DEFORMATION, GEOCHEMISTRY, AND GEOCHRONOLOGY OF EDDINGTON LEUCOGRANITE STOCK RECENTLY DISCOVERED IN THE NORUMBEGA FAULT SYSTEM IN EASTERN-CENTRAL MAINE

A leucogranite stock was recently discovered in Eddington in Chemo Pond 7.5’ quadrangle in eastern-central Maine and named as “Eddington leucogranite stock”. The stock, with an exposed area of 1 km² and elongated in NE direction, occurs within the Norumbega ductile shear zone hosted by Silurian Flume Ridge Formation of the Fredericton trough. The Flume Ridge Formation in the area is presented as a package of predominantly quartzofeldspathic metagreywacke and pelitic slate and phyllite. The leucogranite stock is in fault contact with unmetamorphosed redbeds of Mississippian Great Pond Formation along its NW margin. The leucogranite stock is entirely ductilely sheared into granitic mylonite and ultramylonite with fairly homogenous texture and composition. It is composed dominantly of medium-grained quartz, plagioclase feldspar, and potassic feldspar with minor amounts of muscovite and biotite. Outcrop-scale granitic dikes with coarser-grained texture or pegmatite dikes that intruded in the main-body of the leucogranite are also sheared and mylonitized. Microscopically, quartz is smeared into ribbons and has gone through significant dynamical subgrain-rotation recrystallization; feldspar becomes porphyroclasts; and muscovite is shown as mica fish. Based on its geochemical data, the Eddington leucogranite has an unfractionated orogenic, S-type granite affinity: it is peraluminous and sub-alkaline with high concentrations of SiO₂ (average 69.8 wt%) and Al₂O₃ (average 14.2 wt%), low concentrations of Cr (≤ 70 ppm), Ni (≤ 30 ppm), MgO (average 1.14 wt%), and Fe₂O₃ (average 3.13 wt%), K₂O+Na₂O of 6.64 wt% (average), enriched LREE, depleted Nb, Sr, and Ba, and negative Eu anomalies. Major and trace elemental analysis suggest that it was very likely resulted from partial melting of metagraywacke and pelite sources under collisional tectonic setting. Geochronological analysis of zircons by using LA-ICP-MS U-Pb dating method yielded a U-Pb concordia age of 380.1±2.0 Ma for the leucogranite, which is similar to the age of the initial ductile deformation (380 Ma) of the Norumbega fault system, suggesting syn-ductile shearing origin and possible shear-heating thermodynamic mechanism for the formation of the leucogranitic magmas during later stage of Acadian orogeny.