Abstract A key aim of modern metamorphic geochronology is to constrain precise and accurate rates and timescales of tectonic processes. One promising approach in amphibolite and granulite-facies rocks links the geochronological information recorded in zoned accessory phases such as monazite to the pressure–temperature information recorded in zoned major rock-forming minerals such as garnet. Both phases incorporate rare earth elements (REE) as they crystallize and their equilibrium partitioning behaviour potentially provides a useful way of linking time to temperature. We report REE data from sub-solidus amphibolite-facies metapelites from Bhutan, where overlapping ages, inclusion relationships and Gd/Lu ratios suggest that garnet and monazite co-crystallized. The garnet–monazite REE relationships in these samples show a steeper pattern across the heavy (H)REE than previously reported. The difference between our dataset and the previously reported data may be due to a temperature-dependence on the partition coefficients, disequilibrium in either dataset, differences in monazite chemistry or the presence or absence of a third phase that competed for the available REE during growth. We urge caution against using empirically-derived partition coefficients from natural samples as evidence for, or against, equilibrium of REE-bearing phases until monazite–garnet partitioning behaviour is better constrained.

Abstract Monazite is the mineral of choice in pelitic rocks for providing time constraints on metamorphic rocks and metamorphic processes. However, unlike rock-forming mineral chronometers such as garnet, the petrogenesis of monazite is relatively poorly understood. Consequently, although it is possible to generate precise monazite ages, the significance of the age in metamorphic rocks is often uncertain. In this contribution, we show how the petrogenesis of monazite can be linked to pressure and temperature information. Four complementary approaches, each illustrated by examples, are discussed: (i) the textural relationships of accessory minerals are used to relate the petrogenesis of monazite to that of the rock-forming mineral assemblage, and through this to P-T; (ii) monazite composition, in particular Y content, is used to relate monazite to the rock-forming mineral assemblage, and thus, to P-T; (iii) the bulk compositional control on monazite stability has been empirically determined and this relationship allows the temperature of initial monazite growth to be estimated in a given bulk composition; (iv) monazite-xenotime thermometry is utilized to provide estimates of the temperature of monazite growth. Either individually or combined, these approaches successfully enable monazite age data to be placed in a P-T framework.

Abstract Mineral chronometers, especially accessory minerals using the U–Pb decay system, can reveal important information regarding the environmental conditions and duration of metamorphic–deformation events during the re-working of older rocks. Minerals such as zircon can newly grow during amphibolite facies or granulite facies events, providing direct ages of metamorphism. Pre-existing minerals like monazite, allanite, and titanite can preserve a component of their original age in spite of upper amphibolite facies re-working and very thorough recrystallization of the rock fabric during mylonite development. The degree of Pb loss can be used to deduce, at least semi-quantitatively, the temperature and duration of the subsequent event. In well-studied examples, the relative retentivity of Pb is highly predictable, and this helps place strong constraints on relative closure temperatures, even when laboratory experimental data are lacking or inconclusive. A number of examples are presented from a wide variety of geological environments to illustrate the response of U–Pb isotope systematics within accessory minerals to superimposed deformation, metamorphism and/or mineral growth.

Abstract Results from a comprehensive U-Pb geochronology study of the Kidd Creek deposit and the surrounding Kidd Volcanic Complex are presented. Eleven new zircon and two titanite ages are reported and integrated with U-Pb age results on five related samples, which were published in a previous study. Zircon ages for rhyolite volcanism of the Kidd Volcanic Complex range from 2717.0 + –2 2 . . 5 6 to 2711.5 ± 1.2 Ma. This age range is established on immediate footwall and hanging-wall rhyolites of the Kidd Creek orebody. Since both footwall and hanging-wall rhyolites can be linked to ore formation, the conclusion must be that the giant Kidd Creek deposit is the product of unusually long-lived, albeit episodic, sea-floor hydrothermal activity. Our most precise estimate for the age of footwall rhyolites at Kidd Creek is provided by results on two nearby rhyolites that have been dated at 2716.1 ± 0.6 and 2716.0 ± 0.5 Ma, respectively. The age of at least one large massive sulfide lens at Kidd Creek, the North orebody, has been tightly bracketed between the age of footwall rhyolites and the age of an overlying rhyolite lapillistone horizon dated at 2715.8 ± 1.2 Ma. Hence, ore formation of individual massive sulfide lenses appears to have been rapid and well within the resolution limits of current U-Pb dating techniques. Based on the large number of ages, it appears that volcanic activity of the Kidd Volcanic Complex can be divided into four general phases, each of which is supported by at least one or more high-precision ages: phase I, onset of bimodal komatiite and rhyolite volcanism, probably as early as 2717.7 ± 1.1 Ma, and extrusion of the footwall assemblage at Kidd Creek at ca. 2716 Ma; phase II extrusion of ca. 2714 Ma rhyolites; phase III, extrusion of ca. 2711 Ma rhyolite, including the quartz porphyritic hanging-wall rhyolite at Kidd Creek; and phase IV, extrusion of the hanging-wall basalt sequence and intrusion of subvolcanic gabbro sills sometime after 2711 Ma. The Kidd Creek deposit probably formed along the axial zone of a slow-spreading rift basin that developed during extension of an older volcanic-arc assemblage. This older arc assemblage, which included ca. 2723 and 2735 Ma components, is probably represented by the Deloro Group and correlative assemblages exposed to the south of Timmins. Rifting of the older volcanic substrate, and partial melting to produce the rhyolites, was induced by the arrival of a hot mantle plume that gave rise to the komatiites of the Kidd-Munro assemblage. Graywacke turbidites in the Kidd Creek area are all younger than ca. 2699 Ma and do not form the deeper stratigraphic footwall to the deposit. Instead, the graywackes probably overlie, unconformably to disconformably, all the volcanic assemblages in the region. From the sub -sequent protracted structural-metamorphic evolution in the area, two discrete events have been dated: the 2663.3 ± 3.3 Ma intrusion of the Prosser porphyry granitoid stock and a discrete 2639.1 ± 7.2 Ma metamorphic-hydrothermal event. The timing of both events corresponds closely to ages for granulite facies metamorphic events in lower crustal rocks of the nearby Kapuskasing structural zone.