ONE OF THESE THINGS IS NOT LIKE THE OTHERS

The terrane concept, originally defined in the N. American Cordillera, was quickly adopted to the Appalachian orogen. The concept is an extremely straight forward one – if a fault-bounded fragment of crust is not like its surroundings it may be ‘exotic’ to the area and was and thus tectonically transferred to its current location. However, despite its simplicity it is now apparent that distinguishing which terranes are far traveled, i.e. truly exotic, versus those that may have had minimal transport is extremely difficult. Even trying to determine which areas of crust are parts of a single geologically heterogeneous terrane or are themselves unique terranes is controversial. In some cases the isotopic composition of otherwise geologically and geochronologically similar terranes is an ideal forensic tool (e.g. comparing Avalonia from Cadomia), however in many cases the fragments in question are too similar to be distinguished by whole-rock isotope analysis. Isotopic analysis of minerals may prove to be a more robust strategy. Examples of how mineral isotope compositions might help in determining which terranes are indeed "not like the others" will be presented.