Trace element and age characteristics of zircons in granulite xenoliths from the Udachnaya kimberlite pipe, Siberia

Abstract

Trace element and SHRIMP U–Pb geochronological data are presented for zircons from granulite xenoliths from the Devonian Udachnaya kimberlite pipe, situated in the central part of the Siberian craton (66°05′N, 110°45′ E). The xenoliths include three two-pyroxene garnet granulites, one garnet granulite lacking orthopyroxene, three felsic garnet granulites, one kinzigite and one amphibolized garnet granulite. Most of them are clearly distinct from the local upper crustal lithologies and have pressure and temperature estimates indicative of derivation from the lower crust. Zircons from these granulites show a record of several magmatic and metamorphic events, with different domains revealed by CL imaging and variations in REE content and Th/U ratios. The first event is shown by magmatic zircon in the cores of some grains that yield discordant Archaean to Proterozoic ages. All other domains of the studied zircons have textures typical for metamorphic zircons. They have Proterozoic ages and were formed by subsolidus growth or as a result of solid-state recrystallization of pre-existing zircon. They give a range of concordant ages from 1.94 to 1.81 Ga, in which several distinct episodes can be distinguished that relate to the collision of terranes during the accretion of the Siberian craton, accompanied by formation of granite melts. During these events the xenolith protoliths were metamorphosed under granulite-facies conditions at pressures corresponding to lower crustal depths, except for the kinzigite that is probably derived from shallower depths.