STABILITY FIELD OF ALLANITE AND MONAZITE IN THE BARROVIAN-TYPE METAPELITES OF THE MIYAR VALLEY (SE ZANSKAR): IMPLICATIONS FOR PETROCHRONOLOGY

The textural and chemical evolution of allanite and monazite along a well constrained prograde metamorphic suite in the Greater Himalayan Sequence of Zanskar was investigated to determine the P-T conditions for the crystallization of these two REE accessory phases. The results of this study reveal that: (1) allanite is the stable REE accessory phase in the biotite and garnet zone, and (2) allanite disappears at the staurolite-in isograd, simultaneously with the occurrence of the first metamorphic monazite. Both monazite and allanite occur as inclusions in staurolite, indicating that the breakdown of allanite and the formation of monazite proceeded during staurolite crystallization. Staurolite growth modelling indicates that staurolite crystallized between 580 and 610 °C, thus setting the lower temperature limit for the monazite forming reaction at ~600 °C. Preservation of allanite and monazite inclusions in garnet constrains the garnet molar composition when the first monazite was overgrown and subsequently encompassed by the garnet crystallization front. Garnet growth modelling and the intersection of isopleths reveal that the monazite closest to the garnet core was overgrown by the garnet advancing crystallization front at 590 °C, which establishes an upper temperature limit for monazite crystallization. Significantly, the substitution of allanite by monazite occurs in close spatial proximity, i.e. at similar P-T conditions, in all rock types investigated, from Al-rich metapelites to more psammitic metasedimentary rocks. This indicates that major silicate phases, such as staurolite and garnet, do not play a significant role in the monazite forming reaction. Our data show that the occurrence of the first metamorphic monazite in these rocks was mainly determined by the P-T conditions, not by bulk chemical composition. In Barrovian terrains, dating prograde monazite in metapelites thus means constraining the time when these rocks reached the 600 °C isotherm.