A detailed field and petrographic investigation was made of part of the plutonic belt in extreme southeastern Maine to determine the age and origin of the plutonic rocks. Work was concentrated in the Calais area, Maine, because of the diversity of plutonic rock types and the presence of sediments which could be dated by contained fossils.

The area is underlain by metasedimentary, volcanic, and sedimentary rocks, and several plutonic rock types which range in age from Cambro-Ordovician to Late Devonian. The oldest rocks, the Cambro-Ordovician Charlotte Group, consist largely of isoclinally folded quartzites, phyllites, slates, and schists. The Silurian succession consists of two major groups: (1) a thick sequence of inter-layered shale and rhyolite extrusives of Middle Silurian age and (2) rhyolite and basalt extrusives of Late Silurian age. The youngest stratified unit, the Late Devonian Perry Formation, consists of a thick sequence of coarse clastic sediments and interbedded basalt flows.

The plutonic rocks of the area are chiefly norites, gabbros, diorites, and granites. Structural and petrographic relationships suggest that the bodies of mafic rocks are remnants of a large stratiform body emplaced in sediments of the Charlotte Group prior to or during regional deformation of this group and that the present isolation of the masses is a result of transverse faulting and partial assimilation and replacement by younger plutonic bodies. All the diorites and granites were emplaced in the post-Silurian-pre-Upper Devonian interval, as they exhibit intrusive contacts toward rock of Late Silurian age but have contributed detritus to the Perry Formation. Petrographic characteristics indicate that all the granites and diorites are related and are probably of about the same age, although granites invariably show intrusive relationships toward diorites. Contact features establish that the Red Beach Granite is slightly younger than the other granites and diorites; mineralogical and textural features indicate that it formed under near-surface conditions.

The presence of plutonic rocks in a region characterized by low-grade metamorphism suggests a magmatic derivation. Contact and petrographic characteristics confirm a magmatic origin, although, locally, granitization operated as a subordinate process. Distribution of the plutonic rocks in a linear belt suggests that emplacement occurred along a zone of thrust faulting traversed by numerous cross faults.

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