8 Over fifty new Nd isotope analyses are presented for high-grade orthogneisses from 9 Algonquin Park and surrounding region in order to map major Grenvillian thrust boundaries. Nd 10 model ages display a consistent geographical pattern that allows detailed mapping of the 11 boundary between the Algonquin and Muskoka domains, here interpreted as the local trajectory 12 of the Ottawan-age Allochthon Boundary Thrust (ABT). The ABT is underlain by a domain with 13 Paleoproterozoic Nd model ages,

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... terpreted as a tectonic duplex entrained onto the base of the 14 main allochthon. The boundaries determined using Nd isotope mapping are consistent with field 15 mapping and with remotely sensed aeromagnetic and digital elevation data. The precise location 16 of the ABT can be observed in a road-cut on Highway 60, on the north shore of the Lake of Two 42 Based on its large size (7723 square km) and proximity to major population centres, 43 Algonquin Park could form a public showcase of the geology of the Precambrian Shield. 44 However, its geology has been difficult to unravel for a number of reasons. Some difficulties 45 arise from limited vehicle access to the park interior and extensive glacial cover. However, the 46 main difficulty is the high degree of crustal exhumation experienced by Grenvillian gneisses. As