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Archean felsic volcanic rocks (or Archean felsic volcanics, AFVs) are volcanic rocks that were formed during explosive eruption in the Archean Eon from 4 to 2.5 billion years ago. Rocks that are formed in the early Earth are uncommon in the present[1]. AFVs are even rarer because they only contribute to 10–20% in the Archean greenstone belt rock units[2]. Crucial AFVs are distributed only in the remnants of Archean volcano-sedimentary sequences, known as greenstone belts.

Different from modern felsic eruption under the air, Archean felsic volcanic activities commonly occur in submarine environments. The eruption forms two types of volcanic rocks based on composition: dacite and rhyolite[3]. They can be distinguished by studying their mineral assemblages, rock chemistry and rock layer relationship in the sequences.

AFVs only resemble a small fraction of volcanic activities, but the key to reconstruct what happened in Archean — timing of ancient geological events, is hidden in these chests[4][5].

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This rare group is distinguished by mineralogy and geochemistry, accompanied by stratigraphy in the volcano-sedimentary sequences in the greenstone belt. The volcanics also resemble minor components in the calc-alkaline basalt-andesite-dacite-rhyolite (BADR) association, that form the component Archean supracrustal belts or low-grade metamorphosed greenstone belts[6][7][3].

While Tonalite-Trondhjemite-Granodiorite (TTG), as a felsic plutonic association, is the most prevalent rock type in Archean terranes, it is still uncertain that of whether TTG granitoids are the plutonic equivalents of AFVs[2][8]. A number of debates argue whether a link exhibits between TTG and AFVs[9][10].

Occurrence

Felsic volcanic rocks are rare in Archean cratons[1]. Dacites and rhyolites on average only contribute to ~15-20% in volcanic rocks of greenstone belts[2].

Generally, stable Archean cratons, that survived from tectonic destruction, possess a dome-and-keel geometry. Keels are also known as the greenstone belt that are intruded by domal batholith (magma chamber) of felsic TTG suites[6]. In such composition duality, greenstone belts resemble the supracrustal rock and it is dominated by volcano-sedimentary units[7][9][11][5]. Some volcanic sequences, for example the Warrawoona Group of Eastern Pibara Craton, can be several kilometers thick[12][13]. However, ultramafic and mafic units make up substantial volume of the volcanic units. The remaining volcanic units are extensive but thin felsic volcanic layers, such as Duffer Formation of the Warrawoona Group[12].

A map showing examples greenstone belts with documented Archean Felsic Volcanics localities. See citations in the table.

Modern volcanic processes along with its products could be observed and recorded[14]. Yet, erosion, constantly removing surface materials from the source, leads to sampling bias when studying the Archean supracrustal rocks back in deep time[2].

Examples of AFVs occurrence in greenstone belts
Greenstone Belt Craton Country Age of AFVs (Ma)
Warrawoona[4][5] Eastern Pilbara Craton Australia 3468 ± 2[15]
Marda Complex[16] Yilgarn Craton Australia 2676 ± 4[17]
Gadag-Chitradurga[7] Dharwar Craton India
Central Lapland[18] East European Craton Finland
Wishbone[19] Slave Craton Canada
Abitibi[20] Superior Craton Canada
Bulawayan[21][22] Superior Craton Canada
Barberton[9][23] Kaapvaal Craton South Africa 3445 ± 3[24]

Characteristics

Mineralogy and texture

By the definition of "felsic", the volcanic rocks are rich in quartz and feldspars. Typical mineral assemblage is quartz + feldspar (albite/oligoclase) + amphibole (chlorite) + micas (biotite and/or muscovite)[17]. The mineralogy seems similar with modern rhyolites and dacites. Volcanics are aphanitic, whereas some exhibits porphyritic texture that certain larger minerals (phenocrysts) are visible by eyes.

Illustrated sketch of fiamme - recrystalised quartz with flame-like ending points, in Archean Woman Lake rhyolitic tuff, Uchi, Superior Craton,Canada. Adopted and modified from photograph of Thurston (1980)[25].

AFVs also include felsic tuff that was formed from consolidation of tephra, composed of volcanic ash, glass shards and lithic fragments. Reported eutaxitic tuff from Superior Province, Canada[26], contains lenticular fiamme. When hot pumice deposits rapidly, it is recrystallised and welded into quartz with flame-like ending tips. This texture represents a hot vapour-phase emplacement of the fragmented volcanic materials on the Earth's surface.

Flow bands are present in massive, uniform dacitic flow during the movement of lava[17]. When the viscous lava flow encounters a surface, friction drags the mobile lava and forms internal banding.

Structureless hyaloclastite is commonly found in AFVs[12][17][25][27]. In submarine environments, water quenches and cools lava rapidly during volcanic eruption. The flow is fragmented and form glassy volcanic breccia.

Geochemistry

The assemblages of AFVs are calc-alkaline in the whole-rock composition[28]. Such magmatic series indicates fractional crystallisation of magma leading to low magnesium and iron content in the rock and 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 AFVs felsic.

Average composition of felsic volcanics from Condie (1993)[2]
Time SiO2 (wt%) Na2O+K2O (wt%) TAS Classification[29]
Archean 72.2–73.0 6.4–6.8 Dacite–Rhyolite
Post-Archean 73.0–73.6 7.0–8.0 Rhyolite

Dacite and rhyolite are characterised by high silica (SiO2) content from 62 to 78 wt%[29]. The average composition of AFVs in greenstone belts is between dacite to rhyolite[2]. In comparison, the average composition after Archean (<2.5 Ga) is alike rhyolite, indicating a more felsic shift in felsic volcanism[2]. However, this average may be biased because of weathering right after deposition or metamorphism during later stages of deformation[9].

In addition, AFVs have high zircon abundance. Incompatible elements, like zirconium, are reluctant to substitute into early-forming crystals so they tend to remain in the melt. In strongly fractionated felsic magma, zircon is easily saturated and, as a result, widespread in felsic rocks[30]. Timing of felsic volcanism and tectonic constraints can be identified by radiometric dating and isotopic analysis.

AFVs show a different trend in rare-earth elements (REE). They are depleted in Light REE but enriched in Heavy REE compared to post-Archean felsic volcanics[2].

Archean Felsic Volcanism

Eruption style

In Archean, underwater felsic eruptions are identified[17][27][31]. It is evident by coarse volcanic breccia in place, hyaloclastite or underwater pyroclastic deposits (clastic rock, composed by tephra only). Since felsic magma is viscous, volcanic eruptions that form dacite or rhyolite are explosive and violent. The Archean felsic eruption may be assigned to Vesuvius eruption type in the present day[17].

Rhyolitic flows are not uncommon in Archean, although it is less common in the modern volcanic environment[31]. It is predicted that viscous felsic eruption often causes pyroclastic flow (hot, dense gas with volcanic fragments) instead of fluid lava flow. However, if the rhyolitic lava is is still molten during extrusion, it can behave and flow like fluid flow[27][32].

Subaqueous deposits

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[27]). (b) Submarine lava dome, based on the Gold Lake dome and flow complex (modified from Lambert et al., 1990)[33]. Illustration adopted from Sylvester et al. (1997) in de Wit & Ashwal (1997)[34].

Felsic lava flow and lava dome are the two types of underwater deposits formed by AFVs. Documented Archean lava structures are distinct from post-Archean felsic lava because of underwater eruption[27]. Different aspects show that, unlike modern volcanoes, Archean felsic lava deposits rapidly and quenches afterwards[12][17][27][33]. When water is in touch with lava flow. It quickly cools the lava down, causing the lava to solidify, break up as clast and accumulate on the flow fronts to form breccia[17].

Lava flow

Effusive felsic lava flows elongate several kilometres long. During an eruption, lava continuously wells out from the feeder dyke, then starts to flow outward on the sea floor. Due to quenching, lava is rapidly fragmented to form breccia[32]. A new lobe of lava is injected inside the breccia but it is cooled down slower and push the flow further outwards[27].

Lava dome

Short, stocky dome with subsequent pyroclastic deposits extend less than few kilometres long.

Stratigraphic significance

AFVs is important to deduce absolute age of the rock units in greenstone belts. Felsic eruptions are episodic so that the felsic volcanic layers are distinctive stratigraphic units[5]. Also, AFVs are distributed vastly across long distances because of its extensive deposition. However, the rock sequences of greenstone belts are commonly disputed by later deformation, such as regional folding or intrusion of granitoids. By identifying these felsic sequences and dating their time of formation, stratigraphic units of different locations can be correlated despite the obstacles or discontinuity in between AFVs.

Timing of volcanism

The geochronology of Archean events strongly relies on U-Pb dating or Lu-Hf dating. Since mafic rocks, such as basalt, is lack of zircon, only the age of felsic rocks can be dated among the volcanic rocks in greenstone belts. As AFVs 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[5].

In addition, because of absolute dating, by determining the specific ages of the rocks, the episodic occurrence and the duration of felsic volcanism are revealed.

It is observed that TTG suites intruded the supracrustal volcanic rocks of similar age and composition in Archean cratons[35]. The pluton might represent the exposed batholith which stored felsic magma before eruption[36].

Although the two rock types have similar age and geochemistry, there is a lack of equivalents in the opposite of extrusive or intrusive rocks. Some plutons contain tonalites that are more silicic than andesites, the dominant intermediate volcanic rock in the belt[37]. Conversely, for example, no plutonic equivalents are found for the heavy REE-enriched rhyolite of the Superior Province[37].

Implications in Archean tectonics

Despite its rarity in greenstone belt, AFVs play a crucial role in Archean tectonics. It is the only product that provide evidence in felsic volcanism, possibly reflecting the behaviour of its plutonic equivalents.

Stagnant-lid tectonics[9]

Heat-pipe[38]

See Also

References

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