Copeland, and 2 co-authors reported in 1988 that the simplest interpretation of uranium-lead mineral ages in igneous rocks embodies the assumption that melting of pre-existing rocks leads to homogenization of the lead isotopes. thus resetting the radiometric 'clock'. Zircons often violate this assumption by only partially dissolving and preserving a component of lead 'inherited' from before the melting episode, but until now such behavior has not been observed in monazite. Here the authors report that U-Pb data for zircons and monazites from a Tertiary granite from the Himalayas, in which zircons and monazites contain inherited radiogenie lead. An analysis of a single entirely igneous zircon crystal plots on concordia at 19.5± 0.4 Myr. Eight monazite fractions lie on a line from an upper intercept of 471 ± 10 Myr to the youngest fraction, which plots slightly above concordia at 21 Myr. The presence of excess 206Pb in the youngest monazite fractions indicates that the monazite data represent a mixing line between young magmatic monazite and an older inherited component, rather than a resetting of 471 Myr monazites at ~20 Myr. These data suggest a closure temperature for Pb in monazite significantly higher than previously published estimates.
The authors concluded that since Sawka and Harrison (1986) observed that U-Pb monazite results from S-type granites are often older than independently established crystallization ages, and speculated that this might reflect Pb* accumulation in monazite after crystallization by dehydration of LREE-rich hydrous phosphates (such as rhabdophane) during prograde metamorphism of a metasedimentary lower crust. Although this mechanism may ultimately account for the origin of the restitic monazite, the authors' data clearly suggest that this monazite growth did not occur during the Tertiary orogenic cycle, but during a Palaeozoic event.
The identification of inherited Pb* in the monazites in this sample suggests caution should be used in interpreting U -Pb monazite ages from S-type granites where only one or two 'concordant' fractions were analysed, especially in magmas with high U/Th. Proper interpretation of monazite U-Pb systematics requires full understanding of its pattern of discordance based on analysis of multiple fractions of monazite.
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