Mineralogy and geochemistry of nephelinites and phonolites from the Sadiman volcano,

northern Tanzania

Zaitsev A.N.^*,**, Wenzel T.^***, Markl G.^***, Spratt J.^**, Petrov S.V.^*, Williams, C.T.^**

^* St. Petersburg State University, St. Petersburg, Russia; ^** Natural History Museum, London, UK;

^*** Tuebingen University, Tuebingen, Germany

burbankite@gmail.com

 

Sadiman volcano is 4.0-4.8 Ma old and is located in the Crater Highlands area in northern Tanzania (Bagdasaryan et al., 1973; Mollel 2007). It is an eroded and poorly exposed stratovolcano 2870 m high consisting of tuffs and agglomerates with clasts of nephelinites and melilitites, with lava flows of nephelinites and phonolites (Pickering 1964; Dawson 2008). Field observations show that it formed of interlayered tuffs and nephelinitic lavas. Small xenoliths of ijolites occur in stream deposits on the north slope of the volcano (Hay 1976) and in nephelinites exposed on the south-east ridge of the volcano. Phonolites were observed only as small xenoliths (<1 cm) in the nephelinites. It is suggested that Sadiman volcano also contains carbonatitic rocks (Dawson 2008), but none were observed in the field.

Sadiman volcano is considered to be a source of carbonatite-melilitites tuffs in the Laetoli area where numerous fossils and fossilised footprints from human ancestors are known (Hay 1978, 1986), thus Sadiman is of special interest as a possible source of the tuffs.

Nephelinites range from phenocryst-poor to phenocryst-rich rocks (Fig. 1); the latter seem to be cumulates. Nepheline (3.4-8.2 wt% K₂O) and clinopyroxene are the principal phenocryst minerals. Two types of clinopyroxene are observed: both are diopsides, one with normal zonation (low-sodium core and sodium enriched rim) and the second with a sodium-enriched green-core. Rarely observed phenocrysts are melanite-shorlomite (8.5-21.7 wt% TiO₂), perovskite (0.8-4.9 wt% REE₂O₃, 0.6-1.6 wt% Nb₂O₅) or wollastonite. Microphenocrysts and groundmass consist of diopside, nepheline, sanidine, titanite, sodalite and apatite in various proportions. Perovskite also occurs as corroded relicts in titanite crystals. Aenigmatite (in growndmass and as a reaction rim on titanite), magnetite, and rarely pyrrhotite are accessory minerals.

 

 

Fig. 1. (a) phonolitic nephelinite with wollastonite, (b) cumulate nephelinite. BSE images.

 

Nephelinites are low-magnesium rocks (Mg# = 0.17-0.26) with a peralkaline index ranging between 0.88 and 1.21. On a volatile-free basis these rocks contain 46.3-52.6 wt% SiO₂ and 10.5-15.9 wt% Na₂O+K₂O. On the TAS classification diagram data points form a broad field (Fig. 2) and the data do not support existence of two distinct nephelinite groups at Sadiman volcano as it was suggested by Mollel (2007) and Dawson (2008). Mineralogical and geochemical data suggest that they are highly evolved phonolitic nephelinites.

 

 

 

Fig. 2. TAS diagram for Sadiman lavas.

Filled square – authors data, empty square – data from Mollel (2007) and Dawson (2008).

 

 

Phonolites are phenocryst-rich rocks with anorthoclase (5.8-7.0 wt% Na₂O and 5.7-8.4 wt% K₂O) and nepheline (2.2-2.9 wt% K₂O) being the principal minerals. Diopside and titanite are minor phases.

No evidence was found for the presence of åkermanite, alumoåkermanite  or primary calcite in any of the samples studied. Minerals which can be considered as indicators of carbonatitic activity, and known e.g. from the close Kerimasi carbonatitic volcano (nyerereite, pyrochlore,  baddeleyite, kimzeyite or kerimaste (Zaitsev 2009, Zaitsev et al. 2010)), have not been found at Sadiman. Only secondary calcite, formed during low-temperature alteration of lavas and tuffs, occurs as isolated anhedral crystals and forms veinlets.

All available data do not currently support the occurrence of melilitites and carbonatites at Sadiman volcano. Consequently, we conclude that this volcano is not the source of Laetoli tuffs.

This research was supported by Alexander von Humboldt Stiftung.

 

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