Archaean

The Archaean, or Archean Eon extends from 3.8 billion years ago until 2.5 billion years ago.

The Eon was formerly called the Archaeozoic, and is subdivided into Eoarchean (3.8-3.6 Ga), Paleoarchean (3.6-3.2 Ga), Mesoarchean (3.2-2.8 Ga), and Neoarchean (2.8-2.5 Ga) Eras or into early (to 3.3Ga), middle (to 2.9 Ga), and late Archaean.

Life arose about 4 billion years ago, at the close of the Hadean Eon, and Cyanobacteria comprise the earliest known microfossils. The Cyanobacteria built large stromatolite reefs and dominated life for more than 2 billion years (~3.5 Ga to ~1 Ga). The atmosphere remained reducing for much of the Achaean, though Cyanobacteria began to generate oxygen after approximately 2.8 to 2.7 Ga.

Earth's heat flux during the Archaean was much higher than current levels: early in the Archaean the flux was almost triple current levels, while heat flows fell to twice current levels by the end of the Archaean. As a result of the high heat flow, the Earth had numerous hot spots, very active volcanoes, and rift valleys. Earth continued to experience bombardment by extrasolar debris plus the late heavy bombardment until 3.5 Ga. The majority of Archean rocks still in existence are crystalline cratonic remnants, which include unusual lavas (e.g., komatiite), intrusive igneous rocks such as great melt sheets, and voluminous plutonic masses of granite, diorite, ultramafic to mafic layered intrusions, anorthosites and monzonites known as sanukitoids.

Scientists continue to actively debate the contribution of plate tectonic activity in the formation of Archaean crustal configurations. Large continents did not form until late in the Archean, and the felsic protocontinents probably formed at hotspots over mantle plumes rather than at subduction zones. Protocontinents probably resulted from:
● igneous differentiation of mafic rocks to produce intermediate and felsic rocks
● mafic magma melting of more felsic rocks
● granitization of intermediate rocks
● partial melting of mafic rock
● metamorphic alteration of felsic sedimentary rocks.
Much continental material may have been lost if rocks were not sufficiently buoyant or were consumed at energetic subduction zones.[2]

The oldest rocks so far discovered on Earth are:
1) Jack Hills, Western Australia, a 4.4 Ga detrital zircon (sample W74) in the Jack Hills metaconglomerate, Eranondoo Hill. More at Earliest Piece of Earth [news article]
2) The Acasta Gneisses near Canada’s Great Slave Lake (4.03 Ga)
3) The Isua Supracrustal rocks of West Greenland (3.7 to 3.8 Ga)
4) Northern Michigan (3.5-3.7 Ga)
5) Swaziland (3.4-3.5 Ga)

Archaean Supercontinents and continents:
Vaalbara is Earth's theorized first supercontinent, which, according to radiometric data, existed by 3.3 billion years ago (3.3 Ga) and possibly even as far back as 3.6 Ga. Geochronological and palaeomagnetic evidence suggests Valbaara began to break up ~2.8 Ga.

Ur was the first known continent, which probably formed 3 billion years ago in the early Archean Eon.

By about 2.7 Ga, Neoarchean sanukitoid cratons plus new continental crust accreted to form another of Earth's earliest supercontinents, Kenorland.

Two contrasting mechanisms have been proposed for Archean tectonics: “vertical tectonism” and “horizontal tectonism”. Vertical tectonism is attributed to density inversions inherent between denser volcanic sequences (greenstones) and underlying less dense sialic material (granitoids), which result in the buoyant rising of granitoids (diapirism) and sinking of greenstones (sagduction). Horizontal tectonism in the Archean is assumed to be similar (though probably not identical) to present-day plate tectonics, so is characterized by regional scale horizontal motion (drift) and resulting interactions of plates or microplates. Vertical tectonism can lead to local horizontal movement, and horizontal tectonism commonly results in vertical movement, as in collisional zones. However, regional scale horizontal displacements and regional scale strike-slip faults (or shear zones) cannot be explained by vertical tectonism and most likely results from horizontal tectonism. Vertical and horizontal tectonism were not necessarily mutually exclusive.

While Archean geologists now accept that processes similar to present-day plate tectonics existed in some form in the Archean, particularly in the Neoarchean, studies continue to document evidence for vertical tectonism. Recognition of synchronous vertical and horizontal tectonism proves challenging because it is often difficult to differentiate structures formed by synchronous processes from those formed by one of the two processes followed by the other. It has been suggested that the granite-greenstone patterns in the Dharwar craton in India resulted from the interplay of diapirism and bulk horizontal inhomogeneous contraction. It has also been suggested that the kinematics of a high strain zone in the northwestern Superior craton resulted from synchronous horizontal and vertical tectonism. [Lin]

◙ subduction zone magmas ◙

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Slave craton

The 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