Dating of Granulite Facies Metamorphism and Depletion: SIMS Chronometry of Micron-Scale Monazite Inclusion in Garnet

Abstract

It is difficult to obtain precise ages that give reliable information on the timing of the highest temperature history in granulite terrains. This is because the isotopic closure temperatures of most minerals are significantly lower than those achieved during granulite grade metamorphism. Monazite is a common accessory mineral in highgrade metamorphic rocks and, unlike zircon, is rarely inherited. It appears to retain Pb even at high temperature (725__+25~ Copeland et al., 1988) and offers potential to provide reliable chronological information of the high-grade parts of metamorphic thermal history as already demonstrated (DeWolf et al., 1993; Zhu et al., 1993). Of particular importance is the relatively common occurrence of monazite as inclusions in garnet. Because garnet has a very high closure temperature for Pb-isotopes (> 800~ Mezger et al., 1989), it may serve to armour monazite inclusion and provide age information on the higher temperature part of the record. It may even be possible to obtain the age information on the prograde part of granulite metamorphism from the monazite inclusion in garnet. Here we report the results of in situ ion probe dating of monazite occurring as micron-scale inclusions in garnet as well as discrete grains of monazite in matrix from the samples of the Lewisian complex.