A PETROCHEMICAL ASSESSMENT OF THE HIGH SIO2 STONE MOUNTAIN (NC) PLUTON

The 60km² Stone Mountain (NC) pluton, a member of the Spruce Pine Plutonic Suite of the eastern Blue Ridge of NC, intruded the Alligator Back metamorphic suite of the Gossan Lead thrust block. Field relationships along the eastern contact show an intrusive complex with at least twelve cross-cutting phases. The marginal phases are fine- to medium-grained (~0.05-2 mm) muscovite-biotite tonalite to granodiorite. The interior of the pluton is largely medium- to coarse-grained (~1 mm-1 cm) leucocratic muscovite-biotite quartz monzonite to granite. Whole-rock major-element compositions for one margin sample (e.g., SiO₂ 71 wt.%; FeO 1.6 wt.%; K₂O 1.1 wt.%; Na₂O 5.3 wt.%; Rb 40 ppm; Sr 1300 ppm) and four interior samples (e.g., SiO₂ 72-73.5 wt.%; FeO 0.75-1.2 wt.%; K₂O 2.4-3.6 wt.%; Na₂O ~5-5.9 wt.%; Rb 60-80 ppm; Sr 800-1000 ppm) show that the pluton is peraluminous, and is generally similar to eastern Blue Ridge plutons (e.g., Whiteside, Rabun, Looking Glass, Pink Beds, Mount Airy, Spruce Pine), and to most Alleghanian NC Piedmont plutons (e.g., Rolesville, Sims, Castalia). Overall trace-element abundances resemble those of most NC plutons (e.g., Zr ~100-150 ppm; Nb 3-6 ppm; Y 5-8 ppm; Ba ~950-1300 ppm). However, Sr is comparatively enriched, 800-1300 ppm, and correlates inversely with Rb, which probably reflects decreasing modal plagioclase and increasing microcline (Plag ~An₁₆ & K-fldsp ~Or₉₂ in one analyzed sample). For regional comparison, the Stone Mountain pluton differs from NC Piedmont granitoid rocks by lower K₂O and Rb, and higher Na₂O and Sr (e.g. Rolesville largely Rb 125-300 ppm; K₂O 3-5 wt.%; Na₂O 3.5-5 wt.%). In conclusion, Stone Mountain rocks are high in SiO₂, consistent with the tonalite-trondhjemite-granodiorite series defined by contemporary eastern Blue Ridge plutonic rocks. Small, but systematic compositional variation is probably due to varying percentages of source melting, heterogeneous source material, or country rock assimilation that increased Sr relative to Sr concentrations in other eastern Blue Ridge plutonic rocks.