ELECTRON MICROPROBE AGES OF MONAZITES FROM THE WINDING STAIR GAP, EASTERN BLUE RIDGE

The Winding Stair Gap, NW of Franklin, North Carolina, exposes some of the highest grade metamorphic rocks of the eastern Blue Ridge province. These rocks include K-feldspar (Kspar)+ sillimanite (Sill) – bearing metapelites, garnet (Gt) – hornblende (Hb) gneisses, Gt – botite (Bt) granofelses, orthopyroxene granulites, quartzofeldspathic gneisses, and scapolite bearing calcsilicates. Gt – bearing leucosomes closely associated with the pelitic schists indicate the onset of partial melting. Previous P-T estimates for these rocks range from 670°C, 6.5 kbar to 850 – 900°C, 7 – 10 kbar (e.g. Absher & McSween, 1985; Eckert et al., 1989; Moecher et al., 2004), with Moecher et al. (2004) suggesting a counterclockwise P-T path with an isobaric cooling segment during which kyanite crystallized across the predominant foliation. Peak metamorphism was determined as having been Taconic based on U-Pb ages in the range 458 – 509 Ma obtained by TIMS and SHRIMP dating of zircons from the metapelites and leucosomes (e.g. Miller et al., 2000; Moecher et al., 2004). A Grenvillian event was detected in the detrital core of a zircon crystal from the same outcrop. Monazites from the same rocks yielded Th-Pb SHRIMP ages of 430 – 450 Ma (Miller et al., 1998).

In this study, we report electron microprobe ages for three monazite crystals included in biotite from a metapelitic schist with the mineral assemblage: Bt – Gt – Sill – Qz – Plagioclase – Kspar, along with minor kyanite (Ky) and muscovite (Musc). The sample is characterized by two textural generations of Bt: a coarse grained Bt1 associated with Ky and Musc defining an early foliation, and a finer grained Bt2 associated with Sill. Monazite inclusions in both generations of Bt are characterized by patchy zoning. Analytical profiles across three grains yield average ages of 399± 56 Ma, 411± 56 Ma, and 335 ± 48 Ma. Only one grain yielded ages > 500 Ma for some of its interior domains, with the youngest ages (290 – 300 Ma) typically recorded closer to their rims. None of the grains record Precambrian ages. These data suggest that peak metamorphism in the eastern Blue Ridge was Taconic (~ 470 Ma) during which monazite crystals that record an earlier (Potomac; > 500 Ma) event characterized by the assemblage Ky + Bt1+ Musc + Gt ± St, were partially resorbed and overgrown by new rims. The younger ages of 290 – 350 Ma probably reflect loss of Pb from monazite in response to the Acadian and/or the Alleghanian orogenies.