COOLING AND INFERRED UPLIFT EROSION HISTORY OF THE GRENVILLE OROGEN, ONTARIO - CONSTRAINTS FROM AR-40/AR-39 THERMOCHRONOLOGY

Stepwise Ar-40/Ar-39 degassing experiments of 57 mineral separates of hornblende, muscovite, biotite, and perthitic microcline have been used in conjunction with petrologic observations to place regional constraints on the postmetamorphic cooling and the inferred uplift and erosion history of the Grenville Orogen in Ontario. The Ar-40/Ar-39 data support an interpretation of slow, nearly uniform cooling (1-degrees-4-degrees-C/m.y.) from temperatures of approximately 500-degrees-C to below approximately 150-degrees-C. In the Central Gneiss Belt (CGB) hornblendes cooled through Ar closure between 930 and 1025 Ma, whereas in the Central Metasedimentary Belt (CMB) hornblendes record the following range in Ar-40/Ar-39 cooling ages: 1104 Ma in the Frontenac terrance, 1007-1067 Ma in the Sharbot Lake terrane, 919-1026 Ma in the Elzevir terrane, and 972 Ma in the Central Metasedimentary Belt Boundary Zone. Regional uplift/erosion rates of 0.03-0.14 km/m.y. have been estimated for the Grenville Orogen in Ontario based on the Ar-40/Ar-39 data, a model retrograde P-T path for rocks of the CGB, and an upper time constraint provided by flat, overlying Cambrian and Ordovician sediments. These erosion rates are consistent with rates estimated for other Proterozoic or Archean granulite terranes but are an order of magnitude slower than active orogens such as the Alps and Himalayas. A regular variation in hornblende Ar-40/Ar-39 cooling ages is observed in rocks that traverse highly strained often mylonitic shear zones that separate the four major terranes of the CMB. The pattern of Ar-40/Ar-39 ages is interpreted to reflect late-tectonic extension, consistent with field observations in the Central Metasedimentary Belt Boundary Zone and elsewhere in the CMB. Up to 13 km of vertical displacement is inferred for some rocks in the CMB between the time they cooled below closure to argon diffusion in hornblende (approximately 500-degrees-C) and their exposure at the surface (approximately 25-degrees-C).