Low-pressure metamorphism and anatexis of Carolina Slate Belt Phyllites in the contact aureole of the Lilesville Pluton, North Carolina, USA

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In the contact aureole of the Lilesville granite and comagmatic Pee Dee gabbro, N.C., greenschist-facies phyllites of the Carolina slate belt have been overprinted by a series of metamorphic reactions producing opx-bearing hornfelses and migmatitic gneisses. In the exterior aureole the slate belt assemblage (chl+ms+ep+ab+qz) gives way to the continuous reaction assemblages (chl+bt+cd+ ms+ab±ep+qz), (bt+cd+ms+An8–29+qz), (bt+cd+ kf+ms+pl+qz), (bt+cd+als±ms+kf+pl+qz), and (bt+cd+ga+kf+pl+qz), from lowest to highest grade. The interior aureole, interpreted as part of the floor of the granite, bears the continuous and discontinuous reaction assemblages (bt+cd+als+kf+pl+qz),(bt+cd+kf+ pl+qz), and, near the gabbro, (bt+cd+ga+opx+kf+ pl+qz). The leucosomes of the migmatitic interior aureole are predominantly trondhjemites with the assemblage (An35–45+qz±bt±cd±kf). Restites in the migmatitic interior aureole contain the AFM assemblages (bt), (bt + cd), (bt+cd+als), and (bt+cd+ga), plus kf, An40–50, and qz. Contact metamorphism was isobaric at 4.0–5.1 or 2.0–3.5 kb depending on choice of aluminosilicate triple point; temperatures reached 650° C in the migmatitic interior aureole and approached 750° C near the gabbro; \({\text{P}}_{{\text{H}}_{\text{2}} {\text{O}}}\) was less than 0.8 in the migmatites, and was lower in the interior aureole and in the high grade exterior aureole. Partial melting in the migmatitic interior aureole took place during dynamothermal metamorphism caused by the magmatic diapir. Incipient melting occurred by the reaction bt+cd+kf+pl+qz+w = liquid. The melt was H2O-undersaturated and coefficients of the reactants were weighted heavily toward the felsic minerals; the proportion of felsic minerals in the leucosomes was controlled in part by modal abundance of kf, pl, and qz available for melting. The incorporation of K into biotite by subsolidus reactions, coupled with the high thermal stability and low solubility of biotite in a felsic melt, are responsible for the trondhjemitic composition of the early anatectic liquids.