Alkali Basalt Associations of the Continents
OIBs & Continental Alkali-Basalt-Basinites

There are not a great many alkali basalt associations within the continents, most tend to be on the oceanic islands or at continental margins, especially along block faults or incipient spreading centres such as the E. African Rift.

The Cameroon Line

South-east of Nigeria in the Gulf of Benin are a series of active volcanic islands, Pagalu, Sao Thome, Principe, Fernado Poo, and inland Mt Cameroon, (13,500ft) lying along an apparent fault that crosses the continental margin.. Fitton & Dunlop (1985, EPSL 72) claim there is no change in composition on either side of the continental-oceanic conjunction.

|__| Pagalu.


Small centres occur in Europe. The Pliocene-Quaternary Catalonian Volcanics in NE Spain (Coy-Yll, Gunn & Travaria, 1974, Acta.Geol.Hispanica) are quite ordinary basanites, but the jumillites and verites of southern Spain, (Borley,1967.Min.Mag.36) are variable but highly potassic with up to 6% K2O at 50-60% SiO2. Basanites of West Germany in the Laachersee area (Schminke et al...) are again unremarkable as are small areas of alkali basalt in the south of France.

The East African Rift, (Western branch)

The western rift is a great graben which extends for 750 miles from L. Albert to L Tanganyika and beyond through L. Nyasa.

About 100 miles west of Lake Victoria within the main Rift Zone are three areas of most remarkable potassic volcanics. North of Lake Edward is the Ruwenzori Range and the Toro Ankole province. A little further south between L. Edward and Lake Kivu is an area variously described as the Bufumbira, the Virunga or Birunga or Lake Kivu province. It includes several active and dormant cones, Nyamuragira, Nyiragongo, (11,380 ft), Karisimbi, (14,850ft), Mikeno (14,700ft), Visoki and Muhavura. Nyiragongo especially, has been subject to much attention because of a pit crater within a 3,600ft deep caldera with an active lava lake. This appeared in 1928 but in 1977 a flank emission of lava drained the lake killing 70 people. Since 1982 the lava lake has built up again.

More lavas appear south of Lake Kivu.
The lava lake in Nyiragongo

The lava, about 300ft below the lip of the inner pit crater (seen in the foreground), is circulating round an "island" of rock which has broken from the wall

(Photo: Tazieff; about 1972)

Haroun Tazieff, the well-known volcanologist, has made an especial study of this mountain. The rocks of the region are variously described as potassic basanites, leucite basalts or leucite nepheline melilite basalts. Stress is laid on their variability in different centres. The nepheline is usually potassic and may be kalsilite.
Oddly, the data do not show any great differences between centres nor are most rocks abnormally potassic (see MORB normalised diagram below). In fact the basanites appear similar to those of Tristan or Gough Islands.
However, some highly fractionated lavas are very potassic with 3-5% K2O, very low silica (35-40%) and oddly high CaO at low Mg. Melilite is usually found only in high Ca rocks. Whether these oddities are due to ingestion and desilication of limestone (but no blocks of wollastonite (CaSiO3) are reported), or whether as the leucite-nepheline tends to aggregate in what Tazieff calls "nests" and the very high K is due to poor sampling it is not possible to tell.
East Virunga Province. volcanoes (Rwanda)
Lake Kivu Basanites
Menegai Trachytes

The East Branch Of The E.A.R.S. (East African Rift System)

A rift system begins at the Afar Triple Point in Ethiopia where the spreading centres of the Red Sea, the Gulf of Aden and the African rift join.
The latter trends south through the Afar region (with several active shield volcanoes) towards Lake Rudolf, (Turkana) where there are the Turkana alkali basalts, through L. Naivasa with the Navaisa commendites, past Mt Kenya and Nairobi and the section known as the Kenyan Rift or the Gregory rift to Lake Natron. At the northen end of this is a large volcano, Shombole with the carbonatite Oldoinyo Lengai at the southern end and several massive alkali basalt cones including Meru and Kilimanjaro to the east. The Kenyan - Gregory Rift does not link up with the western branch between L Tanganyika and L. Nyasa but seems to fade.

Rather than E-type MORBs which might be expected in an incipient spreading centre, the lavas are seemingly all alkaline, mainly potassic in the western branch and mainly sodic in the eastern.

Shombole is an extinct nephelinite - phonolite-carbonatite volcano on the northern shore of L. Natron in the Eastern Rift. The fingerprint shows a negative K, a sure sign of a sodic magma though the level of LILE is very high.
Oldoinyo Lengai, the only active carbonatite volcano in the world, lies a few miles south of Lake Natron rising 6000ft above the Eastern Rift Valley.
Peralkaline nephelinites, Oldoinyo Lengai (E.Africa)


Kilimanjaro (19,500ft)
Ol Doinyo Lengai
Carbonatites in the caldera of Ol Doinyo Lengai
South of Lake Natron in the Gregory Valley is Ngorongoro and 8 other cones, the only active one being Ol Doinyo Lengai which erupts sodic carbonatite.

Kerimasi, an extinct carbonatite cone lies immediately to the south east. Four other carbonatite cones are known in the general area of northern Tanzania, as well as Kisingiri in west central Kenya (Peterson, 1989)

Oddly, there seems to be no recent data for even such a well known centre as Kilimanjaro.

The Navaisa perakaline commendites reported under "Rhyolites-Trachytes-Phonolites" lie about 30mi NW of Nairobi.

A group from Penn State University headed by Dr Tanya Furman is working on the EARS and have obligingly allowed us to see all the data. While some series from near the Afar Triangle are transitional the rest as far south as southern Lake Tanganyika, are alkaline, sometimes highly so. Zr/Nb, La/Lu and isotopes overlap with oceanic OIBs. Small volume flows of differing composition and little fractionation seem to be typical.No characteristics of a spreading centre can be seen. Has the the bifurcating spreading of the Read Sea and the Horn of Africa resulted in a series of south propagating tensional faults, but without true spreading? The EARS is more than a little puzzling.


In the Turiy Peninsula, (part of the Kola Peninsula, northern Russia) melilitites, mela-nephelinites grade into nephelinites and carbonatites.

The carbonatites are notable for their high Sr, Ba, La, Ce and to a lesser extent, Nd. These are similar to the "Bermudite" melanephelinites but while they have the same Fe levels, they have only half the TiO2.


In north-eastern China is a large field of basanites, AOB's and tephrite, the Holocene Jingpohu series, Zhang et al, 2002, Geoch.Jour. 36, 133-153. Carbon ages give 2470 - 3490 yrs. The series are quite potassic and minor leucite is found in the tephrites.

Fingerprint for the Jingpohu basanite-tephrites. Cs and Pb have been interpolated.
REE for Jingpohu. Note wide separation of Ce - Nd compared with MORB.

Once again we see liitle diffence between continental and oceanic AOB's.


The Monteregian Hills

A line of the eroded bases of a series of Lower Tertiary-Cretaceous alkaline centres extend along a line east of the city of Montreal. Mount Royal itself is the best known. The core of the mountain is a black-coarse-grained cumulate of titanaugite, (Montrealite). The standard rock, the Mount Royal Gabbro (MRG) comes from this. There also a wide range of related intrusives, inchscale layered steeply-dipping leucogabbros, syenites, microsyenites and lamprophyres often with coarse kaersutite. No lava flows remain. At Oka to the west are carbonatites with a high REE content.

|__| The Mount Royal "Gabbro".


Alkali Basalts – Syenites of West Texas

The Terlingua region of West Texas is located in the Big Bend of the Rio Grande (or Rio Brava) between the Chisos Mountains and the Mexican border. A small basaltic province of Early Tertiary age is intruded into and overlies shallow water Cretaceous sediments. The lavas are mainly of alkali basalt (the Alamo Creek Basalts) and the intrusions are of syenogabbro and syenodiorite. This area has been a special project for many years of Dr Max Carman of the University of Houston and his students while I also assisted at times.

The rocks are of special interest as the sills show late stage segregations of ocelli, veins, bands, layers, cylinders etc of a micro-syenitic (or trachy-phonolite) residue, (see Carman, Cameron, Cameron and Gunn, 1969). It demonstrates in a small way the means by which large syenite intrusions and trachyte flows originate. These features are best seen in the 400ft thick Rattlesnake Mtn Sill.

The main rock is of clinopyroxene and plagioclase of about An 50-60. The syenitic layers have biotite, opaques, sodic plagioclase and alkali feldspar. Carman recognises an earlier plagioclase –rich syenite followed by a plagioclase-poor syenite.

Rattlesnake Mtn Aerial view of the Rattlesnake Mtn Sill, with sedimentary rocks showing below. To the left is Terlingua Creek which flows into the Rio Grande, the Chisos Mts lie to the north (right). The country is semi-desert with yucca, ocatillo, mesquite, creosote bush, saguaro and sagebrush. Apart from intermittent mining for mercury and some ranching, the area is mainly uninhabited.
Ocelli of syenite in upper margin rocks.
Thin veins of syenite formed in partings in the crystal mesh.
Bands of up to 12inch thickness or more in syenodiorite.

Note the belt sagging to the right, this is, after all, Texas!

Interdigitating sill margin.
Thin section of syenodiorite with clinopyroxene-plagioclase.
Thin sections of syenite., with some plag and clinopyroxene, but also biotite and heavy residua of K/Spar. This is presumably a plag-rich syenite.
A tree-like vertical autointrusion of syenite.
A "cylinder". These appear to be diapirs where a layer has accumulated risen through the crystal mesh.
A general view of the intrusion showing the "stair steps" created by weathering. These may reflect circulation patterns in cooling magma and are not uncommon in all sills.

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