TRACE-ELEMENT AND ISOTOPIC CHARACTERIZATION OF MAFIC CUMULATES IN A FOSSIL MANTLE DIAPIR (OMAN OPHIOLITE)

This paper is devoted to an exploratory geochemical study (trace eleme nts, Sr-Nd isotopes) of a suite of cumulate features cropping out in t he mantle harzburgites of Oman. The cumulates are concentrically distr ibuted around a fossil mantle diapir, From the centre to the periphery of the diapir, their mineralogy becomes more and more evolved, from t roctolite to olivine gabbro and gabbronorite, and their textural and f ield characteristics point to injection in a cooler environment. Nd is otopic data are consistent with a mantle origin for all these cumulate s. The mantle source is heterogeneous on a small scale (6.09 < epsilon (Nd) < 10.15) but this heterogeneity and the average epsilon(Nd) (appr oximate to 8) are comparable to those of Indian Ocean mid-ocean ridge basalts (MORB's). An origin of this cumulate suite, largely by fractio nal crystallization from liquids with MORE characteristics, is support ed by major and compatible trace element variations. However compatibl e elements show that it is unlikely that all these cumulates are deriv ed by different degrees of fractional crystallization from the same li quid. This conclusion is corroborated by incompatible trace element da ta. The calculated liquids in equilibrium with these cumulates have MO RE-like REE patterns. However, variations in REE patterns from one lit hological group to the next, and within each lithological group, imply a pronounced heterogeneity in the REE content of the equilibrium liqu ids that clearly does not result from a simple fractional crystallizat ion process. Liquids in equilibrium with the troctolites have a higher range in [La](n)/[Yb](n) than liquids in equilibrium with the olivine gabbros. The most evolved cumulates (gabbronorites) have REE patterns that might imply a more complex origin for their parental melt involv ing mixing of MORE-like liquids with melts from a LREE-depleted source .