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, Al2O3, TiO2 and Na2O 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. δ18O values for clinopyroxenes from mantle pyroxenites in ultramafic massifs tend to be in the range 5.0–5.8‰, i.e. typical mantle values.