| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Journal of Petrology | Volume 44 | Number 1 | Pages 93-112 | 2003
© Oxford University Press 2003
The Petrology of Basanite–Tephrite Intrusions in the Erongo Complex and Implications for a Plume Origin of Cretaceous Alkaline Complexes in Namibia
1GEOFORSCHUNGSZENTRUM POTSDAM, TELEGRAFENBERG, 14473 POTSDAM, GERMANY
2INSTITUT FÜR GEOWISSENSCHAFTEN UND LITHOSPHÄRENFORSCHUNG, UNIVERSITÄT GIESSEN, 35390 GIESSEN, GERMANY
RECEIVED December 6, 2001; ACCEPTED July 12, 2002
 |
ABSTRACT |
|---|
 TOP ABSTRACT INTRODUCTIONGEOLOGICAL SETTINGSAMPLE SELECTION AND ANALYTICAL...PETROGRAPHYWHOLE-ROCK COMPOSITIONSSr-Nd-Pb ISOTOPE COMPOSITIONSDISCUSSIONCONCLUSIONSAPPENDIXREFERENCES |
|---|
Basanite intrusions from the Early Cretaceous Erongo complex, Namibia, have compositions consistent with near-primary mantle melts derived from a depth of at least 100 km. These rocks provide a key reference for the mantle component(s) involved in breakup-related magmatism in this region. Initial Sr–Nd–Pb isotope ratios of the Erongo basanites and associated tephrites and phonotephrites (87Sr/86Sr = 0·70425–0·70465;
Nd = +1·8 to +2·7; 206Pb/204Pb = 18·63–18·91) are independent of the degree of differentiation and correspond closely to an estimated range for the Tristan plume at 130 Ma. Incompatible trace element ratios also overlap with ratios of ocean island basalt (OIB) from the South Atlantic islands of Tristan da Cunha, Gough and Inaccessible associated with the modern Tristan hotspot. The Tristan plume signature of Erongo basanite–tephrite intrusions is shared by at least six other Early Cretaceous mafic alkaline complexes in Namibia, whereas the associated flood basalts in general lack a plume signature. We attribute the contrast in mantle sources for the flood basalts and alkaline complexes to their relative timing with respect to lithospheric thinning. Thick lithosphere during the main flood basalt event prevented direct melting of the Tristan plume and magmas were generated mostly from the lithosphere. The alkaline complexes intruded later, when the lithosphere was sufficiently thinned to allow decompression melting of the underlying plume mantle. KEY WORDS: Sr–Nd–Pb isotopes; Namibia; plume; basanite; petrogenesis
|
INTRODUCTION |
|---|
|
TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTINGSAMPLE SELECTION AND ANALYTICAL...PETROGRAPHYWHOLE-ROCK COMPOSITIONSSr-Nd-Pb ISOTOPE COMPOSITIONSDISCUSSIONCONCLUSIONSAPPENDIXREFERENCES |
|---|
A central issue in understanding the rift-related Cretaceous magmatism in the South Atlantic, and the development of volcanic rifted margins in general, is the relative role of plume mantle vs lithospheric mantle in magma genesis. This has been addressed in many investigations of flood basalts from the Paraná–Etendeka province, and it is well established that the erupted basalts represent chemically evolved magmas whose isotopic and incompatible element ratios are inconsistent with a plume source alone (e.g. Peate, 1997; Hawkesworth et al., 1999; Marques et al., 1999). The mantle source(s) of the flood basalts remains controversial. The issue is complicated by the lack of primary magma compositions, and by the possibility that crustal contamination masks the pristine mantle signature (Arndt & Christensen, 1992; Arndt et al., 1993). The only rare examples of near-primary magmas from the Etendeka flood basalts are Mg- and Fe-rich flows from the base of the sequence. These have been variously termed Tafelkop-type basalts or LTZ.H type basalts (Milner & Le Roex, 1996; Ewart et al., 1998) and ferropicrites (Gibson et al., 2000).
Many continental flood basalt provinces contain intrusive complexes or central volcanoes. Petrogenetic studies of these offer a different perspective on the mantle from the flood basalts, as shown in studies from the East African Rift (e.g. Bell & Tilton, 2001; Späth et al., 2001), the Deccan province (Simonetti et al., 1998) and the Afar region (Baker et al., 1997). This is so because the intrusive complexes and central volcanoes are commonly dominated by alkaline magmas, which are derived by smaller degrees of partial melting and at greater depths than the tholeiitic flood basalts. Furthermore, the relatively high Sr and Nd concentrations in the alkaline magmas compared with the tholeiites make their Sr–Nd isotope composition more resistant to change by crustal contamination.
Early Cretaceous intrusive complexes containing carbonatite, lamprophyre, alkaline gabbro and nepheline syenite are common in the Damaraland province of Namibia (Fig. 1). Previous studies established that many of these have radiogenic isotope and trace element compositions suggesting an origin from the Tristan plume (Milner & Le Roex, 1996; Le Roex & Lanyon, 1998; Harris et al., 1999; Trumbull et al., 2000). However, most of the rocks previously studied have highly evolved compositions and the relevance of their composition to that of the primary mantle source(s) is open to interpretation. The Erongo complex represents a key locality in this respect because it contains late-intrusive plugs and dykes of basanite whose compositions are consistent with an origin as near-primary mantle-derived magmas. The basanites occur in association with more differentiated lithologies including tephrite, phonotephrite, nephelinite and foidite. In this paper we report the geochemical and isotopic composition of the mafic plug lithologies and demonstrate that they represent a comagmatic series related through fractional crystallization to a parental basanite magma. We show that the initial isotope ratios of the Erongo plug samples correspond to those estimated for the ancestral Tristan plume at 130 Ma, and that this plume signature is dominant in all mafic and alkaline units from the Damaraland complexes studied so far.
|
|
GEOLOGICAL SETTING |
|---|
|
TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTINGSAMPLE SELECTION AND ANALYTICAL...PETROGRAPHYWHOLE-ROCK COMPOSITIONSSr-Nd-Pb ISOTOPE COMPOSITIONSDISCUSSIONCONCLUSIONSAPPENDIXREFERENCES |
|---|
The Pan-African Damara Fold Belt in NW Namibia was intruded in the Early Cretaceous by a number of subvolcanic complexes (Fig. 1) that include a wide range of rock types: carbonatite, alkaline and tholeiitic gabbro, syenite, peralkaline granite and peraluminous granite (Martin et al., 1960; Harris, 1995; Trumbull et al., 2000). Field relations suggest that the intrusive complexes are younger than the Etendeka flood basalts, although isotopic ages for the volcanic and intrusive rocks overlap in the range of 137–124 Ma (Milner et al., 1995; Renne et al., 1996; Schmitt et al., 2000).
Erongo is the largest of the Damaraland igneous complexes, with a diameter of 
35 km. Country rocks are late Precambrian to early Cambrian granites and metasedimentary rocks of the Damara Sequence. The Erongo complex consists of a central massif dominated by dacitic to rhyolitic volcanic rocks, and with peripheral intrusions of granite around it (Fig. 1). The base of the central massif is made up of tholeiitic basalt flows whose geochemical and isotopic compositions suggest they may be erosional remnants of the Etendeka flood basalts (Blümel et al., 1979; Wigand et al., 2001). The main units of the central massif are dacitic to rhyodacitic pyroclastic rocks and lavas whose total thickness reaches 1000 m. These felsic volcanics are intruded in the central part of the complex by a granodiorite stock, which is compositionally equivalent to the volcanic units, shows gradational contacts with them and is considered to represent magma resurgence in the vent area (Blümel et al., 1979; Pirajno, 1990). The peripheral intrusions consist of tourmaline-bearing biotite granite, which also occurs as sills and dykes cutting the central massif.
Apart from the basal basalts, mafic units in the Erongo complex are intrusive, emplaced late in the magmatic sequence, and they are concentrated in the NW part of the complex. The most prominent mafic intrusion is a ring dyke of coarse tholeiitic dolerite (Fig. 1). Other intrusions are alkaline, and these include alkali basalt dykes cutting the Erongo granite and a cluster of basanite–tephrite plugs and dykes in dacitic tuffs of the central massif. The latter rocks are the focus of this paper. The concentration of mafic intrusions in the NW sector of the Erongo complex coincides with a strong positive aeromagnetic anomaly in that area (Fig. 1), which is suggested to represent an unexposed mafic intrusion (Eberle & Hutchins, 1996).
The first description of the Erongo basanite–tephrite plugs by Patel (1988) identified at least 40 individual intrusions within a 4 km2 area. The intrusions are oval to circular in outcrop and have diameters between 5 and 50 m. In places, they are resistant to weathering and stand out as mounds 5–10 m above the surroundings. The lithologies include basanite, tephrite, foidite and phonotephrite. Patel (1988) stated that phonotephrite is the most abundant rock type in the plugs and that basanites are restricted to ‘dyke-like bodies’. Our field observations and sampling showed that basanite and tephrite are also components of the plugs, and we also found a diatreme consisting of basanite–tephrite and phonotephrite matrix with a heterogeneous clast assemblage including altered basement granite, diorite and lamprophyre.
The plugs have not been dated directly but are obviously younger than the tuffs they intrude, which have yielded U–Pb zircon ages of 135 ± 3 Ma (Piranjo et al., 2000). Minimum age constraints for the plugs are lacking, but they are likely to have intruded not long after the other Erongo units, because analogous mafic and alkaline intrusions in other Damaraland complexes that have been dated gave the same age as the Erongo tuffs (e.g. nepheline syenite, lamprophyre and alkali gabbro from Okenyenya, Cape Cross, Messum and Okorusu: Milner et al., 1995; Renne et al., 1996) and there are no reported post-Early Cretaceous igneous events in this part of Namibia. Furthermore, compositionally similar alkaline basaltic dykes in the Erongo granite show flexures that suggest the granite was not fully solidified at the time of dyke intrusion (Blümel et al., 1979).
|
SAMPLE SELECTION AND ANALYTICAL METHODS |
|---|
|
TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTINGSAMPLE SELECTION AND ANALYTICAL...PETROGRAPHYWHOLE-ROCK COMPOSITIONSSr-Nd-Pb ISOTOPE COMPOSITIONSDISCUSSIONCONCLUSIONSAPPENDIXREFERENCES |
|---|
A total of 50 samples from seven alkaline plugs were selected for whole-rock chemical analysis after exclusion of visibly altered samples and those containing more than one lithology (e.g. breccia, veined samples, etc.). Full trace element and isotopic analyses were performed on a subset of 11 samples, which cover the full compositional range. Table 1 presents representative chemical compositions for the suite of samples and the complete dataset is available for downloading from the Journal of Petrology web site at http://www.petrology.oupjournals.org. Major and most trace elements were analysed at the University of Giessen by X-ray fluorescence (XRF; Philips PX 1400) using fused and pressed powder discs. H2O was determined by Karl-Fischer-reaction and CO2 was measured coulometrically. FeO was determined by titration, and F by ion-selective electrode. Sulphur was analysed by IR absorption. The rare earth elements were determined by inductively coupled plasma atomic emission spectrometry (ICP-AES) in Giessen using the method described by Zuleger & Erzinger (1988). Concentrations of U, Th and Pb were determined by inductively coupled plasma mass spectrometry (ICP-MS) at the GFZ Potsdam. Mineral analyses were performed by electron microprobe using a CAMECA SX 50 instrument at the University of Giessen and a CAMECA SX-100 instrument at the GFZ Potsdam. Procedures at both laboratories were the same. Calibration was made using natural and synthetic mineral standards and operating conditions were 15 kV accelerating voltage and 20 nA beam current. PAP corrections were applied using CAMECA software (Pouchou & Pichoir, 1984). Representative phenocryst compositions are given in Table 2.
|
|
Isotope ratios of Sr, Nd and Pb were determined on unleached whole-rock samples at the GFZ Potsdam following the methods described in detail by Romer et al. (2001). Sr and Nd separation followed standard ion exchange techniques (Biorad AG50Wx8 for Sr and REE, HDEHP-coated Teflon for Nd). Lead was separated using Biorad AG1x8 in 0·5 ml Teflon columns using an HCl–HBr ion exchange technique. Strontium was loaded on single Ta filament and measured with a VG Sector 54-30 mass spectrometer using dynamic multi-collection. Mass fractionation was corrected for by normalizing to the ratio of 86Sr/88Sr = 0·1194. Repeated measurement of Sr reference material NBS 987 gave 87Sr/86Sr values of 0·710249 ± 4 (2
m, n = 12). Neodymium was loaded on double Re filaments and measured with dynamic multi-collection on a Finnigan MAT 262 mass spectrometer. The 143Nd/144Nd data were normalized to 146Nd/144Nd = 0·7219. During the period of this study the La Jolla Nd reference material yielded a value of 143Nd/144Nd = 0·511850 ± 4 (2m, n = 14). Lead was loaded together with silica-gel and H3PO4 on single Re filaments and its isotopic composition was determined at 1200–1250°C on a Finnigan MAT 262 mass spectrometer using static multi-collection. Instrumental fractionation was corrected with 0·1%/a.m.u. as determined from repeated measurement of Pb standard NBS 981. The reported Pb isotope ratios are known to better than 0·1% at the 2 level. Total procedural blanks were <50 pg Sr, <50 pg Nd and 30 pg Pb. The Sr–Nd–Pb isotope data are reported in Table 3.
|
|
PETROGRAPHY |
|---|
|
TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTINGSAMPLE SELECTION AND ANALYTICAL...PETROGRAPHYWHOLE-ROCK COMPOSITIONSSr-Nd-Pb ISOTOPE COMPOSITIONSDISCUSSIONCONCLUSIONSAPPENDIXREFERENCES |
|---|
The nomenclature for rocks from the Erongo alkaline plugs used here is based on their bulk chemical composition, with further distinctions based on the presence or absence of primary amphibole, and on the CIPW-normative olivine and nepheline contents. We follow Le Maitre et al. (1989) in distinguishing basanite and tephrite as having more than, or less than 10 wt % normative olivine, respectively. Classified in this way, the samples include tephrites, basanites, amphibole foidites and amphibole phonotephrites, with subordinate tephri-phonolites. Mineral compositions have not been studied in detail and the compositional ranges given below are based on electron microprobe measurements of mafic phases in only a few representative samples of basanite and phonotephrite (Table 2).
The samples from the Erongo alkaline plugs have hypabyssal seriate to porphyritic textures. The basanites and tephrites are fine- to medium-grained, porphyritic, mesocratic rocks with up to 30 vol. % phenocrysts of euhedral Ti-augite (to 5 mm) and euhedral to subhedral olivine (0·5–2 mm). These are set in a fine-grained groundmass consisting of poikilitic, subhedral to anhedral nepheline and alkali feldspar, subhedral plagioclase, clinopyroxene and magnetite (commonly 2–3 vol. %). Accessory minerals include acicular apatite and titanite. The mafic silicates are generally fresh, although in some samples clinopyroxene is rimmed by overgrowths of brown anhedral biotite and opaque minerals, and olivine is partially replaced at the rims by iddingsite. Feldspars and nepheline are partially clouded with fine sericite and the groundmass in several samples contains anhedral brown biotite, sericite and scattered carbonate. The degree of secondary mineral overgrowth is greatest in the most differentiated samples.
Olivine grains in the basanite samples are zoned, with a forsterite-rich core (Fo85–90), and rims with fayalite contents up to 30 mol % (Table 2). Some olivine grains include reddish-brown crystals of spinel (not analysed). Olivine in tephrite and phonotephrite samples is less magnesian and the grains are commonly unzoned, with compositions in the range of Fo72Fa28 to Fo85Fa15. Clinopyroxenes in the basanites and tephrites show similar compositions (total range: Wo49–52En33–38Fs10–15, Table 2). Euhedral olivine and clinopyroxene generally coexist. Olivine is partly or entirely included by pyroxene in some samples, thus it appears that olivine crystallization preceded pyroxene. Magnetite (Usp11–26) occurs rarely as inclusions in pyroxene and is an abundant phase in the groundmass.
The amphibole-bearing foidites and phonotephrites are less mafic and generally finer in grain size than the basanite–tephrites but they have similar porphyritic to seriate, hypabyssal textures. Euhedral phenocrysts of brown amphibole and clinopyroxene (20–30 vol. %) are set in a matrix of anhedral to subhedral nepheline and alkali feldspar, with subordinate plagioclase laths, accessory apatite and abundant magnetite. Olivine is rare and, where present, generally altered. Amphibole (pargasite with average formula [Na0·6,K0·3][Ca1·9Na0·1][Mg2·7Fe1·4Ti0·4Al0·3][Al2·1Si5·9]O22OH2; see Table 2) occurs both as phenocrysts and as fine needles in the matrix, and it was also observed as secondary overgrowths on clinopyroxene. In some samples, brown biotite occurs as a late subhedral phase associated with the secondary amphibole. Analcite is present locally as subhedral, rounded grains in the matrix
|
WHOLE-ROCK COMPOSITIONS |
|---|
|
TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTINGSAMPLE SELECTION AND ANALYTICAL...PETROGRAPHYWHOLE-ROCK COMPOSITIONSSr-Nd-Pb ISOTOPE COMPOSITIONSDISCUSSIONCONCLUSIONSAPPENDIXREFERENCES |
|---|
All samples from the Erongo plugs are nepheline-normative and their overall position in the total-alkali–silica classification diagram is shown in Fig. 2. They show no overlap with the other igneous units of the Erongo complex, which are subalkaline. These other units are not of concern in this paper but for comparison we include data for the ring dyke tholeiite and the felsic units in Fig. 2. The basanite and tephrite samples have SiO2 contents of 41–46 wt % and total alkalis between 4 and 10 wt % and MgO between 5 and 12·5 wt %. Values of the Mg-number [100Mg/(Mg + Fe2+)] are 75–81 for the basanites and 64–77 for the tephrites. The amphibole-bearing phonotephrites and foidites overlap in composition with the more evolved tephrite samples (Fig. 3). They have SiO2 contents from 43 to 51 wt % and a range of MgO concentration from 1 to 5 wt % (with one exception at 7%). The growth of secondary sericite, biotite and carbonate in the groundmass may be the cause of low analytical totals of some of the most differentiated samples (Table 1) but the coherence of element variation trends described below suggests that the bulk composition of the samples has not been seriously affected.
|
|
Major element trends against MgO as a differentiation index commonly show inflections at the transition from basanite–tephrite to phonotephrite and foidite (Fig. 3). Concentrations of TiO2 and total Fe are constant or decrease slightly in the basanite–tephrite samples, then decrease strongly in the more evolved samples. P2O5 contents first increase until about 7–8 wt % MgO and then decline sharply with further differentiation. The Al2O3 and CaO trends also show inflections at about 7–8 wt % MgO. As discussed in more detail below, these trends are consistent with fractionation influenced by changing mineral assemblages: dominantly olivine + clinopyroxene in the basanite–tephrite group, and clinopyroxene + amphibole ± apatite ± oxides in the phonotephrites and foidites.
Trace elements that behave incompatibly throughout the compositional range from basanite to phonotephrite include Rb (40–180 ppm), Th (5–35 ppm), Zr (110–300 ppm), Nb (75–225 ppm) and Ba (900–2400 ppm). Strontium also increases with falling MgO concentration, then decreases in the most differentiated samples with <3 wt % MgO. Yttrium variations show inflected trends very similar to those of P2O5 and this suggests that the decrease in Y contents in the more evolved samples is due to crystallization of apatite. Inflections in slope of the Cr and Sc plots at about 5–6 wt % MgO (Fig. 3) mirror those of FeO(tot) and TiO2. The total rare earth element (REE) concentrations and the slope of chondrite-normalized distribution patterns (La/Sm ratio) increase with differentiation (Fig. 4). None of the samples analysed for REE shows a significant Eu anomaly and this suggests negligible fractionation of plagioclase, which is in keeping with the lack of plagioclase in the phenocryst assemblage and the increase in Sr concentration with differentiation in all but the most evolved samples.
|
Figure 5 summarizes the trace element contents of the Erongo tephrite and basanite samples with MgO >6 wt % in a multi-element diagram normalized to an average composition of oceanic island basalts from Tristan da Cunha [data from Weaver et al. (1987) and Le Roex et al. (1990)]. The main point of this diagram is the overall similarity between the least-evolved Erongo basanites and the Tristan ocean island basalt (OIB) average. The greatest differences between the two are higher Ba and lower Zr and Ti in the Erongo samples. It should be noted also that the Erongo tephrite samples show similar trace element patterns to the basanites but higher concentrations, particularly for the more incompatible elements in the left part of the diagram. The similarity of Erongo basanites to South Atlantic OIB is also shown by their incompatible element ratios. The La/Nb ratio in particular is commonly used as a discriminant for mantle sources, as the two elements are similarly incompatible during melting or crystallization in basaltic systems (e.g. Saunders et al., 1992; Gibson et al., 1996). In the Erongo samples, the concentrations of La and Nb both increase about 2–3 times with differentiation, but the La/Nb ratio is less variable, between 0·56 and 0·76. This range agrees well with the average value of 0·68 for plume-related OIB according to the data compilation of Gibson et al. (1996) and with the average value of 0·75 for the Tristan OIB samples with >6 wt % MgO from Le Roex et al. (1990). In contrast, the average ratios in both the low-Ti and high-Ti Paraná flood basalts are 1·61 and 1·56, respectively (Gibson et al., 1996).
|
|
Sr–Nd–Pb ISOTOPE COMPOSITIONS |
|---|
|
TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTINGSAMPLE SELECTION AND ANALYTICAL...PETROGRAPHYWHOLE-ROCK COMPOSITIONSSr-Nd-Pb ISOTOPE COMPOSITIONSDISCUSSIONCONCLUSIONSAPPENDIXREFERENCES |
|---|
The samples chosen for isotopic analysis (Table 3) represent four separate intrusive plugs and cover a compositional range from 12 to 3 wt % MgO. Despite this wide compositional range, the samples display little variation in their radiogenic isotope compositions. The initial Sr and Nd isotope ratios of the Erongo plugs (Fig. 6a) correspond closely to the present-day ratios of the Tristan hotspot OIB and to the estimated Tristan plume composition at 130 Ma (see Discussion and Appendix). This diagram also includes a comparison of published data for mafic alkaline units from six other Damaraland complexes, all calculated for a common age of 130 Ma. The compilation shows that the initial Sr and Nd isotope ratios of mafic alkaline rocks from all complexes for which data are available overlap with the composition of the Tristan plume. It should be noted that we do not include data from quartz-oversaturated units from Okenyenya and Messum (tholeiitic gabbros and quartz syenites), whose oxygen isotope compositions provide evidence for strong crustal contamination (Martinez et al., 1996; Harris et al., 1999).
|
Figure 6b contrasts the isotope compositions of the Damaraland mafic alkaline units (rectangular field) with basalts from the Etendeka and Paraná sequences. By far the most common types of flood basalts in this province, including both the high-Ti and low-Ti groups, have lower initial Nd isotopic ratios and higher initial Sr isotopic ratios than the Damaraland complexes, and their more enriched isotopic signatures have been explained by derivation from a lithospheric mantle source (e.g. Garland et al., 1996; Turner et al., 1996; Hawkesworth et al., 1999; Marques et al., 1999). The only known basalts that plot in the rectangular field are the Tafelkop-type lavas, which occur in a restricted area of Namibia at the base of the Etendeka sequence [LTZ.H. of Ewart et al. (1998) and ferropicrites of Gibson et al. (2000)]. Marsh et al. (2001) suggested that the Tafelkop basalts erupted from the Doros complex (Fig. 1), so their connection with the Damaraland complexes may be closer than to the flood basalts. Another compositionally distinctive and rare group of basaltic rocks are the Horingbaai dolerites, which occur as late-stage dykes in the coastal region of Namibia and have mid-ocean ridge basalt (MORB)-like isotope ratios and chemical characteristics.
The Pb isotope compositions of the Erongo plugs (Fig. 7) are uniform and more radiogenic than most of the Paraná–Etendeka flood basalts, the exception being the Tafelkop basalts and ferropicrites. The Erongo samples overlap with the Okenyenya complex at the radiogenic end of its compositional range, and their 206Pb/204Pb compositions, in particular, are slightly higher than the inferred range for the Tristan plume at 130 Ma. The most reasonable explanation for these features is contamination of the basanite–tephrite magmas by crustal Pb, which is reasonable as the plugs were emplaced through continental crust and the Tristan OIB lavas were not. Supporting evidence for this explanation is the overlap between the Erongo plugs data and the compositional field of Pan-African Damara granites.
|
|
DISCUSSION |
|---|
|
TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTINGSAMPLE SELECTION AND ANALYTICAL...PETROGRAPHYWHOLE-ROCK COMPOSITIONSSr-Nd-Pb ISOTOPE COMPOSITIONSDISCUSSIONCONCLUSIONSAPPENDIXREFERENCES |
|---|
Source depths
A large body of experimental evidence demonstrates the dependence of the bulk composition of primary mantle melts on pressure (e.g. Kushiro, 1968, 1996; Jaques & Green, 1980) and suggests that silica-undersaturated and alkaline magmas such as the Erongo basanites originate from a relatively deep source. Equations that relate bulk-rock SiO2 contents to the pressure of melting based on peridotite melting experiments have been suggested by several workers and applied to estimate the depth of basaltic magma sources (e.g. Albarède, 1992; Scarrow & Cox, 1995; Haase, 1996). The Erongo basanites have about 43–44 wt % SiO2 with the exception of sample FNA105 with 40·3 wt % (Fig. 2), and these values correspond to pressures of 2·9–4·4 GPa (4·5–5·6 GPa for FNA105) using the pressure–composition regressions given in the sources cited. Assuming a thickness of 35 km for the continental crust (Bauer et al., 2000) with an average density of 2800 kg/m3, and a density of 3300 kg/m3 for the upper mantle, pressures of 3–5 GPa correspond to depths of roughly 100–160 km. The FeO contents of primary mantle melts also vary with the pressure of melting, although in contrast to SiO2, variations in initial mantle composition are also important (Langmuir et al., 1992) and preclude general predictive equations as for SiO2. Compared with model curves for equilibrium or fractional melting of fertile peridotite at different pressures given by Langmuir et al. (1992, figs. 47, 48), the basanite FeO(total) contents of
10 wt % are consistent with an onset of melting at a minimum of 3 GPa pressure. We interpret these pressures as minimum estimates for the onset of melt generation, as mantle melting can take place over a considerable depth interval and the bulk magma produced, especially in a plume scenario, may be an aggregate of melt increments derived from different depths (e.g. Klein & Langmuir, 1987).
Mantle melting at depths below 80 km is likely to involve garnet lherzolite and to produce melts relatively depleted in the heavy REE (HREE). The chondrite-normalized ratios of Sm/Yb in the Erongo basanites are 4–6, and these values are consistent with the initiation of melting in the presence of residual garnet (McKenzie & O’Nions, 1991; Ellam, 1992).
Fractionation of the Erongo plug magmas
The regular variation trends of major and trace elements with MgO in samples of the Erongo plugs and the homogeneity in their initial Sr–Nd–Pb isotope ratios throughout the range of compositions suggest that the basanite–tephrite–phonotephrite intrusions represent a comagmatic series related by fractional crystallization. Furthermore, the consistent inflections in differentiation trends for many elements suggest changes in the crystallizing assemblages. This interpretation was tested by calculating an equilibrium crystallization model using the MELTS thermodynamic approach (Ghiorso & Sack 1995) and by standard least-squares mixing calculations based on the observed rock and mineral compositions (Table 4). The starting composition for both models is basanite sample FNA26, which has the highest MgO and Ni contents, and the lowest contents of Na2O, K2O, Sr and Ba of the Erongo basanite–tephrite group.
|
MELTS models were run at various pressure from 50 to 500 MPa and oxygen fugacity of QFM (quartz–fayalite–magnetite) ± 3 log units. Evidence for low-pressure fractionation is given by the fact that the whole-rock compositions plot along the 1 bar olivine–diopside–plagioclase cotectic (Fig. 8) determined experimentally by Sack et al. (1987). Also, higher pressure on the system favours crystallization of clinopyroxene over olivine, and the rock compositions show that early differentiation involved a drop in MgO and Ni but no change in CaO, suggesting that clinopyroxene was not involved. We found the best fit to observed differentiation trends at conditions of 100 MPa and fO2 = QFM + 1 (Fig. 9). The model predicts initial crystallization of 9 wt % forsterite-rich olivine, followed by cotectic crystallization of olivine and clinopyroxene until the melt is 50% crystallized, after which spinel and then apatite join the solid assemblage (Table 4). This sequence of crystallization is in good agreement with the inflections in differentiation trends for Ca, Al, Sc, Fe, Ti, Y and P (Figs 3 and 9). The model olivine compositions of Fo90–85 match the observed olivine core compositions, and the model clinopyroxene composition (Na0·01–0·02Ca0·91–0·93Fe2+0·09–0·10Mg0·69–0·79Fe3+0·10–0·15Ti0·04–0·08Al0·27–0·35Si1·67–1·78) also agrees with observed compositions (Table 2).
|
|
Least-squares mixing models treat fractionation from basanite to phonotephrite in three stages to allow changes in the fractionating assemblage (Table 5). Mineral compositions used are from Table 2 with the exception of apatite, which was assumed to be pure Ca5(PO4)3. Solutions are considered acceptable that gave a value of unity or less for the sum of squared residuals (
r2). The first stage of basanite fractionation (FNA26 to FNA33, from 12·4 to 11·3 wt % MgO) can be fitted satisfactorily by 4·2 wt % crystallization of olivine alone (r2 = 0·66) as predicted by the MELTS results. For the next stage, from 11·3 to 9·7 wt % MgO (FNA33 to FNA37), clinopyroxene is needed in the crystallizing assemblage (r2 = 1·4 for ol and 0·59 for ol–cpx). Adding magnetite to the model results in a negative coefficient (i.e. consumption) for that phase. The degree of crystallization at the end of stage 2 is 14 wt %, which agrees with the MELTS prediction for cotectic olivine–clinopyroxene crystallization beyond 10 wt % crystallization (Table 4). Further fractionation within the tephrite series (FNA37 to FNA109, from 9·7 to 5·2 wt % MgO) requires additional phases (r2 = 3·6 for ol–cpx) but with apatite the fit is rather poor (r2 = 1·4 for ol–cpx–ap) and including magnetite in the model results in its consumption. Hornblende is part of the phenocryst assemblage in sample FNA109, and allowing it to crystallize produces a good fit to the data (r2 = 0·49 for ol–cpx–hb–ap). This result cannot be compared directly with the thermodynamic model because hornblende is not currently part of the calilbrated phases in the MELTS software. Overall, however, the conclusion of this modelling is that crystallization of the observed mineral phases in the Erongo plugs can account for the compositional variations observed, and the changes in crystallization assemblages deduced from the MELTS thermodynamic analysis are consistent with the least-squares modelling within the limitations discussed.
|
The Tristan plume and its role in magma genesis
Past discussions of the role of the Tristan plume in the genesis of flood basalts and mafic intrusions in the South Atlantic have shown that the isotopic composition of the plume mantle in the Early Cretaceous is not well defined. This uncertainty limits any attempt to estimate relative proportions of mantle components and of the crustal contribution to the magma genesis. Ewart et al. (1998) used a value of Nd = +6 for a plume end-member in their interpretation of the Etendeka basalts. Le Roex & Lanyon (1998) estimated a range of Nd values from +1 to -5 for the Tristan plume at 124 Ma, whereas Gibson et al. (1999) favoured a range in Nd from 0 to +10 for the ‘proto-Tristan plume’. The most commonly used reference for the Tristan plume composition is that of OIB from the South Atlantic islands of Gough, Tristan da Cunha and Inaccessible (Le Roex, 1985; Le Roex et al., 1990; Cliff et al., 1991, respectively), which are thought to represent magmas produced by the present-day Tristan hotspot on oceanic crust. We used these OIB data to derive a new estimate for the plume composition at 130 Ma based on isotopic ratios and element concentrations from selected samples that are closest to representing primary magmas based on their MgO and Ni concentrations. From the concentrations of Rb, Sr, Sm, Nd, U, Th and Pb in the selected dataset, we then applied batch melting models to calculate bulk Rb/Sr, Sm/Nd and U–Th/Pb ratios in the mantle source (see Appendix), and applied these ratios to back-calculate the isotopic composition of the ancestral plume. These calculations yielded the following range of values for the Tristan plume at 130 Ma: 87Sr/86Sr = 0·7041–0·7052; 143Nd/144Nd = 0·51241–0·51255 (Nd -1·5 to +1·5), 206Pb/204Pb = 17·94–18·66, 207Pb/204Pb = 15·49–15·59, 208Pb/204Pb = 38·24–39·08.
This estimate for the ancestral Tristan plume composition is shown in Figs 6 and 7 along with compilations of isotope ratios from the Damaraland complexes and the Paraná–Etendeka flood basalts. The Tristan plume signature is an overriding feature of mafic alkaline units from the intrusive complexes in Namibia whereas it is nearly absent in the associated flood basalts, the only exceptions being the Tafelkop-type high-Mg lavas from the base of the Etendeka sequence in Namibia [which may be derived from the Doros complex, according to Marsh et al. (2001)]. Thus, there appears to be a fundamental difference in the mantle sources between the mafic intrusive complexes and coeval flood basalts from the same province. Similar examples of different mantle signatures between continental flood basalts and coeval alkaline centers have been found in other rift settings. For example, flood basalts in the Deccan province show a range of isotope ratios between those of the associated Reunion plume and enriched sub-continental lithospheric mantle (SCLM), whereas contemporary alkaline complexes have more uniform compositions that overlap with the plume range (Simonetti et al., 1998). Similarly, Quaternary alkaline basalts in SW Yemen have isotopic compositions like those of the Afar plume, whereas Oligocene flood basalts from the same region are more varied in composition and suggest a mixed plume–SCLM source (Baker et al., 1997). In general, two explanations for the dominance of a plume signature in alkaline complexes from these settings can be considered. One is that the alkaline magmas are produced by selective, low-degree melting of enriched patches of lithospheric mantle that were metasomatized by fluids or melts from the underlying plume. This is the suggestion favoured in studies of the Deccan and Afar regions cited above, in other studies of continental rifts including the Red Sea (Stein & Hofmann, 1992) and the East African Rift (Späth et al., 2001), and also in some oceanic settings (Grand Comore Island: Class et al., 1998; Kerguelen: Mattielli et al., 1999). The alternative explanation is that the alkaline magmas are derived directly by low-degree melting of plume mantle, as was argued by Bell & Tilton (2001) for the alkaline and carbonatite magmas in the East African Rift.
Le Roex & Lanyon (1998) explained the plume-like isotopic signature of Damaraland lamprophyres and carbonatites in Namibia by a model of plume-induced metasomatism of the lithosphere. They argued that the rising Tristan plume produced metasomatized zones in the lithosphere, which were remelted after the flood basalt event to form late-stage alkaline and carbonatite magmas with a plume-like isotopic signature. However, we suggest that a direct plume origin for the mafic alkaline complexes accounts better for the relative age constraints and petrogenetic models for the regional flood basalts. Available isotopic ages of the Etendeka basalts and Damaraland complexes overlap, but wherever flood basalts are in contact with the complexes, field relations show that the basalts are older (Messum: Korn & Martin, 1954; Erongo: Pirajno, 1990; Brandberg: Schmitt et al., 2000). The prevailing concept for genesis of the Paraná–Etendeka flood basalts, although not universally accepted, implies that they were derived dominantly from the lithospheric mantle, with melting enhanced by heat and volatile flux from the underlying Tristan plume (e.g. Turner et al., 1996; Marques et al., 1999). If so, the flood basalts represent a major episode of partial melting in the lithospheric mantle, and this is likely to have removed any easily fusible metasomatized regions with a distinct plume signature that may have been present.
On the other hand, Garland et al. (1996) and Peate & Hawkesworth (1996) pointed out that the late-stage Esmeralda-type basalts in the Paraná sequence have geochemical signatures indicating a significant sub-lithospheric source. They proposed that the transition from lithospheric to asthenospheric magma sources for the Paraná basalts is due to progressive lithospheric thinning, and cited similar examples from the Ferrar and Siberian provinces. We suggest that this change from lithospheric to sub-lithospheric mantle sources with progressive extension can explain the appearance of plume-derived magmas in the Damaraland complexes and their absence in the main flood basalt sequence. However, the Esmeralda basalts lack a plume-like signature similar to that of the intrusive complexes and instead, the sub-lithospheric component involved in their genesis is thought to have a depleted, MORB-like composition with Nd values of +4 to +8 (Peate & Hawkesworth, 1996). This feature is also shared by the late-stage Horingbaai dykes in Namibia (Hawkesworth et al., 1984; Thompson et al., 2001). An explanation for this difference in source characteristics between the mafic alkaline complexes and the Esmeralda–Horingbaii-type magmas can be different depths of melting in the mantle as shown schematically in Fig. 10. The initial depth of melting for the Erongo basanite magmas, inferred from their major element compositions, is at least 100 km, whereas Garland et al. (1996) estimated a source depth of 50 km for the Esmeralda basalts using the same approach.
|
One observation that disagrees with this simple model of progressive thinning of the lithosphere to explain different mantle sources is that the Tafelkop or ferropicrite lavas from the base of the Etendeka sequence also have a plume-like isotopic signature (Figs 6 and 7), yet they clearly pre-date the main flood basalt event. Gibson et al. (2000) discussed the possible origins of these basalts in the context of peridotite melting models and concluded that they represent 10% partial melting of unusually Fe-rich peridotite ‘streaks’ presumed to be located in the Tristan plume head. Such material would have a lower solidus temperature than more magnesian peridotite and so can be expected to melt earlier, before the large-scale melting of lithospheric mantle dominated the flood basalt system. How these plume melts managed to penetrate the still-thick lithosphere is a matter of speculation but an important factor could be pre-existing weak zones under the Damara fold belt. The Tafelkop-type basalts have a restricted extent near the Doros complex (Marsh et al., 2001), and the NE–SW alignment of the Doros and other Damaraland intrusive complexes (Fig. 1) has long been suggested to reflect local zones of weakness in the Damara crust (e.g. Marsh, 1973; Milner & Le Roex, 1996).
|
CONCLUSIONS |
|---|
|
TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTINGSAMPLE SELECTION AND ANALYTICAL...PETROGRAPHYWHOLE-ROCK COMPOSITIONSSr-Nd-Pb ISOTOPE COMPOSITIONSDISCUSSIONCONCLUSIONSAPPENDIXREFERENCES |
|---|
Geochemical and petrological evidence suggests that basanite–tephrite–phonotephrite intrusive plugs from the Early Cretaceous Erongo igneous complex in Namibia represent a comagmatic series derived from primary basanitic melts of the Tristan mantle plume. Compared with South Atlantic OIB from islands above the Tristan hotspot, which represent plume magmas extruded on oceanic crust, the Erongo plugs have similar initial Sr and Nd isotope ratios but a wider spread in Pb concentrations and higher 208Pb/204Pb and 207Pb/204Pb values, which we attribute to minor contamination by continental crust. Petrogenetic modelling suggests that the parental basanite magmas originated at >100 km depth and that fractional crystallization of olivine, clinopyroxene, spinel and apatite at shallow crustal levels produced the more differentiated tephrite and phonotephrite lithologies.
An estimation of the Sr, Nd and Pb isotopic composition of the ancestral Tristan plume at 130 Ma based on the composition of basalts from the present-day Tristan hotspot reveals a close correspondence to the Erongo plugs and to mafic alkaline units from six other Cretaceous complexes in the Damaraland province of NW Namibia. In contrast, closely associated tholeiitic flood basalts of the Paraná–Etendeka province have a very different isotopic signature and are thought to represent partial melts of metasomatized lithospheric mantle.
The Damaraland complexes are slightly younger than the Etendeka flood basalts in Namibia and we suggest that the contrasting source characteristics between the flood basalts and intrusive complexes are related to differences in timing and location of magma production in the mantle. A previous interpretation of the plume signature in the Damaraland alkaline rocks suggested selective melting of mantle lithosphere that had previously been metasomatized by the Tristan plume (Le Roex & Lanyon, 1998). We argue that the mafic alkaline magmas are better explained by direct derivation from the plume as a result of thermal thinning and extension of the lithosphere during the later stages of igneous activity in the Paraná–Etendeka province. In this scenario, continued extension after generation of the flood basalts from the upper mantle thinned the lithosphere enough to promote melting of the underlying plume, which is consistent with the calculated depth of melt generation for the Erongo basanite.
|
APPENDIX |
|---|
|
TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTINGSAMPLE SELECTION AND ANALYTICAL...PETROGRAPHYWHOLE-ROCK COMPOSITIONSSr-Nd-Pb ISOTOPE COMPOSITIONSDISCUSSIONCONCLUSIONSAPPENDIXREFERENCES |
|---|
The primary composition of Tristan hotspot OIB
Compositional data were selected from published studies of Gough Island, Tristan da Cunha and Inaccessible Island (Table A1). The purpose was to define the composition of near-primary OIB magmas from the Tristan hotspot. The sample selection was made on the basis of MgO–Ni variations after rejecting samples identified in the original sources as being altered or containing foreign material. Following Hart & Davis (1978), primary mantle melts should contain 11–13 wt % MgO and 300–360 ppm Ni. Allowing that fractionation of 10% forsterite-rich olivine has little effect on incompatible trace element concentrations in the melt, we use a range of 8–13 wt % MgO and 120–350 ppm Ni as the discriminant for ‘near-primary’ samples. For the compilation of isotopic ratios (Table A2), which are unaffected by crystal fractionation, we relaxed the ‘near-primary’ discrimination and screened analyses only according to evidence for alteration or near-surface contamination given in the original reference.
|
|
Calculation of the OIB mantle source composition
The trace element composition of the OIB magma source (Rb, Sr, Nd, Sm, Pb, U and Th) was calculated using a batch melt equation and bulk mineral–melt partition coefficients derived from values suggested by Sims & DePaolo (1997) for OIB genesis and the observed degree of element incompatibility relative to the primitive mantle [compilations of Hofmann (1988) and Sun & McDonough (1989)]. To allow for uncertainties in the melting model and partition coefficients, we calculated values for a 10-fold variation in degree of melting (0·5–5%) and the range of bulk distribution coefficents listed in Table A3. The resulting mean values and range of source ratios Rb/Sr, Sm/Nd, Th/Pb, U/Pb, 238U/204Pb and 232Th/238U are also shown in the table. For comparison, Le Roex & Lanyon (1998) gave estimates for the Tristan plume at 124 Ma of Rb/Sr = 0·05, Sm/Nd = 0·2, 238U/204Pb = 15 and 232Th/238U = 4, which are in excellent agreement with our mean values.
|
|
ACKNOWLEDGEMENTS |
|---|
Our sincere thanks go to Franco Pirajno for sharing his observations and insights relating to the alkaline plugs at Erongo, and to Rolf Emmermann and Marcus Wigand for discussions on the geology and geochemistry of the Erongo complex. Heike Rothe and Martin Zimmer are thanked for their help with U–Th–Pb analyses. Fieldwork was financed by the GeoForschungsZentrum Potsdam and aided by co-operation with the Geological Survey of Namibia. Journal reviews by Goonie Marsh and Tony Ewart, and editorial comments by Marjorie Wilson helped us improve the manuscript.
|
FOOTNOTES |
|---|
*Corresponding author: Telephone: +49-331-2881495. Fax: +49-331-2881474. E-mail: bobby{at}gfz-potsdam.de
Present address: Instituto de Geociencias, Universidade de Brasilia, Brasilia DF 70910-900, Brazil
|
REFERENCES |
|---|
|
TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTINGSAMPLE SELECTION AND ANALYTICAL...PETROGRAPHYWHOLE-ROCK COMPOSITIONSSr-Nd-Pb ISOTOPE COMPOSITIONSDISCUSSIONCONCLUSIONSAPPENDIXREFERENCES |
|---|
Albarède, F. (1992). How deep do common basaltic magmas form and differentiate? Journal of Geophysical Research 97, 10997–11009.[CrossRef]
Anders, E. & Grevesse, N. (1989). Abundances of the elements: meteoritic and solar. Geochimica et Cosmochimica Acta 53, 197–214.[CrossRef][Web of Science]
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.[CrossRef]
Arndt, N. T., Czamanske, G. K., Wooden, J. L. & Fedorenko, V. A. (1993). Mantle and crustal contributions to continental flood volcanism. Tectonophysics 223, 39–52.[CrossRef][Web of Science]
Baker, J. A., Menzies, M., Thirlwall, M. F. & Macpherson, C. G. (1997). Petrogenesis of Quaternary intraplate volcanism, Sana’a, Yemen: plume–lithosphere interaction and polybaric melt hybridization. Journal of Petrology 38, 1359–1390.[CrossRef][Web of Science]
Bauer, K., Neben, S., Schreckenberger, B., Emmermann, R., Hinz, K., Fechner, N., Gohl, K., Schulze, A., Trumbull, R. B. & Weber, K. (2000). Deep structure of the Namibia continental margin as derived from integrated geophysical studies—the MAMBA experiment. Journal of Geophysical Research 105, 25829–25853.[CrossRef]
Bell, K. & Tilton, G. R. (2001). Nd, Pb and Sr isotope compositions of East African carbonatites: evidence for mantle mixing and plume inhomogeneity. Journal of Petrology 42, 1927–1945.
Blümel, W. D., Emmermann, R. & Hüser, K. (1979). Der Erongo. Geowissenschaftliche Beschreibung und Deutung eines südwestafrikanischen Vulkankomplexes. Windhoek: Southwest African Scientific Society.
Bryan, W. B., Finger, L. W. & Chayes, F. (1969) Estimating proportions in petrographic mixing equations by least-squares approximation. Science 163, 926–927.
Class, C., Goldstein, S. L., Altherr, R. & Bachélery, P. (1998). The process of plume–lithosphere interaction in the ocean basins—the case of Grande Comore. Journal of Petrology 39, 881–903.[CrossRef][Web of Science]
Cliff, R. A., Baker, P. E. & Bateer, N. J. (1991). Geochemistry of Inaccessible Island volcanics. Chemical Geology 92, 251–260.[CrossRef][Web of Science]
Eberle, D. & Hutchins, D. G. (1996). Compilation of the Namibian airborne magnetic surveys. Procedures, problems and results. Journal of African Earth Science 22, 191–205.[CrossRef]
Ellam, R. M. (1992). Lithospheric thickness as a control on basalt geochemistry. Geology 20, 153–156.
Ewart, A., Milner, S. C., Armstrong, R. A. & Duncan, A. R. (1998). Etendeka volcanism of the Goboboseb Mountains and Messum Igneous Complex, Namibia. Part I: Geochemical evidence of early Cretaceous Tristan plume melts and the role of crustal contamination in the Paraná–Etendeka CFB. Journal of Petrology 39, 191–225.
Garland, F., Turner, S. & Hawkesworth, C. (1996). Shifts in the source of the Paraná basalts through time. Lithos 37, 223–243.[CrossRef][Web of Science]
Ghiorso, M. S. & Sack, R. O. (1995). Chemical mass transfer in magmatic processes IV. A revised and internally consistent thermodynamic model for the interpolation and extrapolation of liquid–solid equilibria in magmatic systems at elevated temperatures and pressures. Contributions to Mineralogy and Petrology 119, 197–212.[Web of Science]
Gibson, S. A., Thompson, R. N., Dickin, A. P. & Leonardos, O. H. (1996). High-Ti and low-Ti mafic potassic magmas: key to plume–lithosphere interactions and continental flood basalt genesis. Earth and Planetary Science Letters 141, 325–341.[CrossRef][Web of Science]
Gibson, S. A., Thompson, R. N., Leonardos, O. H., Dickin, A. P. & Mitchell, J. G. (1999). The limited extent of plume–lithosphere interactions during continental flood-basalt genesis: geochemical evidence from Cretaceous magmatism in southern Brazil. Contributions to Mineralogy and Petrology 137, 147–169.[CrossRef][Web of Science]
Gibson, S. A., Thompson, R. N. & Dickin, A. P. (2000). Ferropicrites: geochemical evidence for Fe-rich streaks in upwelling mantle plumes. Earth and Planetary Science Letters 174, 355–374.[CrossRef][Web of Science]
Haase, K. M. (1996). The relationship between the age of the lithosphere and the composition of oceanic magmas: constraints on partial melting, mantle sources and the thermal structure of the plates. Earth and Planetary Science Letters 144, 75–92.[CrossRef][Web of Science]
Harris, C. (1995). Oxygen isotope geochemistry of the Mesozoic anorogenic complexes of Damaraland, northwest Namibia: evidence for crustal contamination and its effect on silica saturation. Contributions to Mineralogy and Petrology 122, 308–321.[CrossRef][Web of Science]
Harris, C., Marsh, J. S. & Milner, S. C. (1999). Petrology of the alkaline core of the Messum igneous complex, Namibia: evidence for the progressively decreasing effect of crustal contamination. Journal of Petrology 40, 1377–1397.[CrossRef][Web of Science]
Hart, S. R. (1988). Heterogeneous mantle domains: signatures, genesis and mixing chronologies. Earth and Planetary Science Letters 90, 272–296.
Hart, S. R. & Davis, K. E. (1978). Nickel partitioning between olivine and silicate melt. Earth and Planetary Science Letters 40, 203–219.[CrossRef][Web of Science]
Hawkesworth, C. J., Marsh, J. S., Duncan, A. R., Erlank, A. J. & Norry, M. J. (1984). The role of continental lithosphere in the generation of the Karoo volcanic rocks: evidence from combined Nd- and Sr-isotope studies. Geological Society of South Africa Special Publication 13, 341–354.
Hawkesworth, C. J., Kelley, S., Turner, S., Le Roex, A. & Storey, B. (1999). Mantle processes during Gondwana break-up and dispersal. Journal of African Earth Science 28, 239–261.[CrossRef]
Hofmann, A. W. (1988). Chemical differentiation of the Earth: the relationship between mantle, continental crust and oceanic crust. Earth and Planetary Science Letters 90, 297–314.[CrossRef][Web of Science]
Jaques, A. L. & Green, D. H. (1980). Anhydrous melting of peridotite at 0–15 kb pressure and the genesis of tholeiitic basalts. Contributions to Mineralogy and Petrology 73, 287–310.[CrossRef][Web of Science]
Jung, S., Mezger, K. & Hoernes, S. (2001). Trace element and isotopic (Sr, Nd, Pb, O) arguments for a mid-crustal origin of Pan-African garnet-bearing S-type granites from the Damara Orogen (Namibia). Precambrian Research 110, 325–355.[CrossRef][Web of Science]
Klein, E. M. & Langmuir, C. H. (1987). Global correlations of ocean ridge basalt chemistry with axial depth and crustal thickness. Journal of Geophysical Research 92, 8089–8115.
Korn, H. & Martin, H. (1954). The Messum igneous complex in South West Africa. Transactions of the Geological Society of South Africa 57, 83–124.
Kushiro, I. (1968). Composition of magmas formed by partial zone melting of the Earth’s mantle. Journal of Geophysical Research 73, 619–634.
Kushiro, I. (1996). Partial melting of a fertile mantle peridotite at high pressure: an experimental study using aggregates of diamond. In: Basu, A. & Hart, S. (eds) Earth Processes: Reading the Isotopic Clock. Geophysical Monograph, American Geophysical Union 95, 109–122.
Langmuir, C. H., Klein, E. M. & Plank, T. (1992). Petrological systematics of mid-ocean ridge basalts: constraints on melt generation beneath ocean ridges. In: Morgan, J. P., Blackman, D. K. & Sinton, J. M. (eds) Mantle Flow and Melt Generation at Mid-Ocean Ridges. Washington, DC: American Geophysical Union, pp. 183–280.
Le Maitre, R. W., Bateman, P., Dudek, A. & Keller, J. (1989). A Classification of Igneous Rocks and Glossary of Terms: Recommendations of the International Union of Geological Sciences Subcommission on the Systematics of Igneous Rocks. Oxford: Blackwell Scientific.
Le Roex, A. P. (1985). Geochemistry, mineralogy and magmatic evolution of the basaltic and trachytic lavas from Gough Island, South Atlantic. Journal of Petrology 26, 149–186.
Le Roex, A. P. & Lanyon, R. (1998). Isotope and trace element geochemistry of Cretaceous Damaraland lamprophyres and carbonatites, Northwest Namibia: evidence for plume–lithosphere interactions. Journal of Petrology 39, 1117–1146.
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.
Marques, L. S., Dupré, B. & Piccirillo, E. M. (1999). Mantle source compositions of the Paraná Magmatic Province (southern Brazil): evidence from trace element and Sr–Nd–Pb isotope geochemistry. Journal of Geodynamics 28, 439–458.[CrossRef][Web of Science]
Marsh, J. S. (1973) Relationships between transform directions and alkaline igneous rock lineaments in Africa and South America. Earth and Planetary Science Letters 18, 317–323.[CrossRef][Web of Science]
Marsh, J. S., Ewart, A., Milner, S. C., Duncan, A. R. & Miller, R. McG. (2001). The Etendeka Igneous Province: magma types and their stratigraphic distribution with implications for the evolution of the Paraná–Etendeka flood basalt province. Bulletin of Volcanology 62, 464–486.[CrossRef][Web of Science]
Martin, H., Mathias, M. & Simpson, E. W. S. (1960). The Damaraland sub-volcanic ring complexes in South West Africa. Report of the 21st International Geologic Congress, Copenhagen, Part 13, pp. 156–174.
Martinez, I. A., Harris, C., LeRoex, A. P. & Milner, S. C. (1996) Oxygen isotope evidence for extensive crustal contamination in the Okenyenya complex, Namibia. Geochimica et Cosmochimica Acta 60, 4497–4508.[CrossRef][Web of Science]
Mattielli, N., Weis, D., Scoates, J. S., Shimizu, N., Mennessier, J.-P., Grégoire, M., Cottin, J.-Y. & Giret, A. (1999). Evolution of heterogeneous lithospheric mantle in a plume environment beneath the Kerguelen Archipelago. Journal of Petrology 40, 1721–1744.[CrossRef][Web of Science]
McDermott, F. & Hawkesworth, C. J. (1990). Intracrustal recycling and upper-crustal evolution: a case study from the Pan-African Damara mobile belt, central Namibia. Chemical Geology 83, 263–280.[CrossRef][Web of Science]
McKenzie, D. & O’Nions, R. K. (1991). Partial melt distributions from inversion of rare earth element concentrations. Journal of Petrology 32, 1021–1091.
Milner, S. C. (ed.) (1997). Geological Map of Namibia Sheet 2114—Omaruru, 1:250 000. Windhoek: Ministry of Mines and Energy, Geological Survey of Namibia.
Milner, S. C. & Le Roex, A. P. (1996). Isotope characteristics of the Okenyenya igneous complex northwestern Namibia: constraints on the composition of the early Tristan plume and the origin of the EM1 mantle component. Earth and Planetary Science Letters 141, 277–291.[CrossRef][Web of Science]
Milner, S. C., Le Roex, A. P. & O’Connor, J. M. (1995). Age of Mesozoic igneous rocks in northwestern Namibia and their relationship to continental breakup. Journal of the Geological Society, London 152, 97–104.
Mingram, B., Trumbull, R. B., Littmann, S. & Gerstenberger, H. (2000). A petrogenetic study of anorogenic felsic magmatism in the Cretaceous Paresis ring complex, Namibia: evidence for crust–mantle hybridization. Lithos 54, 1–22.[CrossRef][Web of Science]
Patel, S. (1988) The petrology of the alkali intrusive rocks from the Erongo Volcanic Complex. B.Sc. thesis, Rhodes University, Grahamstown, South Africa.
Peate, D. W. (1997). The Paraná–Etendeka Province. In: Mahoney, J. J. & Coffin, M. F. (eds) Large Igneous Provinces. Washington, DC: American Geophysical Union, pp. 217–246.
Peate, D. W. & Hawkesworth, C. (1996). Lithospheric to asthenospheric transition in low-Ti flood basalts from southern Paraná, Brazil. Chemical Geology 127, 1–24.[CrossRef][Web of Science]
Pirajno, F. (1990). Geology, geochemistry and mineralization of the Erongo Volcanic Complex, Namibia. South African Journal of Geology 93, 485–504.[Abstract]
Pirajno, F., Phillips, D. & Armstrong, R. A. (2000). Volcanology and eruptive histories of the Erongo volcanic complex and the Paresis igneous complex, Namibia: implications for mineral deposit styles. Communications of the Geological Survey of Namibia 12, 301–312.
Pouchou, J. L. & Pichoir, F. (1984). Un nouveau modèle de calcul pour la microanalyse quantitative par spectrométrie de rayons X. La Recherche Aérospatiale 3, 167–192.
Renne, P. R., Glen, J. M., Milner, S. C. & Duncan, A. R. (1996). Age of Etendeka flood volcanism and associated intrusions in southwestern Africa. Geology 24, 659–662.
Ridley, W. I. (1970). The petrology of the Las Canadas volcanoes, Tenerife, Canary Islands. Contributions to Mineralogy and Petrology 26, 124–160.[CrossRef][Web of Science]
Romer, R. L., Förster, H.-J. & Breitkreuz, C. (2001). Intracontinental extensional magmatism with a subduction fingerprint: the late Carboniferous Halle Volcanic Complex (Germany). Contributions to Mineralogy and Petrology 141, 201–221.[Web of Science]
Sack, R. C., Walker, D. & Carmichael, I. S. E. (1987). Experimental petrology of alkalic lavas: constraints on cotectics of multiple saturation in natural basic liquids. Contributions to Mineralogy and Petrology 96, 1–23.[CrossRef][Web of Science]
Saunders, A. D., Storey, M., Kent, R. W. & Norry, M. J. (1992) Consequences of plume–lithosphere interaction. 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.
Scarrow, J. H. & Cox, K. G. (1995) Basalts generated by decompressive adiabatic melting of a mantle plume: a case study from the Isle of Skye, NW Scotland. Journal of Petrology 36, 3–22.
Schmitt, A. K., Emmermann, R., Trumbull, R. B., Bühn, B. & Henjes-Kunst, F. (2000). Petrogenesis and 40Ar/39Ar geochronology of the Brandberg complex, Namibia: evidence for a major mantle contribution in metaluminous and peralkaline granites. Journal of Petrology 41, 1207–1239.
Simonetti, A., Goldstein, S. L., Schmidberger, S. S. & Viladkar, S. G. (1998). Geochemical and Nd, Pb, and Sr isotope data from Deccan alkaline complexes—inferences for mantle sources and plume–lithosphere interaction. Journal of Petrology 39, 1847–1864.[CrossRef][Web of Science]
Sims, K. W. W. & DePaolo, D. J. (1997). Inferences about mantle magma sources from incompatible element concentration ratios in oceanic basalts. Geochimica et Cosmochimica Acta 61, 765–784.[CrossRef][Web of Science]
Späth, A., Le Roex, A. P. & Opiyo-Akech, N. (2001). Plume–lithosphere interaction and the origin of continental rift-related alkaline volcanism—the Chyulu Hills volcanic province, Southern Kenya. Journal of Petrology 42, 765–787.
Stein, M. & Hofmann, A. (1992). Fossil plume head beneath the Arabian lithosphere? Earth and Planetary Science Letters 114, 193–209.[CrossRef][Web of Science]
Sun, S. S. (1980). Lead isotopic study of young volcanic rocks from mid-ocean ridges, ocean islands and island arcs. Philosophical Transactions of the Royal Society of London, Series A 297, 409–455.[CrossRef]
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.
Thompson, R. N., Gibson, S. A., Dickin, A. P. & Smith, D. P. (2001). Early Cretaceous basalt and picrite dykes of the southern Etendeka region, NW Namibia: windows into the role of the Tristan mantle plume in Paraná–Etendeka magmatism. Journal of Petrology 42, 2049–2081.
Trumbull, R. B., Emmermann, R., Bühn, B., Gerstenberger, H., Mingram, B., Schmitt, A. & Volker, F. (2000). Insights on the genesis of the Cretaceous Damaraland igneous complexes in Namibia: the Nd- and Sr- isotopic perspective. Communications of the Geological Survey of Namibia 12, 313–324.
Turner, S. P., Hawkesworth, C. J., Gallagher, K., Stewart, K., Peate, D. W. & Mantovani, M. S. 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.[CrossRef]
Weaver, B. L., Wood, D. A., Tarney, J. & Joron, J. L. (1987). Geochemistry of ocean island basalts from the South Atlantic: Ascension, Bouvet, St. Helena, Gough and Tristan da Cunha. In: Fitton, J. G. & Upton, B. G. J. (eds) Alkaline Igneous Rocks. Geological Society, London, Special Publications 30, 253–267.
White, W. M. & Hofmann, A. W. (1982). Sr and Nd isotope geochemistry of oceanic basalts and mantle evolution. Nature 296, 821–825.[CrossRef]
Wigand, M., Trumbull, R. B. & Emmermann, R. (2001). Crust and mantle contributions to anorogenic magmatism: a case study of the Cretaceous Erongo Complex, Namibia. EUG XI. Journal of Conference Abstracts 6(1), 474.
Zuleger, E. & Erzinger, J. (1988). Determination of REE and Y in silicate minerals with ICP-AES. Fresenius Zeitschift für Analytische Chemie 332, 140–143.[CrossRef]
CiteULike 
Connotea 
Del.icio.us What's this?
This article has been cited by other articles:
|
|
 |
|
  R. B. Trumbull, D. L. Reid, C. de Beer, D. van Acken, and R. L. Romer Magmatism and continental breakup at the west margin of southern Africa: A geochemical comparison of dolerite dikes from northwestern Namibia and the Western Cape South African Journal of Geology, September 1, 2007; 110(2-3): 477 - 502. [Abstract] [Full Text] [PDF]
|
|
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||