DISTINGUISHING THE NAHANT GABBRO AND SALEM GABBRO-DIORITE BASED ON THEIR PETROLOGY AND GEOCHEMICAL SIGNATURE

The Avalon Terrane formed during the late Proterozoic as a volcanic arc that was originally part of Gondwana and later rifted away from the supercontinent. During this rifting process, the Nahant Gabbro (NG) formed ~488 Ma and later stages of rifting within Avalon led to the intrusion of the Salem Gabbro-Diorite (SGD; 431-425 Ma) and other plutons as part of the Mid-Paleozoic plutonic belt. Avalon gradually made its way across the Atlantic and collided with the modified margin of Laurentia, contributing to the formation of the Appalachian Mountains. The SGD and the NG outcrops are adjacent and due to their similarities in rock types they can be hard to distinguish in the field. In this study we collected five samples in four different localities (Nahant, Marblehead, Highland pluton, and Forest River pluton) to establish possible differences in petrology and geochemistry.

Petrographic and modal analyses are performed on thin sections, while major and trace elements are obtained via whole rock XRF and LA-ICP-MS. The NG is phaneritic with coarse plagioclase, pyroxene, and amphibole consistently ranging between 3.4 and 1.5 mm. Samples collected from Marblehead, Highland Pluton and Forest River display a porphyritic-phaneritic texture with plagioclase phenocrysts ranging from 2.8 x 0.3 to 1.1 x 0.5 mm. Finally, a component of the Highland Pluton is phaneritic, although fine grained, and displays a “dalmatian” pattern with clusters of mafic minerals. On a TAS diagram, samples from Nahant and Marblehead are basalts while the samples from the Highland Pluton and Forest River plot along a trend from basalt to trachyandesite. Samples from Nahant, Mablehead and the dalmatian sample from the Highland Pluton are consistently more primitive with high MgO, low SiO2 and are depleted in incompatible trace elements as compared to the SGD. At this time we are unable to conclusively discriminate between the two plutonic bodies confirming the significance of this study.