GEOCHEMICAL CONSTRAINTS ON THE PETROGENESIS OF DIAMOND FACIES PYROXENITES FROM THE BENI BOUSERA PERIDOTITE MASSIF, NORTH MOROCCO

The petrogenesis of pyroxenite layers within the Beni Bousera peridotite massif is investigated by means of elemental and Nd Sr-Pb-O-S isotope analyses. The light rare earth element (LREE) depleted nature of many of the pyroxenites, their wide variation in composition, and lack of correlation between incompatible elements and fractionation indices preclude them from representing crystallized melts from a peridotitic source. The physical characteristics of the pyroxenites and their large (greater than a factor of 20) range in Ni rule out partial melting as the cause of their petrological and geochemical diversity. Major and compatible trace element geochemistry is consistent with formation of most of the pyroxenite suite via high-pressure crystal segregation in magma conduits intruding the peridotites. These magmas crystallized clinopyroxene, orthopyroxene, and garnet. The pressure of crystallization is constrained to be above approximately 45 kbar from the presence of graphitized diamonds in pyroxenite layers. Lack of correlation between fractionation indices and highly incompatible elements and the wide variation in incompatible element abundances suggest that the suite did not form from genetically related magmas. The presence of positive ad negative Eu anomalies (Eu/Eu* = 0.54-2.0) in pyroxenites which crystallized at pressures much greater than the plagioclase stability field (approximately 45 kbar) suggests that the parental magmas originated from precursors which formed in the crust. Oxygen isotope compositions of coexisting minerals in the pyroxenites indicate high-temperature equilibration but deltaO-18 values vary from +4.9 to +9.3 parts per thousand, ruling out their derivation from the host peridotites or other normal mantle sources. The extreme O-isotope variation, together with deltaS-34 values of up to + 13 parts per thousand. in sulphides included within CPX strongly suggests that the melts from which the pyroxenites crystallized were derived from hydrothermally altered, subducted oceanic lithosphere. Extreme initial radiogenic isotope variation in the pyroxenites (epsilon(Nd) + 26 to - 9, Sr-87/Sr-86 0.7025-0.71 10, Pb-206/Pb-204 18.21-19.90) support such an origin but also require a component with ancient, high U/Pb and Th/Pb in their source to explain the high DELTA7/4 and DELTA8/4 values of some pyroxenites. This component may be subducted hemi-pelagic sediment. Further evidence for a sediment component in the pyroxenites is provided by isotopically light carbon in the graphite pyroxenites (deltaC-13 - 16 to - 28 parts per thousand). Parent daughter isotopes in the pyroxenites are strongly decoupled, making estimation of formation ages speculative. The decoupling occurred recently (<200 Ma), probably as a result of partial melting associated with diapiric upwelling and emplacement of the massif into the crust from the diamond stability field. This late partial melting event further depleted the pyroxenites in incompatible elements. The variably altered nature of the subducted protolith and complex history of trace element fractionation of the pyroxenites has largely obscured geochemical mixing trends. However, Nd Pb isotope systematics indicate that incorporation of the component with high U/Pb-Th/Pb occurred relatively recently (<200 Ma) for some pyroxenites. Other pyroxenites do not show evidence for incorporation of such a component and may be substantially older Tectonic, geophysical, and isotopic constraints indicate formation of the pyroxenites in the mantle wedge above a subducting slab during the Cretaceous. Physical and chemical evidence for high-pressure fractionation seen in most of the pyroxenites precludes them from simply representing ancient subducted oceanic lithosphere, thinned by diffusion. However, the petrological and isotopic diversity of the massif support the concept of a 'marble cake' mantle capable of producing the observed geochemical diversity seen in oceanic magmas.