Petrology of the Halifax County complex, North Carolina, Southern Appalachians: constraints from petrography, mineral chemistry, and geothermobarometry

Abstract

The Halifax County complex (HCC) is a meta-ultramafic/metamafic body that crops out along the easternmost exposed part of the Carolina superterrane in northeastern North Carolina. Petrographic and mineral chemistry studies were undertaken to place some constraints on the evolution of the HCC. Halifax County Complex amphiboles are zoned, with hornblende cores and actinolitic rims. Feldspar minerals span the whole plagioclase- and potassium-feldspar spectrum. Evolved olivines (Fo69–75) are primary, and pyroxenes plot in the enstatite, pigeonite, augite, and diopside fields. Low TiO2 (<0.8 wt.%) clinopyroxenes and highly calcic plagioclases are consistent with origin of the HCC at an island arc setting. Chlorites are characterized by wide variations in their Si atoms per formula unit but have restricted total Fe concentrations. Potassium feldspar in the HCC likely originated during adularization or potassification. Chlorite thermometry yields temperatures of formation of 241–300 °C. Application of hornblende-plagioclase thermometers yields average temperatures of 648 °C consistent with amphibolite facies conditions as well as greenschist facies conditions, at pressures of up to 6.5 kbar. Both clinopyroxene and evolved olivine compositions are consistent with an island arc origin for the HCC. Amphibolite facies metamorphisms recorded by HCC rocks likely represent metamorphism of the HCC during arc–arc terrane collision, whereas greenschist metamorphism is interpreted to record the temperatures of thrusting of the HCC onto its present location at pressures of <3 kbar in Alleghanian times during the final assembly of the Appalachians. Results reported here have implications for the origin of comparable metamorphosed mafic-ultramafic rocks encountered along ancient orogenic belts worldwide.