The significance of Paleoproterozoic zircon in carbonatite dikes associated with the Bayan Obo REE-Nb-Fe deposit

Sensitive high resolution secondary ion mass spectrometry (SHRIMP II), laser ablation inductively coupled plasma mass spectrometry (ICP-MS) and laser Raman spectroscopy have been used to determine the ages in zircons from carbonatite dikes associated with the Bayan Obo giant REE-Nb-Fe deposit, Inner Mongolia, China. Analyses of small amounts of zircon extracted from large samples of three carbonatite dikes yield late Palaeoproterozoic ²⁰⁷Pb/²⁰⁶Pb ages of 1894 ± 27, 1944 ± 20 and 1956 ± 9 Ma. One sample also contained inherited grains with ages up to ∼2.55 Ga. Zircon grains, even from a single rock, have a wide range of REE patterns, suggesting that they are probably inherited zircons trapped during magma genesis, not zircons crystallized from a single magma. None of the zircon grains has the extremely high Th/U characteristic of the Bayan Obo ore deposit or of the associated carbonatites. Further, mineral inclusions in the zircon identified by Raman spectroscopy are all silicate minerals (quartz and feldspars), consistent with crystallization of the zircon from silicate, not from carbonate, magmas. The Palaeoproterozoic zircons in the Bayan Obo carbonatite dikes did not crystallize from carbonatite magma at the time of dike emplacement, but were derived from wall rock contamination. The ages obtained from the zircons are consistent with ages measured on basement igneous rocks elsewhere in the Bayan Obo region.