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

Abstract

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.

Introduction

Of the U–Th-rich accessory phases commonly used in geochronology, monazite is perhaps the most useful for determining the timing of metamorphism in amphibolite and higher grade granitic and pelitic rocks (e.g. [1], [2]). This light rare earth elements (LREEs) phosphate is a relatively common accessory mineral in a variety of rock types [3], [4], usually has high concentrations of U and Th, and typically has minimal concentrations of common Pb [4]. These characteristics coupled with a minor risk of isotopic inheritance [4] and a high resistance to Pb loss [4], [5] have led to the growing use of monazite as a chronological tool in metamorphic rocks (e.g. [6]). However, unlike common rock-forming minerals such as garnet, the metamorphic reactions responsible for the formation and breakdown of such accessory phases are poorly understood, despite numerous studies over the last decade (e.g. [7] and references therein). It is, therefore, difficult to combine the precise chronological information provided by monazite with the pressure–temperature determinations of rock-forming minerals. Thus, despite its chronological utility, the application of metamorphic monazite U–Th–Pb dating to geological problems is severely limited, and will remain so until reactions responsible for the breakdown and formation of monazite are not only recognised but also calibrated in pressure–temperature space.

In this contribution, we apply secondary ion mass spectrometry techniques to provide in situ U–Th–Pb isotopic measurements of monazites, both included within garnet porphyroblasts and in the matrix of a rock. This approach has the enormous advantage over conventional U–Th–Pb investigations, or more traditional ion probe techniques, in that the textural relationships of monazite are retained. Although the technique has been used before [8], this contribution involves several novel and important advances. Firstly, we present a full U–Th–Pb dataset for the analysed monazites so that we are not reliant simply on ²⁰⁷Pb–²⁰⁶Pb [8], [9] or ²³²Th–²⁰⁸Pb ages [10]. This approach allows us to apply the critical test of concordancy to our data. Secondly, we also present estimates of the timing of local garnet growth, independently established using Sm–Nd chronometry (this study, [11]). We are thus able to demonstrate, for the first time, that the ages of monazite inclusions in garnet are similar to, or slightly older than, the occluding porphyroblast. As a result, we can combine precise U–Th–Pb ages on monazite inclusions with thermobarometric data from garnet. Finally, we show that matrix monazites commonly yield more variable and younger ages than the included grains. By integrating detailed REE and trace element data with textural observations, we are able to distinguish several growth phases of monazite during regional metamorphism and estimate the conditions under which this growth occurred.