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Mineral Resources > Online Spatial Data > Geology > by state > Massachusetts Geology

Geologic units in Middlesex county, Massachusetts

Paxton Formation (Silurian) at surface, covers 0.1 % of this area

Paxton Formation - Sulfidic mica schist where mapped separately. The Paxton, here of group rank, includes strata formerly mapped in CT as the Hebron Formation and in MA as the Paxton Formation. It conformably overlies the Oakdale Formation and structurally and conformably underlies the Brimfield Group. It is undivided in central MA; in northeast CT and adjacent MA it is divided into the Dudley and Southbridge Formations. Age is Late Proterozoic(?) based on the intrusion of 440 m.y. Hedgehog Hill gneiss into the overlying Brimfield Group and an age of 1188 m.y. for detrital zircons from the Paxton (Pease, 1989).

Lithology: mica schist

Berwick Formation (Silurian) at surface, covers 0.3 % of this area

Berwick Formation - Mica schist. In New Hampshire: Used as Berwick Formation of Merrimack Group. Consists of purple biotite-feldspar granofels or schist. Contains interbeds of calcsilicate granofels and minor metapelites. Includes Gove Member, mapped separately. Stratigraphic sequence with respect to Eliot Formation is uncertain. Age of all formations in Merrimack Group changed to Ordovician(?) to Silurian(?) based on isotopic age determinations of approx 440 and 420 Ma from detrital zircons from Berwick by J.N. Aleinikoff (oral commun., 1994) (Lyons and others, 1997).

Lithology: mica schist

Fitchburg Complex (Lower Devonian or younger) at surface, covers 0.2 % of this area

Fitchburg Complex - Dfgd containing many zones of foliated biotite-muscovite granite gneiss and inclusions of mica schist and feldspathic granulite.

Lithology: granitic gneiss; mica schist; phyllite; granulite

Massabesic Gneiss Complex (Late Proterozoic) at surface, covers < 0.1 % of this area

Massabesic Gneiss Complex - Migmatite consisting of pink, foliated biotite granite intruding gneissic and granulose metasedimentary and metavolcanic rocks in southeastern New Hampshire.

Lithology: migmatite; granite; metasedimentary rock; metavolcanic rock

Quincy Granite (Lower Silurian or Upper Ordovician) at surface, covers < 0.1 % of this area

Quincy Granite - Alkalic granite containing riebeckite and aegirine. Intrudes CAbw and PZZc (?). Quincy Granite intrudes Middle Cambrian Braintree Argillite. Southern contact is with chemically and mineralogically similar Blue Hills Granite Porphyry. Fresh rock is dark gray to dark green, weathering to buff brown or salmon. Bounded on the northwest by Blue Hills thrust and on the west by Neponset fault. No clasts of Quincy observed in Early Pennsylvanian Pondville Conglomerate, but clasts of overlying and probably related Blue Hills Granite Porphyry are common in Pondville. Combined area of Quincy and Blue Hills rocks is 55 sq km. Small mass of "Quincy Granite" east of Woonsocket, RI, near RI-MA border is similar in texture to Quincy Granite at Quincy, MA, but is more peralkaline. The mass near Woonsocket was thought at the time of compilation of the MA State geologic map (Zen and others, 1983) to be same general age as granite at Quincy. Age of mass at Woonsocket has been more recently determined to be Devonian or possibly Carboniferous, rather than Late Ordovician and Silurian (Hermes and Zartman (1985) (Wones and Goldsmith, 1991).

Lithology: alkali-granite (alaskite)

Sharpners Pond Diorite (Silurian) at surface, covers 2 % of this area

Sharpners Pond Diorite - Non-foliated, medium-grained equigranular biotite-hornblende tonalite and diorite. Intrudes Soagr, OZn, OZf.

Lithology: tonalite; diorite

Gray biotite granite (Permian) at surface, covers < 0.1 % of this area

Gray biotite granite - Contains minor muscovite. Found in Milford quadrangle.

Lithology: granite

Diorite and tonalite (Devonian and Silurian) at surface, covers 2 % of this area

Diorite and tonalite - Includes Dracut Diorite, tonalite near the Ayer Granite, and equivalents of the Exeter Diorite of New Hampshire; intrudes Sb.

Lithology: diorite; tonalite

Metamorphosed mafic to felsic flow, and volcaniclastic and hypabyssal intrusive rocks (Proterozoic Z) at surface, covers 8 % of this area

Metamorphosed mafic to felsic flow, and volcaniclastic and hypabyssal intrusive rocks - Includes some diorite and gabbro north and northwest of Boston.

Lithology: metavolcanic rock; mixed clastic/volcanic; diorite; gabbro

Two-mica granite of northern and southeastern New Hampshire (Early - Late Devonian) at surface, covers < 0.1 % of this area

Two-mica granite of northern and southeastern New Hampshire - Similar to Concord Granite.

Lithology: granite

Andover Granite (Silurian or Ordovician) at surface, covers 10 % of this area

Andover Granite - Light- to medium-gray, foliated, medium-to coarse-grained -muscovite-biotite granite; pegmatite masses common. Includes Acton Granite (Silurian or Ordovician). Intrudes OZn.

Lithology: granite; pegmatite

Rangeley Formation, undivided (Lower Silurian (Llandoverian)) at surface, covers < 0.1 % of this area

Rangeley Formation, undivided.

Lithology: metasedimentary rock; calc-silicate rock; meta-argillite; quartzite; pelitic schist; granofels; felsic metavolcanic rock

Berwick Formation (Silurian) at surface, covers 7 % of this area

Berwick Formation - Thin- to thick-bedded metamorphosed calcareous sandstone, siltstone, and minor muscovite schist. In New Hampshire: Used as Berwick Formation of Merrimack Group. Consists of purple biotite-feldspar granofels or schist. Contains interbeds of calcsilicate granofels and minor metapelites. Includes Gove Member, mapped separately. Stratigraphic sequence with respect to Eliot Formation is uncertain. Age of all formations in Merrimack Group changed to Ordovician(?) to Silurian(?) based on isotopic age determinations of approx 440 and 420 Ma from detrital zircons from Berwick by J.N. Aleinikoff (oral commun., 1994) (Lyons and others, 1997).

Lithology: metasedimentary rock; mica schist

Chelmsford Granite (Lower Devonian) at surface, covers 2 % of this area

Chelmsford Granite - Light-gray, even and medium-grained, muscovite-bearing granite; locally foliated; intrudes Sb.

Lithology: granite

Blackstone Group (Proterozoic Z) at surface, covers 0.1 % of this area

Blackstone Group - Undivided - Quartzite, schist, phyllite, marble, and metavolcanic rocks.

Lithology: quartzite; schist; phyllite; marble; metavolcanic rock

Roxbury Conglomerate (Proterozoic Z to earliest Paleozoic) at surface, covers 1 % of this area

Roxbury Conglomerate - Conglomerate, sandstone, siltstone, argillite, and melaphyre. Consists of Brookline, Dorchester, and Squantum Members. Roxbury Conglomerate forms base of Boston Bay Group. Divided into Brookline, Dorchester, and Squantum Members. Conglomerate in Brookline Member contains clasts of Dedham Granite, quartzite (possibly from Westboro Formation), and volcanic rock from underlying Mattapan Volcanic Complex. Dorchester Member consists of interbedded argillite and sandstone and forms an intermediate unit between Brookline Member and overlying Cambridge Argillite. Uppermost Squantum Member is a distinctive diamictite which appears to pinch out in northern part of basin. Brighton Melaphyre lies within Brookline Member and consists of mafic volcanic rocks (quartz keratophyre, keratophyre, and spilite). Roxbury clearly lies nonconformably on Dedham Granite near Hull, MA; can be traced continuously over Mattapan Volcanic Complex. Age is Proterozoic Z and possibly Early Cambrian (Goldsmith, 1991).

Lithology: conglomerate; sandstone; siltstone; argillite; mafic volcanic rock

Diabase dikes and sills (Lower Jurassic) at surface, covers 0.2 % of this area

Diabase dikes and sills.

Lithology: diabase

Fitchburg Complex (Lower Devonian or younger) at surface, covers 1 % of this area

Fitchburg Complex - Light-gray to white, medium-grained, weakly foliated muscovite-biotite granite; commonly contains white pegmatite bearing muscovite and tourmaline; may include some granite of late Paleozoic age; locally intrudes Dfgrg, Dfgd, and Dl.

Lithology: granite; pegmatite

Ayer Granite (Lower Silurian and Upper Ordovician?) at surface, covers 2 % of this area

Ayer Granite - Devens-Long Pond facies. Equigranular to porphyritic gneissic biotite granite and granodiorite. 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).

Lithology: granite; granodiorite

Tadmuck Brook Schist (Silurian?, Ordovician, or Proterozoic Z) at surface, covers 1 % of this area

Tadmuck Brook Schist - Andalusite phyllite and sillimanite schist, partly sulfidic; local quartzite in upper part.

Lithology: phyllite; schist; quartzite

Metamorphosed felsic metavolcanic rocks (Proterozoic Z) at surface, covers 0.6 % of this area

Metamorphosed felsic metavolcanic rocks .

Lithology: felsic metavolcanic rock

Nashoba Formation (Ordovician or Proterozoic Z) at surface, covers 12 % of this area

Nashoba Formation - Sillimanite schist and gneiss, partly sulfidic, amphibolite, biotite gneiss, calc-silicate gneiss and marble. Nashoba Formation occurs in Nashoba zone of eastern MA. Consists of interlayered sillimanite-bearing, partly sulfidic schist and gneiss, calc-silicate gneiss, and subordinate quartzite and marble. Protoliths were probably volcanogenic sediments interlayered with limy marine sediments. Bell and Alvord (1976) divided Nashoba into 10 members on basis of lithology. Amphibolite is most abundant near presumed base, namely in Boxford Member. Skehan and Abu-Moustafa (1976) divided Nashoba into 30 members based on section in Wachusett-Marlborough tunnel. Although Bell and Alvord's and Skehan and Moustafa's sections contain similar lithologies, Bell and Alvord's is much thicker, and Boxford Member is not readily identified in Skehan and Abu-Moustafa's. Subdivision of Nashoba is conjectural south of Marlborough and Shrewsbury. On MA State bedrock map (Zen and others, 1983) only Boxford Member is separated out from the rest of the Nashoba because this unit was the only member clearly recognized in several area. A definite sequence of members probably does not exist anywhere in the Nashoba because of lenticularity of assemblages and repeated rock types, both of which could be accounted for by either sedimentary or tectonic processes. Although Castle (1965) considered Fish Brook to be either a premetamorphic intrusive rock or a core gneiss of intrusive or sedimentary ancestry, Bell and Alvord (1976) considered it to be volcanic or volcaniclastic in origin. Zircons in Fish Brook are certainly volcanic in origin and yield a date of 730 +/-26 Ma (Olszewski, 1980). If the rock were a core gneiss, that date would apply only to the Fish Brook and not to surrounding rocks; but, Bell and Alvord (1976) believe Fish Brook to be part of the Marlboro Formation-Nashoba Formation sequence and therefore the date does apply to the sequence. In addition, a 1500 Ma date for Shawsheen Gneiss [reference not given] helps bracket age of Marlboro-Nashoba sequence. An upper limit for the sequence was established from the 430 +/-5 Ma age of intruding Sharpers Pond Diorite and 450 +/-23 Ma age of the intruding Andover Granite (Zartman and Naylor, 1984). Although age on MA State bedrock map is shown as Proterozoic Z or Ordovician (due to uncertainty regarding actual rocks sampled by Olszewski and a strong belief that rocks of Nashoba zone correlated with Ordovician rocks to the west), author now feels that rocks of Nashoba zone (except for Tadmuck Brook Schist) are all Proterozoic, but that they are unlike the Proterozoic rocks of neighboring Milford-Dedham zone. [no formal age change made in this report] (Goldsmith, 1991).

Lithology: schist; gneiss; amphibolite; biotite gneiss; marble

Merrimack Group, Berwick Formation (Ordovician? - Silurian?) at surface, covers < 0.1 % of this area

Merrimack Group, Berwick Formation - Purple biotite-quartz-feldspar granofels or schist and interbeds of calc-silicate granofels and minor metapelites. Stratigraphic sequence with respect to Eliot Formation uncertain

Lithology: granofels; quartz-feldspar schist; meta-argillite

Dedham Granite (Proterozoic Z) at surface, covers 1 % of this area

Dedham Granite - Gray granite to granodiorite more mafic than Zdgr north of Boston. Intrudes Zw, Zv. Extensive calc-alkaline plutons separated by Boston basin have long been mapped as Dedham. Those to the north of Boston and studied in this report, are referred to as Dedham North. Crystallization ages for the Dedham North suite (based on titanites and zircons) have been determined at 607+/-4 Ma, while ages for the Lynn are slightly younger at 596+/-3 Ma. Both are clearly part of the Late Proterozoic magmatic event. Dates on two samples from Sheffield Heights indicate that the diorite and granite are part of the Dedham North suite. The Dedham south and west of Boston has been dated at 630+/15 Ma (Zartman and Naylor, 1984). Dedham North Granite has a compositionally highly variable suite ranging from leucogranites to granodiorites, tonalites, and quartz diorite. The granites originated by partial melting of a sedimentary protolith, while the intermediate members show a mixing of granitic magma and mafic magma (Hepburn and others, 1993).

Lithology: granite; granodiorite

Ayer Granodiorite (Early Silurian) at surface, covers < 0.1 % of this area

Ayer Granodiorite - Gneissic granite to tonalite, locally coarsely porphyritic and muscovitic, southeastern New Hampshire.

Lithology: granodiorite; granite; tonalite

Oakdale Formation (Silurian) at surface, covers 4 % of this area

Oakdale Formation - Metamorphosed thin-bedded, pelitic and calcareous siltstone and muscovite schist, probably low-grade equivalent of Paxton Formation. The Oakdale Formation is here revised to include strata previously mapped in CT and adjacent MA as the Hebron Formation and the Scotland Schist. The Scotland Member (Pease, 1980) is renamed the Scotland Schist Member of the Oakdale. The Oakdale is a homogeneous, calcareous metasiltstone at the base of a thick stratigraphic sequence in a geosyncline terrane and extends from NH to the Honey Hill fault in eastern CT. In central eastern CT it underlies the Hebron Formation; in northeast CT and adjacent MA it underlies conformably the Dudley Formation of the Paxton Group; in central MA it underlies the Paxton Group undivided. The lower part of the Oakdale is cut out along the Clinton-Newbury fault zone. Thickness in type area is about 1500 m. Correlative with the Gove Member of the Berwick Formation in NH and the Gonic Formation in ME. Age is Late Proterozoic(?) based on intrusion of 440 Ma Hedgehog Hill gneiss in the upper part of the Brimfield Group at the top of the stratigraphic sequence, and an age of 1188 Ma for detrital zircons from the Paxton in north-central MA (Pease, 1989).

Lithology: metasedimentary rock; mica schist

Lower part of Rangeley Formation (Lower Silurian (Llandoverian)) at surface, covers < 0.1 % of this area

Lower part of Rangeley Formation - Gray, thinly laminated (5-25 mm) metapelite with local lentils of turbidites and thin quartz conglomerates in western New Hampshire. Sparse calc-silicate pods and coticule. Probably equivalent to member B of Rangeley Formation of Maine.

Lithology: meta-argillite; meta-conglomerate; calc-silicate rock

Fish Brook Gneiss (Ordovician or Proterozoic Z) at surface, covers 0.9 % of this area

Fish Brook Gneiss - Light-gray, biotite-plagioclase quartz gneiss; distinctive "swirl-form" foliation.

Lithology: granitic gneiss

Fitchburg Complex (Lower Devonian or younger) at surface, covers 0.1 % of this area

Fitchburg Complex -Dark-gray, strongly foliated biotite granodiorite to tonalite gneiss; resembles Dht; intrudes and contains inclusions of Dl, some mappable; locally cut by sills identical to Dfgrg.

Lithology: granitic gneiss; mica schist; phyllite

Fitchburg Complex (Lower Devonian or younger) at surface, covers 1 % of this area

Fitchburg Complex - Light-gray, strongly foliated biotite-muscovite granite to granodiorite gneiss; common small to very large inclusions of Dl, some mapped separately.

Lithology: granitic gneiss; mica schist; phyllite

Cambridge Argillite (Proterozoic Z to earliest Paleozoic) at surface, covers 4 % of this area

Cambridge Argillite - Gray argillite and minor quartzite; rare sandstone and conglomerate. Contains acritarchs. Cambridge Argillite of Boston Bay Group contains sandy horizons which are in some places quartzite. Most prominent are Milton quartzite unit of Billings (1976), and Tufts Quartzite Member (described by Billings, 1929, and LaForge, 1932) in northern part of basin. Red sandstone and sandy argillite in Chelsea, Revere, and Milton-Quincy areas intertongue with green argillite (Kaye, 1980). Red beds lie above cleaner quartzites such as Tufts and Milton units. Core analysis by D.A. Ashenden (Metropolitan District Commission, 1980, written commun.) indicates that Cambridge and Braintree Argillites are identical. Age of Boston Bay rocks has been controversial and was once thought to be Cambrian to Pennsylvanian (the latter based on lithologic similarity to rocks of Narragansett basin and now discredited plant fossils). Age of Cambridge and of Boston Bay Group as a whole is Proterozoic Z and possibly Early Cambrian based on presence of acritarchs in Cambridge. Acritarchs are diagnostic species that ranges in age from Proterozoic Z to Early Cambrian, but is most abundant in Proterozoic Z time (Lenk and others, 1982; [also see Goldsmith and others, 1982]). Age is also supported by the following: 1) plant fossils so numerous in strata of Narragansett basin are absent in the Boston Bay Group strata, 2) Late Ordovician and Early Silurian Quincy Granite contains argillite inclusions that are on strike with Cambridge Argillite, and 3) Boston Bay Group stratigraphy is primarily marine, not similar to terrestrial stratigraphy of Narragansett basin (Goldsmith, 1991).

Lithology: argillite; quartzite; sandstone; conglomerate

Scituate Granite Gneiss (Proterozoic Z) at surface, covers 1.0 % of this area

Scituate Granite Gneiss - Gneissic granite containing biotite in small clots. Equivalent to part of former Northbridge Granite Gneiss (usage now abandoned). Gradational with Zhg.

Lithology: granite

Orange-pink, rusty-weathering, medium- to coarse-grained biotite granite to granodiorite (Silurian) at surface, covers 2 % of this area

Orange-pink, rusty-weathering, medium- to coarse-grained biotite granite to granodiorite - Locally porphyritic. Intrudes Ssqd.

Lithology: granite; granodiorite

Tower Hill Quartzite (Silurian) at surface, covers 0.1 % of this area

Tower Hill Quartzite - Quartzite and phyllite.

Lithology: quartzite; phyllite

Dedham Granite (Proterozoic Z) at surface, covers 2 % of this area

Dedham Granite - Light grayish-pink to greenish-gray, equigranular to slightly porphyritic, variably altered, granite south and west of Boston. Includes dioritic rock near Scituate and Cohasset and Barefoot Hills Quartz Monzonite of Lyons (1969) and Lyons and Wolfe (1971). Intrudes Zdi, Zgb, Zb, Zv. Extensive calc-alkaline plutons separated by Boston basin have long been mapped as Dedham. Those to the north of Boston and studied in this report, are referred to as Dedham North. Crystallization ages for the Dedham North suite (based on titanites and zircons) have been determined at 607+/-4 Ma, while ages for the Lynn are slightly younger at 596+/-3 Ma. Both are clearly part of the Late Proterozoic magmatic event. Dates on two samples from Sheffield Heights indicate that the diorite and granite are part of the Dedham North suite. The Dedham south and west of Boston has been dated at 630+/15 Ma (Zartman and Naylor, 1984). Dedham North Granite has a compositionally highly variable suite ranging from leucogranites to granodiorites, tonalites, and quartz diorite. The granites originated by partial melting of a sedimentary protolith, while the intermediate members show a mixing of granitic magma and mafic magma (Hepburn and others, 1993).

Lithology: granite; diorite; quartz monzonite

Diorite (Proterozoic Z) at surface, covers 0.3 % of this area

Diorite - Medium-grained hornblende diorite metamorphosed in part to amphibolite and hornblende gneiss.

Lithology: diorite; amphibolite; mafic gneiss

Nashoba Formation (Ordovician or Proterozoic Z) at surface, covers 3 % of this area

Nashoba Formation - Boxford Member - Thin bedded to massive amphibolite, minor biotite gneiss. Of the 10 members of the Nashoba Formation defined by Bell and Alvord (1976), only amphibolitic Boxford Member, at the presumed base of Nashoba is separated out on MA State bedrock map of Zen and others (1983) because it is the only member clearly identified in several locations. Although age on MA State bedrock map is shown as Proterozoic Z or Ordovician (due to uncertainty regarding actual rocks sampled by Olszewski and a strong belief that rocks of Nashoba zone correlated with Ordovician rocks to the west), author now feels that rocks of Nashoba zone (except for Tadmuck Brook Schist) are all Proterozoic, but that they are unlike the Proterozoic rocks of neighboring Milford-Dedham zone. [No formal age change is made in this report.] (Goldsmith, 1991).

Lithology: amphibolite; biotite gneiss

Upper part of Rangeley Formation (Lower Silurian (Llandoverian)) at surface, covers < 0.1 % of this area

Upper part of Rangeley Formation - Rusty-weathering, pelitic schist, metasandstone, and local coarse-grained metasandstone lentils; calc-silicate pods common; minor coticule. Probably equivalent to member C of Rangeley Formation of Maine.

Lithology: pelitic schist; metasedimentary rock; calc-silicate rock

Peabody Granite (Middle Devonian) at surface, covers 0.4 % of this area

Peabody Granite - Alkalic granite containing ferro-hornblende. Intrudes Zgb, Zdngr.

Lithology: alkali-granite (alaskite)

Marlboro Formation (Ordovician, Cambrian, or Proterozoic Z) at surface, covers 2 % of this area

Marlboro Formation - Thinly layered amphibolite, biotite schist and gneiss, minor calc-silicate granofels and felsic granofels.

Lithology: amphibolite; mica schist; biotite gneiss; granofels

Milford granite (Proterozoic Z) at surface, covers 0.9 % of this area

Milford Granite - Mafic phase. Gray, seriate to sub-porphyritic granite to granodiorite, mafic minerals tend to be in clots; locally gneissic. Intrudes Zb. Occupies an area of about 100 sq km. Central mass near Milford is elliptical and is divided into and mapped as a light-colored phase and a dark-colored phase; dark-colored phase defines an irregular border for largest of light-colored plutons. Intrudes Blackstone Group rocks and Ponaganset Gneiss, but was deformed with them at some later unknown time. Isotopic age of 630 +/-15 Ma was determined using U-Pb methods on zircon by Zartman and Naylor (1984). Characterized by salmon-pink color, bluish quartz on weathered surfaces, and lineations defined by lenticular mosaics of quartz and oriented patches of biotite; texture contrast strongly with that of Dedham Granite (Wones and Goldsmith, 1991).

Lithology: granite; granodiorite

Granodiorite of the Indian Head pluton (Precambrian to Paleozoic) at surface, covers 1 % of this area

Granodiorite of the Indian Head pluton - Light-gray to pinkish-gray, fine- to medium-grained biotite granodiorite, and gray fine-grained hornblende-biotite tonalite. Intrudes OZm.

Lithology: granodiorite; tonalite

Straw Hollow Diorite and Assabet Quartz Diorite undifferentiated (Silurian) at surface, covers 0.9 % of this area

Straw Hollow Diorite and Assabet Quartz Diorite undifferentiated - Gray, medium-grained, slightly-foliated biotite-hornblende diorite and quartz diorite. Intrudes OZn.

Lithology: diorite; quartz diorite

Diorite and gabbro (Proterozoic Z) at surface, covers 5 % of this area

Diorite and gabbro - Complex of diorite and gabbro, subordinate metavolcanic rocks and intrusive granite and granodiorite.

Lithology: diorite; gabbro; metavolcanic rock; granite; granodiorite

Lynn Volcanic Complex (Lower Devonian, Silurian, or Proterozoic Z) at surface, covers 0.9 % of this area

Lynn Volcanic Complex - Rhyolite, agglomerate and tuff.

Lithology: rhyolite; volcanic breccia (agglomerate); tuff

Ayer Granite, Clinton facies (Lower Silurian) at surface, covers 0.5 % of this area

Ayer Granite - Clinton facies. Porphyritic biotite granite with a non-porphyritic border phase; intrudes Sb. 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).

Lithology: granite

Spaulding Tonalite (Early Devonian) at surface, covers < 0.1 % of this area

Spaulding Tonalite (Spaulding Quartz Diorite of Fowler-Billings, 1949) - Weakly foliated to nonfoliated, spotted biotite quartz diorite, tonalite, granodiorite, and granite; garnet and muscovite may or may not be present.

Lithology: quartz diorite; tonalite; granodiorite; granite

Milford granite (Proterozoic Z) at surface, covers 0.9 % of this area

Milford Granite - Light-gray to pale orange-pink biotite granite; biotite tends to be in clots or short streaks, quartz granular; locally gneissic. Intrudes Zb. Occupies an area of about 100 sq km. Central mass near Milford is elliptical and is divided into and mapped as a light-colored phase and a dark-colored phase; dark-colored phase defines an irregular border for largest of light-colored plutons. Intrudes Blackstone Group rocks and Ponaganset Gneiss, but was deformed with them at some later unknown time. Isotopic age of 630 +/-15 Ma was determined using U-Pb methods on zircon by Zartman and Naylor (1984). Characterized by salmon-pink color, bluish quartz on weathered surfaces, and lineations defined by lenticular mosaics of quartz and oriented patches of biotite; texture contrast strongly with that of Dedham Granite (Wones and Goldsmith, 1991).

Lithology: granite

Shawsheen Gneiss (Ordovician or Proterozoic Z) at surface, covers 2 % of this area

Shawsheen Gneiss - Sillimanite gneiss, sulfidic at base; minor amphibolite.

Lithology: gneiss; amphibolite

Mattapan Volcanic Complex (Proterozoic Z or younger) at surface, covers 0.7 % of this area

Mattapan Volcanic Complex - Rhyolite, melaphyre, agglomerate, and tuff. Mattapan Volcanic Complex is found in west and southwest part of Boston basin and beyond, and to the south in Blue Hills. Similar in lithology to Lynn Volcanic Complex. Both units consist largely of partly porphyritic rhyolite and rhyodacite flows, welded ash-flow tuffs, vitric tuff, lapilli tuff, lithic tuff, flow breccias, breccia pipes, and extrusion domes. Mattapan's rhyolite and rhyodacites are thinner and less varied in composition and texture than Lynn's, and volcanic breccias are absent in Lynn. Both units are reported to lie nonconformably on Dedham Granite and unnamed plutonic-volcanic complex of eastern MA; however, Mattapan has been observed as dikes and stocks cutting Dedham Granite (Billings, 1976; Kaye and Zartman, 1980; Chute, 1966), and other workers have pointed out evidence that some of Mattapan may be penecontemporaneous with younger phases of Dedham batholith. Westwood Granite may be intrusive equivalent of Mattapan. Mattapan is conformably and fairly continuously overlain by Roxbury Conglomerate of Boston Bay Group. LaForge (19832) cautioned against identifying rocks interbedded within Mattapan that look like Roxbury, but are still part of Mattapan. Metavolcanic rocks in Blue Hills resembling Mattapan are assigned to Mattapan on State bedrock map (Zen and others, 1983); for years they were considered to be either Silurian and Devonian, or Carboniferous; however, they are chemically and mineralogically distinct from Ordovician and Silurian Blue Hills [sic] Granite Porphyry. Author follows usage of Chute (1966) who could find no difference between volcanic rocks in Blue Hills area and Mattapan rocks, and thus these rocks are assigned to Mattapan. Proterozoic Z age is based on U-Th-Pb zircon date of 602 +/-3 Ma (Zartman, in Kaye and Zartman, 1980). Although Billings (1979) questions reliability of zircon ages from volcanic rocks, discovery of Proterozoic Z acritarchs in overlying Cambridge Argillite indicates zircon age is appropriate (Goldsmith, 1991).

Lithology: rhyolite; mafic volcanic rock; volcanic breccia (agglomerate); tuff

Gabbro (Proterozoic Z) at surface, covers 0.4 % of this area

Gabbro - Hornblende gabbro and hornblende-pyroxene gabbro metamorphosed in part to hornblende gneiss and amphibolite.

Lithology: gabbro; mafic gneiss; amphibolite

Massive to weakly foliated, pink and gray, fine- to medium-grained biotite granite (Pennsylvanian) at surface, covers 1 % of this area

Massive to weakly foliated, pink and gray, fine- to medium-grained biotite granite - In the Townsend area; commonly contains pink magnetite-bearing pegmatite identical to granite of Milford, New Hampshire; intrudes OZma and Sp.

Lithology: granite; pegmatite

Roxbury Conglomerate (Proterozoic Z to earliest Paleozoic) at surface, covers 0.5 % of this area

Roxbury Conglomerate - Melaphyre in the Roxbury Conglomerate. Brighton Melaphyre lies within Brookline Member and consists of mafic volcanic rocks (quartz keratophyre, keratophyre, and spilite). Roxbury clearly lies nonconformably on Dedham Granite near Hull, MA; can be traced continuously over Mattapan Volcanic Complex. Age is Proterozoic Z and possibly Early Cambrian (Goldsmith, 1991).

Lithology: mafic volcanic rock

Littleton Formation (Lower Devonian) at surface, covers 2 % of this area

Littleton Formation - Black to gray aluminous mica schist, quartzose schist, and aluminous phyllite.

Lithology: schist; phyllite

Massabesic Gneiss Complex (Ordovician and Proterozoic Z) at surface, covers 0.3 % of this area

Massabesic Gneiss Complex - Biotite-feldspar paragneiss of Proterozoic Z age intruded by potassium-feldspar-rich gneiss of Ordovician age.

Lithology: paragneiss; gneiss

Paxton Formation (Silurian) at surface, covers 1 % of this area

Paxton Formation - Undifferentiated biotite granofels, calc-silicate granofels, and sulfidic schist. The Paxton, here of group rank, includes strata formerly mapped in CT as the Hebron Formation and in MA as the Paxton Formation. It conformably overlies the Oakdale Formation and structurally and conformably underlies the Brimfield Group. It is undivided in central MA; in northeast CT and adjacent MA it is divided into the Dudley and Southbridge Formations. Age is Late Proterozoic(?) based on the intrusion of 440 m.y. Hedgehog Hill gneiss into the overlying Brimfield Group and an age of 1188 m.y. for detrital zircons from the Paxton (Pease, 1989).

Lithology: granofels; schist

Hope Valley Alaskite Gneiss (Proterozoic Z) at surface, covers 0.9 % of this area

Hope Valley Alaskite Gneiss - Mafic-poor gneissic granite, locally muscovitic. Gradational with Zsg. Late Proterozoic Hope Valley Alaskite Gneiss occurs as one of several plutonic rocks in Milford antiform. Forms tabular masses along west side of Rhode Island anticlinorium from southern RI and eastern CT to northwestern RI; flanks west side of Milford anticlinorium and terminates at north end of anticlinorium in MA. Color is light pink to tan. Intrudes Plainfield Formation in CT and Blackstone Group rocks in RI. Isotopic age of 630 Ma by U/Pb methods on zircon is reported by Zartman and Naylor (1984) from a sample in MA. Age of 601 +/-5 Ma by U/Pb methods on zircon is reported by Hermes and Zartman (1985) from a sample in RI (Wones and Goldsmith, 1991).

Lithology: alkali-granite (alaskite)

Westboro Formation (Proterozoic Z) at surface, covers 2 % of this area

Westboro Formation - Quartzite, schist, calc-silicate quartzite, and amphibolite. Consists of quartzite and argillite in Saugus and Lynnfield areas. Westboro Formation consists primarily of orthoquartzite and subordinate mica schist, calc-silicate rock, amphibolite, and quartzitic biotite gneiss and schist. Westboro as portrayed by Nelson (1974), Bell and Alvord (1976), and Hepburn and DiNitto (1978) are correlative [with varying certainty]. West and south of Boston, disconnected masses of quartzite and associated rocks are shown on State bedrock map by Zen and others (1983) as Westboro although not continuous with belts mapped by Nelson (1974) or Hepburn and DiNitto (1978). Includes isolated quartzite masses mapped by Castle (1964) in Reading area. On State bedrock map, arbitrarily includes thin quartzite mapped as Burlington Formation by Bell and Alvord (1976) because the units are similar and to reduce number of small units on State map. Rocks mapped as Rice Gneiss by Nelson (1974) were included in unnamed metamorphosed mafic and felsic volcanic unit on State bedrock map, but author now feels it should be either a part of Westboro, or a separate unit below it. Westboro in Framingham area and to the northeast is overlain by unnamed assemblage of metamorphosed mafic and felsic volcanic rocks. South of town of Westborough, Westboro is truncated by Bloody Bluff-Lake Char fault system. Intruded by Proterozoic Z batholithic rocks. Equivalent to Plainfield Formation of eastern CT because it lies in same strike belt, and is probably equivalent to Quinnville Quartzite and unnamed mica schist and phyllite of Blackstone Group. Contact between Westboro and Blackstone is arbitrary on State bedrock map on basis of proximity of isolated exposures of the two units to their respective type areas. No area of continuous exposure exists between Westboro and Blackstone (Goldsmith, 1991).

Lithology: quartzite; schist; amphibolite; argillite

Biotite granite (Proterozoic Z) at surface, covers 5 % of this area

Biotite granite - Light-gray to grayish-pink, biotite granite, locally foliated. Mafic minerals less prominent than in Milford Granite but granular quartz common. Includes mafic-poor granite similar to Zhg. Intrudes Zdi, Agb, and Zv.

Lithology: granite

Ayer Granite (Lower Silurian) at surface, covers 1 % of this area

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).

Lithology: granite; tonalite