Origin and significance of spinel and garnet pyroxenites in the shallow lithospheric mantle: Ultramafic massifs in orogenic belts in Western Europe and NW Africa

Abstract

Anhydrous spinel and garnet pyroxenites form a small (< 10%) but integral part of the shallow sub-continental lithospheric mantle. They occur as layers or dykes within tectonically-emplaced ultramafic massifs and display lithologies that range from orthopyroxenite through websterites to clinopyroxenite, with or without olivine. The ultramafic massifs in orogenic belts of western Europe and Morocco contain many different varieties of mantle pyroxenites that show a wide range of major and trace element compositions. Their bulk compositions tend to vary between at least three end-members: two of these are similar to the consitutent clinopyroxene and orthopyroxene, while the third component has concentrations of MgO, CaO, Al 2O 3, TiO 2 and Na 2O that are close to basaltic levels and may indicate the former presence of trapped basaltic melt. These pyroxenites are probably cumulates in the broadest sense. Nevertheless, some pyroxenites from Beni Bousera and Ronda have whole rock compositions that are similar to that of bulk oceanic crust. Most mantle pyroxenites have LREE-depleted whole-rock REE patterns, although with lower absolute concentrations than MORB or bulk oceanic crust, and most lack Eu anomalies. Trace element patterns in clinopyroxenes from pyroxenites are also usually LREE-depleted. Most clinopyroxenes show depletions in Zr, Hf and Sr, although those from the Cabo Ortegal complex (NW Spain) display LREE-enrichment and significant Sr peaks, attributed to the influence of subduction-related fluids or melts. δ 18O values for clinopyroxenes from mantle pyroxenites in ultramafic massifs tend to be in the range 5.0–5.8‰, i.e. typical mantle values. Rare garnet pyroxenite layers from a few massifs show higher values that reflect recycling of a crustal component. Similarly, most Sr, Nd and Pb isotopic ratios from pyroxenites fall in the depleted mantle field, although with a slightly wider range of values than is typical for shallow sub-continental mantle peridotites from Europe. Most mantle pyroxenites are best explained as products of crystal accumulation of mantle-derived magmas, together with variable amounts of trapped interstitial magma. Others may be the products of interaction between magma and peridotite or older pyroxenite wall-rock. Only samples from a few massifs (e.g. garnet pyroxenites from Beni Bousera and Ronda) display strong evidence of being related to recycled oceanic crust or melts of oceanic crust. Even fewer mantle pyroxenites show good evidence for derivation either by incipient melting of the host peridotite or by metamorphic segregation. Pyroxenites are low-MgO, high CaO, high Al 2O 3 lithologies that provide a well-documented example of enrichment of the lithospheric mantle. If such material were recycled into the asthenosphere, mantle pyroxenites might melt preferentially to the host peridotite. However, the precise role of mantle pyroxenites in magmagenesis could be difficult to track, as many of their isotope and trace element compositions (e.g Sm/Nd ratios) are often similar to those of their host mantle peridotites.