Archean felsic volcanic rocks
Archean felsic volcanic rocks are felsic volcanic rocks that were formed in the Archean Eon. The term "felsic" means that the rocks have silica content of 62–78%. Given that the Earth formed at ~4.5 billion year ago, Archean felsic volcanic rocks provide clues on the Earth's first volcanic activities on the Earth's surface started 500 million years after the Earth's formation.
As the Archean Earth was hotter than the present, formation of felsic volcanic rocks may differ from the modern plate tectonics.
Archean felsic volcanic rocks are distributed only in the preserved Archean greenstone belts, where deformed sequences of volcanic-sedimentary rocks are common. Felsic volcanic rocks are rare in the early Earth and only contribute to less 20% of rocks in the Archean greenstone belts worldwide. Nonetheless, mafic volcanic rocks occupy about 50% in the greenstone belts. Thus, felsic volcanic rocks are rare members in the Archean terranes.
Archean felsic volcanic activities commonly occur in submarine environments. The composition of Archean felsic volcanic rocks are equivalent to a spectrum between dacite and rhyolite. They can be distinguished by their mineral assemblages, rock chemistry and rock layer relationship in the sequences.
Archean felsic volcanic rocks are utilised to date the timing of geological events and match distant rock units in separated Archean cratons. They are important to reconstruct Archean geological environments.
Felsic granitoids are the most prevalent rock type in Archean terranes. These intrusive felsic igneous rocks include TTG suites that contributes over half the portion of Archean cratons. They have implications in finding how the felsic volcanic rocks were formed and related to the granitoids.
Occurrence
Archean felsic volcanic rocks are only preserved in Archean cratons. A craton is an ancient stable continental block. Also, a craton has survived from plate tectonics that pull apart, collide or tear continents. On average, the felsic volcanic rocks only contribute to ≈15-20% in volcanic rocks of greenstone belts. See Figure 2 and Table 1 for Examples of Archean felsic volcanic rocks occurrence.All Archean felsic volcanic rocks are distributed in greenstone belts. In Archean cratons, greenstone belts represent supracrustal rocks formed at the Earth's surface and the belts are dominated by volcano-sedimentary sequences. Some volcanic sequences can be several kilometers thick, such as the Warrawoona Group of Eastern Pilbara Craton. However, ultramafic and mafic units make up the major volume of the volcanic units. The remaining volcanic units are extensive but thin felsic volcanic layers, such as Duffer Formation of the Warrawoona Group. The greenstone belts may be subsequently intruded by dome-shaped magma chambers. The intrusion deformed the felsic volcanic rocks along with the volcano-sedimentary sequences.
Observing modern volcanic processes is relatively easier than observing Archean volcanism, because erosion constantly started removing earlier formed materials. So, studying the Archean supracrustal rocks back in deep time may be subjected to sampling bias.
| Felsic volcanic units/localities | Age | Greenstone belt | Craton | Country/Region |
| Duffer Formation | 3468 ± 2 | Warrawoona | Eastern Pilbara Craton | Australia |
| Marda Tank | 2734 ± 3 | Marda Volcanic Complex | Yilgarn Craton | Australia |
| Kallehadlu Felsic Volcanics | 2677 ± 2 | Gadag-Chitradurga | Dharwar Craton | India |
| Kovero schist belt | 2754 ± 6 | Ilomantsi | Baltic Shield | Finland |
| Sample SM/GR/93/57 | 3710 ± 4 | Isua | North Atlantic Craton | Greenland |
| Musk massive sulphide deposit | 2689.3 +2.4/-1.8 | Yellowknife | Slave Province | Canada |
| Blake River Group | 2694.1±4.5 | Abitibi | Superior Province | Canada |
| Upper Michipicoten volcanic sequences | 2696 ± 2 | Wawa | Superior Province | Canada |
| Bulawayan Group | 2615 ± 28 | Harare | Zimbabwean Craton | Zimbabwe |
| Onverwacht Group | 3445 ± 3 | Barberton | Kaapvaal Craton | South Africa |
Characteristics
Mineralogy and texture
The meaning of "felsic" refers to high silica content from 62 to 78 wt% in rock. In terms of mineralogy, the felsic volcanic rocks are rich in feldspar and quartz. A typical mineral assemblage is quartz + feldspar + amphibole + micas. The mineralogy seems similar with modern rhyolites and dacites. The volcanics are aphanitic, whereas some exhibits porphyritic texture that certain larger minerals are visible by eyes.Felsic volcanic rocks also include felsic tuff that was formed when tephra was consolidated. Tuff is composed of volcanic ash, glass shards and lithic fragments. Reported eutaxitic tuff from Superior Province, Canada, contains lenticular fiamme. When hot pumice deposits on a cool surface, it is rapidly cooled, recrystallised and welded into quartz with flame-like ending tips. The eutaxiitc texture represents a hot vapour-phase emplacement of the fragmented volcanic materials on the Earth's surface.
Flow bands are present in massive, uniform felsic lava flow units. When the viscous lava flow encounters a surface, friction drags the mobile lava and forms internal banding.
Structureless hyaloclastite is commonly found in Archean felsic volcanic rocks. In submarine environments, water quenches and cools lava rapidly during volcanic eruption. The flow is fragmented and form glassy volcanic breccia.
Geochemistry
The composition of Archean felsic volcanic rocks are in the calc-alkaline series. Such magmatic series indicates that fractional crystallisation of magma occurs during cooling. 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 the volcanic rocks felsic.Dacite and rhyolite are characterised by high silica content from 62 to 78 wt%. The average composition of felsic volcanic rocks in Archean greenstone belts is between dacite to rhyolite. In comparison, the modern felsic volcanic rock average composition is similar to rhyolite, indicating a more felsic shift with greater alkali content in felsic volcanism. However, the composition may be biased because of weathering right after deposition or metamorphism during later stages of deformation.
| Time | SiO2 | Na2O+K2O | Rock Classification |
| Archean | 72.2–73.0 | 6.4–6.8 | Dacite–Rhyolite |
| Post-Archean | 73.0–73.6 | 7.0–8.0 | Rhyolite |
In addition, Archean felsic volcanic rocks 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. As a result, zircon is common in felsic rocks. Timing of felsic volcanism and tectonic constraints can be identified by radiometric dating and isotopic analysis.
Eruption style
In Archean, underwater eruptions of felsic lava were common. Submarine eruption is evident by coarse volcanic breccia formed in situ, hyaloclastite or underwater pyroclastic deposits. 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.Submarine rhyolitic flows were widespread in Archean but it is uncommon in the modern volcanic environment. Viscous felsic eruption often causes pyroclastic flow instead of fluid lava flow. However, if the rhyolitic lava is still molten during eruption, it can behave and flow like fluid flow.
Subaqueous deposits
Felsic lava flow and lava dome are the two common types of underwater deposits formed by Archean felsic volcanic rocks. Documented Archean lava structures are distinctive from post-Archean felsic lava because underwater eruptions are so rare in post-Archean. The dacitic or rhyolitic lava flows are quenched right after the eruption. When water is in touch with the flow, the lava quickly cools down. Finally, The lava solidifies and breaks up as clasts, and the clasts accumulate on the flow fronts to form breccia.Lava flow
felsic lava flows elongate several kilometres long. 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. A new lobe of lava is injected inside the breccia but it is cooled down slower and push the flow further outwards.Lava dome
Short, stocky dome with subsequent pyroclastic deposits extend less than few kilometres long. When explosion eruption occurs, volcanic fragments would be deposited by violent pyroclastic flows. Coarse breccia would be formed as a result. Submarine sediments would subsequently be deposited along the steep flank of the volcano. Submarine landslides would occur to form turbidites.Stratigraphic significance
Archean felsic volcanic rocks 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. Also, felsic volcanic rocks 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 felsic volcanic units.Timing of volcanism
The geochronology of Archean events strongly relies on U-Pb dating and Lu-Hf dating. Since mafic rocks are lack of zircon, only the age of felsic rocks can be dated among the volcanic rocks in greenstone belts. 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. So, the time of occurrence and the duration of volcanic episodes can be revealed.Relationships between Archean felsic volcanic rocks and granitoids
From TTG to GMS granitoids
Two plutonic, igneous rock suites forms 50% of Archean cratons. They are Tonalite-Trondhjemite-Granodiorite suites and Granite-Monzonite-Syenite suites in chronological order. They are magma chambers that later formed the volcanics on the Earth's surface by volcanic eruption. Later they intruded the supracrustal rocks of similar age and composition in Archean. The uprising magma bodies deformed the surface greenstone belt in a cratonic scale.| Relative age | Granitoid | Important mineral present | Magma origin |
| Older | Tonalite-Trondhjemite-Granodiorite | Na-rich plagioclase + garnet + amphibole | hydrated mafic crust |
| Younger | Granite-Monzonite-Syenite | K-feldspar | felsic crust |
The two kinds of granitoids have different magma origins: melting of water-rich mafic materials formed older sodium-rich TTG and melting of felsic materials formed younger potassium-rich GMS. They imply gradual chemical changes of the in the magma and the Earth's crust.
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. On contrary, the chemical compositions of some felsic volcanic rocks are similar to that of GMS but they are much older than GMS. For example, a GMS-like rhyolite unit in the Abitibi Greenstone Belt has no plutonic equivalent in the same period. The composition of felsic volcanic rocks are being altered concurrently with shifting granitoid composition.Possible relationships
The older GMS-like felsic volcanic rocks formed with similar age of TTG has two implications:- 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.
- 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.
Limitation