Alkaline Rocks and Carbonatites of the World

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Anahef (Amadror)

stripes

Occurrence number: 
003-00-006
Country: 
Algeria
Region: 
Hoggar (Ahaggar) Recent volcanism
Location: 
Longitude: 7.17, Latitude: 24.25

The Anahef region, referred to as Amadror by Remy (1967), lies between the volcanic fields of Adrar n'Ajjer and Atakor and differs from the rest of the Hoggar volcanic province in containing not only extensive areas of extrusive rocks but also numerous intrusions. The igneous rocks can be divided into the four groups of (1) the lava field of Taharaq-Imadouezene and various outliers extending for 20 km to the northeast, (2) a number of volcanic centres scattered across these lava fields, (3) intrusive centres including a 7 km long complex and (4) a volcanic field comprising numerous small areas of basalt, trachyte and phonolite which is located north of Serouenout and extends for about 60 km to the northeast, nearly as far as the Adrar n'Ajjer volcanic field. The Taharaq-Imadouezene volcanic fields extend for some 30 km north-south and form a plateau with an elevation of 3-500 m. The flows comprise olivine basalts and andesites (trachybasalt by TAS nomenclature of IUGS) with some analcime basanites (Remy, 1967). The basanites contain phenocrysts of augite, small crystals of nepheline and plagioclase (An35) and a little amphibole and apatite in a glassy base with much analcime. Half a dozen volcanic centres are described briefly by Remy (1967); they post-date the lavas. Imadouezene is an extrusive mass of trachyte two km in diameter with associated trachyte dykes. The rock contains phenocrysts of orthoclase and biotite and microlites including aegirine. Amekchou Sud is another trachyte edifice 1.5 km across and 80 m high around which are a number of concentric trachyte dykes. The trachyte contains rare phenocrysts of orthoclase and albite and small groups of aegirine and aegirine-augite crystals; sparse barkevikite has arfvedsonite rims and there is a little fluorite and quartz. The 1 km diameter Amekchou Nord centre is also of a peralkaline trachyte which contains a little sodic amphibole including arfvedsonite. Tindi and Iharhen are extrusive trachyte bodies lying to the north of the Taharaq-Imadouezene volcanic field around both of which are dykes of trachyte containing sodic amphibole and aegirine.Remy (1967) divides the intrusive centres into two types. The most numerous are characterised by silica saturated to over saturated rocks, predominantly syenite and monzonite, with fewer basic complexes in three of which occur essexite, nepheline monzonite and nepheline syenite. The largest of the intrusive centres of the first group is Tellerteba (Remy, 1967) which is 8x5 km, has an elevation of 1200 m, and consists of arcuate intrusions together with a central cylinder of extrusive volcanic rocks accompanied by lava flows which form vertical cliffs 600 m high. Rock types include trachyte, rhyolite, syenite and andesite. Andesite forms an outer screen on the southern margin and is partly brecciated. Rhyolite occupies much of the southern end of the complex; it contains alkali feldspar phenocrysts and iron oxides which pseudomorph either aegirine or sodic amphibole. Syenite forms a major arcuate body on the northern margin and an area within the rhyolites. It comprises perthite, biotite, quartz, sodic amphibole, in part riebeckite, rare apatite and zircon. The central trachytes form an outer area of flows which are penetrated by a central plug, that is essentially syenite. Both comprise alkali feldspar, as phenocrysts and microlites, quartz, biotite, aegirine, a sodic amphibole, magnetite and a little apatite. There are numerous cone sheets as well as radial dykes around the intrusion, most of which are peralkaline trachytes. The Assegafi intrusion is located a few kilometres east of Tellerteba, is oval in shape, 2x2.5 km in diameter and surrounded by lavas of the Taharaq field. The central area is of trachyte and trachyte breccia flows that are surrounded by two crescents of syenite; there is a system of concentric dykes around the southern margin. The syenite is composed of perthite, biotite, acicular aegirine, sodic amphibole and analcime and the dykes are microsyenite of a similar mineralogy. The central trachytes consist of alternate flows and thick breccia units up to 100 m thick. Phenocrysts of perthite and biotite are set in a groundmass of orthoclase, aegirine, sodic amphibole and biotite with rare interstitial quartz. Ebeggui is a sub-circular syenite plug 1.5 km in diameter cutting basaltic lavas which is itself penetrated by a 400 m diameter neck of trachyte and a small intrusion of essexite, nepheline monzonite and microsyenite. The syenite contains quartz, orthoclase with rims of plagioclase (An25-50), sodic amphibole, aegirine-augite, biotite and accessories. The essexite is a rock of plagioclase (An25-50), orthoclase, amphibole, biotite, augite with aegirine-augite rims, analcime and nepheline and this grades into monzonites which contain less feldspathoid, no biotite and have a lower colour index. Tezzefi is located southeast of Ebeggui and is an sodic amphibole-bearing syenite intrusion of 1.6x0.6 km with a circular, 500 m diameter trachytic plug to the east of it. Southwest of Tezzefi is the 300 m high monzonite plug of In Kaoukane to the south of which is an essexite intrusion cutting Precambrian metamorphic rocks. In Roundoum is an sodic amphibole-bearing monzonite stock situated on the northern margin of the Anahef area.Of the three basic intrusive centres including alkaline rocks Achkal is an isolated intrusion in the northwest of the area and is the most complex. It is 4 km in diameter and of high relief with a central pillar of rhyolite and trachyte surrounded by arcuate sheets of syenite, monzonite, diorite, essexite, gabbro and diorite. The syenite is exceptionally coarse, with orthoclase crystals 2-4 cm long, and forms a complete ring around the central plug. It contains green-rimmed pyroxene, rare sodic amphibole, a little nepheline and alteration products after feldspathoid. The monzonite, diorite and essexite grade into each other and develop an arc around the central plug. They comprise orthoclase, intermediate plagioclase, augite, biotite and amphibole with nepheline and analcime in the essexite. Taklit lies on the northern margin of the area and is an isolated intrusion consisting of essexite and nepheline monzonite. The essexite form a circular 700 m diameter plug and consists of plagioclase (An50), a little orthoclase, nepheline, abundant crystals of hauyne and analcime, rare olivine, augite, biotite and amphibole. The monzonite occupies two overlapping intrusions to the northeast of the essexite and consists of orthoclase, some plagioclase (An25-30), nepheline, hauyne, analcime and the same mafic minerals as the essexite. Taharaq North is 1.6x0.8 km and intrusive into Taharaq lavas. Early syenite is cut by monzonite and this in turn by shonkinite and finally by a circular plug of nepheline syenite. The syenites and monzonites contain orthoclase in the former and perthite and plagioclase (An45) in the latter; both have augite/aegirine and biotite and rare analcime, nepheline and hauyne. The shonkinite and nepheline syenite are similar mineralogically to the monzonite (Remy, 1967). The Serouenout lava field consists of dozens of small, and several larger (up to 5 km across), outcrops of basalt, trachyte and phonolite (Fabre, 1976, V1,4A, p. 351). No details of field relationships, petrology or chemistry have been traced. Except for the plateau lavas, Remy (1967) describes in some detail all the Anahef occurrences and includes 36 analyses and numerous field photographs.

Age: 
Probably Tertiary and Quaternary.
References: 
FABRE, J. 1976. Introduction a la geologie du Sahara Algerien. Societe Nationale d'Edition et de Diffusion, Alger. 422 pp.LELUBRE, M. 1952. Recherches sur la géologie de L'Ahaggar central et occidental (Sahara Central). Bulletin du Service de la Carte Géologique de l'Algérie, Ser. 2, 22: 1-385.REMY, J.-M. 1967. Étude géologique et volcanologique du sud-est de l'Amadror en Ahaggar (Sahara Central). Sciences de la Terre, Mémoire. Nancy. 10: 1-189.
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