Precise UPb zircon ages of Neoproterozoic plutons in the southern Appalachian Blue Ridge and their implications for the initial rifting of Laurentia

Abstract

Late Precambrian plutonic rocks of the Crossnore Complex in the North Carolina Blue Ridge are considered to have formed in response to rifting of the Laurentian continent. Previously published ages are equivocal, 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 for similar plutonic rocks in the central Appalachian Blue Ridge of Virginia; 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. Thus, our new data, plus published data, suggest that Laurentian intra-continental rifting in the southern and central Appalachians was initiated at different times in different places. Available high-precision UPb analyses of zircon indicate that this rift-related magmatism occurred over a span of at least 30–40 Ma. Core-bearing zircon from the Beech metagranite of the Crossnore Complex has a UPb concordia upper-intercept age of 1424 ± 29 Ma, which is interpreted as the age of at least some of the source rocks for the Crossnore Complex. The existence of Mesoproterozoic crust in this region increases the known extent of rocks of this age. Other Crossnore-type plutons exhibit less inheritance of old zircon, possibly the result of enhanced dissolution of old zircon due to magma temperature differences, or due to a more reactive chemical environment.