CHARACTERIZATION AND CLASSIFICATION OF THE PEABODY AND QUINCY PLUTONS IN THE EASTERN MASSACHUSETTS IGNEOUS COMPLEX

Plutonic and volcanic sequences in Eastern Massachusetts have been studied since the 1850s accounting for their detailed mineralogy, chronology and classification. This study focuses on the Quincy and Peabody plutons and their temporal and formational relations to the Cape Ann plutonic complex. They all have reported late Ordovican to Devonian ages (Zartman and Marvin, 1971; Zartman, 1977; Hermes and Zartman, 1985), are largely undeformed, have A-type chemistry, and are seemingly anorogenic. The Cape Ann pluton serves as a well-studied baseline for other plutons within the Boston-Avalon terrane. The Quincy granite is mineralogically similar to the Cape Ann granite, bearing aegirine, riebeckite, and perthite and is an hypersolvus syenogranite constituting an earlier phase of long-lived igneous activity resulting in the three aforementioned plutons. Interestly, we found no ilmenite or magnetite in our samples. The presence of LREE silicates, including monazite, aenigmatite, and possible hingganite (Ce), karnasurtite (Ce), and/or percleveite (Ce) are remarkable but also are REE carbonates including bastnasite/parasite. All of this constrains the formation of the Quincy pluton and represents its earliest fractionation from its parent magma. The Quincy granite also contains the rare occurrence of native arsenic with some Fe in solid solution. The Peabody granite serves as the youngest end member to the igneous activity, but is sometimes overlooked due to its similarity to the Cape Ann granite. It is a quartz syenite to syenogranite and is likewise hypersolvus. Two-pyroxene analysis on mafic enclaves allow the determination crystallization temperature during mingling and limited mixing of magma in the Peabody granite. These A-type granites have similar igneous characteristics, but may have intruded over a range of age along with mixing and fractionation resulting in a complex history.