Occurrence and Stability of Allanite and Monazite in the Greater Himalayan Sequence, Dhauliganga Valley, Garhwal Himalaya

The occurrence and relative stability of metamorphic allanite and monazite in metapelites of the Greater Himalayan Sequence (GHS) in the Dhauliganga valley, Garhwal Himalaya have been studied. The GHS is marked by kyanite grade metamorphism in the lower and middle structural levels and sillimanite grade metamorphism in the upper structural level. The occurrence of allanite is restricted to the lower structural level, while monazite occurs in the middle and upper structural levels. The allanite-out reaction occurred at similar to 660 degrees C and 9.5 kbar. Monazite shows widely varying ThO2 content which can be explained mostly by brabantite substitution. The rocks of the GHS of the Dhauliganga valley display inverted metamorphic sequence and the allanite-out reaction is the result of prograde metamorphism.