The Robert's Arm Group is a Middle Ordovician.fault-bounded, spilitized. bimodal volcanic to shallow plutonic assemblage within the northern Dunnage tectonostratigraphic zone. On the north it is nonconformably overlain by Silurian Springdale Group red beds, and both are separated from the Lower Ordovician Lushs Bight ophiolitic terrane by the Lobster Cove Fault. The group itself is complexly faulted and comprises an older terrane of tholeiitic to alkaline basalt slices overlain by one of more calc-alkaline slices. The tholeiitic basalts are interleaved with fine elastics of Cresent Lake Formation and with quartz-phyric felsites accompanied by substantial pyroclastics, whereas the calc-alkaline terrane includes quartz-amygdaloidal felsites with less common pyroclastics, concentrated in major centres that locally grade into granite. Erratic lithium enrichment of the basalts is spatially related to the basal fault contact of the group but spreads upward where deformation of underlying shales is strongest and disruption of the overlying basalts is greatest. The Robert's Arm Group evolved at the outer edge of a back arc basin beginning with deep water eruption of tholeiitic basalts before abduction of ophiolite in western Newfoundland. Subsequent more calc-alkaline volcanism occurred in neritic to littoral environments likely about the time of continental collision. After southeastward thrusting of Robert's Arm Group and deposition of Springdale Group, the dips of both were steepened by Devonian tectonism. Oroclinal bending of the basalts accompanied late right lateral movements on the Lobster Cove Fault. Alkaline dykes of presumed Mesozoic age were emplaced during opening of the modern Atlantic Ocean.

The western edge of Rae Province, prior to indentation of Slave Province, is conceived as a compressional tectonic margin in which Archean plutonic rocks were intruded by syntectonic granites of 2.4–2.3 Ga age as a result of eastward subduction. Subsequently this margin was intruded by the 2.0–1.90 Ga granites that characterize the Taltson magmatic zone. The latter granites engulf remnants of a widespread supracrustal assemblage of lower granulite facies metamorphic grade, the age of which has heretofore been unknown. We use U–Pb zircon and monazite geochronology to limit the age of cessation of deposition of these metasediments in a pre-Taltson granite basin to between 2.13 and 2.09 Ga.Similarities in geochronology and isotope geochemistry between western Rae Province and Buffalo Head domain, together with the presence of mafic to ultramafic rocks both within the basin and along the western Rae margin, suggest that basin formation was by rifting. Influx of 2.15 Ga detrital zircons probably from the west, and high-grade metamorphism accompanying basin closure at 2.09 Ga, suggest an eastward (inward) movement of magmatism at that time. A second similar eastward migration of magmatism occurred in association with the Slave–Churchill collision (2.0–1.9 Ga). These relations suggest a complex record of crustal accretion within Buffalo Head and Chinchaga domains, the details of which remain to be established.

The Strait of Belle Isle area comprises three principal geological terranes and each is described separately. Precambrian terrane. The oldest rocks in the area are widespread leucocratic to melanocratic biotite-quartz-felspar gneiss including some quartz-rich gneiss, pelitic gneiss, amphibolite, and minor calcareous gneiss of Helikian or earlier age. The gneiss was intruded by plutons of the anorthositic suite, commencing with metagabbro throughout the area and continuing with mangerite and hornblende granite north of the Strait of Belle Isle. Small bodies of foliated granite were probably emplaced early in this sequence. Regional folding along northeast-trending axes accompanied by amphibolite to granulite facies metamorphism probably occurred after intrusion of the metagabbro and before emplacement of the hornblende granite. Later, more hydrous megacrystic to massive granitic plutons were emplaced diapirically, mostly within the gneiss of the northern Long Range. Phyllonite, up to 300 m thick, was formed by outward thrusting along the northeast margin of the megacrystic pluton at Canada Bay. Gneiss and megacrystic granite along the east margin of the area were folded along northeast-trending axes and subjected to greenschist fad es metamorphism during or soon after emplacement of the megacrystic plutons. Small bodies of fine grained, massive to foliated, mostly leucocratic rocks of probable cataclastic origin are present near the east coast of the Long Range. The protolith of these bodies is of the same age as the Grenville basement, but cataclasis is of late Grenville or later age or both. Normal faulting followed by reverse faulting is evident on Belle Isle and similar faults are present along the margin of the Long Range Grenville inlier. A zone of greenschist facies metamorphism of Ordovician age crosses the Precambrian rocks along the east margin of the area and may extend to the southern end of the Grenville inlier. Lower Paleozoic autochthonous strata. Hadrynian, Cambrian, and Ordovician autochthonous strata of the Strait of Belle Isle area are part of an easterly thickening wedge of strata extending to the southwest and underlying much of the northern Gulf of St. Lawrence. The strata were deposited on a shelf that gave way easterly to a shelf edge and then to a turbiditic fades which formed along the former continental slope and rise. The oldest strata comprise quartzite, conglomerate, siltstone, and minor basalt of the Bateau Formation on Belle Isle. Tholeiitic diabase dykes of the Long Range dyke swarm of latest Hadrynian age intrude the Bateau Formation and are feeders to tholeiitic basalt flows of the overlying Lighthouse Cove Formation. These flows are interlayered with basal arkosic sandstone, conglomerate and siltstone of the succeeding Bradore Formation. In all, these strata are as much as 700 m thick. A regolith is widely evident on the basement complex beneath the Bradore beds. The Cambrian to Middle Ordovician is represented by up to 1500 m of shallow water marine carbonates and some shale and quartz sandstone derived from the Precambrian rocks to the northwest as the proto- Atlantic ocean waters gradually transgressed to the northwest. Lower Ordovician carbonate of the St. George Group consists of about 600 m of calcitic dolomite and dolomitic limestone, which exhibits subtle lateral changes from one rock type to the other. Fossils are not abundant and include graptolites, trilobites, and gastropods. Carbonate in the top of the group contains zinc occurrences. Following the Lower Ordovician deposition, the entire continental shelf was uplifted and exposed to subaerial erosion. After perhaps 5 million years, the shelf again subsided and 500 m of marine limestone of the Table Head Group was then deposited in a Middle Ordovician epicontinental sea upon a karst surface. Water on the shelf then deepened, and detritus for the first time came from the east when the Hare Bay Allochthon was being moved westward on to the shelf. The resulting black shale and minor greywacke of the Goose Tickle Formation, as much as 500 m thick, were ·deposited gradationally upon the carbonate. Hare Bay Allochthon. The Allochthon consists of a variety of sedimentary and volcanic and plutonic rocks including greywacke, polymictic conglomerate, quartz-pebble conglomerate, siltstone, shale, mafic pillow lava, peridotite, harzburgite, dunite, gabbro, and diorite. These rocks make up six contrasting rock assemblages which are separated by thrust faults. The lower structural slices consist of sedimentary rocks and the highest structural slice consists of an ophiolite suite. Melange zones commonly separate the thrust slices. One of these zones contains large exotic blocks. The commonest blocks are serpentinized peridotite, mafic volcanic rocks, amphibolite, foliated gabbro, greywacke, diorite, hornblendite and hornblende-biotite schist. These are contained in a matrix of black and green shale. Most rock types of the blocks may be matched directly to rocks in nearby structural slices. The rocks of the Hare Bay Allochthon originated in the east, in the region near what is now Notre Dame Bay, and record a part of the development and destruction of the ancient continental margin of eastern North America.

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Felsites from the Roberts Arm Group have yielded a Rb–Sr isochron age of 447 ± 7 Ma, with an initial 87Sr/86Sr of 0.7063, and a genetically related granite pluton has yielded an isochron age of 464 ± 13 Ma, with an initial ratio of 0.7064. The Mansfield Cove oceanic plagiogranite complex, which is in fault contact with the Roberts Arm Group, has yielded a minimum zircon 207Pb/206Pb age of 594 ± 10 Ma. This plagiogranite, and basaltic rocks of the Hall Hill complex which partly surround it, thus provide a basement of suitable age upon which the Roberts Arm Group may have been deposited. The age of this basement corresponds to that of the opening of the lapetus Ocean.The stratigraphic age of the Roberts Arm Group, indicated by these ages, is Middle or possibly Late Ordovician. The volcanics were deposited either before or slightly after the Caradocian black shales of the Notre Dame Bay region. In the former case, Ordovician volcanism around western Notre Dame Bay ceased at about the time of obduction of the allochthons of western Newfoundland. In the latter, volcanism of the Roberts Arm Group marks a north to westward shift of volcanism within the central mobile belt of Newfoundland that occurred in late Middle or early Late Ordovician time. If the Buchans Group is of Silurian age and correlative with the Roberts Arm Group then this lithostratigraphic unit is probably diachronous, younging to the south.