Abstract
Metasedimentary rocks with ca. 3.0 Ga depositional ages exposed at Mt. Narryer, Western Australia contain Hadean (>4.0 Ga) detrital zircons. Previous studies have demonstrated that detrital zircons from the metasedimentary rocks define two distinctive age groups, an older group from 4.28-4.00 Ga and a younger group from 3.75-3.25 Ga. We have studied the age of monazites from the metasedimentary rocks relative to zircon U-Pb ages to constrain their provenance as well as post-depositional thermal history. About 450 monazite grains were separated from five rock samples and their U-(Th-)Pb isotopes were measured by LA-ICPMS. All monazite populations show a prominent peak at 2.70-2.65 Ga in the 207Pb/206Pb age histograms, indicating monazite recrystallization and/or growth during the metamorphism at that time. The older pre-depositional monazite population shows a small peak at 3.6 Ga and more dominant peaks at 3.30-3.25 Ga, approximately corresponding to the minimum age of the younger zircon group. Interestingly, no Hadean monazites have been found here: the oldest monazites identified have a maximum age of ca. 3.6 Ga. The lack of Hadean monazite can be interpreted as indicating that the source rocks experienced a sequence of metamorphic events from ca. 3.6, or alternatively, the source rocks might be mafic-intermediate in composition, rather than felsic, and hence did not contain igneous monazite.
