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.
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.
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.
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.
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.
Pink to gray, fine-grained, locally porphyritic felsite (phenocrysts of quartz and Carlsbad-twinned orthoclase). Includes some hypabyssal intrusive rocks (feeder dikes and sills), and some probable lava flows. Contains accessory biotite, opaque minerals, zircon, monzonite, fluorite, sphene, apatite, and epidote. Quartz phenocrysts commonly are embayed and partly resorbed; matrix is fine-grained intergrowth of quartz and feldspar, probably formed from devitrified glass. Includes some rock mapped formerly as Spencer Hill Volcanics and Maskerchugg Granite.
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.
Dark-gray, pink, greenish, and purplish tuff and epiclastic rock (sandstone, breccia, conglomerate) containing abundant volcanic detritus. Tuff may include ignimbrite and ash-flow tuff. Fiamme common in some rock mapped formerly as Spencer Hill Volcanics.
Dark-gray to black, fine- to medium-grained, massive to faintly foliated rock, containing plagioclase, microcline, quartz, and accessory hornblende, biotite, sphene, apatite, and opaque minerals. Includes some rock previously mapped as Cowesett Granite.
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.
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.
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.
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.
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.
Light-pink to gray., medium- to coarse-grained, generally porphyritic, well-lineated and locally foliated granitic gneiss, composed of microcline, quartz, albite or orthoclase, biotite, hornblende, and magnetite. Megacrysts of microcline up to 3 cm long; lineation formed by splotches of biotite or by rods or quartz.
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.
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.
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).
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.
Gray medium-grained biotite gneiss, commonly with porphyroclasts of microcline/orthoclase, and amphibolite gneiss. Compositions range from quartz diorite to granodiorite. Composed of plagioclase, quartz, microcline/orthoclase, biotite, opaque minerals (commonly as conspicuous magnetite octahedra); accessory apatite, zircon, sphene; secondary chlorite. Varies from massive to layered, with strong penetrative foliation and lineation defined by major minerals. Includes some rock mapped formerly as Blackstone Series in southwestern and westernmost Rhode Island.
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.
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.
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.
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.