Tracing the origin of zircon megacrysts in Triassic sediments of northeastern Siberian craton with implications to diamond paucity of craton-edge subcontinental lithospheric mantle

Abstract

We report combined UPb ages, Hf isotopes, and trace-element compositions of 116 mantle zircon megacrysts from the unusual diamond-bearing sedimentary rocks of the northeastern Siberian craton to constrain the age and geochemical affinity of their parental rocks as well as their possible relationship to diamond-rich placers of northern Yakutia. The textures, trace-element systematics, and 206Pb238U dates of zircon megacrysts are evidence of their formation from kimberlites or related alkaline mantle melts and are similar to those of zircon from most Siberian kimberlites. The exclusively Triassic ages of zircon (247–202 Ma) fit the Triassic kimberlite eruption ages and those of related ultramafic-mafic magmatism in northern Siberia, which has conventionally been considered to sample the diamond-barren Proterozoic subcontinental lithospheric mantle. The chemical compositions of zircon megacrysts largely overlap with those of mantle zircon from both diamondiferous Paleozoic and diamond-barren Late Triassic and Late Jurassic kimberlite fields and indicate kimberlitic parental melts of significantly variable composition. Zircon megacrysts reveal 176Hf/177Hf heterogeneities (εHf(t) of +1.7 to +10.5), which, combined with a range of parental melt compositions, favor a chemically-heterogeneous sublithospheric source, which comprises a mixture of moderately depleted peridotitic and enriched pyroxene-rich (pyroxenitic, lherzolitic, or eclogitic) domains. As favored mechanisms, local incipient melting of variable degrees and carbonate-dominated melt fractionation en route to the surface might produce alkaline melts of contrasting bulk compositions within a spatially-limited mantle section. The zircon observations in this study indicate that the Triassic Bulkur rocks as well as – potentially – all the diamond-rich placers in northeastern Siberian craton may be sourced from one or a few coeval occurrences of unexposed kimberlites, which should have sampled the yet diamondiferous Triassic mantle. Considering the existing data for placer diamonds and specific K-Fe-Ti enriched composition of kimberlites in northeastern Siberian craton, we propose that chemical and LuHf isotopic signatures of the Bulkur zircon, bulk kimberlite compositions and diamond growth and storage in northeastern Siberia might be governed by the same processes of Mesozoic interaction between the hot ascending asthenosphere, oceanic slabs and lower cratonic lithosphere.