J. Allan Donaldson

Enriched Archean lithospheric mantle beneath western Churchill Province tapped during Paleoproterozoic orogenesis

Ultrapotassic rocks of the Christopher Island Formation (Baker Lake basin) were emplaced across an enormous area (240 000 km 2 minimum) of the western Churchill Province ca. 1.83 Ga. These rocks extend across the Snowbird zone, a geophysical feature postulated by others to represent a Paleoproterozoic suture that welded the Rae and Hearne domains. Minette dikes and flows of the Rae and Hearne domains display identical ϵ Nd, 1830 values and incompatible element patterns, and thus appear to have originated from a common lithospheric-mantle source. Christopher Island Nd model ages cluster at 2.8 Ga, and ϵ Nd data from one Archean lamprophyre and three 2.45 to ca. 2.2 Ga mafic suites suggest that enriched lithospheric-mantle sources beneath both the Rae and Hearne domains existed well before ca. 1.83 Ga, inconsistent with Paleoproterozoic suturing along the Snowbird zone. In contrast to commonly invoked models that envisage melting of local enriched domains, Christopher Island ultrapotassic rocks appear to have originated from an extensive reservoir. We suggest that such a reservoir was created during an Archean metasomatic event, perhaps owing to flat subduction, and that it remained in nearly complete isolation until tapped during Paleoproterozoic extension related to squeezing of western Churchill crust between flanking Wopmay and Trans-Hudson orogens.

Archaean quartz arenites in the Canadian Shield: examples from the Superior and Churchill Provinces

Abstract Units of remarkably pure Archaean quartz arenite occur in the northwestern part of the Superior Province and in the northern terrane of the Western Churchill Province (Rae Province) of the Canadian Shield. In the Superior Province, silica-cemented quartz arenites of Archaean age are well preserved in several greenstone belts. The example from the Keeyask Lake sedimentary assemblage displays tabular–planar and trough cross-beds, ripple marks, reactivation surfaces and pebble lag deposits. In spite of penetrative deformation and greenschist-grade metamorphism, primary textures are extremely well preserved, showing framework grains to be very well rounded and sorted. The succession of Keeyask Lake quartz-arenite beds is overlain by siltstones containing small-scale stratiform, domal and columnar stromatolites. A shallow-marine environment of deposition is inferred. Detrital heavy minerals include pyrite, magnetite, zircon, tourmaline, apatite, sphene and topaz. In the northern part of the Western Churchill Province (Rae Province), Archaean quartz arenites occur in northeasterly trending belts where intense structural deformation has in most places obscured or obliterated primary textures and structures. This has led to speculation that some of these units are metachert or recrystallized vein quartz, but local preservation of primary textures and structures provides clear evidence of epiclastic origin. In the example described herein, quartz arenites of the Woodburn Lake Group display sparse occurrences of trough and tabular–planar cross-beds, channels, ripple marks and pebble lag deposits. Probable environments of deposition for these quartz arenites include fluvial systems and shallow-marine shelf settings. The occurrence of unequivocal quartz-arenite clasts in beds of intercalated conglomerate provides direct evidence of at least two episodes of accumulation of almost pure quartz sand. Thin sections and polished slabs reveal frameworks of clastic quartz grains with little to no matrix (now mainly muscovite), and rare detrital grains of accessory heavy minerals, predominantly zircon and opaque iron oxides. Pyrite and other sulphides have been introduced along fractures, but some intergranular sulphide grains may be of detrital origin. The principal source for the quartz arenites in both areas must have been quartz-rich granitoid rocks. Conditions of intense chemical weathering are indicated. The widespread occurrence of extremely mature quartz arenites throughout Archaean terranes of the Canadian Shield, and in other shields of the world, are suggestive of crustal stability during early Earth history. The association of quartz arenites and ultramafic rocks, uncharacteristic of younger terranes, is now recognized in many Archaean greenstone belts of the Canadian Shield.

Basinal and shelf sedimentation in relation to the archaean-proterozoic boundary

Abstract In most countries the Archaean-Proterozoic boundary has been designated as an unconformity between the oldest shelf-type quartz arenites and a basement consisting of greenstone belts intruded by granitic rocks, or high-grade gneisses. The quartz arenites have been characterized as shallow-water shelf deposits, and sediments in the greenstone belts as resedimented basinal wackes of volcanogenic origin. Radiometric ages for rocks of the Canadian Shield led to initial designation of the Archaean-Proterozoic boundary as thus defined at about 2.5 Ga. Subsequent age determinations have clearly established that a number of quartz arenites originally assigned to the Proterozoic are 2.7 – 2.94 Ga old (e.g., Witwatersrand and Pongola Supergroups of South Africa). To accommodate these old quartz arenites, some have suggested that the Arch aean-Proterozoic boundary should be regarded as diachronous, embracing a span of time about equivalent to that of the Phanerozoic. However, quartz arenites also are abundant in 3.2 — 3.5 Ga old greenstone belts and high-grade metamorphic terranes of South Africa, Western Australia and the U.S.A. Likewise, large regions of basinal volcanogenic sediments occur in post-2.5 Ga terranes. This greatly expands the diachroneity of geological units that were initially regarded as either uniquely Archaean or Proterozoic. We therefore recommend that chronometric and chronostratigraphic sense be retained for the Archaean-Proterozoic boundary, divested of the requirement for all quartz arenites to be Proterozoic. According to this analysis of the Precambrian record, shelf sediments have increased in abundance relative to basinal sediments through time, but at different rates in different areas. This change is attributable to progressive cratonization of shield areas, a process that was strongly diachronous on a worldwide scale.

Paleoclimatology of Nonacho Basin (early proterozoic), Northwest territories, Canada

Abstract Numerous features of paleoclimatic significance are preserved in terrestrial strata of Nonacho Basin (Early Proterozoic). Minimal chemical weathering is indicated by relic corestones, grus and talus, and the absence of chemical alteration zones at the basal unconformity. Humid conditions are indicated by: (1) dominance of siliciclastic lacustrine deposits (even in lakes that had restricted drainage); (2) paucity of caliche in extensive alluvial fan deposits; (3) near-absence of debris flows in alluvial fan deposits; (4) absence of large-scale high-angle cross-stratification in eolian deposits; and (5) penecontemporaneous variegated redbeds (drab sandstones; red mudstones). Minimal chemical weathering under humid conditions implies a cold climate. This is supported by published paleomagnetic data humid conditions implies a cold climate. This is supported by published paleomagnetic data which indicate that Nonacho Basin probably formed at paleolatitudes of 40° – 55°, by talus deposits (formed as a result of frost action), and by penecontemporaneous stratiform sandstone breccias for which the early cementing agent probably was ice. High elevations probably contributed to the low temperatures. The Nonacho Group probably was deposited in a moist continental climate with cold winters characterized by repeated freeze—thaw cycles. Local chemically precipitated lacustrine sediments and caliche indicate brief dry intervals.