Abstract

The graphite-bearing Pickering gneiss and intercalated Franklin marble are part of an aggregation of crystalline rocks in the Honey Brook Upland of the southeastern Pennsylvania Piedmont Uplands that consists of a charnockitic basement overlain by a suite of volcanic rocks and intruded by the Honey Brook Anorthosite massif. Carbon isotopic analysis indicates that graphite formation in the Pickering gneiss and Franklin marble is related to metamorphism of organic-rich muds and carbonates accompanied by localized fluid flow associated with the development of anatectic pegmatites. Graphite in fine-grained Pickering gneisses of upper amphibolite and lower granulite facies has an average δ13C = -23.8 per mil relative to PDB and ranges in value from -17.4 to -29.6, indicating an organic source. Calcite from the Franklin marble and associated calcareous gneisses has an average δ13C = +0.4, as would be expected with a sedimentary carbonate protolith. Coarse-grained Pickering gneisses, termed "anatectic pegmatites" in this study, of amphibolite facies have an average δ13C = -18.7 and range in value from -14.0 to -26.9, whereas those of granulite facies have an average δ13C = -10.9 and range in value from -8.9 to -12.4. At the time of anatexis, metamorphic fluids may have facilitated exchange of high δ13C carbon from the carbonates with low δ13C carbon of the partially melted fine-grained gneisses, thus producing the intermediate values of δ13C of graphite in the anatectic pegmatites. Calcite-graphite isotope thermometry yields estimates of 680 ± 40 °C for upper amphibolite-facies and 748 ± 40 °C for granulite-facies metamorphism in the upland.

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