ANATOMY OF THE MIGMATITE-GRANITE COMPLEX, SOUTHWESTERN MAINE

Long mapped as a single massive body of granite representing a single Carboniferous magmatic episode, the c. 5000 km² Migmatite-Granite Complex (MGC) of southwestern Maine, as it is now referred, instead records crustal melting and deformation over a period of c. 100 Myr. That history includes Devonian syntectonic regional high grade metamorphism, migmatite formation and magmatism, Devonian through Carboniferous(?) regional deformation as temperature waned to sub-solidus conditions, and finally the late-tectonic intrusion of widespread Permian granite.

Rims from zircon crystals extracted from migmatite melanosomes in two distinct locations yielded a pooled U-Pb age of 376 +/- 15 Ma (LA-MC-ICP-MS). Inherited cores of the same crystals record ages ranging from middle Proterozoic to Ordovician. Individual undeformed granite bodies, ranging from meter- to kilometer-scale, crop out within the migmatites locally. Although there is considerable textural variability, granitic rocks selected for U-Pb dating were all equigranular and fine-grained. However, despite broad textural similarity, these rocks record ages in two populations. Some of the granites in the MGC crystallized coevally with the migmatites, including one single-crystal U-Pb age at 381 +/- 1 Ma. Otherwise similar granites have Permian ages, such as a sample from New Gloucester at 288 +/- 13 Ma.

Central to the MGC is the c. 400 km² Sebago pluton. Despite its central position in the MGC, the pluton is far too young to be at all related to migmatites exposed in the Complex. Exposures of texturally varied granites along the pluton’s NE margin suggest pluton construction may have extended over c. 15 Myr. Rocks with magmatic foliation, in places schlieric, record crystallization from 308 to 293 Ma (U-Pb zircon rims). Equigranular two-mica granite from two distinct parts of the pluton’s interior crystallized around 293 Ma (both U-Pb zircon and monazite).