WANDERING CAROLINA: TRACKING EXOTIC TERRANES WITH DETRITAL ZIRCONS

U-Pb dating of detrital zircons is increasingly being used to help constrain the paleogeography of exotic terranes. Here we report U-Pb dates of detrital zircons from the Aaron, Asbill Pond, and Richtex formations, within the southern Appalachian Carolina terrane.

The Aaron Formation, which overlies the c. 610 Ma Hyco Formation, consists of pebble-conglomerates, sandstones, and tuffaceous siltstones. Abundant zircons were extracted from both a sandstone and a conglomerate and dated using a SHRIMP II ion-microprobe. The majority of needle-shaped zircons from both rocks range from c. 650-610 Ma, ages typical of the Carolina terrane. The youngest zircon has a 238U/206Pb date of c. 578 Ma, providing a maximum age of the formation. Pre-Neoproterozoic zircons range in age from 3.2-1.13 Ga. Of these 31 zircons, 77% are 1.8-1.1 Ga, 16% are 2.5-1.8 Ga, and two grains are 2.7 Ga and 3.2 Ga.

The Asbill Pond Formation consists of quartz-rich sandstones and mudstones. Overlying the sandstones are mudstones containing Middle Cambrian trilobites, thus placing a minimum age on the formation. The youngest detrital zircons analyzed are c. 530 Ma, establishing the maximum age of the formation as Early Cambrian. Of the 40 pre-Neoproterozoic zircons analyzed 25% are 1.8-1.2 Ga, 58% are 2.5-1.8 Ga, and 17% are 2.85-2.6 Ga.

The Richtex Formation is an unfossilliferous sequence of mudstones, siltstones and wackes. The youngest detrital zircons are c. 535 Ma, constraining the maximum depositional age as Early Cambrian. Of the 25 pre-Neoproterozoic zircons 8% are 1.8-1.27 Ga, 76% are 2.5-1.8 Ga, and 16% are 3.6-2.7 Ga.

The bulk of all of the Paleoproterozoic zircons fall in the narrow interval of 2.2-2.0 Ga. Because there was virtually no magmatic activity in Laurentia during that interval it is unlikely that Laurentia was an important sediment source. Western Gondwana, however, contains extensive regions of 2.2-2.0 Ga crust. It is thus suggested that the pre-Neoproterozoic zircons were derived from the South American region of Gondwana, which contains abundant Mesoproterozoic and Paleoproterozoic crust. The shift from the earlier predominance of Mesoproterozoic zircons to Paleoproterozoic zircons in the younger rocks suggests that the terrane was traveling away from South America and towards the West African craton.