The volcano Wau-an-Namus and further crater-like structures in context
 with the large flood basalt field of
Al-Haruj (Libya)

Norbert Brügge, Germany
Dipl. - Geol.

1. Al Haruj flood basalt field

The Al-Haruj is part of an alkaline basaltic intracontinental flood basalt field in central Libya. The northern part the Al-Haruj is marked as Jabal Al-Aswad. The smaller southern part is marked as Jabal Al-Abiyad.
Trigger of the volcanic activity in the Al-Haruj and the neighboring areas could have been a so-called mantle-plume, which probably already in the Mesozoic from the outer Earth's core to the frontier between mantle and crust penetrated and remained for a long time under the African plate. When a plume hits the bottom of the lithosphere, it is partially melted and thinned. This causes the Earth's crust to stretch, causing certain sections of the Earth's surface to sink and rifts were formed (tectonic trenches). The Al-Haruj is located in the so-called Hun-Graben.
The eruptions of the magma took place in the Al-Haruj on this fault zone. The eruption activity periodically flooded the sediments in area. Pyroclastites (ashes, lapilli and tuffs, etc.) are almost completely absent in Al-Haruj. Petrographically, the volcanic rocks in Al-Haruj are mainly alkaline basalts with olivine and pyroxene as well as feldspars. Such (mostly potassium-rich) rocks are typical of continental rift zones.
Morphologically, flood basalt fields and lava flows predominate in the Al-Haruj. Shield volcanoes, cinder cones as well as diatrems and pit-crater sit on the flood basalts. The oldest lavas in the Al-Haruj are about 5.3 million years old, the youngest probably only thousands of years.

A photo of the central part of the volcanic chain exhibiting large shield volcanoes
 with wide pit craters on their tops and smaller pit craters on their flanks (black arrows)

Ruin of a volcano in the southern Al-Abiyad area

Geochemistry and Petrology of Basic Volcanic Rocks of Jabal Al Haruj Al-Aswad, Libya
Al-Hafdh & El-Shaafi -- International Journal of Geosciences, 2015, 6, 109-144

"The Al Haruj Intra-continental Volcanic Province is the largest part of the extensive volcanic activity in Libya which is considered to be a typical within plate basalts. The volcano-tectonics evolution of this province, as well as its origin, are still widely disputed. According to K-Ar dating previously studied, the volcanic activity started in the Late Miocene and lasted until at least the Late Pleistocene. The field may still be volcanically active. The mafic rocks of Jabal Al Haruj have been classified into six major phases or groups. These phases have been differentiated using Landsat images together with aerial photographs of different scales as well as field observations. The topographic forms of the earliest phase are highly eroded while the forms of the latest phase are usually fresh and very well preserved as regards primary features. Mafic lavas of this field consist of alkali basalts to olivine tholeiites (transitional basalt) which contain olivine as essential constituent together with clinopyroxene, plagioclase and glass. The basalt exhibits intergranular, intersertal, ophitic and subophitic relations. Amygdaloidal and glomerporphyritic textures are also observed. The basaltic rocks of different ages and from different localities are petrographically rather similar. Phenocrysts of olivine probably the result from slow cooling in crustal magma chambers prior to eruptions, suggesting that magmas ascended slowly through the crust. 109 samples have been carefully collected from various phases, some of these samples have been chosen for major and trace elements analyses, using XRF in order to determine the characteristics of the mantle source and investigate crustal interaction. The major and trace elements revealed a slightly significant chemical diversity among the phases and within each phase. The normative classification of most of these rocks shows close agreement with their modal classification. A vague correlation between MgO and most major oxides in the studied samples suggests different degrees of partial melting rather than fractional crystallization. A characteristic feature of the studied volcanic rocks is the relatively constant ratios of certain incompatible trace elements (Nb/Zr, Rb/Zr), which provides strong evidence of a common source. In addition, the rocks display similar patterns of the peaks and troughs; this strongly suggests that they have a common parent and common subsequent processes. The compatible transitional metals Ni (81 - 193 ppm) and Cr contents (238 - 361 ppm) and relatively low Mg# (Mg/(Mg + Fet)) (52 - 62) give an indication that the studied basaltic rocks have slightly to moderately fractionated olivine and/or spinel. The magmatism of this volcanic field seems to be related to reactivation of pre-existing structures during the passive rifting of the Sirt Basin that most likely produced in response to convergence between European and African plates since Jurassic until Holocene times."


Volcanoes in the central Al-Haruj

This lava ropes in the northern Al-Haruj are thousands years old in maximum

"Black Hole"

Massive basalt at a hole

Young lava nearby Al-Fogaha

Typical terrain in the Al-Haruj

2. Volcano Wau-en-Namus

"The Wau-en-Namus lies southeast of Al-Haruj. The cone itself consists of a massive sequence of poorly sorted lapilli agglomerate that appears well stratified from a distance. The agglomerate, locally enclosing bombs and some ash, shows no mesoscopic welding effects and is somewhat friable. It is thus deduced that ejecta must have considerably cooled prior to cone build up. However, at a small area of the cone there is congealed twisted spatter in the form of ribbon bombs attesting perhaps to the waning eruptive activity of the volcano as the magma reservoir emptied and pressure dropped.
The slopes of the cone are marked by huge rills in places and in others by curling up and outward bulging thin crust of ash and sand in the vicinity of gashes and cracks, both suggesting escaping volcanic gases. The underside of the crust in such localities shows nodular to botroidal deposits of white salts and yellow sulfur. The latter may also occur as small thin tablets scattered around or in the form of yellow staining on adjacent rock blocks. All of these features appear recent and suggest continued, slow activity of a dormant volcano.
Repeated eruption at the site is attested to by locally slumped agglomerate strata, truncation of the present cone of two earlier structures to the west, and a collapsed dome on the northern side where a shallow, bowl-like crater had developed."

"The Wau-en-Namus is a remote and isolated volcanic caldera complex composed of alkalibasaltic scoria, located in southern Libya at 24°56′N, 17°45′E, about 70 km south of the Pliocene-to-Holocene Al-Haruj volcanic field. The Wau-en-Namus caldera has a diameter of about 4 km and is surrounded by an extensive halo of black ash and lapilli. No lava flows are present at the crater, but bombs and blocks of various types of basalt, olivine megacrysts and upper mantle xenoliths were ejected from the central cinder cone that rises about 140 m above the floor of the caldera. A reliable 40Ar/39Ar age determination of the host basalts was impossible due to the extremely low radiogenic content of the matrix. Step-heating of the whole-rock material, however, suggests that the K-rich matrix is very young."

Lapilli on the caldera ground

Ash of the crater cone

Mafic and silica-rich glasses in mantle xenoliths from Wau-en-Namus, Libya: Textural and geochemical evidence for peridotite–melt reactions
C. Miller et al. --
Lithos 128-131 (2012) 11–26

"Anhydrous spinel peridotite xenoliths in Quaternary nepheline-basanite and melilite- or sodalite-bearing lavas of the Wau-en-Namus volcano  range from lherzolites to harzburgites recording melt extraction in a shallow setting (≤2 GPa). Primary clinopyroxenes have distinct trace element characteristics documenting LILE (large ion lithophile element) depletion or enrichment events predating the formation of glass pockets and veins in the xenoliths. These glasses are aluminous and alkali-rich, range in composition fromultrabasic to silicic (43–67 wt.% SiO2) and may contain empty vugs and micro-phenocrysts of olivine, clinopyroxene, spinel, plagioclase, sodalite, apatite that are similar in composition to phenocrysts in the host lavas. Reactions of infiltrating melt and xenolith minerals are documented by diffuse Fe–Ca-rich rims of olivine in contact with glass, and by spongy-textured reaction domains caused by incongruent dissolution of primary pyroxenes and spinel. Some glasses have trace element characteristics similar to that of the host Ne-basanite, suggesting they were derived from the same source during entrainment and transport to the surface. Incompatible element enrichment and Sr–Nd isotopic compositions of the analyzed host lava are similar to HIMU (high μ; μ=238Pb/204Pb)-type magmas, but the Pb isotopic composition is less radiogenic compared to other intra-plate Neogene magmatic rocks from N Africa."

BSE images illustrating micro-textures in Wau en Namus xenoliths.

a -  Homogeneous Cpx-1 in lherzolite (i) sample L2.

b -  Type (Ia) glass pocket containing phenocrysts of Cpx-3 and Sp-3. Note the feeder veinlets (type Ib glass) that crosscut primary olivine, the undulating olivine-glass contact and the necking off of anhedral olivine from the main grain. The diffuse light gray margins at the edge of Ol-1 in contact with type Ia and Ib glasses indicate the development of Fe–Ca-rich diffusion zones; sample L2 (lherz (i)).

c -  Detail of type (Ia) glass pocket with newly crystallized euhedral Ol-3, Sp-3 and zoned Cpx-3; sample H124b (hzb (iii)).

d -  Type (Ia) glass pocket containing euhedral sodalite (S) in addition to Ap (A=white skeletal crystals), Ol-3 and Cpx-3 phenocrysts, sample H124b (hzb (iii)).

e -  Veining of primary olivine by former melt (type Ib glass). Note the Cpx-3 phenocrysts and the development of Fe–Ca-rich diffusion zones at the undulating olivine–glass contact; sampleW13 (lherz (i)).

f -  Type Ib glass crosscutting Ol-1 and infiltrating along
the olivine/spinel boundary. Note the sieve-texture development of Sp-2 at the margin of Sp-1 where it is in contact with glass; sample L2 (lherz (i)).


a - BSE image showing incongruent breakdown of primary Opx to Ol-2 and silica-rich type IIa glass. Note the sieve textured reaction rim consisting of Sp-2 and Al-rich glass that surrounds primary spinel; sampleWL3 (lherz (i)).

b -  X-ray elemental map (Mg) of reaction zone consisting of Ol-2 and type IIa glass (black) developed on primary Opx; sample W128 (hzb (iii).

BSE images illustrating (c) development of spongy-textured Cpx-2 domains on grain boundaries of Cpx-1 and along a fracture; sample L4 (lherz (i)) and (d) detail of spongy-textured Cpx-2 reaction domain, with dark blebs representing type IIb glass. Note Fe–Ni-sulfide droplets and glass at the contact with Ol-1; sample L4 (lherz (i)).


3. Crater-shaped and dyke structures (South of Jabal Abiyad)

One crater similar of Oasis crater (Kufra region)
Position: 17° 20' 12" E and 25° 13' 07" N

Christian Laroubine, Societe Astronomique de France, presents some Landsat - pictures from centrally Libya, on which a strikingly field of crater-like structures was to be seen. These craters-like structures have a surprising similarity to the craters in the Gilf Kebir region (Egypt).
After searching of the craters-field was to be ascertained, that could be a context to the Tertiary flood basalts in Haruj. The new craters-field is found near in the southwest of the large basalt shield of the Al-Haruj. There is a large ring structure and many small craters. Some are covered evidently with outflows of basalt. One crater has a similarity to the Oasis crater (nearby Kufra). The large ring-structure is crossed by long dykes.


Striking section of crater-field: 16 24'  E and 24° 50' N

Details of craters and dykes

Ring-structure with cross-cut of long dykes

Stretched craters, locally basalt - outflows probable