U-PB DATING OF ZIRCON FROM THE LOWER CRUSTAL XENOLITHS, UDACHNAYA PIPE, YAKUTIA

Abstract

This paper presents results of zircon dating of xenoliths from the Udachnaya Pipe. Mafic two-pyroxene garnet granulites (samples uk1 and uk2), mafic amphibole-bearing garnet granulite (uk11), and intermediate garnet granulite (uk5) were taken for age determination. The mafic two-pyroxene garnet granulites correspond to tholeiitic basalts. Xenolith uk1 shows differentiated REE distribution (Lan/Ybn = 3.9); depletion in Ti, Nb, and other HFSE; and enrichment in LILE, including Pb, which is typical of island-arc tholeiites. Sample uk 21 has a flat PM-normalized REE pattern with an insignificant Nb maximum and Pb minimum. Its protoliths was presumably ocean floor basalt. Xenolith uk11 differs from other xenoliths in the high content of incompatible elements, which can be related to contamination by host kimberlites or metasomatism. Granulite uk5 is characterized by elevated contents of Al2O3 and alkalis, as well as a fractionated REE pattern with LREE enrichment, HREE depletion, and Eu maximum (Eu/Eu* = 1.32). This geochemistry suggests a monzonitic composition of the protolith.