IGNEOUS PETROGENESIS AND SUPERIMPOSED DEFORMATION OF THE BLINN HILL PLUTONIC COMPLEX, EAST-CENTRAL MAINE

The Blinn Hill plutonic complex is a 25 km² suite of Late Silurian granitoid rocks within the Ordovician Liberty-Orrington lithotectonic belt in east-central Maine. These rocks have never been studied in detail and, given their age, provide insight into tectonic processes associated with Late Silurian–Devonian orogenesis in this region.

Previous maps depicted the Blinn Hill pluton as a single intrusive body, however, detailed mapping associated with this study indicate the plutonic rocks can be divided into four northeast-trending units that have been variably deformed. From northwest to southeast these units include: (1) mylonitic granodiorite, (2) foliated biotite granodiorite, (3) foliated two-mica granite, and (4) muscovite granite and migmatite. The mylonitic granodiorite is associated with the Norumbega fault system (NFS) and thus the northwestern contact of the complex is interpreted to be tectonic. Migmatites along the southeastern margin of the complex suggest an intrusive contact in this region. Fabrics within the central two units are parallel to fabrics in the surrounding country rocks.

Identical trace element signatures of the mylonitic granodiorite and foliated biotite granodiorite units suggest these were once equivalents but are now texturally different due to variations in the intensity of superimposed deformation. Similar trace element patterns of the granodiorites and the two-mica granite suggest these two rock types originated from the same source and carry the trace element signatures of crustal sources and subduction involvement. The tectonic environment that produced these magmas was likely either subduction during the later stages of Salinic orogeny, or extension-related partial melting of crustal rocks during post-subduction extension associated with the early stages of the Acadian orogeny.

The rocks of the plutonic complex were first deformed during the main compressive stages of the Acadian orogeny in the Early to Middle Devonian which resulted in the development of a steep dipping northeast-trending penetrative foliation. This eventually transitioned into dextral transpressional stresses later in the Devonian which caused partitioning of mylonitic deformation along the northwestern margin of the plutonic complex during the later stages of the Acadian orogeny.