LEAD-ISOTOPE ANALYSES OF LEACHED FELDSPARS - CONSTRAINTS ON THE EARLY CRUSTAL HISTORY OF THE GRENVILLE-OROGEN

Lead isotopic data on leached K-feldspars, plagioclases, and muscovite provide constraints on the premetamorphic evolution of the Grenville Orogen in Ontario and New York. A boundary between reworked Archean crust to the northwest and new mantle-derived crust generated during mid-Proterozoic times to the southeast is identified within the northern portion of the Gneiss Belt of Ontario. The identification of juvenile mid-Proterozoic crust at the southeastern margin of the Orogen is consistent with previous investigations based on neodymium model ages. However, near the Grenville Front lead isotopes indicate the presence of reworked Archean crust rather than primary early Proterozoic crust postulated on the basis of neodymium model ages. This discrepancy between the lead and neodymium ''isotopic terranes'' underscores the necessity of using a multidisciplinary approach to identify possible terrane boundaries. Only through a combination of the lead with neodymium isotopes is it possible to distinguish between crust that formed during a single major orogenic cycle and crust that represents a mixture of old with new mantle-derived material. The combined neodymium and lead data imply that the Grenville Orogen in Ontario consists essentially of reworked Archean crust and juvenile mantle derived material generated around 1.5 Ga. The Grenville Orogen involved Archean crust that was depleted in thorium and uranium several hundred Ma before high grade metamorphism at ca. 1.1 Ga. Such a depletion may have a significant impact on the geotherm achieved during orogeny and may explain the lower metamorphic thermal gradients observed for the Gneiss Belt.