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 carrie d out in situ on monazite (micro-inclusions within garnet as well as matrix grains) from rocks recovered from two chronologically well-constrained are as of the Himalayan orogen. Monazite inclusions within garnets from three s amples yield ages of similar to 44-36 Ma, whereas matrix grains within one sample are typically younger (30-26 Ma). Y depletion of these younger matri x grains indicates that they grew after garnet had crystallised. The recogn ition of episodic monazite growth during regional metamorphism, first at gr eenschist facies (before garnet growth) and secondly at upper amphibolite f acies (postdating garnet growth), allows relative growth ages of the occlud ing garnet to be calculated. These are in excellent agreement with Sm-Nd ga rnet ages from surrounding units. This approach not only provides a ready m eans of obtaining porphyroblast growth ages but also allows the combination of precise U-Th-Pb data from metamorphic monazite with thermobarometric in formation obtained from rock-forming minerals. (C) 2000 Elsevier Science B. V. All rights reserved.