ACADIAN PLUTONISM IN THE INNER PIEDMONT AND EASTERN BLUE RIDGE, NORTH CAROLINA AND NORTHERN GEORGIA

Plutonism is the most obvious manifestation of Acadian activity in the Piedmont Terrane (Inner Piedmont [IP] + Eastern Blue Ridge [EBR]). Determining source materials and conditions of magma generation can provide critical constraints on Acadian processes in the southern Appalachians.

Current data indicate that Acadian plutons in the EBR of NC and northern GA range from 360-390 Ma and contain abundant Grenville zircon inheritance (e.g., Sinha et al., 1999; Miller et al., 2000; Waters et al., 2000). The Mount Airy, Stone Mountain, Pink Beds, Looking Glass, Rabun, and Mount Yonah plutons and Spruce Pine pegmatites are peraluminous, uniformly felsic intrusions. With the exception of the highly evolved Spruce Pine pegmatites, these intrusions are low K₂O (1-3 wt%) trondjhemites to granodiorites, enriched in Sr (400-800 ppm), strongly depleted in HREE (1-3 x chond.) and HFSE, and lack significant Eu anomalies. Initial Sr and Nd isotope ratios reveal a trend from strongly depleted toward mature continental crust (0.704-0.707, e(Nd) +5 to -2); d¹⁸O likewise ranges from +6.5 to +9. We interpret the melting events to have occurred deep in tectonically thickened crust that included both juvenile mafic and continental (sedimentary?) materials, leaving garnet-bearing, feldspar-poor residue.

Acadian plutonism appears to have been even sparser in the IP and restricted to its eastern portion. Zircon TIMS and ion probe data suggest that the strongly peraluminous Cherryville and Toluca plutons of NC are both of late Acadian age (McSween et al., 1991; our preliminary data). Initial analyses indicate that these plutons are poor in Sr (100-200 ppm), less depleted in HREE (3-4 x chond.), have negative Eu anomalies, and are relatively rich in K₂O (~4 wt%). The Cherryville pluton is more isotopically evolved than those of the EBR (Sr 0.712, Nd -6; McSween et al., 1991). Both plutons have Grenville zircon inheritance. Acadian plutons of the eastern IP were apparently generated from different sources than those of the EBR (lacking juvenile material) and probably originated at shallower depths.