SrNdPb isotopic and trace element evidence for crustal contamination of plume-derived flood basalts: Oligocene flood volcanism in western Yemen

Geochimicaet CosmochimicaActa, Vol. 60, No. I4, pp. 2559-2581, 1996 Copyright© 1996 ElsevierScience Ltd Printed in the USA. All rights reserved 0016-7037/96 $15.00 + .00

Pergamon

P I I S0016-7037(96) 00105-6

Sr-Nd-Pb isotopic and trace element evidence for crustal contamination of plume-derived flood basalts: Oligocene flood volcanism in western Yemen J. A. BAKER, M. F. THIRLWALL,and M. A. MENZIES Department of Geology, Royal Holloway University of London, Egham, Surrey TW20 0EX, UK (Received May 11, 1994; accepted in revisedform March 26, 1996)

A b s t r a c t - - O l i g o c e n e flood basalts from western Yemen have a relatively limited range in initial isotopic composition compared with other continental flood basalts: 875r/86Sr = 0.70365-0.70555; 143Nd/J44Nd = 0.51292-0.51248 (eN~ = +6.0 to - 2 . 4 ) ; 2°6pb/2°4pb = 17.9-19.3. Most compositions lie outside the isotopic ranges of temporally and spatially appropriate mantle source compositions observed in this area, i.e., Red Sea/Gulf of Aden MORB mantle, the Afar plume, and Pan-African lithospheric mantle. Correlations between indices of fractionation, silica, and isotope ratios suggest that crustal contamination has substantially modified the primary isotopic and incompatible trace element characteristics of the flood basalts. However, significant scatter in these correlations was produced by: (a) the heterogeneous isotopic composition of Pan-African crust; (b) the difference in susceptibility of magmas to contamination as a result of variable incompatible trace element contents in primary melts produced by differing degrees of partial melting; (c) the presence or absence of plagioclase as a fractionating phase generating complex contamination trajectories for Sr; (d) sampling over a wide area not representing a single coherent magmatic system; and (e) variation in contamination mechanisms from assimilation associated with fractionation (AFC) to assimilation by hot mafic magmas with little concomitant fractionation. The presence of plagioclase as a fractionating phase in some suites that were undergoing AFC requires assimilation to have taken place within the crust and, coupled with the limited LREE-enrichment accompanying isotopic variations, excludes the possibility that an AFC-type process took place during magma transfer through the lithospheric mantle. Isotopic compositions of some of the inferred crustal assimilants are similar to those postulated by other workers for an enriched lithospheric mantle source of many flood basalts in southwestern Yemen, Ethiopia, and Djibouti. The western Yemen flood basalts contain 0 - 3 0 % crust which largely swamps their primary lead isotopic signature, but the primary SrNd isotopic signature is close to that of the least contaminated and isotopically most depleted flood basalts. LREE/HFSE and LILE/HFSE ratios also correlate with isotopic data as a result of crustal contamination. However, Nb/La and K/Nb ratios of > 1.1 and < 150, respectively, in least contaminated samples require an OIB-like source. The pre-contamination isotopic signature is estimated to be: STSr/86Sr 0.7036; J43Nd/144Nd - 0.51292; 2°6pb/2°4pb ~ 18.4-19.0. This, coupled with low LILE/HFSE ratios, suggest the source has characteristics akin to the Afar plume. A mantle source isotopically more depleted than Bulk Earth, but not as depleted as MORB, coupled with LILE depletion, also characterises other examples of plume-derived flood volcanism. This mantle reservoir is responsible for the second largest outbursts of volcanism on Earth and has radiogenic isotopic characteristics akin to PREMA mantle, but the incompatible trace element signature of HIMU mantle. 1. INTRODUCTION AND REGIONAL SETTING

have demonstrated that preserved flood volcanism in western Yemen was erupted between 31 and 26 Ma (Baker et al., 1996). A minimum emplacement rate of ca. 0.03 km3/yr is obtained for flood volcanism at the Afro-Arabian triple junction, given that these dates are based on whole rock KAr rather than 4°Ar/39Ar age data, which are not yet available for the Ethiopian part of the province, and that the volume estimate does not take into account intrusive and underplated igneous material. This rate is similar in magnitude to minimum estimates for the Columbia River basalt province (0.1 km3/yr; Coffin and Eldholm, 1993) and the Hawaiian island chain (0.16 krn3/yr; Watson and McKenzie, 1989), but orders of magnitude less than the larger examples of flood volcanism, e.g., Deccan T r a p s - - 2 - 8 km3/yr; Ontong-Java oceanic plateau--22 km3/yr (Coffin and Eldholm, 1993). The voluminous magmatism produced in Ethiopia, Djibouti, and Yemen is considered to be related to an upwelling, anomalously hot, mantle plume (e.g., Vidal et al., 1991).

The Afro-Arabian triple junction, located at the apex of the Red Sea and Gulf of Aden rifts, is the site of the youngest continental flood basalt province that is associated with nascent oceanic basins. Approximately 350,000 km 3 of basalt and rhyolitic pyroclastic rocks were erupted in Ethiopia, Eritrea, Djibouti, and Yemen (Fig. 1 ) prior to oceanisation of the rifts (Mohr and Zanettin, 1988). Subsequent smaller volume, and more alkaline volcanism, has persisted in more localised rift zones of Ethiopia, Eritrea, and Djibouti, and in discrete fields on the Arabian peninsula, including Yemen (e.g., Camp and Roohol, 1992). Whole rock K-Ar dating suggests that the bulk of the main flood volcanic phase in Yemen was erupted from 31-18 Ma, although volcanism may have commenced as early as 40 Ma (Civetta et al., 1978; Menzies et al., 1990; Manetti et al., 1991 ; A1-Kadasi, 1995). More recent 4°Ar/39Ar studies 2559

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FIG. 1. Simplified geological map of western Yemen (modified after Davison et al., 1994) illustrating the location of the four stratigraphic sections from which samples were collected: (1) Wadi Lahima/Jabal Hufash; (2) Sana'a; (3) northeast Sana'a and; (4) Jihana (south Sana'a). Section 1 is from a region that underwent significant crustal extension immediately after volcanism, located in the Yemen rift mountains. Sections 2, 3, and 4 are from unextended terrain on the shoulder of the rifted region.

The lack of extensive flood volcanic rocks along the rest of the Red Sea/Gulf of Aden rift provides qualitative evidence for the role of a mantle plume as a heat source or melt source during generation of flood volcanism at the Afro-Arabian triple junction. Volcanism associated with lithospheric extension, such as flood volcanism, is now widely considered to be a product of decompression melting (McKenzie and Bickle, 1988; White and McKenzie, 1989). However, it is unclear whether the mantle material undergoing melting is plume material, entrained asthenosphere within the plume head, or lithospheric mantle (LM). Melt generation within subductionmodified LM has been advocated in order to explain the common continental signature of flood basalts (i.e., low 143Nd/14ZNd and N b / L a ratios; e.g., Hawkesworth et al., 1984, 1988). As the LM is considered to be cold and refractory, proponents of LM melt generation have suggested that hydration of the LM, during past subduction events, effectively lo'wers the solidus permitting large degrees of melting at realistic mantle temperatures (Gallagher and Hawkesworth, 1992). Alternatively, incompatible-element-enriched small melt fractions from the LM (lamproites) may contaminate asthenospheric melts and dominate their isotopic and incompatible trace element systematics (EUam and Cox, 1991).

Yemen is an ideal place to study the relative contributions of mantle source regions to flood magmatism. Recent study of dredge basalts from the Gulf of Aden and Red Sea spreading ridges (Schilling et al., 1992; Volker et al., 1993) has defined the isotopic compositions of local depleted MORB mantle and also an isotopically less depleted mantle component, the Afar mantle plume, which coincides with a bathymetric high on the Gulf of Aden ridge. Work by Vidal et al. (1991) and Deniel et al. (1994) on Oligocene (25 Ma) to Recent basalts erupted in Djibouti, close to the proposed current location of the Afar plume, has also helped to define the composition of the Afar plume, although older basalts were thought to be derived from ancient and isotopically heterogeneous LM. Menzies and Murthy (1980) documented enriched neodymium isotopic compositions for clinopyroxenes separated from LM mantle xenoliths entrained in intraplate volcanoes from southwestern Yemen (Ataq). However, new Sr-Nd-Pb isotopic analysis of fifteen Ataq mantle xenoliths, including some of the same samples studied by Menzies and Murthy (1980), has revealed that no mantle clinopyroxenes have enriched isotopic signatures, i.e., ~43Nd/144Nd -- 0.5125 (J. A. Baker, unpubl, data). Acidleached clinopyroxene and pargasite from spinel ± pargasite peridotite xenoliths from Ataq and Kod Ali (southern Red Sea island) all have 87Sr/86Sr < 0.7036 and t43Nd/144Nd

Oligocene flood volcanism in western Yemen

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Fir. 2. Schematic west to east variation of the Oligocene flood volcanic stratigraphy in western Yemen. Locations of the three stratigraphic sections are shown in Fig. 1. Samples were collected from these three sections whose basal basalt flows yield 4°Ar/39Arages of 29-31 Ma (Baker et al., 1996), and an additional section to the south of Sana'a (Jihana; basal basalt 4°Ar/39Arage = 30.4 Ma) which is not illustrated in this figure.

= 0.51290 _+ 3 (J. A. Baker et al., unpubl, data). These values are consistent with the generally depleted Sr-Nd isotopic ratios reported for LM samples entrained in Saudi Arabian alkali basalts (Henjes-Kunst et al., 1990; Blusztajn et al., 1995), and all but one sample from the ultramafic body of Zabargad Island (Brueckner et al., 1988). We present here major and trace element data and Sr-NdPb isotopic data for Oligocene flood basalts from western Yemen which is at the northern periphery of the Ethiopian/ Yemen flood basalt province (Fig. 1). We identify an OIBor plume-like mantle component in primitive compositions that seems to be a pervasive feature of volcanic rocks erupted in this region. However, we present strong evidence that most magmas have interacted with Pan-African continental crust en route to the surface masking their primary isotopic and trace element signatures. In contrast, Chazot and Bertrand (1993), like Vidal et al. (1991), Deniel et al. (1994), and Hart et al. (1989) in studies of Djiboutian and Ethiopian volcanism, concluded that most of the isotopic heterogeneity in flood basalts from southwestern Yemen was a function of mixing between asthenospheric- or plume-derived magmas with isotopically enriched LM-derived magmas (high 87Sr/865r and low J43Nd/fa4Nd ratios). Interestingly, the samples of Vidal et al. ( 1991 ), Deniel et al. (1994), Hart et al. (1989), and Chazot and Bertrand ( 1993 ) that have supposed enriched LM signatures are isotopically similar to some of our inferred crustal contaminants. Yet again, the relative roles of crust and LM remains a contentious issue in a continental flood basalt province. 2. VOLCANISM IN WESTERN YEMEN The Oligocene flood volcanic stratigraphy of western Yemen is schematically illustrated in Fig. 2. The volcanic rocks were erupted through Pan-African lithosphere, a com-

plex mosaic of Late Proterozoic to Archean basement terranes accreted ca. 600 Ma ago, onto a thin ( < 3 km) cover of Jurassic limestone and Cretaceous-Paleocene arkosic sandstones (Fig. 1 ). The sedimentary rocks immediately underlying the flood volcanic rocks record an upward change from marginal marine sedimentation to subaerial lateritic paleosols, suggesting sediment starvation and, in turn, prevolcanic surface uplift. From east (rift shoulder) to west (rift margin) there is a systematic change in the thickness and character of the volcanic pile. In the west, the pile is at its thickest (>1500 m) and comprises a thick monotonous sequence of basal basalt flows overlain by a complex section of rhyolitic and basaltic pyroclastic rocks. Moving to the east, the volcanic pile thins substantially from 1500 m near Sana'a to less than 300 m about 50 km northeast of Sana'a. Here, only a thin sequence of basal basalt flows are present, overlain by spectacular welded rhyolitic ignimbrite and associated airfall tuff units, in turn unconformably overlain by a younger series of intraplate basalts. The rhyolitic pyroclastic rocks thicken and coarsen towards the west, consistent with eruption from caldera centres marked by unroofed A-type granite plutons near the Red Sea rift margin (Fig. 1; Capaldi et al., 1987). 4°mr/ 39mr dating indicates that magmatism commenced between 31 and 29 Ma at the four stratigraphic sections considered in this paper, and that all the basaltic rocks were erupted in a period < 2 - 3 myr (Baker et al., 1996). Each stratigraphic section is a composite of several sections logged and sampled over an area of ca. 10 km 2 at each locality. Basaltic flows vary markedly in thickness from ca. 1 m to >50 m. Most flows are simple tabular units, although in the east there are lateral thickness variations indicative of ponding within a subdued volcanic topography. The largest flows have been traced in the field only over distances of

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ca. 5 km, although individual flows have been traced over distances of >50 km on the basis of petrography and major and trace element chemistry. Evidence for time gaps and erosional periods between basaltic eruptions are rare although small lenses of lacustrine volcaniclastic sediments are occasionally present between rhyolitic pyroclastic units. No angular unconformities exist in the volcanic pile, requiring upper crustal extension in this part of Yemen to postdate flood volcanism. Sampling sought to provide stratigraphically evenlyspaced samples and to document the full variety of lithologies present in each section. This paper presents and discusses geochemical data of the basaltic samples split into four geographical groupings: Wadi Lahima/Jabal Hufash (rift margin); Sana'a; northeast Sana'a; and Jihana (south Sana'a) (rift shoulder). The flood basalts are typically olivine + clinopyroxene -+ plagioclase _+ Fe-Ti oxide-phyric with phenocrysts set in a holocrystalline to hypocrystalline groundmass. Alteration of samples is minor with olivine often still in pristine condition. Mafic samples are dominated by olivine and clinopyroxene phenocrysts with plagioclase (and minor Fe-Ti oxides) joining the fractionating assemblage at lower MgO contents (ca. 6 wt% MgO). Crustal xenoliths have not been identified in any of the basalt flows.

3. GEOCHEMISTRY 3.1. Analytical Techniques Major and trace element and Sr-Nd-Pb isotopic data for fifty-two basaltic samples are presented in Table 1. Major and most trace element data were determined on fused glass discs and pressed powder pellets, respectively, by XRF spectrometry at Royal Holloway University of London (RHUL). Analytical reproducibility of presented XRF major and trace element data presented or used in Table 1 is better than ( -+2 sd): SiO2 -+ 0.3 wt%; A1203 -+ 0.1 wt%; Fe203 -+ 0.05 wt%; MgO _+ 0.10 wt%; CaO _+ 0.05 wt%; Na20 _+ 0.1 wt%; K20 -+ 0.004 wt%; TiO2 _+ 0.01 wt%; MnO _+ 0.01 wt%; PeOs _+ 0.01 wt%; Ni _+ 1 ppm; Cr _+ 1.5 ppm; V _+ 1.5 ppm; Sc _+ 1 ppm; Cu -+ 1.5 ppm; Zn _+ 1 ppm; CI _+ 20 ppm; Ga _+ 1 ppm; Pb _+ 0.5 ppm; Sr _+ 2 ppm; Rb -+ 0.5 ppm; Ba -+ 5 ppm; Zr -+_ 1 ppm; Nb _+ 0.5 ppm; Th _+ 0.4 ppm; Y _+ 0.6 ppm; La _+ 2 ppm; Ce -+ 2 ppm; Nd -+ 1 ppm. At high trace element concentration levels reproducibility increases to _+1%. Lead and thorium were determined using extended count times and calibrated against a set of international and in-house standards with Pb and Th determined by isotope dilution (Pb) and neutron activation (Th) analysis. Samarium, neodymium (and lanthanum and cerium), and uranium, which were used to age correct isotopic data, were determined by thermal ionisation mass spectrometric isotope dilution (ID) analysis, with <0.1% reproducibility on Srn/ Nd ratios and < 1% reproducibility on U contents. Srn/Nd ratios of nineteen samples for which no REE ID data were available for were estimated from a correlation constructed between La/Nd and Srn/Nd for samples where ID data were available. For the seven samples that U ID data were also not available, the U content used in age correcting samples was estimated by assuming Th/U = 4.

Strontium, neodymium, and lead isotopic data were determined, after conventional chemical separation, on a VG354 multicollector mass spectrometer at RHUL. Strontium and neodymium isotope analyses were determined in multidynamic mode as outlined in Thirlwall ( 1991 ). In-run internal precision of strontium and neodymium isotope analyses was better than _+0.000012 and _+0.000006, respectively (2 se). External precision or reproducibility of Sr and Nd data is better than _+0.000018 and _+0.000013 (n > 50; 2 sd) and ratios are reported relative to values of 0.710250 for SRM987 and 0.511424 for an in-house laboratory Nd standard. This neodymium isotopic ratio corresponds to 0.511860 and 0.512638 for the international standards La Jolla and BCR-1, respectively. Lead isotopic data, collected in static multicollector mode, were normalised for mass fractionation by comparison with repeated analyses of SRM981. External precision or reproducibility of SRM981 shows the reproducibility of sample lead isotope analyses is ca. -+0.010, _+0.012 and _+0.030 (2 sd) for 2°6pb/2°4pb, 2°7pb/2O4pb, and 2°8pb/2°4pb, respectively, significantly more than the typical within-run precision (2 se) of -+0.004, _+0.004, and _+0.007. Sample powders for strontium isotope analysis were leached in hot 6 M HC1 for one hour and repeatedly rinsed in ultraclean water prior to dissolution. This leaching procedure removes most of the Rb-bearing material (i.e., groundmass) from the powder leaving a residue of clinopyroxene and occasionally plagioclase. As both phases would have Rb/Sr 0.1 the strontium isotopic ratio measured on this residue in these relatively young rocks ( < 3 0 Ma) can be considered to be the initial ratio. Differences between 87Sr/86Sr ratios measured on the same sample, leached and unleached, typically approximated the relevant age correction. Similar experiments to evaluate the effect of leaching on the Nd-Pb isotopic composition of samples revealed little difference between leached and unleached analyses. Hence, all Nd-Pb isotope analyses are of unleached samples. Total procedural blanks for the Sr, Nd, and Pb separation procedure were ca. 1.5, 0.3, and 0.5 ng, respectively, and are negligible. Neodymium and lead isotopic data are age corrected based on 4°Ar/39Ar ages determined for representative samples from the same stratigraphic sections. The Nd correction is small (<0.000025) and rather insensitive to the age and small variations in Sm/Nd ratios exhibited by these samples. The Pb correction is less precise and is largely dependent on the quality of long-counting-time Pb and Th XRF data. Significant differences in U/Pb (0.07-0.87) and Th/Pb (0.42-3.8) ratios exist between different samples that warrant age correcting Pb data, but unfortunately relative uncertainties in U/Pb and Th/Pb ratios are sometimes ca. _+25% and -+40%, respectively. Age corrections to Pb data were 0.019-0.24 for 2°rpb/2°4pb, 0.04-0.36 for 2°spb/2°4pb, and insignificant for 2°vpb/2°4pb (~0.01). Calculated initial 2°rpb/2°4pb and 2°spb/2°4pb isotope ratios thus are about two or three times less precise than measured data, although the uncertainty is different for each sample depending on the Pb and Th contents. All the western Yemen isotopic data presented in figures throughout this paper are calculated initial compositions. Comparative isotopic data are not age corrected as full elemental data are not available for these sam-

Oligocene flood volcanism in western Yemen pies (i.e., Sm, Nd, Th, U, and Pb). Source references for comparative data are given in figure captions.

14

3.2. Major Element Data

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The most striking compositional feature of the volcanic pile is the bimodality with respect to SiO= (Fig. 3a). Of the >100 samples of all types analysed as part of this study only five have silica contents between 55 and 68 wt% SiO2. However, herein we restrict our discussion to the trace element and isotopic systematics of the basaltic rocks, although isotopic data for andesitic and rhyolitic samples are annotated on some figures. From the total alkalis-silica classification diagram (TAS; Fig. 3b) it is apparent that the western Yemen flood basalts comprise a variety of basaltic rock types (i.e., basanite, basalt sensu stricto, hawaiite, basaltic andesite, and basaltic trachyandesite), although most samples straddle the subalkalinealkaline divide in the basalt field. Samples are initially divided into four groups on the basis of geographic location, and each of these four groups is further subdivided into a lower (LS; open symbols) and upper series (US; filled symbols ) based on ratios of very incompatible (VICE) to moderately incompatible (MICE) trace elements (Baker, 1996). The US / L S transition is marked by a sharp change in VICE/ MICE ratios (Fig. 4; e.g., C e / Y and Nb/Zr) with the US having higher VICE/MICE ratios. In the case of the three sections close to Sana'a, this subdivision also corresponds to the change from exclusively basaltic to bimodal flood volcanism. While some of the scatter in the TAS plot is a function of alteration and crystal accumulation, it is clear that some of the US rocks are also more silica-undersaturated than the LS. MgO contents vary from 14-3 wt% in the basaltic rocks, but most samples have < 8 wt% (Fig. 3c) and have apparently undergone crystal fractionation of ol + cpx ± plag ± Fe-Ti oxides (Baker, 1996). There is substantial scatter in silica contents from 4 1 - 5 3 wt% SiO2 over the MgO range displayed by the basalts. While the SiO2 and MgO contents of some of the most highly porphyritic samples are clearly not melt values (e.g., JB335), SiO2 contents of aphyric and primitive samples range from 4 3 - 5 3 wt%, and there are two primary silica contents at ~ 4 3 - 4 4 and ~ 4 7 - 4 8 wt% SiO2. While the most magnesian samples (JB281 and JB 11) are ol + cpx-phyric, Mg numbers approaching 0.70 and olivine compositions of Fos8 suggest that these samples, although somewhat accumulative, are near primary magmas. A notable feature of the MgO-SiO2 plot is the within-suite increases in SiO= observed with decreasing MgO exhibited by most individual sample groups at MgO < 6 - 8 wt%. The silica increases can not be solely attributed to ol + cpx + plag fractionation, and require large amounts of Fe-Ti oxide fractionation or addition of siliceous material to the basalts during fractionation.

3.3. Trace Element Data Primitive-mantle-normalised multi-element diagrams of Oligocene flood basalts are highly variable, particularly with respect to the LILE, Th, and U (Fig. 5). However, three

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FIG. 3. (a) Total alkalis-silica classification (TAS; Le Baset al., 1986) diagram highlighting the marked bimodality of Yemen flood volcanism with respect to silica. (b) Non-accumulative mafic rocks plot in a number of the basaltic fields including basanite, basalt sensu stricto, trachybasalt (hawaiite), basaltic andesite, and basaltic trachyandesite, but most samples straddle the subalkaline-alkaline divides of Irvine and Baragar (1971) and MacDonald (1968). Highly altered or accumulative samples often plot away from the main body of data and the sample numbers of these rocks are annotated by their symbols. (c) MgO vs. SiO2 plot of Oligocene flood basalts. Note the within-suite increases in SiO2 at MgO < 6-8 wt% that can not be solely attributed to olivine + clinopyroxene _+ plagioclase fractionation. SiO2 contents of primitive and nonaccumulative samples vary from 43-48 wt%. Symbols: Wadi Lahima/ Jabal Hufash section = squares; Sana'a section = circles; northeast Sana'a section = diamonds; Jihana (south Sana'a) section = triangles; open symbols = lower series; filled symbols = upper series. Figures include some data from Baker (1996) for which isotopic data are not available.

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MgO or higher silica contents (Table 1), respectively, or those which are more silica-saturated are more likely to have patterns like (2) or (3). However, an equally important observation is that very different rock types from different groups can have the same shaped multi-element patterns, e.g., JB281 (basalt) and JB231II (basanite) = type (1); JB129 (hypersthene-normative basalt), and JB172 (nepheline-normative basalt) = type (2). This suggests that while some of the major element variation (e.g., MgO or SiO:) within each sample group is related to the processes that generates the LILE/actinide trace element differences, some of the major element differences between different sample groups are not always related to this trace element heterogeneity. In contrast, however, it is the less silica-saturated or more silica-undersaturated US samples that are characterised by higher VICE/MICE ratios than the LS. However, the Wadi Lahima/Jabal Hufash US and some Sana'a US samples have similar major element compositions to their LS counterparts despite having clearly higher VICE/MICE ratios. Andesitic and rhyolitic samples have marked negative St, Ti, and sometimes Ba anomalies that reflect extensive fractionation of feldspar and Fe-Ti oxides (Fig. 5).

r, I,~

m

3.4. S r - N d I s o t o p e D a t a

400 "

400'

_% ....

-

200

•

0

2

3

Ce/Y

4

~=E:

[3 r q ~ r n .

1.0

tu tU

[] ,

1.5

D,

OLLI

.-lEO

.

2.0

,

2.5

-

,

3.0

-

3.5

CeN

FIG. 4. Stratigraphic variations in Ce/Y ratios. Ratios of very incompatible (VICE) to moderately incompatible (MICE) trace elements increase abruptly in the upper part of each section considered in this study, and provides the basis for dividing the volcanic stratigraphy into an upper (US) and lower series (LS). In the case of the three sections close to Sana'a, this subdivision also corresponds in a change to bimodal basalt-rhyolite volcanism.

endmember types of patterns can be recognised: ( 1 ) Relatively smooth and LILE depleted (Rb, K, and sometimes Ba and Pb) patterns that peak at Nb e.g., JB281, J B l l , and JB231II. (2) Multi-element patterns with marked positive Ba anomalies (relative to Th, U, and Rb) that are associated with the formation of negative Th and U, and sometimes Nb, anomalies, and less marked negative or sometimes positive K and Pb anomalies, e.g., Wadi Lahima/Jabal Hufash LS and some US; Jihana LS; JB282 (Sana'a LS); JB327, JB328, and JB330 (Sana'a US). (3) Multi-element patterns with general enrichment in all the LILE (Rb, Ba, K, and Pb), and Th and U compared with (1) e.g., some Wadi Lahima/Jabal Hufash US samples--JB84 and JB85; the Sana'a LS samples, except JB281 and JB282. Such samples may have positive Rb anomalies relative to Ba. Superimposed on this variability is the tendency again, as illustrated in Fig. 4, for nearly all the US samples to have higher VICE/MICE ratios than the LS samples. Considering samples within each group, those with lower

The western Yemen flood basalts display the following range in initial Sr-Nd isotopic composition: 87Sr/86Sr = 0.70365-0.70555; 143Nd/144Nd = 0.51292-0.51248; (Table 1; Fig. 6). These ratios are less extreme than many other examples of continental flood volcanism (e.g., Carlson, 1991). The western Yemen volcanic rocks have substantially higher 87Sr/86Sr and lower 143Nd/J44Nd ratios than Gulf of Aden/Red Sea MORB (Fig. 6a). However, samples with the lowest 87Sr/86Sr ratios approach the Sr-Nd isotopic composition of the Afar plume and some LM clinopyroxenes of the Arabian shield, and this composition is close to the HIMU and PREMA endmembers of Zindler and Hart (1986). In detail, the Yemen basalts apparently form two groups (Fig. 6b and c); one of increasing 87Sr/86Sr at relatively constant 14~Nd/~44Nd ratios and the other with a somewhat steeper Sr-Nd isotopic array. The shallow Sr-Nd isotopic array is dominated by samples from the Sana'a LS section, although a few samples from the other sections also fall on this trend. Unpublished data for rhyolitic volcanic rocks intercalated with the US extend the shallow array in Sr-Nd isotopic space (Fig. 6b; Baker, 1996). Compared with southwestern Yemen basalts (Chazot and Bertrand, 1993), data for western Yemen basalts reported herein have somewhat lower J43Nd/~44Nd, but similar 87Sr/S6Sr ratios (Fig. 6a). Flood basalts from Ethiopia (Hart et al., 1989) and Djibouti (Vidal et al., 1991 ) form a similar array in Sr-Nd isotopic space as the Yemen basalts (Fig. 6a), but they have a greater proportion of samples with high 87Sr/86Sr and low L43Nd/la4Nd ratios. Crustal compositions in this region (Fig. 6d) encompass a wide range in present-day Sr-Nd isotopic compositions depending on the age and nature of the crust. While most LM xenoliths from this region have depleted compositions

Oligocene flood volcanism in western Yemen

2569

100

IO0

e-

E E t,, 100

o

• JB172

-" - ~

D JB166

North-east Sana'a (upper series)

o

RbBaTh U Nb K LaCePbSr P NdZrSmEuli GdDyY ErYb Lu. R

JB271

~

andesitic and rhyolitic rocks

r

Y

b

LU

100 e¢Q

E E

.¢-

10 ~121BJ

o

~

o r

series)

o JB136 • JB139 o JB141

~ Jihana(upperseries)

RbBaTh U Nb K LaCePbSrP NdZrSmEuTiGdDyY ErYbLu RbBaThU Nb K LaCePbSrP NdZrSmEuTiGdDyY ErYbLu Q~

100

E E

.¢-

10 ¢J

o o JB259 • JB282

Sana'a(lower series)

JB23111o JB330 JB327 • JB335

RbBaTh U Nb K LaCePbSr P NdZrSmEuTiGdDyY ErYbLu RbBaThU Nb K LaCePbSrP NdZrSmEuTiGdDyY Er YbLu 100

E

• JB11 • JB66 o JB59 • JB51

Wadi Lahima/Jabal Hufash(lower series)

• JB79 ' ~• JBSO = oJB84 ~ J B 8 5

" Wadi~Lahm i aJ/abaiHufash(upper series)

'-

RbBaTh U Nb K LaCePt)SrPNdZrSmEuTtGdDyY ErYbLu RbBaThU Nb K LaCePbSrP NdZrSmEuTiGclDyY ErYbLu

FIG. 5. Primitive-mantle-normalisedmultielement diagrams for representative Oligocene flood basalts (normalisation values from Sun and McDonough, 1989). The multielement patterns display considerable variation, particularly with respect to the LILE, Th, and U, and are considered further in the text.

(Fig. 6a), a small number of analyses (3) from the study of Blusztajn et al. ( 1995 ) on S audi Arabian xenoliths do have 143Nd/144Nd ratios that approach 0.5125, and a significant number of samples from Tanzania have very enriched SrNd isotopic compositions (875r/86Sr = 0.7034-0.836; 143Nd/ 144Nd = 0.51284-0.51127; not shown in Fig. 6; Cohen et al., 1984). Both the crustal rocks and enriched LM xenoliths might be suitable endmembers to create the observed isotopic variation in the flood basalts. Alternatively, the relatively limited isotopic variability might be a function of asthenospheric or plume heterogeneity with no little or no lithospheric involvement.

3.5. Lead Isotope Data Western Yemen flood basalts display a wide and scattered range in initial lead isotopic composition: 2°6pb/2~Pb = 17.9-19.3; 2°7pb/2°4pb = 15.52-15.59; 2°spb/Z°4pb = 37.7-38.9 (Fig. 7). A plot of 2°6pb/2°~Pb vs. 2°7pb/z°4pb (Fig. 7a and c) shows few clear trends principally as a result of the limited range in 2°7pb/2°4pb ratios exhibited by these samples. However, three general features are evident in these figures: (a) a decrease in 2°6pb/Z°4pb at a nearly constant 2°7pb/2°4pb ratio (ca. 15.58) by some samples from the Sana'a, northeast Sana'a and Jihana sections towards a postu-

2570

J.A. Baker, M. F. Thirlwall, and M. A. Menzies 0.51295

0

0.5136 0,51290

QO "0

Z

cn Aam •

0.51285

0.5132

[]

re

'ID

z

0.51280

•

0

oOA •

[]

e , o o%

•

0,5128 0.51275

6c 0.5124 0.702

0.703

0.704

0.705

0.51270 0,706 0.7036 0.5135 •

0.51295

=;area enlarged i in 6c

i

i

0.7039

0.7042

Late Proterozoic mafic granulites (Yemen; Saudi Arabia)

|

0.7045

0.7048

Oligocene flood volcanics

0.51285

"a Z

~ C 3 ~

~ +

0.5125 •

-+

0.51275

Late Proterozoic upper

~ - - crust (Arabian shield)

Early =roterozoic

(~-

0.51265 0.5115 " 0.51255

~

6b

0.51245 0.7036

6d

, 0.7046

i

0.7056

87Sr/88Sr

0.7066

~.

sTSr/86Sr >> 1

. .

^¢=. . . . . . u~^~~ s l l i c I C upper crust ~T~'zania/~° (Early Proterozoic Late Archean?; - ~ ~ ~ Sudan)

"0

Z

++

0.5105 0.70

~

silicic lower crust (Early Proterozoic - Late

Archean?; Sudan)

i

i

=

0.72

0.74

0.76

0.78

87SrP6Sr

FIG. 6. (a) Sr-Nd isotopic composition of western Yemen samples analysed in this study compared with MORB, Arabian LM, the Afar plume, and other continental volcanism at the Afro-Arabian triple junction; (b) 87Sr/S6Sr vs. 143Nd/t44Nd plot of western Yemen flood basalts (herein), and unpublished andesite and rhyolite data (Baker, 1996) - CI, C2 and C3 are isotopic components defined on the basis of lead isotopic compositions; (c) expanded view of 87Sr/S6Sr vs. ~4SNd/~44Ndplot of western Yemen flood basalts from (b); (d) present-day Sr-Nd isotopic compositions of crustal rocks from Arabia and northeast Africa. Comparative data from: MORB/Afar plume--Schilling et al. (1992); Afar plume--Vidal et al. (1991); LM--Henjes-Kunst et al. (1990); Blusztajn et al. (1995); J. A. Baker (unpubl. data); southwestern Yemen--Chazot and Bertrand (1993); Ethiopia--Hart et al. (1989); Djibouti--Vidal et al. ( 1991 ); Deniel et al. (1994). Crustal data taken from: Sudan--Davidson and Wilson ( 1989); Early Proterozoic mafic granulites--Cohen et al. (1984); Late Proterozoic mafic granulites--McGuire and Stern (1993) and G. Chazot and J. A. Baker (unpubl. data); Late Proterozoic upper crust (Saudi Arabia)--Duyverman et al. (1982) and Hegner and Pallister (1989). Same symbols for western Yemen samples as Fig. 3b and 3c, with additional symbols representing: + = rhyolites; × = andesites.

lated mixing component C1; (b) a number of samples (mostly Wadi Lahima/Jabal Hufash LS samples) with lower 2°7pb/z°4pb ratios than trend (a) at 2 ° 6 p b / 2 ° 4 p b ~ 18.3; and (c) samples with high 2°6pb/z°4pb ratios that fall in the Afar plume field (Sana'a LS samples and Wadi Lahima/Jabal Hufash US samples). These three trends are also evident in a plot of 2°6pb/2°4pb vs. 2°spb/z°4pb (Fig. 7b,d). The first group of samples extend to low 2°6pb/2°4pb ratios, with a marked increase in A 8 / 4 values (C1). The second extend to unradiogenic Pb compositions crudely parallel to the N H R L ( C 2 ) . The final group display an increase in 2°6pb/z°4pb ratios, accompanied by a decrease in 2°spb/z°4pb ratios and A 8 / 4 values ( C 3 ) . In detail, though, these trends are a little misleading. For example, two of the Sana'a LS samples which lie on the trend towards C1 in the z°Vpb/z°4pb plot, trend toward C2 in the 2°8pb/2°4pb plot. Samples with high A 8 / 4 values extend to much lower 143Nd/J44Nd ratios (field with solid outline in

Fig. 6b) than samples closer to the N H R L (field with dashed outline in Fig. 6b). Although some samples fall in the Afar plume or M O R B fields in binary lead isotopic plots (e.g., C3 in the 2°Tpb/2°4pb plot), in multi-isotopic Pb space few, if any, of the samples actually overlap these mantle components. Samples that fall closest to one of these two mantle fields include some Wadi Lahima/Jabal Hufash US samples, Sana'a LS and US sampies, northeast Sana'a US samples, and Jihana US samples. In detail, these samples almost overlap the least radiogenic part of the Afar plume field, and the field for hydrated LM from southwestern Yemen which was recently metasomatised by the Afar plume (Baker et al., 1995). Kod Ali pyroxenites have lead isotopic compositions quite unlike those of the volcanic rocks. Similarly, although the volcanic samples with unradiogenic Pb (C2) extend towards the field for depleted Arabian LM, these LM samples have much higher ~43Nd/ 144Nd ratios ( >0.513 ) than the volcanic samples.

Oligocene flood volcanism in western Yemen 39.6

"

2 sd uncertainty smaller than symbol size

39.1 ..Q

oeo,o,oo Arabian

D.

38.6 I1.

o= oJ

39.5 '

hydrated LM - Ataq (southern Yemen

\

Kod All pyroxenites

m,,,~/y/ , ~

,~IMIXF '~

~."

~ ,,, plume

39.0'

." '

~

~NN~l~

_

E~1

38.5"

Cl

C3 \

~

MORB

37.6 ~ 15,66 "

,~,~"

. . . . . .

LM-cpx " analyses A

38.1 "t

2571

I

7b

~

hydratedLM - Ataq

38.0'

37.5 '

Kod AU pyroxenites ~-~\~-X~b

(southern Yemen)

.......

,Q

~5 [] m ~ J f ~ f a r lS.54,

p.u e

D.

15.48 ' depleted Arabian /

15.42 17.7

2 sO uncertainty ~~ zsouncenamry

MORB

LM - cpx analyses v

. 18.2

.

.

.

18.7

7a

. 19.2

2O6pbFO4pb

19.7

17.5

18.0

18.5

19,0

19.5

2O6pb/=O4pb

FIG. 7. (a) and (b) Lead isotope systematics of western Yemen flood basalts compared with the fields for Red Sea/Gulf of Aden MORB, the Afar plume, and Arabian shield LM. (c) and (d) Lead isotope systematics of western Yemen flood basalts, andesites, and rhyolites compared with data for flood basalts from southwesternYemen, Ethiopia, and Djibouti. Uncertainties are related to mass fractionation and do not include uncertainties that result from age correction. Analytical uncertainty is smaller than symbol size in the case of 2°6pb/2°4pband 2°spb/2°4pbratios. Same symbols as Fig. 6b. Sources of comparative data are cited with Fig. 6.

Sr-Nd isotopically enriched mantle xenoliths from Tanzania have lead isotopic compositions marked by extremely unradiogenic Pb (2°6pb/2°4pb = 15.6-17.3; not illustrated in Fig. 7; Cohen et al., 1984). These xenoliths have lead isotopic compositions that are distinct from the western Yemen flood basalts and, in particular, are characterised by too low 2°spb/2°4Pb at a given 2°7pb/2°4pb to have produced the trend to unradiogenic Pb observed in the basalts (not shown). With one exception, andesitic and rhyolitic samples overlap the lead isotopic composition of the basaltic rocks (Fig. 7c,d). The exception is an andesitic sample that has a considerably less radiogenic Pb composition than the basalts. The rhyolite samples form two groups, one of which extends to component C3, while the other lies close to the basaltic samples that approach the Afar plume field. None of the andesitic and rhyolitic samples are marked by high A7/4 and A 8 / 4 values like the basaltic samples that vector towards C1. Continental volcanic rocks from Djibouti, Ethiopia, and southwestern Yemen have broadly similar lead isotopic compositions to the western Yemen flood volcanic rocks (Fig. 7c,d). However, Djiboutian samples extend to marginally higher 2°7pb/2°4pb and 2°spb/2°4Pb at given 2°6pb/2°4pbratios than the western Yemen, southwestem Yemen, or Ethiopian volcanic rocks. The Djiboutian samples also do not extend to low 2°6pb/2°4pb and low 2°7pb/2°4pb ratios like the aforementioned samples.

Continental crust in the Afro-Arabian region has a wide range in lead isotopic compositions (Fig. 8). Many of the western Yemen samples lie within the field for Late Proterozoic crust from Saudi Arabia, but the C1 data array extends out of this field and towards Early Proterozoic or older crust, like that from Tanzania and Sudan.

3.6. Sr-Nd and Lead Isotopic Correlations SVSr/S6Sr or ~43Nd/J44Nd ratios generally do not correlate well with lead isotopic composition (Fig. 9), probably as a result of the rather variable lead isotopic composition of components contributing to the western Yemen flood volcanic rocks. Some of the apparent mixing arrays identified in lead isotopic space are clearly marked by sympathetic changes in 87Sr/86Sr or ~43Nd/1~Nd ratios. For example, in Fig. 9 increasing 87Sr/86Sr ratios are coupled with increasing A8/4 along trend C1 and decreasing A 8 / 4 along trend C3. Little correlated change in STSr/86Sr ratios with lead isotopic composition are observed with the group of samples that vector towards C2, although in the case of the Wadi Lahima/ Jabal Hufash LS samples 2°6pb/z°4Pb ratios do decrease slightly with increasing 87Sr/86Sr ratios (not shown), and in the case of the Wadi Lahima/Jabal Hufash US and some Sana'a LS samples, A8/4 values decrease with increasing 87Sr/S6Sr ratios (excluding those vectoring towards C3). While samples vectoring towards lead isotopic components

2572

J.A. Baker, M. F. Thirlwall, and M. A. Menzies 16.1

heavily crustally-contaminated, Oligocene trachytes and rhyolites (Yemen; Djibouti) \

15.9.

~

N

silicic lower crust (Eady Proterozoi;-~'-~ ~ LateArchean?;// \ ~

J~

a. g

mafic granulites (Early 15.5, Proterozoic - Tanzania; Late Proterozoic - Yemen and / (~" Saudi Arabia) /

15.3 ' ~ - - ' 15.5

" 16.5

S

,

r-, \ \ sUicic 'percrust (Early Proterozoic Late Archean?' ..~^-~ '

Suuow\ \

\

Late Proterozoic upper crust

.

17.5

(Saudi Ar.abia) 18.5

19.5

2O6pbFO4pb

FIG. 8. Present-day Pan-African Pb crustal compositions. The Oligocene flood basalts from western Yemen lie within or extend towards compositions appropriate for Pan-African crust in this region, although the crustal samples encompass a large range in lead isotopic composition. The group of samples that trend towards C1 extend out of the field defined by Late Proterozoic upper crust from Saudi Arabia, and towards older lower crustal compositions. Data sources: Late Proterozoic Saudi Arabian upper crnst--Hegner and Pallister (1989); Sudanese crust--Davidson and Wilson (1989); mafic granulites--Cohen et al. (1984), Altherr et al. (1990), and G. Chazot and J. A. Baker (unpubl. data); crustally contaminated Oligocene silicic volcanic rocks--Chazot and Bertrand ( 1993 ), Deniel et al. (1994), and Baker (1996).

3.7. Trace Element-Isotopic Correlations Incompatible trace element data correlate with radiogenic isotopic composition. Increasing 875r]86Sr ratios are accompanied by generally higher large ion lithophile abundances (LILE; not shown). However, ratios of V I C E - - B a , K, Th, U, Nb, L a - - w h i c h are unlikely to be fractionated by fractional crystallization or partial melting are more useful to examine than elemental abundances, as correlations with isotopic composition will reflect variable contributions from different mantle or crustal sources. N b / L a ratios are particularly useful because both Nb and La are relatively immobile during alteration, and asthenospheric- or plume-derived melts have high N b / L a (e.g., Weaver, 1991), whereas continental crust has low N b / L a (e.g., Taylor and McLennan, 1985), and enriched LM is also believed to have low N b / L a (e.g., Hawkesworth et al., 1990). Although the relatively small contrast in N b / L a ratios between crust and mantle means this ratio is less sensitive to the presence of a lithospheric component than L I L E /

180

120

C1 and C3 are clearly marked by increasing 87Sr/86Sr and decreasing t43Nd/]44Nd ratios (Fig. 9), those samples that do not form part of these arrays have much more variable Sr-Nd-Pb isotopic compositions that do not seem to define simple mixing arrays. This either requires a number of additional isotopically distinct C2-1ike components, or that the samples contain contributions from three or more components, e.g., the Wadi Lahima/Jabal Hufash US and some Sana'a LS samples might be the result of combining C2 + C3 with a starting composition close to the Afar plume, when the position of these samples in multi-isotopic space is considered. The rhyolitic rocks which do not form part of the trend to C3 have elevated 87Sr/S6Sr ratios at 2°6pb/2°4pb = 18.618.7 and an andesite sample also has very high 87Sr/86Sr ratios at 2°6pb/2°4pb = 18.5. Once again, these seem to represent yet more isotopically distinct components contributing to flood volcanism or multi-component contributions to flood volcanism. The most striking general feature of the Sr-Nd-Pb correlations is that the western Yemen data forms fan-shaped fields, whereby greater lead isotopic heterogeneity is observed at higher 875r/86Sr and lower 143Nd/144Nd ratios. Samples with the lowest 875r/S6Sr ratios have 2°rpb/2°4pb = 18.4-18.9, 2°spb/2°apb = 38.2-38.8, and A 8 / 4 = +10 to +40, lying close to the Afar plume field. Djiboutian, Ethiopian (not shown), and southwestern Yemen (not shown) samples display decreasing 2°6pb/2°4pb ratios, and increasing A 7 / 4 and A 8 / 4 values (Fig. 9), with increasing 87Sr/86Sr and decreasing ~43Nd/aaaNd ratios, respectively. These trends are precisely the same as those exhibited by western Yemen samples vectoring towards C1.

60

Afar ~ • plume~:~

O'

+~1-co

+

~'-~ -60 0.7025

• C3 .m.._)~ i

i

0.7035

0,7045

+ i

0,7055

0.7065

87Sr/B6Sr 19.4

Afar plume

19.0

,,~

mm

/

18.2 x

continental volcanic

C1 "(----" rocks fro m Djibouti • 17.8 0.51245

+/ 0.51255

0.51265

0.51275

0.51285

0.51295

143Nd#44Nd

FIG. 9. 87Sr/86Sr vs. A8/4 and 143Nd/l+~Nd vs. 2°6pb/2~pb. At low 87Sr/S6Sr and high 143Nd/t44Nd ratios the western Yemen data converge into the Afar plume field. Lead isotopic arrays towards C1 and C3 are marked by correlated increases in 87Sr/86Sr and decreases in ]43Nd/l~Nd ratios, but the C2 array is more complex and probably reflects either multi-component contributions to flood volcanism or lead isotopic heterogeneity of this component. Note the general dispersion in lead isotopic compositions at higher 87Sr/86Sr and lower 143Nd/144Nd ratios, and the limited L43Nd/~44Ndvariations produced by contributions from components C2 and C3 compared with C1. Same symbols as Fig. 6b.

Oligocene flood volcanism in western Yemen

2573

120

A" •a

60'

eo

•

o OOo

.o •

%o o~jo ° o --0

o

•

,~ =o.O~ o

~o

••

~

o on

• ./"/"0 0

•

o o

oo

•

-60 19.4'

~" C 3 ~ t ,,Q

•

19.0'

"~o.

o 00~ o

O

g

o

,. %0" o'g l

oo

18.6' O0 0 ~

o~

•

18.2'

o

oO

•

,..~

°

A

A

. - - ,'-t 17.8 0.7056

0.7051

! 0.7046'

•

A

if)

o

•

0.7036 0.6

().8

• e O

A

o

0

o . ~ o J

o

•

oo-# .o.t

1'.0

&mm

Oo

A

[~:>um

0.7041' 0

•

•e o

1'.2

Nb/La

'~ooB.~ ° 1.4 50

250

450

650

850

K/Nb

FIG. 10. Nb/La and K/Nb ratios vs. 87Sr/86Sr, z°6pb/2°4pb and A8/4. Note the generally decreasing Nb/La and increasing K/Nb ratios, respectively, with increasing 87Sr/86Sr ratios, and increasing dispersion in lead isotopic compositions with decreasing Nb/La and increasing K/Nb ratios. Same symbols as Fig. 3.

HFSE ratios, it does not suffer from the affects of secondary alteration. Nb/La ratios in the western Yemen basaltic rocks vary from 0.66-1.32 (Fig. 10), although all but three samples have Nb/La >0.9 which is considered typical of oceanic basalts (Weaver, 1991). Nb/La ratios decrease generally with increasing g7Sr/86Sr and decreasing J43Nd/~44Nd ratios, and individual sample suites often exhibit clearer correlations between Nb/La and 87Sr/86Sr or 143Nd/~44Nd ratios (e.g., Fig. 10). Moreover, Nb/La ratios also clearly decrease moving towards lead isotopic components C1 and C3. Although the Wadi Lahima/Jabal Hufash LS samples exhibit quite a large range in Nb/La (1.3-1.0), equivalent to that of most of the other samples, they show little change in 875r/86Sr o r ]43Nd/144Nd ratios with decreasing Nb/La ratios. There is, however, some evidence of a decrease in Nb/La with 2°6pb/2°4pb ratios moving towards C2. Ba/Nb (not shown) and K/Nb ratios also correlate with radiogenic isotopic composition (Fig. 10), although the trends are more scattered, perhaps as the result of secondary alteration. Variations in LILE/HFSE ratios (ca. 700-800%) are considerably larger than that of Nb/La (ca. 100%) which

indicates a greater contrast in LILE/HFSE ratios between the Sr-Nd isotopically depleted and enriched components than in LREE/HFSE ratios. Ba/Nb and K/Nb ratios both generally increase with increasing 87Sr/S6Sr and decreasing ~43Nd/]44Nd ratios, respectively. Samples with the lowest LILE/HFSE and highest HFSE/LREE ratios clearly have the most restricted Sr-Nd-Pb isotopic compositions that converge on the following composition: 87Sr/S6Sr = 0.7037; J43Nd/144Nd = 0.51292; 2°6pb/2°4pb ~ 18.7; A 8 / 4 = + 3 0 60. Moreover, there is again a clear tendency for samples to have higher LILE/HFSE ratios moving towards lead isotopic compositions C1, C2, and C3. 4. DISCUSSION 4.1. C o m p o n e n t s Contributing to Flood V o l c a n i s m

Western Yemen flood volcanic rocks display significant Sr-Nd-Pb isotopic and incompatible trace element heterogeneity, which must reflect variable contributions from different mantle and/or crustal components. In detail, Sr-Nd-Pb isotopic and incompatible trace element data apparently define at least four general components (Figs. 5 - 1 0 ) : (1) A

2574

J.A. Baker, M. F. Thirlwall, and M. A. Menzies

depleted component with 878r/86Sr ~ 0.7036-0.7037 and 143Nd/~a4Nd - 0.5129 which approaches the composition inferred for the Afar plume. This component is marked by a lead isotopic composition that lies close to the MORBAfar plume array, and sometimes near the unradiogenic part of the Afar plume field. Samples with this isotopic composition have low LILE/HFSE and high HFSE/LREE ratios. Most samples vector away from this composition towards components with more enriched isotopic compositions and higher LILE/HFSE and lower HFSE/LREE ratios that are outlined below. (2) C1 = a component with a low 2°6pb/ 2°4pb ratio and high A 8 / 4 and A 7 / 4 values, which is also responsible for the most isotopically enriched and extreme samples that fall along the steep Sr-Nd isotopic array. Jihana LS and US samples, and some Sana'a and northeast Sana'a US samples vector towards this component. (3) C2 = a component with low z°rpb/2°4pb and relatively low A 8 / 4 and A 7 / 4 values. This component contributes to samples that fall along the steep Sr-Nd isotopic array, although the increase in 87Sr/86Sr and decrease in 143Nd/144Nd ratios is much more limited than (2) above, and some samples retain rather depleted Sr-Nd isotopic compositions. Most Wadi Lahima/Jabal Hufash LS and some US samples, two Sana'a LS samples, and one sample each from the Sana'a US and northeast Sana'a LS sections contain substantial contributions from this component. Many of the samples with substantial C2 Pb display poor correlations between strontium or neodymium and lead isotopic composition that leads to the inference that this component is not simply confined to the region of C2 on Fig. 7. It is likely that this component has a variable lead isotopic composition that lies sub-parallel to the NHRL. (4) C3 = a component with high 2°rPb/2°4pb, relatively low 2°8pb/2°4pb, and negative A8/4. This is associated with higher 87Sr/86Sr ratios than C2, but again only produced small decreases in 143Nd/144Nd ratios. Sana'a LS and northeast Sana'a LS samples, and some Wadi Lahima/ Jabal Hufash US samples, along with a number of the rhyolite samples contain substantial contributions from this component.

4.2. Crustal Contamination vs. Mantle Heterogeneity Much of the variability in incompatible trace element ratios and radiogenic isotopic composition encompassed by the western Yemen flood basalts is less than that exhibited by oceanic basalts (e.g., Weaver, 1991; Zindler and Hart, 1986). However, most of the Yemen basalts fall outside the Sr-Nd-Pb isotopic fields measured to date for local LM, local depleted MORB mantle, and the Afar plume (Figs. 6 and 7). Those which fall close to any of these fields on a twodimensional plot (e.g., 2°6pb/2O4pb vs. 2°7pb/2°4pb; Fig. 7) do not in multi-isotopic space. The cause of this disparity could be attributed to asthenospheric heterogeneity (presently not documented in this region), or to interactions with the lithosphere including crustal contamination, melt generation in an enriched LM, or perhaps a more complex contamination process within the LM, e.g., mixing with small amounts of lamproitic melts. Note that suitable LM compositions have not been clearly identified in studies of Arabian

shield LM (Henjes-Kunst et al., 1990; Blusztajn et al., 1995; J. A. Baker, unpubl, data). Plots of differentiation indices, MgO and Sit2, against isotopic composition and LILE/HFSE and HFSE/LREE ratios display (weak) correlated trends typical of combined assimilation and fractional crystallization (AFC; Fig. 11 ). Individual sample suites often exhibit tighter correlations than the data viewed as a whole, e.g., the Sana'a LS. The trends are particularly striking for plots of SiO2 vs. 878r/ 86Sr and the within-suite MgO-SiO2 covariation (Fig. 3) exhibited by the basalts suggests that SiO2 contents at MgO < 6 - 8 wt% were primarily governed by crustal contamination processes. However, samples from the Jihana and northeast Sana'a US and some Sana'a US sections may have had different, low, primary SiO2 contents ( < 4 4 wt%) compared to the bulk of the samples (ca. 4 7 - 4 8 wt% SiO2). These samples represent smaller degrees of partial melting, as exemplified by lower Zr/Nb and higher Ce/Y and La/Yb ratios (Fig. 4; Baker, 1996), and will be considered further elsewhere. A similar correlation between S i t 2 and 87Sr/86Sr was reported by Manetti et al. (1991) for a reconnaissance set of samples from north, west, and south Yemen and was interpreted as the result of crustal contamination. The presence of plagioclase phenocrysts in some suites of progressively contaminated samples (e.g., Sana'a LS), and the fact that the basalts plot between the 10 kbar and 1 atm ol + cpx +_ plag cotectics on phase diagrams (e.g., Stolper, 1980; not shown) seems to precludes the possibility that addition of lamproitic melts from enriched LM took place (e.g., Ellam and Cox, 1991 ) and is responsible for the observed isotopic variations. Only two samples with > 6 wt% MgO have 87Sr/86Sr > 0.7041, and samples with MgO < 6 wt% had begun to fractionate plagioclase in crustal magma chambers (Baker, 1996). Addition of small degree melts from enriched LM is also unlikely to have produced the SiO2-87Sr/86Sr covariations, as such lamproitic melts do not have SiO2 contents significantly higher than the flood basalts (ca. 53 wt% SiO2; Ellam and Cox, 1991). Moreover, addition of lamproitic melts from enriched LM to the Nuanetsi picrites from the Karoo province resulted in extreme LREE enrichment (almost an order of magnitude change in La/Yb over five CNd units; Ellam and Cox, 1991); no such LREE enrichment coupled with enriched isotopic compositions is observed in the western Yemen basalt suite. The trends between MgO or SiO2 and ~43Nd/~44Nd ratios (Fig. 11 ) are less clear than the relationships with 878r/86Sr, presumably because Nd is less sensitive to contamination than strontium isotopic composition. Possible reasons for the greater shift in strontium isotopic composition during contamination include: (a) the greater contrast between primitive magmas and Pan-African crust in strontium isotopic composition (0.7035 vs. >0.7100), compared with neodymium isotopic composition (0.5129 vs. 0.5132-0.51 10; Fig. 6d); and (b) strontium being more compatible in the assimilating and fractionating basalts than Nd, due to plagioclase crystallization, rendering Sr to be progressively more susceptible to crustal contamination. The isotopic compositions of andesitic and rhyolitic units from the flood volcanic pile are consistent with an origin by further AFC of some of the basalts (Fig. 11 ; J. A. Baker, unpubl, data).

Oligocene flood volcanism in western Yemen

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Although the trends described above are somewhat scattered, we argue that this is not evidence against AFC processes, but rather is likely to be a inevitable consequence of this process operating on the Yemen flood basalts. Five simple explanations can account for the scattered contamination trajectories: (1) Pan-African upper continental crust is extremely heterogeneous, with a wide range in present-day isotopic composition, e.g., Figs. 6d and 8 (Stacey et al., 1980; Duyverman et al., 1982; Stacey and Stoeser, 1983; Stacey and Hedge, 1984; Davidson and Wilson, 1989; Hegner and Pallister, 1989; Kr6ner et al., 1991; Stern and Kr6ner, 1993 ). Furthermore, the lower crust is likely to have distinct strontium and lead isotopic compositions from the upper crust, e.g., lower crustal granulite xenoliths from Saudi Ara-

bia and Yemen have rather low 87Sr/S6Sr and moderately high a43Nd/144Nd ratios (Fig. 6d). These data, and those for older Sudanese lower crust (Fig. 8), also indicate the lower crust has a heterogeneous and generally unradiogenic lead isotopic composition compared with present-day upper crustal compositions. (2) Plagioclase crystallization, or its absence, will have a distinctive impact on AFC trajectories (e.g., DePaolo, 1981). Many of the basalts that follow the shallow Sr-Nd isotopic array have plagioclase as the dominant phenocryst phase, and this trend is further extended by the rhyolitic rocks that have undergone extensive feldspar fractionation. In contrast, most of the basalts on the steeper Sr-Nd array have only fractionated olivine and clinopyroxene. (3) Large variations in ratios of VICE to MICE suggest

2576

J.A. Baker, M. F. Thirlwall, and M. A. Menzies

the Yemen basalts are the product of variable degrees of partial melting (e.g., La/Yb = 7.5-20; Ce/Y = 1.2-3.5 at 143Nd/144Nd 0.5 129). Consequently, widely different incompatible trace element contents were present in the parental magmas prior to contamination. This would have produced markedly different magma:crust concentration ratios for Sr, Nd, and Pb rendering melt batches to be different in their respective susceptibility to crustal contamination. (4) The Yemen samples come from a number of locations and stratigraphic sections; geographically and temporally unrelated samples are unlikely to represent the products of a single magmatic plumbing system. Hence, AFC trends will be scattered unless single stratigraphic sections are considered where batches of magma produced by similar degrees of partial melting have traversed crust of the same composition. Even if a single section were considered, flood basalt flows have the potential to travel large distances and flows erupted from different localities can interdigitate at some intermediate point. These effects will be accentuated by the complex crustal architecture in western Yemen (Fig. 1 ). (5) Although most samples form trends suggestive of combined AFC, two high MgO samples (JB129 and JB172; 7.2 and 11.0 wt%, respectively) have high 87Sr/86Sr ratios. Such rocks might represent melts of enriched LM, or these samples may represent a contrasting style of contamination whereby hot highMgO basalts rapidly assimilate crustal rocks without fractionating substantially (e.g., Thirlwall and Jones, 1983; Devey and Cox, 1987; Kerr et al., 1995). JB129 and JB172 may be examples of this as they fall on the same Pb contamination trajectory (C l; Fig. 7) as other, lower MgO, samples from Jihana, Sana'a and northeast Sana'a that produce clear AFC trends (Fig. 1 1 ). These samples also have the most extreme non-mantle-like oxygen isotopic compositions of the Yemen flood basalt suite (Baker et al., 1994). ~

4.3. Modelling Contamination Processes Figure 12 illustrates AFC curves in Sr-Nd isotopic space for contamination of LS and US flood basalts (which had different primary Sr and Nd contents) by crustal components with variable 143Nd/~44Nd ratios and Sr contents. Two curves for each model are also presented using bulk distribution coefficients for Sr of 0.1 and 2.0, which can be considered minimum and maximum possible values during crystallization of the Yemen flood basalts. Several important features are evident from these simplistic models. First, the samples on the shallow Sr-Nd isotopic array (e.g., most Sana'a LS) can be satisfactorily modelled by assimilation of Late Proterozoic crust with 143Nd/144Nd ~ 0.5125 and 87Sr/S6Sr 0.7100. Second, samples on the steeper Sr-Nd isotopic array can be satisfactorily modelled by assimilation of a crustal component with much lower ~43Nd/144Nd ratios (e.g., Late Archean crust). However, only very small amounts of crust would then be needed to produce the neodymium isotopic variation of many of these samples, in particular the Wadi Lahima/Jabal Hufash samples. This is clearly at odds with the similar disturbance of incompatible trace element signatures/ratios of LS samples on the steep and shallow Sr-Nd isotopic arrays (e.g., Wadi Lahima/Jabal Hufash LS vs. Sana'a LS; Figs. 5 and 10) at the same 875r/86Sr ratio.

A more satisfactory explanation for these samples is that they were contaminated by a crustal component with 143Nd/ 144Nd ~ 0.5125, but 875r/86Sr "~ 0.7100, which can adequately model the isotopic variation observed in these samples (not shown). In contrast, the samples with very low ~43Nd/~44Nd ratios do require a crustal component with H3Nd/t44Nd ~ 0.5125 in order to generate the observed isotopic variation at realistic levels of contamination ( < 5 0 % ) . Finally, from examination of the AFC trajectories with different bulk distribution coefficients for Sr and the same crustal composition, it is apparent that a variable bulk distribution coefficient for Sr during AFC can not account for the spread of data in Sr-Nd isotopic space; isotopic heterogeneity of the crustal contaminant is required. Interpretation of the different Sr-Nd trajectories as assimilation of three broad types of crust is entirely consistent with the lead isotopic data presented in Fig. 7. The samples with the lowest 143Nd/144Nd ratios extend to higher A 7 / 4 and A 8 / 4 values (C1) than the samples with more restricted ~43Nd/H4Nd ratios (C2 and C3). We note that these samples ( C I ) are restricted to the Sana'a section and sections to the east of Sana'a and were probably erupted through the region of reworked Archean basement (143Nd/~44Nd < 0.5125) from the east of the study transect in Fig. 1, as opposed to the other samples which were probably erupted through juvenile Neoproterozoic crust ( ~43Nd/144Nd -> 0.5125 ) closer to the Red Sea rift margin. Finally, it is interesting to speculate that components CI and C2, which produced multielement patterns in contaminated samples with positive Ba anomalies relative to Rb, Th, and U (Fig. 5) is lower crust, whereas component C3 which contributed to the most evolved samples (and rhyolites), and produced general enrichment in all the LILE and Th and U (Fig. 5), is upper crust. Quantifying the amount of crust assimilated by the western Yemen basalts is difficult given the problems in choosing appropriate isotopic and trace element compositions of the clearly heterogeneous Pan-African crustal contaminant. However, the modelling presented in Fig. 12 suggests levels of contamination up to 20-30%.

4.4. Is Enriched LM Responsible for Lithospheric Signatures in Flood Basalts from Ethiopia, Djibouti, and Southwestern Yemen? Complex isotopic variations result from crustal contamination in the western Yemen basalt suite which was erupted in a brief period < 3 myr. is it, therefore, possible that such contamination, rather than an enriched LM contribution, is responsible for similar characteristics in basalts from Ethiopia, southwestern Yemen, and Djibouti? 4.4.1. Southwestern Yemen

Chazot and Bertrand ( 1993 ) propose that a lack of correlation between 875r/86Sr and MgO or K / P is evidence against crustal contamination being the primary control on isotopic variation in southwestern Yemen flood basalts. The lack of correlation with MgO is not surprising given that variable Dsr values are likely, controlled by the presence or absence

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of plagioclase crystallization in a wide variety of genetically unrelated rocks, which in this case were erupted and intruded over a > 10 myr period. The lack of a correlation between K / P and 875r/86Sr in their sample suite is also not surprising; K / P in the western Yemen basalt suites does not always correlate with 87Sr/86Sr and certainly does not clearly do so when all the samples are considered together. Moreover, we caution against use of K as a petrogenetic indicator in the Chazot and Bertrand (1993) sample suite given the very high LOI exhibited by some samples (up to 10 wt%). 4.4.2. E t h i o p i a a n d D j i b o u t i

Ethiopian samples (Hart et al., 1989) also overlap the region occupied by the Yemen flood basalts although few coherent trends evident (Fig. 6a). The lowest ~43Nd/~44Nd ratios are associated with lead isotopic compositions similar to our inferred C1 contaminant. While Hart et al. (1989) identified some clearly crustally contaminated samples, they

argued that the lack of correlation between isotopic composition and Mg number, SiO2 or K / P in most samples suggested the involvement of enriched LM, in particular in some relatively high M g O basalts. The lack of clear AFC trends may be a function of grouping together a diverse suite of samples with ages from ca. 30 to 0 Ma which should not be expected to show clear correlated trends. We only observe correlations when geographically and stratigraphically linked samples are compared, and this is not always the case; if data for Late Oligocene and Quaternary intraplate basalts from western Yemen (Baker, 1996) are also added to Fig. 11 even poorer correlations are observed (not shown). We are, therefore, not convinced that crustal assimilation can be excluded as the process responsible for much of the isotopic variation (to enriched compositions) in the Ethiopian flood basalt suite. Oligocene to Recent basalts from Djibouti display an age progression from a component with low 2°6pb/2°4pb ratios and high z&2°7pb and A2°sPb values to a component with radiogenic 2°6pb/ZO4pb lying close to the N H R L (Vidal et al.,

2578

J.A. Baker, M. F. Thirlwall, and M. A. Menzies

1991; Deniel et al., 1994). Their oldest and most enriched samples show a good correlation between silica content and 87Sr/S6Sr which is suggestive of crustal assimilation. Again though, it can be argued that this sample set which comprises a few samples of a number of formations spanning an age range of 25 Ma, may well lack the tight correlated AFC trends typical of a single magmatic system. In summary, we argue that existing studies of continental volcanism at the Afro-Arabian triple junction lack the sample resolution and necessary data to identify crustal contamination, e.g., none of the studies outlined above present isotopic data for more than five samples from the period 3 0 - 2 5 Ma. As such the commonly invoked role of enriched LM in this flood basalt province must be viewed with some scepticism. We note that the higher proportion of samples from southwestern Yemen, Djibouti, and Ethiopia with low 143Nd/H4Nd ratios than western Yemen might reflect predominance of an older crustal contaminant in these regions than western Yemen. Oxygen isotope analysis of mineral separates (Baker et al., 1994) may provide the key to resolving the nature of the low 143Nd/144Nd and high A2°Vpb component in the Yemen/ Ethiopian flood basalt province. The only published oxygen isotope data, of whole rock samples from Djibouti (Barrat et al., 1993), revealed that samples with low ~43Nd/~44Nd ratios had crustal oxygen isotope ratios (6L80 > 8.0%~) in contrast to primitive values of ca. 5.5-6.0 for uncontaminated samples with high t43Nd/~44Nd. Similar, elevated, oxygen isotope ratios have been reported for western Yemen samples (Baker et al., 1994), but over the whole province oxygen isotope data will be equivocal as large amounts of contamination by low-6 ~80 Pan-African crust (e.g., Davidson and Wilson, 1989) is required to produce any measurable shifts in the oxygen isotope composition of the contaminated magmas.

4.5. The Primary Isotopic and Trace Element Signature of Yemen Flood Basalts and Plume-Related Flood Volcanism The primary incompatible trace element and isotopic signature of the western Yemen flood basalts is characterised by three important features: ( 1 ) LILE depletion, most obviously marked by the negative K anomalies in Fig. 5. Samples with K-depletion include both silica-undersaturated basanites and hypersthene-normative tholeiites. K/Nb, Ba/Th, Ba/La, and Ba/Nb ratios of 150, 50, 5, and 5 approach values typical of HIMU OIB (Weaver, 1991). Studies of Late Cenozoic intraplate basalts from Djibouti suggest that the Afar plume is also characterised by low K/Nb ratios (Deniel et al., 1994). (2) A depleted Sr-Nd isotopic signature relative to Bulk Earth, but not as depleted as N-MORB that is similar to HIMU or PREMA mantle and many ocean island basalts (Zindler and Hart, 1986). The low LILE/HFSE ratios of least contaminated samples precludes the possibility that the Sr-Nd isotopic signature is significantly more depleted than that of the least contaminated samples in our sample suite and also preclude it being N-MORB ( K / N b --270-290; e.g., Weaver, 1991). (3) The least contaminated Yemen flood basalts in this study have relatively radiogenic lead

isotope ratios (2°6pb/2°4pb ~ 18.7) with A 7 / 4 and A 8 / 4 values that overlap the Afar plume-MORB array, but the Pb composition does not approach the extremely radiogenic values of HIMU OIB, i.e., 2°6pb/2°4pb > 21.0. Although the strontium and neodymium isotopic compositions of least contaminated samples approach the Afar plume composition (Figs. 6, 7, and 9), it is no surprise that few samples overlap it in lead isotopic space. Lead isotopic compositions of plume-derived magmas are highly susceptible to crustal contamination, because of the high Pb content of crust relative to the basaltic magmas. Some of the samples with depleted Sr-Nd isotopic compositions and low LILE/HFSE ratios do approach the unradiogenic part of the Afar plume field. However, the lead isotopic variations in the Yemen basalts appear to be largely a function of assimilation of isotopically heterogeneous continental crust, and preclude identification of any possible mantle lead isotopic heterogeneity. LILE depletion, coupled with a Sr-Nd isotopic signature intermediate between MORB and Bulk Earth, and sometimes relatively radiogenic Pb also characterises other examples of plume-related flood volcanism. Saunders et al. (1992) noted the distinctively low K/Nb ratios and general LILEdepletion (Ba and Sr?) relative to HFSE found in some other continental flood basalt provinces: Madagascar ferrotholeiites, Greenland basalts, and Deccan Ambenali basalts. Negative-K anomalies, but not Ba-depletion, are also a feature of the Deccan Mahabaleshwar basalts. Some oceanic plateau basalts including those of the Nauru Basin, Manihiki Plateau, and Caribbean Basin (Marriner and Tarney, unpubl, data) also have negative K anomalies, but data for such rocks must be interpreted with care due to submarine alteration. LILE depletion is also a common feature of many OIB with 2°6pb/ 2°4pb - 19 and Sr-Nd isotopic signatures intermediate between MORB and Bulk Earth (e.g., Halliday et al., 1995). Although Hoernle and Schminke (1993) have recently demonstrated that pairs of VICE like K, Nb, and La may be fractionated by small degrees of partial melting, we doubt that this is likely at the relatively large degrees of melting inferred to produce flood basalts. Similarly, the low LILE/ HFSE ratios of the western Yemen samples are unlikely to be a product of LILE retention in a minor potassic phase such as phlogopite, at these degrees of melting, unless small quantities of LILE-depleted melt are added to the magma during interaction and ascent through the LM, for which there is little isotopic evidence. The key point is that negative K anomalies of similar magnitude in the basanites and tholeiites precludes partial melting being responsible for the LILE depletion. At present the principal postulated explanation for low LILE/HFSE ratios in HIMU-OIB has been that subductionprocessing of recycled oceanic crust removes the LILE relative to the HFSE (e.g., Saunders et al., 1988). However, uncontaminated flood basalts never have 2°6pb/2°4pb approaching the values (>21.0) of typical HIMU OIB like St. Helena. This may be because the flood basalt source has lower U/Pb than the source of these HIMU basalts, or because the recycled oceanic crust has simply had longer to time-integrate its high U/Pb in the case of HIMU OIB (Thirlwall et al., 1994; Thirlwall, 1995). However, explanation of plume-generated flood volca-

Oligocene flood volcanism in western Yemen nism as the product of recycling relatively immature oceanic crust seems to be at odds with helium isotopic evidence. Most examples of flood volcanism or their related plumes, for which helium isotope data are available, have R[Ra significantly higher than MORB suggesting there is some primordial (lower mantle?) contribution to flood volcanism, e.g., Afar, Iceland, and Rtunion (Condomines et al., 1983; Kurz et al., 1985; Poreda et al., 1986; Graham et al., 1990; Greisshaber et al., 1994). However, HIMU ocean islands do not share this helium signature (Graham et al., 1992). Low LILE/HFSE ratios are not considered to be characteristic of lower or primordial mantle (e.g., Weaver, 1991 ) and seem to preclude a major role for primitive mantle in flood volcanism. It is conceivable that the helium isotopic differences between HIMU OIB and flood volcanism merely reflects the scale of the respective plumes; larger and deeper seated plumes probably produce flood volcanism and are more likely to ascend from levels in the deep mantle where primordial helium is entrained in the plume. Regardless of the nature of plumes that produce flood volcanism, further work is warranted to identify uncontaminated trace element and isotopic compositions that can be used to elucidate the origin of this mantle reservoir that seems to be responsible for the second largest examples of volcanism on Earth. 4.6. A Chemically Heterogeneous Plume or Entrainment of MORB Mantle? Chazot and Bertrand ( 1993 ) proposed that the asthenospheric or plume component in magmas from southwestern Yemen approached has a more depleted or MORB-like signature (i.e., 143Nd/In4Nd > 0.5130; Fig. 6a) than the Afar plume. This is supported by the average K/Nb ratio (ca. 280) of five samples with 143Nd/t44Nd ~> 0.5130, a K/Nb ratio close to the average MORB or primitive mantle value (Weaver, 1991 ), although their samples are somewhat altered. Given the high K/Nb and generally low t43Nd/~a4Nd ratios of Pan-African continental crust there is no way of accounting for the difference between the western (herein) and southwestern Yemen basalts except by melting distinct mantle source compositions. As these basalts were produced at the same time, 3 0 - 2 0 Ma, these spatial differences in geochemistry must represent melting of a heterogeneous mantle source. We interpret the spatial difference in geochemistry to have been the result of melting a heterogeneous plume head comprising the isotopically less depleted component identified in this study and either entrained, but not chemically mixed, more depleted MORB (?) mantle, or a more depleted plume component sensu stricto. Simple laboratory experiments (Griffiths and Campbell, 1990) predict that starting plume heads should comprise a mixture of plume material and entrained lower/upper mantle stirred into the plume head during ascent. Whilst the concept of a simple HIMU-like Afar starting plume impinging on the Afro-Arabian triple junction at ca. 30 Ma and producing flood volcanism is appealing (Schilling et al., 1992), the existence of this plume is highly speculative. However, the spatial distribution of these mantle components under Yemen at 3 0 - 2 0 Ma might represent melting

2579

of a flattened plume torus at the northern periphery of the influence of the Afar plume with the more MORB-like southwestern Yemen basalts derived from the plume-absent window between the torus and central stem of the plume located further to the south. Such a model requires the Afar plume to have been present beneath the Arabian lithosphere for at least 30 Ma. 5. CONCLUSIONS Geochemical data on Oligocene flood basalts from western Yemen provide important insights into the interaction between the Afar plume and Pan-African lithosphere during magmagenesis. Our main conclusions are: Variable SiO2 at constant MgO contents in the flood basalts in part reflects the effects of variable degrees of partial melting, but within-suite MgO-SiO2 covariations are principally the result of assimilation of continental crust. Yemen flood basalts define complex trends on Sr-Nd-Pb isotope plots and almost always have compositions that lie outside the range of possible mantle reservoirs that could have contributed to flood volcanism (plume, MORB, and LM) in this region. However, variations in incompatible trace element ratios and radiogenic isotopic ratios are still largely within the range defined by oceanic basalts. Correlations between indices of fractionation, or SIO2, and isotopic data suggest that crustal contamination has modified the isotopic and trace element composition of most of the basalts. The Sr-Nd-Pb isotopic composition of the contaminants is variable and includes juvenile Late Proterozoic Pan-African crust and also a much older crustal component, and may also include lower and upper crustal components. Modelling of Sr-Nd isotopic data requires incorporation of 0 - 3 0 % crust in the basaltic samples. Most samples contain 0 - 1 0 % crust which swamps their primary lead isotope characteristics, but not does not as dramatically affect 875r/86Sr and ~43Nd/~44Nd ratios. Crustal contamination in western Yemen has produced complex chemical and isotopic trends due to the heterogeneous composition of Pan-African crust, the occurrence or lack of plagioclase crystallization, differences in primary melt incompatible trace element contents, contrasting styles of contamination, and the vagaries of comparing samples that do not represent simple monogenetic magmatic systems. No isotopically enriched LM can be demonstrated to have contributed to flood volcanism in western Yemen and its role in the petrogenesis of other flood basalt suites in the Ethiopian flood basalt province must be questioned in light of the complex crustal contamination effects identified within our sample suite. This, of course, does not preclude the possibility of a LM contribution to flood volcanism as much Arabian LM has a Sr-Nd isotopic composition that is similar to the Afar plume. Least contaminated samples carry a HIMU-like signature of low LILE/HFSE ratios, but with radiogenic isotopic compositions that are PREMA- or FOZO-like (Zindler and Hart, 1986; Hart, 1994). Furthermore, their neodymium isotopic composition, though depleted, is substantially lower than that of MORB. Such characteristics are common chemical and isotopic features of plume-derived flood basalts. The

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incompatible trace element and isotopic signature of the western Y e m e n flood basalts suggests they were derived from the Afar plume which impinged at the base of Arabian lithosphere at least 30 Ma. The low pre-contamination L I L E / HFSE ratios of the Y e m e n flood basalts and other related volcanic suites is a distinctive signature of m a n y plumederived continental and oceanic flood basalts and is not like primitive mantle. Acknowledgments--Gerry Ingram and Giz Marriner are thanked for technical assistance with isotopic and XRF analyses. Fieldwork in Yemen was supported by the Royal Society and the British Council. Mohamed A1-Kadasi is thanked for making available unpublished K-Ar data. Mohamed AI-Kadasi, Abdulkarim Subbary, and the Geology Department of Sana a University are thanked for providing logistic support in the field. Stan Hart and an anonymous reviewer, and Associate Editor Julie Morris, are thanked for their constructive criticism of this manuscript. Editorial handling: J. D. Morris

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