Journal of Petrology Volume 42 Number 4 Pages 765-787 2001
© Oxford University Press 2001
Plume–Lithosphere Interaction and the Origin of Continental Rift-related Alkaline Volcanism—the Chyulu Hills Volcanic Province, Southern Kenya
1DEPARTMENT OF GEOLOGICAL SCIENCES, UNIVERSITY OF CAPE TOWN, RONDEBOSCH, 7701, SOUTH AFRICA
2DEPARTMENT OF GEOLOGY, UNIVERSITY OF NAIROBI, PO BOX 30197, NAIROBI, KENYA
Received November 8, 1999; Revised typescript accepted July 10, 2000
 |
ABSTRACT |
|---|
 TOP ABSTRACT INTRODUCTIONGEOLOGICAL SETTINGBULK-ROCK GEOCHEMISTRYMELTING MODELSEVIDENCE FOR AN AMPHIBOLE...DISCUSSIONREFERENCES |
|---|
Geochemical data are presented for primitive alkaline lavas from the Chyulu Hills Volcanic Province of southern Kenya, situated some 100 km east of the Kenya Rift Valley. In addition to their primitive compositions, a striking and ubiquitous feature is a strong but variable depletion in K relative to other highly incompatible elements when normalized to primitive mantle values. Semi-quantitative models are developed that best explain the petrogenesis of these lavas in terms of partial melting of a source that contained residual amphibole (but not phlogopite). The presence of amphibole implies a source in the subcontinental lithosphere rather than the asthenosphere. It is suggested that the amphibole is of metasomatic origin and was precipitated in the lithospheric mantle by infiltrating fluids and/or melts derived from rising mantle plume material. A raised geotherm as a consequence of the continued ascent of the plume material led to dehydration melting of the metasomatized mantle and generation of the Chyulu Hills lavas. It is proposed that the Chyulu Hills Volcanic Province represents an analogue for the earliest stages of continental rift initiation, during which interaction between a plume and initially refractory lithosphere may lead to the generation of lithospheric melts.
KEY WORDS: rift-related alkaline volcanism; residual amphibole; subcontinental lithosphere
|
INTRODUCTION |
|---|
|
TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTINGBULK-ROCK GEOCHEMISTRYMELTING MODELSEVIDENCE FOR AN AMPHIBOLE...DISCUSSIONREFERENCES |
|---|
The geochemistry of mafic, alkaline volcanic rocks erupted in continental areas can potentially yield valuable information about the nature of large parts of the Earth’s interior that are otherwise inaccessible. In combination with geophysical evidence and clues from mantle-derived xenoliths, such geochemical data hold the key to understanding the composition and evolution of the Earth’s mantle. The purpose of this contribution is to investigate aspects of the geochemistry and mineralogy of the mantle source region of the relatively primitive, rift-related, alkaline lavas of the Quaternary Chyulu Hills Volcanic Province (CHVP) of southern Kenya (Fig. 1).
|
The voluminous alkaline magmatism commonly associated with continental rifting, of which the East African Rift System (Fig. 1) is a type example, has been the subject of geochemical investigations for decades (e.g. Williams, 1970
; Baker et al., 1971; Baker, 1987). One problem frequently addressed by recent studies is the identification of the source regions of primitive, mafic, rift-related magmas. A primary concern is centred around whether they are derived from the subcontinental lithospheric mantle (SCLM) or from sublithospheric sources in the asthenosphere or in mantle plumes. Some workers have argued that the large volumes of lava generated during continental rifting are produced exclusively, or at least predominantly, by asthenospheric melting (e.g. McKenzie & Bickle, 1988; White & McKenzie, 1989; Arndt & Christensen, 1992), the lithospheric mantle being considered too refractory to yield significant quantities of magma. Others have suggested that a large proportion of the lava erupted during the various stages of continental break-up may have its origin in a metasomatically enriched SCLM (e.g. Gallagher & Hawkesworth, 1992; Turner et al., 1996). In light of this dichotomy of melting models for rift-related continental magmatism, it is of relevance that several investigations have shown that the lithosphere beneath parts of East Africa has experienced pervasive metasomatic alteration and enrichment (e.g. Rudnick et al., 1993; Furman, 1995; Paslick et al., 1995). The debate over ‘active’ (plume-driven) vs ‘passive’ (plate-driven) continental rifting has provided a second major focus of interest. In recent years, plume-related models of rifting and rift magmatism have tended to dominate the literature about the East African Rift System, and one or more mantle plumes have been postulated beneath the Kenya Rift Valley or the nearby Tanzanian Craton (e.g. Karson & Curtis, 1989; Burke, 1996; Mechie et al. 1997; Simiyu & Keller, 1997).
The interpretation of geochemical data and the application of mathematical melting models to primitive continental rift-related lavas such as those of the CHVP can provide crucial insights into important issues such as magma source regions (continental lithosphere, asthenosphere or plume) and the infiltration and metasomatic modification of the SCLM by plume-derived fluids and melts, and may also allow constraints to be placed on melting processes and mantle source compositions (mineralogy and geochemistry).
|
GEOLOGICAL SETTING |
|---|
|
TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTINGBULK-ROCK GEOCHEMISTRYMELTING MODELSEVIDENCE FOR AN AMPHIBOLE...DISCUSSIONREFERENCES |
|---|
The CHVP is a NW–SE aligned volcanic field located between 100 and 250 km east of the southern Kenya Rift Valley (Fig. 1), covering an area of
2840 km2. The origin of this and several other off-rift igneous provinces in Kenya (e.g. Huri Hills, Nyambeni Hills) has previously been attributed either to tectonic and pre-existing structural controls (e.g. Bosworth 1987, 1989), or to rising mantle plume material (e.g. Karson & Curtis, 1989; Burke, 1996). Most recent studies, however, favour a synthesis of these two approaches and suggest that the volcanic activity is related to the exploitation of pre-existing zones of lithospheric weakness by small diapirs of plume material derived from a common, larger, mantle plume located beneath the rift valley itself or the Tanzanian Craton (e.g. Smith, 1994; Mechie et al., 1997; Ritter & Kaspar, 1997). Geophysical studies have revealed the presence of a partially molten, low seismic velocity zone in the lithospheric mantle below the CHVP (e.g. Ritter & Kaspar, 1997), but the small size of this anomaly and the absence of a connection to the asthenosphere argues strongly against an origin by direct melting of a rising mantle plume (Ritter et al., 1995; Ritter & Kaspar, 1997).
The lavas erupted in the CHVP can be divided into the Pleistocene Northern Chyulu Hills lavas and the Holocene Southern Chyulu Hills lavas (Saggerson, 1963, 1968; Haug & Strecker, 1995; Späth et al., 2000). On the basis of their geographical location and geochemistry, the Northern Chyulu Hills lavas can be assigned to one of three units: the Sultan Hamud–Emali–Simba lavas (further subdivided into a high-Ti nephelinite group, a low-Ti nephelinite–basanite group and a basanite–alkali basalt group), the Nguu lavas and the Ngatatema lavas, whereas the Southern Chyulu Hills lavas comprise the Southern Chyulu Hills basanites, alkali basalts and hawaiites and the Mzima-type transitional basalts (Späth et al., 2000).
The Northern Chyulu Hills lavas tend to be fresh to somewhat altered, vesicular, microporphyritic and porphyritic rocks containing predominantly phenocrysts of olivine, fewer microphenocrysts of clinopyroxene and Fe–Ti oxide, and rarely plagioclase and Cr-spinel. The fine-grained to cryptocrystalline and in some cases partly glassy groundmass typically contains abundant clinopyroxene and plagioclase, less common Fe–Ti oxide, olivine and nepheline, as well as accessory apatite and Cr-spinel. The Southern Chyulu Hills basanites, alkali basalts and hawaiites are mostly fresh, predominantly olivine phyric rocks with a fine-grained to cryptocrystalline and partly glassy groundmass of clinopyroxene, plagioclase, lesser Fe–Ti oxide, olivine and accessory apatite. The Mzima-type transitional basalts, which straddle the dividing line between alkalic and tholeiitic lavas (Macdonald & Katsura, 1964) on the total alkali–silica diagram (Fig. 2), are fresh to slightly altered rocks that tend to be somewhat seriate textured. They range from slightly porphyritic and microporphyritic lavas with a fine-grained to cryptocrystalline and partly glassy groundmass of clinopyroxene, plagioclase, Fe–Ti oxide, olivine and accessory apatite, to comparatively coarse-grained rocks consisting of interlocking crystals of plagioclase, olivine and clinopyroxene with interstitial patches of finer-grained clinopyroxene, plagioclase, Fe–Ti oxide and glass.
|
|
BULK-ROCK GEOCHEMISTRY |
|---|
|
TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTINGBULK-ROCK GEOCHEMISTRYMELTING MODELSEVIDENCE FOR AN AMPHIBOLE...DISCUSSIONREFERENCES |
|---|
Major and trace element data for representative Chyulu Hills lavas are presented in Table 1 (the analytical procedures followed are outlined in the caption to Table 1). The complete dataset may be downloaded from the Journal of Petrology Web site at http://www.petrology.oupjournals.org. The rocks range in composition from nephelinites, basanites and nepheline-normative hawaiites and alkali basalts to hypersthene-normative subalkali basalts (Fig. 2). The Northern Chyulu Hills lavas cover a relatively limited range in mg-number from 0·50 to 0·76 and show only poor correlations in most major element variation diagrams (Fig. 2), although broad negative trends are apparent for some elements (e.g. Al2O3 and TiO2 vs mg-number). The lavas of the high-Ti nephelinite group of the Sultan Hamud–Emali–Simba lavas show negative correlations in plots of Al2O3, TiO2 and FeO vs mg-number and have elevated TiO2 (6·37–8·10 wt %) and FeO (12·32–13·82 wt %), but comparatively low Al2O3 (6·86–8·65 wt %) concentrations. The Southern Chyulu Hills lavas similarly show only limited differentiation (mg-number = 0·55–0·72), but define more well-constrained linear trends than their northern counterparts in several major element variation diagrams (Fig. 2). In general terms, the major element variations of the Chyulu Hills lavas are consistent with an evolution dominated by olivine fractionation and to a much lesser extent clinopyroxene fractionation, but argue against significant involvement of feldspar and Fe–Ti oxide (Späth et al., 2000
). A broad trend of decreasing CaO/Al2O3 with decreasing mg-number, particularly for the Southern Chyulu Hills lavas (Fig. 2), confirms the role of clinopyroxene and supports the absence of significant feldspar fractionation.
|
All of the lavas exhibit significant and variable enrichment in incompatible elements (Zr/Nb = 2·93–6·14; Ba/Nb = 4·61–13·2; chondrite-normalized La/Sm = 2·35–5·08) and in the light rare earth elements (LREE) over the heavy rare earth elements (HREE) (Fig. 3). The Northern Chyulu Hills lavas have variable, but generally steeper chondrite-normalized REE patterns (chondrite-normalized La/Yb = 20·7–60·0) than the Southern Chyulu Hills lavas (chondrite-normalized La/Yb = 7·7–37·1). A general age progression from the oldest lavas in the northwest of the province to progressively younger rocks in the southeast (Saggerson, 1963, 1968; Haug & Strecker, 1995) coincides with a concomitant decrease in the degree of silica undersaturation and incompatible trace element enrichment. The bulk-rock major and trace element data for the primitive Chyulu Hills lavas are consistent with an age-progressive increase in the degree of partial melting and a concurrent decrease in the depth of melting from the Pleistocene northwest of the province to the Holocene southeast (Späth et al., 2000).
|
A striking geochemical feature of the Chyulu Hills lavas is a significant relative depletion in K, compared with elements expected to be of similar incompatibility (e.g. Th, Nb, La), when normalized to primitive mantle values (Fig. 3; K/Nb = 9·16–66·0, compared with 258 for N-MORB and 351 for primitive mantle; Sun & McDonough, 1989). In addition, the older, more silica-undersaturated, more incompatible element enriched lavas from the northern part of the CHVP display more pronounced relative K depletions (K/Nb = 9·16–36·8) than the progressively younger, less silica-undersaturated, less incompatible element enriched rocks from the southern CHVP. The least silica-undersaturated, least incompatible element enriched lavas, the Mzima-type transitional basalts, also have the least dramatic relative K depletions (K/Nb = 46·8–66·0). Relative depletions of K comparable with those observed in the CHVP have been reported for mafic lavas from a number of volcanic provinces (e.g. Wilson & Downes, 1991; Hoernle & Schmincke, 1993; Halliday et al., 1995; Späth et al., 1996; Class & Goldstein, 1997), including the Cenozoic Laisamis–Merille region (Freerk-Parpatt, 1992) and the Huri Hills (Class et al., 1994), two volcanic centres located east of the Kenya Rift Valley (Fig. 1).
Nd, Sr and Pb isotope data for selected primitive Chyulu Hills lavas are presented in Table 2 (the analytical procedures followed are described in the footnotes to Table 2). With the exception of the Mzima-type transitional basalts (87Sr/86Sr = 0·70385–0·70485; 143Nd/144Nd = 0·51269–0·51281), the samples define a moderately tight cluster in the depleted quadrant of the Nd–Sr isotope correlation diagram (87Sr/86Sr = 0·70330–0·70372; 143Nd/144Nd = 0·51277–0·51284), indicative of derivation from a common source with time-integrated depletion in incompatible trace elements (Fig. 4). This cluster of data lies close to the composition of lavas from a number of volcanic provinces on or near the African plate such as the Huri Hills of northern Kenya (Class et al., 1994), the Arabian Peninsula (Altherr et al., 1990), the Ahaggar province of North Africa (Allègre et al., 1981) and the islands of the Comores Archipelago in the Indian Ocean (Späth et al., 1996; Class et al., 1998). In addition, the general region of Nd–Sr isotope space occupied by these Chyulu Hills lavas appears to be common to several other volcanic provinces in and around East Africa, defining the low 87Sr/86Sr, high 143Nd/144Nd end of a series of elongate fields extending towards compositions of higher 87Sr/86Sr and lower 143Nd/144Nd. Thus the Nd and Sr isotopic data for lavas from Ethiopia (Hart et al., 1989; Stewart & Rogers, 1996), the Kenya Rift Valley (Norry et al., 1980; Davies & Macdonald, 1987), northern Tanzania (Cohen et al., 1984; Rudnick et al., 1993; Paslick et al., 1995, 1996), the Comores (Späth et al., 1996; Class et al., 1998) and Napak volcano (Simonetti & Bell, 1994) form a fan-like arrangement in Fig. 4 with a common low 87Sr/86Sr, high 143Nd/144Nd component, but different high 87Sr/86Sr, low 143Nd/144Nd endmembers.
|
|
The Northern Chyulu Hills lavas plot slightly below the Northern Hemisphere Reference Line (NHRL) of Hart (1984) in the 207Pb/204Pb vs 206Pb/204Pb diagram, but slightly above it in the 208Pb/204Pb vs 206Pb/204Pb diagram (Fig. 5). The Southern Chyulu Hills basanites, alkali basalts and hawaiites have somewhat lower 206Pb/204Pb and 208Pb/204Pb ratios. Consistent with the Nd–Sr isotope data, most of the Chyulu Hills lavas show similarities in Pb–Pb isotope space with lavas from several other volcanic provinces on the African plate (e.g. Comores, Ahaggar, Kenya Rift Valley, northern Tanzania).
|
The Mzima-type transitional basalts show considerable variability in the Nd–Sr correlation diagram (Fig. 4) and define a trend from the region of the diagram occupied by the rest of the Chyulu Hills lavas towards moderately lower 143Nd/144Nd, but significantly higher 87Sr/86Sr. This trend parallels the Nd–Sr data for lavas from Ethiopia (Hart et al., 1989; Stewart & Rogers, 1996), the Afar–Yemen region (Betton & Civetta, 1984; Vidal et al., 1991; Schilling et al., 1992; Marty et al., 1993; Deniel et al., 1994; Baker et al., 1997) and the Virunga province in the western branch of the East African Rift System (Vollmer & Norry, 1983a, 1983b; De Mulder, 1985; Rogers et al., 1992). Comparable Nd, Sr and Pb isotope characteristics have been ascribed to the influence of both continental crustal (e.g. Yemen: Baker et al., 1996, 1997) as well as metasomatically enriched lithospheric mantle components (e.g. Virunga: Rogers et al., 1992). In terms of hypothetical mantle endmember compositions, the Mzima-type transitional basalts are displaced from the remaining Chyulu Hills lavas towards EM II in the 207Pb/204Pb vs 206Pb/204Pb diagram, but they trend towards EM I with respect to 208Pb/204Pb (Fig. 5).
|
MELTING MODELS |
|---|
|
TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTINGBULK-ROCK GEOCHEMISTRYMELTING MODELSEVIDENCE FOR AN AMPHIBOLE...DISCUSSIONREFERENCES |
|---|
To facilitate the quantitative analysis of the geochemical data presented here, the original primary melt compositions of a selection of fresh, sparsely to moderately porphyritic, primitive Chyulu Hills lavas (mg-number 0·63) were estimated by correcting for the effects of crystal fractionation in a manner similar to that described by other workers (e.g. Furman, 1995
). As olivine is by far the predominant phenocryst phase in the primitive lavas and is suggested to have largely controlled their differentiation, with only relatively small amounts of clinopyroxene and insignificant feldspar fractionation (Späth et al., 2000), it is proposed that their primary magma compositions may be reasonably estimated by correcting for olivine fractionation. Equilibrium olivine was added to bulk-rock major element compositions in 1% increments until an assumed primary melt composition with an mg-number of 0·72 was reached. On average, <10% of olivine fractionation was required to achieve primary compositions (see Table 1). Trace element abundances in the theoretical primary magmas were then calculated using the Rayleigh fractionation equation, the required amount of olivine fractionation, and the mineral–melt partition coefficients listed in Table 3.
|
In investigating the melting processes involved in the formation of the Chyulu Hills lavas, we focus on the REE compositions of a cogenetic subgroup of fractionation-corrected Chyulu Hills lavas (including representatives of the low-Ti nephelinite–basanite and the basanite–alkali basalt groups of the Sultan Hamud–Emali–Simba lavas, the Ngatatema lavas and the Southern Chyulu Hills basanites, alkali basalts and hawaiites) that is considered to represent estimates of primary melt compositions generated by variable degrees of partial melting of a common mantle source. The lavas of this cogenetic suite all have similar Sr, Nd and Pb isotope ratios (Table 2; Figs 4 and 5). Their chondrite-normalized REE patterns (Fig. 6a) show considerable LREE enrichment, very similar slopes and a general correlation between the degree of REE enrichment, silica undersaturation and age (the older, more silica-undersaturated nephelinitic to basanitic lavas being more REE enriched than the younger, less silica-undersaturated, basanitic to alkali basaltic samples). The constrained forward modelling method of Feigenson et al. (1996) may be used to determine which of the various trace element melting equations best describe the REE variations in this suite of lavas. Potential source compositions were calculated for a selected sample using different melting equations with the source parameters listed in Table 4 and partition coefficients presented in Table 3. The validity of a particular melting model may be tested by evaluating whether or not variable degrees of partial melting of one of these potential sources are capable of generating the overall range of REE compositions observed for the full suite of lavas. In Figs 6 and 7 source compositions were calculated using sample AS-101, assuming 3% melting (with the exception of the continuous melting model in Fig. 7b for which 5% melting was assumed) and the relevant melting equation. It should be noted that amphibole is included as a constituent mantle phase—the need for amphibole in the source is developed fully in the following section.
|
|
, source pattern. Also shown is a shaded field indicating the range of REE compositions of the cogenetic suite of fractionation-corrected Chyulu Hills lavas. The mineral–melt partition coefficients listed in Table
|
) of 0·05. (c) Chondrite-normalized REE patterns for equilibrium batch melts of an amphibole-bearing garnet lherzolite source containing 5% garnet. (d) Chondrite-normalized REE patterns for equilibrium batch melts of an amphibole-bearing garnet lherzolite source containing 1% garnet. The source composition is identical to that used in Fig. 6b. Also shown is a shaded field indicating the range of REE compositions of the fractionation-corrected lavas of the high-Ti nephelinite group of the Sultan Hamud–Emali–Simba lavas. •, model melt patterns;
The REE compositions of a series of model melts formed by equilibrium batch melting of a theoretical source (calculated from sample AS-101) are depicted in Fig. 6b, together with a field representing the range of REE compositions observed for the cogenetic suite of Chyulu Hills lavas. It is clear that equilibrium batch melting of a common, amphibole-bearing, spinel lherzolite source is capable of producing a series of model melts with REE patterns that correspond fairly well to the observed data. Over a melting range of 10%, these model melts cover the abundance ranges observed for both the LREE and the HREE. The slight discrepancy between the observed and model data in the middle REE (MREE) range could be removed by adjusting the mineral–melt partition coefficients used (within acceptable limits) or smoothing the source REE pattern. The source composition that provides the best results shows significant LREE enrichment (as is the case in all other constrained forward models investigated). Sources with flatter REE patterns are incapable of producing melts with compositions comparable with those of the lavas of the CHVP. Furthermore, mathematical inversion of the REE data according to the method of Feigenson and co-workers (e.g. Hofmann & Feigenson, 1983; Feigenson et al., 1983, 1996) results exclusively in LREE-enriched sources.
Constrained forward modelling reveals that both fractional melting and continuous or ‘dynamic’ melting with a low residual source porosity (Langmuir et al., 1977; McKenzie, 1985; Albarède, 1995) of garnet-free as well as garnet-bearing lherzolitic sources are incapable of generating a series of melts with REE compositions comparable with those of the cogenetic suite of Chyulu Hills lavas (e.g. Fig. 7a). Only melting models that involve the pooling and accumulation of melt fractions, such as aggregated fractional melting and continuous melting with relatively high residual source porosity (Langmuir et al., 1977; McKenzie, 1985; Albarède, 1995), can produce melts with REE patterns that approach those of the Chyulu Hills lavas, but are little different from simple batch melts (e.g. Fig. 7b).
To evaluate the role of garnet in the source of the Chyulu Hills lavas, constrained forward models were conducted on garnet-bearing source mineralogies. Equilibrium batch melts generated by variable degrees of partial melting of a source containing 5% garnet display non-parallel REE patterns that fan out increasingly towards the LREE (Fig. 7c). Although this source is capable of generating the range of LREE concentrations observed for degrees of melting from 1 to 10%, it cannot produce the observed range in the HREE within this melting interval. Even a very small quantity (1%) of garnet in the source is capable of substantially buffering the HREE entering the melt and results in an HREE abundance range of just over half of the observed range. The REE data for the CHVP lavas are therefore inconsistent with having been produced by melting of a common garnet-bearing source.
Some Chyulu Hills lavas not included in the cogenetic suite considered here (the high-Ti nephelinite group of the Sultan Hamud–Emali–Simba lavas) have distinct REE patterns with slopes in the MREE to HREE range that are notably steeper (chondrite-normalized La/Yb ratios 45·2–60·0) than those of the other lavas from the CHVP (chondrite-normalized La/Yb ratios 7·7–51·7). Constrained forward modelling demonstrates that the REE data for this group of lavas are consistent with an origin by small degrees (1–3%) of equilibrium batch melting of a source that is geochemically equivalent to that giving rise to the remaining Chyulu Hills lavas, but contains a small quantity (1%) of garnet (Fig. 7d). The lavas of the CHVP may thus have a common, geochemically relatively homogeneous source that extends across the garnet–spinel transition in the mantle.
|
EVIDENCE FOR AN AMPHIBOLE-BEARING MANTLE SOURCE |
|---|
|
TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTINGBULK-ROCK GEOCHEMISTRYMELTING MODELSEVIDENCE FOR AN AMPHIBOLE...DISCUSSIONREFERENCES |
|---|
In this section, we present evidence suggesting that the relative K depletion in the lavas of the CHVP is the result of the presence of residual amphibole during melting of the source of these rocks.
Major element considerations
The most plausible explanations for relative K depletion in alkaline lavas involve the fractionation of a K-rich mineral (e.g. amphibole or phlogopite), or partial melting of a source containing a residual K-bearing phase. The absence of amphibole or phlogopite phenocrysts or their relicts in the Chyulu Hills lavas argues against the first option. Furthermore, considerable amounts of amphibole or phlogopite fractionation (>10–20%) would have been required to generate relative K depletions comparable with those observed for the lavas of the CHVP (e.g. Späth et al., 1996). Such large degrees of amphibole or phlogopite fractionation, presumably accompanied by the precipitation of other mafic minerals, are inconsistent with the very primitive nature of many of the lavas investigated in this study.
Both amphibole and phlogopite are important hydrous, K-bearing constituents of the upper mantle, which have been suggested to play a significant role in the genesis of mafic alkaline magmas and in the distribution of elements such as K and Rb in these melts (e.g. Beswick, 1976; Greenough, 1988). Several workers have postulated the presence of residual amphibole or phlogopite in the mantle source regions of alkaline basaltic lavas (e.g. Mertes & Schmincke, 1985; Wilson & Downes, 1991; Hoernle & Schmincke, 1993; Class et al., 1994). Späth et al. (1996) and Class & Goldstein (1997) have independently demonstrated that primitive lavas from the Comores Archipelago that show significant relative K depletions can be modelled as partial melts of an amphibole-bearing mantle source. The presence of amphibole and phlogopite in the mantle is consistent with experimental evidence and is confirmed by numerous reports of the occurrence of these minerals in mantle-derived xenoliths and as mantle xenocrysts hosted in alkaline volcanic rocks (e.g. Best, 1974; Wilshire et al., 1980; Erlank et al., 1987; Gamble & Kyle, 1987; Harte et al., 1987). Henjes-Kunst & Altherr (1992) found phlogopite in a xenolith from the CHVP, and amphibole and phlogopite are present in mantle xenoliths from other areas in Kenya and northern Tanzania, such as Marsabit (Henjes-Kunst & Altherr, 1992), Lashaine (Dawson et al., 1970; Dawson & Smith, 1973), Pello Hill (Dawson & Smith, 1988) and Deeti (Johnson et al., 1997).
In evaluating the role of partial melting of an amphibole- or phlogopite-bearing mantle source, we again focus on the cogenetic subgroup of fractionation-corrected Chyulu Hills lavas discussed in the previous section. All of the rocks in this suite have incompatible element enriched mantle-normalized patterns of broadly similar shape with overall negative slopes between Nb and Yb, but positive slopes between Rb and Th (Fig. 8). The most striking features, however, are the significant and variable relative depletion in K and the curious ‘pinching’ behaviour of Ti, some samples showing slight relative Ti enrichment, others slight depletion (Fig. 8).
|
Partial melting of a four-phase spinel lherzolite mantle source with slight incompatible element enrichment cannot produce any relative depletion in K (Fig. 9; for details of the parameters used in the melting model see caption to Fig. 9). The diagram also shows that although considerable relative Ti depletion can be generated in model melts if Ti partition coefficients at the high end of the published range are used (e.g. Hart & Dunn, 1993; Kelemen et al., 1993), the absolute Ti abundances produced over an interval of partial melting will not remain constant, but decrease with increasing degree of melting. Both K and Ti are expected to behave incompatibly during partial melting of a lherzolitic mantle source and, when plotted against another incompatible element such as La, should display a positive correlation for a cogenetic suite of lavas produced by variable degrees of partial melting. This is not the case for the subgroup of lavas considered here, which show relatively constant concentrations of K and Ti over a considerable range of La abundances (Fig. 10). This behaviour is consistent with partial melting of a source that contains a residual K- and Ti-bearing phase capable of buffering the concentrations of these elements in the melts produced. The most likely candidates for such a residual mantle mineral are amphibole and phlogopite (e.g. Mengel & Green, 1986; Greenough, 1988).
|
|
When attempting to model the behaviour of K and Ti during melting of mantle containing residual amphibole or phlogopite, it is important to bear in mind that both of these minerals can typically contain several wt % of K2O and TiO2. These elements can therefore not simply be considered as trace elements that follow Henry’s Law, and their distribution between amphibole or phlogopite and a melt is not strictly governed by conventional trace element melting equations involving mineral–melt partition coefficients. It is more appropriate to treat K and Ti as stoichiometric components of these phases when modelling partial melting processes. The approach taken here is analogous to that described by Greenough (1988) and Späth et al. (1996). If it is assumed that the entire budget of a particular element is contained in a single mineral, and if the element is a stoichiometric constituent of that phase, then the concentration of the element in the melt will be given by the product of the concentration of the element in the host mineral and the portion of melt composed of the host mineral for as long as the host mineral remains residual in the source. The concentration of the element in the melt is independent of either the degree of partial melting or the abundance of the residual mineral in the source. Assuming that the proportion in which the host mineral enters the melt remains constant throughout the melting interval, the concentration of the element of interest in the melt will also remain constant. Only once the last amount of the mineral has melted will the concentration of the element in the melt change (i.e. decrease) as a result of mass dilution on further melting.
For an amphibole- or phlogopite-bearing peridotitic mantle source, the assumption that the bulk source budget of K is contained in a single phase (i.e. either amphibole or phlogopite) is probably met to a close approximation, as the common mantle minerals olivine, clinopyroxene, orthopyroxene, spinel and garnet contain insignificant quantities of this element. The approach outlined above allows a first-order estimate of whether an amphibole- or a phlogopite-bearing mantle source can be expected to yield melts with K and Ti concentrations that are comparable with those observed in the fractionation-corrected, primary lavas from the CHVP. Both minerals are hydrous, early melting phases that enter into small-degree partial melts in considerable proportions, their melt modes during mantle melting having been estimated at between 0·5 and 0·7 (e.g. Wass & Rogers, 1980; Greenough, 1988). Given this range in melt modes, the ranges of K and Ti abundances in mantle amphibole (K2O 0·0–2·5 wt %; TiO2 0·0–8·82 wt %; Dawson & Smith, 1973; Best, 1974; Zanetti et al., 1996) and phlogopite (K2O 6·19–10·7 wt %; TiO2 0·0–9·13 wt %; Dawson et al., 1970; Dawson & Smith, 1973; Erlank et al., 1987; Harte et al., 1987; Zanetti et al., 1996), the upper and lower limits of K and Ti abundances expected in partial melts produced from amphibole- and phlogopite-bearing mantle sources can be calculated. The results of such calculations are shown graphically in Fig. 10. The upper diagram in this figure illustrates the fact that all of the fractionation-corrected Chyulu Hills lavas considered here have K abundances that fall within the range of concentrations expected for mantle melts generated in the presence of residual, kaersutitic or pargasitic amphibole. Partial melts of mantle sources that contain either the K-rich amphibole K-richterite, or phlogopite as their solitary K-bearing mineral will have K concentrations that are very much higher than those observed in the lavas under consideration. The K abundance data for these rocks are therefore consistent with a derivation by partial melting of a mantle source containing pargasitic or kaersutitic amphibole, but inconsistent with partial melting of a K-richterite- or a phlogopite-bearing source. Even if unrealistically low melt modes of 0·3 or 0·2 are used for phlogopite, the resultant melts are richer in K than the lavas from the CHVP. Expressing the results of this exercise in a slightly different way, if the lavas considered here were produced by partial melting in the presence of a single, residual K-bearing mineral (and given their K concentrations and assuming melt modes between 0·5 and 0·7), then this phase contained between 0·91 and 2·1 wt % of K2O—abundances well within the range reported for mantle amphibole, but much lower than those expected for K-richterite or phlogopite.
Although the Ti concentrations of the fractionation-corrected lavas are also consistent with partial melting in the presence of residual amphibole, they cannot be used to distinguish between partial melting of an amphibole-bearing source and a phlogopite-bearing source, as such sources are expected to generate melts with similar, overlapping ranges in Ti concentrations (Fig. 10, lower diagram). The situation in the case of Ti is furthermore complicated by the fact that a number of potential mantle Fe–Ti oxide minerals contain considerable abundances of this element (Haggerty et al., 1983; Erlank et al., 1987; Harte et al., 1987; Haggerty, 1991) and even clinopyroxene and garnet may have small, but non-trivial TiO2 contents (e.g. Harte et al., 1987). Even though the assumption of a single Ti-bearing phase is thus possibly compromised, the conclusion that the Ti contents of these lavas are consistent with partial melting of an amphibole-bearing source remains valid nevertheless.
Trace element considerations
Elements other than K and Ti may be used to establish whether the lavas of the CHVP were generated by partial melting of a source containing amphibole or phlogopite. Nb, Ba and Rb occur as trace components in both amphibole and phlogopite, and their behaviour during partial melting may be quantified using conventional trace element melting equations and mineral–melt partition coefficients. During partial melting, Nb is expected to behave incompatibly with regard to both amphibole and phlogopite (DNbphlog = 0·14; DNbamph = 0·2; McKenzie & O’Nions, 1991; Adam et al., 1993; Dalpé & Baker, 1994). In contrast, Ba and Rb are moderately incompatible with respect to amphibole (DBaamph = 0·5; DRbamph = 0·3; le Roex et al., 1990; Adam et al., 1993), but compatible with respect to phlogopite (DBaphlog = 2·9; DRbphlog = 5·8; Adam et al., 1993). Ba and Rb should therefore behave differently during partial melting of amphibole-bearing mantle sources and phlogopite-bearing mantle sources. Figure 11 shows the theoretical variation of Nb, Rb and Ba as a consequence of variable degrees of equilibrium batch melting of hypothetical amphibole-bearing and phlogopite-bearing mantle sources up to and beyond the exhaustion of these phases (for details of the melting models, see caption to Fig. 11). In the diagram of Nb vs Ba, the Chyulu Hills lavas follow the amphibole lherzolite melting trend fairly closely, the more silica-undersaturated, more incompatible element enriched lavas generally being produced by smaller degrees of partial melting than their less silica-undersaturated, less incompatible element enriched counterparts. The positive correlation of the data points in this diagram would also be broadly consistent with partial melting of an amphibole-free spinel lherzolite source, but importantly, it is entirely inconsistent with partial melting of a phlogopite-bearing source, for which the Ba concentration of the melt would be largely buffered and more or less constant until the last phlogopite had melted. In the Nb vs Rb diagram, the Southern Chyulu Hills basanites, alkali basalts and hawaiites are consistent with melting of an amphibole-bearing source, whereas most of the fractionation-corrected Northern Chyulu Hills lavas are displaced above the amphibole lherzolite melting trend.
|
Figure 12 illustrates that the K and Ti anomalies of the Chyulu Hills lavas can be generated by melting of an amphibole-bearing source using reasonable geochemical and mineralogical mantle source compositions and established mineral–melt partition coefficients. The upper diagram in Fig. 12 shows model incompatible element patterns generated by non-modal equilibrium batch melting of a hypothetical, incompatible element enriched, amphibole-bearing spinel lherzolite mantle source (both K and Ti were modelled as stoichiometric constituent of amphibole; for details of the melting model, see caption to Fig. 12). For comparison, the lower diagram in Fig. 12 shows a summarized version of Fig. 8, displaying the range of patterns observed for the Chyulu Hills lavas and illustrating the general correlation between the degree of relative K depletion, the degree of silica undersaturation, the degree of incompatible element enrichment and age.
|
). The degrees of partial melting (F) shown range from 1 to 20%. The incompatible element enriched source used is shown at the bottom of the diagram and is identical to the one used in Fig. While amphibole remains residual in the source (i.e. between 1 and 9% of partial melting), the calculated mantle-normalized incompatible element patterns mimic those of the lavas very closely. Furthermore, the peculiar ‘pinching’ behaviour at Ti is well reproduced. The smallest degrees of partial melting produce the incompatible element patterns with the greatest incompatible element enrichments and the most severe relative K depletions. With increasing degrees of partial melting, the degree of incompatible element enrichment and the magnitude of the relative K depletion decrease. As soon as all of the amphibole in the source has been exhausted, the relative K depletions in the melts disappear and the formerly constant Ti concentrations start to decrease.
Although the amount of amphibole in the model source used here (i.e. 5%) is considered reasonable and reproduces the observed incompatible element patterns of the Chyulu Hills lavas fairly well, the true amphibole content of the source of these rocks is relatively unconstrained by the model. As noted above, the amount of amphibole in the source does not affect the absolute concentration of K in the magmas produced, but only determines the melting interval over which these magmas will have relative K depletions. Similarly, the relative degrees of partial melting in the model shown in Fig. 12 are consistent with the range of rock types it aims to explain (nephelinites, basanites, alkali basalts), although the absolute degrees of partial melting involved in their formation are only poorly constrained by this model. Overall, however, Fig. 12 illustrates that variable degrees of partial melting of a geologically plausible mantle source (both in terms of mineralogy and geochemistry) containing a reasonable amount of typical mantle amphibole can explain the behaviour of K and Ti observed in the lavas of the CHVP.
Using mantle mineral compositions from the literature and the melt modes used in the trace element models, the major element compositions of magmas produced from an appropriate amphibole-bearing spinel lherzolite source can be estimated by simple mass balance calculations. Such calculations result in model magmas with more or less basanitic to alkali basaltic compositions (typical approximate composition: SiO2, 45%; Al2O3, 9%; FeO, 11%; MgO, 15%; CaO, 11%; Na2O, 3%) that are broadly consistent with the observed compositions of Chyulu Hills lavas. Although these kinds of models give a good first approximation of major element compositions, they are poorly constrained, as mineral compositions in mantle-derived xenoliths (including those of amphibole, which has the greatest influence on resultant bulk magma compositions) show considerable compositional variability. Furthermore, the reaction coefficients of mantle minerals during melting of hydrous peridotite are not well constrained [recent work by Kinzler et al. (1997) is related to anhydrous melting at shallow depth], and although the melting proportions of minerals such as olivine and orthopyroxene are not particularly important when modelling incompatible trace element behaviour, their melting proportions are crucial for major element models, as olivine and orthopyroxene are relatively rich in important major elements such as SiO2, FeO and MgO. It is interesting to note that high-pressure melting experiments on a model metasomatized peridotite led Mengel & Green (1986) to suggest that the breakdown of amphibole is the key process in the production of basanitic magmas.
|
DISCUSSION |
|---|
|
TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTINGBULK-ROCK GEOCHEMISTRYMELTING MODELSEVIDENCE FOR AN AMPHIBOLE...DISCUSSIONREFERENCES |
|---|
Experimental estimates of the upper stability limit of pargasitic or kaersutitic amphibole range from 21 kbar to >30 kbar (e.g. Olafsson & Eggler, 1983
; Mengel & Green, 1986), representing depths of 70 km to somewhat more than 95 km. Under water-saturated conditions, amphibole has been demonstrated to coexist stably with melt in a small temperature interval above the peridotite solidus at pressures of up to 25 kbar (Mengel & Green, 1986) and the presence of F may increase further its stability above the solidus (Foley, 1991). Considering the estimated lithospheric thickness of 110 km beneath the CHVP (Henjes-Kunst & Altherr, 1992), the inferred presence of residual pargasitic or kaersutitic amphibole during the production of the Chyulu Hills lavas implies source regions located towards the base of the subcontinental lithospheric mantle (SCLM), rather than in the asthenosphere. Furthermore, a comparison of the stability field of amphibole with typical asthenosphere and plume adiabats shows amphibole to be thermally unstable in both the convecting, asthenospheric upper mantle and in thermal mantle plumes, but stable in old SCLM (Class & Goldstein, 1997). A lithospheric source is also supported by the presence of lithospheric mantle peridotite inclusions in some of the oldest Northern Chyulu Hills lavas (Henjes-Kunst & Altherr, 1992), and is in accordance with evidence for a metasomatized lithosphere beneath northern Tanzania (Rudnick et al., 1993; Paslick et al., 1995; Johnson et al., 1997) and other parts of East Africa (e.g. Vollmer & Norry, 1983a, 1983b; Cohen et al., 1984; Rogers et al., 1992; Furman, 1995). The lavas erupted in the CHVP are therefore interpreted to have their origin in the SCLM, and not by direct partial melting of a rising mantle plume.
The Nd and Sr isotope characteristics of the Chyulu Hills lavas require their derivation from a time-integrated incompatible element depleted source region, whereas their incompatible element enriched compositions (taken in conjunction with the isotope data) and the inferred presence of residual amphibole in their source regions imply derivation from a recently enriched source. This enrichment process is most likely to have involved modal metasomatism, during which the infiltration of incompatible element- and volatile-rich metasomatic melts or fluids led to a pervasive geochemical modification of the host material and the precipitation of metasomatic minerals, in this case amphibole (e.g. Best, 1974; Erlank et al., 1987; Fabriès et al., 1989; Zanetti et al., 1996). Although the age of this enrichment event is poorly constrained, it must have occurred recently enough not to have significantly affected the isotope composition of the lavas. A source evolution by ancient melt extraction followed by recent re-enrichment such as that suggested here has been proposed for a number of volcanic provinces to explain the commonly observed decoupling of isotopic and trace element behaviour in young, mafic, alkaline lavas (e.g. Clague & Frey, 1982). We suggest therefore that the CHVP is underlain by a widespread region in the SCLM that experienced enrichment by infiltration and percolation of metasomatic fluids and/or melts, resulting in pervasive modal metasomatism and the introduction of small amounts (on the order of 5%) of pargasitic or kaersutitic amphibole.
Although direct melting of a mantle plume is excluded for the formation of the Chyulu Hills lavas, hot plume material may have provided the fluids that metasomatized the lithosphere, as well as the heat source required to initiate partial melting in the SCLM. The relatively restricted range in isotopic compositions among the majority of the lavas of the CHVP (excluding the Mzima-type transitional basalts) suggests their generation from a common, albeit slightly heterogeneous source. Old SCLM is, however, commonly assumed to have radiogenic isotope characteristics trending towards the EM endmember components with elevated 87Sr/86Sr, but low 143Nd/144Nd (e.g. Zindler & Hart, 1986; Hawkesworth et al., 1990), unlike those of the CHVP. The isotopic composition of the CHVP source region is much more typical of plume-derived ocean island lavas. The isotope data for the Chyulu Hills lavas with the exception of the Mzima-type transitional basalts may thus best be explained as reflecting the composition of the fluids and/or melts responsible for the recent geochemical enrichment and modal metasomatism of the underlying SCLM. The isotopic composition of these metasomatic fluids and/or melts is interpreted to largely reflect the composition of the mantle plume from which they were derived. The same plume also provided the thermal energy required to conductively heat the lithosphere and induce partial melting.
There is clear evidence for the presence of a partially molten, low seismic velocity zone beneath the CHVP (e.g. Ritter & Kaspar, 1997). The relatively small size of this anomaly and the absence of a direct connection between this region and the asthenospheric mantle, however, argue strongly against this low-velocity zone reflecting the depth to which an underlying mantle plume has risen (Ritter et al., 1995; Ritter & Kaspar, 1997). The restriction of the low-velocity zone beneath the CHVP to a maximum depth of 70 km lends further support to the contention of strictly lithospheric sources for the lavas erupted in this province. Current geophysical models place a large mantle plume beneath the rift valley or the Tanzanian Craton (e.g. Karson & Curtis, 1989; Mechie et al., 1997; Ritter & Kaspar, 1997), and large-scale tectonic or pre-existing structural control, possibly along the lines of the models proposed by Bosworth (1987), Ellis & King (1991) or Smith (1994), may have caused the deflection and channelling of comparatively minor quantities of hot, rising plume material towards the future location of the CHVP 150 km to the east. Fluids and/or melts originating from this plume material caused modal metasomatism of the SCLM beneath the CHVP. Partial melting within the SCLM was subsequently initiated through conductive heating to temperatures above the volatile-depressed lithospheric solidus (e.g. McKenzie, 1989; Gallagher & Hawkesworth, 1992). The geochemistry of the magmas generated in this fashion is dominated by the amphibole-rich, early-melting component precipitated by the metasomatizing fluids and/or melts. These lavas have inherited not only their incompatible element enriched, K-depleted geochemical signatures from this component, but also their Nd, Sr and Pb isotope compositions. Isotopic compositions identical to those of the majority of the Chyulu Hills lavas appear to represent a common component in the eruptive products of a number of volcanic provinces on and around the African plate (Fig. 4). By analogy with the CHVP, several other Cenozoic volcanic provinces to the east of and approximately equidistant from the Kenya Rift Valley (e.g. Marsabit, Huri Hills, Nyambeni Hills, Mt Kenya, Kilimanjaro), and possibly even the older, off-rift volcanic centres of eastern Uganda and western Kenya (e.g. Mt Elgon, Kadam, Napak) may have had similar origins.
The Nd, Sr and Pb isotope characteristics of the Mzima-type transitional basalts are most conveniently explained by binary mixing between a widespread source component located in the East African SCLM, represented by the Northern Chyulu Hills lavas and suggested above to have been generated by plume-derived metasomatic fluids and/or melts, and a second component, characterized by comparatively higher 87Sr/86Sr and 207Pb/204Pb, but lower 143Nd/144Nd, 208Pb/204Pb and 206Pb/204Pb ratios, that shows similarities with both EM I and EM II. The fact that two of the Southern Chyulu Hills basanites, alkali basalts and hawaiites have Pb isotope compositions intermediate between the Northern Chyulu Hills lavas and the Mzima-type transitional basalts suggests that these lavas may also have had contributions from this second source component. This component could be located within the SCLM or within the continental crust. The task of distinguishing between these two possibilities is made difficult by the non-existence of isotopic data for crustal material from beneath the Chyulu Hills and the very considerable isotopic variability of the Afro-Arabian continental crust in general (e.g. Baker et al., 1996, 1997; Möller et al., 1998). There is, however, no unequivocal evidence for crustal contamination in any of the primitive lavas of the CHVP. Furthermore, the major and trace element geochemistry of the Mzima-type transitional basalts is inconsistent with combined assimilation of continental crustal material and fractional crystallization, but may be explained by variable degrees of partial melting of a common, isotopically heterogeneous source located within the SCLM. We propose that the comparatively larger degrees of partial melting required for the formation of the Mzima-type transitional basalts compared with all other Chyulu Hills lavas resulted in the additional incorporation of the pre-metasomatized SCLM, considered to have EM-type isotopic and trace element characteristics, leading to elevated 87Sr/86Sr, but lower 143Nd/144Nd. For the CHVP as a whole, the preferred petrogenetic model involves a common lithospheric mantle source region with an EM-type radiogenic isotope and trace element composition that has experienced recent and pervasive modal metasomatism by mantle plume-derived fluids and/or melts that introduced an early-melting component with comparatively lower 87Sr/86Sr and higher 143Nd/144Nd ratios characteristic of the mantle plume. Small-degree melts incorporated predominantly the early-melting component and have plume-like isotope compositions, whereas larger-degree melts (the Mzima-type transitional basalts) assimilated significant quantities of EM-type SCLM to drive their isotope compositions to notably higher 87Sr/86Sr and lower 143Nd/144Nd ratios.
The lavas of the CHVP were thus produced by partial melting of lithospheric, incompatible element enriched, amphibole-bearing source materials, during which there was a general decrease in the depth of melting, a concomitant increase in the degree of melting and a migration of volcanic activity from the northwest to the southeast with time (Späth et al., 2000). The observed increase in the degree of partial melting with time is consistent with quantitative results for Turner et al.’s (1996) model of lithospheric melting through conductive heating by a mantle plume.
The petrogenetic model for the CHVP presented has implications for the interaction between sub-lithospheric diapirs and the SCLM, melt generation in the SCLM and possibly even the initiation of continental rifting involving actively upwelling mantle plumes in general (Fig. 13). Old, geochemically depleted, lithosphere is too dry to yield significant quantities of basaltic melts even when conductively heated by a rising, anomalously hot mantle plume (e.g. Menzies, 1992). Fluids and volatile-rich small-degree melts derived from and advancing ahead of such an upwelling mantle plume may, however, infiltrate and metasomatize the SCLM and introduce a ubiquitous, solidus-depressing, early-melting, amphibole-rich component with geochemical characteristics that reflect those of the plume material itself. Upon continued ascent, the plume conductively heats and partially melts the metasomatized SCLM by a process of dehydration melting (e.g. McKenzie, 1989; Gallagher & Hawkesworth, 1992; Turner et al., 1996). The geochemistry of the first, small-degree melts generated is dominated by the plume-derived metasomatic component and reflects the composition of the plume. Subsequent larger degrees of melting lead to significant incorporation of the pre-metasomatic lithospheric ‘wallrock’ and to magmas with compositions representing a mixture between those of the plume-derived metasomatic component and old SCLM.
|
Melt generation in the metasomatized SCLM above a rising mantle plume may be expected to lead to considerable structural weakening of the lithosphere that may also facilitate the initiation of its thermal erosion and thinning, allowing the plume to ascend to progressively shallower levels, causing updoming and eventually leading to continental rifting and break-up. Upon the initiation of lithospheric thinning and rifting (particularly as the extension factor, ß, exceeds a value of 1·3 for a 100 km thick lithosphere; McKenzie & Bickle, 1988), the rising mantle plume itself will melt to increasingly larger degrees by decompression melting and magmas derived from this sub-lithospheric source may be expected to start to predominate over melts generated within the SCLM (e.g. Gallagher & Hawkesworth, 1992, 1994; Hawkesworth et al., 1992; Turner et al., 1996). Lithospheric as well as sub-lithospheric melts may thus dominate magmatic activity in extensional continental settings at various stages of development. Although pre-existing structural features, the location and size of ascending mantle plumes, as well as external lithospheric stresses are undoubtedly the controlling factors in the initiation and geometry of continental rifting and break-up, it is proposed that pervasive metasomatism by plume-derived fluids and/or melts may play an important role in pre-conditioning the initially dry, refractory SCLM not only for subsequent partial melting, but also for lithospheric weakening, aiding in lithospheric thinning, thermal erosion and rifting [compare the models of McKenzie (1989) and Saunders et al. (1992)].
|
ACKNOWLEDGEMENTS |
|---|
Reviews by R. M. Ellam, R. Macdonald and A. D. Saunders contributed significantly to improving the quality of this paper. This work was made possible through generous logistical support from the University of Nairobi (in particular, Dr Simon Mangarere Onywere and Mr H. Nyali), the Kenya Wildlife Service and the Kenya Academy of Sciences. Financial support from the University of Cape Town and the Foundation for Research Development is gratefully acknowledged by A.S. and A.l.R.
|
FOOTNOTES |
|---|
*Corresponding author. Tel: +27-21-6502911. Fax: +27-21-6503783. E-mail: aspath{at}geology.uct.ac.za
Extended data set can be found at: http://www.petrology.oupjournals.org
|
REFERENCES |
|---|
|
TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTINGBULK-ROCK GEOCHEMISTRYMELTING MODELSEVIDENCE FOR AN AMPHIBOLE...DISCUSSIONREFERENCES |
|---|
Adam, J., Green, T. H. & Sie, S. H. (1993). Proton microprobe determined partitioning of Rb, Sr, Ba, Y, Zr, Nb and Ta between experimentally produced amphiboles and silicate melts with variable F content. Chemical Geology 109, 29–49.[Web of Science]
Albarède, F. (1995). Introduction to Geochemical Modelling. Cambridge: Cambridge University Press, pp. 477–525.
Allègre, C. J., Dupré, B., Lambret, B. & Richard, P. (1981). The subcontinental versus suboceanic debate, I. Lead–neodymium–strontium isotopes in primary alkali basalts from a shield area: the Ahaggar volcanic suite. Earth and Planetary Science Letters 52, 85–92.[Web of Science]
Altherr, R., Henjes-Kunst, F. & Baumann, A. (1990). Asthenosphere versus lithosphere as possible sources for basaltic magmas erupted during formation of the Red Sea: constraints from Sr, Pb and Nd isotopes. Earth and Planetary Science Letters 96, 269–286.
Arndt, N. T. & Christensen, U. (1992). The role of lithospheric mantle in continental flood volcanism: thermal and geochemical constraints. Journal of Geophysical Research 97, 10967–10981.
Baker, B. H. (1987). Outline of the petrology of the Kenya rift alkaline province. In: Fitton, J. G. & Upton, B. G. J. (eds) Alkaline Igneous Rocks. Geological Society, London, Special Publications 30, 293–311.
Baker, B. H., Williams, L. A. J., Miller, J. A. & Fitch, F. J. (1971). Sequence and geochronology of the Kenya rift volcanics. Tectonophysics 11, 191–215.
Baker, J. A., Thirlwall, M. F. & Menzies, M. A. (1996). Sr–Nd–Pb isotopic and trace element evidence for crustal contamination of plume-derived flood basalts: Oligocene flood volcanism in western Yemen. Geochimica et Cosmochimica Acta 60, 2559–2581.
Baker, J. A., Menzies, M. A., Thirlwall, M. F. & Macpherson, C. G. (1997). Petrogenesis of Quaternary intraplate volcanism, Sana’a, Yemen: implications for plume–lithosphere interaction and polybaric melt hybridization. Journal of Petrology 38, 1359–1390.
Bell, K. & Peterson, T. (1991). Nd and Sr isotope systematics of Shombole volcano, East Africa, and the links between nephelinites, phonolites, and carbonatites. Geology 19, 582–585.
Best, M. G. (1974). Mantle-derived amphibole within inclusions in alkalic–basaltic lavas. Journal of Geophysical Research 79, 2107–2113.
Beswick, A. E. (1976). K and Rb relations in basalts and other mantle derived materials. Is phlogopite the key? Geochimica et Cosmochimica Acta 40, 1167–1183.[Web of Science]
Betton, P. J. & Civetta, L. (1984). Strontium and neodymium isotopic evidence for the heterogeneous nature and development of the mantle beneath Afar (Ethiopia). Earth and Planetary Science Letters 71, 59–70.[Web of Science]
Bosworth, W. (1987). Off-axis volcanism in the Gregory Rift, East Africa: implications for models of continental rifting. Geology 15, 397–400.
Bosworth, W. (1989). Basin and Range style tectonics in East Africa. Journal of African Earth Sciences 8, 191–201.
Burke, K. (1996). The Africa plate. South African Journal of Geology 99, 341–409.[Web of Science]
Chaffey, D. J., Cliff, R. A. & Wilson, B. M. (1989). Characterisation of the St Helena magma source. In: Saunders, A. D. & Norry, M. J. (eds) Magmatism in the Ocean Basins. Geological Society, London, Special Publications 42, 257–276.
Chazot, G., Menzies, M. A. & Harte, B. (1996). Determination of partition coefficients between apatite, clinopyroxene, amphibole, and melt in natural spinel lherzolites from Yemen: implications for wet melting of the lithospheric mantle. Geochimica et Cosmochimica Acta 60, 423–437.[Web of Science]
Clague, D. A. & Frey, F. A. (1982). Petrology and trace element geochemistry of the Honolulu Volcanics, Oahu: implications for the oceanic mantle below Hawaii. Journal of Petrology 23, 447–504.
Class, C. & Goldstein, S. L. (1997). Plume–lithosphere interactions in the ocean basins: constraints from the source mineralogy. Earth and Planetary Science Letters 150, 245–260.[Web of Science]
Class, C., Altherr, R., Volker, F., Eberz, G. & McCulloch, M. T. (1994). Geochemistry of Pliocene to Quaternary alkali basalts from the Huri Hills, northern Kenya. Chemical Geology 113, 1–22.
Class, C., Goldstein, S. L., Altherr, R. & Bachèlery, P. (1998). The process of plume–lithosphere interactions in the ocean basins—the case of Grande Comore. Journal of Petrology 39, 881–903.
Cohen, R. S., O’Nions, R. K. & Dawson, J. B. (1984). Isotope geochemistry of xenoliths from East Africa: implications for development of mantle reservoirs and their interaction. Earth and Planetary Science Letters 68, 209–220.
Consolmagno, G. J. & Drake, M. J. (1976). Equivalence of equations describing trace element distribution during equilibrium partial melting. Geochimica et Cosmochimica Acta 40, 1421–1422.
Dalpé, C. & Baker, D. R. (1994). Partition coefficients for rare-earth elements between calcic amphibole and Ti-rich basanitic glass at 1·5 GPa, 1100°C. Mineralogical Magazine 58A, 207–208.
Davies, G. R. & Macdonald, R. (1987). Crustal influences in the petrogenesis of the Naivasha basalt–comendite complex: combined trace element and Sr–Nd–Pb isotope constraints. Journal of Petrology 28, 1009–1031.
Dawson, J. B. & Smith, J. V. (1973). Alkalic pyroxenite xenoliths from the Lashaine volcano, northern Tanzania. Journal of Petrology 14, 113–131.
Dawson, J. B. & Smith, J. V. (1988). 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 100, 510–527.
Dawson, J. B., Powell, D. G. & Reid, A. M. (1970). Ultrabasic xenoliths and lava from the Lashaine volcano, northern Tanzania. Journal of Petrology 11, 519–548.
De Mulder, M. (1985). The Karisimbi volcano (Virunga). Koninklijk Museum voor Midden-Afrika–Tervuren, Belgie, Annalen 8, 101 pp.
Deniel, C., Vidal, P., Coulon, C., Vellutini, P.-J. & Piguet, P. (1994). Temporal evolution of mantle sources during continental rifting: the volcanism of Djibouti (Afar). Journal of Geophysical Research 99, 2853–2869.
Duncan, A. R., Erlank, A. J. & Betton, P. J. (1984). Analytical techniques and database descriptions. Geological Society of South Africa, Special Publication 13, 389–395.
Ellis, M. & King, G. (1991). Structural control of flank volcanism in continental rifts. Science 254, 839–842.
Erlank, A. J., Waters, F. G., Hawkesworth, C. J., Haggerty, S. E., Allsopp, H. L., Rickard, R. S. & Menzies, M. (1987). Evidence for mantle metasomatism in peridotitic nodules from the Kimberley pipes, South Africa. In: Menzies, M. A. & Hawkesworth, C. J. (eds) Mantle Metasomatism. London: Academic Press, pp. 221–311.
Fabriès, J., Bodinier, J.-L., Dupuy, C., Lorand, J.-P. & Benkerrou, C. (1989). Evidence for modal metasomatism in the orogenic spinel lherzolite body from Caussou (northeastern Pyrenees, France). Journal of Petrology 30, 199–228.
Feigenson, M. D., Hofmann, A. W. & Spera, F. J. (1983). Case studies on the origin of basalt. II. The transition from tholeiitic to alkalic volcanism on Kohala volcano, Hawaii. Contributions to Mineralogy and Petrology 84, 390–405.[Web of Science]
Feigenson, M. D., Patino, L. C. & Carr, M. J. (1996). Constraints on partial melting imposed by rare earth element variations in Mauna Kea basalts. Journal of Geophysical Research 101, 11815–11829.
Foley, S. (1991). High-pressure stability of the fluor- and hydroxy-endmembers of pargasite and K-richterite. Geochimica et Cosmochimica Acta 55, 2689–2694.[Web of Science]
Freerk-Parpatt, M. (1992). Geochemical characteristics of basalts from the Laisamis–Merille area (northern Kenya). Sonderforschungsbereich 108. Spannung und Spannungsumwandlung in der Lithosphäre. Berichtsband für die Jahre 1990–1992. Teil A. Universität Karlsruhe, pp. 139–159.
Frey, F. A., Green, D. H. & Roy, S. D. (1978). Integrated models of basalt petrogenesis: a study of quartz tholeiites to olivine melilitites from south eastern Australia utilizing geochemical and experimental petrological data. Journal of Petrology 19, 463–513.
Furman, T. (1995). Melting of metasomatized subcontinental lithosphere: undersaturated mafic lavas from Rungwe, Tanzania. Contributions to Mineralogy and Petrology 122, 97–115.
Gallagher, K. & Hawkesworth, C. (1992). Dehydration melting and the generation of continental flood basalts. Nature 358, 57–59.
Gallagher, K. & Hawkesworth, C. (1994). Mantle plumes, continental magmatism and asymmetry in the South Atlantic. Earth and Planetary Science Letters 123, 105–117.
Gamble, J. A. & Kyle, P. R. (1987). The origins of glass and amphibole in spinel–wehrlite xenoliths from Foster Crater, McMurdo Volcanic Group, Antarctica. Journal of Petrology 28, 755–779.
Greenough, J. D. (1988). Minor phases in the Earth’s mantle: evidence from trace- and minor-element patterns in primitive alkaline magmas. Chemical Geology 69, 177–192.
Haggerty, S. E. (1991). Oxide mineralogy of the upper mantle. In: Lindsley, D. H. (ed.) Oxide Minerals: Petrologic and Magnetic Significance. Mineralogical Society of America, Reviews in Mineralogy 25, 355–416.
Haggerty, S. E., Smyth, J. R., Erlank, A. J., Rickard, R. S. & Danchin, R. V. (1983). Lindsleyite (Ba) and mathiasite (K): two new chromium-titanates in the crichtonite series from the upper mantle. American Mineralogist 68, 494–505.[Abstract]
Halliday, A. N., Lee, D.-C., Tommasini, S., Davies, G. R., Paslick, C. R., Fitton, J. G. & James, D. E. (1995). Incompatible trace elements in OIB and MORB and source enrichment in the sub-oceanic mantle. Earth and Planetary Science Letters 133, 379–395.
Hanson, G. N. (1980). Rare earth elements in petrogenetic studies of igneous rocks. Annual Review of Earth and Planetary Sciences 8, 371–406.[Web of Science]
Hart, S. R. (1984). A large-scale isotope anomaly in the southern hemisphere mantle. Nature 309, 753–757.
Hart, S. R. & Brooks, C. (1977). The geochemistry and evolution of early Precambrian mantle. Contributions to Mineralogy and Petrology 61, 109–128.
Hart, S. R. & Dunn, T. (1993). Experimental cpx/melt partitioning of 24 trace elements. Contributions to Mineralogy and Petrology 113, 1–8.[Web of Science]
Hart, W. K., WoldeGabriel, G., Walter, R. C. & Mertzman, S. A. (1989). Basaltic volcanism in Ethiopia: constraints on continental rifting and mantle interactions. Journal of Geophysical Research 94, 7731–7748.
Harte, B., Winterburn, P. A. & Gurney, J. J. (1987). Metasomatism and enrichment phenomena in garnet peridotite facies mantle xenoliths from the Matsoku kimberlite pipe, Lesotho. In: Menzies, M. A. & Hawkesworth, C. J. (eds) Mantle Metasomatism. London: Academic Press, pp. 145–220.
Haug, G. H. & Strecker, M. R. (1995). Volcano-tectonic evolution of the Chyulu Hills and implications for the regional stress field in Kenya. Geology 23, 165–168.
Hawkesworth, C. J., Kempton, P. D., Rogers, N. W., Ellam, R. M. & van Calsteren, P. W. (1990). Continental mantle lithosphere, and shallow level enrichment processes in the Earth’s mantle. Earth and Planetary Science Letters 96, 256–268.[Web of Science]
Hawkesworth, C. J., Gallagher, K., Kelley, S., Mantovani, M., Peate, D. W., Regelous, M. & Rogers, N. W. (1992). Paraná magmatism and the opening of the South Atlantic. In: Storey, B. C., Alabaster, T. & Pankhurst, R. J. (eds) Magmatism and the Causes of Continental Break-up. Geological Society, London, Special Publications 68, 221–240.
Henjes-Kunst, F. & Altherr, R. (1992). Metamorphic petrology of xenoliths from Kenya and northern Tanzania and implications for geotherms and lithospheric structures. Journal of Petrology 33, 1125–1156.
Hertogen, J. & Gijbels, R. (1976). Calculation of trace element fractionation during partial melting. Geochimica et Cosmochimica Acta 40, 313–322.
Hoernle, K. & Schmincke, H.-U. (1993). The petrology of the tholeiites through melilite nephelinites on Gran Canaria, Canary Islands: crystal fractionation, accumulation, and depths of melting. Journal of Petrology 34, 573–597.
Hofmann, A. W. & Feigenson, M. D. (1983). Case studies of basalt I. Theory and reassessment of Grenada basalts. Contributions to Mineralogy and Petrology 84, 382–389.
Johnson, K. T. M., Dick, H. J. B. & Shimizu, N. (1990). Melting in the oceanic upper mantle: an ion microprobe study of diopsides in abyssal peridotites. Journal of Geophysical Research 95, 2661–2678.
Johnson, L. H., Jones, A. P., Church, A. A. & Taylor, W. R. (1997). Ultramafic xenoliths and megacrysts from a melilitite tuff cone, Deeti, northern Tanzania. Journal of African Earth Sciences 25, 29–42.[Web of Science]
Karson, J. A. & Curtis, P. C. (1989). Tectonic and magmatic processes in the eastern branch of the East African Rift and implications for magmatically active continental rifts. Journal of African Earth Sciences 8, 431–453.
Kelemen, P. B., Shimizu, N. & Dunn, T. (1993). Relative depletion of niobium in some arc magmas and the continental crust: partitioning of K, Nb, La and Ce during melt/rock reaction in the upper mantle. Earth and Planetary Science Letters 120, 111–134.[Web of Science]
Kinzler, R. J. (1997). Melting of mantle peridotite at pressures approaching the spinel to garnet transition: application to mid-ocean ridge basalt petrogenesis. Journal of Geophysical Research 102, 853–874.
Langmuir, H., Bender, J. F., Bence, A. E., Hanson, G. N. & Taylor, S. R. (1977). Petrogenesis of basalts from the FAMOUS area: Mid-Atlantic Ridge. Earth and Planetary Science Letters 36, 133–156.[Web of Science]
Le Bas, M. J., Le Maitre, R. W., Streckeisen, A. & Zanettin, B. (1986). A chemical classification of volcanic rocks based on the total alkali–silica diagram. Journal of Petrology 27, 745–750.
Lemarchand, F., Villemant, B. & Calas, G. (1987). Trace element distribution coefficients in alkaline series. Geochimica et Cosmochimica Acta 51, 1071–1081.
le Roex, A. P. (1980). Geochemistry and mineralogy of selected Atlantic Ocean basalts. Ph.D. thesis, University of Cape Town.
le Roex, A. P. & Dick, H. J. B. (1981). Petrography and geochemistry of basaltic rocks from the Conrad fracture zone on the America–Antarctica Ridge. Earth and Planetary Science Letters 54, 117–138.[Web of Science]
le Roex, A. P. & Watkins, R. T. (1990). Analysis of rare-earth elements in geological samples by gradient ion chromatography: an alternative to ICP and INAA. Chemical Geology 88, 151–162.
le Roex, A. P., Erlank, A. J. & Needham, H. D. (1981). Geochemical and mineralogical evidence for the occurrence of at least three distinct magma types in the ‘Famous’ region. Contributions to Mineralogy and Petrology 77, 24–37.
le Roex, A. P., Cliff, R. A. & Adair, B. J. I. (1990). Tristan da Cunha, South Atlantic: geochemistry and petrogenesis of a basanite–phonolite lava series. Journal of Petrology 31, 779–812.
Macdonald, G. A. & Katsura, T. (1964). Chemical composition of Hawaiian lavas. Journal Petrology 5, 82–133.
Mahoney, J. J. (1987). An isotopic survey of Pacific oceanic plateaus: implications for their nature and origin. In: Keating, B. H., Fryer, P., Batiza, R. & Boehlert, G. W. (eds) Seamounts, Islands, and Atolls. Geophysical Monograph Series, American Geophysical Union 43, 207–220.
Manhes, G., Minster, J. F. & Allègre, C. J. (1978). Comparative uranium–thorium–lead and rubidium–strontium of St. Séverin amphoterite: consequences for early solar system chronology. Earth and Planetary Science Letters 39, 14–24.
Marty, B., Appora, I., Barrat, J.-A. A., Deniel, C., Vellutini, P. & Vidal, P. (1993). He, Ar, Sr, Nd and Pb isotopes in volcanic rocks from Afar: evidence for a primitive mantle component and constraints on magmatic sources. Geochemical Journal 27, 219–228.
McKenzie, D. (1985). The extraction of magma from the crust and mantle. Earth and Planetary Science Letters 74, 81–91.
McKenzie, D. (1989). Some remarks on the movement of small melt fractions in the mantle. Earth and Planetary Science Letters 95, 53–72.[Web of Science]
McKenzie, D. & Bickle, M. J. (1988). The volume and composition of melt generated by extension of the lithosphere. Journal of Petrology 29, 625–679.
McKenzie, D. & O’Nions, R. K. (1991). Partial melt distributions from inversion of rare earth element concentrations. Journal of Petrology 32, 1021–1091.
Mechie, J., Keller, G. R., Prodehl, C., Khan, M. A. & Gaciri, S. J. (1997). A model for the structure, composition and evolution of the Kenya rift. Tectonophysics 278, 95–119.
Mengel, K. & Green, D. H. (1986). Stability of amphibole and phlogopite in metasomatised peridotite under water-saturated and water-undersaturated conditions. In: Ross, J., Jaques, A. L., Ferguson, J., Green, D. H., O’Reilly, S. Y., Danchin, R. V. & Janse, A. J. A.. (eds) Kimberlites and Related Rocks, Volume 1. Carlton (Australia): Blackwell Scientific, pp. 571–581.
Menzies, M. A. (1992). The lower lithosphere as a major source for continental flood basalts: a re-appraisal. In: Storey, B. C., Alabaster, T. & Pankhurst, R. J. (eds) Magmatism and the Causes of Continental Break-up. Geological Society, London, Special Publications 68, 31–39.
Mertes, H. & Schmincke, H.-U. (1985). Mafic potassic lavas of the Quaternary West Eifel volcanic field I. Major and trace elements. Contributions to Mineralogy and Petrology 89, 330–345.[Web of Science]
Möller, A., Mezger, K. & Schenk, V. (1998). Crustal age domains and the evolution of the continental crust in the Mozambique Belt of Tanzania: combined Sm–Nd, Rb–Sr, and Pb–Pb isotopic evidence. Journal of Petrology 39, 749–783.
Norry, M. J., Truckle, P. H., Lippard, S. J., Hawkesworth, C. J., Weaver, S. D. & Marriner, G. F. (1980). Isotopic and trace element evidence from lavas, bearing on mantle heterogeneity beneath Kenya. Philosophical Transactions of the Royal Society, London, Series A 297, 259–271.
Olafsson, M. & Eggler, D. H. (1983). Phase relations of amphibole, amphibole–carbonate, and phlogopite–carbonate peridotite: petrologic constraints on the asthenosphere. Earth and Planetary Science Letters 64, 305–315.
Paslick, C., Halliday, A., James, D. & Dawson, J. B. (1995). Enrichment of the continental lithosphere by OIB melts: isotopic evidence from the volcanic province of northern Tanzania. Earth and Planetary Science Letters 130, 109–126.
Paslick, C. R., Halliday, A. N., Lange, R. A., James, D. & Dawson, J. B. (1996). Indirect crustal contamination: evidence from isotopic and chemical disequilibria in minerals from alkali basalts and nephelinites from northern Tanzania. Contributions to Mineralogy and Petrology 125, 277–292.
Ritter, J. R. R. & Kaspar, T. (1997). A tomography study of the Chyulu Hills, Kenya. Tectonophysics 278, 149–169.
Ritter, J. R. R., Fuchs, K., Kaspar, T., Lange, F. E. I., Nyambok, I. O. & Stangl, R. L. (1995). Seismic images illustrate the deep roots of the Chyulu Hills volcanic area, Kenya. EOS Transactions, American Geophysical Union 76, 273–278.
Rogers, N. W., De Mulder, M. & Hawkesworth, C. J. (1992). An enriched mantle source for potassic basanites: evidence from Karisimbi volcano, Virunga volcanic province, Rwanda. Contributions to Mineralogy and Petrology 111, 543–556.
Rudnick, R. L., McDonough, W. F. & Chappell, B. W. (1993). Carbonatite metasomatism in the northern Tanzanian mantle: petrographic and geochemical characteristics. Earth and Planetary Science Letters 114, 463–475.[Web of Science]
Saggerson, E. P. (1963). Geology of the Simba–Kibwezi area. Geological Survey of Kenya, Report 58.
Saggerson, E. P. (1968). Eclogite nodules associated with alkaline olivine basalts, Kenya. Geologische Rundschau 57, 890–903.
Saunders, A. D., Storey, M., Kent, R. W. & Norry, M. J. (1992). Consequences of plume–lithosphere interactions. In: Storey, B. C., Alabaster, T. & Pankhurst, R. J. (eds) Magmatism and the Causes of Continental Break-up. Geological Society, London, Special Publications 68, 41–60.
Schilling, J.-G., Kingsley, R. H., Hanan, B. B. & McCully, B. L. (1992). Nd–Sr–Pb isotopic variations along the Gulf of Aden: evidence for Afar plume–continental lithosphere interaction. Journal of Geophysical Research 97, 10927–10966.
Simiyu, S. M. & Keller, G. R. (1997). An integrated analysis of lithospheric structure across the East African plateau based on gravity anomalies and recent seismic studies. Tectonophysics 278, 291–313.
Simonetti, A. & Bell, K. (1994). Nd, Pb and Sr isotopic data from the Napak carbonatite–nephelinite centre, eastern Uganda: an example of open-system crystal fractionation. Contributions to Mineralogy and Petrology 115, 356–366.
Simonetti, A. & Bell, K. (1995). Nd, Pb and Sr isotopic data from the Mount Elgon volcano, eastern Uganda–western Kenya: implications for the origin and evolution of nephelinite lavas. Lithos 36, 141–153.
Smith, M. (1994). Stratigraphic and structural constraints on mechanisms of active rifting in the Gregory Rift, Kenya. Tectonophysics 236, 3–22.
Späth, A., le Roex, A. P. & Duncan, R. A. (1996). The geochemistry of lavas from the Comores Archipelago, western Indian Ocean: petrogenesis and mantle source region characteristics. Journal of Petrology 37, 961–991.
Späth, A., le Roex, A. P. & Opiyo-Akech, N. (2000). The petrology of the Chyulu Hills Volcanic Province, southern Kenya. Journal of African Earth Sciences 31, 337–358.[Web of Science]
Stewart, K. & Rogers, N. (1996). Mantle plume and lithosphere contributions to basalts from southern Ethiopia. Earth and Planetary Science Letters 139, 195–211.
Sun, S. S. & Hanson, G. N. (1975). Origin of Ross Island basanitoids and limitations upon the heterogeneity of mantle sources for alkali basalts and nephelinites. Contributions to Mineralogy and Petrology 52, 77–106.
Sun, S.-s. & McDonough, W. F. (1989). Chemical and isotopic systematics of oceanic basalts: implications for mantle composition and processes. In: Saunders, A. D. & Norry, M. J. (eds) Magmatism in the Ocean Basins. Geological Society, London, Special Publications 42, 313–345.
Turner, S., Hawkesworth, C., Gallagher, K., Stewart, K., Peate, D. & Mantovani, M. (1996). Mantle plumes, flood basalts, and thermal models for melt generation beneath continents: assessment of a conductive heating model and application to the Paraná. Journal of Geophysical Research 101, 11503–11518.
Vidal, P., Deniel, C., Vellutini, P. J., Piguet, P., Coulon, C., Vincent, J. & Audin, J. (1991). Changes of mantle sources in the course of a rift evolution: the Afar case. Geophysical Research Letters 18, 1913–1916.[Web of Science]
Vollmer, R. & Norry, M. J. (1983a). Unusual isotopic variations in Nyiragongo nephelinites. Nature 301, 141–143.
Vollmer, R. & Norry, M. J. (1983b). Possible origin of K-rich volcanic rocks from Virunga, East Africa, by metasomatism of continental crustal material: Pb, Nd and Sr isotopic evidence. Earth and Planetary Science Letters 64, 374–386.
Wass, S. Y. & Rogers, N. W. (1980). Mantle metasomatism—precursor to continental alkaline volcanism. Geochimica et Cosmochimica Acta 44, 1811–1823.[Web of Science]
White, R. & McKenzie, D. (1989). Magmatism at rift zones: the generation of volcanic continental margins and flood basalts. Journal of Geophysical Research 94, 7685–7729.
Williams, L. A. J. (1970). The volcanics of the Gregory Rift Valley, East Africa. Bulletin Volcanologique 34, 439–465.
Wilshire, H. G., Nielson Pike, J. E., Meyer, C. E. & Schwarzman, E. C. (1980). Amphibole-rich veins in lherzolite xenoliths, Dish Hill and Deadman Lake, California. American Journal of Science 280-A, 576–593.[Web of Science]
Wilson, M. & Downes, H. (1991). Tertiary–Quaternary extension-related alkaline magmatism in Western and Central Europe. Journal of Petrology 32, 811–849.
Zanetti, A., Vannucci, R., Bottazzi, P., Oberti, R. & Ottolini, L. (1996). Infiltration metasomatism at Lherz as monitored by systematic ion-microprobe investigations close to a hornblendite vein. Chemical Geology 134, 113–133.[Web of Science]
Zindler, A. (1980). Geochemical processes in the Earth’s mantle and the nature of crust–mantle interactions: evidence from studies of Nd and Sr isotope ratios in mantle-derived igneous rocks and lherzolite nodules. Ph.D. thesis, Massachusetts Institute of Technology.
Zindler, A. & Hart, S. (1986). Chemical geodynamics. Annual Review of Earth and Planetary Sciences 14, 493–571.[Web of Science]
Zindler, A., Hart, S. R. & Frey, F. A. (1979). Nd and Sr isotope ratios and rare earth element abundances in Reykjanes Peninsula basalts: evidence for mantle heterogeneity beneath Iceland. Earth and Planetary Science Letters 45, 249–262.[Web of Science]
CiteULike 
Connotea 
Del.icio.us What's this?
This article has been cited by other articles:
|
|
 |
|
  J. B. Dawson Chapter 8 Regional comparisons, petrochemistry and petrogenesis Geological Society, London, Memoirs, January 1, 2008; 33(1): 79 - 89. [Full Text] [PDF]
|
|
|
|
 |
|
 A. Ulianov, O. Muntener, P. Ulmer, and T. Pettke Entrained Macrocryst Minerals as a Key to the Source Region of Olivine Nephelinites: Humberg, Kaiserstuhl, Germany J. Petrology, June 1, 2007; 48(6): 1079 - 1118. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 M. Lopez-Plaza, M. Peinado, F.-J. Lopez-Moro, M. D. Rodriguez-Alonso, A. Carnicero, M. P. Franco, J. C. Gonzalo, and M. Navidad Contrasting mantle sources and processes involved in a peri-Gondwanan terrane: A case study of pre-Variscan mafic intrusives from the autochthon of the Central Iberian Zone Geological Society of America Special Papers, January 1, 2007; 423(0): 297 - 313. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
B. KAESER, A. KALT, and T. PETTKE Evolution of the Lithospheric Mantle beneath the Marsabit Volcanic Field (Northern Kenya): Constraints from Textural, P-T and Geochemical Studies on Xenoliths J. Petrology, November 1, 2006; 47(11): 2149 - 2184. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
K. S. PANTER, J. BLUSZTAJN, S. R. HART, P. R. KYLE, R. ESSER, and W. C. MCINTOSH The Origin of HIMU in the SW Pacific: Evidence from Intraplate Volcanism in Southern New Zealand and Subantarctic Islands J. Petrology, September 1, 2006; 47(9): 1673 - 1704. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
T. FURMAN, K. M. KALETA, J. G. BRYCE, and B. B. HANAN Tertiary Mafic Lavas of Turkana, Kenya: Constraints on East African Plume Structure and the Occurrence of High-{micro} Volcanism in Africa J. Petrology, June 1, 2006; 47(6): 1221 - 1244. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
A. ULIANOV and A. KALT Mg-Al Sapphirine- and Ca-Al Hibonite-bearing Granulite Xenoliths from the Chyulu Hills Volcanic Field, Kenya J. Petrology, May 1, 2006; 47(5): 901 - 927. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
Y. WEINSTEIN, O. NAVON, R. ALTHERR, and M. STEIN The Role of Lithospheric Mantle Heterogeneity in the Generation of Plio-Pleistocene Alkali Basaltic Suites from NW Harrat Ash Shaam (Israel) J. Petrology, May 1, 2006; 47(5): 1017 - 1050. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
M. BECKER and A. P. L. ROEX Geochemistry of South African On- and Off-craton, Group I and Group II Kimberlites: Petrogenesis and Source Region Evolution J. Petrology, April 1, 2006; 47(4): 673 - 703. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 N.W. Rogers Basaltic magmatism and the geodynamics of the East African Rift System Geological Society, London, Special Publications, January 1, 2006; 259(1): 77 - 93. [Abstract] [PDF]
|
|
|
|
|
|
T. Furman, J. Bryce, T. Rooney, B. Hanan, G. Yirgu, and D. Ayalew Heads and tails: 30 million years of the Afar plume Geological Society, London, Special Publications, January 1, 2006; 259(1): 95 - 119. [Abstract] [PDF]
|
|
|
|
|
|
R. N. THOMPSON, C. J. OTTLEY, P. M. SMITH, D. G. PEARSON, A. P. DICKIN, M. A. MORRISON, P. T. LEAT, and S. A. GIBSON Source of the Quaternary Alkalic Basalts, Picrites and Basanites of the Potrillo Volcanic Field, New Mexico, USA: Lithosphere or Convecting Mantle? J. Petrology, August 1, 2005; 46(8): 1603 - 1643. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
J. S. JOHNSON, S. A. GIBSON, R. N. THOMPSON, and G. M. NOWELL Volcanism in the Vitim Volcanic Field, Siberia: Geochemical Evidence for a Mantle Plume Beneath the Baikal Rift Zone J. Petrology, July 1, 2005; 46(7): 1309 - 1344. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
J. A. WOLFF, M. C. ROWE, R. TEASDALE, J. N. GARDNER, F. C. RAMOS, and C. E. HEIKOOP Petrogenesis of Pre-caldera Mafic Lavas, Jemez Mountains Volcanic Field (New Mexico, USA) J. Petrology, February 1, 2005; 46(2): 407 - 439. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
T. FURMAN, J. G. BRYCE, J. KARSON, and A. IOTTI East African Rift System (EARS) Plume Structure: Insights from Quaternary Mafic Lavas of Turkana, Kenya J. Petrology, May 1, 2004; 45(5): 1069 - 1088. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
B. KIEFFER, N. ARNDT, H. LAPIERRE, F. BASTIEN, D. BOSCH, A. PECHER, G. YIRGU, D. AYALEW, D. WEIS, D. A. JERRAM, et al. Flood and Shield Basalts from Ethiopia: Magmas from the African Superswell J. Petrology, April 1, 2004; 45(4): 793 - 834. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
A. P. LE ROEX, D. R. BELL, and P. DAVIS Petrogenesis of Group I Kimberlites from Kimberley, South Africa: Evidence from Bulk-rock Geochemistry J. Petrology, December 1, 2003; 44(12): 2261 - 2286. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 M. Marks, M. Marks, and G. Markl Ilimaussaq 'en miniature': closed-system fractionation in an agpaitic dyke rock from the Gardar Province, South Greenland (contribution to the mineralogy of Ilimaussaq no. 117) Mineralogical Magazine, October 1, 2003; 67(5): 893 - 919. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
M. MARKS, T. VENNEMANN, W. SIEBEL, and G. MARKL Quantification of Magmatic and Hydrothermal Processes in a Peralkaline Syenite-Alkali Granite Complex Based on Textures, Phase Equilibria, and Stable and Radiogenic Isotopes J. Petrology, July 1, 2003; 44(7): 1247 - 1280. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
L. M. LARSEN, A. K. PEDERSEN, B. SUNDVOLL, and R. FREI Alkali Picrites Formed by Melting of Old Metasomatized Lithospheric Mantle: Manitdlat Member, Vaigat Formation, Palaeocene of West Greenland J. Petrology, January 1, 2003; 44(1): 3 - 38. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
R. B. TRUMBULL, B. BUHN, R. L. ROMER, and F. VOLKER The Petrology of Basanite-Tephrite Intrusions in the Erongo Complex and Implications for a Plume Origin of Cretaceous Alkaline Complexes in Namibia J. Petrology, January 1, 2003; 44(1): 93 - 112. [Abstract] [Full Text] [PDF]
|
|
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||