CLOSURE TEMPERATURES OF THE SM-ND SYSTEM IN METAMORPHIC GARNETS

Garnet-whole rock and garnet-mineral isochrons were determined on granulite facies gneisses and amphibolites from the Archean Pikwitonei Granulite Domain of the Superior Province, and the Proterozoic Central Gneiss Belt and Adirondack Highlands of the Grenville orogen. The Sm-Nd ages obtained from Archean garnets 0.1-0.5 cm in length are 30-110 Ma younger than the U-Pb ages obtained on the same garnets and also younger than the time of the last regional metamorphism, as determined by the growth ages of the youngest metamorphic garnets and zircons, Similarly, the Sm-Nd ages obtained from Proterozoic garnets with a diameter of 0.1-5 cm are younger than the time of the last regional metamorphism and similar or younger than cooling ages obtained on sphenes from the same sample or from the same geologic setting. Only the core of a garnet with a diameter of ca. 30 cm and without abundant inclusions may record the time of garnet growth. Comparison of the Sm-Nd ages with other geochronologic data and temperature estimates leads to the conclusion that the closure temperature for the Sm-Nd system in garnets analyzed in this study is ca. 600 +/- 30-degrees-C. Only garnets with radii much larger than 5 cm may record Sm-Nd growth ages in upper amphibolite facies rocks from slowly cooled terranes. Garnets from higher grade terranes yield cooling ages that define the retrograde history of metamorphic terranes.