The Moosehorn Igneous Complex of southwestern New Brunswick and southeastern Maine: Petrology, geochemistry and tectonic setting

The Moosehorn Igneous Complex in the area of Calais, Maine, and St. Stephen, New Brunswick, is part of the Coastal Maine magmatic province (CMMP). Based on a compilation of previous work, new field mapping, and petrochemical study, the Moosehorn Igneous Complex (MIC) consists of 5 units: Staples Mountain Gabbro, St. Stephen pluton, Calais quartz diorite, Baring Granite, and Elliott Mountain quartz diorite. In contrast to the sill-like Staples Mountain Gabbro, the St. Stephen pluton consists of a core of dunite and troctolite, surrounded by olivine gabbro and gabbro. The Calais quartz diorite is areally the most extensive component of the MIC, and is commingled with mainly medium grained biotite monzogranite to syenogranite of the Baring Granite. The Elliott Mountain quartz diorite has distinctive ophitic texture and is the youngest unit of the MIC. Mineralogical and chemical features indicate that all 5 units are cogenetic, a conclusion supported by similar U-Pb (zircon) crystallization age for the Baring Granite (421.6 +/− 0.3 Ma) and 40Ar/ 39Ar (phlogopite) cooling age of 421 +/− 5 Ma for the St. Stephen pluton. Mafic to intermediate rocks of the Bocabec Pluton of the Saint George Batholith (SGB) are similar to the Calais quartz diorite and gabbroic part of the St. Stephen pluton, whereas the granitic part of the Bocabec Pluton is similar to the Baring Granite, which is of the same age as the Utopia Granite of the SGB. Plutons of the MIC have petrological features similar to other intrusive complexes in the CMMP, and formed by melting of Mascarene-Ellsworth basement crust brought about by southeastward subduction associated with amalgamation of the Miramichi-St. Croix and Ellsworth-Mascarene terranes, for which the MIC provides a minimum age.