Ayer Granite (Lower Silurian)
Ayer Granite - Granite to tonalite, partly porphyritic; locally gneissic, locally muscovitic; may include rocks older than Silurian; intrudes Sb and So. Ayer Granite is divided into the Clinton facies and the Devens-Long Pond facies (Gore, 1976). In addition, there are some masses not assigned to either facies that intrude Berwick Formation west and northwest of Lawrence, and that intrude Paxton and Oakdale Formations south of Worcester and west of probable southern continuation of Wekepeke fault. Radiometric ages obtained for facies of Ayer pose problems in assigning ages to unfossiliferous sedimentary rocks they intrude. Clinton facies has a well-defined U-Pb zircon age of 433 +/-5 Ma (Zartman and Naylor, 1984) that authors cite as Early Silurian; Devens-Long Pond facies has a similar age. This age greatly compresses the time available for deposition, burial, deformation, and metamorphism of Berwick and Paxton if these units are truly Silurian. Some of the Ayer not assigned to a facies may have been more properly correlated with Early Devonian Chelmsford Granite and muscovite-biotite granite at Millstone Hill. Bodies south of Worcester may be more properly correlated with Canterbury Gneiss of CT, which lies on strike with Ayer and has Early Devonian age of 329 +/-9 Ma (Zartman and Naylor, 1984). Zartman and Naylor (1984) believe Ayer Granite has same age range as Newburyport Complex. It is quite possible, based on textural and mineralogical differences that the two facies should be separate units, representing different magmatic events (Wones and Goldsmith, 1991).
Newburyport Complex (Silurian or Ordovician)
Newburyport Complex - Gray, medium-grained tonalite and granodiorite. Newburyport Complex was divided into two facies, tonalitic granodiorite and granite, by Shride (1971). Tonalitic facies was originally termed Newburyport Quartz Diorite and included dioritic rocks north of Clinton-Newbury fault zone that are now called Sharpners Pond Diorite in Nashoba zone, and Topsfield Granodiorite in Milford-Dedham zone. These correlations are no longer tenable due to differences in age and composition. Therefore, Newburyport Complex is restricted to the two facies present in Newburyport area. Rocks formerly mapped as Newburyport Quartz Diorite and Salem Gabbro-Diorite, except for gabbros at Salem Neck, MA, are included in undifferentiated diorite and gabbro unit (Zdigb) on MA State Geologic Map (Zen and others, 1983), largely because they could not be mapped separately at 1:250,000 scale. Unit Zdigb also includes mafic dikes and sills that are probably younger or contemporaneous. Most of the dioritic rocks northeast of Boston previously assigned to Newburyport Quartz Diorite are now assigned to an undifferentiated diorite unit (Zdi) on MA State Geologic Map. Newburyport Complex forms a large mass near Newburyport and a small one to its west, both truncated by Clinton-Newbury fault. Tonalite and granodiorite facies occupies core of Newburyport Complex at Newburyport and is intruded to the north by granite facies; described as medium to dark gray in fresh rock, weathering to both green and red, fine to medium grained, and highly variable in mineralogy. A U-Pb zircon age of 455 +/-15 Ma was determined by Zartman and Naylor (1984) for the tonalite. Granite facies intrudes both the Kittery Formation and the tonalite and granodiorite facies and covers an area of about 45 sq km. Described as light gray to dark gray, buff weathering, and porphyritic. No radiometric ages available for granite facies, but it is conceivable that the two facies are different in age. [Papers presented as chapters in U.S. Geological Survey Professional Paper 1366 are intended as explanations and (or) revisions to MA State bedrock geologic map of Zen and others (1983) at scale of 1:250,000.] (Wones and Goldsmith, 1991).