Chemical and textural record of REE accessory phases along a well constrained prograde pelitic suite: implications for allanite and monazite petrochronology

Because metamorphism comprises a complex sequence of structural changes and chemical reactions that can be extended over several millions of years, metamorphic rocks cannot in general be said to have "an age". As a consequence, interpreting radiometric age data from metamorphic rocks requires first to establish the behavior of the isotopic system used for dating relative to the P-T conditions that a metamorphic rock experienced. As the U-Th-Pb system in monazite and allanite is not easily reset during subsequent temperature increase, allanite and monazite ages are interpreted as reflecting crystallization ages. Consequently, to interpret allanite and monazite crystallization ages, it is crucial to determine the physical conditions of their crystallization.

A detailed account of the textural and chemical evolution of allanite and monazite along a well constrained prograde metamorphic suite of detrital sediments of the Greater Himalaya of Zanskar reveals that: (1) allanite is the stable REE accessory phase in the biotite and garnet zone and (2) at the staurolite-in isograd, allanite disappears abruptly from the metamorphic assemblage, simultaneously with the occurrence of the first metamorphic monazite. The finding of both monazite and allanite as inclusion in staurolite indicates that the breakdown of allanite and the formation of monazite occurred during staurolite crystallization. The preservation of allanite and monazite inclusions in garnet core and rim respectively allows to accurately identify the P-T conditions of the formation of the first metamorphic monazite. Thermobarometry indicates that, in both cases, monazite crystallization occurred at 600 °C. The fact that the substitution of allanite by monazite occurs spatially close, i.e. at similar P-T conditions, in all types of rocks whether they are Al-rich metapelites or more psammitic metasediments indicates that major silicate phases like staurolite or garnet do not play a significant role in the monazite forming reaction. Our data shows that the occurrence of the first metamorphic monazites in the rocks is mainly dependent on the pressure-temperature conditions and not on the bulk chemistry of the rocks. Dating monazite in metapelites means thus constraining the timing when these rocks reached the 600 °C isotherm.