IMPLICATIONS OF A CAMBRIAN (530 MA) TECTONOTHERMAL EVENT IN THE SMITH RIVER ALLOCHTHON, SOUTHERN VIRGINIA

The Smith River Allochthon (SRA) is a prominent tectonic element in the southern Appalachian Piedmont Zone; the zone is composed mainly of gneisses, schists, and granitoids of unknown crustal affinity that lie between native Laurentian rocks to the west and documented exotic Neoproterozoic magmatic arc rocks of the Carolina Zone to the east. Many investigators consider a large portion of the Piedmont Zone to be peri-Laurentian in origin. The SRA is a regional scale fault-bound block of multiply deformed, high grade metaclastic rocks of previously unknown age that structurally overlie native Laurentian rocks. New Th-U-Pb monazite geochronological data (both TIMS and EPMA) from metaclastic rocks in the SRA indicate that they were subjected to an Early Cambrian middle-amhibolite facies tectonothermal event at about 530 Ma. Specifically, preliminary microprobe dating of monazites in pelitic schist from the western edge and central part of the SRA yields ages of 528 and 530 Ma (+/-10), respectively. A staurolite U-Pb age of 534 Ma (+/-12) (Lanzirotti and Hanson, 1997) was previously reported from the central part of the SRA. These significant new data are startling in that an event of this age has never been fully documented in Appalachian rocks immediately adjacent to the Laurentian margin. In addition, preliminary microprobe data also strongly suggest the presence of very low-Th and Y detrital cores with an age of 685 Ma in some monazites from both western and central SRA samples.

The eastern Laurentian margin was undergoing the rift-to-drift transition responsible for the formation of the Iapetus ocean in the Early Cambrian. Therefore, the new data indicate that the SRA is exotic with respect to Laurentia and also suggest that rocks of the SRA are of peri-Gondwanan affinity. The Early Cambrian event may therefore serve as a tectonic tracer, linking the SRA and the Piedmont Zone to specific Gondwanan sources. In addition, the geometry of an exotic terrane immediately emplaced onto native Laurentian rocks without an intervening, peri-Laurentian, oceanic/magmatic arc-tract is unique in the Appalachians and may well place added constraints on both the extent of the Taconic orogeny and recently proposed tectonic models for the northern Appalachian Laurentian margin.