Alkaline plume-magmatism of continents and oceans and roots of alkaline plumes

Lazarenkov V.G.

State Mining Institute named after G.V. Plekhanov, Saint Petersburg, Russia.

 

According to up-to-date concepts, the plume is a fluid-heat flow, which might be mainly hydrogenous and generated in the Dlayer of the liquid Earths core (Dobretsov et al., 2000, 2008; Letnikov, 2001; Brandon, 2005). Its magmatic realization the plume-magmatism takes place in the asthenospheric Earths layer at depths down to 600-700 km. The similar level of depth is suggested by seismic data for formation of potassic igneous rocks of Java Island (Whitford, Nicholes, 1976). The plume moves in form of a fluid phase with properties of super conductivity, super permeability and probably, with the super ascension rate from the depth of D-layer (about 2900 km) up to the boundary of asthenosphere bottom.

Alkaline magmatism is first of all a continental phenomenon, and thus; it is developing under cratons: platforms and shields, and in a lesser degree in folded areas, mainly in hard median blocks (accretionary terranes). In margins of continents the alkaline magmatism is observed in pericontinental areas of the passive ocean-continent transition zones of Atlantic type (table 1).

In the oceans alkaline magmatism is developed weakly, as compared to continental crust, and the mere series of oceanic alkaline formations seems to be clearly contracted, shortcut. There, the signs of plumes with high alkalinity can be observed, in first turn, on islands located in continent-ocean transition zones near passive Atlantic type margins. For instance, on Cape Verde Islands, on Canaries in the Atlantic Ocean or on Comoro Islands in the Indian Ocean. Thus, the temporal series of the Canaries volcanic complexes have the following composition: tholeitic (Cretaceous-Paleogene), alkali-rhyolite-trachyte-alkali-basaltic (13.7-13.0 Ma) phonolite-trachyte-alkalibasaltic (4.4-3.4 Ma). The temporal series of Comoro Islands complexes is as following: alkaline-basalts (Miocene), phonolite-trachyte-alkali-basalts (Miocene-Pleistocene), leucitite-phonolite-alkalibasalts (Emerick, Duncan, 1982). Temporal series of insular alcaline margin complexes of perioceanic rifts (Canary Islands, Comoro Islands, Cape Verde Islands, Islands of the Gulf of Guinea, etc.) correspond to the first members in the series of continental alkaline volcanic complexes.

Table 1.

Temporal series of volcanic formations in rift zones (Lazarenkov, 1988)

 

Continental

Oceanic

Intercontinental

Pericontinental

Inter-platform rise blocks

Mid-oceanic ridges

leucitic

leucitic

alkali-rhyolitic

alkali-rhyolitic

alkali-trachyte-trachytic

alkali-trachyte-trachytic

phonolitic

phonolitic

blue earth

blue earth

phonolite- leucititic with carbonatite

phonolite- leucititic with carbonatite

basanite-phonolite- leucititic

 

phonolite-trachyte-alkali-basaltic

phonolite-trachyte-alkali-basaltic

phonolite-trachyte-alkali-basaltic

 

rhyolite (alkali-rhyolite) trachyte alkali-basaltic

rhyolite (alkali-rhyolite) trachyte alkali-basaltic

rhyolite (alkali-rhyolite) trachyte alkali-basaltic

rhyolite (alkali-rhyolite) trachyte alkali-basaltic

alkali-basaltic

alkali-basaltic

alkali-basaltic

alkali-basaltic

basaltic

basaltic

basaltic

basaltic

Note. Series are given in order from the earlier to later ones (from bottom upward).

 

The signs of plumes with high alkalinity are observed on the lifted inter-plate blocks of oceanic islands located on oceanic platforms (thalassocratons) - on Seychelles and Kerguelen Island in the Indian Ocean, on Hawaiian Islands in the Pacific Ocean, etc. Thickness of crust in such platforms exceeds in 2-5 times the usual thickness of oceanic crust up to 20-40 km, and its cross-section is similar sometimes to the geologic section of continental crust, for instance, beneath Seychelles (Ben-Abraham et al., 1984). The oceanic crust with increased thickness is a product of a long-lasted geological history and alkaline rocks of such mature crust correspond to alkali-basalts and phonolite-trachyte-alkali-basalts complexes, for example, of Hawaiian Islands.

Finally, some islands are located on the mid-ocean ridges; for instance, the large island of Iceland in the mid-Atlantic ridge. Iceland, considered as a hot spot, has in the basement thin oceanic crust as thick as 10 km, and the island is characterized by acid rhyolite-(alkali-rhyolite)-alkali-basalt volcanism. From  formational point of view, Iceland is not the unique place with rhyolite and alkali-rhyolite magmatism occurrence, because the South Atlantic chain of islands strung along the mid-Atlantic ridge (Ascension, Tristan da Cunha, Gof, Buve islands) is also characterized by occurrence of rhyolitic extrusive rocks (rhyolite-basaltic province of the Atlantic Ocean).

Besides series of alkaline formations, there is one more important index of alkaline igneous activity in continents and oceans that is the rate of alkaline rock occurrence within those largest geotectonic structures. Its value is near 0,1% for continents, than in the oceans it makes maximum several parts per ten thousand, more probably some parts per hundred thousand, or even some parts per million, i.e. it is extremely low (Lazarenkov, 1988). There are not known petrographic provinces with more or less broad aureole of alkaline rocks occurrence in oceans.

Consequently, if continent areas were the place for active alkaline magmatism, the far more extensive oceanic areas didnt relate to such zones. From the concept of plumes this phenomenon may be regarded in the way that the Earths continents did be areas of intensive and continual alkali gaseous drainage during the long-lasted period of time: at least, from the end of Archaean - 2.7-2.5 Ga (Kogarko, 2004), but in vast areas of oceans the alkaline plumes remained weakly developed and appeared, in this way, mainly in Cretaceous - Cenozoic time, i.e. within last 0.14 Ga.

What might be the reason for strong and continual alkaline plume-magmatism under continents and the weak, short-live appearance of this phenomenon in oceans? According to the geochemistry studies of the largest oceanic ultrabasic solid masses, for example, the world largest solid mass Dyu Syud in New Caledonia (4950 км2), and investigations of mantle inclusions in oceanic alkali-basalts, in particular, at Hawaiian Islands, the ultrabasic oceanic mantle is poor in alkalis and incoherent elements, at least down to the bottom of asthenosphere. Obviously, it didnt have much influence upon the plumes chemistry, in terms of variations in its alkalinity and contents of incoherent elements. Thus, it is quite possible that oceanic mantle didnt produce alkaline plumes for this reason. Judging from the Earths geological development history, it may be assumed that envelop of the primary mantle had one and the same genesis and more or less similar ultrabasic composition over the whole Earth as under future oceans, so under future continents. And if it was so, it may be supposed, with the unique initial composition of oceanic and continental mantle, that continental mantle, like the oceanic one, was not the source of alkaline plumes and associated foidaphile elements. Therefore, this source was probably connected with the liquid Earths core, first of all with its outer layer D.

Conclusion: Comparison between alkaline formations developed in continents and oceans leads to assumption that the source of alkaline plumes was the liquid Earths core under the continents and, in far more seldom cases, under oceans.

 

References:

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Lazarenkov V.G. Formational analysis alkaline rock continents and oceans. The formations analysis of alkaline rocks of continents and oceans. Leningrad: Nedra, 1988. 236 p.

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Whitford D.F., Nicholls I.A. A Potassium variation in lavas across the Sunda arc Java and Bali. // Volcanism in Australia, Amsterdam. 1976. P. 63-75.

 


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