PRECISE U-PB ZIRCON AGES OF NEOPROTEROZOIC PLUTONS IN THE SOUTHERN APPALACHIAN BLUE-RIDGE AND THEIR IMPLICATIONS FOR THE INITIAL RIFTING OFLAURENTIA

Late Precambrian plutonic rocks of the Crossnore Complex in the North Carolina Blue Ridge are considered to have formed in response to rifti ng of the Laurentian continent. Previously published ages are equivoca l, suggesting only that these rocks crystallized 690-820 Ma. New U-Pb data indicate that the Crossnore Complex formed at 741 +/- 3 Ma. This age is 10-40 Ma older than has been obtained by other investigators fo r similar plutonic rocks in the central Appalachian Blue Ridge of Virg inia; our age for the Crossnore Complex is 5-20 Ma younger than the U- Pb zircon age reported for the Mount Rogers volcanics in Virginia. Thu s, our new data, plus published data, suggest that Laurentian intra-co ntinental rifting in the southern and central Appalachians was initiat ed at different times in different places. Available high-precision U- Pb analyses of zircon indicate that this rift-related magmatism occurr ed over a span of at least 30-40 Ma. Core-bearing zircon from the Beec h metagranite of the Crossnore Complex has a U-Pb concordia upper-inte rcept age of 1424 +/- 29 Ma, which is interpreted as the age of at lea st some of the source rocks for the Crossnore Complex. The existence o f Mesoproterozoic crust in this region increases the known extent of r ocks of this age. Other Crossnore-type plutons exhibit less inheritanc e of old zircon, possibly the result of enhanced dissolution of old zi rcon due to magma temperature differences, or due to a more reactive c hemical environment.