Formally named the South Penobscot Intrusive Suite. Divided into 5 informal mapped lithologic units: Granite of Grey Ridge. Porphyritic variant of granite of Grey Ridge. Granite of Camp Stream. Plutonic rocks of the border zone. Gabbro, locally with cumulate layering. Composed of an irregular border zone of mixed mafic and granitic rocks surrounding a composite central core of granite. Outcrop area is crudely oval in shape, measuring about 7 by 12 km. Age is tentatively assigned to the Devonian based on Rb-Sr whole-rock age of 436+/-9 Ma and U-Pb-Th zircon ages ranging from 424.4 Ma to 308.2 (isotopic ages are considered preliminary).

Source: GNU records (USGS DDS-6; Reston GNULEX).

South Penobscot Intrusive Suite consists of granite of Gray Ridge (youngest), granite of Camp Stream, gabbro, and plutonic rocks of the border zone. Age changed to Silurian based on isotopic dating. Although Stewart and others (1988) report a Rb-Sr whole-rock age of 436+/-9 Ma for a suite of samples ranging in composition from quartz diorite to granite, they questioned this age because the intrusion formed a contact-metamorphic aureole on Castine Volcanics. Brookins and others (1973) correlated Castine with fossiliferous Early Devonian rocks in Vinalhaven quad. However, U-Pb zircon date from Castine (Ruitenberg and others, 1993) indicates that Castine may be Early Ordovician. Thus, Rb-Sr age cited in Stewart and others (1988) may be accepted for South Penobscot Intrusive Suite as a whole, but its youngest phase, the porphyritic facies of granite of Gray Ridge, was dated by U-Pb methods as Middle Silurian (424+/-6 Ma) (Stewart and others, 1988).

Source: GNU records (USGS DDS-6; Reston GNULEX).