Little and Large: Implications for Preservation of Radiogenic-Pb in Titanite. An example from the Albany–Fraser Orogen, Australia

Abstract:

Titanite typically contains more non-radiogenic Pb than zircon, nonetheless it can preserve useful age information that complements geochronology from other datable phases. Titanite is more reactive than zircon and it interacts more readily with other major phases. As a result titanite dates frequently indicate the time of cooling below a blocking temperature (in reality an interval). The role of titanite grain size is important as it has a bearing on the extent to which titanite U-Pb ages reflect diffusive Pb loss or pristine formation ages. We demonstrate that titanite collected through the Albany-Fraser Orogen, across an uplifted refractory lower crustal block, can record thermal overprints apparently lacking in the zircon record. Two zones of the Albany-Fraser Orogen are the Biranup and Fraser Zones, each with a distinctive Proterozoic history but unequivocally part of the reworked margin of the Archean Yilgarn Craton. A dichotomy exists in the zircon geochronology record of this area, in that within the older Biranup Zone, Stage II overprinting (1225–1140 Ma) is widespread whereas in the younger Fraser Zone, Stage I (1345–1260 Ma) is dominant, with Stage II apparently absent. Although, most metamorphic titanite in the Fraser Zone records an age of 1307 ± 17 Ma, reflecting closure to radiogenic-Pb mobility after Stage I metamorphism, small titanite grains reveal Stage II overprinting with a mean reset age of 1205 ± 16 Ma. In contrast, titanite from metasediments within the Biranup Zone principally record ages of 1203 ± 6 Ma and 1153 ± 27 Ma reflecting cooling after prolonged Stage II metamorphism. Thermochronological modelling indicates that small titanite grains in the Fraser Zone would be reset during Stage II overprinting at temperatures of 695-725°C. Larger titanite crystals would not be reset by this thermal overprint. This result is similar to phase equilibrium modelling from the Biranup Zone that indicates temperatures of 670–680 °C during Stage II metamorphism. An in-situ U-Pb analysis strategy for titanite that targets a range of grain sizes has the potential to reveal differential resetting and thus place important controls on the thermal history.