The inhibited response of accessory minerals during high-temperature reworking

Abstract

U–Pb zircon and monazite geochronology are considered to be among the most efficient and reliable methods for constraining the timing of high-temperature (HT) metamorphic events. However, the reliability of these chronometers is coupled to their ability to participate in reactions. A case study examining the responsiveness of zircon and monazite has been conducted using granulite facies metapelitic and metamafic lithologies in the Warumpi Province, central Australia. In some instances, metapelitic granulites from this locality are polymetamorphic, with an early M1 assemblage containing orthopyroxene, cordierite, biotite, quartz, ilmenite and magnetite, and an M2 assemblage represented by garnet, sillimanite, orthopyroxene, cordierite, biotite, sapphirine, ilmenite and magnetite. M2 metamorphism is linked to HT peak conditions of 8–10 kbar and 850–915°C. Detrital and metamorphic zircon and monazite from these rocks dominantly record U–Pb dates of 1670–1610 Ma and have trace element compositions suggesting they grew prior to peak M2 garnet in the rock. Lu–Hf geochronology from M2 garnet gives ages of c. 1150 Ma. Zircon and monazite are therefore suggested to have remained largely inert during HT metamorphism. We attribute the relatively minor response of zircon and monazite during high-temperature Mesoproterozoic metamorphism to the localized development of refractory bulk compositions at c. 1630 Ma during M1 metamorphism. This created refractory Mg–Al-rich bulk compositions that were unable to undergo significant partial melting, despite experiencing subsequent temperatures of ~900°C at c. 1150 Ma. In contrast, metapelitic and metamafic rocks in the area that did not develop refractory bulk compositions during M1 metamorphism were able to partially melt and record c. 1150 Ma accessory mineral U–Pb ages. These results contribute to a small, but growing number of case studies investigating the systematics of the U–Pb system in zircon and monazite in polymetamorphic HT terranes and their apparent resistance to isotopic resetting. Where disequilibrium is apparent, garnet Lu–Hf geochronology can form an important tool to interrogate the significance of accessory U–Pb ages. In the Warumpi Province in central Australia, c. 1640 Ma zircon U–Pb ages had previously been interpreted to reflect the formation of HT garnet-bearing granulites during a collisional event. Instead, the garnet-bearing assemblages formed at c. 1150 Ma during the Mesoproterozoic, calling into question the existence of a late Palaeoproterozoic collisional system in central Australia.