Inherited and Disturbed U-Pb Ages in Zircon and Titanite from an Archaean Syenite: Implications for the Stability of U-Pb Systems in These Minerals

R. T. Pidgeon School of Applied Geology, Curtin University of Technology, Bentley, Western Australia, 6102, Australia

D. Bosch Laboratoiré de Géochronologie-Géochemie-Petrologie, U.R.A. 1763, Case courrier 066,

U.M.II., place E.Bataillon, 34095 Montpellier Cedex 5, France

O. Bruguier Laboratoiré de Géochronologie-Géochemie-Petrologie, U.R.A. 1763, Case courrier 066,

U.M.II., place E.Bataillon, 34095 Montpellier Cedex 5, France

Zircon and titanite from the Archaean Katrine syenite show inherited U-Pb systems. Conventional U-Pb analyses on abraded, transparent, single zircons from two samples from the Katrine syenite from southwestern Australia, provide a concordant to slightly discordant data-set with a mean age of 2654±5Ma, together with a number of more discordant data points. SHRIMP analyses on euhedrally zoned zircon and zoned rims of zircons with cores from the same zircon population give the same age. The euhedral zircon zoning pattern is interpreted as having formed during magmatic crystallisation and this age of 2654±5Ma is interpreted as dating crystallisation of the syenite magma. However, SHRIMP analyses on zircon cores and unzoned subhedral zircons give a complex set of inherited ages which we interpret as a ca 3250 Ma inherited component which has lost Pb during a strong disturbance at 2654Ma, probably during anatexis of the source rocks which must have occurred only shortly before crystallisation of the syenite magma, and also by long term diffusion or episodically during a relatively recent event. There is no obvious geological event in the Phanerozoic that can be assigned to this disturbance, unless Pb was lost during uplift and weathering. Titanite is also present in the syenite and data-points from conventional U-Pb analyses of titanite also fall on a reverse discordia between ca 3250 Ma and ca 2650 Ma. These data-points also show some indication of a recent isotopic disturbance but not to the extent of the SHRIMP analytical points on the zircon cores. It follows from the interpretation of the zircon data that the titanite retains a memory of the age of
the source rocks and has survived anatexis followed by
crystallisation of the syenite. Titanite occurs in the syenite as euhedral crystals, which are attributed to the 2654Ma
crystallisation event, and as irregular shaped grains which might be older. Analyses were made of a range of grain types and no correlation between inheritance and morphology or colour was found. The titanites also have a variety of inclusions. The largest of these are small rounded to subhedral apatite grains which may well have been incorporated at the time of crystallisation of the titanite. Other bodies consist
of irregularly shaped masses of two or more interlocking
crystals, generally quartz and a calcium silicate phase dominated by rare earth elements, which may be a grothite variety of sphene. These bodies are interpreted as secondary products from the breakdown of previous inclusions or titanite itself. Monazite is known to be more robust than titanite and the presence of monazite inclusions could explain the titanite U-Pb behaviour. Some of the analyses with the greatest amount of inherited 3250Ma Pb have the highest 208Pb/206Pb ratios which would support the presence of a phase like monazite. However, no inclusions of monazite were found. Nevertheless, whereas care was taken to avoid inclusions, we cannot be absolutely sure that some micron sized, ca 3250Ma monazite grains are not present in the titanites. Our present interpretation is that the inherited U-Pb system in the titanite is due to the presence of extremely small inclusions of monazite rather than the alternative explanation that 3250Ma titanite grains have survived magmatism
and crystallisation at 2654Ma without complete loss of
radiogenic Pb.