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Zirconolite: A new U-Pb chronometer for mafic igneous rocks

¹ School of Earth and Geographical Sciences, University of Western Australia, Crawley, WA 6009, Australia

Precise dates for mafic igneous events are essential for tectonic reconstructions and understanding mantle dynamics, mass extinctions, and paleoclimate. Zirconolite (CaZrTi₂O₇) is a uranium-bearing accessory mineral, found in a wide range of terrestrial and lunar rocks, that has been largely overlooked as a chronometer. In situ U-Pb geochronology of zirconolite from three dolerite intrusions in Western Australia demonstrates that it yields emplacement ages that are more precise than those obtained from coexisting zircon and baddeleyite. Dikes in the Stirling Range Formation give a zirconolite ²⁰⁷Pb/ ²⁰⁶Pb age of 1218 ± 3 Ma, indistinguishable from the less precise dates obtained from zircon (1215 ± 10 Ma) and baddeleyite (1217 ± 39 Ma) and coincident with dike emplacement in the adjacent craton margin and peak metamorphism in the Albany-Fraser orogen. Zirconolite from the 755 Ma Mundine Well dike swarm yields a ²⁰⁷Pb/²⁰⁶Pb age of 754 ± 5 Ma. Sills intruding the Proterozoic Manganese Group contain zirconolite crystals that give a ²⁰⁷Pb/²⁰⁶Pb age of 523 ± 14 Ma. Despite high U contents (550 ppm to 14,000 ppm) and greenschist facies metamorphism, zirconolite in these samples is apparently unaffected by loss of radiogenic Pb. Because of its remarkable properties for U-Pb geochronology, it may soon become the dominant tool for dating mafic igneous rocks as young as 500 Ma by ion microprobe, and thus will prove especially valuable in reconstructing Precambrian geologic history.

Key Words: zirconolite • dolerite • SHRIMP • geochronology • mafic rocks • zircon • baddeleyite

This article has been cited by other articles:

				S. Bengtson, B. Rasmussen, and B. Krapez The Paleoproterozoic megascopic Stirling biota Paleobiology, June 1, 2007; 33(3): 351 - 381. [Abstract] [Full Text] [PDF]