Alkaline Rocks and Carbonatites of the World

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Occurrence number: 
Chilwa Province
Longitude: 35.38, Latitude: -15.13

The overlapping complexes of Mongolowe, Chaone, Chikala and Chinduzi, the last described separately (No. 103-00-026), are the most extensive centres of nepheline syenite-syenite in the northern part of the Chilwa Province. The work of Garson (1960) and Stillman and Cox (1960) on Chikala, of Vail and Monkman (1960) on Chaone and Vail and Mallick (1965) on Mongolowe, all of which is reviewed by Bloomfield (1965a), demonstrated that each centre comprises a complex series of intrusions that generally take an annular form. The whole complex of intrusions forms an east-west line of low hills 40 km in length that lie north of the Zomba-Malosa complex (No. 103-00-028) of granite and syenite, with which they come in contact along the southern margin of the Mongolowe centre. Although the field relationships are not clear, dating indicates that Zomba-Malosa is somewhat younger than the nepheline syenite centres. The complexes are intruded into Precambrian gneisses which in places are fenitized. In the northwest the outer contacts are obscured by Tertiary sediments while the Chikala complex is nearly surrounded by beach deposits of Lake Chilwa. All three centres comprise successive intrusions of nepheline syenite and syenite that show some degree of annularity but this form is best developed in Chikala which, on all but its western side, consists of inward dipping sheets of perthosite and various mineralogical and textural types of nepheline syenite. Nearer the centre of the complex, however, the intrusions are nearer akin to ring-dykes. The rocks have traditionally been classified as foyaite and microfoyaite, pulaskite, coarse and fine pulaskite and perthosite. There have been some minor variations in the definitions of these rock types but essentially the perthosites are alkali feldspar (perthite) rocks with up to 5% quartz. The pulaskites contain up to 5% nepheline and 5-50% mafic minerals and the foyaites include 5-50% feldspathoids, principally nepheline although sodalite is plentiful in places. Perthosite is the least abundant rock type being confined to a few marginal areas of the Mongolowe and Chaone complexes, where the field relationships appear to indicate that it was the first phase to be emplaced, but forming more extensive sheets on the lower eastern slopes of Chikala. The perthosite is a coarse rock of plates of perthite, in some varieties a little quartz, ferroedenite and more rarely an arfvedsonitic amphibole, often with cores of aegirine-augite, and a little brown biotite. The pulaskites and foyaites range from very coarse rocks with alkali feldspar crystals over 1 cm in diameter and patches of mafic minerals of a similar size to finer-grained homogeneous varieties. The feldspar is perthitic and displays a wide range of complex exsolution and replacement textures. Nepheline may be interstitial or form stout prisms or rounded patches in the feldspar; it is generally fresh with only minor alteration to cancrinite. Interstitial sodalite is present in some of the foyaites and in Chikala may develop as approximately circular patches poikilitically including myriads of small feldspars. The principal mafic phase is katophorite but a wide range of compositions is present ranging from hastingsite and hornblende through katophorite and richterite to magnesio-arfvedsonite (Woolley and Platt, 1986). Pyroxene ranges from salite to aegirine-augite and Woolley and Platt (1986) have shown that the trend of aegirine-enrichment is slightly different for each centre while the pyroxenes of the perthosites define a quite distinct trend from salite to hedenbergite with no detectable trend towards aegirine. Biotite is a ubiquitous but minor phase that defines a trend of increasing Fe2+ (Woolley and Platt, 1986), and an opaque oxide, titanite and apatite are normal accessories with zircon sometimes present. In the northwestern part of the Chaone complex, in southwestern Chikala and in a zone at the contact of these two complexes are areas of metamorphosed basic volcanic rock the largest of which is about 1 km2; xenoliths of the same rock are to be found throughout the two complexes. The mapping seems to indicate that the larger areas have been preserved by downfaulting. These rocks appear to have been lavas, which indicates the relatively shallow crustal level represented by the present erosion surface. Petrographically they are compact, medium-grained, melanocratic rocks that in places display a vague foliation. They consist principally of a pale-green pyroxene and hornblende, which may be concentrated in monomineralic layers. An opaque phase is abundant and biotite occurs in some samples. Nepheline is usually the sole felsic phase with alkali feldspar present in more mesocratic examples. Eight analyses indicate compositions close to average nephelinite (Woolley and Jones, 1987), but a sample analysed by Stillman and Cox (1960) proved to be close to an average nepheline tephrite. There are numerous dykes both within the complexes and in the areas of Precambrian gneiss to the north and south, as depicted on Plate 12 of Bloomfield (1965a). Two main groups of dykes are represented: a group including microsyenite, solvsbergite, rhyolite, microgranite and felsite, which are probably related to the Zomba-Malosa centre (No. 103-00-028), and a phonolite group most of which probably emanate from the Mongolowe-Chaone-Chikala centres but which includes in the north dykes that may have originated at Junguni (No. 103-00-025). The phonolites are texturally rather variable and alkali feldspar, nepheline, aegirine, sodic amphibole and biotite may form phenocrysts. The groundmass is generally a felted mass of prismatic alkali feldspar and nepheline, generally with numerous tiny needles of aegirine and commonly biotite. Zeolite, probably natrolite, is plentiful in some examples and sodalite may occur. Collected analyses of rocks are given in Bloomfield (1965) and numerous analyses are to be found in Woolley and Jones (1987).

Pb-( dating of zircon from Chaone gave 138(14 Ma (Bloomfield, 1961) and K-Ar on amphibole from Chikala 123(3 Ma (Eby et al., 1995). Fission-track ages from titanite were as follows: Mongolowe 125(16.6, 132.6(8.4 and 137.1(8.7 Ma; Chaone 126.0(13.4 Ma; Chikala 122.6(11.4 Ma (Eby et al., 1995).
BLOOMFIELD, K. 1961. The age of the Chilwa Alkaline Province. Records, Geological Survey of Malawi, 1: 95-100.BLOOMFIELD, K. 1965a. The geology of the Zomba area. Bulletin, Geological Survey of Malawi, 16: 1-193.EBY, G.N., RODEN-TICE, M., KRUEGER, H.L., EWING, W., FAXON, E.H. and WOOLLEY, A.R. 1995. Geochronology and cooling history of the northern part of the Chilwa Alkaline Province, Malawi. Journal of African Earth Sciences, 20: 275-88.GARSON, M.S. 1960. The geology of the Lake Chilwa area. Bulletin, Geological Survey of Nyasaland, 12: 1-67.STILLMAN, C.J. and COX, K.G. 1960. The Chikala Hill syenite-complex of southern Nyasaland. Transactions and Proceedings of the Geological Society of South Africa, 63: 99-117.VAIL, J.R. and MALLICK, D.I.J. 1965. The Mongolowe Hills nepheline-syenite ring-complex, southern Malawi. Records, Geological Survey of Malawi, 3: 49-60.VAIL, J.R. and MONKMAN, L.J. 1960. A geological reconnaissance survey of the Chaone Hill ring complex, southern Nyasaland. Transactions and Proceedings of the Geological Society of South Africa, 63: 119-35.WOOLLEY, A.R. and PLATT, R.G. 1986. The mineralogy of nepheline syenite complexes from the northern part of the Chilwa province, Malawi. Mineralogical Magazine, 50: 597-610.WOOLLEY, A.R. and JONES, G.C. 1987. The petrochemistry of the northern part of the Chilwa alkaline province, Malawi. In J.G. Fitton and B.G.J. Upton (eds), Alkaline igneous rocks. 335-55. Geological Society of London, Special Publication 30.
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