AGE AND HF ISOTOPIC PROVINCIALITY OF GRENVILLE CRUST IN LAURENTIA: IMPLICATIONS FOR ZIRCON-BASED PROVENANCE STUDIES

From the earliest studies of detrital zircons in the Neoproterozoic and Phanerozoic clastic sedimentary rocks of Laurentia, it has been evident that Mesoproterozoic zircons had been widely distributed in space and time. Occurrences range from the Appalachians (e.g., Chilhowee) to the western Cordillera (e.g., Wood Canyon) and from Sonora to the Canadian arctic. This “Grenville Flood” of detritus has often been attributed to westward transport from the Appalachian highlands by a continent-spanning fluvial system(s). A fluvial system of this magnitude would be comparable to the modern Amazonian system, which transports Andean zircons over 3000 km to the Amazon delta. To confirm the existence of such a trans-continental system requires that the Mesoproterozoic zircons in clastic sedimentary rocks of the western cordillera can be traced to the Appalachians, as opposed to other Laurentian Mesoproterozoic crust (e.g., West Texas, Sonora, Colorado, Mid-continent Rift, etc.). In an effort to resolve this issue we have analyzed the U-Pb (SHRIMP) and Lu-Hf (LA-ICP-MS) systematics of individual zircons from Grenville (s.l.) gneisses and granites in the Appalachian orogen from Maine to Alabama in order to develop a reference section for Appalachian Grenville zircons. Hf isotopic compositions were measured in the same domains analyzed by the ion probe for U-Pb ages to maximize coherence of the age-Hf data. These data (>1000 analyses) define a continuum of εHf values that show both spatial and temporal provinciality. The lack of a comparable database for non-Appalachian Mesoproterozoic crust in North America limits the uniqueness of the Appalachian array for provenance determinations, but our data strongly suggest that Appalachian Grenville-age zircons were transported across Laurentia beginning in the Neoproterozoic. The relatively low abundance of older Mesoproterozoic zircons (e.g., comparable to the intervening 1.3-1.5 Ga Granite-Rhyolite Province of the mid-continent) in these same sedimentary rocks suggests that Appalachian Grenville crust was being extensively eroded shortly after its formation and well before the beginning of the Appalachian orogenies. It is likely this flood of Grenville detritus quickly blanketed much of the Precambrian crust of the mid-continent as it moved westward.