Archean felsic volcanic rocks
Archean felsic volcanic rocks are
As the Archean Earth was hotter than the present, formation of felsic volcanic rocks may differ from the modern plate tectonics.[5][6][7]
Archean felsic volcanic rocks are distributed only in the preserved Archean greenstone belts, where deformed sequences of volcanic-sedimentary rocks are common.[5][6][8] Felsic volcanic rocks are rare in the early Earth and only contribute to less 20% of rocks in the Archean greenstone belts worldwide.[6] In contrast, mafic volcanic rocks (such as basalt and komatiite, silicate content <52%[3]) occupy about 50% in the greenstone belts.[6] Thus, felsic volcanic rocks are rare members in the Archean terranes.
Archean felsic volcanic activities commonly occur in submarine environments.[7] The composition of Archean felsic volcanic rocks are equivalent to a spectrum between dacite and rhyolite.[5] They can be distinguished by their mineral assemblages, rock chemistry and rock layer relationship in the sequences.[7]
Archean felsic volcanic rocks are utilised to date the timing of geological events and match distant rock units in separated Archean cratons.[9] They are important to reconstruct Archean geological environments.[10][11]
Felsic
Occurrence
Archean felsic volcanic rocks are only preserved in Archean cratons.[8] A craton is an ancient stable continental block.[13] Also, a craton has survived from plate tectonics that pull apart, collide or tear continents.[13] On average, the felsic volcanic rocks only contribute to ≈15-20% in volcanic rocks of greenstone belts.[6] See Figure 2 and Table 1 for Examples of Archean felsic volcanic rocks occurrence.
All Archean felsic volcanic rocks are distributed in greenstone belts.[6] In Archean cratons, greenstone belts represent supracrustal rocks formed at the Earth's surface and the belts are dominated by volcano-sedimentary sequences.[9][11][14][15][16] Some volcanic sequences can be several kilometers thick, such as the Warrawoona Group of Eastern Pilbara Craton.[17][18] However, ultramafic and mafic units make up the major volume of the volcanic units.[18] The remaining volcanic units are extensive but thin felsic volcanic layers, such as Duffer Formation of the Warrawoona Group.[17] The greenstone belts may be subsequently intruded by dome-shaped magma chambers.[19] The intrusion deformed the felsic volcanic rocks along with the volcano-sedimentary sequences.[5]
Observing modern volcanic processes is relatively easier than observing Archean volcanism, because erosion constantly started removing earlier formed materials.[20] So, studying the Archean supracrustal rocks back in deep time may be subjected to sampling bias.[6]
Felsic volcanic units/localities | Age (Ma) | Greenstone belt | Craton | Country/Region |
---|---|---|---|---|
Duffer Formation[11][10] | 3468 ± 2[21] | Warrawoona | Eastern Pilbara Craton | Australia |
Marda Tank[22] | 2734 ± 3[23] | Marda Volcanic Complex | Yilgarn Craton | Australia |
Kallehadlu Felsic Volcanics[15] | 2677 ± 2[24] | Gadag-Chitradurga | Dharwar Craton | India |
Kovero schist belt[25] | 2754 ± 6[25] | Ilomantsi | Baltic Shield | Finland |
Sample SM/GR/93/57[26][27] | 3710 ± 4[27] | Isua | North Atlantic Craton | Greenland |
Musk massive sulphide deposit[28] | 2689.3 +2.4/-1.8[28] | Yellowknife | Slave Province | Canada |
2694.1±4.5[31] | Abitibi | Superior Province | Canada | |
Upper Michipicoten volcanic sequences[32] | 2696 ± 2[33] | Wawa | Superior Province | Canada |
Bulawayan Group[34] | 2615 ± 28[34] | Harare | Zimbabwean Craton | Zimbabwe |
Onverwacht Group[35] | 3445 ± 3[35] | Barberton | Kaapvaal Craton | South Africa |
Characteristics
Mineralogy and texture
The meaning of "felsic" refers to high silica (SiO2) content from 62 to 78 wt% in rock.[3] In terms of mineralogy, the felsic volcanic rocks are rich in feldspar and quartz.[36] A typical mineral assemblage is quartz + feldspar (albite/oligoclase) + amphibole (chlorite) + micas (biotite and/or muscovite).[36] The mineralogy seems similar with modern rhyolites and dacites.[36] The volcanics are aphanitic, whereas some exhibits porphyritic texture that certain larger minerals (phenocrysts) are visible by eyes.[37]
Felsic volcanic rocks also include felsic
Flow bands are present in massive, uniform felsic lava flow units.[36] When the viscous lava flow encounters a surface, friction drags the mobile lava and forms internal banding.[36]
Structureless
Geochemistry
The composition of Archean felsic volcanic rocks falls in the calc-alkaline series.[32] Such magmatic series indicate that fractional crystallisation of magma occurred during cooling. Magnesium and iron content in the rock are low, and it forms dacite or rhyolite. Magma is a mixture of various minerals. When minerals crystallise from the molten magma, they are progressively removed and dissociated from the melt. The last proportion of the melt is strongly fractionated, causing richness in quartz and feldspars that make the volcanic rocks felsic.
Dacite and rhyolite are characterised by high
Time | SiO2 (wt%) | Na2O+K2O (wt%) | Rock Classification[3] |
---|---|---|---|
Archean | 72.2–73.0 | 6.4–6.8 | Dacite–Rhyolite |
Post-Archean | 73.0–73.6 | 7.0–8.0 | Rhyolite |
Archean felsic volcanic rocks also have high zircon abundance. Incompatible elements, like zirconium, are reluctant to substitute into early-forming crystals.[17] As a result, they tend to remain in the melt. In strongly fractionated felsic magma, zircon is easily saturated. As a result, zircon is common in felsic rocks.[38] The timing of felsic volcanism and tectonic constraints can be identified by radiometric dating and isotopic analysis.[17]
Eruption style
In the Archean aeon, underwater eruptions of felsic lava were common.
Submarine rhyolitic flows were widespread in the Archean but are uncommon in the modern volcanic environment.[39] Viscous felsic eruption often causes pyroclastic flow (hot, dense gas with volcanic fragments) instead of fluid lava flow. However, if the rhyolitic lava is still molten during eruption, it can behave and flow like fluid lava.[7][40]
Subaqueous deposits
Lava flow
Effusive felsic lava flows elongate several kilometres. During an eruption, lava continuously wells out from the vent, then starts to flow outward on the sea floor. Due to quenching, lava is rapidly fragmented to form breccia.[40] A new lobe of lava is injected inside the breccia but it is cooled less quickly, and pushes the flow further outwards.[7]
Lava dome
Short, stocky dome with subsequent pyroclastic deposits extend less than few kilometres long. When explosive eruption occurs, volcanic fragments would be deposited by violent
Stratigraphic significance
Archean felsic volcanic rocks are important in determining
Timing of volcanism
The geochronology of Archean events strongly relies on U-Pb dating[11][26] and Lu-Hf dating.[42] Since mafic rocks (contain low silica content, such as basalt) are lack of zircon, only the age of felsic rocks can be dated among the volcanic rocks in greenstone belts.[14] As felsic volcanic rocks are episodically deposited in between mafic layers, the age range of a particular mafic layer can be constrained by the upper and lower felsic volcanic layers.[11] Thus the time of occurrence and the duration of volcanic episodes can be revealed.[17]
Relationships between Archean felsic volcanic rocks and granitoids
From TTG to GMS granitoids
Two plutonic, igneous rock suites form 50% of Archean cratons.
Relative age | Granitoid | Important mineral present | Magma origin |
---|---|---|---|
Older (1st granitoid) | Tonalite-trondhjemite-granodiorite (TTG)
|
Na-rich plagioclase + garnet + amphibole | hydrated mafic crust |
Younger (2nd granitoid) | Granite-Monzonite-Syenite (GMS) | K-feldspar
|
felsic crust |
The two kinds of granitoids have different magma origins: (a) melting of water-rich mafic materials formed older sodium-rich TTG and (b) melting of felsic materials (e.g. TTG and/or sediments[44]) formed younger potassium-rich GMS (see Table 3).[9][43] They imply gradual chemical changes in the magma and the Earth's crust.[9]
Conflicting compositions
Records of Archean felsic volcanic rocks shows a peculiar trend. The eruption of felsic volcanic rocks and plutonic activities in Archean are largely synchronised as show in overlapping zircon ages.
Possible relationships
The older GMS-like felsic volcanic rocks formed with similar age of TTG has two implications:[9]
- GMS may have intruded the crust and GMS-like volcanics at a very shallow depth. Later, intense erosion rips up all GMS suites and deposited at a proximal distance. If this is true, then GMS and TTG intruded the crust together at the same time. No solid evidence is present yet but the irregular geochemical fingerprints may link both to TTG or GMS.[9]
- GMS is concentrated at the upper crust and TTG at deeper intermediate crust. Later, GMS as well as GMS-like volcanics are eroded and deposit as sediments. The detrital zircons may show a range of mixed GMS and TTG geochemical signature.[9]
Limitation
Revealing the relationship between Archean felsic volcanic rocks and the granitoids may be difficult. It is because weathering alters the geochemical signatures of the felsic rocks above the Earth's surface.[45] The earliest weathering record can be traced back to 3.8 Ga during Eoarchean.[45] Potassium is enriched but sodium is depleted in these weathered felsic rocks.[45] Altered feldspars in the rocks may result in such anomalous signatures.[45]
See also
References
- ^ a b Giles, Christopher William (1980). A comparative study of Archaean and Proterozoic felsic volcanic associations in Southern Australia / by Chris W. Giles (Thesis).
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- ^ a b Vaasjoki, M., Sorjonen-Ward, P. and Lavikainen, S. (1993). U-Pb age delerminations and sulfide Pb-Pb Characteristics from the late Archean Hattu Schist Belt, Ilomantsi, eastern Finland. Geological Survev of Finland, Special Paper 17, 103-131
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