Monte Muambe Monte Muambe is an inactive volcano located east of Moatize in the Tete Province of Mozambique. The volcano is 780 meters high, has a 6 km external diameter with a caldera A caldera ( ) is a large cauldron-like hollow that forms shor ...

is a volcanic

caldera A caldera ( ) is a large cauldron-like hollow that forms shortly after the emptying of a magma chamber in a volcano eruption. When large volumes of magma are erupted over a short time, structural support for the rock above the magma chamber is ...

located south-east of Moatize in Tete Province of Mozambique. Morphologically, Monte Muambe is a ring-shaped feature, composed of

Karroo The Karoo ( ; from the Afrikaans borrowing of the South Khoekhoe !Orakobab or Khoemana word ''ǃ’Aukarob'' "Hardveld") is a semi-desert natural region of South Africa. No exact definition of what constitutes the Karoo is available, so its ext ...

sandstones which rise from peneplain plateau at altitude between 250 m in the south and 300 m in the north. The rim of the caldera forms a well-developed circular ridge of altered and indurated Karroo sandstones ( gritstones) rising approximately 500 m above the plain of unaltered Karroo sediments. Due to hardness of altered Karroo sediments, after continuous, long-time erosion, Monte Muambe at present is a well-developed circular ridge (caldera), basin-shaped, and steep-sided with a crater-like depression inside. The core of the circular structure is 200 m lower than its periphery.

Geology of adjacent area

The region of

is situated on the southern margin of the Tete gabbro-diorite complex of the

Precambrian The Precambrian (or Pre-Cambrian, sometimes abbreviated pꞒ, or Cryptozoic) is the earliest part of Earth's history, set before the current Phanerozoic Eon. The Precambrian is so named because it preceded the Cambrian, the first period of the ...

age, in a depression filled in the Mesozoic by sediments and volcanic activity products of the Karroo System.

Karroo System

Karroo sediments are in a tectonic relationship with older rocks. The boundary of the Tete gabbro-diorite complex is E -W striking dislocation. These sediments form an east-west trending belt, gently dipping to southeast in the sedimentary formation and volcanic rocks of Monte Muambe area. The succession normally progresses from north to south. From the oldest formations in the north as represented by Serie Tilitos ad Serie Produtiva with coal seams (Beaufort), sedimentation progressed continuously to fossiliferous sandstones, often cross-bedded, including clay-marly beds (Stromberg) - which closed the Karroo sedimentation. Volcanic formation of Stromberg Series consists of amygdaloid basalts plates in the form of lava flows over Karroo sandstones, and dolerite in the form of dykes. Post-Karroo deposits are commonly represented by sandstones and conglomerates with interstratified Mesozoic or younger volcanic rocks. In the Monte Muambe region they are recognized as rocks of Lupata Series. This geologic formation is divided into Lower Lupata Sandstones and Upper Lupata Series. Older sandstones consider to include also

rhyolite Rhyolite ( ) is the most silica-rich of volcanic rocks. It is generally glassy or fine-grained (aphanitic) in texture, but may be porphyritic, containing larger mineral crystals (phenocrysts) in an otherwise fine-grained groundmass. The mineral ...

and younger include

alkaline In chemistry, an alkali (; from ar, القلوي, al-qaly, lit=ashes of the saltwort) is a base (chemistry), basic, ionic compound, ionic salt (chemistry), salt of an alkali metal or an alkaline earth metal. An alkali can also be defined as ...

rocks. Only the Upper Lupata Sandstones are developed in Monte Muambe region. The formation consists of two members:

clastic Clastic rocks are composed of fragments, or clasts, of pre-existing minerals and rock. A clast is a fragment of geological detritus,Essentials of Geology, 3rd Ed, Stephen Marshak, p. G-3 chunks, and smaller grains of rock broken off other rocks ...

, mostly feldspathic various grained sandstones, and volcano erupted - alkali lavas. The intrusive members are

syenite Syenite is a coarse-grained intrusive igneous rock with a general composition similar to that of granite, but deficient in quartz, which, if present at all, occurs in relatively small concentrations (< 5%). Some syenites contain larger proport ...

and carbonatite. The complete sedimentary sequence is separated in Lupata region, about 10 km southeast of Monte Muambe, in Chincongolo area, near to Lupata Gorge, and consists of Upper Lupata Sandstones formation, intrusive rocks (syenite), carbonatites (intrusive and effusive products) and alkaline extrusive rocks. According to field findings geological units in the region as follows: 1. Coarse-grained and conglomeratic sandstones which transgressive overlain basalts, gently dipping to the south. 2. Agglomerates which developed from sandstones are composed mainly of fragments of Karroo sediments, volcanic carbonatite material, and syenite fragments - cemented with carbonate. The

pyroclastic Pyroclastic rocks (derived from the el, πῦρ, links=no, meaning fire; and , meaning broken) are clastic rocks composed of rock fragments produced and ejected by explosive volcanic eruptions. The individual rock fragments are known as pyroc ...

series base consists of agglomerate with carbonatite volcanic bombs and lapilii. 3. Alkali lava flows which cover sedimentary sequence forming tabular igneous bodies elongated to south, represented by rhomb porphyry trachytes and hyalotrahytes Summarized the major events and characteristics of Lupata Series their time of extent was from Lower Jurassic to Lower Cretaceous, and from that time Monte Muambe, as a part of volcanic system of the

East African Rift The East African Rift (EAR) or East African Rift System (EARS) is an active continental rift zone in East Africa. The EAR began developing around the onset of the Miocene, 22–25 million years ago. In the past it was considered to be part of a ...

, represent carbonatite extinct volcano.

Carbonatitic Complex

During the Lupate Series formation, in the phase of alkali magmatism, syenite and later carbonatite were also intruded. The concave bottom of caldera, about 5 km in diameter, consists of carbonatite, agglomerate, tuff, feldspathic rocks ( fenites) and basic dykes. The bottom periphery, 0.5 km wide, is mostly built of feldspathic rocks and little or no carbonatite. The central circular part of highly dissected, craggy hills of carbonatite is asymmetrical and about 200 m lower than Karroo ridge. It is composed of relics of carbonatite intrusion, now appearing as carbonatite ring and subsided central part. The carbonatite ring is characterized by vertical scarps on inner side, with surface which often shows irregular knots, nodes, or veins of more resistant (silicate) material. Laterite covers are formed on both sides of carbonatite ring with accumulation of residual ferruginous-manganese minerals. Carbonatitic rocks are characterized by considerable variation, or great variety of textures, while the mineral composition is quite uniform and chiefly independent of texture. According to texture of carbonatite, can be distinguished: hypidiomorphic granular, medium-grained, pseudoporphyric and trachytic. The most abundant are carbonatites with granular texture, while porphyric and trachytoid carbonatites are much less abundant. Porphyric carbonatite are formed by fast cooling of semi-crystallized magma, or in contact with cool environment. The primary crystallized and fast crystallized calcites form phenocrysts and fine-grained groundmass, respectively. The slower cooled magma obtained granular texture. Carbonatitic magma was probably separated during the differentiation and crystallization of syenites and alkali basic rocks. The primary silicate magma was enriched with alkalies, carbon-dioxide, calcium and deficient with silica. During the alkali syenite

crystallization Crystallization is the process by which solid forms, where the atoms or molecules are highly organized into a structure known as a crystal. Some ways by which crystals form are precipitating from a solution, freezing, or more rarely deposi ...

, the carbonate portion of silicate magma was separated forming carbonatitic magma. Alkalies were already linked with feldspars and feldspathoids both in alkali syenites and in metasomatic fenites to be contained by residual carbonate. That is why carbonatites contained very low alkalies. The carbonatitic magma, after the melt separation, was intruded in Karroo sandstones, or earlier formed fenites. The bottom periphery of caldera, 0.5 km wide, is mostly built of feldspathic rocks ( fenites), and little or no carbonatite. Under the name the fenites, several types of rocks are separated: fenised sandstones, syenite-fenite, syenite-fenite with nepheline, and feldspar breccias. The fenised sandstones and syenite-fenite form aureoles around carbonatites. The main characteristic for all fenites is metasomatic substitution and replacement of primary constituents by K-feldspar minerals. At the end of carbonatitic volcano activity, hydrothermal stage have led to the formation of exploitable mineral resources, both in carbonatites and fenites. Afonso R S : 'A Geologia de Mocambique'' , Impresa Nacional de Mocamique, Maputo, 1976.
Bettencourt Dias M :'' Geologia do Monte Muambe''
Koscal M, Kachamila J, Stefanovic M, Janjic M : 'Fluorite mineralization of Monte Muambe cerbonatite complex, Mozambique '
Summary of World Congr. Non-Met Minerals pg 103-114, Beograd, 1985.

References

{{coord missing, Mozambique

Geology of adjacent area

Karroo System

Carbonatitic Complex

See also

References