzolites (cpx 12–14%) sometimes with pargasitic amphibole, and highly refractory spinel harzburgites (cpx ≤ 1%) with coarse-grained granular textures. The lherzolites range from slightly depleted to moderately LREE-enriched with flat HREE patterns between 1.5 and 2.7 × chondrite (Ch). In contrast, the harzburgites are extremely depleted in HREE (down to 0.2 × Ch) and strongly LREE-enriched (La_N/Yb_N = 12.3–17.2). LA-ICP-MS analyses of clinopyroxene and amphibole of the lherzolites highlight variable degrees of LREE depletion (HREE up to 13 × Ch, La_N/Yb_N down to 0.01), with the exception of a single sample in which both clinopyroxene and amphibole are LREE-enriched (La_N/Yb_N up to 19). In the harzburgites, clinopyroxenes display totally different REE distributions, characterized by extreme HREE depletion (down to 0.4 × Ch) and upward convex positively fractionated middle-light REE patterns (Nd_N/Yb_N up to 20.7 × Ch; La_N/Yb_N up to 12 × Ch). Sr–Nd–Hf isotopic data for both whole-rocks and cpx separates, coherently indicate depleted mantle (DM) compositions for the lherzolites (εSr = − 15 to − 26, εNd = + 9 to + 17, εHf = + 18 to + 68) and enriched mantle (EM) compositions for the harzburgites (εSr = − 10 to + 36, εNd = − 1 to − 6, εHf = + 3 to + 8). Modelling of the clinopyroxene REE data and isotopic systematics suggest that some lherzolites were affected by pre-Paleozoic (0.6–1 By) low-degree partial melting processes, while others probably reflect some extent of refertilization of the mantle protolith by metasomatizing melts similar to the Triassic rift-related tholeiites reported from several Pyrenean localities. The harzburgites represent extreme refractory residua, resulting from a complex depletion history due to multistage melt extraction as often observed in the cratonic mantle. The distinctive REE patterns and isotopic systematics of their clinopyroxenes suggest that the harzburgites were formed by the interaction of an ultra-depleted peridotite matrix with highly alkaline basic melts similar in composition to the Permo-Triassic alkaline lamprophyres which are widespread within the Iberian plate. Lherzolites possibly represent younger lithosphere (accreted asthenosphere?) up-lifted and juxtaposed to the older subcontinental lithospheric mantle (harzburgites) during the post-Variscan rifting of the Iberian margin. These two genetically different, but adjoining, mantle domains intimately mingled along the northern Iberian margin during the subsequent plate convergence processes, leading to the close association of harzburgites and lherzolites observed in the Olot mantle xenoliths and in some Pyrenean peridotite massifs.

Introduction

The Quaternary anorogenic volcanic district of Olot (NE Spain; Fig. 1) is located on the southern edge of the Pyrenean collisional orogenic belt, where Alpine tectonic phases (Larrasoaña et al., 2003, Pomar et al., 2005) are superimposed on older Variscan structures (Martì, 1996, Laumonier, 1998). During the Cenozoic, this sector of the paleo-European continental margin faced a NW-dipping subduction zone, which generated orogenic magmatism and ultimately led to the opening of the Liguro–Provencal and Valencia Trough inter-arc basins (Beccaluva et al., 1987, Martí et al., 1992, Beccaluva et al., 2005), and to the anticlockwise rotation of the Sardinia–Corsica block. These tectono-magmatic events may have played a significant role in the evolution of the mantle lithosphere beneath the area. Information on the nature of the lithospheric mantle beneath the Iberian plate is relatively scarce and is mainly based on indirect evidence from studies of Cenozoic basic volcanism (Cebriá and López-Ruiz, 1996, Benito et al., 1999, Cebrià et al., 2000). Direct studies of mantle lithologies have so far focused on the Alpine-type peridotites of the Pyrenees and Ronda massifs (Reisberg et al., 1989, Downes et al., 1991, Lenoir et al., 2001). Although mantle xenolith occurrences in alkaline basalts have been reported from several localities in Spain (Ancochea and Nixon, 1987), recent detailed investigations are available only for xenoliths from Tallante in the Betic Cordillera (Beccaluva et al., 2004).

Here we present new major and trace element and Sr–Nd–Hf isotope data for bulk-rock and constituent minerals of mantle xenoliths from Olot (Fig. 1). These data provide important new constraints on the evolution of the lithospheric mantle beneath the north-eastern part of the Iberian plate, as well as its relationships with the complex tectono-magmatic events of the area. Analytical methods are reported in the Appendix.