Pb isotope mapping of differentially uplifted Archean basement: a case study from the Grenville Province, Ontario

Abstract

Uranium mobility during regional metamorphism typically causes the continental crust to become stratified in the U/Pb ratio, and hence, over time, Pb isotope ratio. This means that Pb isotope analysis of old gneiss terranes might be used as a means of estimating the original burial depth of the crust and its subsequent uplift history. The approach would be useful if other measures of burial depth, such as mineral chemistry, have been erased by younger metamorphic events, a situation found in the NW Grenville Province. To test the usefulness of this approach, whole-rock Pb isotope data were collected for Grenville gneisses in order to investigate their burial-uplift history. Results were compared with Levack gneiss data from Sudbury, immediately north of the study area. Several Grenville gneisses have Pb isotope ratios close to the least radiogenic Levack gneiss signature, believed to be exhumed from >20 km depth. Hence, it is concluded that gneisses immediately SE of the Grenville Front Tectonic Zone represent deeply eroded Archean basement, exhumed in the Grenville orogeny. On the other hand, high Pb isotope ratios near the Archean–Proterozoic suture suggest the preservation of Archean upper crustal rocks in this vicinity, consistent with the burial of the craton margin under an accreted Paleoproterozoic arc. It is concluded that Pb isotope mapping is a useful tool to study the exhumation of Archean crust in highly metamorphosed orogenic belts such as the Grenville Province.