Ion microprobe age and geochemistry of southern appalachian basement, with implications for Proterozoic and Paleozoic reconstructions

Abstract

Ion microprobe U–Pb analyses of zircons from basement units in the southern Appalachians, combined with supporting isotopic compositions and major and trace element geochemistry, have delineated a granitic magmatic pulse ∼1165–1150 Ma. The pulse is manifested by the Watauga River Gneiss (western Blue Ridge), Toxaway, Wiley, and Sutton Creek gneisses (eastern Blue Ridge), Pilot Mountain and Grassy Creek gneisses (Sauratown Mountains window), and possibly the Forbush gneiss (∼1140 Ma, Inner Piedmont or Sauratown Mountains window) and Cranberry-Mine Layered Gneiss (∼1190 Ma, western Blue Ridge). Additional samples analyzed include the Blowing Rock Gneiss (∼1080 Ma, Grandfather Mountain window), and the Carvers Gap Granulite Gneiss (∼1.8 Ga) and Cloudland Granulite Gneiss (detrital cores ∼1.2–1.8 Ga) from the Mars Hill terrane. Age data were evaluated by calculating concordia ages and concordia probability plots using 206 Pb*/ 238 U and 207 Pb*/ 206 Pb* data simultaneously. Rocks in the main magmatic pulse are granitic (63–72 wt.% SiO 2 ), but elevated in K and incompatible trace elements compared to typical subduction-related magmas, and initial Nd ratios cluster tightly near CHUR. Mars Hill terrane samples are distinct in age, geochemistry (poorer in K and incompatible elements), and isotopic compositions (ε Nd −7.6 and −5 at 1.0 Ga). Zircons from almost all samples have metamorphic rims that yield ages ∼1030 Ma, with the exception of the Blowing Rock Gneiss. Ages of Grenvillian magmatism and metamorphism are similar to reported ages from the northern Blue Ridge of Virginia, the Adirondack Highlands, the Central Metasedimentary Belt (Canadian Grenville Province), and the Llano uplift of Texas. This suggests the entire southeastern margin of Laurentia has a similar history ∼1.2–1.0 Ga. Although consistent with known ages in Laurentia, the presence of ∼1.8 Ga rocks and T DM ages commonly >1.6 Ga is inconsistent with the inferred 1.6 Ga margin of Laurentia. This suggests either that the 1.4–1.5 Ga mid-continent terrane separates older portions of Laurentia, that this region was exotic, or that it was a rifted fragment of Laurentia reattached during Grenville orogeny. Surprisingly few Paleozoic metamorphic zircon rims have been identified, but the few analyses obtained from samples in the eastern Blue Ridge yield late Acadian ages (∼350 Ma). The similarity of basement units across the southern Appalachians suggests a relationship between these provinces and that the Piedmont terrane is not exotic to Laurentia during the Appalachian orogenic cycle.