The Lu-Hf system in apatite represents a robust dating tool for assessing the extent, timing and duration of high-grade metamorphic processes. Until recently, garnet has been the only common phase in high-grade terranes suitable for Lu-Hf geochronology. However, apatite has elevated $^{176}$Lu/$^{177}$Hf ratios compared to garnet (up to 90 versus less than 30 [e.g., 1]). The $^{176}$Lu/$^{177}$Hf ratios are heterogeneous on a mm-scale allowing, in some cases, for dating of single crystals [2]. Moreover, mineral inclusions in apatite can be largely eliminated by dissolution of the host apatite in highly dilute acid [2]; whereas one of the major challenges in garnet geochronology is assessing the influence from possible inherited inclusions [e.g., 3].

Apatite from three Proterozoic terranes yields Lu-Hf ages that are consistently older than their respective Pb step leaching (PbSL) ages. Isotopic closure for the Lu-Hf system, therefore, occurs prior to U-Pb closure in this mineral. In the Adirondack Lowlands, where H$_{2}$O activity was low, Lu-Hf ages of 1270 Ma and 1230 Ma are obtained for 3 cm and 0.3 cm diameter apatite crystals, respectively. These ages significantly predate the peak metamorphism conditions during the Elzevirian orogeny in the Adirondack Lowlands that reached upper amphibolite facies at 1170-1130 Ma. In contrast, apatite from the fluid-rich Otter Lake area and Black Hills record surprisingly young Lu-Hf and PbSL ages, presumably reflecting late exchange facilitated by fluids. The Lu-Hf ages for metamorphic apatites are shown to be controlled by temperature and grain size, and, in areas where fluid activity was high during peak metamorphism and exhumation, by fluid composition.

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