DO SEDIMENTS LIE (OR ARE THEY JUST COY)? NEW APPROACHES IN ADVANCING PROVENANCE STUDIES

Clastic sedimentary rocks have the potential to hold vast amounts of information about the past tectonic history of orogenic regions, including the timing of major deformational/metamorphic events, relationships between distinct terranes, the paleogeographic origin of terranes, and the timing of accretion. Unlocking this vault of information requires deciphering the details of the provenance of clastic sedimentary rocks within orogenic belts. Early provenance studies involved identifying minerals and their modes and assigning proportions of generic tectonic environments to such minerals. Such petrographic-based studies are valuable, often identifying significant aspects of the sedimentary rock, but can't detect potential differences between lithologically similar rock types. Petrography thus become augmented by chemical characterization of clastic rocks. Such studies provide advanced information about rocks with similar mineral modes but unique chemical characteristics. Further advances involved determination of the age of mineral grains in a sedimentary rock. Early studies involved the dating of a bulk group of detrital minerals, but analytical advances now allow the routine dating of single crystals thus providing more discrete age information. Detrital zircon geochronology is an example, and it has now become almost common place in provenance studies. A disturbing aspect of such studies, however, is that Appalachian orogenic events have gone undetected in syn-tectonic sedimentary rocks. The most spectacular example is the lack of Alleghanian-age zircon in Carboniferous sedimentary rocks in the Appalachians. This lack of a tectonic ‘high fidelity' signal might, however, be a result of inappropriate analytical protocols. For example, the selection of large zircon crystals might preclude those derived from volcanic terranes and the analysis of only cores of grains would preclude information obtainable from analysis of metamorphic rims. In addition, other minerals might prove to be better suited for detrital chronological studies – e.g. detrital monazite might prove to better preserve the metamorphic/tectonic history of an area. New analytical protocols and analyses of different minerals are thus being investigated to try to achieve the next level in provenance research.