REINVESTIGATION OF THE ORIGIN OF MARBLEHEAD NECK PLUTONIC AND VOLCANIC ROCKS USING INTEGRATED FIELD, PETROLOGICAL, AND GEOCHRONOLOGICAL ANALYSIS

Marblehead Neck sits within the Salem block of the Avalon terrane, adjacent to the Dedham-Milford Zone. The island is separated from the Late Silurian Salem Gabbro-Diorite on the mainland by a fault of unknown motion, referred to here as the Marblehead Harbor fault. 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 on Marblehead Neck and their relationship to the local granodiorite through field mapping, petrographic analysis, ICP-MS LA U-Pb zircon geochronology, Hf isotope geochemistry, and a review of previous geochemical analyses. During field mapping, three additional lithologies were identified; a dacitic subvolcanic unit, a pink alkali granite, and a very localized fluvial sedimentary carapice overlaying the volcanics. On Marblehead Neck, the granodiorite displays a possible chilled margin against the dacite. This relationship warrants further investigation based on expected relationships between the Dedham, and the older Middlesex Fells and younger Lynn Volcanics. The pink alkali granite, which intrudes the granodiorite, could be a younger phase of the Dedham or a younger Late Silurian intrusion related to the Cape Ann Plutonic Suite. Steeply dipping fiamme within the ignimbrites and the sequence of exposed lithologies suggests the entire section is steeply tilted. All lithologies are transected by a prominent set of NW-SE basalt and diabase dikes. Clast lithologies in the largely epiclastic carapace includes fragments of the Westboro Quartzite supporting an Edicarian age, similar to the Roxbury Conglomerate of the Boston Bay Group. If the sequence is Ediacaran, the Marblehead Harbor Fault may be an oblique left-lateral fault potentially offsetting the Melrose sub-block to the south, or a high angle dip slip fault that juxtaposes the Ediacaran rocks against the Salem Gabbro Diorite. Results of field studies, when integrated with petrological, geochronological, and geochemical analysis, will provide a better characterization of Marblehead igneous rocks and its relationship to Dedham-Milford Zone.