Mafurite

Mafurite: An ultrabasic rock consisting of phenocrysts of olivine and minor pyroxene in a groundmass of diopside and kalsilite with small amounts of perovskite, olivine and biotite. It is a kamafugitic rock and now regarded as an olivine-pyroxene kalsilitite. (Le Maitre, 2005)

Kalsilite-bearing rocks (kamafugitic rocks)

Kamafugite is a term describing several kalsilite-bearing rocks alkaline volcanic rocks. These rocks are characterized by the presence of modal kalsilite, melilite and perovskite as a result of their extreme silica undersaturation, low Al, K-enrichment and extremely high Ca.
The name kamafugite (Sahama, 1974) is an acronym made up of its most conspieuous rock types Katungite, Mafurite and Ugandite, after alkaline mafic volcanics occurring in the Toro-Ankole volcanic province (Uganda).
IUGS recommendations for the kalsilite-bearing rocks eliminate the local names but retain the term kamafugite as a rock series name. Kamafugitic rocks have low Al2O3 but high CaO (usually > 12%), and represent primitive ultrapotassic rocks which are rare yet found on all continents.

Since Holmes and Harwood (1932) first described the upper Pleistocene (<50 ka) kalsilite-bearing lavas that were named later by Sahama (1974) as kamafugite series in South-west Uganda, kamafugites have been reported in several locations worldwide, in particular the well-studied type examples such as the middle Pleistocene kamafugites in Italy and the late Cretaceous suite in Brazil. However, the origin of kamafugites remains not well understood. An important observation is that most kamafugites worldwide coexist with carbonatites, including all the cases mentioned above.

Kamafugitic rocks can be classified as:

Mafurite: Olivine-pyroxene kalsilitite;
Katungite: Kalsilite-leucite-olivine melilititi;
Ugandite: Pyroxene-olivine leucitite;
Venanzite: Kalsilite-phlogopite-olivine-leucite melilitite;
Coppaelite: Kalsilite-phlogopite melilititi;

The Bunyaruguru volcanic field

The Toro-Ankole volcanic field (Fig.1) of southwestern Uganda contains the archetypal kamafugites, a series of rare silica-undersaturated, calcium-rich igneous rocks. The absence of plagioclase distinguishes these from lavas occurring in the rest of the western rift branch. Although the compositions and petrographic characteristics of the rocks from these famous localities are used as a benchmark for strongly silica-undersaturated potassic rocks all over the world. From north to south these are distinguished: (a) Rusekere; (b) Fort PortaI; (c) Ndali, formerly known as Kyatwa; (d) Katwe-Kikorongo; (e) Bunyaruguru, formerly known as Kichwamba, and (f) Katunga.
Eruptives in Toro-Ankole occur as numerous tuffs, small explosion craters and maars rich in bombs and ejected blocks, and frequently carry rounded ultramafic nodules dominated by pyroxenites and glimmerites. The dominance of explosive volcanic activity indicates the volatile-rich nature of the magmatism. The oldest known Toro-Ankole rock is a lava with a K-Ar age of 46 ka, whereas volcanics from the youngest region (Fort Portal) range from 6000 to 4000 years.

toroankole2017.jpg

Fig.1: Sketch map of the Toro-Ankole volcanic province (Uganda). Modified from Holmes, A. (1945).



The Bunyaruguru volcanic field (Fig.2), also known as Kichwamba, lies along the eastern side of the Rift Valley, south of Lake George, and extend over an area of 37x7 Km. The field consists principally of tuff, that extend over the Rift Valley floor, together with rare occurrence of lava. Numerous explosion crater have been blown through the tuff, and there is one isolated crater, the Kasenyi crater, that lies to the NW of the main field. In the northern part of the field the tuff grade into Pleistocene to recent sand, silt and gravel, and to the south the tuffs overlie Precambrian schist. Holmes in a number of paper has classified the volcanic rocks of the Bunyaruguru volcanic field into four major type: Ugandite (augite+leucite), Mafurite (augite+kalsilite), Katungite (melilite+leucite glass+leucite) and kalsilite-katungite (melilite+kalsilite). All the four types also contain abudant olivine, perovskite and opaque phases, sometime with biotite and glass.

bunyaruguru.jpg

Fig.2: The Bunyaruguru volcanic field.



Petrogenesis
The Ugandan kamafugites are volatile-rich, potassic to ultrapotassic, silica undersaturated (SiO2 = 31.8-41.8 wt.%) volcanic rocks. They are MgO-rich (6 to 22.5 wt.% MgO) and possess high Mg#. An unusual chemical feature of the Ugandan kamafugites is their low Al2O3 (< 8.0 wt.%) coupled with high CaO contents (> 16.6 wt.%) relative to other ultrapotassic rocks.
In terms of major elements, kamafugites in general, are most similar to ultramafic lamprophyres, which are characteristic of continental rifts that cut through thick, cold lithosphere. However, end-member katungite, mafurite and ugandite are chemically distinct in terms of major elements, and they cannot be related through fractional crystallization.
The compositional differences between kamafugitic rock types point to differences in depth or degree of melting of the mantle source. Major differences in source composition can be excluded because of the similar REE distribution pattern and nearly constant incompatible trace element ratios. Numerous experiments have been performed to evaluate the source mineralogy of kamafugitic magma. These show that clinopyroxene, phlogopite, CO2 and H20 play an important role in the genesis of kamafugites, possibly as widespread mica clinopyroxenite, or wehrlitic veins in mantle peridotite at the base of the lithosphere.

Many authors consider the East African Rift kamafugites as a the initial stage continental rifting. The strongly potassic character of kamafugites may be explained by melting phlogopite-bearing veins at > 100 km depth. Towards the south, melting occurs at shallower levels, involving amphibole and phlogopite to produce olivine melilitites and basanites (Virunga), and amphibole alone further south to produce tholeiites in the Kivu field.

Sample doned by F.Stoppa (University of Chieti, Italy)


Bibliography



• Holmes, A. (1945). Leucitized granite xenoliths from the potash-rich lavas of Bunyaruguru, Southwest Uganda. Am. J. Sci., 243, 313-332.
• Tappe, S., Foley, S. F., & Pearson, D. G. (2003). The kamafugites of Uganda: a mineralogical and geochemical comparison with their Italian and Brazilian analogues. Periodico di Mineralogia, 72, 51-77.
• Lavecchia, G., Stoppa, F., & Creati, N. (2006). Carbonatites and kamafugites in Italy: mantle-derived rocks that challenge subduction. Annals of Geophysics, 49(1), 389-402.


Photo
Foto
mafurite(1).jpg

Olivine crystals (partially altered by iddingsite) and pyroxene crystals within a kalsilite-rich groundmass (the small colorless crystals). Mafurite from Bunyaruguru, Uganda. PPL image, 2x (Field of view = 7mm)
mafurite(2).jpg

Olivine crystals (partially altered by iddingsite) within a kalsilite-rich groundmass (the small colorless crystals). Mafurite from Bunyaruguru, Uganda. PPL image, 2x (Field of view = 7mm)
mafurite(3).jpg

Olivine crystals (partially altered by iddingsite) within a kalsilite-rich groundmass (the small I order gray crystals). Mafurite from Bunyaruguru, Uganda. XPL image, 2x (Field of view = 7mm)
mafurite(4).jpg

Olivine crystals (partially altered by iddingsite) within a kalsilite-rich groundmass (the small colorless crystals). Mafurite from Bunyaruguru, Uganda. PPL image, 2x (Field of view = 7mm)
mafurite(5).jpg

Olivine crystals (partially altered by iddingsite) within a kalsilite-rich groundmass (the small I order gray crystals). Mafurite from Bunyaruguru, Uganda. XPL image, 2x (Field of view = 7mm)
mafurite(6).jpg

Olivine crystals (partially altered by iddingsite) within a kalsilite-rich groundmass (the small colorless crystals). Mafurite from Bunyaruguru, Uganda. PPL image, 2x (Field of view = 7mm)
mafurite(7).jpg

Olivine crystals (partially altered by iddingsite) within a kalsilite-rich groundmass (the small I order gray crystals). Mafurite from Bunyaruguru, Uganda. XPL image, 2x (Field of view = 7mm)
mafurite(8).jpg

Olivine crystals (partially altered by iddingsite) within a kalsilite-rich groundmass (the small colorless crystals). Mafurite from Bunyaruguru, Uganda. PPL image, 2x (Field of view = 7mm)
mafurite(9).jpg

Olivine crystals (partially altered by iddingsite) within a kalsilite-rich groundmass (the small I order gray crystals). Mafurite from Bunyaruguru, Uganda. XPL image, 2x (Field of view = 7mm)
mafurite(12).jpg

Olivine crystals (partially altered by iddingsite) within a kalsilite-rich groundmass (the small colorless crystals). Mafurite from Bunyaruguru, Uganda. PPL image, 2x (Field of view = 7mm)
mafurite(13).jpg

Olivine crystals (partially altered by iddingsite) within a kalsilite-rich groundmass (the small I order gray crystals). Mafurite from Bunyaruguru, Uganda. XPL image, 2x (Field of view = 7mm)
mafurite(14).jpg

Olivine crystals (partially altered by iddingsite) within a kalsilite-rich groundmass (the small colorless crystals). Mafurite from Bunyaruguru, Uganda. PPL image, 2x (Field of view = 7mm)
mafurite(15).jpg

Olivine crystals (partially altered by iddingsite) within a kalsilite-rich groundmass (the small I order gray crystals). Mafurite from Bunyaruguru, Uganda. XPL image, 2x (Field of view = 7mm)
mafurite(16).jpg

Olivine crystals (partially altered by iddingsite) within a kalsilite-rich groundmass (the small colorless crystals). Mafurite from Bunyaruguru, Uganda. PPL image, 2x (Field of view = 7mm)
mafurite(17).jpg

Olivine crystals (partially altered by iddingsite) within a kalsilite-rich groundmass (the small colorless crystals). Mafurite from Bunyaruguru, Uganda. PPL image, 2x (Field of view = 7mm)
mafurite(18).jpg

Olivine crystals (partially altered by iddingsite) within a kalsilite-rich groundmass (the small I order gray crystals). Mafurite from Bunyaruguru, Uganda. XPL image, 2x (Field of view = 7mm)
mafurite(19).jpg

Dunitic xenolith. Mafurite from Bunyaruguru, Uganda. PPL image, 2x (Field of view = 7mm)
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Dunitic xenolith. Mafurite from Bunyaruguru, Uganda. XPL image, 2x (Field of view = 7mm)
mafurite(21).jpg

Olivine crystals (partially altered by iddingsite) within a kalsilite-rich groundmass (the small colorless crystals). Mafurite from Bunyaruguru, Uganda. PPL image, 2x (Field of view = 7mm)
mafurite(22).jpg

Olivine crystals (partially altered by iddingsite) and pyroxene crystals within a kalsilite-rich groundmass (the small colorless crystals). Mafurite from Bunyaruguru, Uganda. PPL image, 2x (Field of view = 7mm)
mafurite(23).jpg

Olivine crystals (partially altered by iddingsite) and pyroxene crystals within a kalsilite-rich groundmass (the small I order gray crystals). Mafurite from Bunyaruguru, Uganda. XPL image, 2x (Field of view = 7mm)
mafurite(24).jpg

Olivine crystals (partially altered by iddingsite) within a kalsilite-rich groundmass (the small colorless crystals). Mafurite from Bunyaruguru, Uganda. PPL image, 2x (Field of view = 7mm)
mafurite(26).jpg

Olivine crystals (partially altered by iddingsite) within a kalsilite-rich groundmass (the small colorless crystals). Mafurite from Bunyaruguru, Uganda. PPL image, 2x (Field of view = 7mm)
mafurite(27).jpg

Olivine crystals (partially altered by iddingsite) within a kalsilite-rich groundmass (the small I order gray crystals). Mafurite from Bunyaruguru, Uganda. XPL image, 2x (Field of view = 7mm)
mafurite(28).jpg

Olivine crystals (partially altered by iddingsite) within a kalsilite-rich groundmass (the small colorless crystals). Mafurite from Bunyaruguru, Uganda. PPL image, 2x (Field of view = 7mm)
mafurite(29).jpg

Olivine crystals (partially altered by iddingsite) within a kalsilite-rich groundmass (the small I order gray crystals). Mafurite from Bunyaruguru, Uganda. XPL image, 2x (Field of view = 7mm)
mafurite(30).jpg

Olivine crystals (partially altered by iddingsite) within a kalsilite-rich groundmass (the small I order gray crystals). Mafurite from Bunyaruguru, Uganda. XPL image, 2x (Field of view = 7mm)
mafurite(32).jpg

Olivine crystals (partially altered by iddingsite) within a kalsilite-rich groundmass (the small colorless crystals). Mafurite from Bunyaruguru, Uganda. PPL image, 10x (Field of view = 2mm)
mafurite(33).jpg

Olivine crystals (partially altered by iddingsite) within a kalsilite-rich groundmass (the small I order gray crystals). Mafurite from Bunyaruguru, Uganda. XPL image, 10x (Field of view = 2mm)
mafurite(35).jpg

Olivine crystals (partially altered by iddingsite) within a kalsilite-rich groundmass (the small I order gray crystals). Mafurite from Bunyaruguru, Uganda. XPL image, 10x (Field of view = 2mm)
mafurite(36).jpg

Olivine crystals (partially altered by iddingsite) within a kalsilite-rich groundmass (the small colorless crystals). Mafurite from Bunyaruguru, Uganda. PPL image, 10x (Field of view = 2mm)
mafurite(37).jpg

Olivine crystals (partially altered by iddingsite) within a kalsilite-rich groundmass (the small colorless crystals). Mafurite from Bunyaruguru, Uganda. PPL image, 10x (Field of view = 2mm)
mafurite(38).jpg

Olivine crystals (partially altered by iddingsite) and pyroxene within a kalsilite-rich groundmass (the small colorless crystals). Mafurite from Bunyaruguru, Uganda. PPL image, 10x (Field of view = 2mm)
mafurite(39).jpg

Olivine crystals (partially altered by iddingsite) and pyroxene within a kalsilite-rich groundmass (the small I order gray crystals). Mafurite from Bunyaruguru, Uganda. XPL image, 10x (Field of view = 2mm)
mafurite(40).jpg

Olivine crystals (partially altered by iddingsite) within a kalsilite-rich groundmass (the small colorless crystals). Mafurite from Bunyaruguru, Uganda. PPL image, 10x (Field of view = 2mm)
mafurite(41).jpg

Olivine crystals (partially altered by iddingsite) within a kalsilite-rich groundmass (the small colorless crystals). Mafurite from Bunyaruguru, Uganda. PPL image, 10x (Field of view = 2mm)
mafurite(42).jpg

Olivine crystals (partially altered by iddingsite) within a kalsilite-rich groundmass (the small colorless crystals). Mafurite from Bunyaruguru, Uganda. PPL image, 10x (Field of view = 2mm)
mafurite(43).jpg

Olivine crystals (partially altered by iddingsite) and pyroxene within a kalsilite-rich groundmass (the small colorless crystals). Mafurite from Bunyaruguru, Uganda. PPL image, 10x (Field of view = 2mm)
mafurite(44).jpg

Olivine crystals (partially altered by iddingsite) and pyroxene within a kalsilite-rich groundmass (the small I order gray crystals). Mafurite from Bunyaruguru, Uganda. XPL image, 10x (Field of view = 2mm)
mafurite(45).jpg

Olivine crystals (partially altered by iddingsite) and pyroxene within a kalsilite-rich groundmass (the small colorless crystals). Mafurite from Bunyaruguru, Uganda. PPL image, 10x (Field of view = 2mm)
mafurite(46).jpg

Olivine crystals (partially altered by iddingsite) within a kalsilite-rich groundmass (the small colorless crystals). Mafurite from Bunyaruguru, Uganda. PPL image, 10x (Field of view = 2mm)
mafurite(47).jpg

Olivine crystals (partially altered by iddingsite) within a kalsilite-rich groundmass (the small I order gray crystals). Mafurite from Bunyaruguru, Uganda. XPL image, 10x (Field of view = 2mm)
mafurite(50).jpg

Olivine crystals (partially altered by iddingsite) within a kalsilite-rich groundmass (the small colorless crystals). Mafurite from Bunyaruguru, Uganda. PPL image, 10x (Field of view = 2mm)
mafurite(48).jpg

Olivine crystals (partially altered by iddingsite) within a kalsilite-rich groundmass (the small colorless crystals). Mafurite from Bunyaruguru, Uganda. PPL image, 10x (Field of view = 2mm)
mafurite(49).jpg

Olivine crystals (partially altered by iddingsite) within a kalsilite-rich groundmass (the small I order gray crystals). Mafurite from Bunyaruguru, Uganda. XPL image, 10x (Field of view = 2mm)