Geology of Part of the Southwestern Brushy Mountains, Inner Piedmont, North Carolina, and the Geochemistry of Western Inner Piedmont Migmatite

The newly defined boundary between the western and eastern Inner Piedmont belts has been the focus of several recent tectonic studies in the southern Appalachians. Specifically, the Brindle Creek fault was recognized in southwestern North Carolina as a thrust that juxtaposed Siluro-Devonian-age paragneisses (now-known) and Devonian and Mississippian granites atop Neoproterozoic- to Cambrian-age paragneisses, Middle Ordovician volcanic and metasedimentary rocks, and Ordovician intrusive units. Detailed geologic mapping in the Kings Creek 7.5-minute quadrangle, southwestern Brushy Mountains, Caldwell County, North Carolina, revealed the northeastern continuation of the thrust. Recognition of the northeastern extension of the fault, and analysis of small -scale map compilations indicate the thrust is a major (crustal­scale) boundary. Structural data denote six deformation events in the southwestern Brushy Mountains. Penetrative and partially penetrative fabrics are attributed to syn-peak and post-peak metamorphic deformation (Neoacadian and earliest Alleghenian; D2 and D3, respectively). Changes in the orientations of D2 and D3 structures were observed in the study area; foliations, mineral stretching lineations, and isoclinal and tight fold axes in the northwestern two thirds of the quadrangle are strongly oriented northeast-southwest. Structural orientations in the southeastern comer of the quadrangle vary, but are dominantly oriented east-west, southeast-northwest, and north-south. The structural transition represents Neoacadian transpressional flow caused by buttressing of plastically deforming thrust sheets against the primordial Brevard fault zone. Mylonites immediately adjacent to a second thrust fault (the Green Mountain/Mill Spring thrust) also confirm a significant component of dextral strike-slip displacement. Two episodes of post-Neoacadian folding (F4 and F5) complexly refolded F2 (isoclinal) and F3 (tight) folds. Multiple joint sets are also attributed to Alleghanian and Mesozoic-Cenozoic deformation. A zone of intensely migmatized amphibolite and biotite-hornblende granodiorite was identified in the immediate footwall of the Brindle Creek thrust in the Kings Creek quadrangle and extrapolated ~110 km to the southwest. Detailed geologic mapping has locally placed the migmatite in the same tectonostratigraphic position as the Poor Mountain Formation; intense migmatization, however, has obscured virtually all primary sedimentological evidence that could establish an unequivocal migmatite protolith. U-Pb ion microprobe (SHRIMP RG) geochronology of zoned zircons indicate the migmatite protolith has a Middle Ordovician crystallization age (zircon cores), and was subsequently metamorphosed at 342 Ma and 330 Ma (zircon rims). The crystallization age is coeval with the Poor Mountain Quartzite member. Whole-rock trace and rare earth element chemical data from melanocratic and leucocratic constituents of the migmatite also suggest partial derivation from the Poor Mountain Amphibolite. The highly fractionated migmatite leucosome, an in situ anatectic melt, is enriched in low compatibility trace elements and depleted in high compatibility trace elements. A subset of melanocratic samples has similar concentrations of low compatibility trace elements as the Poor Mountain Amphibolite, but is depleted in higher compatibility elements. The same samples have nearly identical flat REE trends as Poor Mountain Amphibolite samples, but are slightly REE depleted. Emplacement of the hot Brindle Creek thrust sheet in the Neoacadian likely resulted in intense migmatization of the already hot Poor Mountain Formation and adjacent units