Origin of chemical heterogeneities in the subcontinental lithospheric mantleinsights from the westernmost Mediterranean orogenic peridotites and allied crustal rocks

Laburpena

Understanding the mechanisms and processes responsible for the generation of mantle heterogeneities is crucial for improving our knowledge about the composition and differentiation of the Earth at different scales. Orogenic peridotite massifs provide an exceptional opportunity to investigate in situ the nature and scale of compositional mantle heterogeneities. This Ph.D. thesis aims to shed new light on the role of melt-rock reaction processes and recycling of oceanic and continental crustal material in creating chemical heterogeneities in the mantle. To achieve this objective, this study focuses on mantle rocks exposed in orogenic peridotites from the Betic-Rif Cordillera in the westernmost Mediterranean, and their allied crustal rocks. Two processes related to the development of geochemical heterogeneities in the Earth’s upper mantle are addressed in this work: (i) the formation of secondary lherzolites by melt-rock reaction processes, and (ii) the genesis of ultra-high pressure garnet pyroxenites and its implications for crustal recycling into the mantle. Furthermore, this thesis explores (iii) to what extent Betic crustal rocks were involved in the geochemical signature of magmatism recorded in the subcontinental lithospheric mantle (SCLM), and in the Miocene Alborán Sea basin volcanism. The westernmost Mediterranean records an Alpine history of subduction and slab roll-back that resulted in the creation of the Alborán Sea basin. Mantle rocks and their allied crustal rocks in the Betic-Rif belt hence provide a unique natural laboratory for studying large scale recycling of crustal components in the SCLM. In order to address how melt-rock reaction processes are recorded in the SCLM, a large geochemical database of peridotites from the four tectono-metamorphic domains of the Beni Bousera orogenic massif (Rif Belt, N. Morocco) was processed. This study reveals that variations in bulk rock major and minor elements, Mg-no. and modal proportions of lherzolites, as well as their clinopyroxene trace element compositions, are inconsistent with simple partial melting and mainly resulted from different reactions between melts and depleted peridotites. On the other hand, up to 30% melting at < 3 GPa and cryptic metasomatism can account for the geochemical variations of most harzburgites. In garnet-spinel mylonites, melting and melt-rock reactions are masked by tectonic mixing with garnet pyroxenites and subsolidus re-equilibration. In the rest of the massif, lherzolites were mostly produced by refertilization of a refractory protolith (Mg-no. ~ 91, olivine ~ 70%, clinopyroxene/orthopyroxene = 0.4) via two distinct near solidus, melt-rock reactions: (1) clinopyroxene and orthopyroxene precipitation and olivine consumption at melt/rock ratios < 0.75 and variable mass ratio between crystallized minerals and infiltrated melt (R), which are recorded quite homogeneously throughout the massif; and (2) dissolution of orthopyroxene and precipitation of clinopyroxene and olivine at melt/rock ratios ≤ 1 and R = 0.2 – 0.3, which affected mainly the Ariègite-Seiland and Seiland domains. The distribution of secondary lherzolites in the massif suggests that the first refertilization reaction occurred prior to the differentiation of the Beni Bousera mantle section into petro-structural zones, whereas the second reaction was associated to the development of the tectono-metamorphic domains during exhumation. These data support a secondary, refertilization-related, origin for most lherzolites in orogenic peridotite massifs. The genesis of ultra-high pressure garnet pyroxenites and its implications for crustal recycling into the mantle were investigated through a detailed major and trace element and Sr-Nd-Pb-Hf isotopic study of garnet pyroxenites from the Ronda and Beni Bousera massifs (Betic-Rif Belt, westernmost Mediterranean). The results obtained show that the analyzed garnet pyroxenites not only derived from recycled oceanic crust, but also from recycled lower continental crust spatially associated with the orogenic massifs. Recycled continental crust is often invoked as the source of the Enriched Mantle (EM) isotopic component sampled by some oceanic basalts. This new isotopic evidence lends further support to the hypothesis that ultra-high pressure garnet pyroxenites can be a proxy for the study of the Earth’s mantle sampled by oceanic and continental basalts. The existence of exotic recycled oceanic and co-lithospheric continental crust components in the subcontinental lithospheric mantle requires new recipes for the generation of the Earth’s Marble Cake Mantle. To evaluate the role of the Betic crustal rocks (southern Spain) in the geochemical signature of the magmatism recorded in the SCLM of the westernmost Mediterranean, and in the Miocene Alborán Sea basin volcanism, this thesis presents a detailed trace element and Sr-Nd-Pb isotope study of the western Alpujárride metamorphic basement and the pre-Miocene Flysch sediments. The results indicate that Nd model ages are consistent with an increasing detrital input from the Alborán domain to the Flysch Trough in the western Mediterranean during the late Oligocene. The Alpujárride metamorphic crustal rocks derived from Archean-Paleoproterozoic terranes located along the northern margin of Gondwana in the Neoproterozoic. The heterogeneous isotopic signatures of the Alpujárride units indicate that they have different sedimentary protoliths and underwent contrasted Variscan and pre-Variscan tectono-magmatic evolutions. Melts/fluids derived from the western Alpujárride gneisses contaminated the mantle source of the Ronda high-Mg pyroxenite dykes, implying that the Alpujárride lower crust underthrusted the subcontinental lithospheric mantle of the Alborán domain generating subduction-like magmatism in the late Oligocene. The western Alpujárride upper crust is involved in the Neogene volcanism of the Alborán Sea basin, but only contaminated some LREE-enriched calc-alkaline lavas during their ascent close to the continental margins. On the other hand, tholeiitic lavas in the centre of the basin show no isotopic evidence of crustal assimilation. This indicates that most of the crust in the central Alborán Sea accreted by Miocene tholeiitic magmatism and that Alpujárride lower crust is absent and likely foundered close to the continental margins of the basin.