GEOCHRONOLOGICAL CONSTRAINTS ON GRENVILLIAN DEFORMATIONAL EVENTS IN THE BLUE RIDGE, BALDWIN GAP QUADRANGLE, NW NORTH CAROLINA

The Laurel Creek pluton is a large, typically massive to slightly foliated granite which intruded an older deformed complex of strongly foliated granite and polydeformed biotite gneiss. Zircons from samples of the Laurel Creek pluton at two localities near Laurel Creek yielded consistent ages. One sample had a concordia upper intercept age of 1055+/-5 Ma from 15 of 20 zircons with an average age from 7 zircons (<1% discordant) also of 1055+/-5 Ma. The other sample, from closer to the margin of the pluton, had two populations of zircons. Clear grains (19 of 23 were <1% discordant) had a weighted average of 207Pb/206Pb ages of 1049+/-8 Ma. Larger brown grains were discordant with mixed individual ages suggesting partial resetting of older zircons. Only 3 brown grains were <1% discordant with an average age of 1059+/-8 Ma. Along Roundabout Creek a small dike of massive granite cuts across both strongly foliated granite and polydeformed biotite gneiss. Eleven of 23 grains (<1% discordant) had a weighted average age of 1040+/-19 Ma. The age range of xenocrystic grains from all 3 localities of Laurel Creek granite is generally ~1100-1200 Ma with a few ages >1200 Ma. The older strongly foliated granite along Roundabout Creek contains a large number (20) of discordant zircon (~1200-1250 Ma, ca.1400 Ma and ca.1600-1700 Ma). A few (<1% discordant) grains suggest a protolith age of 1165+/-18 Ma with a few zircons reset at 1043+/-14 Ma. The Laurel Creek granite (~1055 Ma) post-dated the deformational event which deformed the strongly foliated older granite (~1165 Ma) which post-dated both earlier magmatic events from which older (>1200 Ma) xenocrystic zircons were derived and an earlier deformational event that affected the biotite gneiss.