A persistent problem in New England tectonics has been to identify the eastern continental mass whose collision with composite Laurentia may have caused the Acadian orogeny. The St. Croix belt in coastal Maine is a candidate for the northwestern edge of the Acadian collider.

In the Ordovician, Cambrian and Precambrian(?) rocks of the St. Croix belt west of Penobscot Bay, early NW-directed recumbent folds with axial plane schistosity including andalusite, were deformed by upright folds with crenulation cleavage, then metamorphosed again up to sillimanite grade. The upright folds are cut by bt-ms-gar granite plutons at Northport (ca. 421 Ma) and Youngtown (ca. 420 Ma), and hbl granite (ca. 421 Ma) of the composite Spruce Head pluton. (Ages previously published.) The relationship of the plutons to metamorphism is less clear. A sil+Ksp migmatitic zone extends from the southwest end of the belt (Warren) northeasterly through Lincolnville Center to Pitcher Pond, approx. 30 km long though less than 2 km wide in places. Grade decreases to garnet zone both to NW and to SE. This ridge of high metamorphic grade suggests a local heat source, perhaps represented by the small bodies of metagabbro and metadiorite in the belt. Migmatites are deformed together with the metamorphic foliation, and are crosscut by dikes of the granitic plutons. Yet, metatexite and diatexite near the granitic plutons postdate upright folds, showing that some migmatite is younger. Also, hbl diorite and hybrid rocks commingled with granite appear to be unmetamorphosed, so probably represent a second influx of mafic magma. Small melt pockets localized in boudin necklines indicate melting was still occurring during emplacement of the 420-421 plutons. Because the undeformed plutons approximately track the high grade zone, they may represent the last stages of the metamorphic/anatectic event, allowing that all the metamorphic and plutonic events occurred during a short time.

This welt may represent a deeper level of the plutonic complex represented by the late Silurian plutonic-volcanic suite to the southeast (Pleasant Bay to Vinalhaven) that intrudes the Ellsworth/coastal volcanic belt with its late Silurian European Province faunas and Old World Realm Early Devonian faunas, exotic to Laurentia.