Nd and Sr isotopic constraints on the source of Alleghanian granites in the Raleigh metamorphic belt and Eastern slate belt, southern Appalachians, U.S.A

Pennsylvanian-Permian granites that intrude the Raleigh metamorphic belt and the Eastern slate belt of the southern Appalachian orogen account for ∼ 40% of the total volume of granite produced during the Alleghanian orogeny. We report major- and trace-element concentrations and initial Nd and Sr isotopic compositions from 31 samples of Allegahanian granites that intrude these belts to further constrain the nature of the sources of the granites. Nd isotopic compositions of three Raleigh metamorphic belt orthogneisses and two Eastern slate belt metavolcanic rocks are also presented to further constrain the nature of the crust in this region of the orogen. Major- and trace-element concentrations of Raleigh metamorphic belt and Eastern slate belt Alleghanian granites are similar for every element examined. Major-element concentrations of the Alleghanian granites indicate that they are high K₂O (3-5 wt%), slightly peraluminous [Al₂O₃/(CaO + Na₂O + K₂O) = 0.96-1.12]. Trace-element compositions of the Alleghanian granites are similar to those of granites produced in both volcanic arc and continental collision settings. Initial Nd isotopic compositions of Raleigh metamorphic belt granites range from ε_{Nd(300 Ma)} -4.4 to +3.0 and initial ⁸⁷Sr/⁸⁶Sr ratios from 0.703 to 0.708. Eastern slate belt granites have θ_Nd(300Ma) between -2.7 and +2.2, and initial ⁸⁷Sr/⁸⁶Sr ratios range between 0.703 and 0.706. Depleted mantle model ages of granites from both belts range from 600 to 1070 Ma. Gneisses from the Raleigh metamorphic belt and volcanic rocks of the Eastern slate belt have ε_{Nd(300 Ma)} values of -4.4 to +4.4, similar to those of the Alleghanian granites, suggesting that anatexis of the crust in these belts could have produced the granites. The neodymium isotopic compositions of the gneisses from the Raleigh metamorphic belt are significantly more radiogenic than that of Grenville crust. The granites that intrude the Raleigh metamorphic belt also show no evidence of interaction with Grenvillian crust, suggesting that neither the surface rocks nor unexposed portions of the Raleigh metamorphic belt are predominantly composed of Grenville crust. Thus in contrast to previous suggestions, the Raleigh metamorphic belt is significantly different than the adjoining Goochland terrane, but is similar to the nearby Carolina terrane.