GENESIS OF FE-AL-SI RICH “PINK HORIZONS” IN BUCK CREEK AMPHIBOLITES AT GLADE GAP, NC

Exposures of Buck Creek amphibolites at Glade Gap differ from its other outcrop areas due to the presence of prominent quartz veins and the uncommon occurrence of “pink horizons:” gneissic enclaves of fine-grained white, pink and dark bands. The horizons vary in size from lighter folia within amphibolites to 0.1-0.2 m wide zones rich in pink bands. Foliations within the horizons are consistent with those in surrounding amphibolites, suggesting they reflect local compositional variability.

Thin section petrography and electron microprobe analysis were conducted on several “pink horizon” samples to resolve their mineralogies. Microprobe measurements were conducted at USF via remote operation using the EPMA system at FIU-FCAEM. Prominent in thin section are folia rich in pleochroic blue-green actinolite, interleaved with low-relief minerals (albitic plagioclase and quartz + minor K-feldspar and muscovite), peppered with tiny garnets and minor titanite. Secondary epidote and chlorite also occur. The surrounding amphibolites are mineralogically typical (hornblende + plagioclase, with retrograde epidote and chlorite: “Type 2” amphibolites after MacElhaney and McSween, 1983). The horizons appear to grade mineralogically into the amphibolites on their margins.

Compositionally “pink horizon” garnets are almandines with significant CaO (9-10% wt.) and MnO (5-8%). Elevated Si, Fe, Al, Mn and K in the horizons, as inferred from mineralogy, suggest a sedimentary protolith, possibly montmorillanite or nontronite clays. Clay-rich sediments can be locally preserved as horizons within basalts in oceanic settings due to sedimentation between eruptions. This idea is consistent with the contention of Peterson et al (in press) that the Buck Creek complex represents a fragment of ocean crust formed at a slow spreading ridge, provided the Glade Gap amphibolites reflect basaltic compositions. Alternatively, these horizons may be metamorphosed alteration zones in basalts and gabbros developed via seawater circulation.