Genesis of Ultrapotassic Rocks - Origin of Potassium-rich Silica-deficient Igneous Rocks - page 465

15.14 Possible Causes for the Frequent Occurrence
of K-Rich Silica-Poor Volcanic Rocks in the
Recent Evolutionary History of the Earth

Eruption of K-rich lavas are recent (Table 15.1 ); but it has been recorded by most
petrologists (See Table 15.2 ) that there had been emplacement of very few alkaline
silica-undersaturated rocks during Precambrian (e.g. some shoshonitic rocks and
minor amounts of calc-alkalic lamprophyres), which might indicate the presence of
rather a few regions of subduction-related magmatism and presence of ancient
convective cycles within the Archaean mantle.
It has been argued that scarcity of alkaline rocks during the Precambrian could
be due to one of the following reasons (Blichert-Toft et al. 1996):

1. Preferential destruction of alkaline rocks as the feldspathoids present are rather
facile (Gupta and Fyfe 1975),
2. The majority of alkaline rocks are present in tectonic settings that have little
chance of survival,
3. Rift-related volcanic rocks are easily eroded away,
4. Those formed in the oceanic islands might have been subducted and therefore,
lost.

Although the above mentioned arguments could be correct, judging by the
abundance of K-rich volcanic rocks in the modern geological history, and occur-
rence of komatiites almost exclusively during Archaean, it is most likely that the
thermal-tectonic condition of the mantle during Precambrian was different. On the
basis of experimental studies the estimated temperatures of eruption of komatiites
(Green et al. 1975) were quite high, and according to Campbell et al. (1990) they
represent ancient analogues of modern plume-related magmas. Such high temper-
atures and extensive melting might indicate that alkaline rocks could not have been
formed because they need low degree of melting (Frey et al. 1978). It is possible
that even during ancient time low degree of melts might have been produced at the
cooler margins of the plumes, but Blichert-Toft et al. (1996) consider that they
might have been swamped by lavas of high degree melts from the central hot zone.
This would be similar to absence of alkalic suite from Mid-Oceanic plateau.
According to Green and Taylor (1987), experimental studies indicate that in
presence of CO 2, partial melting of peridotite results in the production of silica-
undersaturated magmas. It is possible that carbon was concentrated in the atmo-
sphere during Archaean, and CO 2 was not introduced to the mantle. For example,
Lowe (1980), stated that carbonates are relatively rare in the Archaean sedimentary
formation and oceanic crust was probably not introduced to the mantle as frequently
as today.
Modern view states that genesis of K-rich magma is related to partial melting of
fertile mantle, because the original mantle has been metasomatized by introduction
of crustal segments. This also involves introduction of hydrous fluid to the

asthenosphere. This mechanism happens more frequently in the modern time