NATURE AND ORIGIN OF ACADIAN PLUTONISM, PIEDMONT TERRANE, NC-GA

Acadian plutonism in the Piedmont Terrane spanned a problematic 50 million year interval in southern Appalachian history. Acadian plutons are widespread but not especially abundant and do not form sizable batholiths. Absence of mafic to intermediate rocks or other evidence for a contemporaneous arc setting rule out a subduction-related origin.

In the Eastern Blue Ridge (EBR) of NC and GA, Acadian plutons include the Mt. Airy and Stone Mountain plutons (~350 Ma; Fetter, Sinha, pers. comm.), the 380 Ma Spruce Pine intrusives (Waters et al., Johnson et al.), and the 390 Ma Pink Beds, 380 Ma Looking Glass, 370 Ma Rabun and 340 Ma Yonah plutons. These mildly peraluminous, low K₂O trondhjemites to granodiorites have high Sr and very low heavy REE concentrations and are relatively primitive isotopically (initial ⁸⁷Sr/⁸⁶Sr 0.704-0.707, e_Nd +5 to -3, d¹⁸O +6-9‰). Zircons have ubiquitous inherited cores, mostly Grenville but less commonly 1.4 Ga and 2.6-2.8 Ga. These data suggest a deep, mixed parentage involving both juvenile mafic and mature crustal contributions.

The only Acadian plutons in the Inner Piedmont (IP) of NC are the ~375 Ma Cherryville and Toluca plutons, both located in the eastern portions of the province. These strongly peraluminous, highly felsic granites are more potassic and have lower Sr and somewhat higher heavy REE than their EBR counterparts. Inherited zircons are relatively sparse, but they have elevated ⁸⁷Sr/⁸⁶Sr and fairly low e_Nd (0.710-0.712, -4 to -5). These data are consistent with derivation entirely from evolved, but younger, crustal rocks, possibly eastern IP metasediments (cf. Bream et al.).

Zircon saturation thermometry for all samples suggests magma generation at <800°C, implying that dehydration melting involving biotite or amphibole was not the trigger. Muscovite dehydration melting is plausible for the IP plutons, but compositions of the EBR plutons are inconsistent with abundant muscovite in their source. An external water source seems necessary for generation of EBR plutons at such low temperatures, possibly a large mass of cool, underthrust, mafic material that dehydrated as it thermally equilibrated. We speculate that these magmas were produced in a syn- or post collisional environment with a cold immature slab underthrust below warmer, more mature continental crust.