Archean felsic volcanic rocks

[5] As the Archean Earth was hotter than the present, formation of felsic volcanic rocks may differ from the modern plate tectonics.

[6] In contrast, mafic volcanic rocks (such as basalt and komatiite, silicate content <52%[3]) occupy about 50% in the greenstone belts.

[7] The composition of Archean felsic volcanic rocks are equivalent to a spectrum between dacite and rhyolite.

[6] These intrusive felsic igneous rocks include TTG suites (Tonalite-trondhjemite-granodiorite) that contributes over half the portion of Archean cratons.

[9][11][14][15][16] Some volcanic sequences can be several kilometers thick, such as the Warrawoona Group of Eastern Pilbara Craton.

[20] So, studying the Archean supracrustal rocks back in deep time may be subjected to sampling bias.

[36] A typical mineral assemblage is quartz + feldspar (albite/oligoclase) + amphibole (chlorite) + micas (biotite and/or muscovite).

[36] The volcanics are aphanitic, whereas some exhibits porphyritic texture that certain larger minerals (phenocrysts) are visible by eyes.

[11][37] Reported eutaxitic tuff from Superior Province, Canada (Figure 3),[37] contains lenticular fiamme.

[37] The eutaxitic texture represents a hot vapour-phase emplacement of the fragmented volcanic materials on the Earth's surface.

The last proportion of the melt is strongly fractionated, causing richness in quartz and feldspars that make the volcanic rocks felsic.

[3] The average composition of felsic volcanic rocks in Archean greenstone belts is between dacite and rhyolite (Table 2).

[38] The timing of felsic volcanism and tectonic constraints can be identified by radiometric dating and isotopic analysis.

[7][36][39] Submarine eruption is evident by coarse volcanic breccia formed in situ, hyaloclastite or underwater pyroclastic deposits (clastic rock, composed of tephra only).

Since felsic magma is viscous, volcanic eruptions that form dacite or rhyolite are explosive and violent.

[36] Submarine rhyolitic flows were widespread in the Archean but are uncommon in the modern volcanic environment.

During an eruption, lava continuously wells out from the vent, then starts to flow outward on the sea floor.

[40] A new lobe of lava is injected inside the breccia but it is cooled less quickly, and pushes the flow further outwards.

[7][17][18][41] However, the rock sequences of greenstone belts are commonly obscured by later deformation, such as regional folding or intrusion of granitoids.

[5] 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).

The eruption of felsic volcanic rocks and plutonic activities in Archean are largely synchronised as show in overlapping zircon ages.

[9] For example, a GMS-like rhyolite unit in the Abitibi Greenstone Belt (abnormally more enriched in potassium and heavy rare-earth elements than other Archean felsic volcanic rocks) has no plutonic equivalent in the same period.

Fig. 1. A schematic diagram showing the formation environment of Archean felsic volcanic rocks. Modified from Giles (1980). [ 1 ] Felsic eruption forms felsic volcanic rocks near the volcano and a spectrum of volcano-sedimentary sequence in the sea in Archean. [ 1 ]
Fig. 2. A map showing examples greenstone belts with documented Archean felsic volcanic rocks localities. See citations in Table 1.
Fig. 3. Archean felsic volcanic rocks have particular characteristic structure. Some are tuffs, formed by volcanic materials from eruption. A significant structure is fiamme, which are recrystallised quartz with flame-like ending points. The illustration is fiamme in Archean Woman Lake rhyolitic tuff, Superior Province, Canada. Adopted and modified from photograph of Thurston (1980). [ 37 ]
Fig. 4. Schematic illustration of documented subaqueous felsic lava deposits. (a) Submarine lava flow, based on Héré Creek rhyolite (modified from De Rosen-Spence et al., 1980 [ 7 ] ). (b) Submarine lava dome, based on the Gold Lake dome and flow complex (modified from Lambert et al., 1990). [ 41 ] Illustration adopted from Sylvester et al. (1997) in de Wit & Ashwal (1997). [ 14 ]
Fig 5. Possible relationship 1 of Archean felsic volcanic rocks and granitoids. GMS may have intruded the crust at a very shallow depth, and later TTG intruded. [ 9 ]
Fig 6. Possible relationship 2 of Archean felsic volcanic rocks and granitoids. GMS and TTG may have intruded the crust at the same time. Yet, GMS was concentrated at the upper crust and TTG at deeper intermediate crust. [ 9 ]