PALEOZOIC HIGH PRESSURE METAMORPHISM IN THE MAIDENS GNEISS, GOOCHLAND TERRANE, VA: EVIDENCE FROM GNEISSES WITH KYANITE + K-FELDSPAR ASSEMBLAGES

The Goochland Terrane in the Piedmont Province of Virginia preserves evidence of an early, high-grade (sillimanite-producing) event that was later overprinted by a lower-grade (kyanite-producing) event. However, two belts of pelitic gneiss in a small area in the northeastern portion of the terrane contain the rare association kyanite + K-feldspar in an assemblage also containing qtz, plag, grt, bt, rut, ilm, +/- mus, grp, po, and py. The overall assemblage in this case (coupled with the absence or paucity of mus) suggests that the kyanite in these rocks is not a retrograde phase, but rather was produced via the “second Al-silicate” reaction at pressures exceeding about 8 kbar where the mus breakdown curve intersects the ky-sil boundary. To test this possibility, we applied conventional thermobarometry to one typical sample. Garnet-biotite thermometry yielded a temperature of ~730-740°C, and the garnet-kyanite-quartz plagioclase (GASP) barometer yielded a pressure of ~10.2 kbar. A calculated pseudosection based on a major element whole-rock analysis of the same sample also indicates a minimum pressure of ~10 kbar for the stability of this assemblage. These rocks thus appear to represent the deepest structural portion of the Goochland Terrane, and their exposure here may reflect some overall tilting of the terrane accompanying southwestward transport during the Alleghanian orogeny. Electron microprobe (chemical) dating of monazite in this and other high-grade rocks indicates a Devonian (~380-400 Ma) age for the granulite-facies event (Shirvell et al. 2004 GSA abstract), consistent with metamorphism accompanying the Acadian orogeny, followed by a strong thermal overprint at ~290 Ma (Alleghanian). This young event produced some retrogression as shown by late mineral growth and textural adjustments in the rocks. The geographic position of the Goochland Terrane during the Acadian is not well-constrained, but other examples of kyanite + K-feldspar-bearing rocks produced at this time occur in New England (e.g., Waterbury and Goshen domes). Given the relative rarity of this assemblage, it is therefore tempting to speculate that the Goochland Terrane represents a far-traveled crustal fragment that was at one time much closer to New England.