The northern part of Baffin Island, in Nunavut, lies within the ca. 3.0–2.5 Ga Committee belt, characterized by episodic granitic plutonism and greenschist- to upper-amphibolite-facies belts of supracrustal rocks. Whereas the entire belt on Baffin Island was likely affected by ca. 2.7 Ga plutonism and associated metamorphism, there is also evidence of a high-grade event at ca. 2.55–2.5 Ga. Evolution of the Baffin orogen in south-central Baffin Island led to generation of the ca. 1.86 Ga Cumberland Batholith, ca. 1.85–1.83 Ga northwest-directed thrusting of the Foxe fold-and-thrust belt over the Dexterity granulite belt (DGB), and variable structural and metamorphic reworking of the Committee belt on northern Baffin Island. Thermobarometric data for granulite-facies rocks in the western DGB indicate ~8.7 kbar paleopressure, interpreted to have been achieved in continental crust thickened by thrust imbrication. Paleopressure decreases gradually northward from the western DGB to <4 kbar, but decreases sharply south of the Isortoq fault zone (IF), which approximates the southern boundary of the DGB. Combined thermobarometric and structural constraints suggest that the IF represents an important crustal-scale structure involved in both NW-directed thrusting and tectonic loading of the DGB, as well as subsequent extensional unroofing of the DGB. Southwest-directed thrusting in the ca. 1.825–1.81 Ga Northeast Baffin thrust belt (NBTB) is interpreted to have produced similar tectonic thickening and ~10.5 kbar pressures along most of the length of this belt on northern Baffin Island, including the eastern part of the DGB. The ~6–8 kbar Archean Bylot Batholith represents a higher structural level that may have contributed to tectonic loading of adjacent high-pressure rocks in the NBTB.
The Franklin intrusions are an extensive swarm of late Hadrynian (latest Proterozoic) diabase dikes that occur in a giant arc from Great Bear Lake eastward to Melville Peninsula, Baffin Island, and northern Ungava. They are chemically and petrologically classified as tholeiites and are probably co-magmatic. Paleomagnetic pole positions and numerous whole-rock of K-Ar age determinations indicate that the dikes were emplaced at low paleolatitudes 650 m.y. ago. They intrude Hadrynian sedimentary sequences that contain features indicative of deposition under warm climatic conditions. The Baffin dikes are subparallel with the northeast coastline of Baffin Island and a pronounced northwest-trending fault system. Intermittent, mainly normal, movement along these faults persisted from the Helikian to the Quaternary and produced a series of graben structures which may be due to the same regional tension as the dikes. Thus Baffin Bay and Davis Strait may have started to form as early as the late Hadrynian and may contain Paleozoic strata. End_of_Article - Last_Page 2480------------
ABSTRACT Small digitate stromatolites with diameters in the range of 0·2–20 mm (ministromatolites) are common in Early and Middle Proterozoic carbonate sequences, and extend stratigraphically from the Archaean to the Holocene. An occurrence of columnar and stratiform types exhibiting a primary or early diagenetic radial‐fibrous fabric and microscopically crinkled (microcrenate) lamination is described from the ˜ 1·9 Gyr old Belcher Supergroup (McLeary Formation) in southeastern Hudson Bay, Canada. The structures, which can be considered to be a variety of tufa, are unusually well preserved because of early diagenetic silicification. Columnar types are referable to Pseudogymnosolen (Asperia) , and are morphologically similar to other occurrences of these taxa in coeval dolostones in northwestern and eastern Canada, where the fabric is normally preserved by a secondary mosaic of dolomite. The textural evidence of angulate cross‐sections and rectilinear divergent patterns indicates that the radial‐fibrous fabric represents primary or very early diagenetic precipitation, and that pseudogymnosolenids with mosaic dolomite originally also had radial‐fibrous structure. The precipitation may have been within, or on, microbial mats.
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.
Cubic casts interpreted as indications of halite crystals and slit-like gashes interpreted as molds of sulphate crystals have been found in the Belcher Group.
The Precambrian rocks on either side of Davis Strait show a similar pattern of events and are interpreted as having formed part of a single shield. Nine major stages in the development of this shield are suggested: (1) formation of an extensive early crust before 3,000 m.y. ago, relicts of which are now preserved as migmatites and high-grade gneisses in the Archean block of eastern Labrador and southwest Greenland; (2) deposition of greenstone belts 2,700-3,000 m.y. ago; (3) plutonic activity in the period 2,500-2,900 m.y. ago, affecting both the greenstone belts and the major parts of the basement on which the greenstones lie; (4) intrusion of numerous basic dike swarms in the general period 2,000-2,600 m.y. ago; (5) deposition of early Proterozoic (Aphebian) geosynclina rocks on the consolidated Archean basement; (6) alteration of these rocks by orogenesis which occurred approximately 1,650-1,850 m.y. ago (the Hudsonian orogeny in Canada and the Ketilidian and Nagssugtoqidian orogenies in Greenland); (7) post-orogenic magmatism--particularly marked in areas affected by Hudsonian metamorphism--which extended from South Greenland through Labrador (This magmatism produced chiefly anorthosites, adamellitic granites, monzonites, and norites, which probably were emplaced between 1,400 and 1,700 m.y. ago, although the areas in which they occur commonly remained thermally active to 1,200 m.y. ago or later.); (8) graben faulting, deposition of molasse sediments, and widespread intrusion of basic dikes, in South Greenland and in parts of Baffin Island, accompany ng and following emplacement of the post-orogenic rocks {Most of these dikes are tholeiitic; however, alkalic and peralkalic intrusions took place locally 1,100-1,300 m.y. ago.); (9) metamorphism and tectonic alteration of the Archean and Proterozoic rocks in the southern part of the Canadian shield by the Grenville orogeny about 900-1,100 m.y. ago. The only effect of the Grenville orogeny in South Greenland was a weak updating of older rocks in areas close to major faults, so that they yield K-Ar ages of about 900-1,000 m.y.
A small assemblage of primitive microscopic fossils is here reported for the first time from the lower part of the Belcher Group in Hudson Bay. The microbiota includes chains and clumps of bacteria, and filamentous and spheroidal structures of probable algal or fungal affinities. In addition, a variety of isolated and clustered spheroids and other structures of probable biologic origin is present. The structures are more than 1600 m.y. old, and occur in a black chert associated with thick dolomite beds considered to be of Aphebian (Early Proterozoic) age. Morphologically comparable Precambrian microfossils occur in the Gunflint Formation (Early Proterozoic, Ontario) and the Bitter Springs Formation (Late Proterozoic, central Australia).
Archaean sedimentary rocks of the North Spirit Lake area show little evidence of having been derived predominantly from associated Archaean volcanic rocks. Instead, compositions of the sediments reflect significant sedimentary and (or) granitoid provenance. A remarkably high content of clastic quartz in thick units of sandstone and conglomerate suggests either reworking of older quartzose sediments, or reduction of the labile constituents in quartz-rich granitoid rocks through prolonged weathering and rigorous transport. Observations for other sedimentary sequences in the region between Red Lake and Lansdowne House suggest that the North Spirit sediments are not unique in the Superior Province. Quartzose sandstones commonly are regarded as atypical of the Archaean, but such rocks arc abundant in northwestern Ontario. Frameworks of many Archaean greywackes actually are richer in quartz than typical greywackes from numerous Proterozoic and Phanerozoic sequences.The concept of rapidly rising volcanic arcs as the sole source of Archaean sedimentary detritus is rejected for the North Spirit area. The volcanies, rather than representing relicts of protocontinents, probably record events removed from initial volcanism in the history of the earth by one or more orogenic cycles. Major unconformities may therefore exist not only between sedimentary and volcanic units, but also between these units and older granitoid rocks.
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