Interpreting Igneous Textures and Compositions of the Marblehead Neck Area for Comparison in Local Formation Event Sequencing

Abstract

This geologic study focuses on Marblehead Neck, a small Island connected by a bridge off Marblehead, MA, characterized by diverse igneous formations. The island, part of the Avalon terrane, has a makeup of igneous units. The prevailing map indicates exposures of Proterozoic Dedham Granite and Lynn Volcanics, yet the region's complexity challenges existing interpretations. These formations, crucial for deciphering the Dedham-Milford Zone's history in the area, prompt comparisons with Dedham Granite, Milford Granite, Lynn-Mattapan Volcanic Complex, and Middlesex Fells Volcanic Complex. Using Petrographic methods and comparing local lithologies, we can come to unravel the lithologies origins. Petrographic analyses of the lithologies reveal that granodiorite consists of plagioclase (60%), quartz (25%), K- feldspars (5%), hornblende (5%), biotite (5%) and includes metasedimentary xenoliths. The 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. These units showed field relations spanning different time periods. Overall, our results indicate the granodiorite unit is part of the Dedham Granite, Granite with the Spot Pond Granite, Rhyolite with the Lynn-Mattapan Volcanic Complex, and Dacite with later subvolcanic activity, possibly related to Lynn feeder dikes or later Pangea volcanism. This approach, incorporating fieldwork, remote reconnaissance, petrographic analysis, and petrographic comparison to known data, aims to refine the understanding of Marblehead Neck's geological evolution within the broader context of the Dedham-Milford Zone.