THE SWANS ISLAND PLUTONIC COMPLEX, CENTRAL MAINE COAST: MAGMATIC EVIDENCE FOR THREE DISTINCT EPISODES OF LITHOSPHERIC EXTENSION IN NORTHERN NEW ENGLAND

The Swans Island Plutonic Complex consists of rocks belonging to four distinct magmatic episodes: 1) Late Silurian(?) bimodal volcanic rocks of the Cranberry Island Series, 2) Late Silurian (424Ma +/- 2) granites and gabbros, 3) Middle Devonian (370 Ma +/- 5 ) rapakivi granites, and 4) a single Early Jurassic basaltic dike. The Late Silurian(?) volcanic rocks are thin, tholeiitic basalt flows and thick, flow-banded rhyolites and lithic lapilli tuffs. Based upon chemical data and analogy with the Cranberry Island series, these rocks are probably the eruptive equivalents of the associated granites and gabbros. The Late Silurian Swans Island (SI) granite-gabbro complex is similar to the Cadillac Mountain complex and the Isle au Haut Igneous complex. Spectacular magma mingling and mixing features between metaluminous biotite-hornblende granite and hornblende gabbro are exposed along the southeastern shore of the island. The SI granite-gabbro complex is intruded by a Middle Devonian rapakivi granite. The age, petrography, and whole-rock chemistry of this intrusion identify it as the easternmost exposure of the Deer Isle Granite (Oak Point phase). Xenoliths, mafic schlieren and enclaves are common in the Oak Point granite, although there are no distinct mafic intrusions associated with this granite. While 50 Ma younger than the SI granite and texturally distinct, the Deer Isle granite is mineralogically and chemically similar, suggesting that the two intrusions share a similar lower crustal source. The final magmatic episode in the region was the intrusion of an enormous (15-30m wide), northeast trending, opx-cpx-plag glomerophyric basaltic dike cutting through the Oak Point granite. Whole-rock chemistry indicates that this dike is the northernmost exposure of the ~200Ma Higganum dike, the largest feeder dike in the Eastern North America flood basalt province. It is proposed that the Late Silurian and Middle Devonian igneous events are the result of two distinct episodes of lithospheric extension associated with the Acadian Orogeny; the Jurassic dike records asthenospheric upwelling associated with continental rifting.