HF ISOTOPES AND TRACE ELEMENT CONTENTS OF DETRITAL ZIRCONS: KEYS TO IMPROVING PROVENANCE RESOLUTION

Detrital zircons preserve not only the U-Pb ages of the rocks from which they crystallized, but also petrogenetic information about their parent rocks based on their Lu and Hf isotopic systematics and trace element abundances (e.g., REE). Collectively, these data can be used to make more refined interpretations of the provenance of individual sedimentary rocks and rock associations. In order to make such interpretations, however, it is essential that all data are from the same, homogeneous domain of each zircon. This is particularly critical for zircons in metasedimentary rocks or for zircons that were formed or affected by metamorphism prior to inclusion within a given sedimentary rock. The most unambiguous information, therefore, derives from spatially resolved measurements of both Lu-Hf systematics by LA-ICP-MS and U-Pb ages by ion microprobe on the same domain in individual grains. This approach can yield robust initial Hf isotopic compositions of zircons and their parent rocks, and thereby provide significant constraints on patterns of crustal evolution within the source terrane. This record can, in fact, be more complete than records obtained from direct measurements of rocks in the source terrane because the zircons can accumulate and be retained in the sedimentary system, even though their parent lithologies are lost to erosion or inaccessible due to burial. Application of this approach is particularly critical for widely dispersed detrital populations such as the “Grenville” zircons of Rodinia. In the southern Appalachians, for example, initial Hf isotopic compositions of detrital Grenville-age zircons (~1.3-0.9 Ga) show relatively small variations in εHf (2.2 +/-1.7), yet define an evolutionary path that lies well below depleted mantle values (ε~14@1.1 Ga). These characteristics indicate continuous mixing of asthenospheric material with older lithosphere and provide a key for identifying detrital zircons from this region.