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North Nyasa Alkaline Province (NNAP), Malawi

The North Nyasa Alkaline Province (NNAP) of Malawi consists of seven late-Precambrian intrusions emplaced along a north-south trend roughly parallel to that of the current rift valley (Figure at right). The intrusions are predominately nepheline syenite, but minor pyroxenite is found at Ilomba and alkali syenite and granite are associated with nepheline syenite at Mphompha.

This is an ongoing study of the petrology, mineral chemistry, geochemistry (major element and XRF and INAA trace elements), geochronology, and isotope geology of the NNAP. The participants are G. N. Eby, University of Massachusetts Lowell, Alan Woolley and Vic Din, Natural History Museum, and Garth Platt, Lakehead University. Publications resulting from the present study and pictures of the field area are found below.

Publications:

Eby, G. N., Woolley, A. R., Din, V., and Platt, G. (1998) Geochemistry and petrogenesis of nepheline syenites: Kasungu-Chipala, Ilomba, and Ulindi nepheline syenite intrusions, North Nyasa Alkaline Province, Malawi. Journal of Petrology 39, 1405-1424.

Woolley, A. R., Platt, R. G., and Eby, G. N. (1996) Relatively aluminous alkali pyroxene in nepheline syenites from Malawi: mineralogical response to metamorphism in alkaline rocks. Canadian Mineralogist 34, 423-434.

Woolley, A. R., Platt, R. G., and Eby, G. N. (1992) Niobian titanite and eudialyte from the Ilomba nepheline syenite complex, north Malawi. Mineralogical Magazine 56, 428-430.

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Geochemistry and Petrogenesis of Nepheline Syenites: Kasungu-Chipala, Ilomba, and Ulindi Nepheline Syenite Intrusions, North Nyasa Alkaline Province, Malawi

Eby, G. N., Woolley, A. R., Din, V., and Platt, G.

The North Nyasa Alkaline Province of Malawi consists of seven late-Precambrian intrusions emplaced along a north-south trend roughly parallel to that of the current rift valley. The intrusions are predominately nepheline syenite, but minor pyroxenite is found at Ilomba and alkali syenite and granite are associated with nepheline syenite at Mphompha. The chemistry and mineralogy of four representative nepheline syenite intrusions (Kasungu, Chipala, Ilomba, and Ulindi) have been investigated. Pyroxenes vary in composition from diopside (pyroxenite) through aegirine-augite to aegirine. Al-rich micas (Ilomba and Ulindi) may reflect primary magmatic compositions. Rocks from three intrusions plot near the minimum in the 1 Kbar Nepheline-Albite-Orthoclase-Kalsilite phase diagram indicating that they represent evolved liquids. Rocks from the fourth intrusion (Ilomba) are apparently cumulates. Nb/Ta, Zr/Hf, and Th/U ratios vary from those typical of OIB to very high ratios which may reflect a hydrothermal overprint. REE patterns are steep, slightly to moderately concave upwards, and lack Eu anomalies. The Ulindi and Ilomba nepheline syenites were derived by nepheline, pyroxene, and titanite fractionation of a basanite/nephelinite while the Kasungu and Chipala nepheline syenites were fractionated from an alkali basalt magma. These magmas originated from a subcontinental OIB-like source.

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Rock chemistry (Excel spreadsheet)

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Relatively Aluminous Alkali Pyroxene in Nepheline Syenites from Malawi: Mineralogical Response to Metamorphism in Alkaline Rocks

Woolley, A. R., Platt, R. G., and Eby, G. N.

The North Nyasa Alkaline Province (NNAP) of central and northern Malawi, eastern central Africa, consists of seven nepheline syenite intrusions which were, to varying degrees, affected by the Mozambique Orogenic event. We have analyzed the constituent clinopyroxene in rocks of five of these intrusions by electron microprobe. Three groups of pyroxenes can be distinguished. (a) Alkali pyroxenes range from diopside through aegirine-augite to aegirine. These have total Al < 0.15 (apfu) and are taken to be of primary magmatic origin. (b) Pyroxenes characterized by Al > 0.15 (apfu), with ^VIAl predominant, form a rim on group-(a) pyroxenes, cut across them and also form, in one intrusion, tiny acicular crystals within nepheline and, locally, feldspar. They comprise aluminian aegirine-augite, aluminian aegirine and omphacite. These pyroxenes are intrerpreted as metamorphic in origin. (c) The third group comprises Al-rich pyroxenes in which ^IVAl is predominant. They are diopside and aluminian diopside exhibiting no trend of alkali enrichment. They are probably of igneous origin but could also have been affected by metamorphism. Although the production of pure jadeitic pyroxenes probably requires a minimum pressure of about 7 kbar, aluminian aegirine with higher contents of Fe³⁺ can probably be generated at much lower pressures. The preservation of primary igneous pyroxenes in the NNAP intrusions probably indicates that neither high pressures nor particularly elevated temperatures were reached. The formation of the aluminous pyroxenes was essentially the result of an isochemical event; in general, metamorphism of agpaitic rocks will give rise to aluminian aegirine and a jadeitic pyroxene, whereas miaskitic rocks will contain aluminian aegirine-augie and an omphacitic pyroxene.

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Niobian Titanite and Eudialyte from the Ilomba Nepheline Syenite Complex, North Malawi

Woolley, A. R., Platt, R. G., and Eby, G. N.

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Photographs of field area.

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