PRELIMINARY PETROLOGICAL, GEOCHRONOLOGICAL, AND GEOCHEMICAL CONSTRAINTS ON THE TIMING AND ORIGIN OF MAGMATISM AT MARBLEHEAD NECK, MA

Marblehead Neck, a small island off Marblehead, MA, is composed of an array of plutonic, subvolcanic, and volcanic rocks located within the Avalon terrane, the Late Proterozoic exotic terrane underlying most of Southeastern Massachusetts. Previous investigations have interpreted these units as part of the Late Proterozoic Dedham Granite and Lynn Volcanics based on location and lithologic similarities. However, accurate field descriptions and relations, precise ages, and geochemistry are largely lacking. We present new petrological, geochronological, and geochemical analyses of these units to better document the timing and origin of magmatism in the Marblehead Neck area. Field mapping reveals various felsic igneous rocks, which include a granodiorite, a pink granite, a subvolcanic porphyry, and abundant rhyolitic flows and pyroclastic rocks. These units are intruded by numerous diabase dikes, not investigated here. Petrographic analyses reveal that granodiorite consists of plagioclase (60%), quartz (25%), K- feldspars (5%), hornblende (5%), biotite (5%) and includes metasedimentary xenoliths. The pink granite intrudes into the granodiorite and consists of K-feldspar (40%), quartz (35%), plagioclase (20%), minor hornblende (5%), and is devoid of metasedimentary xenoliths. The rhyolitic flows are mostly fine-grained groundmass, with some plagioclase phenocrysts and mafic inclusions. The subvolcanic unit is identified as dacite porphyry. The granodiorite was dated using LA-ICPMS. Zircon crystals are euhedral to subhedral that exhibited well-developed igneous CL textures, such as sector and oscillatory zoning, and yield an age of 606.9 ± 0.79 Ma. Overall, our results indicate the granodiorite unit is part of the Dedham Granite, based on the modal composition, inclusions (Westboro xenoliths), and age. Major and trace element analyses are pending and will be used to make correlations with neighboring igneous bodies and better understand the tectonic environment during magma generation and emplacement.