EVIDENCE FOR MAFIC ORDOVICIAN MAGMATISM AND ACADIAN METAMORPHISM IN THE BRUSHY MOUNTAINS, WESTERN INNER PIEDMONT, OF NORTH CAROLINA

Considerable debate persists over the character, lateral extent, conditions, and age of metamorphism in the eastern Blue Ridge and Inner Piedmont belts of the southern Appalachians. Although the amount of reliable geochronologic data is increasing, conclusions are limited due to a paucity of information concerning this region’s structural and thermal histories. New evidence from the western Inner Piedmont and its southeastern boundary, the Brindle Creek fault zone, provides important geochemical and temporal constraints, and confirms the timing of a significant Acadian metamorphic and migmatitic event within the Inner Piedmont.

Rocks of the Brindle Creek fault zone in western North Carolina preserve an Acadian thermal and metamorphic history. The crystallization age of the Walker Top gneiss (~370 Ma), a laterally extensive but disjunctive, megacrystic, biotite-rich orthogneiss, confirms late Devonian magmatic activity in the hanging wall nappe. Ordovician zircon core ages (~460 Ma) from an amphibole-bearing migmatite (Acadian?) in the immediate footwall of the Brindle Creek fault (near Lenoir, NC), are similar to the age of the Poor Mountain Quartzite (collected near Marion, NC) to the west. Based on these ages and the mineral assemblage of these units, the protolith for the migmatite is likely the Poor Mountain Formation of the western Inner Piedmont. Zircon rim ages in the amphibole-bearing migmatite are ~350 Ma, in agreement with other recent monazite and zircon metamorphic ages. Detailed geologic mapping (1:24,000-scale) in the Brushy Mountains (near Lenoir, NC) and in the South Mountains (near Marion, NC) confirms the lateral continuity of both hanging wall and footwall lithostratigraphic units, and upper amphibolite-facies metamorphism throughout. The true areal extent of western Inner Piedmont units, and thus the extent of this Acadian metamorphic event, are unknown.

Geochemical analyses of representative amphibolite samples from the Poor Mountain Formation will constrain the nature of this Ordovician magmatic pulse in the western Inner Piedmont. A comparison of this new data with published data from the central Inner Piedmont of South Carolina will delimit the areal extent and geochemical nature of similar lithologies.