2. Previous studies

Mosonik was first visited by the German geologists Carl Uhlig and Fritz Jaeger in 1904 during a survey of the East African Rift from Lake Manyara to Lake Natron (Uhlig, Reference Uhlig1907; Uhlig & Jaeger, Reference Uhlig and Jaeger1942). Initially, the volcano was considered to consist of basement or sedimentary rocks and that the unusual morphology was a result of faulting; Uhlig subsequently recognized the presence of volcanic rock but, given that no crater was evident, considered the occurrence to be a fissure eruption. Samples collected were subsequently examined by the eminent petrographers Brögger (Reference Brögger1921, pp. 249, 398) and Rosenbusch (Reference Rosenbusch1907, p. 1455) and described as ‘ringite’ (aegirine ‘sövite’) and sodalite nephelinite, respectively.

N. J. Guest (unpub. Ph.D. thesis, Univ. Sheffield, 1953), during a survey of the geology of the western scarp of the rift, initially recognized the Older and Younger Series of volcanic rocks, and at Mosonik the presence of mela-nephelinite, nephelinite and phonolite together with biotite-bearing grey ash. Subsequently, Guest et al. (Reference Guest, James, Pickering and Dawson1961) showed on the Quarter Degree Sheet No. 39 of the Tanzania Geological Survey that Mosonik consisted mainly of nephelinitic–phonolite tuffs and agglomerates, together with an eastern lobe of lava.

Paslick et al. (Reference Paslick, Halliday, James and Dawson1995, Reference Paslick, Halliday, Langem, James and Dawson1996) undertook Sr–Nd–Pb isotope studies of diverse Older and Younger Series volcanic rocks, and presented whole-rock and isotopic compositions for three nephelinites from Mosonik (Paslick et al. Reference Paslick, Halliday, Langem, James and Dawson1996), together with a sample incorrectly termed a ‘basanite’ on the basis of classification using the whole-rock composition in terms of total alkali–silica (TAS) volcanic rock nomenclature (Le Maitre et al. Reference Le Maitre, Streckeisen, Zanettin, Le Bas, Bonin, Bateman, Bellieini, Dudek, Efremova, Keler, Lameyre, Sabine, Schmid, Sørensen and Woolley2002). Petrographic data to support this designation were not presented. Paslick et al. (Reference Paslick, Halliday, James and Dawson1995) included at least one sample from Mosonik in their study, although this was not specifically identified as it was grouped with other diverse Older Extrusive rocks. The different sampling number schemes in these papers preclude identification of this material. The isotopic studies indicated isotopic and ‘chemical’ dis-equilibria in all of the volcanic rocks analysed (see Section 7 below), and no detailed mineralogical study of Mosonik was undertaken, although Paslick et al. (Reference Paslick, Halliday, Langem, James and Dawson1996) presented very limited paragenetic and compositional data for nepheline, titanite, apatite and titanomagnetite.

Dawson (Reference Dawson2008, p. 41) summarized the observations of an expedition by J. B. Dawson, A. N. Mariano and R. H. Mitchell to Mosonik in September 2005 as follows. Carbonate-cemented tuffs and lapilli tuffs blanket the lower northern and western slopes of the mountain and cover the plains to the west. Nepheline- and clinopyroxene-phyric lavas and pyroclastic rocks form the bulk of the volcano. Nephelinite sensu stricto is not the dominant rock type as most samples contain groundmass alkali feldspar and are hence transitional to phonolite, which is present in small amounts. Zaitsev et al. (Reference Zaitsev, Spratt, Shaygin, Wenzel, Zaitseva and Markl2015) and Sedova et al. (Reference Sedova, Zaitsev and Spratt2018) confirmed that the bulk of volcano is composed of diverse phonolitic nephelinites and phonolites and reported that melilite-bearing nephelinite is also present but did not elaborate further on the location of the occurrence. Some modal data for these lavas were presented by Sedova et al. (Reference Sedova, Zaitsev and Spratt2018) together with compositional data for resorbed melilite phenocrysts (akermanite–alumino-akermanite), nepheline and pyroxene (see Section 6 below). Xenoliths of mica pyroxenite, ijolite and country rocks (quartzite and schist) occur in the volcanic rocks and are common in the vicinity of the eastern lobe lava.

The Guest et al. (Reference Guest, James, Pickering and Dawson1961) survey map shows a small occurrence of carbonatite within the volcanic rocks in a northerly trending valley on the western side of the mountain. This carbonatite was not found by the Dawson 2005 expedition (Dawson, Reference Dawson2008), and neither by M. S. Garson in an unpublished United Nations geological survey report (pers. comm. to Dawson, Reference Dawson2008). However, in September 2007, Dawson & Mitchell (unpub.) observed rare, but inaccessible, blocks of carbonatite in the walls of the gorge of the Leshuta River which drains eastwards from the volcano. Subsequent visits to Mosonik by Anatoly Zaitsev and others in 2009 and 2015 have apparently also been unable to locate the Guest et al. (Reference Guest, James, Pickering and Dawson1961) occurrence, perhaps owing to the presence of the extensive vegetation. However, Zaitsev et al. (Reference Zaitsev, Spratt, Shaygin, Wenzel, Zaitseva and Markl2015) and Sedova et al. (Reference Sedova, Zaitsev and Spratt2018) reported the presence of small (<0.5 m) boulders of carbonatite in the longest western valley, although no carbonatites were observed in outcrop. Anatoly Zaitsev (pers. comm.) has also confirmed the presence of carbonatite boulders in the Leshuta River gorge and that a dyke of carbonatite, 2–3 m in width and 10 m in length, occurs in situ in the southwestern part of the volcano. These occurrences demonstrate that carbonatites are indeed present at Mosonik but their rarity indicates that they are volumetrically not significant. These discoveries suggest that the aegirine ‘sövite’ described by Brögger (Reference Brögger1921) indeed originated from Mosonik.

A summit crater or caldera is not present and the col (2° 34.713′ S; 35° 49.535′ E) is composed of phonolitic nephelinite. It is impossible to produce a detailed geological map of the volcano as the extensive vegetation (Fig. 2) coupled with a predominance of ‘wait-a-bit thorns’ precludes detailed or extensive traverses. The Leshuta River gorge exposes debris flows and lahars containing mega-xenoliths of Mosonik volcanic rocks.

This manuscript describes volcanic rocks collected by the 2005 Dawson Expedition and presents the results of petrographic and mineralogical studies together with bulk-rock and isotopic compositional data. Some new Sr–Nd isotopic data for Oldoinyo Lengai and the Recent Natron–Engaruka melilitites in the North Tanzanian Divergence are also included in this paper for comparative purposes. There is no description of the melilite-bearing rocks, carbonatites, pyroclastic rocks and ijolite suite xenoliths occurring at Mosonik in this publication.

3. Analytical methods

Quantitative compositions for minerals were obtained using carbon-coated polished thin-sections at Lakehead University using a Hitachi SU-70 field emission scanning electron microscope (FE-SEM) with an accelerating voltage of 20 kV and beam current of 300 pA. Raw X-ray spectra were analysed with an Oxford AZtec 80 mm/124 eV energy dispersive X-ray spectrometer (EDS) using process times of 60–120 s. Standards used were jadeite (Na, Al), wollastonite (Ca, Si), Mn-hortonolite (Mn, Fe, Mg), orthoclase (K, Al), SrTiO₃ (Sr, Ti), BaSO₄ (Ba, S), zircon (Zr), apatite (P, Ca) and ThNbO₁₂ (Nb). Pyroxene, nepheline and feldspar data were recalculated into structural formulae and end-member components using an in-house APL Mineralogical Program. For nepheline and feldspar all Fe was assumed to be Fe³⁺. For pyroxenes Fe²⁺ and Fe³⁺ were calculated from total Fe on the basis of six atoms of oxygen with all Na expressed as NaFe³⁺Si₂O₆. Garnet data were recalculated into IMA approved end-member components using the Excel program of Locock (Reference Locock2008).

Bulk-rock major and trace elements were determined by Activation Laboratories Ltd (Actlabs) at Ancaster (Ontario) using the 4Lithores – lithium metaborate/tetraborate fusion – inductively coupled plasma mass spectrometry package (see online Supplementary Material).

Sr and Nd isotopic compositions for Mosonik were determined at the University of Alberta (see online Supplementary Material) and for Oldoinyo Lengai and Natron–Engaruka at the Memorial University of Newfoundland (see online Supplementary Material). Potassium ⁴⁰Ar–³⁹Ar ages were determined by laser incremental heating methods at the Geochronology Laboratory of Oregon State University (http://geochronology.ceoas.oregonstate.edu).

4. Petrography of the lavas

Petrographic examination has identified in our sample suite four nepheline-bearing lava types at Mosonik. Three of these contain groundmass alkali feldspar and are best termed phonolitic nephelinites, as they are not nephelinites sensu stricto; the fourth lava type is phonolite. The relative proportions of the lava types cannot be determined because of the poor exposure, although Type 2 nephelinite lavas appear to be dominant. All lavas are antecryst and phenocryst rich (30–60 vol. %), with a heterogeneous population of pyroxenes (see Section 6.a below) indicating that magma mixing has played a significant role in their genesis. Significantly, we thus consider that none of the bulk compositions of the lavas can be considered as representative of liquid compositions. The heterogeneity and alteration of the lavas also precludes the determination of any useful compositional data from the residual fine-grained groundmass.

Type 1 phonolitic nephelinites (Fig. 3) are characterized by anhedral macrocrysts of titanomagnetite, clinopyroxene, clinopyroxene-magnetite intergrowths, round resorbed brown magnesio-hastingsite and discrete apatite, together with euhedral phenocrysts of nepheline, K-zeolite pseudomorphs after nepheline and colourless-to-pale brown, euhedral-to-subhedral clinopyroxene. All are set in a groundmass of fine-grained euhedral prismatic alkali feldspar, prismatic clinopyroxene with thin aegirine mantles, zeolitic pseudomorphs after euhedral nepheline, apatite, titanomagnetite, calcite, natrolite, K–Ca-zeolites and altered glass. Amphibole macrocrysts contain rare prismatic crystals of apatite and are not in equilibrium with their current hosts as they have thin-to-near complete reaction mantles of titanomagnetite, diopside-rich clinopyroxene and minor calcite. Vesicles contain calcite and K–Ca-zeolites. Microxenoliths present are phlogopite plus aluminous spinel and ijolite.

Fig. 3.

Photomicrographs of Type 1 lavas: (a) anhedral and euhedral phenocrysts of clinopyroxene some with pale green mantles and anhedral magnetite set in a groundmass of brown altered nepheline, alkali feldspar and orange-red altered glass (MOS28); (b) orange-brown euhedral and subhedral amphibole phenocrysts/antecrysts with opaque reaction rims of very fine-grained diopside and magnetite. The groundmass consists of colourless prismatic alkali feldspar and altered nepheline (MOS23).

Type 2 phonolitic nephelinites (Fig. 4) are characterized by abundant phenocrysts of euhedral-to-subhedral, complexly zoned brown-to-green clinopyroxenes, euhedral nepheline with titanite and clinopyroxene inclusions, and minor titanite. These are set in a fine-grained groundmass of broken and prismatic light green – dark green clinopyroxene with bright green aegirine-rich mantles, acicular Ti-bearing aegirine, euhedral nepheline (20–30 μm) with pseudomorphs of sodalite and/or zeolite, prismatic (10 × 50 μm) alkali feldspar, sodalite, K–Ca-zeolites, calcite and altered glass. Notably absent from Type 2 lavas are groundmass titanomagnetite and the amphibole macrocrysts characteristic of Type 1 lavas, although resorbed macrocrysts (100–300 μm) of apatite are present.

Fig. 4.

Photomicrographs of Type 2 lavas: (a) phenocrysts and clasts of nepheline (Ne) together with green prismatic phenocrysts and clasts of clinopyroxene and euhedral titanite set in a brown fine-grained groundmass of clinopyroxene, nepheline and alkali feldspar (MOS13); (b) lava dominated by large complex nepheline phenocrysts (Ne) showing resorption and zoning together with subhedral green zoned clinopyroxene set in a fine-grained optically unresolvable groundmass containing nepheline, sodalite, alkali feldspar and potassium feldspar (MOS24).

Type 3 phonolitic nephelinites (Fig. 5) have many similarities to Type 2 lavas with respect to the overall mineralogy and the occurrence of complexly zoned pyroxenes. They differ principally in that they are characterized by the presence of red-brown subhedral-to-euhedral Ti-rich garnets and nepheline which is enriched in K₂O (see Section 6.c below) relative to that in Type 1 and 2 lavas. Typical Type 3 nephelinite lavas contain garnet, titanite and apatite macrocrysts (up to 500 μm). Phenocrysts include oscillatory zoned nepheline with peralkaline glass inclusions and complexly zoned brown-to-green clinopyroxenes with Ti-aegirine mantles. The groundmass contains euhedral (30–50 μm) nepheline, which is commonly pseudomorphed by sodalite; fresh and altered potassium feldspar; acicular Ti-aegirine; small (<50 μm) prismatic Sr-bearing (1–7 wt % SrO) apatite crystals; K–Ca-zeolites; Na-zeolites and altered interstitial glass. The massive flows at the eastern margin of Mosonik (2° 34.431′ S; 35° 48.860′ E) also contains trace amounts of barytolamprophyllite, fluorite, Sr–Ca–rare earth element (REE) carbonates (bastnaesite and -calcio-ancylite-(La)), fluorite and celestite.

Fig. 5.

Photomicrographs of Type 3 lavas: (a) large and small euhedral nepheline (Ne), broken and complexly zoned green clinopyroxene (cpx), rounded red-brown zoned garnet (G) antecrysts set in a very fine-grained brown groundmass of altered and fresh nepheline, alkali feldspar and altered brown glass (MOS 43); (b) crystal-rich lava with euhedral and broken phenocrysts of nepheline (Ne), resorbed phenocrysts of pale green clinopyroxene, euhedral and anhedral red-brown garnet (G) set in a very fine-grained groundmass of green pyroxene altered nepheline, alkali feldspar and altered glass.

Type 4 lavas are phonolites (Fig. 6) consisting of very large (5–10 mm) euhedral phenocrysts of twin-free or Carlsbad-twinned alkali feldspar and nepheline, with both minerals commonly altered at their margins and along cleavage planes to natrolite, and, in some examples, minor calcite. Alkali feldspars in some instances exhibit marginal incipient microcline twinning. Aggregates of subhedral nepheline are common and some are clearly microxenocrysts of nepheline plus pyroxene. Other phenocrysts include euhedral green complexly zoned or zonation-free clinopyroxene occurring as single crystals which are commonly juxtaposed, indicating magma mixing. Aggregates (0.5–1.5 mm) of subhedral-to-anhedral pyroxene represent fragmented cumulates. Euhedral twinned titanite is a trace phenocryst (<2 vol. %).

Fig. 6.

Photomicrographs of Type 4 lavas: (a) euhedral green clinopyroxene (cpx), colourless alkali felspar (afsp) and twinned titanite (T) phenocrysts set a very fine-grained brown matrix of altered nepheline and alkali feldspar with irregular areas of natrolite and calcite (n-c) (MOS35); (b) large euhedral phenocryst of nepheline (Ne) and resorbed prismatic crystal of alkali feldspar (afsp) with small green phenocrysts of clinopyroxene set in a very fine-grained groundmass of altered and fresh nepheline and alkali feldspar (MOS36); (c) crossed-polarized light image of a typical large phenocryst of alkali feldspar (afsp) showing internal crystallographic domains and incipient microcline twinning (MOS37); (d) heterogeneous groundmass of MOS38 showing the contrast between the globular areas of phenocryst-bearing areas and other regions of the groundmass (MOS39).

The groundmass is extremely fine grained and typically consists of small laths of potassium feldspar, euhedra of nepheline and natrolite pseudomorphs, acicular Ti-bearing aegirine, Sr–REE-bearing apatite, calcite and altered glass. The groundmass is, in some examples, heterogeneous and exhibits a spherulitic texture with ovoid areas of relatively coarse-grained material with pyroxene and titanite microphenocrysts set in a finer grained mineralogically similar groundmass which lacks these minerals. Rare macrocrysts of titano-magnetite are surrounded by Ti-aegirine plus titanite mantles.

5. Bulk-rock geochemistry

The bulk-rock compositions of representative lavas are listed in Table 1 and plotted in the Le Maitre et al. (Reference Le Maitre, Streckeisen, Zanettin, Le Bas, Bonin, Bateman, Bellieini, Dudek, Efremova, Keler, Lameyre, Sabine, Schmid, Sørensen and Woolley2002) IUGS TAS diagram (Fig. 7). With respect to TAS, many of the Type 1 nephelinite lavas plot in the fields of tephritic lavas regardless of not containing plagioclase. This incorrect nomenclature arises because the rocks do not represent liquid compositions and their modal mineralogy is dominated by calcic, sodic-calcic and sodic pyroxenes. Thus, all Type 1 lavas plot as phonotephrite or tephrite basanite, whereas some Type 4 phonolites plot as tephriphonolite. Type 2 and 3 nephelinite lavas are all essentially foidites in terms of TAS but are mineralogically phonolitic nephelinites.

Table 1.

Whole-rock compositions of Mosonik volcanic rocks

LOI – loss on ignition.

Fig. 7.

Total alkali versus SiO₂ (wt %) diagram (TAS) for the major-element bulk compositions of lavas from Mosonik (this work), Shombole (Peterson, Reference Peterson1989 a) and Sadiman (Zaitsev et al. Reference Zaitsev, Marks, Wenzel, Spratt, Sharygin, Strekopytov and Markl2012). Mosonik compositions 35–39 are modally phonolites; all other samples are phonolitic nephelinites.

None of the lavas can be considered as strongly peralkaline, as the peralkalinity index (molar (Na₂O + K₂O)/Al₂O₃) ranges from 0.68 to 1.22. Mosonik Type 2 nephelinite lavas are similar to nephelinites from Sadiman (Zaitsev et al. Reference Zaitsev, Marks, Wenzel, Spratt, Sharygin, Strekopytov and Markl2012) and some Shombole (Peterson, Reference Peterson1989 a) lavas in terms of TAS (Fig. 7) and peralkalinity (Sadiman 0.88–1.21; Shombole 0.86–1.37), but do not exhibit the wider compositional variation of the Shombole lavas.

With respect to trace elements the lavas are, in common with the major-element variations, heterogeneous (Table 2). Extended mantle normalized trace-element (aka spider diagrams) diagrams for Mosonik lavas are not presented as we consider that these diagrams are not relevant to magmas derived from metasomatized mantle sources (see Section 8 below), and as the whole-rock compositions certainly do not represent those of liquids. The principal trace-element variation is in respect to that of Type 2 and 3 nephelinite lavas relative to types 1 and 4. Surprisingly, the mineralogically least evolved Type 1 nephelinite lavas are, in terms of trace elements, geochemically similar to the most evolved Type 4 phonolites. This dichotomy is well illustrated by the REE chondrite-normalized distribution patterns (Fig. 8), which shows that Type 1 and 4 lavas are relatively depleted in overall REE abundances, although having similar distribution patterns to Type 2 and 3 nephelinites. The heavy REE (Tm–Lu) enrichment of the phonolites undoubtedly results from minor crustal contamination as suggested by Sr–Nd isotopic data (see Section 7 below). Europium anomalies are not present and attest to the absence of plagioclase fractionation.

Table 2.

Trace-element compositions of Mosonik volcanic rocks

All samples have Cr < 20 ppm; Ni < 20 ppm. Types 1, 2 and 3 are phonolitic nephelinites; Type 4 are phonolites (see text). For analytical information see online Supplementary Material. bld – below limit of detection.

Fig. 8.

Chondrite-normalized (Boynton, Reference Boynton and Henderson1985) rare earth element distribution diagram for representative bulk compositions of Mosonik lava types 1–4.

With respect to high field strength incompatible elements (Table 2), there are no significant correlations with lava type excepting that phonolites have overall lower abundances (ppm) of V (29–42), Sr (449–1566), Y (20.5–23.8), Zr (193–230), Nb (72–100), Ba (943–1158) and higher Zr/Nb ratios (2.1–2.9, av. 2.5) than other lavas (875–3638 Sr; 17–44 Y; 148–316 Zr; 64–201 Nb; 1.5–2.8, av. 1.9 Zr/Nb; 701–1735 Ba). The wide variations in the Sr, REEs, Zr and Nb contents (Fig. 9) reflect the extensive variations in antecryst, phenocryst and groundmass clinopyroxene abundance and composition, Y and Zr in garnet, Sr and Ba in alkali feldspar and Sr in calcite as groundmass phases, coupled with ‘dilution’ by the wide modal variations in nepheline phenocryst content. Because of this complexity none of the trace-element variations can be directly correlated with mineral modal abundances. All lavas are strongly depleted (ppm) in Cu (10–170), Ni (<20), Co (1–41) and Cr (<20).

Fig. 9.

Nb versus Zr (ppm) relationships for lavas from Mosonik (this work) and Sadiman (Zaitsev et al. Reference Zaitsev, Marks, Wenzel, Spratt, Sharygin, Strekopytov and Markl2012).

Compared to Mosonik, the Sadiman lavas (Zaitsev et al. Reference Zaitsev, Marks, Wenzel, Spratt, Sharygin, Strekopytov and Markl2012) are significantly enriched (ppm) in Sr (1530–2460), Ba (1475–2770), Nb (144–230) and Zr (341–483). Figure 9 illustrates the Nb and Zr enrichment of Sadiman relative to Mosonik. Similar trace-element enrichments (not illustrated) relative to Mosonik occur at Shombole (Peterson, Reference Peterson1989 a).

6. Mineralogy of the lavas

6.a. Pyroxene compositional variation

Pyroxenes exhibit the greatest compositional variation of all of the minerals occurring in the Mosonik lavas as a consequence of magma mixing and extensive zoning. Overall the compositional range is from diopside through aegirine-augite/aegirine to Ti-bearing aegirine. Representative compositions of pyroxenes in lava types 1–4 are given in Tables 3–6, and the compositional variation in each type illustrated in terms of the diopside–hedenbergite–aegirine (Di–Hd–Ae) ternary solid solution (mol. %) series in Figure 10. Note that these compositional variation diagrams do not illustrate zoning within individual crystals as the intent is to illustrate the overall compositional range of pyroxenes within a particular lava type. Zoning within individual crystals is complex and varies significantly between adjacent crystals. There is no common pattern of either continuous and/or discrete oscillatory zoning and resorption as illustrated in Figure 11, although these define the overall trends shown in Figure 10.

Table 3.

Representative compositions of pyroxene in Mosonik Type 1 nephelinites (MOS29, 18)

Cats = CaTiAl₂O₆; Tsch = CaAl₂SiO₆; Ae = NaFe³⁺Si₂O₆; Wo = Ca₂Si₂O₆; Di = CaMgSi₂O₆; En = Mg₂Si₂O₆; Hd = CaFe²⁺Si₂O₆. FeO and Fe₂O₃ calculated from total FeO by assigning all Na₂O to NaFe³⁺Si₂O₆.

Table 4.

Representative compositions of pyroxene in Mosonik Type 2 nephelinite lavas

Cats = CaTiAl₂O₆; Tsch = CaAl₂SiO₆; Ae = NaFe³⁺Si₂O₆; Wo = Ca₂Si₂O₆; Di = CaMgSi₂O₆; En = Mg₂Si₂O₆; Hd = CaFe²⁺Si₂O₆. n.d. – not detected. FeO and Fe₂O₃ calculated from total FeO by assigning all Na₂O to NaFe³⁺Si₂O₆.

Table 5.

Representative compositions of pyroxene in Mosonik Type 3 nephelinite lavas

Cats = CaTiAl₂O₆; Tsch = CaAl₂SiO₆; Ae = NaFe³⁺Si₂O₆; Wo = Ca₂Si₂O₆; Di = CaMgSi₂O₆; En = Mg₂Si₂O₆; Hd = CaFe²⁺Si₂O₆. FeO and Fe₂O₃ calculated from total FeO by assigning all Na₂O to NaFe³⁺Si₂O₆.

Table 6.

Representative compositions of pyroxene in Mosonik phonolite

Cats = CaTiAl₂O₆; Tsch = CaAl₂SiO₆; Ae = NaFe³⁺Si₂O₆; Wo = Ca₂Si₂O₆; Di = CaMgSi₂O₆; En = Mg₂Si₂O₆; Hd = CaFe²⁺Si₂O₆. n.d. – not detected. FeO and Fe₂O₃ calculated from total FeO by assigning all Na₂O to NaFe³⁺Si₂O₆.

Fig. 10.

Composition (mol. %) of clinopyroxenes in representative Mosonik lavas expressed in terms of the diopside–hedenbergite–aegirine (Di–Hd–Ae) ternary system: (a) Type 1 nephelinites; (b) Type 2 and 3 nephelinites; (c) Type 4 phonolites; (d) nephelinites and phonolites from Sadiman volcano (Zaitsev et al. Reference Zaitsev, Marks, Wenzel, Spratt, Sharygin, Strekopytov and Markl2012).

Fig. 11.

Photomicrographs illustrating the complex compositional zoning of euhedral and anhedral phenocrystal clinopyroxenes in (a, b) Type 2 and (c) Type 3 Mosonik lavas. The juxtaposition of clinopyroxenes of diverse character are considered to indicate magma mixing.

Pyroxenes in Type 1 nephelinite lavas are the least evolved and range principally in composition from Di₈₅Hd₁₀Ae₅ to Di₆₀Hd₃₀Ae₁₀ (Fig. 10a). One Type 1 lava exhibits a secondary trend originating from the former trend at Di₆₀Hd₃₀Ae₁₀ with evolution to Di₄₀Hd₅Ae₅₅ (Fig. 10a). These data indicate mixing has certainly played a role in the evolution of the Type 1 nephelinite clinopyroxene phenocryst assemblage. Sedova et al. (Reference Sedova, Zaitsev and Spratt2018) has reported similar unevolved pyroxene compositions of Di_85–53Hd_35–10Ae_12–4 in the melilite nephelinites.

Pyroxenes in Type 2 nephelinites show the greatest compositional variation (Fig. 10b). The least evolved pyroxenes follow the main trend of the Type 1 nephelinite pyroxenes but continue this trend to compositions representing increasing Ae contents at essentially constant low Di contents, i.e. Di₈₅Hd₁₀Ae₅ → Di₆₀Hd₃₀Ae₁₀ → Di₁₀Hd₅₀Ae₄₀ → Di₅Hd₅Ae₉₀ → Ae₁₀₀ (Fig. 10b).

Pyroxenes in garnet-bearing Type 3 nephelinites (Fig. 10c) in some respects resemble those in Type 1 and 2 nephelinites. For example, sample MOS43 (Fig. 10c) contains only relatively unevolved pyroxenes similar to those of both Type 1 and Type 2 lava types. In contrast, samples MOS11 and 26 contain Na-rich pyroxenes (Fig. 10c) similar to, but not identical with, evolved Type 2 pyroxenes but with greater Di contents (Di₃₀Hd₂₀Ae₅₀ to Di₁₀Hd₁₀Ae₈₀). The pyroxene compositions suggest that some, but not all, Type 2 lavas incorporated garnets prior to eruption to form Type 3 lavas, and therefore that there are many discrete batches of Type 2 nephelinite, some of which are more evolved in terms of their clinopyroxene and nepheline (see Section 6.c below) assemblages than others. Note that magnetite- and amphibole-bearing Type 1 lavas do not contain garnet antecrysts and Type 3 lavas lack these minerals.

Pyroxenes in the phonolite Type 4 lava exhibit a range in composition primarily from Di₇₀Hd₂₀Ae₁₀ to Di₂₀Hd₄₀Ae₄₀ (Fig. 10d). The majority of these data plot at the Ae-rich end of this compositional range but with no distinct trend evident and represent crystals which are only weakly zoned. The less-evolved Di-rich compositions are representative of the rare complexly zoned antecrysts. The groundmass in some instances contains very small acicular aegirine prisms. Compared to clinopyroxenes in the Sadiman phonolites (Di₂₅Hd₄₀Ae₃₅–Di₅Hd₃₅Ae₆₀), those from Mosonik are relatively poor in Na. Figure 10e indicates that in both volcanoes the phonolites represent the most evolved lavas.

The compositional trends of clinopyroxenes from Mosonik and Sadiman are compared with those of a variety of volcanic and plutonic alkaline rocks in Figure 12. The positions of these trends reflects differing redox conditions and/or peralkalinity of the parental magmas (Mitchell & Platt, Reference Mitchell and Platt1982; Mitchell & Vladykin, Reference Mitchell and Vladykin1996). In this context, neither Mosonik nor Sadiman can be considered as being particularly peralkaline or formed under exceptionally high or low oxygen fugacities. The trends of both suites are very similar to those of volcanic rocks from Uganda (Fig. 12: trends 1 and 2).

Fig. 12.

(a) Compositional trends of clinopyroxenes in types 1–3 phonolitic nephelinites from Mosonik (blue curve; this work) and nephelinites from Sadiman (green curve; Zaitsev et al. Reference Zaitsev, Marks, Wenzel, Spratt, Sharygin, Strekopytov and Markl2012). Dashed black line shows the trend of compositions in MOS25 (see Fig. 10). Also shown are compositional fields for clinopyroxenes in phonolites from Mosonik (M; this work) and Sadiman (S; Zaitsev et al. Reference Zaitsev, Marks, Wenzel, Spratt, Sharygin, Strekopytov and Markl2012). (b) Compositional trends for clinopyroxenes in a variety of alkaline volcanic and plutonic rocks (after Mitchell & Vladykin, Reference Mitchell and Vladykin1996) for comparison with those of Mosonik and Sadiman lavas: 1 – Morotu; 2 – Uganda; 3 – Itapirapua; 4 – South Qoroq; 5 – pantellerite; 6 – Nandewar; 7 – Ilimausaq; 8 – Coldwell ferroaugite syenite; 9 – Turja; 10 – Iron Hill; 11 – Coldwell nepheline syenite; 12 – Little Murun complex; A, B, C – Fen complex (data sources in Mitchell & Platt, Reference Mitchell and Platt1982; Mitchell & Vladykin, Reference Mitchell and Vladykin1996). The Mosonik and Sadiman pyroxene compositional trends approximate those of the mildly peralkaline Ugandan volcanics (trends 1 and 2) with intermediate oxygen fugacities.

6.b. Garnet compositional variation

Garnets in Type 3 nephelinite lavas are enriched in TiO₂ and Fe₂O₃ (Table 7) and exhibit a significant compositional range within the morimotoite–andradite–schorlomite (Mmt–Adr–Slo) ternary solid solution series (Fig. 13). The majority of compositions are morimotoite–andradite ranging from Mmt₆₅Andr₁₀Slo₂₅ to Mmt₃₀Andr₃₅Slo₃₅, reflecting decreasing TiO₂ and increasing Fe₂O₃ contents from core to rim of subhedral crystals. Garnets with oscillatory zoning exhibit the same compositional trend within each zone. Similar Ti-bearing garnets are found at Sadiman (Zaitsev et al. Reference Zaitsev, Marks, Wenzel, Spratt, Sharygin, Strekopytov and Markl2012) and Shombole (Peterson, Reference Peterson1989 a). All garnets contain only trace contents of ZrO₂ (<0.3 wt %) in common with other volcanic morimotoite garnets at Sadiman (0.2–0.4 wt % ZrO₂).

Table 7.

Representative compositions of garnet in Type 3 nephelinite lavas

FeO and Fe₂O₃ calculated on the basis of 8 cations and 12 oxygens following Droop (Reference Droop1987). Garnet end-members calculated using Locock (Reference Locock2008). c – core; r – rim. Slo = schlorlomite (Ca₃Ti₂(SiFe³⁺ ₂)O₁₂); Slo-Al = schlorlomite-Al (Ca₃Ti₂(SiAl₂)O₁₂); Mmt = morimotoite (Ca₃(TiFe²⁺)Si₃O₁₂); Mmt-Mg = morimotoite-Mg (Ca₃(TiMg)Si₃O₁₂); Adr = andradite (Ca₃Fe³⁺ ₂Si₃O₁₂); Skg = skiagite Fe²⁺ ₃Fe³⁺ ₂Si₃O₁₂; Khh = khoharite Mg₃Fe³⁺ ₂Si₃O₁₂

Fig. 13.

Compositions (mol. %) of garnets in Type 3 phonolitic nephelinites from Mosonik.

6.c. Nepheline, sodalite and feldspar compositional variation

Nepheline in all lava types does not exhibit significant major-element compositional variation, and there are no significant differences between phenocrystal and groundmass nepheline compositions (Fig. 14; Table 8). The principal compositional variation is that of K₂O, with nepheline in individual lavas being distinctive, e.g. Type 1 nephelinite lavas 4.8–5.6 wt %; Type 2 nephelinite lavas MOS15 and MOS10 with 4.2–5.4 wt % or 5.7–6.5 wt %, respectively. Nephelines in Type 3 nephelinite lavas are relatively enriched in K₂O (6.1–7.9 wt %) and those in phonolites relatively depleted (2.8–4.2 wt %). With respect to minor elements, all nephelines lack Ca but contain significant amounts of Fe, with groundmass types being relatively enriched (2–4.5 wt % Fe₂O₃) compared to phenocrysts (0.7–1.8 wt % Fe₂O₃). Where zoning of phenocrysts is evident, crystal margins are enriched in Fe relative to cores. Sedova et al. (Reference Sedova, Zaitsev and Spratt2018) has noted that nepheline phenocrysts in the melilite nephelinite are relatively potassic (Ne_70–66Ks_31–8Qtz_0–0.1).

Fig. 14.

Compositions (wt % end-members) of nepheline and feldspar for Mosonik lavas expressed in the quartz–nepheline–kalsilite ternary system (Q–Ne–Ks). Ab – albite; Or – KAlSi₃O₈ (orthoclase).

Table 8.

Representative compositions of nepheline

Composition: (1) phenocryst Type 1 nephelinite; (2 and 3) phenocryst and groundmass, Type 2 nephelinite; (4 and 5) phenocryst core and rim, Type 3; (6) groundmass Type 3 nephelinite; (7 and 8) phenocryst and groundmass, phonolites. Ks = KAlSiO₄; Ne = NaAlSiO₄; FeNe = NaFe³⁺SiO₄; Q = excess SiO₂; □ = cation vacancies calculated after Mitchell (Reference Mitchell1972).

Groundmass nepheline is commonly partially-to-completely pseudomorphed by sodalite in many of the lavas whose composition (wt %; 25.1 Na₂O; 29.0 Al₂O₃; 38.6 SiO₂; 1.8 Fe₂O₃; 0.3 K₂O; 6.14 Cl; 0.5 S; 99.40 Total) does not vary significantly. Sodalite in turn can be replaced by natrolite (13.78 Na₂O; 22.08 Al₂O₃; 53.8 SiO₂ wt %) and unidentified K–Na–Ca zeolites.

Feldspar occurs principally as a groundmass mineral in most lavas, and only the phonolites are characterized by common phenocrystal feldspar. In contrast to nepheline, feldspars exhibit considerable compositional variation (Fig. 14; Tables 9, 10). Groundmass feldspars in Type 1 lavas show the most extensive composition, and individual lavas are characterized by distinct groundmass feldspar compositions. In one amphibole-bearing lava (MOS23) these are zoned from anorthoclase cores (Ab₇₁Or₁₈An₁₁–Ab₆₆Or₂₇An₇) to Ca-free albitic feldspar (Ab₇₂Or₂₈–Ab₅₆O₄₄). Other Type 1 nephelinite lavas contain feldspars ranging in composition from Ab₅₅Or₄₅–Ab₄₁Or₅₉ in sample MOS31 to Ab₂₂Or₇₈–Ab₁₆Or₈₄ in MOS25. Feldspars in Type 2 and 3 nephelinite lavas are more ‘evolved’, in terms of increased K-content, with respect to the Type 1 lava trend of feldspar compositions, and lack anorthoclase and albite-rich types. These range from Ab₂₇Or₇₃–Ab₉Or₉₁ in Type 2 lavas to Type 3 lavas having a restricted range of potassic feldspar compositions of Ab₅Or₉₅–Ab₃Or₉₇ (Fig. 14). One Type 2 lava contains rare (<1 vol. %) phenocrystal alkali feldspar (Ab_50–48Or_50–52) mantled by clinopyroxene. Phenocrystal and groundmass feldspars in the phonolites range from Ab₇₇Or₂₃–Ab₆₆Or₃₄ and Ab₄₁Or₅₉–Ab₅Or₉₅, respectively. Note that the phonolite feldspar phenocrysts are of similar composition in terms of their albite component to those of groundmass anorthoclase in Type 1 lavas but lack CaO.

Table 9.

Representative compositions of groundmass feldspar in Type 1 nephelinite lavas

Compositions: (1 and 2) M23; (3 and 4) M31; (5 and 6) M25. Cs = BaAl₂Si₂O₈; An = CaAl₂Si₂O₈; Ab = NaAlSi₃O₈; FeOr = KFeSi₃O₈; Or = KAlSi₃O₈. Total Fe expressed as Fe₂O₃

Table 10.

Representative compositions of feldspar in Type 2 and 3 nephelinites and phonolite lavas

Compositions: (1 and 2) Type 2 M7; (3 and 4) Type 2 M10; (5) Type 3 M748; (6 and 7) Type 4 phenocrysts phonolite; (8–10) Type 4 groundmass phonolite. Cs = BaAl₂Si₂O₈; Ab = NaAlSi₃O₈; FeOr = KFeSi₃O₈; Or = KAlSi₃O₈. Total Fe expressed as Fe₂O₃

6.d. Magnetite

Groundmass Ti-magnetite occurs only in Type 1 nephelinite lavas principally as opaque euhedral-to-subhedral 5–50 μm single crystals. Less common larger crystals (50–200 μm) typically exhibit convoluted resorbed margins and embayments. Some magnetite occurs intergrown with groundmass pyroxene in reaction mantles formed on amphibole antecrysts. Ilmenite exsolution is absent. Magnetite crystallizes prior to groundmass feldspars.

The magnetites are principally Cr-free, Al- and Mg-poor members of the Mn₂TiO₄–Fe₂TiO₄–Fe₃O₄ (MnUsp–Usp–Mt) solid solution series (Table 11). Those occurring in individual Type 1 nephelinite lavas exhibit a range in composition, e.g. MnUsp_1–4UsP_41–55Mt_42–54 in sample MOS25. They typically show Ti-enrichment from cores to margins, although there is no correlation between crystal morphology and composition. The principal inter-lava variation is in respect to MgO and MnO: e.g. MgO <0.7 wt % and MnO 0.8–2.2 wt % in sample MOS25; MgO 1.3–3.0 wt % and MnO 0.6–1.0 wt % in sample MOS31; MgO <0.5 wt % and MnO 0.8–1.0 wt % in sample MOS25.

Table 11.

Representative compositions of ulvöspinel-magnetite in Type 1 nephelinite lavas

Compositions: (1 and 2) core and rim M25; (3 and 4) core and rim M25; (5 and 6) core and rim M31; (7 and 8) core and rim M31; (9 and 10) single crystals M18. n.d. – not detected. FeO and Fe₂O₃ calculated from stoichiometry (Droop, Reference Droop1987).

6.e. Amphibole

Amphiboles are present only in Type 1 nephelinite lavas. These occur as isolated round light-green-to-brown antecrysts with complex thin reaction mantles of diopsidic pyroxene and Ti-magnetite with subsequent overgrowths (Fig. 3b) of coarser Ti-magnetite, calcite and pyroxene. The latter are similar in composition (Di₇₁Hd₂₃Ac₆–Di₆₅Hd₂₂Ac₁₃) to phenocrystal and groundmass pyroxenes in Type 1 lavas (Fig. 10). The amphiboles are all Ti-bearing magnesio-hastingsite and typically continuously zoned with increasing FeO from core to rim (Table 12). Inclusions of apatite are present in some crystals.

Table 12.

Representative compositions of magnesio-hastingsite

FeO_T – total Fe calculated as FeO; FeO and Fe₂O₃ are calculated using the method of Droop (Reference Droop1987). n.d. – not detected.

6.f. Accessory minerals

Titanite occurs as isolated euhedral phenocrysts (200 μm) in all lavas (<1 vol. %) and as rare euhedral inclusions in nepheline phenocrysts. The composition is near stoichiometric with only minor FeO_T (1–2 wt %) and Nb₂O₅ (<1 wt %) being present. F-bearing (2.3–3.2 wt %) apatite occurs as oval crystals (100–500 μm) in Type 2 and 3 lavas. SrO contents range from 0.7 to 1.9 wt % and REE contents are typically not detectable.

Sr–F-bearing barytolamprophylite ((Na,K)₂(Ba,Sr,Ca)₂(Ti,Fe)₃(Si₂O₇)₂(O,OH,F)₄) occurs as a trace (<<1 vol. %) late-stage interstitial groundmass mineral (5–20 μm) and as thin (<5 m) rims on garnet antecrysts in the Type 3 eastern lava flow. Representative compositions are given in Table 13. The groundmass also contains trace amounts of a Sr–REE–F carbonate as irregular patches (<15 μm) in the interstices between feldspars. This mineral cannot be unambiguously identified and requires further characterization. The mineral is unusual as its composition (wt %: 32–33 La₂O₃; 9.5–11.0 SrO; 10.6–11.3 Nd₂O₃; 4.0 Pr₂O₃; 1.9–2.2 F), is dominated by Sr, Nd and La, and contains minor amounts of other REE₂O₃ but lacks Ce (<0.3 wt % Ce₂O₃), perhaps due to removal as Ce⁴⁺. Possible minerals are Ce-deficient ancylite-(La) or Sr-kozoite-(La).

Table 13.

Representative compositions of barytolamprophyllite

FeO_T – total Fe as FeO; SF – structural formula calculated on the basis of 16 atoms of oxygen.

6.g. Peralkaline glass inclusions in nepheline

Phenocrystal nephelines in some Type 3 lavas contain 15–30 μm spherical inclusions consisting principally of silicate glass. Former gas bubbles and wollastonite crystals are present in some examples and carbonates are apparently absent. Table 14 shows that, although the glass is of diverse composition, all examples are strongly peralkaline, with peralkalinity indices ranging from 6.25 to 13.35, iron-enrichment and significant Cl and S contents. Fluorine is also present but cannot be quantitively determined by EDS methods in the presence of significant amounts of Fe. These data and the presence of barytolamprophyllite suggest that the development of peralkaline lavas might be possible at Mosonik.

Table 14.

Composition of peralkaline glass inclusions in nepheline

P.I. – peralkalinity index molar (Na₂O + K₂O)/Al₂O₃

7. Sr–Nd isotopic relationships

New data for the bulk-rock Sr and Nd isotopic compositions for Mosonik volcanic rocks are given in Table 15 and illustrated in Figures 15–17. Because of the young age of the rocks, their low Rb/Sr and low Sm/Nd ratios coupled with high Sr contents, the isotopic data are not age corrected and are considered to represent initial ratios. Table 15 and Figure 15 indicate that for data obtained in this work there is no correlation between lava type and isotopic compositions. The lavas have distinctly different Sr and Nd isotopic compositions, and Figure 15 shows that all Nd isotopic compositions (ϵ_Nd(0) −3.1 to −8.3) plot below the present day bulk earth chondritic uniform reservoir (CHUR) Nd isotopic composition (ϵ_Nd(0)) implying derivation from an old source enriched in the light REEs (low Sm/Nd) relative to bulk earth, suggesting derivation from depleted mantle sources. Sr isotopic compositions straddle the chondritic bulk earth composition (⁸⁷ Sr/⁸⁶Sr = 0.7047) but are not significantly enriched in radiogenic Sr, suggesting sources possibly with low Rb/Sr ratios and no crustal contamination. Mosonik compositions all plot below the field of Oldoinyo Lengai silicate lavas (Bell & Peterson, Reference Bell and Peterson1991), which are coincident within the East African Carbonatite Line (Bell & Blenkinsop, Reference Bell and Blenkinsop1987).

Table 15.

Isotopic composition of Mosonik, Oldoinyo Lengai and Engaruka volcanic rocks

Accepted values of the standards JNdi-1 and NBS 987 are ¹⁴³Nd/¹⁴⁴Nd = 0.512107 and ⁸⁷Sr/⁸⁶Sr = 0.710240, respectively. Mosonik lavas (types 1–4) were analysed at the University of Alberta (see online Supplementary Material). Oldoinyo Lengai 2007 silicate ash, combeite wollastonite nephelinites (cwn) from North and South Nasira parasitic cones (NC and SC) and Engaruka melilitites (mel) were analysed at Memorial University of Newfoundland (see online Supplementary Material). HMC – Half Moon Crater; AH – Amykron Hill; KS – Kisete; BH – Baboon Hill; BD-108 – Lalarasi.

Fig. 15.

¹⁴³Nd/¹⁴⁴Nd versus ⁸⁷Sr/⁸⁶Sr correlation diagram for lavas from Mosonik (this work; Paslick et al. Reference Paslick, Halliday, Langem, James and Dawson1996), Shombole (Bell & Peterson, Reference Bell and Peterson1991) and Lengai silicate lavas (Bell & Dawson, Reference Bell, Dawson, Bell and Keller1995). Data for granulite xenoliths are from (Cohen et al. Reference Cohen, O’Nions and Dawson1984), and the East African Carbonatite Line (EACL) from Bell & Blenkinsop (Reference Bell and Blenkinsop1987). Mosonik samples are labelled as ‘sample number/lava type’ i.e. 8/1, or P for data from Paslick et al. (Reference Paslick, Halliday, Langem, James and Dawson1996).

Fig. 16.

¹⁴³Nd/¹⁴⁴Nd versus ⁸⁷Sr/⁸⁶Sr correlation diagram for Old Extrusive Series lavas from Mosonik (this work; Paslick et al. Reference Paslick, Halliday, James and Dawson1995, Reference Paslick, Halliday, Langem, James and Dawson1996; Mana et al. Reference Mana, Furman, Carr, Mollel, Mortlock, Feigensen, Turrin and Swisher2012, Reference Mana, Furman, Turrin, Feigenson and Swisher2015) and Sadiman (Zaitsev et al. Reference Zaitsev, McHenry, Savchenok, Strekopytov, Sprat, Humphreys-Williams, Sharygin, Bogomolov, Chakhmouradian, Zaitseva, Arzamastsev, Reguire, Leach, Leach and Mwankunda2019). Labels for Mosonik lavas as in Figure 15.

Fig. 17.

¹⁴³Nd/¹⁴⁴Nd versus ⁸⁷Sr/⁸⁶Sr correlation diagram for lavas from the Engaruka volcanic field and Mosonik (this work; Paslick et al. Reference Paslick, Halliday, Langem, James and Dawson1996) compared with other volcanic rocks of the Older and Newer Extrusive Series (Paslick et al. Reference Paslick, Halliday, James and Dawson1995, Reference Paslick, Halliday, Langem, James and Dawson1996; Mana et al. Reference Mana, Furman, Carr, Mollel, Mortlock, Feigensen, Turrin and Swisher2012, Reference Mana, Furman, Turrin, Feigenson and Swisher2015). Labels for Mosonik lavas as in Figure 15; E – Essimingor; L – Lemagrut; Lo – Loolmalasin; N – Nasira; S – Sadiman. Data for Nasira and Lengai 2007 ash (this work). EACL – East African Carbonatite Line (Bell & Blenkinsop (Reference Bell and Blenkinsop1987); BE – bulk earth ⁸⁷Sr/⁸⁶Sr ratio (0.7047); CHUR = ¹⁴³Nd/¹⁴⁴Nd ratio of 0.512638.

Figure 15 also shows Sr–Nd isotopic compositions for four lavas from Mosonik determined by Paslick et al. (Reference Paslick, Halliday, Langem, James and Dawson1996). These differ significantly from those determined in this work as they plot above the bulk earth CHUR Nd isotopic composition with ϵ_Nd(0) +0.86 to +1.81 and lower ⁸⁷Sr/⁸⁶Sr ratios close to the East African Carbonatite Line. These differences are unlikely to be attributed to analytical errors as data presented for Nd and Sr isotopic standards (Table 15) indicate satisfactory inter-laboratory analytical accuracy and precision between the Universities of Alberta and Michigan analytical protocols. These data significantly extend the extent of Sr–Nd isotopic variation at Mosonik (see below).

Figure 15 also illustrates Sr and Nd isotopic data for nephelinites and phonolites from Shombole (Bell & Peterson, Reference Bell and Peterson1991), a petrologically similar Older Extrusive volcano (1.96–2.0 Ma; Fairhead et al. Reference Fairhead, Mitchell and Williams1972; Peterson, Reference Peterson1989 a) adjacent to Mosonik. These volcanic rocks are similar to those of Mosonik in having a wide range in initial negative ϵ_Nd(0) isotopic compositions. However, they differ with respect to Sr in having much lower initial ⁸⁷Sr/⁸⁶Sr ratios. These data suggest that Mosonik and Shombole lavas are derived from different sources or from similar sources which have undergone different genetic processes, such as different styles of metasomatism. Unlike Shombole, there is no significant correlation between ¹⁴³Nd/¹⁴⁴Nd and the SiO₂ contents of Mosonik lavas. At Mosonik and Shombole ⁸⁷Sr/⁸⁶Sr ratios are not correlated with SiO₂.

Figure 16 illustrates Sr and Nd isotopic compositions of Older Series Volcanics in the North Tanzania Divergence (Paslick et al. Reference Paslick, Halliday, James and Dawson1995, Reference Paslick, Halliday, Langem, James and Dawson1996; Mana et al. Reference Mana, Furman, Turrin, Feigenson and Swisher2015; Zaitsev et al. Reference Zaitsev, McHenry, Savchenok, Strekopytov, Sprat, Humphreys-Williams, Sharygin, Bogomolov, Chakhmouradian, Zaitseva, Arzamastsev, Reguire, Leach, Leach and Mwankunda2019) and shows that each volcanic centre has a distinct isotopic composition. The majority of these data, including the Paslick et al. (Reference Paslick, Halliday, Langem, James and Dawson1996) data for Mosonik, are isotopically different from the new data for the Mosonik volcanics, with only lavas from Sadiman (Zaitsev et al. Reference Zaitsev, McHenry, Savchenok, Strekopytov, Sprat, Humphreys-Williams, Sharygin, Bogomolov, Chakhmouradian, Zaitseva, Arzamastsev, Reguire, Leach, Leach and Mwankunda2019) and some lavas from Essimingor and Lemagrut having relatively similar isotopic compositions to the Mosonik lavas analysed in this work. However, considering the extreme isotopic variation of the Mosonik lavas indicated by inclusion of the Paslick et al. (Reference Paslick, Halliday, James and Dawson1995, Reference Paslick, Halliday, Langem, James and Dawson1996) data, note that the Essimingor and Monduli lavas exhibit a similar wide range in isotopic composition from positive to negative ϵ_Nd(0) signatures (Fig. 16). Although there are limited data, these do suggest that similar processes could have occurred in the development of the isotopic compositions of these three volcanoes. In marked contrast, the Sadiman lavas (Paslick et al. Reference Paslick, Halliday, James and Dawson1995; Zaitsev et al. Reference Zaitsev, McHenry, Savchenok, Strekopytov, Sprat, Humphreys-Williams, Sharygin, Bogomolov, Chakhmouradian, Zaitseva, Arzamastsev, Reguire, Leach, Leach and Mwankunda2019), although heterogeneous, form a near-linear mixing line suggesting derivation from two enriched distinct mantle sources.

Figure 17 shows that most of the Older Volcanics have compositions close to that of CHUR with Gelai and some examples from Essimingor and Loolmalasin having positive ϵ_Nd(0) signatures, whereas Kitumbeine and Tarosero have negative ϵ_Nd(0) signatures. Tarosero is distinct in exhibiting an extremely wide variation in ⁸⁷Sr/⁸⁶Sr ratios, suggesting significant crustal contamination or mixing of ancient sources with diverse Rb/Sr ratios and uniform Sm/Nd ratios.

Figure 17 also includes new data for melilititic lavas of the Engaruka–Natron volcanic field. These lavas are distinct from most other Older and Younger Series volcanics in that they all have Nd isotopic compositions greater than CHUR (i.e. positive ϵ_Nd(0)) and Sr isotopic compositions less than those of bulk earth, implying derivation from depleted mantle sources. Only the Gelai lavas, an Older Series volcanic, have a similar restricted range of isotopic compositions (Fig. 17).

9. Summary and conclusions

This investigation has shown that Mosonik, a deeply dissected polygenetic Older Extrusive Series volcano, is composed principally of diverse phonolitic nephelinite and phonolite lavas, with minor amounts of melilite-bearing nephelinite and carbonatite. Less-evolved olivine nephelinites (sensu stricto), peralkaline nephelinites and basalts are not present at the current level of exposure. At least three types of phonolitic nephelinite are present and distinguished on the presence or absence of amphibole or garnet antecrysts and differing populations of complexly zoned clinopyroxenes. The phenocryst assemblage is typical of hybrid lavas derived from magma mixing. As a consequence of the high modal proportions of clinopyroxene and nepheline phenocrysts and the hybrid character of the lavas, their bulk compositions do not represent those of the liquids from which they crystallized and an unevolved parental magna cannot be defined. This observation renders AFC modelling of major and trace elements as inappropriate. Isotopic data for Mosonik are similar to those previously determined for other Older Extrusive Series rocks. Interpretation of these data is ambiguous and the observed isotopic heterogeneity might reflect mixing of Sr and Nd derived from two distinct mantle sources with or without lower crustal granulite contamination.

It is also proposed that three distinct mantle sources might have contributed to the isotopic character of the Older and Younger Extrusives. The Older Extrusives might be derived by partial melting of ancient metasomatized lithospheric mantle with mixing of Sr and Nd from two sources coupled with minor lower crustal contamination. Partial melting was undoubtedly induced by the plume currently impinging on the Tanzanian craton, and represents the initial interaction of the plume with the cratonic lithosphere. In contrast, the Younger Extrusives as exemplified by the Oldoinyo Lengai nephelinite–carbonatite volcanism could be derived from this ancient metasomatized lithospheric mantle plus a recent plume-derived asthenospheric component and no contamination by crustal material. The genetically distinct Natron–Engaruka melilitites are considered to represent direct adiabatic melting of the Tanzanian plume without lithospheric contributions.

Further investigations at Mosonik should include a more extensive sampling of the eroded edifice; characterization of the large blocks of lava in the debris flows of the Leshuta River Gorge; and the petrology of the suite of ijolite series xenoliths. Clearly, given the paucity of geological, petrological and isotopic studies of all the volcanoes of the North Tanzanian Divergence, especially the basaltic basal sequences, coupled with the current ambiguity regarding their genesis, many further investigations are desirable before any comprehensive petrogenetic scheme for this province can be formulated.