MELT ENRICHMENT OF SHALLOW DEPLETED MANTLE - A DETAILED PETROLOGICAL,TRACE-ELEMENT AND ISOTOPIC STUDY OF MANTLE-DERIVED XENOLITHS AND MEGACRYSTS FROM THE CAMEROON LINE

Major element, trace element and Sr-Nd-Pb isotopic compositions of ult ramafic xenoliths and megacrysts from the continental Cameroon line pr ovide evidence for metasomatism of the upper-most lithospheric mantle by enriched melts during the Mesozoic. The megacrysts probably crystal lized within the lower continental crust from melts similar to the hos t magmas. All the xenoliths originated as depleted residues after the extraction of basaltic melts, but some indicate evidence of interactio n with enriched partial melts before entrainment. The U-Pb isotopic da ta on garnet are consistent with cooling through > 900 degrees C at > 300 Ma. The Sm-Nd isotope systematics in constituent phases appear to have been in equilibrium on a xenolith scale at the time of entrainmen t, indicating derivation from mantle that remained at temperatures > 6 00 degrees C until eruption. Spinel lherzolites that show simple light rare earth element (LREE) depletions are characterized by isotopic co mpositions that are comparable with, but slightly more depleted than A tlantic NMORB, suggesting that the unmetasomatized sub-continental lit hosphere of the Cameroon line may be isotopically similar to that of s ub-oceanic lithosphere. The Nd-depleted mantle model ages of these xen oliths are consistent with late Proterozoic depletion, similar in age to much of the overlying continental crust. In contrast, samples that have LREE-enriched clinopyroxenes (La/Yb = 4.7-9.4) contain trace amou nts of amphibole, are enriched in U and have more radiogenic Pb and Sr . These xenoliths yield U-Pb and Sm-Nd model ages consistent with Meso zoic enrichment, in agreement with the age of enrichment of the source regions of the basalts, as deduced from Pb isotopic compositions. Cli nopyroxenes record three orders of magnitude enrichment in U and LREE accompanied by progressive K depletion associated with the growth of t race amphibole, with K/U ratios that range from 12 000 to 1. The ratio s of the trace elements thought to have similar bulk D in mantle melti ng, Ce/Pb, Ba/Rb and Nd/Sr ratios, display regional variations related to the time integrated history of enrichments indicated by Nd isotopi c compositions. Mass balance calculations suggest that the melts respo nsible for the most recent enrichment of the lithosphere had higher La /Yb and U/Pb than Cameroon line host magmas, and were probably the Pro duct of small degrees of partial melting associated with the earliest stages of the breakup of Pangea.