THE PETROLOGIC AND 40AR/39AR RECORD OF CULMINATING ALLEGHANIAN METAMORPHISM AND WIDESPREAD CORE-COMPLEX STYLE EXHUMATION OF THE SOUTHEASTERN APPALACHIANS

Orogenic belts typically form through tectonic episodes that overlap in space, time and pressure-temperature conditions, with events that may span hundreds of millions of years. Commonly, the interpretation of such events requires structural, crystal-chemical, and isotopic data from rocks with complex histories. The Alleghanian event was the culminating stage of Appalachian orogeny for the southern Appalachians, with record of Carboniferous magmatism and high temperature metamorphism from the Piedmont to the Eastern Blue Ridge as exposed in Georgia. Metamorphic rocks of the Western Blue Ridge are typically polymetamorphic, although rocks within the core of the Murphy Synclinorium (Lay Dam / Mineral Bluff Formations) have been considered to record a single metamorphic event, and to have depositional ages that preclude Middle-Devonian or earlier metamorphism. Garnet porphyroblasts in kyanite-grade assemblages from pelites of the Mineral Bluff formation (collected near Tate, Georgia) preserve simple prograde zoning of major elements, syn-to post kinematic textures with respect to the host foliations, and yield thermobarometric estimates for peak conditions of ca. 7 kbar and 630°C. The most straightforward interpretation of isograds associated with these assemblages is that the metamorphism occurred with the folding to produce the Murphy Synclinorium, as they locally appear folded along and are also locally crosscutting with respect to the axial plane. These structural relationships occur within a large region that can be characterized ⁴⁰Ar/³⁹Ar ages for micas of ca. 330 Ma, interpreted to record a Visean stage of Alleghanian metamorphism. Cooling following Alleghanian metamorphism was locally rapid, with rates of ca. 10-15° C/m.y. through the ⁴⁰Ar retention temperatures of micas, but sufficiently protracted on a regional scale to produce a systematic, regional gradient of ~ 50 m.y. in age from the western Blue Ridge to the Piedmont provinces. The observations that mid-crustal cooling could be locally very rapid, yet also led to extensive and protracted regional age gradients, are consistent with Pennsylvanian-Triassic crustal thinning and exhumation of Appalachian crystalline rocks in metamorphic core complexes, as typical of the Basin and Range province.