Rare Earth and Trace Element Geochemistry of Clinopyroxenes from the Zabargad Peridotite-Pyroxenite Association

Clinopyroxenes in spinel lherzolites and pyroxenites from the island of Zabargad (St. John's Island, Red Sea) were analysed for rare earth and other trace elements using an ion microprobe. The data indicate that the lherzolites represent mantle material compatible with a fertile source for mid-ocean ridge basalts (MORB), whereas depleted lherzolites record multiple processes, involving low-degree partial melting and successive light rare earth element (LREE) enrichment by melt percolation. Further, the ion microprobe data reveal that spinel pyroxenites are related to deep-seated tholeiitic and alkaline magmatism. Tholeiitic melts probably had garnet in the primary crystallizing assemblage, as suggested by the presence of clinopyroxene with low Sr and very high heavy rare earth element (HREE) and Sc abundances. All the clinopyroxenes are marked by significant depletions in Ti and Zr relative to neighbouring REE. Moreover, they form positive sub-parallel trends in Cr vs. Ti and Cr vs. Zr diagrams. These features were not produced during magmatic processes but resulted from complex processes including: (1) garnet breakdown (re-equilibration under spinel facies conditions); (2) early metasomatism (pargasite development); (3) growth of Mg-hornblende during the later stages of evolution.