IDENTIFYING SUSPECT TERRANES, DETERMINING THEIR TIMING OF ACCRETION, IDENTIFYING THEIR 'BIRTHPLACE', AND ASSESSING THEIR ROLE IN THE GROWTH OF CONTINENTAL CRUST: A GEOCHEMICAL PERSPECTIVE

It has been close to thirty years since the concept of ‘suspect terranes' was introduced and broadly accepted by the geological community. The concept was initially applied to the North American Cordillera, but was quickly adapted to other orogens, including the Appalachians. Considerable effort has been expended in identifying discrete Appalachian terranes, attempting to determine possible relationships between terranes, documenting their paleogeographic ‘birthplaces', and determining their timing of accretion to North America.

Although all of the above tasks are fraught with various difficulties, from resetting of paleomagnetic data, overlap in chemical compositions, lack of critical units such as stitching plutons, and non-unique faunas to name just a few, there has been progress at least for some of the more major and most thoroughly studied terranes. Some of that progress is attributable to the collection of chemical and isotopic data sets for whole-rock samples, frequently from igneous rocks within the terrane. Some successful examples of that geochemical approach will be presented. However, continuing to collect geochemical data from what now are considered standard techniques is likely to have diminishing returns. For more substantial progress to be made new approaches will need to be employed, sedimentary and metamorphic units will need to be examined in much greater detail, and individual mineral phases will need to be interrogated for chemical, isotopic and chronological information. Examples of the latter approach include Nd and Sr isotopes in apatite, the analysis of Hf and O isotopic composition of zircon prior to U-Pb geochronology of the same crystal, U-Th-Pb geochronology of monazite in conjunction with rare earth element composition and Nd isotopes. These data may provide entirely new geochemical avenues for terrane analysis, as well as provide the data needed for a critical evaluation of the importance of terranes to net growth of continental crust.