Investigation Of The Volcanics Rocks On Marblehead Neck Through Petrographic, Geochemical, And Field Analyses

Abstract

Marblehead Neck is a rocky island within the Dedham-Milford zone in northeastern Massachusetts and is separated by the younger Late Silurian Salem Gabbro-Diorite on the adjacent mainland of Marblehead by a fault of unknown motion. Although shown on the 1983 bedrock map of MA, to be underlain by the Late Proterozoic Dedham Granodiorite and Lynn Volcanics, attempts to date and definitively correlate these rocks have been unsuccessful. This study focuses on confirming the identity of the volcanics and correlating to local lithologies(Middlesex Fells Complex, Lynn-Mattapan Volcanic Complex, Newbury Volcanics, and extrusives of the Cape Ann Granite) through field mapping, petrographic analysis, and a review of previous geochemical analyses. During field mapping, eight lithologies were identified; metasedimentary rocks, granodiorite, alkali “pink” granite, subvolcanic dacite, pyroclastic volcanics (lapilli tuffs, ignimbrites, flow-banded vitrophyres, and red vitrophyres), lahars, epiclastic carapace, and diabase and basalt intrusions. Contact relations of the lithologies are asfollows: inclusion of metasedimentary rocks within granodiorite; diffusive contact of alkali “pink” granite and granodiorite; granodiorite as an intrusive contact with the dacite (chill margin); inclusions of the lapilli tuff of the pyroclastic volcanic rocks contained within the granodiorite; and epiclastic overlaying the ignimbrites. All the lithologies are transected by NW-SE diabase and basalt dikes. Evidence for a tilting event is preserved in steeply dipping fiamme within the ignimbrites. The granodiorite is interpreted as the Dedham Granite based on modalcomposition, character, and inclusion of the metasedimentary rocks (now interpreted as Westboro Formation). Based on expected relationships between the Dedham the older Middlesex Fells and younger Lynn Volcanics more investigation is warranted for the subvolcanic dacite rocks. The pink alkali granite intrudes the granodiorite and a cold contact is visible further inland. Petrographic analysis reveals Westboro Quartzite clasts within the epiclastic rocks, supporting an Edicarian age. The contact between the Dedham and the volcanics excludes boththe Newbury Volcanics and the Cape Ann Plutonic Complex from consideration of the volcanics relation. Considering Thompson et al. (2007) determined Ediacaran age of 595.8 ± 1.2 Ma for the Lynn Volcanics, an unconformity of approximately 10 Ma excludes the Lynn from being related to the Dedham Granite. Therefore, the volcanics present on Marblehead Neck must belong to Middlesex Fells. This relationship is shown to be inconsistent with field relationships observed at Marblehead Neck and field relations in general north of Boston. Geochemical data of the flow-banded vitrophyres found at Marblehead Neck Lighthouse are similar to a flow-banded rhyolite from the Mattapan Volcanic Complex collected in Milton, MA. Marblehead Neck could be considered a fault block offset of the Melrose subblock (Goldsmith, 1999), or the Marblehead Harbor Fault could be a steeply dipping reverse fault that brought Ediacaran basement rocks to juxtapose against Silurian Salem Gabbro Diorite. The steeply dipping fiamme in the ignimbrites suggest a titling event of the entire section. The lithologies present on Marblehead Neck may offer further insight where exposures around Boston have not. Perhaps the Lynn Volcanics have multiple magmatic events that span a longer time frame then previously suggested. Further study and geochronology efforts of the subvolcanic and volcanic rocks and their relation to the Dedham Granite is essential to fully understand the geologic history of New England.