TIMING OF ALLEGHANIAN MAGMATISM REVISITED

Determining the duration of magmatism during an orogenic event is critical to developing realistic models that describe the evolution of the orogen. Most previous estimates of the duration of Alleghanian magmatism have been based on Rb-Sr geochronological studies of Alleghanian granites. Based on these Rb-Sr dates Alleghanian magmatism lasted for more than 60 m.y. as published dates range from 325-263 Ma.

To test if Alleghanian magmatism occurred for a protracted length of time the ages of several granites have been re-investigated using U-Pb geochronological methods. Two concordant zircon fractions for the York granite indicate a crystallization age of 321 ± 2 Ma, in agreement with a Rb-Sr whole-rock date of 322 ± 12 Ma, and confirm that this pluton is one of the oldest Alleghanian granites yet recognized. An indistinguishable age of 320 ± 2 Ma has been determined for the nearby Churchland granite, considered to be the second largest of the Alleghanian granites. The new U-Pb date, considerably older than the Rb-Sr date of 282 ± 12 Ma, indicates that the Churchland granite is also one of the oldest known bodies. U-Pb zircon and titanite dates were determined from the Winnsboro and Liberty Hill granites to test the Rb-Sr dates of c. 295 Ma for both granites. The U-Pb dates are indistinguishable for both bodies, yielding ages of 309 ± 1 Ma for the Winnsboro granite and 309 +3.8/-1.2 Ma for the Liberty Hill granite, and thus both intrusions were emplaced 15 m.y. earlier than thought. U-Pb analyses of zircon and titanite from the Siloam granite yield an upper intercept age of 304 +3.5/-2.3 Ma, in sharp contrast to the Rb-Sr date of 263 ± 3 Ma.

These new U-Pb dates establish that Alleghanian magmatism lasted for ~ 20 m.y. rather than 60 m.y. and underscores the danger of interpreting any of the Rb-Sr dates of these granites as crystallization ages. Determining if Alleghanian magmatism occurred in distinct pulses or nearly continuously awaits further high-precision U-Pb geochronological studies.