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2008 annual meeting – Spruce Pine Mining District: Little Switzerland, North Carolina
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Feldspar geotherms (Figure 7) indicate a very high pressure for the liquidus of the Spruce
Pine magmas. Errors on the intersections of the feldspar geotherms and the liquidus are
high due to the shallow intersection of these lines (Figure 7), but the pressure of the
intersection is clearly high on the order of 12± kilobars. High pressures (13-17 kilobars)
and moderate temperatures (625-790 degrees C) were attributed to the formation of
eclogite at the base of the Spruce Pine thrust sheet (Adams and Trupe, 1997). These
conditions are above the liquidus of the Spruce Pine magmas (Figure 7) suggesting that
the thrusting and accompanying thickening initiated melting to form the Spruce Pine
magmas.
Muscovite and garnet compositions in the LUF pegmatite are similar to compositions of
these phases in other Spruce Pine granitc rocks (Figures 5 and 6) and probably represent
magmatic compositions. Euhedral epidote included in large crystals of Fe-bearing
muscovite may also represent a magmatic phase. Experiments at 8 kilobars by Naney
(1983) showed epidote is a stable magmatic phase in granodioritic compositions, but
epidote was not a magmatic phase at 2 kilobars. The apparent stability of epidote in the
LUF magmas is consistent with their crystallization at high pressure.
Evidence for subsolidus, post-magmatic recrystallization is widespread in the Spruce
Pine granitic rocks. The common feldspar exsolution forms during subsolidus cooling.
The fine-grained, low-Fe muscovite probably reflects crystallization form either a late
stage magmatic or post-magmatic hydtrothermal fluid. Open-space veins of pumpellyitezoisite
reported from a Spruce Pine pegmatite (Wood and Abbott, 1995) crystallized a
hydrothermal fluid at subsolidus temperatures.
Geochronology of Spruce Pine granitic rocks also reflects subsolidus cooling. Whole
rock and muscovite Rb-Sr and zircon U-Pb ages (Figure 1) reflect magmatic
crystallization ages. Muscovite K-Ar ages are lower (Figure 1), similar to muscovite
ages in the country rocks, and reflect cooling ages. The K-Ar ages reflect about 100 Ma
of cooling from magmatic conditions to the 350 degrees C blocking temperature for Ar in
muscovite. This long cooling period is consistent with a tectonic setting of thickened
continental crust related to thrusting in the Blue Ridge.
Butler (1973) indicated a third deformation-metamorphic event in the Spruce Pine area at
about 250 Ma (Figure 1) This event is not reflected in any of the geochronology either in
the granitic rocks or the country rocks (Figure1). The alteration phases (low Fe
muscovite, pumpellyite-zoisite) may be related to this later event.
CONCLUSIONS
The LUF1 pegmatite and, by analogy, the Spruce Pine magmas, crystallized at high
pressure, near the end of the peak regional metamorphic event. Mineral assemblages and
compositions of the LUF1 pegmatite mimic those of other Spruce Pine granitic rocks.
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