Timing and Pattern of Metamorphism in the Western and Central Blue Ridge, TN and NC: Status and Outstanding Problems

Morcher, David; Massey, Matthew; Tracy, Robert

ABSTRACT: Details regarding the absolute age of regional metamorphism, porphyroblast-matrix microstructural relationships, deformation history, and 3-D orientation of isograds in the Western and Central Blue Ridge of the Great Smoky Mountains and adjacent terranes remain largely uncompiled on a regional scale. The Barrovian progression (chlorite through sillimanite) developed in Great Smoky and Snowbird Group metaclastic rocks of the Western Blue Ridge was shown to continue into migmatitic metapelites of the Central Blue Ridge, with the WBR-CBR boundary (Hayesville Fault) being folded and metamorphosed. Further regional along-strike comparisons of tectonometamorphic histories (e.g., into Blue Ridge Thrust Complex) remain to be elucidated. Staurolite through kyanite grade metapelites and migmatites of the Great Smoky Mountains Barrovian sequence in the eastern WBR are overprinted by a phase of deformation (tight to isoclinal folding) under retrograde conditions that produced recrystallization of matrix biotite, muscovite, and quartz (generally not plagioclase), folding and kinking of kyanite, and growth of biotite at expense of garnet. The foliation and microstructural features generated by this deformation event are widespread and could be used to correlate deformation histories throughout the WBR and CBR. Despite provocative proposals that orogenesis in the WBR and CBR is Acadian or Alleghanian, age data and the distribution of prograde isograds in the WBR and CBR are most consistent with Taconian orogenesis. Precise U-Pb zircon geochronometry on granulites and eclogites in the CBR yield an age of ~458 Ma, interpreted to be the time of peak thermal or baric conditions. This is supported by a dominant monazite Th-U-Pb chemical age mode of ~470 Ma obtained for staurolite and kyanite grade Great Smoky Group metapelites in the WBR. The age of the post-peak metamorphic deformation event is proposed to be Acadian and related to tectonometamorphism in the Eastern Blue Ridge and Inner Piedmont that propagated northwestward into the CBR and WBR. Much of the published geochronology for metapelites and metabasic rocks the Spruce Pine thrust sheet (Rb-Sr, Sm-Nd mineral isochrons and mineral-rock isochrons; U-Pb ID-TIMS analysis) is consistent with an Acadian overprinting of Taconian peak metamorphic assemblages. The meaning of much of the available K-Ar and 40Ar-39Ar geochronology is equivocal owing to complex deformation histories, lack of information on fabric relationships and mineral growth histories, disturbed isotopic systematics, or isotopic inheritance.