Melt Enrichment of Shallow Depleted Mantle: a Detailed Petrological, Trace Element and Isotopic Study of Mantle-Derived Xenoliths and Megacrysts from the Cameroon Line

Abstract

Major element, trace element and Sr-Nd-Pb isotopic compositions ofultramafic xenoliths and megacrysts from the continental Cameroon line provide evidence for metasomatism of the uppermost lithospheric mantle by enriched melts during the Mesozoic The megacrysts probably crystallized within the lower continental crust from melts similar to the host magmas. All the xenoliths originated as depleted residues after the extraction of basaltic melts, but some indicate evidence of interaction with enriched partial melts before entrainment. The U—Pb isotopic data on garnet are consistent with cooling through > 900°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 entrainment, indicating derivation from mantle that remained at temperatures > 600°C until eruption. Spinel Iherzolites that show simple light rare earth element (LREE) depletions are characterized by isotopic compositions that are comparable with, but slightly more depleted than Atlantic NMORB, suggesting that the unmetasomatized sub-continental lithosphere of the Cameroon line may be isotopically similar to that of sub-oceanic lithosphere. The Nd-depleted mantle model ages of these xenoliths 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 amounts 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 Mesozoic enrichment, in agreement with the age of enrichment of the source regions of the basalts, as deduced from Pb isotopic compositions. Clinopyroxenes record three orders of magnitude enrichment in U and LREE accompanied by progressive K depletion associated with the growth of trace amphibole, with K/ U ratios that range from 12 000 to 1. The ratios of the trace elements thought to have similar bulk D in mantle melting, Ce/ Pb, Ba/Rb and Nd/Sr ratios, display regional variations related to the time integrated history of enrichments indicated by Nd isotopic compositions. Mass balance calculations suggest that the melts responsible for the most recent enrichment of the lithosphere had higher La/Yb and U/Pb than Cameroon line host magmas, and were probably the product of small degrees of partial melting associated with the earliest stages of the breakup of Pangea.