CHRONOLOGY OF PALEOZOIC METAMORPHISM AND DEFORMATION IN THE BLUE-RIDGE THRUST COMPLEX, NORTH-CAROLINA AND TENNESSEE

The Blue Ridge province in northwestern North Carolina and northeaster n Tennessee records a multiphase collisional and accretionary history from the Mesoproterozoic through the Paleozoic. To constrain the tecto nothermal evolution in this region, radiometric ages have been determi ned for 23 regionally metamorphosed amphibolites, granitic gneisses, a nd pelitic schists and from mylonites along shear zones that bound thr ust sheets and within an internal shear zone. Samples were collected a long a northwest-southeast traverse across three crystalline thrust sh eets located west of the Grandfather Mountain window. Ordovician metam orphic mineral ages (Llanvirnian to Caradocian) are recorded in the hi ghest thrust sheets within the Blue Ridge thrust complex. Six samples of metapelite and amphibolite from the Spruce Pine thrust sheet and fi ve samples of basement gneiss and metagabbro from the Pumpkin Patch th rust sheet yield Sm-Nd and Rb-Sr garnet ages of 460 to 450 Ma and horn blende ages of 472 to 451 Ma In addition, multiple mineral isochron ag es of 457 +/- 2 and 458 +/- 17 Ma were obtained from a Pumpkin Patch m etagabbro. The garnet ages from the Pumpkin Patch thrust sheet (458, 4 55, and 451 Ma) are similar to those from the structurally overlying S pruce Pine thrust sheet (460, 456, 455, 455, and 450 Ma). Both thrust sheets exhibit similar upper amphibolite-facies conditions. Because of the high closure temperature for garnet, the garnet ages are interpre ted to date growth at or near the peak of Taconic metamorphism. Devoni an metamorphic ages are recognized in the Spruce Pine thrust sheet, wh ere Sm-Nd and Rb-Sr garnet ages of 386 and 393 Ma and mineral isochron ages of 397 +/- 14 and 375 +/- 27 Ma are preserved. Hornblendes recor d similar Ar-40/Ar-39, Sm-Nd, and Rb-Sr ages of 398 to 379 Ma. Devonia n Ar-40/Ar-39 hornblende ages are also recorded in the structurally lo wer Pumpkin Patch thrust sheet. The Devonian mineral ages are interpre ted to date a discrete tectonothermal event, as opposed to uplift and slow cooling from an Ordovician metamorphic event This interpretation is based in part on the fact that Devonian and Ordovician mineral ages are intermixed spatially within the Spruce Pine and Pumpkin Patch thr ust sheets. Ar-40/Ar-39 muscovite ages from regionally metamorphosed r ocks indicate that much of the Blue Ridge slowly to temperatures of ab out 375 degrees C by the Late Mississippian. White micas from mylonite 328, and 336 Ma and Rb-Sr ages of 324, 324, 526, and 326 Ma. These re flect the age of Late Mississippian mylonitization (mica growth below or near respective Sr and AE closure temperatures). The Mississippian mylonitization is interpreted to represent thrusting and initial assem bly of crystalline sheets associated with the Alleghanian orogeny. The composite thrust stack of the Blue Ridge complex was subsequently thr ust northwestward along the Linville Fans fault during middle Alleghan ian orogeny (about 300 Ma).