SR-ND-PB ISOTOPIC AND TRACE-ELEMENT EVIDENCE FOR CRUSTAL CONTAMINATION OF PLUME-DERIVED FLOOD BASALTS - OLIGOCENE FLOOD VOLCANISM IN WESTERN YEMEN

Oligocene flood basalts from western Yemen have a relatively limited r ange in initial isotopic composition compared with other continental f lood basalts: Sr-87/Sr-86 = 0.70365-0.70555; Nd-143/Nd-144 = 0.51292-0 .51248 (is an element of(Nd) = +6.0 to -2.4); Pb-206/Pb-204 = 17.9-19. 3. Most compositions lie outside the isotopic ranges of temporally and spatially appropriate mantle source compositions observed in this are a, i.e., Red Sea/Gulf of Aden MORE mantle, the Afar plume, and Pan-Afr ican lithospheric mantle. Correlations between indices of fractionatio n, silica, and isotope ratios suggest that crustal contamination has s ubstantially modified the primary isotopic and incompatible trace elem ent characteristics of the flood basalts. However, significant scatter in these correlations was produced by: (a) the heterogeneous isotopic composition of Pan-African crust; (b) the difference in susceptibilit y of magmas to contamination as a result of variable incompatible trac e element contents in primary melts produced by differing degrees of p artial melting; (c) the presence or absence of plagioclase as a fracti onating phase generating complex contamination trajectories for Sr; (d ) sampling over a wide area not representing a single coherent magmati c system; and (e) variation in contamination mechanisms from assimilat ion associated with fractionation (AFC) to assimilation by hot mafic m agmas with little concomitant fractionation. The presence of plagiocla se as a fractionating phase in some suites that were undergoing AFC re quires assimilation to have taken place within the crust and, coupled with the limited LREE-enrichment accompanying isotopic variations, exc ludes the possibility that an AFC-type process took place during magma transfer through the lithospheric mantle. Isotopic compositions of so me of the inferred crustal assimilants are similar to those postulated by other workers for an enriched lithospheric mantle source of many f lood basalts in southwestern Yemen, Ethiopia, and Djibouti. The wester n Yemen flood basalts contain 0-30% crust which largely swamps their p rimary lead isotopic signature, but the primary Sr-Nd isotopic signatu re is close to that of the least contaminated and isotopically most de pleted flood basalts. LREE/HFSE and LILE/HFSE ratios also correlate wi th isotopic data as a result of crustal contamination. However, Nb/La and K/Nb ratios of >1.1 and <150, respectively, in least contaminated samp!es require an OIB-like source. The pre-contamination isotopic sig nature is estimated to be: Sr-87/Sr-86 approximate to 0.7036; Nd-143/N d-144 approximate to 0.51292; Pb-206/Pb-204 approximate to 18.4-19.0. This, coupled with low LILE/HFSE ratios, suggest the source has charac teristics akin to the Afar plume. A mantle source isotopically more de pleted than Bulk Earth, but not as depleted as MORE, coupled with LILE depletion, also characterises other examples of plume-derived flood v olcanism. This mantle reservoir is responsible for the second largest outbursts of volcanism on Earth and has radiogenic isotopic characteri stics akin to PREMA mantle, but the incompatible trace element signatu re of HIMU mantle.