Petrology and geochemistry of granulite xenoliths from Udachnaya and Komsomolskaya kimberlite pipes, Siberia

Lower crustal xenoliths from the Udachnaya and Komsomolskaya kimberlite pipes in Siberia are mainly meta-igneous mafic garnet granulites, with subordinate feldspar-rich garnet granulites. Pressure and temperature estimates are interpreted as the conditions in the lower crust at the time of the last granulite-facies metamorphic event (800–890°C) followed by cooling to 610–720°C with a pressure decrease from 1·2 to 0·8 GPa. Most of the xenoliths show minor alteration. Leaching experiments demonstrate that their isotopic, major and trace element compositions have been affected by interaction not only with the host kimberlite but also with a fluid mobilized from local sedimentary country rocks. To obtain unambiguous compositional data we have calculated the composition of selected samples using modal analyses, electron microprobe and laser ablation inductively coupled plasma mass spectrometry data for constituent minerals. The reconstructed protoliths of most of the xenoliths were Fe-tholeiites of intraplate affinity, similar to some Archean basalts, whereas the others show characteristics of subduction-related magmas. However, the mafic granulites are strongly depleted in Rb, Th and U, which were removed by a small-degree partial melt. A protolith age of c. 3 Ga is supported by a disturbed Sm–Nd isochron, Nd and Hf model ages, and published U–Pb ages of zircon cores