Southeastern Section–56th Annual Meeting (29–30 March 2007)

Paper No. 10
Presentation Time: 4:20 PM


WILSON, Shelbi E. and OWENS, Brent, Department of Geology, College of William and Mary, Williamsburg, VA 23187,

Kyanite quartzite is a distinctive and locally economically important rock type that occurs in numerous places in Virginia's Piedmont Province, primarily in the Chopawamsic and Milton terranes.  A similar occurrence is located at Hagers Mountain, North Carolina within the Hyco Formation in the Carolina terrane.  Hagers Mountain (HM) also appears to lie within the Hyco shear zone, the tectonic boundary between the Carolina and Milton terranes.  In contrast to the VA occurrences, rocks at HM contain additional chloritoid and this mineralogical difference prompted the current investigation.  Moreover, we wanted to investigate whether models for the origin of kyanite quartzite in VA could be applied at HM.  All HM rocks are dominated by fine-grained quartz and kyanite.  Kyanite is typically white in hand sample, a distinctive feature that these rocks share with the VA occurrences.  Other minerals include white mica, variable amounts of chloritoid, and sporadic rutile, ilmenite(?), and zircon.  Although pyrite is ubiquitous in the VA occurrences, it is notably lacking at HM.  Major element compositions of six samples are dominated by SiO2 and Al2O3, with negligible amounts of CaO, Na2O, K2O, MgO, and MnO.  Amounts of Fe2O3(T) are typically <0.4 wt%, but correlate in a crude fashion with the amount of chloritoid; one sample with abundant chloritoid contains ~3.3 wt%.  Although bulk compositional contrasts (particularly for Fe) might explain the presence of chloritoid here and its absence in the VA rocks, no such contrasts are apparent.  However, pyrite is the dominant Fe-bearing phase in the VA rocks, and its absence at HM may reflect a paucity of S in the petrogenesis of these rocks.  Recent models for the origin of the VA rocks involve severe leaching in high-sulfidation hydrothermal fluids (Owens & Pasek, 2005 GSA abstract), resulting in quartz-kaolinite protoliths.  A key feature consistent with this idea is an anomalous depletion in Ga relative to Al in most VA rocks.  Significantly, HM rocks do not show this depletion.  Thus, contrasts in hydrothermal fluid composition may account for the slight mineralogical and compositional differences between HM and the VA rocks.  Alternatively, the HM protoliths may have originated in a different way, perhaps as a consequence of severe leaching in a weathering environment.