Résumé
The genesis of ultra-high pressure (UHP) garnet pyroxenites in orogenic peridotite massifs and its implications on the formation of chemical heterogeneities in the mantle and on basalt petrogenesis are still not fully understood. Some UHP (diamond-bearing) garnet pyroxenites have isotopic, and major and trace element compositions similar to the recycled oceanic crustal component observed in oceanic basalts [1-6]. These pyroxenites hence provide an exceptional opportunity to investigate in situ the nature and scale of the Earth's mantle chemical heterogeneities. Here, we present an integrated geochemical study of UHP garnet pyroxenites from the Ronda (Betic Belt, S. Spain) and Beni Bousera (Rif Belt, N. Morocco) peridotite massifs. This investigation encompasses, in the same sample, bulk rock major and trace elements, as well as Sr-Nd-Pb-Hf isotopic analyses. According to their Al (sub 2) O (sub 3) content, we classify UHP garnet pyroxenites into three groups that have distinct trace elements and Sr-Nd-Pb-Hf isotopic signatures. Group A pyroxenites (Al (sub 2) O (sub 3) : 15 - 17.5 wt. %) are characterized by low initial (super 87) Sr/ (super 86) Sr, relatively high (super 143) Nd/ (super 144) Nd, (super 206) Pb/ (super 204) Pb and (super 176) Hf/ (super 177) Hf ratios, and highly variable (super 207) Pb/ (super 204) Pb and (super 208) Pb/ (super 204) Pb ratios. Group B pyroxenites (Al (sub 2) O (sub 3) < 14 wt. %) have isotopic signatures characterized by relatively high initial (super 87) Sr/ (super 86) Sr and low (super 143) Nd/ (super 144) Nd, (super 206) Pb/ (super 204) Pb and (super 176) Hf/ (super 177) Hf ratios. Group C pyroxenites (Al (sub 2) O (sub 3) approximately 15 wt. %) display relatively low initial (super 87) Sr/ (super 86) Sr and (super 206) Pb/ (super 204) Pb ratios, high (super 143) Nd/ (super 144) Nd and (super 176) Hf/ (super 177) Hf ratios, and (super 207) Pb/ (super 204) Pb and (super 208) Pb/ (super 204) Pb ratios similar to Group B pyroxenites. The major and trace element, and isotopic compositions of the studied Ronda and Beni Bousera UHP garnet pyroxenites lend support to the "Marble Cake Mantle" model [7] for the genesis of these pyroxenites. This model envisions the mantle source of oceanic basalts as a melange of subducted, ancient oceanic crust - represented by garnet pyroxenites in orogenic peridotites - intimately mixed with peridotites by mantle convection. The present study reveals, however, that besides this exotic component of ancient recycled oceanic crust, the genesis of these pyroxenites requires a previously unnoticed component of recycled lower continental crust akin to the lower crustal section of the lithosphere where these UHP garnet pyroxenites now reside in. The results of this study provide a new recipe for the marble cake hypothesis for the genesis of UHP garnet pyroxenites in orogenic peridotites. Furthermore, it establishes a connection between the genesis of UHP pyroxenites, the composition of the continental crust and the generation of Earth's mantle heterogeneities. References: Pearson, D. G., Davies, G. R. & Nixon, P. H. (1993). Geochemical constraints on the petrogenesis of diamond facies pyroxenites from the Beni Bousera peridotite massif, North Morocco. Journal of Petrology 34, 125-172. Blichert-Toft, J., Albarede, F. & Kornprobst, J. (1999). Lu-Hf Isotope systematics of garnet pyroxenites from Beni Bousera, Morocco: implications for basalt origin. Science 283, 1303-1306. Garrido, C. J. & Bodinier, J. L. (1999). Diversity of mafic rocks in the Ronda peridotite: Evidence for pervasive melt-rock reaction during heating of subcontinental lithosphere by upwelling asthenosphere. Journal of Petrology 40, 729-754. Marchesi, C., Garrido, C. J., Bosch, D., Bodinier, J. L., Gervilla, F., Hidas, K., 2013. Mantle refertilization by melts of crustal-derived garnet pyroxenite: Evidence from the Ronda peridotite massif, southern Spain. Earth and Planetary Science Letters 362, 66-75. doi: 10.1016/j.epsl.2012.11.047. Marchesi, C., Dale, C. W., Garrido, C. J., Pearson, D. G., Bosch, D., Bodinier, J. L., Gervilla, F., Hidas, K., 2014. Fractionation of highly siderophile elements in refertilized mantle: Implications for the Os isotope composition of basalts. Earth and Planetary Science Letters 400, 33-44. doi: 10.1016/j.epsl.2014.05.025 Montanini, A. & Tribuzio, R. (2015). Evolution of recycled crust within the mantle: Constraints from the garnet pyroxenites of the External Ligurian ophiolites (northern Apennines, Italy). Geology 43, 911-914. Allegre, C. J. & Turcotte, D. L. (1986). Implications of a two-component marble-cake mantle. Nature 323, 123-127. [Copyright Author(s) 2017. CC Attribution 3.0 License: https://creativecommons.org/licenses/by/3.0/legalcode]