Elsevier

Lithos

Volume 152, 1 November 2012, Pages 66-83
Lithos

Mineralogy, geochemistry and petrology of the phonolitic to nephelinitic Sadiman volcano, Crater Highlands, Tanzania

https://doi.org/10.1016/j.lithos.2012.03.001Get rights and content

Abstract

Sadiman volcano is located in the Crater Highlands area of northern Tanzania, which lies next to the western escarpment of the Gregory rift—a part of the eastern branch of the East African Rift system. It consists of interlayered phonolitic tuffs, tuff breccias (with blocks of nephelinites) and nephelinitic lava flows. Rare xenoliths of phonolite lava and ijolite were observed within the nephelinite lavas with ijolite blocks occurring in phonolitic tuffs. No evidence for the presence of melilite-bearing and/or carbonatitic rocks was found during this study. On the basis of petrography, mineralogy and geochemistry the nephelinites are divided into highly porphyritic nephelinite, wollastonite nephelinite and phonolitic nephelinite, the latter of which is the dominant variety at Sadiman. Nepheline + clinopyroxene + titanite ± perovskite ± andradite–schorlomite ± wollastonite ± sanidine ± sodalite are the principle pheno- and microphenocryst phases. The nephelinites are highly evolved (Mg# = 0.17–0.26) alkaline to peralkaline (AI = 0.88–1.21) rocks enriched in incompatible elements such as Rb, Ba, Th, U, Nb, Pb, Ta, Sr and light REEs, and strongly depleted in P and Ti. This suggests derivation from an enriched mantle source and fractionation of apatite and Ti-rich mineral(s). Primary melt inclusions in nepheline phenocrysts (Thomogenization = 860–1100 °C) indicate enrichment of volatile components in the melts, particularly of fluorine (up to 1.8 wt.% in silicate glass) resulting in the formation of daughter fluorite in partly and complete crystallized inclusions. The Sadiman nephelinites crystallized under relatively oxidizing conditions (above the FMQ buffer), which differ from the reducing conditions reported for trachytic and pantelleritic rocks from other parts of the Gregory rift. Similar rock types and relatively oxidizing conditions are known from Oldoinyo Lengai and other localities, all of which are closely associated with carbonatites. By analogy, we conclude that andradite–schorlomite-rich nephelinites may indicate a pre-stage on the evolutionary path towards carbonatitic magmatism.

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:

  • 1

    – 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

References (80)

  • F. Neukirchen et al.

    The Lava sequence of the East African Rift escarpment in the Oldoinyo Lengai—Lake Natron sector, Tanzania

    Journal of African Earth Sciences

    (2010)
  • C. Paslick et al.

    Enrichment of the continental lithosphere by OIB melts: isotopic evidence from the volcanic province of northern Tanzania

    Earth and Planetary Science Letters

    (1995)
  • Th. Platz et al.

    Low-pressure fractionation of the Nyiragongo volcanic rocks, Virunga Province, D.R. Congo

    Journal of Volcanology and Geothermal Research

    (2004)
  • M. Powell

    The crystallisation history of the Igdlerfigssalik nepheline syenite intrusion, Greenland

    Lithos

    (1978)
  • M. Ren et al.

    Application of the QUILF thermobarometer to the peralkaline trachytes and pantellerites of the Eburru volcanic complex, East African Rift, Kenya

    Lithos

    (2006)
  • V.V. Sharygin et al.

    Silicate–natrocarbonatite liquid immiscibility in 1917 eruption combeite–wollastonite nephelinite, Oldoinyo Lengai Volcano, Tanzania: Melt inclusion study

    Lithos

    (2012)
  • H. Tazieff

    Mt. Niragongo: renewed activity of the lava lake

    Journal of Volcanology and Geothermal Research

    (1984)
  • D. Wiedenmann et al.

    Melilite-group minerals at Oldoinyo Lengai, Tanzania

    Lithos

    (2010)
  • A.N. Zaitsev et al.

    Was Sadiman volcano a source for the Laetoli Footprint Tuff?

    Journal of Human Evolution

    (2011)
  • G.P. Bagdasaryan et al.

    Age of volcanic rocks in the rift zones of East Africa

    Geochemistry International

    (1973)
  • T. Yu. Bazarova et al.

    Magmatic Crystallisation Based on the Study of Melt Inclusions

    (1975)
  • K. Bell

    Radiogenic isotope constraints on relationships between carbonatites and associated silicate rocks—a brief review

    Journal of Petrology

    (1998)
  • K. Bell et al.

    Nd and Sr isotope systematics of Shombole volcano, East Africa, and the links between nephelinites, phonolites, and carbonatites

    Geology

    (1991)
  • K. Bell et al.

    Nd and Sr isotope systematics of the active carbonatite volcano, Oldoinyo Lengai

  • K. Bell et al.

    Nd, Pb and Sr isotopic compositions of East African carbonatites: evidence for mantle mixing and plume inhomogeneity

    Journal of Petrology

    (2001)
  • J. Berger et al.

    The role of fractional crystallization and late-stage peralkaline melt segregation in the mineralogical evolution of Cenozoic nephelinites/phonolites from Saghro (SE Morocco)

    Mineralogical Magazine

    (2009)
  • V.V. Belousov et al.

    East-African Rift System, Vol. 1–3

    (1974)
  • R. Berman

    Internally consistent thermodynamic data for minerals in the system Na2O–K2O–CaO–MgO–FeO–Fe2O3–Al2O3–SiO2–TiO2–H2O–CO2

    Journal of Petrology

    (1988)
  • R.G. Berman et al.

    Geo-Calc; a software for calculation and display of P–T–X phase diagrams

    American Mineralogist

    (1987)
  • Church, A.A., 1996. The petrology of the Kerimasi carbonatite volcano and the carbonatites of Oldoinyo Lengai with a...
  • M.A. Cosca et al.

    Activity-composition relations for the join grossular-andradite and application to calc-silicate assemblages

    Abstracts with Programs—Geological Society of America

    (1986)
  • J.B. Dawson

    Peralkaline nephelinite–natrocarbonatite relationships at Oldoinyo Lengai, Tanzania

    Journal of Petrology

    (1998)
  • J.B. Dawson

    The Gregory rift Valley and Neogene–Recent volcanoes of northern Tanzania

  • J.B. Dawson et al.

    Metasomatised and veined upper-mantle xenoliths from Pello Hill, Tanzania: evidence for anomalously light mantle beneath the Tanzanian sector of the East African Rift Valley

    Contributions to Mineralogy and Petrology

    (1988)
  • J.B. Dawson et al.

    A comparative study of bulk rock and mineral chemistry of olivine melilitites and associated rocks from East and South Africa

    Neues Jahrbuch für Mineralogie, Abhandlungen

    (1985)
  • G. Faure

    Origin of Igneous Rocks. The Isotopic Evidence

    (2001)
  • M. Ferrat et al.

    A single procedure for the accurate and precise quantification of the rare earth elements, Sc, Y, Th and Pb in dust and peat for provenance tracing in climate and environmental studies

    Talanta

    (2012)
  • A. Foster et al.

    Tectonic development of the northern Tanzanian sector of the east African rift System

    Journal of the Geological Society of London

    (1997)
  • M.L. Fuhrman et al.

    Ternary-feldspar modeling and thermometry

    American Mineralogist

    (1988)
  • E. Green et al.

    An order–disorder model for omphacitic pyroxenes in the system jadeite–diopside–hedenbergite–acmite, with application to eclogitic rocks

    American Mineralogist

    (2007)
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