PROVENANCE IMPLICATIONS OF HF ISOTOPIC COMPOSITIONS OF ZIRCONS FROM APPALACHIAN GRENVILLE-AGE PLUTONS AND ORTHOGNEISSES

Numerous workers (e.g., Rainbird, Stewart, Gehrels, Barth, etc.) have shown that Grenville-age (0.95-1.4 Ga) zircons are common constituents of Neoproterozoic and Cambrian psammitic rocks throughout North America and that these grains are persistent constituents in younger sediments (e.g., Navajo sandstone and other units in the Cordilleran miogecline). Although some locations are devoid of Grenville-age grains, they are strongly dominant in others. The cause and source of this “Grenville Flood” in western North America, in particular, has been commonly attributed to a combination of above average fertility of Laurentian Grenville magmas for production of zircon and a continent-wide drainage system extending west from the Appalachians. Because Grenville-age crystalline basement is also known from locales other than the Appalachians, several attempts have been made to use the Hf isotopic compositions of these detrital zircons to test the Appalachian source hypothesis. The critical missing link in this approach is a “type section” of Grenville-age crystalline sources in the Appalachians. Consequently, we have determined U-Pb age by SHRIMP and Lu-Hf isotopic systematics by laser ablation MC-ICP-MS of zircons from Grenville-age plutons and orthogneisses over a wide geographic distribution from Vermont to Alabama. These data show a clear pattern of progressively decreasing age (1370 Ma to 1030 Ma) and initial Hf isotopic compositions (eHf of +8 to 0) from north to south along the orogenic strike, with more dispersion of values at southern latitudes. Although the present level of erosion may not be representative of material eroded in the Neoproterozoic, strongly suggest that the most northerly and earliest stages of Grenvillian magmatism had a different petrogenesis than magmas produced further south and at a younger age. Data from potential non-Appalachian sources (e.g., Precordillera terrane, West Texas, Mexico, Baltica, etc.) are not extensive, but do overlap with values from some Appalachian regions. In addition to the provenance implications, these data are also important for understanding Grenville orogenesis in terms of times of magmatism, extent of recycling of older crust, and the likelihood of Amazonian terrane transfer during the formation of Rodinia.