DIVERSITY OF BASEMENT ROCKS ALONG THE EASTERN-WESTERN BLUE RIDGE CONTACT IN NORTHWESTERN NORTH CAROLINA

The eastern-western Blue Ridge contact in northwestern NC is a tectonic feature that separates Neoproterozoic/early Paleozoic (?) metamorphic rocks of the Ashe Metamorphic Suite (AMS) to the east from Grenvillian-age basement rocks to the west. Interpretations of this contact include a pre-metamorphic (Taconian) thrust fault, a post-metamorphic (Alleghanian) thrust fault, and a syn-metamorphic (Acadian) dextral strike-slip fault. A diverse assemblage of basement rocks occurs along this contact, and includes granitic gneisses, layered gneisses, high-grade mafic gneisses, and amphibolitic units. Grenvillian ages have been reported for many of these units, while others are interpreted as Grenvillian based on field relationships. Reported isotopic ages in basement units are ~900-1800 Ma, with the majority in the 1000-1200 Ma range. Previous workers have interpreted basement massifs in this area as a combination of Grenvillian metamorphic and intrusive rocks. Neoproterozoic magmatic rocks (Crossnore and Bakersville Intrusive Suites) intrude many of the basement units. Near the Grandfather Mountain window (GMW), Alleghanian thrusting emplaced AMS rocks over Grenvillian-age gneiss (Cranberry Gneiss, Cranberry Mine Layered Gneiss, Valle Crucis gneiss) of the Linville Falls thrust sheet. West of the GMW, the Acadian Burnsville fault juxtaposes AMS rocks and a group of basement rocks termed the Pumpkin Patch Metamorphic Suite (PPMS). The PPMS consists of migmatitic layered Mesoproterozoic gneisses containing amphibolite to granulite facies assemblages. Gneissic layering in these rocks exhibits refolded folds, and is crosscut by Bakersville mafic intrusive rocks. The Meadlock Mountain gneiss is a distinctive mafic gneiss that records high T-high P metamorphism within the PPMS. Additional PPMS units include the Cloudland, Carvers Gap, Jacks Creek, and Earlies Gap gneisses. Field relationships and isotopic ages indicate Mesoproterozoic assemblage of basement terranes, followed by Neoproterozoic magmatism and Paleozoic tectonism. Ultimately, the PPMS and AMS were emplaced as a composite tectonic unit by thrusting at the base of the Fries-Gossan Lead thrust sheet during Alleghanian assembly of the Blue Ridge thrust complex.