Melrose Green rhyolite: Its geologic setting and petrographic and geochemical characteristics

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Fine-grained lithic material called "Melrose Green" is common in Middle Archaic to Late Woodland Period archaeological sites in southeastern New England. A quarried source of this material crops out on a hillside in Melrose, Massachusetts. At this site, a crystal-rich volcanic tuff is intruded by three fine-grained felsic dikes composed of rock similar to archaeological materials identified as Melrose Green. Locally, parts of the dikes are pockmarked and saddle-shaped in cross-section due to removal of quarried material. The aphanitic dike rock is characterized by sparse feldspar microphenocrysts in a matrix of granular quartz and feldspar. Epidote, titanite, opaque minerals, and calcite are accessory minerals. Flow-bands and relict shard texture are also present. The rock is of rhyolitic-dacitic composition. Some previous workers interpreted Melrose Green artifacts as chert or argillite. Field relationships, petrography, and geochemistry demonstrate that Melrose Green is of igneous parentage. While geochemistry shows that Melrose Green is similar to felsic volcanic rocks of the nearby Lynn-Mattapan Volcanics, the trace element composition of the dikes distinguish them from most other known prehistoric quarry sites within the Lynn-Mattapan complex. Chipping debris from the dikes is widespread at the Melrose quarry site, demonstrating that the dikes were extensively worked for sources of raw material. Three artifacts macroscopically similar to Melrose Green rhyolite were collected from three coastal sites within Boston Harbor. Analyses showed that two of these are of igneous origin and generally similar to rocks of the Lynn-Mattapan complex. Only one of these yielded data consistent with a source from the dike outcrops in Melrose. The third artifact is most likely a non-igneous argillite. These examples show the importance of using petrographic and geochemical analysis to correctly characterize macroscopically similar lithic material. © 2001 Academic Press.

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Journal of Archaeological Science