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Several groups of Aphebian layered rocks in the northeastern Canadian Shield have been correlated because of lithologic and stratigraphic similarities and alignment of groups and structural trends. Most of these layered rocks lie in three distinct fold belts which from south to north have been named the Dorset, Foxe, and the Committee Fold Belts.The widespread occurrence of Aphebian outliers between the fold belts indicates that Aphebian strata probably originally covered the entire area from the Circum–Ungava Geosyncline or Fold Belt east to the Dorset Fold Belt and north to the Committee Fold Belt. The name Baffin Geosyncline is proposed for this depositional zone. Aphebian layered rocks in the two last-named fold belts were probably deposited in marginal mio-eugeosynclinal zones of the main geosyncline and are mainly meta-shale, meta-graywacke, and metamorphosed basic volcanic rocks and associated basic and ultrabasic intrusions. The Aphebian rocks of the Dorset and Foxe Fold Belts were deposited in the central shelf zone of the Baffin Geosyncline and are mainly meta-shale, meta-graywacke, rusty quartz-rich gneiss, marble, and quartzite.The Aphebian layered rocks have been intruded by large granitic plutons and have been metamorphosed to amphibolite and granulite facies of regional metamorphism. Age determinations and structural data indicate that a mid-Aphebian orogeny affected much of the northern part of this region 2000–2200 m.y. ago, and that the whole region was strongly affected by the Hudsonian orogeny.

Mistastin Lake, Labrador, latitude 55°53′ N, longitude 63°18′ W, is a roughly circular lake with a large central island lying in a marked depression. A meteorite impact origin is indicated by shock metamorphism features and fracturing of anorthosite and adamellite on the central island and of inclusions in flat-lying igneous rocks on the western shore. The crater had an original diameter of about 20 km and an age, by whole-rock K–Ar determination, of 202 ± 25 m.y.

The area occupies 168 000 km2 of the northeastern mainland Precambrian Shield. Landforms range from low plains in the southwest to alpine mountains in the northeast, where peaks are over 1500 m above sea level. Except for some mountain tops the entire area was covered by ice during the Pleistocene. Parts of two structural provinces are present-the Nain, which forms a block up to 95 km wide and 500 km long along the Labrador coast, and the Churchill, which comprises the remainder. Most of the Nain Province is underlain by Archean migmatite, gneissic granite, granulite, granite, granodiorite, paragneiss and amphibolite, and includes some rocks 3600 Ma old. Small areas of Aphebian strata (the Ramah, Mugford and Snyder groups) lie unconformably on these rocks. In the Makkovik Subprovince rocks consist chiefly of the Aphebian Aillik Group and basic and acidic intrusive rocks. The Churchill Province not only is underlain mostly by Aphebian migmatite, granitic gneiss, granodiorite and granulite, but also includes Aphebian stratiform rocks of the Kaniapiskau Supergroup, the Lake Harbour Formation, and unassigned metasedimentary and mafic rocks. Paleohelikian batholiths of adamellite and anorthosite and allied rocks intrude the older rocks of both provinces. Scattered, small occurrences of rocks, including diabase dykes, range in age from Archean to Triassic. Metamorphism in the Nain Province is dominantly in the amphibolite facies with local granulite and greenschist. In the Churchill Province, although amphibolite facies is the most common, a progressive increase from greenschist in the southwest to granulite in the northeast is evident. Structures in the Nain Province are northerly overall with well defined folds and west-dipping thrust faults in the Ramah Group and underlying rocks, whereas in the Churchill Province structures trend northwest to north-northwest with well defined folds and east-dipping thrust faults in the extreme southwest in the Kaniapiskau Supergroup. In the northeast there is a zone of well defined folds, east of which a broad zone of mylonite forms much of the contact with the Nain Province. There are no known mineral deposits.

The Meguma group of lithic greywacke, feldspathic quartzite, slate siltstone, and argillite is Early Ordovician or older in age and has undergone both regional and contact metamorphism. Both types of metamorphism have resulted in recrystallization and locally in orientation of newly formed minerals. Metasomatism and retrogressive metamorphism are subordinate and only locally important. Regionally metamorphosed rocks are divided into greenschist and almandine–amphibolite facies, although some assemblages cannot be assigned with certainty. Locally, biotite and garnet isograds are mappable within the greenschist zone.Relationships between regional metamorphism and structural elements (folding) show that deformation preceded regional metamorphism. Intrusion of granitic rocks has produced a zone of contact metamorphism (hornblende–hornfels facies) that is superimposed upon regional greenschist facies rocks, which shows that granite emplacement occurred after the regional grade was reached. Gold–quartz veins are confined to areas lying in the greenschist zone of regional metamorphism, which suggests that the almandine–amphibolite zone is not favorable.