Geologic units in Providence county, Rhode Island

Esmond Igneous Suite - augen granite gneiss (Late Proterozoic) at surface, covers 20 % of this area

Pale- to dark-gray, medium- to coarse-grained, granite gneiss characterized by large feldspar porphyroclasts (> 1 cm). Composition ranges from tonalite to granodiorite to quartz monzonite. Composed of sodic plagioclase, quartz, microcline/orthoclase, biotite, opaque minerals; hornblende and epidote common in more mafic types; accessory apatite, sphene, allanite, and zircon; secondary chlorite. Typically massive, but with strong penetrative foliation and lineation defined by major minerals. Commonly contains structurally conformable layers of amphibolite. Includes most rock mapped formerly as Ponaganset Granite Gneiss and Ten Rod Granite Gneiss.

Narragansett Bay Group - Rhode Island Formation (Pennsylvanian) at surface, covers 14 % of this area

In northern Rhode Island, consists of gray to black, fine- to coarse-grained quartz arenite, litharenite, shale, and conglomerate, with minor beds of anthracite and meta-anthracite. In southern Rhode Island, consists of meta-sandstone, meta-conglomerate, schist, carbonaceous schist, and graphite. Plant fossils are common.

Scituate Igneous Suite - granite (Devonian) at surface, covers 12 % of this area

Gray to pink, coarse-grained, porphyritic to subporphyritic subsolvus granite (phenocrysts of perthite, microcline, and plagioclase). Contains microcline, perthite, plagioclase, quartz, and accessory biotite, hornblende, allanite, zircon, apatite, sphene, fluorite, and opaque minerals; minor garnet is present in some varieties; some secondary muscovite, calcite, and epidote. Ovoid clots of mafic and accessory minerals are locally prominent. Some rock is massive, but most is commonly lineated and foliated, especially in the west and south near the Hope Valley shear zone. Includes most rock mapped formerly as Scituate Granite Gneiss, and some rock mapped as Hope Valley Alaskite Gneiss.

Esmond Igneous Suite - granite (Late Proterozoic) at surface, covers 9 % of this area

Gray, tan, greenish, or pale-pink, medium- to coarse-grained, mainly equigranular rock. Contains microcline, perthite, plagioclase, quartz, and accessory biotite, epidote, zircon, allanite, monazite, apatite, sphene, and opaque minerals; secondary muscovite, chlorite, and calcite. Mainly massive, but locally foliated and lineated. Includes rock mapped formerly as Esmond Granite.

Harmony Group - Absalona Formation (Late Proterozoic? or older?) at surface, covers 8 % of this area

Gray, medium- to coarse-grained biotite granite gneiss characterized by alkali feldspar porphyroblasts. Consists of biotite, hornblende, quartz, microcline, microperthite, albite, epidote, garnet, and chlorite. Subordinate amounts of quartz-biotite schist, amphibolite, and quartzite occur as layers and isolated bodies.

Scituate Igneous Suite - alkali-feldspar granite (Devonian) at surface, covers 5 % of this area

Gray to pink, medium- to coarse-grained, porphyritic to subporphyritic alkali-feldspar granite (phenocrysts of perthite). Contains perthite, quartz, and accessory biotite, hornblende, and lesser riebeckite, aegerine, zircon, allanite, sphene, fluorite, and opaque minerals. Plagioclase is sparse to absent. Secondary muscovite, chlorite, and calcite. Ovoid clots of mafic and accessory minerals are locally prominent. Rock is a hypersolvus, mildly peralkaline granite. Fabric ranges from massive to locally lineated and foliated. Includes rock mapped formerly in part as Scituate Granite Gneiss, Cowesett Granite, and perthitic Cowesett Granite.

Blackstone Group - undifferentiated rock (Late Proterozoic? or older?) at surface, covers 4 % of this area

Consists of associations of the above rock types. Locally cut by granitic rocks of the Esmond Igneous Suite, resulting in migmatitic appearance. Includes rock mapped formerly as undifferentiated Blackstone Series and migmatite.

Esmond Igneous Suite - granite gneiss (Late Proterozoic) at surface, covers 4 % of this area

Pale-gray to pale-pink, fine- to medium-grained granite gneiss, rarely with porphyroclasts of microcline/orthoclase. Composition ranges from quartz monzonite to granite. Composed of sodic plagioclase, quartz, microcline, biotite, opaque minerals; minor muscovite common, garnet more rare; accessory apatite and zircon; sphene and hornblende present in some rocks; secondary chlorite. Typically massive, but with strong penetrative foliation and lineation defined by major minerals. Commonly associated with, and in part gradational into, augen gneiss. Includes rock mapped formerly in part as Ponaganset Granite Gneiss, Scituate Granite Gneiss, Ten Rod Granite Gneiss, and Hope Valley Alaskite Gneiss.

Blackstone Group - epidote and biotite schist (Late Proterozoic? or older?) at surface, covers 4 % of this area

Green to gray, fine-grained, massive to thinly-bedded mica schist, quartzite, and marble. Schist consists of quartz plus chlorite, muscovite, and/or biotite. Includes rock mapped formerly as Sneech Pond Schist, Mussey Brook Schist, and marble.

Esmond Igneous Suite - mafic/intermediate rock (Late Proterozoic) at surface, covers 3 % of this area

Dark-gray, purple, or black, medium- to coarse-grained rock that may contain plagioclase, quartz, clinopyroxene, hornblende, olivine, and accessory biotite, epidote, sphene, zircon, apatite, and opaque minerals; secondary chlorite, sericite, and saussurite. Massive to variably foliated. Composition includes tonalite, quartz diorite, diorite, and gabbro. Includes rock mapped formerly as quartz diorite.

Other metasedimentary rocks - metaclastic rock, undivided (Pennsylvanian? or Late Proterozoic?) at surface, covers 3 % of this area

Red to gray to green polymict conglomerate, sandstone, and shale. Variably metamorphosed. Includes rock mapped formerly as Bellingham Conglomerate.

Blackstone Group - quartzite (Late Proterozoic? or older?) at surface, covers 2 % of this area

Gray, medium-grained, massive to thinly bedded quartzite; commonly tectonically interleaved with other units of the Blackstone Group. Interpreted as olistostromal deposits. Includes rock mapped formerly as Quinnville Quartzite.

Blackstone Group - greenstone, amphibolite, serpentinite (Late Proterozoic? or older?) at surface, covers 2 % of this area

Dark-green, massive to foliated metamorphosed gabbroic and basaltic rock interlayered with epiclastic rock; local pillow-structures preserved. Consists primarily of epidote, actinolite, chlorite, and plagioclase. Includes rock mapped formerly as Hunting Hill Greenstone.

Harmony Group - Woonasquatucket Formation (Late Proterozoic? or older?) at surface, covers 2 % of this area

Gray, medium-grained felsic gneiss, composed of microcline-microperthite, quartz, albite, muscovite, and biotite. Commonly phyllonitic.

Sterling Igneous Suite - granite gneiss (Late Proterozoic) at surface, covers 2 % of this area

Pale pink to gray, medium-grained granite gneiss, commonly with small porphyroclasts of microcline/orthoclase. Similar to alaskite gneiss, but with more than 3% dark minerals. Composition is granite with generally less than 3% dark minerals. Compositions range from quartz monzonite to granite. Composed of sodic plagioclase, quartz, microcline, biotite, opaque minerals; minor muscovite common, garnet more rare; accessory apatite and zircon; sphene present in some rocks; secondary chlorite. Typically is compositionally homogeneous, with strong foliation and locally well-developed lineation defined by major minerals. Includes some rock mapped formerly as Ten Rod Granite Gneiss, Hope Valley Alaskite Gneiss, Potter Hill Granite Gneiss, and Scituate Granite Gneiss.

Esmond Igneous Suite - granodiorite (Late Proterozoic) at surface, covers 1 % of this area

Gray, tan, greenish, or pale-pink, medium- to coarse-grained, mainly porphyritic rock (phenocrysts of microcline). Contains microcline, perthite, plagioclase, quartz, and accessory biotite, epidote, zircon, allanite, monazite, apatite, sphene, and opaque minerals; secondary muscovite, chlorite, and calcite. Mainly massive. Includes rock mapped formerly as Grant Mills Granodiorite.

Esmond Igneous Suite - fine-grained granite (Late Proterozoic) at surface, covers 0.6 % of this area

Gray to pale-pink, fine-grained, equigranular rock, containing microcline, perthite, plagioclase, quartz, and accessory biotite, epidote, sphene, zircon, monazite, apatite, and opaque minerals. Secondary muscovite, chlorite, and calcite. Mainly massive. Includes rock mapped formerly as fine-grained Esmond Granite.

Scituate Igneous Suite - fine-grained granite (Devonian) at surface, covers 0.6 % of this area

Gray to pink, fine-grained, equigranular to locally porphyritic granite (phenocrysts of perthite and plagioclase). Contains microcline, perthite, plagioclase, quartz, and accessory biotite, zircon, allanite, sphene, monazite, fluorite, and opaque minerals; minor garnet is present in some varieties. Partly massive, but locally lineated and foliated. Includes rock mapped formerly as fine-grained Scituate Granite Gneiss, and some rock mapped as Hope Valley Alaskite Gneiss.

Sterling Igneous Suite - alaskite gneiss (Late Proterozoic) at surface, covers 0.5 % of this area

Pale pink, orange, or gray, fine- to medium-grained granite gneiss, rarely with porphyroclasts of microcline/orthoclase. Composition is granite with generally less than 3% dark minerals. Composed of sodic plagioclase, quartz, microcline/orthoclase, minor biotite, and opaque minerals; minor muscovite (in part secondary), and rare garnet and sphene in some rocks; accessory apatite and zircon; secondary chlorite. Varies from massive to layered. Strongly foliated and locally well lineated. Includes most rock mapped formerly as Hope Valley Alaskite Gneiss.

Narragansett Bay Group - Wamsutta Formation (Pennsylvanian) at surface, covers 0.4 % of this area

Red sandstone, shale, and conglomerate, locally containing abundant volcanic detritus as clasts and matrix. Plant fossil localities occur in adjacent Massachusetts. Minor, but significant amounts of interstratified bimodal-composition volcanic lava flows consisting of alkalic basalt (locally pillowed) and rhyolite are present in adjacent Massachusetts.

alkali-feldspar granite of Cumberland (Devonian? or Mississippian?) at surface, covers 0.3 % of this area

Gray to blue-gray. medium- to coarse-grained, equigranular to porphyritic alkali-feldspar granite (phenocrysts of perthite). Massive to locally foliated. Contains perthite, quartz, abundant accessory riebeckite and aegerine, and lesser astrophyllite, anegmatite, zircon, allanite, fluorite, and opaque minerals. Plagioclase is notably absent. Hypersolvus, strongly peralkaline granite. Includes rock mapped formerly as Rhode Island Quincy Granite.

Esmond Igneous Suite - felsic volcaniclastic rock (Late Proterozoic) at surface, covers 0.3 % of this area

Light-colored to dark-gray ash flow tuff, ignimbrite, and felsite; may include some shallow-level intrusive rock. Phenocrysts of quartz, orthoclase, and plagioclase in a fine-grained matrix of quartz, feldspar, biotite, and opaque minerals. Some varieties contain fiamme and lithic fragments. Includes rock mapped formerly as Pennsylvanian felsite at Diamond Hill.

Scituate Igneous Suite - diorite/gabbro (Devonian) at surface, covers 0.2 % of this area

Fine- to medium-grained, equigranular, massive to slightly foliated rock, composed of plagioclase, hornblende, clinopyroxene, and quartz, with accessory biotite, sphene, apatite, and opaque minerals; secondary chlorite, epidote, and saussurite. Varieties include quartz diorite, diorite, and gabbro.

gabbro/diorite (Late Proterozoic? to Devonian?) at surface, covers 0.2 % of this area

Dark-gray to black, fine- to medium-grained, equigranular, faintly foliated rock, composed of plagioclase, clinopyroxene, hornblende, biotite, and accessory sphene, apatite, and opaque minerals. Includes quartz diorite, diorite, and gabbro.

Rhode Island Formation (Upper and Middle Pennsylvanian) at surface, covers 0.2 % of this area

Sandstone, graywacke, shale and conglomerate; minor beds of meta-anthracite. Fossil plants. Rhode Island Formation is thickest and most extensive formation in Narragansett basin. Does not extend to Norfolk basin. Consists of gray sandstone and siltstone and lesser amounts of gray to black shale, gray conglomerate, and coal beds 10 m thick. Interfingers with Wamsutta Formation in Narragansett basin. In places overlies Dedham Granite. Age is Middle and Late Pennsylvanian (Goldsmith, 1991).

Plainfield Formation (Late Proterozoic? or older?) at surface, covers < 0.1 % of this area

Pale-tan to gray, fine-grained quartzite, and pale- to medium-gray quartz-mica schist. Quartzite is massive to layered (bedded?). Includes Plainfield Formation and some rock mapped formerly as Blackstone Series.

Harmony Group - Nipsachuck Formation (Late Proterozoic? or older?) at surface, covers < 0.1 % of this area

Gray to tan, fine- to medium-grained granite gneiss, characterized by biotite streaks. Consists of biotite, muscovite, quartz, microperthite, and sodic plagioclase.

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

Quartzite, schist, phyllite, marble, and metavolcanic rocks.

Ponaganset Gneiss (Proterozoic Z?) at surface, covers < 0.1 % of this area

Dark-gray, coarse-grained, porphyritic, well-foliated gneiss, composed of oligoclase, quartz, microcline (mostly as megacrysts up to 8 cm long), biotite, magnetite, and generally hornblende; also garnet and muscovite where hornblende is absent. Sterling Plutonic Suite is here restricted to the Hope Valley terrane. (The Hope Valley together with the Esmond-Dedham terrane make up the Avalon superterrane of this report.) The Ponaganset Gneiss and the Ten Rod Granite Gneiss lie within the Esmond-Dedham terrane and are therefore removed from the Sterling. Ponaganset lies east of the Hope Valley shear zone and extends from RI into MA. [Apparently does not occur in CT.] Predominantly granite, ranges to tonalite. Gray to light gray, some pink, generally medium- to coarse-grained, but ranges from fine-grained to porphyritic. Alkali K-feldspar, plagioclase, quartz, and biotite are major minerals. The Ponaganset is interpreted as an intensely deformed phase of the Esmond Plutonic Suite. Strong lineation is defined by elongate aggregates of quartz and feldspar and trains of biotite and hornblende. Age is Late Proterozoic based on correlation with Northbridge Gneiss (MA), which has been dated at 557+/-4 Ma by Zartman and Naylor (1984) (Skehan and Rast, 1990).

diabase (Triassic?) at surface, covers < 0.1 % of this area

Greenish-gray to gray-black, fine-grained, commonly porphyritic, generally massive dike rock composed of plagioclase, augite, opaque minerals, olivine pseudomorphed by serpentine and chlorite minerals, and sparse quartz.

Scituate Granite Gneiss (Proterozoic Z) at surface, covers < 0.1 % of this area

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

Narragansett Bay Group - Pondville Conglomerate (Pennsylvanian) at surface, covers < 0.1 % of this area

Gray to greenish conglomerate with sand-sized matrix, interbedded with quartz arenite and litharenite; typically lenticular and discontinuous. At type locality (Pondville Station, Massachusetts), unit consists of interbedded red and green slate, siltstone, arkose, and quartzite-pebble conglomerate.

Wamsutta Formation (Middle and Lower Pennsylvanian) at surface, covers < 0.1 % of this area

Red to pink, well-sorted conglomerate, graywacke, sandstone, and shale; fossil plants. Wamsutta Formation occurs in Narragansett and Norfolk basins. Consists of conglomerate, lithic graywacke, sandstone, and shale. Also contains rhyolite and basalt horizons near Attleboro. Northwest of Attleboro, Wamsutta overlies Diamond Hill Felsite as used by Skehan and Murray (in Skehan and others, 1979). Volcanic rocks similar to Diamond Hill Felsite crop out west of Lake Pearl, between Franklin and Wrentham, on west flank of Norfolk basin. These are shown within Wamsutta Formation on MA State bedrock map of Zen and others (1983) because of their proximity to Diamond Hill. They also resemble Proterozoic Z Mattapan Volcanic Complex. Chute (1966) described lenses of carbonate rock in red and green shale in Wamsutta in Norwood quad. Limestone also observed in rocks mapped as Wamsutta adjacent to exposed Dedham Granite at Manchester Pond Reservoir (J.P. Schafer, 1982, oral commun.). Red and green shales may actually be Cambrian. Upper member of Pondville Conglomerate grades into and interfingers with Wamsutta; in turn, Wamsutta interfingers with Rhode Island Formation in northwest part of Narragansett basin. Nonconformably overlies Dedham Granite. Partly equivalent to Rhode Island Formation. Age is Early and Middle Pennsylvanian. Contains a few plant fossils (Goldsmith, 1991).

vein quartz (Triassic?) at surface, covers < 0.1 % of this area

En echelon and crosscutting veins (millimeter to meter widths) of massive quartz; crystal terminations common in vugs and open cavities. Contains local concentrations of cryptocrystalline jasper and quartz, and irregular distributions of hematite and goethite, some in botryoidal masses. Locally present are abundant rectangular crystals of barite (up to 2 cm), or hollow molds after dissolved barite. Age uncertain, but rocks lack deformation features present in nearby Pennsylvanian rocks of the Narragansett Basin; emplacement most likely postdates Alleghanian deformation

Ponaganset Gneiss (Proterozoic Z) at surface, covers < 0.1 % of this area

Gneissic biotite granite containing megacrysts of microcline; biotite in coarse streaks and patches. Equivalent to part of former Northbridge Granite Gneiss (usage now abandoned). Sterling Plutonic Suite is here restricted to the Hope Valley terrane. (The Hope Valley together with the Esmond-Dedham terrane make up the Avalon superterrane of this report.) The Ponaganset Gneiss and the Ten Rod Granite Gneiss lie within the Esmond-Dedham terrane and are therefore removed from the Sterling. Ponaganset lies east of the Hope Valley shear zone and extends from RI into MA. [Apparently does not occur in CT.] Predominantly granite, ranges to tonalite. Gray to light gray, some pink, generally medium- to coarse-grained, but ranges from fine-grained to porphyritic. Alkali K-feldspar, plagioclase, quartz, and biotite are major minerals. The Ponaganset is interpreted as an intensely deformed phase of the Esmond Plutonic Suite. Strong lineation is defined by elongate aggregates of quartz and feldspar and trains of biotite and hornblende. Age is Late Proterozoic based on correlation with Northbridge Gneiss (MA), which has been dated at 557+/-4 Ma by Zartman and Naylor (1984) (Skehan and Rast, 1990).

cumberlandite (Late Proterozoic? to Devonian?) at surface, covers < 0.1 % of this area

The designated state rock of Rhode Island. Black, medium-grained, porphyritic melatroctolite (phenocrysts of plagioclase), composed of magnetite, ilmenite, olivine, plagioclase, and accessory hercynite spinel; secondary chlorite and saussurite. Abundance of magnetite and ilmenite (up to 70% of rock) results in characteristic high specific gravity. The rock is massive, but locally exhibits a flow lamination caused by the planar orientation of plagioclase laths.

Esmond Granite (Proterozoic Z) at surface, covers < 0.1 % of this area

Light-gray to pink biotite granite, generally slightly altered, locally foliated. Gradational with Zgmgd. Intrudes Zsg. Esmond Granite occupies nearly 100 sq km in RI and forms an elongate pluton southeast of Woonsocket, RI, of which only the northern part is in MA. Occurs in Dedham batholith. Consists of mottled red and green, mainly massive granite. Coarse-grained facies intrudes Blackstone Group rocks and a related tonalite; fine-grained facies intrudes the tonalite and coarse-grained facies (Hermes and Zartman, 1985). Contact relations with Milford Granite not known, although phases of Milford resemble Esmond. Age of 621 +/-8 Ma (U/Pb methods on zircon by Hermes and Zartman, 1985) is close to age of Milford (Wones and Goldsmith, 1991).

Scituate Igneous Suite - monzonite/monzodiorite (Devonian) at surface, covers < 0.1 % of this area

Dark-gray to pink, fine- to medium-grained, equigranular rock, containing plagioclase, microcline, quartz, and accessory biotite, garnet, zircon, apatite, allanite, and opaque minerals; secondary muscovite and epidote. Mainly foliated. Varieties include quartz monzonite, monzonite, and monzodiorite. Includes some rock mapped formerly as Cowesett Granite.

Bellingham Conglomerate (Pennsylvanian, Cambrian or Proterozoic Z) at surface, covers < 0.1 % of this area

Red and gray metamorphosed conglomerate, sandstone, graywacke, and shale. Bellingham Conglomerate consists of conglomerate and lithic graywacke interbedded with chlorite phyllite and is confined to Bellingham basin. Also contains some volcanic rocks (rhyolite porphyry in roadcut on MA Hwy 146 at Premisy Hill west of Woonsocket, and felsite porphyry in the Franklin area east of Bellingham). Conglomerate contains pebbles of quartzite from adjacent Blackstone Group rocks and typical blue quartz of Milford Granite, so sediments are locally derived. Exposures on east side of Woonsocket Hill, southeast of Woonsocket, RI, show cliffs of steeply dipping, thin-bedded, white to gray quartzite of Blackstone Group standing above green schistose conglomerate containing many flattened white to gray quartzite pebbles and interbedded green calcareous quartz schist. Contact is probably a fault, but source of pebbles is quite obvious. In the same area, schist of Blackstone Group is difficult to distinguish from those of the Bellingham because of low-grade metamorphism of Blackstone rocks; it is probable that some of the low-grade Blackstone Rocks mapped northwest of Woonsocket in Blackstone River valley are part of Bellingham. The two rock units have been traditionally distinguished in the past by presence or absence of epidote (Warren and Powers, 1914), but this needs further study. Age is uncertain. Rocks have customarily been correlated with those of Pennsylvanian Narragansett basin; however, rocks in some exposures, such as the one at River St and Blackstone St in Woonsocket resemble Proterozoic Z Roxbury Conglomerate in Boston basin. Skehan and others (1979) suggest that Bellingham may have a similar age to that of Roxbury. This is supported by observation that Bellingham is a structural trough extending southwest from Boston basin and separating primarily Proterozoic Z granitoids from altered, but nongneissic, Proterozoic granitoids (Wones and Goldsmith, 1991). In deference to tradition, and because Proterozoic Z age has not been proven, age is shown on MA State bedrock map of Zen and others as Proterozoic Z to Pennsylvanian [map actually has age of Proterozoic Z, Cambrian, or Pennsylvanian, which differs from age stated in this report.] (Goldsmith, 1991).

Diorite (Proterozoic Z) at surface, covers < 0.1 % of this area

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

Wamsutta Formation (Middle and Lower Pennsylvanian) at surface, covers < 0.1 % of this area

Rhyolite and mafic volcanic rocks in the Wamsutta Formation. Wamsutta Formation occurs in Narragansett and Norfolk basins. Consists of conglomerate, lithic graywacke, sandstone, and shale. Also contains rhyolite and basalt horizons near Attleboro. Northwest of Attleboro, Wamsutta overlies Diamond Hill Felsite as used by Skehan and Murray (in Skehan and others, 1979). Volcanic rocks similar to Diamond Hill Felsite crop out west of Lake Pearl, between Franklin and Wrentham, on west flank of Norfolk basin. These are shown within Wamsutta Formation on MA State bedrock map of Zen and others (1983) because of their proximity to Diamond Hill. They also resemble Proterozoic Z Mattapan Volcanic Complex. Chute (1966) described lenses of carbonate rock in red and green shale in Wamsutta in Norwood quad. Limestone also observed in rocks mapped as Wamsutta adjacent to exposed Dedham Granite at Manchester Pond Reservoir (J.P. Schafer, 1982, oral commun.). Red and green shales may actually be Cambrian. Upper member of Pondville Conglomerate grades into and interfingers with Wamsutta; in turn, Wamsutta interfingers with Rhode Island Formation in northwest part of Narragansett basin. Nonconformably overlies Dedham Granite. Partly equivalent to Rhode Island Formation. Age is Early and Middle Pennsylvanian. Contains a few plant fossils (Goldsmith, 1991).

Hope Valley Alaskite Gneiss (Proterozoic Z?) at surface, covers < 0.1 % of this area

Light-pink to gray, medium- to coarse-grained, locally porphyritic, variably lineated and foliated alaskitic gneiss, composed of microcline, quartz, albite or oligoclase, and minor magnetite, and locally biotite and muscovite. Lineation formed by rods of quartz. Locally contains quartz-sillimanite nodules.

Dedham Granite (Proterozoic Z) at surface, covers < 0.1 % of this area

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).

Hoppin Formation (Cambrian) at surface, covers < 0.1 % of this area

Quartz arenite, argillite, and minor limestone; contains fossils of Lower Cambrian age.

Hope Valley Alaskite Gneiss (Proterozoic Z) at surface, covers < 0.1 % of this area

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).