U-PB DATING OF MONAZITE AND ITS APPLICATION TO GEOLOGICAL PROBLEMS

Monazite is an underutilized mineral in U-Pb geochronological studies of crustal rocks. It occurs as an acessory mineral in a wide variety of rocks, including granite, pegmatite, felsic volcanic ash, felsic gneiss, pelitic schist and gneiss of medium to high metamorphic grade, and low-grade metasedimentary rocks, and as a detrital mineral in clastic and metaclastic sediments. In geochronological applications, it can be used to date the crystallization of igneous rocks, determine the age of metamorphism in metamorphic rocks of variable metamorphic grade, and determine the age and neodymium isotopic characteristics of source materials of both igneous and sedimentary rocks. It is particularly useful in the dating of peraluminous granitic rocks where zircon inheritance often precludes a precise U-Pb age for magmatic zircon. The U-Pb systematics of the mineral are not without complexity, however. Being a mineral that favors incorporation of Th relative to U, it can contain considerable amounts of excess Pb-206 derived from initially incorporated Th-230, an intermediate decay product of U-238. Corrections for this effect can be made using the Th/U ratio of the host rock, but these corrections may not always be valid. Monazite is known to be capable of preserving inheritance in a manner similar to that zircon, and it can lose Pb during episodic or prolonged heating events of uppermost amphibolite and granulite facies metamorphic grades. Monazite is less retentive of Pb than zircon during high-temperature igneous and metamorphic processes, and a few studies of its behavior suggest that its closure temperature is approximately 725 +/- 25-degrees-C. Examples of U-Pb systematics from most of the above situations are presented in this paper to illustrate both the utility and complexity of monazite in geochronological studies in an attempt to encourage more widespread application of this dating method.