The significance of monazite U–Th–Pb age data in metamorphic assemblages; a combined study of monazite and garnet chronometry

In this study, a coupled U–Th–Pb isotopic and EMP chemical study was carried out in situ on monazite (micro-inclusions within garnet as well as matrix grains) from rocks recovered from two chronologically well-constrained areas of the Himalayan orogen. Monazite inclusions within garnets from three samples yield ages of ∼44–36 Ma, whereas matrix grains within one sample are typically younger (30–26 Ma). Y depletion of these younger matrix grains indicates that they grew after garnet had crystallised. The recognition of episodic monazite growth during regional metamorphism, first at greenschist facies (before garnet growth) and secondly at upper amphibolite facies (post-dating garnet growth), allows relative growth ages of the occluding garnet to be calculated. These are in excellent agreement with Sm–Nd garnet ages from surrounding units. This approach not only provides a ready means of obtaining porphyroblast growth ages but also allows the combination of precise U–Th–Pb data from metamorphic monazite with thermobarometric information obtained from rock-forming minerals.