Zonation of monazite in metamorphic rocks and its implications for high temperature thermochronology: a case study from the Lewisian terrain

Abstract

Monazite is a valuable chronometer for many geological processes, and a proper interpretation for the age derived from monazite requires an adequate understanding of the behaviour of this phase and its chemical and isotopic systems during geological processes. This contribution reports the results of a detailed study on chemical and isotopic zonation of monazite in Lewisian granulites, which is of general significance for monazite geochronology in high-grade metamorphic rocks. Detailed investigations on two Lewisian granulite samples using SEM, EPMA and SIMS have revealed that Lewisian monazites frequently show petrographic, chemical and age zonation. The petrographical zonation, which reflects the chemical compositional variation, can be characterised as follows on the basis of the BSE images: (1) concentric zoning, (2) patchy zoning, and (3) `intergrowth-like' zoning. The chondrite-normalised REE distribution patterns are remarkably similar between different zones of single monazite grains. This rules out the possibility that any portion of a zoned monazite in these two samples is detrital in origin. Some possible mechanisms are proposed for the formation of monazite zonation. These include: (1) intergrowth of monazite crystals with different composition; (2) episodic growth or regrowth of monazite in response to the change of environment conditions during thermal event(s), which may involve recrystallisation or replacement of the original crystals. In short, the diversity of zoning patterns in monazites observed in this study cannot be explained assuming a single mechanism. Accordingly, interpretation of the chemical and age information from any zoned monazite requires an adequate understanding of its formation mechanism. Line scans of Th, U, Pb, La, Ce, Nd, Sm, Ca and Si across monazite zones have been performed on two monazite grains which experienced granulite facies metamorphism. The very sharp boundaries in these line scans indicate that the diffusive transport of these elements in monazite is low even at the temperature of granulite facies metamorphism. The effective closure of monazite grains may also depend upon other factors such as the armouring effects of the host phase, and recrystallisation of the mineral itself. Thus great care needs to be taken for geological application of the closure temperature concept.