15 Petrology and Tectonics of the Southern Appalachians

Thursday, 29 March 2007
1:20 PM - 5:20 PM

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

Paper number: 15-2
Program: THEME SESSIONS
Time: 1:40 PM – 2:00 PM

Authors

Abstract

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.