Paleogeology

Paleogeology, Paleoclimate, in relation to Evolution of Life on Earth

Slave craton

northwestern Canadian Shield courtesy of USGS. Click to go to larger original. In this new map, shield rocks are subdivided into dozens of units distinguished by age, lithology and originThe Slave craton is a complex containing ancient collisional orogenic structures and accreted fragments, which now sits at the northeast of the North America craton. (map, geological map, very simplified, maximum protolith age, cross-section, northern Slave mafic intrusions).

(The Slave Province is "A" in the image at left - magnify - legend)

(more images at  Geomorphology, lithology, geological history of the Slave craton.)

Rock formations of the Slave Craton comprise ancient crust including the Acasta Gneiss Complex (A), which contains the oldest known intact Earth rocks. The craton also contains juvenile arcs, mature arcs, and intervening accretionary prism material. All tectonostratigraphic units are cut by strike-slip faults and later-emplaced granitic plutons, and many strata show evidence of late extensional collapse structures.

The Slave craton is a mere fragment of ancient crust, surrounded by Paleoproterozoic rifted margins. Cratons such as the Slave are remnants that preserve parts of the much larger, ancient pre-tectonic and tectonic systems in which they were generated. The Slave craton originated from the break-up of a much larger late Archean landmass and preserves a complex and spatially heterogeneous record of crustal growth that spans nearly 1.5 billion years. (garnets in subcontinental lithospheric mantle (SCLM))

large late Archean supercontinent Kenorland (~2.7 Ga)
_____________________________
late Archean parental supercraton Sclavia Slave _ + Rae (~2 Ga) Laurentia
__________________________________________________________+
_________________________________________Baltica, Ukraine, Amazonia, Australia
_____________________________________________ and possibly Siberia, North China and Kalahari
________________________________________________

______________________Columbia (Nuna, Hudsonland, Hudsonia) 1.8-1.5 Ga

The original landmass could have been the speculative late Archean supercontinent Kenorland, followed by, or perhaps more likely instead, a smaller landmass referred to as the supercraton Sclavia (a late Archean supercraton of unknown size and configuration, which is considered to be parental to the Slave craton).

The Archean gneisses were united at the core of the Slave protocontinent, at least 2.9 Ga. In the nucleus of the old continent, juxtaposed rocks sometimes differ by a billion years in age, probably indicating episodic volcanic eruptions, fed by broad plumes of rock ascending periodically from the deep mantle, rather than having resulted from gradual tectonic accretion of crust at plate boundaries. Such lava flows presumably gradually built up the continental nuclei as a part of mantle mafic-ultramafic and crustal acid magmatism.

Within the later Laurentian/Nuna supercontinent, the Archean rocks subsequently experienced uplift, possibly because of a hot mantle plume. (time chart, Yellowknife domain time chart) Elevation of the continent caused erosion, creating the unconformity. Two volcanic layers above the unconformity date to a little over 2.8 Ga. Subsequently the plume dissipated, and the region sank beneath an ancient ocean, accumulating sediments: quartz-rich sandstone, and then the banded iron formations (Yellowknife Supergroup). The volcanics accumulated as the Slave protocontinent was rifted apart about 2.8-2.7 Ga. (Hadean to Mesoarchean basement of CSBC)

The Slave craton has thick sequences of tholeiitic greenstone sequence date from ~2.7 Ga, younger arc-like sequences from 2.69-2.61 Ga, and extensive turbidite sequences from about 2.68 to 2.62 Ga. Syn-orogenic conglomerates were deposited about 2.6 Ga. Some of Canada's largest volcanogenic massive sulfide (E) deposits lie in arc-like sequences formed above the attenuated basement and in progressively widening, juvenile, back-arc-like basins. (evolution of Slave craton)

Archean cratons of similar age (Zimbabwe, Wyoming) also have quartzite layers and banded iron formations lying over gneisses. Geologist, Wouter Bleeker of the Geological Survey of Canada (GSC) says, "These may be several pieces of a larger continent of which the Slave nucleus is just one remnant."

Parts of the Central Slave Basement Complex contain quartzite gneiss similar to the quartzite found in the Southern Cross Province of the Yilgarn craton.

Many geologists dispute any possibility that plate tectonics could have operate on the early Earth. The planet was much hotter 3-4 billion Ga – too hot, many believe, for formation of rigid continental plates. Further, the hot surface layer would have been too buoyant to sink in subduction zones, preventing development of plate-tectonic cycles. Eventually Earth cooled sufficiently for the crust to form rigid, less buoyant plates, so that plate tectonics evolved as the dominant geophysical force. Bleeker speculates that the breakup of the Slave nucleus some 2.8-2.7 Ga could mark the beginning of plate tectonics.

The oldest parts of the Slave province lack the long, narrow belts of accreted and deformed rocks that are key signatures of plate tectonics. Mapping of the Slave craton has even disconfirmed some supposed relics of ancient plate tectonics.

Key elements of the geology of the Slave craton are illustrated in field photographs on the Slave Craton website, refer to cross-section of the craton (legend):
A. The Acasta gneisses at their discovery site, with ancient tonalites (4.03 Ga) on left.
B. Basal quartzites of the Central Slave Cover Group overlying basement of the Central Slave Basement Complex. To right, in low foreground are low-weathering basement gneisses. The dark ridge in background has ca. 2.7 Ga basalts overlying the quartzites.
C. Syn-Kam Group quartz-porphyritic tonalite intrusion (ca. 2713 Ma), silling into the northern part of the Yellowknife greenstone belt. The large sill-like body is cut by somewhat younger mafic dykes that likely fed the upper part of the greenstone belt. Inset altered quartz-porphyritic tonalite.
D. Quartz porphyritic rhyolite breccia with carbonate matrix, typical for the uppermost part of 2690-2660 Ma felsic volcanic edifices.
E. Massive sulphide mineralization of the Sunrise deposit, associated with ca. 2670 Ma felsic volcanic rocks just below the interface with the Burwash Formation turbidites.
F. Thickly bedded sandy turbidites typical of the Burwash Formation in its type area east of Yellowknife. Oblique areal photo shows F1 syncline refolded by north-northwest trending F2 folds.
G. Silicate facies iron formation interlayered with turbiditic greywackes, George Lake, northeastern Slave. This banded iron formation hosts significant epigenetic gold mineralization.
H. Passive margin strata of the Coronation Supergroup (Epworth Group) overlying the western margin of the rifted Slave craton, structurally at the base of Wopmay orogen.
I. Dense Proterozoic mafic dyke swarms cutting extended Slave crust and its cover

Legend for Field photos accompanying cross-section:
B. Typical 2.95 Ga foliated tonalites of the Central Slave Basement Complex with transposed 2734 Ma mafic dykes.
C, D & E. Basal quartz pebble conglomerate, fuchsitic quartzite, and banded iron formation of the Central Slave Cover Group that overlies the basement complex.
F. Variolitic pillow basalts of the Kam Group, Yellowknife.
G. Syn-Kam Group K-feldspar porphyritic granodiorite pluton in basement below greenstone belts.
H. Polymict conglomerate, including 10-30 cm granitoid cobbles, which occurs locally at the base of the younger, 2.69-2.66 Ga, volcanic cycle.
I. Carbonate-cemented rhyolite breccia typical for the younger volcanic cycle.
Well-preserved sub-biotite grade turbidites in the core of the Yellowknife structural basin, showing graded bedding and load casts.
J. Aerial photo of large scale, upright, fold structures in turbidites of the Yellowknife structural basin.
K. Late-tectonic conglomerates, formation: Acasta Gneisses, 2; Acasta Gneisses; AG complex; hand-specimen: Acasta Gneiss, 2; Tonalite gneiss; Acasta Gneiss, 2; close-up: Acasta Gneiss; sem: Acasta Gneiss - sem; article (1999); Acasta gneiss and another old zircon; abstract, 2 pdf; article; news; Microstructure of Neoarchean zircon from the Acasta gneiss complex ... (pdf); websites: The Slave Craton: Geological and Metallogenic Evolution : Abstract
Introduction : Ancient Basement Complex : The Cover Sequence : Ca. 2.73-2.70 Ga Tholeiitic Volcanism : Post-2.70 Ga Volcanism : Ca. 2.68-2.66 Ga Sedimentation : Ca. 2.65-2.63 Ga Closure Of The Burwash Basin : Post-2.63 Ga Turbidites : 2.60-2.58 Ma, Final Orogenesis : Cratonization And Beyond : Summary : References : Table : Figures : Appendix

Labels: , , , , , ,

Links to this post:

Create a Link

<< Home

. . . stratifying since 10/06/06