Mineralogy, geochemistry and petrology of the phonolitic to nephelinitic Sadiman volcano, Crater Highlands, Tanzania
Highlights
► Sadiman volcano consists of phonolitic tuffs and nephelinite lavas. ► The nephelinites are highly evolved alkaline to peralkaline rocks. ► Mineral assemblages and chemical composition suggest rock crystallization at oxidized conditions. ► Sadiman records an evolved nephelinitic magma chamber just before carbonatite exsolution.
Introduction
Sadiman volcano (3°11′S, 35°25′E), also known as Satiman, is part of the Neogene–Quaternary volcanic complexes forming the Crater Highlands area in northern Tanzania (Fig. 1a) (Dawson, 2008). This area lies next to the western escarpment of the Gregory rift which is part of the eastern branch of the East African Rift System. The Crater Highlands, also termed as the Ngorongoro Volcanic Highlands, consist of several large volcanic complexes including the 2.4–2.2 Ma Lemagarut and the 2.25–2.0 Ma Ngorongoro basalt–trachybasalt–trachyandesite volcanoes (Ngorongoro also contains trachydacite) as well as the 1.6–1.5 Ma Oldeani basalt–trachyandesite volcano (Dawson, 2008; Mollel et al., 2008; 2011).
Sadiman is a highly eroded high stratovolcano whose top lies at 2870 m above see level, rising for about 400–500 m above the Malanja depression. It is located between Lemagarut, Oldeani and Ngorongoro (Fig. 1a). The volcano is well known having been previously cited as the source of the Laetoli Footprint Tuff, where 3.66 Ma Australopithecus afarensis footprints were discovered in 1976 (Harrison, 2011a, Leakey and Hay, 1979). However, it has recently been suggested that available geological, mineralogical and geochemical data for the Sadiman volcano do not currently support this hypothesis (Zaitsev et al., 2011). Sadiman is also believed to be a source of the Wembere-Manonga sediments (Manonga Valley), located about 170 km southwest of Laetoli (Harrison, 2011b, Mollel et al., 2011, Mutakyahwa, 1997).
In this paper we report new data from 28 samples of Sadiman lavas, tuffs and epiclastic rocks from (i) east and summit ridge outcrops, (ii) a stream channel and outcrops at the base of the north side of the volcano (Fig. 1b), and (iii) unspecified localities within Sadiman (Belousov et al., 1974). The major goals of this study are to (1) evaluate the occurrence of various rock types at Sadiman, (2) establish a detailed mineralogical and geochemical characterization of the Sadiman rocks, and (3) constrain the petrological evolution of the nephelinites based on intensive parameters such as temperature, silica activity (aSiO2) and oxygen fugacity (fO2).
Section snippets
Geological background
Sadiman is a highly vegetated, poorly exposed volcano and only a limited number of outcrops are known, e.g. six outcrops have been identified by Mollel et al. (2011) whereas sixteen outcrops have been mapped in this study. According to Pickering's (1964) QDS 52 “Endulen” map (Fig. 1b) “Sadiman … made up largely of nephelinitic tuffs and agglomerates with subsidiary lava flows. Nephelinites and phonolites cap a prominent ridge on the south-east side of the volcano”. Pebbles, cobbles and boulders
Analytical methods
Polished thin sections of lavas and tuffs and epoxi blocks with fractions of heavy minerals were studied using scanning electron microscopy (SEM—JEOL 5900LV) and energy-dispersive X-ray microanalysis (EDX—Oxford instruments INCA) at the Natural History Museum (NHM), London. SEM and EDX were also used for the study of sodalite. Wavelength-dispersive electron probe microanalyses (WD-EPMA) of major, minor and accessory minerals were obtained using a JEOL 8900 Superprobe at Tübingen University (TU)
Petrography and mineral chemistry
Our field data, optical petrography and electron microscopy studies show that nephelinites are the major effusive rocks at Sadiman volcano; phonolite lavas were observed only as small xenoliths (< 1 cm in size) in the nephelinites (Zaitsev et al., 2011). Nephelinites range from phenocryst-poor to phenocryst-rich rocks with crystalline groundmass, and phonolites are phenocryst-rich rocks. Rarely, nephelinites contain xenoliths of annite-bearing ijolite (0.5–25 cm in size).
On the basis of their
Bulk geochemistry
The bulk rock compositions (Table 3) show that the studied nephelinites are low-magnesium rocks (atomic Mg/(Mg + Fe) = 0.17–0.26) with an alkalinity index [molar (Na + K)/Al] 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. In the total alkali–silica (TAS) diagram, data form a broad field with the majority of points plotting in the foidite and phonolite fields (Fig. 9) (this study and Dawson, 2008, Paslick et al., 1995 and
Melt inclusions
Primary silicate melt inclusions were observed in phenocrysts and microphenocrysts of the Sadiman nephelinites—they occur in nepheline, titanite, diopside–hedenbergite, apatite, wollastonite and sanidine and are most abundant in nepheline. Primary silicate melt inclusions, together with crystal inclusions (diopside, titanite, apatite), commonly outline the growth zones in host nepheline phenocrysts. Primary fluid and sulfide inclusions are rare; trails of secondary melt inclusions occur
Evaluation of rock types composing Sadiman
Despite the limited number of outcrops at Sadiman, the available data suggest a multiphase evolution of the volcano with punctuated effusive (lavas) and explosive (ashes—now tuffs) eruptions. Published and new data presented in this work show that nephelinite is the major effusive rock type at Sadiman (Dawson, 2008, Hay, 1976, Mollel, 2007, Mollel et al., 2011, Paslick et al., 1995, Zaitsev et al., 2011). New data on the petrography, mineralogy and geochemistry of the nephelinites suggest that
Conclusions
The results of the present study of the Sadiman volcano combined with previously published data discussed above lead to following conclusions:
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– Nephelinite lavas and phonolitic tuffs are the principle rock types at the Sadiman volcano, while phonolite lavas are very rare rocks. Available to date geological, mineralogical and geochemical data do not support the occurrence of melilite-bearing rocks and carbonatites (or natrocarbonatites) at Sadiman. The occurrence of ijolitic rocks indicates the
Acknowledgments
We thank Dr. Mike Kobrick (Project Scientist, Shuttle Radar Topography Mission, Jet Propulsion Lab, Pasadena, USA) for permission to publish Fig. 1a, which was generated by Dr. Robert Crippen. We thank Catherine Unsworth for help with the ICP-AES analyses. We are grateful to Nelson Eby and two anonymous reviewers for constructive comments. This research was supported by the Alexander von Humboldt Stiftung (Germany), St. Petersburg State University, the Natural History Museum (UK) and the
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