ZIRCONS IN CRATONIC MANTLE ECLOGITES: INSIGHTS INTO SUBCONTINENTAL LITHOSPHERIC MANTLE EVOLUTION AND METASOMATISM

Zircon is a trace constituent of a number of kimberlite-borne, cratonic lithospheric mantle eclogites from the Kaapvaal craton of southern Africa. Although representing less than 2% of the lithospheric mantle, eclogites provide important insights into its geodynamic evolution. New ID-TIMS U-Pb and MC-ICPMS Lu-Hf isotopic measurements on the same single zircon grains have revealed numerous episodes of growth that provide insights into the timing of eclogite genesis and subsequent metasomatism of the subcontinental lithospheric mantle. Near-concordant Archean (3.06 to 2.63 Ga) dates have been measured for zircons from eclogites in the Roberts Victor, Lace, and Lovedale kimberlites, providing minimum ages for eclogite formation and confirming the participation of eclogitic lithospheric components in Archean geodynamic processes. These Late Archean zircons also unequivocally establish the closed-system behavior of zircon with respect to Pb diffusion in these lithospheric mantle samples, and confirm the nature of recorded isotopic dates as crystal growth ages. It remains unclear whether the Archean zircons record the eclogite protolith age, or subsequent growth catalyzed by metasomatic fluids/melts. While there are similarities between zircon ages in several eclogites and the timing of intracratonic mafic magmatism, the presence of several different episodes of Archean zircon growth in the same xenolith are suggestive of repeated crystallization in response to metasomatic fluid/melt infiltration, rather than magmatic crystallization from a basaltic melt. In nearly all of these eclogites, Archean zircon is accompanied by younger (1.07 to 0.16 Ga) growth that must date fluid/melt-induced mantle metasomatism. A compilation of all available metasomatic growth ages and Hf isotope compositions reveals that zircon growth associated with periods of continental extension has radiogenic Hf, preserving a unique record of infiltration of asthenosphere-derived metasomatizing agents into the subcontinental mantle. By contrast, zircon growth coeval with periods of active plate subduction beneath the Kaapvaal craton is characterized by unradiogenic Hf, which may reflect the catalytic effect of subduction-related fluids to remobilize ancient incompatible element enriched domains in the lithosphere.