MICROSCALE SECONDARY PHASES IN DETRITAL ZIRCON RECORD >1 GA OF METAMORPHISM IN THE YILGARN CRATON

Crystallization ages of detrital zircons are often used to constrain the provenance of clastic sediments, due to the retentive nature of the U-Pb isotopic system in zircon during later heating and sedimentary transport. Diagenetic and metamorphic xenotime and monazite are common in metasediments, and monazite and xenotime grown epitaxially on or filling void space within detrital zircons can constrain both their post-depositional metamorphic history and the potential for inherited pre-depositional phosphates acquired in previous sedimentary cycles. However, the spatial scale of these features necessitates a very small analytical footprint. In order to investigate how well the metamorphic history of these systems can be resolved using phases grown within detrital zircon, we investigated the ²⁰⁷Pb/²⁰⁶Pb ages (using the CAMECA ims1290 ion microprobe) and chemistry (using EPMA) of secondary phosphates within detrital zircon on the 3-5 μm scale in Archean quartzites of relatively well-known metamorphic history from the Jack Hills and Mt. Narryer belts in the Yilgarn Craton of Western Australia. Secondary phosphates at Mt. Narryer dominantly reflect a known ca. 2.6 Ga Yilgarn Craton-wide metamorphic event, but at least one earlier event is also preserved. The greenschist facies Jack Hills samples preserve a >1 Ga metamorphic history, with ages differing by up to 0.5 Ga within a single inclusion. Xenotime age and chemistry reflects the known metamorphic history from matrix phosphates. We then investigated metamorphic xenotime age and chemistry in the Maynard Hills supracrustal belt (Southern Cross terrane, Yilgarn Craton), which has a less well-constrained metamorphic history. Secondary xenotime in detrital zircons record the 2.6 Ga Yilgarn-wide event as well as a broader spectrum of ages ranging from syn-depositional (ca. 3.1 Ga) to 1 Ga, and their chemistry suggests growth at higher metamorphic grade than at Jack Hills. This previously undocumented Proterozoic metamorphism in the Maynard Hills supracrustal belt may have implications for the preservation of geochemical and paleomagnetic information in Hadean zircons from this belt.