Journal of Petrology

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Journal of Petrology | Volume 44 | Number 9 | Pages 1657-1679 | 2003
© Oxford University Press 2003

Petrogenesis of the Largest Intraplate Volcanic Field on the Arabian Plate (Jordan): a Mixed Lithosphere–Asthenosphere Source Activated by Lithospheric Extension

JULIA E. SHAW^1,2,*, JOEL A. BAKER¹, MARTIN A. MENZIES², MATTHEW F. THIRLWALL² and KHALIL M. IBRAHIM³

¹ DANISH LITHOSPHERE CENTRE, ØSTER VOLDGADE 10, COPENHAGEN, 1350 K, DENMARK
² DEPARTMENT OF GEOLOGY, ROYAL HOLLOWAY UNIVERSITY OF LONDON, EGHAM TW20 0EX, UK
³ DEPARTMENT OF EARTH AND ENVIRONMENTAL SCIENCES, HASHEMITE UNIVERSITY OF JORDAN, ZARQA, JORDAN

^* Corresponding author. Telephone: ++45 3814 2665. Fax: ++45 3311 0878. E-mail: jes{at}dlc.ku.dk

RECEIVED JULY 1, 2002; ACCEPTED MARCH 12, 2003

	ABSTRACT

TOP ABSTRACT INTRODUCTION GEOLOGICAL SETTING ANALYTICAL TECHNIQUES PETROGRAPHY MAJOR AND TRACE ELEMENT... Sr-Nd-Pb ISOTOPE CHEMISTRY DISCUSSION CONCLUSIONS SUPPLEMENTARY DATA REFERENCES

 
Miocene to Recent volcanism in northwestern Arabia produced the largest intraplate volcanic field on the Arabian plate (Harrat Ash Shaam, Jordan). The chemically and isotopically diverse volcanic field comprises mafic alkali basalts and basanites. The magmas underwent limited fractional crystallization of ol ± cpx ± plag and rare samples have assimilated up to 20% of Late Proterozoic crust en route to the surface. However, there are subtle Sr–Nd–Pb isotopic variations (⁸⁷Sr/⁸⁶Sr = 0·70305–0·70377, ¹⁴³Nd/¹⁴⁴Nd = 0·51297–0·51285, ²⁰⁶Pb/²⁰⁴Pb = 18·8–19·2), which exhibit marked correlations with major elements, incompatible trace element ratios and abundances in relatively primitive basalts (MgO >8·5 wt %), and cannot be explained by fractional crystallization and crustal contamination alone. Instead, the data require polybaric melting of heterogeneous sources. Semi-quantitative melt modelling suggests that this heterogeneity is the result of small degree melts (2–5%) from spinel- and garnet-facies mantle, inferred to be shallow Arabian lithosphere, that mixed with smaller degree melts (<1%) from a predominantly deep garnet-bearing asthenospheric(?) source with ocean island basalt characteristics. The latter may be a ubiquitous part of the asthenosphere but is preferentially tapped at small degrees of partial melting. Volcanism in Jordan appears to be the result of melting lithospheric mantle in response to lithospheric extension. With time, thinning of the lithosphere allowed progressively deeper mantle (asthenosphere?) to be activated and melts from this to mix with the shallower lithospheric mantle melts. Although Jordanian intraplate volcanism is isotopically similar to examples of Late Cenozoic volcanism throughout the Arabian peninsula (Israel, Saudi Arabia), subtle chemical and isotopic differences between Yemen and Jordan intraplate volcanism suggest that the Afar plume has not been channelled northwestwards beneath the Arabian plate and played no role in producing the northern Saudi Arabian and Jordan intraplate volcanic fields.

KEY WORDS: asthenosphere; intraplate volcanism; Jordan; lithospheric mantle; Sr–Nd–Pb isotopes

	INTRODUCTION

TOP ABSTRACT INTRODUCTION GEOLOGICAL SETTING ANALYTICAL TECHNIQUES PETROGRAPHY MAJOR AND TRACE ELEMENT... Sr-Nd-Pb ISOTOPE CHEMISTRY DISCUSSION CONCLUSIONS SUPPLEMENTARY DATA REFERENCES

 
The rift systems comprising the Afro-Arabian triple junction, the Red Sea and Dead Sea, as well as the continental volcanic fields that encompass this area, have been studied intensively in recent years (e.g. Freund et al., 1968; Bohannon et al., 1989; Menzies et al., 1992; Schilling et al., 1992). Oligocene flood volcanism at the Afro-Arabian triple junction in Yemen, Ethiopia, Eritrea and Djibouti was associated with continental extension and possibly the presence of a mantle plume (Chazot & Bertrand, 1993; Deniel et al., 1994; Baker et al., 1996; Stewart & Rogers, 1996; Pik et al., 1999). However, magma genesis and the geodynamic implications of the numerous intraplate volcanic fields on the Arabian plate are less clear (Camp & Roobol, 1992). In the last 15 Myr, widespread intraplate volcanism has occurred along the western margin of the Arabian peninsula (i.e. Jordan, Syria, Israel, Yemen and Saudi Arabia). This intraplate volcanism post-dates the break-up of Africa and Arabia, opening of the Red Sea, and Oligocene flood volcanism in Yemen and Ethiopia (e.g. Camp & Roobol, 1992; Baker et al., 1997). Although areally extensive, the Late Cenozoic intraplate volcanism is volumetrically insignificant compared with Oligocene flood volcanism in Yemen and Ethiopia (12 000 km³ vs 350 000 km³), which may reflect lower mantle temperatures and/or less lithospheric extension than the environment responsible for the large-volume flood volcanic province (White & McKenzie, 1989).

The Afar plume has been linked with Oligocene flood volcanism in Yemen and Ethiopia and also contributed to Pliocene–Recent intraplate volcanism in Yemen (Baker et al., 1996, 1997; Pik et al., 1999). Baker et al. (1997, 1998) suggested that the Yemen late-stage intraplate fields were produced by small amounts of lithospheric extension, which resulted in melting of shallow mantle that was previously metasomatized and hydrated by the Afar plume. However, magma genesis and the geodynamic implications of volcanism in northwestern Arabia farther from the influence of the Afar plume are not so clear cut. A normal mantle potential temperature of 1280°C requires a stretching factor (initial/final lithosphere thickness) of 2–5 for partial melting of dry mantle to occur (McKenzie & Bickle, 1988). This prompted Camp & Roobol (1992) to suggest that, given the small stretching factor, anomalously hot mantle is required to produce alkaline volcanism in Saudi Arabia. They hypothesized that a lobe of hot asthenosphere, believed to have originated in the Afar region, may have been channelled NW along pre-existing lithospheric flexures (e.g. Ebinger & Sleep, 1998) or that a separate mantle plume existed beneath Saudi Arabia. In contrast, Altherr et al. (1990) and Stein & Hofmann (1992) suggested that older shallow mantle sources produced the Saudi Arabian and Israeli alkali basalts. Stein & Hofmann (1992) speculated that an old (>200 Ma) ‘fossilized’ plume head had been stored in the sub-continental lithosphere and that volcanism was periodically triggered by thinning or rifting of the lithosphere.

This study focuses on the largest intraplate volcanic field on the Arabian plate (Harrat Ash Shaam). Using elemental and isotopic data, we address the following aspects of the petrogenesis: (1) the extent to which crustal processes have affected these magmas; (2) the nature of the mantle sources and the possibility that the Afar plume has been channelled northwards beneath the Arabian plate; (3) temporal and spatial relationships within the intraplate volcanic field; (4) the relationship between magma genesis of Harrat Ash Shaam and evolution of the larger Red Sea area.

	GEOLOGICAL SETTING

TOP ABSTRACT INTRODUCTION GEOLOGICAL SETTING ANALYTICAL TECHNIQUES PETROGRAPHY MAJOR AND TRACE ELEMENT... Sr-Nd-Pb ISOTOPE CHEMISTRY DISCUSSION CONCLUSIONS SUPPLEMENTARY DATA REFERENCES

 
Jordan is located at the northern end of the Red Sea where the Dead Sea plate boundary marks the transition from fully fledged oceanic rifting (i.e. central Red Sea) to trans-tensional faulting in a continental setting (Lyberis, 1988). Juvenile Pan-African basement that crops out in SW Jordan has been dated at 630–540 Ma (Ibrahim & McCourt, 1995). This basement is overlain by a series of continental and marine sandstones and limestones of Cambrian to Oligocene age. NW–SE-striking Mesozoic graben structures formed during the northeastward drift of the Arabian plate and are filled with <7 km of Cretaceous–Paleogene marine sediments. Closure of the Tethys Ocean resulted in Early Neogene regional uplift, east–west extensional tectonics, formation of the present-day Dead Sea plate boundary and initiation of compressional structures at right angles to the Mesozoic grabens (Guba & Mustafa, 1988).

Volcanism in Jordan occurs as sporadic small volcanic centres along the eastern side of the Dead Sea plate boundary and as an areally extensive intraplate volcanic field to the NE of the plate boundary (Fig. 1). Volcanism is clearly associated with continental rifting and inception of the Dead Sea plate boundary, but the exact relationship between magmatism and tectonics and the processes that have produced melt generation are unclear. Barberi et al. (1979) reported that alkaline volcanism in Jordan, similar to other Arabian intraplate volcanic fields, was erupted through two main fissure systems. Along the eastern margin of the Dead Sea rift the fissures trend east–west, and on the large basaltic plateau the fissures trend NW–SE. Volcanism probably commenced during the Miocene (14 Ma) and has continued to the present day, producing a laterally extensive volcanic field (46 000 km²) (K. M. Ibrahim, unpublished data, 1999), which is the largest single intraplate field on the Arabian plate. The volcanic field extends from Syria through Jordan and into Saudi Arabia and varies in thickness from 100 m to 1·5 km, reaching an altitude of 1800 m on the Syrian plateau. The volcanic field in Jordan itself covers an area of 11 400 km² (Guba & Mustafa, 1988) and is typically <150 m thick. The volcanic field comprises a series of subhorizontal lava flows <25 m thick, numerous scoria cones, extensional faults, and large fissure eruptions that emanated from dykes <100 km in length that strike NW–SE and north–south, forming elongated ridges (K. M. Ibrahim, unpublished data, 1999). Scoria cones and fissures occur throughout the area that is covered by lava flows.

View larger version (59K): [in this window] [in a new window]   Fig. 1. Map of the Miocene–Recent intraplate volcanic field Harrat Ash Shaam, and the numerous small volcanic outcrops along the eastern margin of the Dead Sea plate boundary. Inset shows regional map of the Afro-Arabian area.

 
K–Ar dates from Harrat Ash Shaam (Ilani et al., 2001) range from 0·05 to 40 Ma. Interestingly, only a small number of ages (5%) >14 Ma have been reported, suggesting that little volcanism occurred before 14 Ma. Volcanism appears to have been concentrated in two periods: from 0 to 8 Ma (with the majority of ages <5 Ma) and an earlier period from 8 to 14 Ma (most ages from 8 to 12 Ma). The distinct age groups will be referred to in later geochemical discussions as ‘younger’ and ‘older’. These newly published K–Ar ages confirm earlier work by Barberi et al. (1979), who reported K–Ar ages of <0·5 Ma to 13·7 Ma for Harrat Ash Shaam. Historical eruptions were also recorded in the 17th century (Tarawneh et al., 2000). K–Ar ages from numerous intraplate volcanic fields in Saudi Arabia (Camp & Roobol, 1989, 1992; Camp et al., 1991, 1992) define a period of volcanism from 0 to 12 Ma, but few ages of volcanic activity are older than 7 Ma. This followed a period of sporadic, volumetrically insignificant volcanism from 12 to 20 Ma and an older magmatic period from 20 to 32 Ma. The majority of volcanism in both Saudi Arabia and Jordan therefore appears to be younger than 12 Ma.

	ANALYTICAL TECHNIQUES

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Ninety-three samples were collected by the National Resources Authority of Jordan and the Geological Survey of Israel in 1999. The sampling covers the entire stratigraphic section (Ilani et al., 2001) and a large proportion of the volcanic field. These samples and further sampling carried out by the authors in 2000 provide the basis for this geochemical study. Representative major and trace element and Sr–Nd–Pb isotopic data are presented in Tables 1, 1A, 2, and 2A respectively. The full dataset is available online and may be downloaded from the Journal of Petrology website at http://www.petrology.oupjournals.org. Major element data were determined on fused glass discs by X-ray fluorescence spectrometry (XRF) at the Geological Institute, University of Copenhagen. Trace elements were determined on pressed powder pellets by XRF at Royal Holloway University of London (RHUL). Rare earth elements (REE) were determined on a small subset of samples at RHUL. Light rare earth elements (LREE) were determined by thermal ionization mass spectrometry (TIMS) isotope dilution analysis after chemical separation using anion exchange resin. Heavy rare earth elements (HREE) were passed through an additional set of anion exchange columns and analysed on an IsoProbe multiple-collector inductively coupled plasma mass spectrometer (MC-ICP-MS). External reproducibility for replicate LREE and HREE analyses is <2·5% and <3·3% (2SD), respectively. However, elemental ratios have significantly better reproducibility (Sm/Nd 0·03%, Dy/Yb 0·04% 2SD, n = 7), reflecting the fact that weighing errors control the reproducibility of the concentration determinations. External reproducibility of BHVO-1 is generally <1% for the LREE and slightly higher (1–2%) for the HREE.

View this table: [in this window] [in a new window]   Table 1: Representative major and trace element data and K–Ar ages for Jordan intraplate volcanic rocks (Harrat Ash Shaam)

 

View this table: [in this window] [in a new window]   Table 1A: Table 1 (continued)

 

View this table: [in this window] [in a new window]   Table 2: REE isotope dilution and Nd–Sr–Pb isotopic data for Jordanian intraplate volcanic rocks (Harrat Ash Shaam)

 

View this table: [in this window] [in a new window]   Table 2A: Table 2 (continued)

 
Sr–Nd–Pb isotope analyses were determined on an Axiom MC-ICP-MS at the Danish Lithosphere Centre (DLC), Copenhagen, after chemical separation using cation (Nd and Sr) and anion (Pb) exchange resins. For Sr isotope analysis, 1 g of powder was leached in 6 M HCl for 1 h at 190°C then repeatedly rinsed with MQ H₂O. Further details on analytical procedures for Nd and Sr have been described by Luais et al. (1997) and Waight et al. (2002), respectively. Waight et al. (2002) demonstrated that replicate analyses of standards and aqueous solutions are comparable to the reproducibilities given by TIMS analyses. Similarly, Nd isotopic analyses of BCR-1 (0·512634 ± 5) and BHVO-1 (0·513002 ± 5) by MC-ICP-MS at the DLC are within error of TIMS analyses. Internal precision (2SE) was typically better than ±0·000015 for Sr and ±0·00001 for Nd. External reproducibility of Sr and Nd isotope ratios is better than 0·00002 and 0·00001 (2SD), respectively. A standard was run after every four samples. Sr is reported relative to values of 0·71025 for SRM987 and Nd is reported relative to 0·512125 for an in-house AMES Nd and mixed AMES Sm–Nd standard. This Nd isotope ratio corresponds to a value of 0·51186 for La Jolla. Pb isotopic data were determined on 0·07–0·09 g of clean hand-picked rock chips. These were rinsed in MQ H₂O and leached for 1 h in hot 6 M HCl, then rinsed repeatedly in MQ H₂O before digestion and standard anion exchange separation. Pb isotopes were corrected for instrumental mass bias by internal second element normalization using Tl, and the exponential mass fractionation law. A Tl/Pb ratio of 0·7 and a ²⁰⁵Tl/²⁰³Tl ratio of 2·3889 were used, and variable Tl/Pb ratios produced no change in ²⁰⁶Pb/²⁰⁴Pb over a range of Tl/Pb ratios (0–1·5) (100 ppm). Internal within-run precision of ²⁰⁶Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb ratios was generally better than (2SE) ±0·0021, ±0·0017 and ±0·0047, respectively, although three samples had slightly worse internal precision (±0·003, ±0·0026 and ±0·0055), as a result of poor Pb yields during the chemical separation procedure. Pb isotope ratios are reported relative to ²⁰⁶Pb/²⁰⁴Pb = 16·937, ²⁰⁷Pb/²⁰⁴Pb = 15·491 and ²⁰⁸Pb/²⁰⁴Pb = 36·720 for SRM 981 with an external precision of ±0·0035, ±0·0019 and ±0·0055 (2SD, n = 8) obtained during the course of this work. Procedural blanks were 0·5 ng, 0·05 ng and 0·075 ng for Sr, Nd and Pb, respectively, and are insignificant to this study.

	PETROGRAPHY

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The Jordan intraplate volcanic rocks contain 5–20 modal % phenocrysts of olivine ± plagioclase ± clinopyroxene. Olivine phenocrysts vary in size from <0·5 to 3 mm and commonly have iddingsite rims or are completely altered to iddingsite. This is most common in basalts situated in the southeastern part of the intraplate field, where samples are considerably more weathered. In hand specimen, some samples are characterized by caliche lined vesicles and veins within the rock. It was not always possible to avoid this when selecting chips for crushing for chemical and isotopic analysis. Only one sample (JS049) appears to have accumulated olivine. The more primitive samples have predominantly small olivine phenocrysts (generally <0·5 mm) within a cryptocrystalline groundmass. Plagioclase and clinopyroxene phenocrysts are less abundant than olivine. Evolved rocks are more coarse grained, with abundant plagioclase and clinopyroxene as well as olivine phenocrysts, and the phenocrysts are much larger (olivine phenocrysts <3 mm) than in the more primitive samples. The groundmass is microcrystalline and consists of plag ± cpx ± ol ± Fe–Ti oxides. Phenocrysts are euhedral to subhedral in shape, and olivine and clinopyroxene are skeletal in some cases.

	MAJOR AND TRACE ELEMENT CHEMISTRY

TOP ABSTRACT INTRODUCTION GEOLOGICAL SETTING ANALYTICAL TECHNIQUES PETROGRAPHY MAJOR AND TRACE ELEMENT... Sr-Nd-Pb ISOTOPE CHEMISTRY DISCUSSION CONCLUSIONS SUPPLEMENTARY DATA REFERENCES

 
Chemical classification
The Harrat Ash Shaam volcanic rocks exhibit a wide range of silica contents (41·0–49·6 wt %) and can be classified as alkali basalts (64%), basanites (32%), hawaiites (2%) and picrobasalts (2%) using the total alkalis vs silica classification scheme (Le Bas et al., 1986) (Fig. 2). The majority of samples are alkaline according to the alkaline–subalkaline classification system of MacDonald (1968), and there does not appear to be any significant change in composition in the erupted lavas from Miocene to Recent time. Compared with rock suites from the intraplate volcanic fields in Saudi Arabia and Yemen, the Jordan intraplate volcanic rocks lack more evolved compositions (Camp & Roobol, 1989; Camp et al., 1991, 1992; Baker et al., 1997). Miocene to Recent volcanic rocks from Israel have similar compositions to the Jordanian rocks (Stein & Hofmann, 1992).

View larger version (31K): [in this window] [in a new window]   Fig. 2. Total alkalis vs silica diagram (Le Bas et al., 1986) highlighting the wide range of silica saturation and the alkalic nature of the volcanic rocks. The dashed line represents the field for Saudi Arabian intraplate volcanic rocks (Altherr et al., 1990) and the continuous line represents the field for Yemen intraplate volcanic rocks (Baker et al., 1997). There is a lack of evolved compositions in Jordan compared with intraplate volcanic rocks from Yemen and Saudi Arabia. The alkaline–subalkaline divide is from MacDonald (1968). , young series rocks; , old series rocks.

 
Major and trace element observations
MgO versus other major elements
MgO contents vary from 12·2 to 4·2 wt %, although most samples are restricted to 7–11 wt % (Fig. 3a–h). SiO₂, Al₂O₃ and CaO show a general increase with decreasing MgO contents. FeO (total) decreases with decreasing MgO. P₂O₅, Na₂O, K₂O and TiO₂ show little co-variation with MgO content, and there is generally large variation and scatter within the dataset. Fractionation crystallization vectors (Fig. 3a–h) show schematically that fractionation of olivine and possibly clinopyroxene has taken place, although fractionation of these phases alone can clearly not be responsible for all the major element variability. For example, at 8·5–10 wt % MgO there is significant variation in major elements, which is comparable with the entire variation displayed by the dataset as a whole. Both the younger and older series show broadly similar element concentrations and ranges.

View larger version (32K): [in this window] [in a new window]   Fig. 3. (a–h) MgO vs other major element data. Diagrams are annotated with crystal fractionation vectors. The position of the vectors corresponds approximately to that estimated for a primary magma composition. Symbols are the same as in Fig. 2.

 
Trace element variation
Chondrite-normalized REE patterns highlight that the Jordan intraplate volcanic rocks are strongly and variably LREE enriched relative to chondrites (La_N = 29–167) (Fig. 4a and b). All the rocks have small positive Eu anomalies (Eu/Eu^* 1·05–1·16). REE patterns for different samples also cross, reflecting variable LREE/HREE ratios. Primitive-mantle-normalized patterns commonly peak at Nb, and are depleted in Rb relative to Ba (Fig. 4c and d). The most primitive samples have negative K and Pb anomalies, and similar patterns to some ocean-island basalts (OIB), e.g. St Helena (Weaver et al., 1987; Chaffey et al., 1989). Jordanian incompatible trace element ratios are similar to HIMU-type OIB; e.g. K/Nb 144, La/Nb 0·72, Th/La 0·09, Ba/Nb 5·6 for HIMU OIB (Weaver, 1991) vs K/Nb 176, La/Nb 0·66, Th/La 0·11, Ba/Nb 5·8 for Jordan. In contrast to primitive samples, some of the more evolved rocks have larger positive Ba and positive, or absent, K and Pb anomalies.

View larger version (27K): [in this window] [in a new window]   Fig. 4. (a, b) Chondrite-normalized REE diagrams, and (c, d) primitive-mantle-normalized multi-element diagrams for selected Jordan samples. (a) and (c) are primitive samples with MgO >9 wt %; (b) and (d) are evolved samples with MgO <9 wt %. Normalization values from Sun & McDonough (1989). A primitive-mantle-normalized ocean-island basalt (St Helena) has a similar pattern to the most primitive Jordan samples (data from Weaver et al., 1987; Chaffey et al., 1989).

 

	Sr–Nd–Pb ISOTOPE CHEMISTRY

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Sr–Nd isotopic composition
The Jordanian intraplate volcanic rocks display the following range in Sr and Nd isotope ratios: ⁸⁷Sr/⁸⁶Sr = 0·70305–0·70415, ¹⁴³Nd/¹⁴⁴Nd = 0·51297–0·51285. Sr and Nd isotope ratios are strongly correlated, although there are some samples that lie above the main array. There are no distinct compositional differences between the younger and older series rocks (Fig. 5a). Cenozoic volcanic rock compositions from Israel, Saudi Arabia and Yemen are plotted for comparison in Figure 5a. Jordanian intraplate volcanic rocks extend to lower Sr isotopic values than Yemen intraplate volcanics, whereas some Saudi Arabian intraplate volcanic rocks have slightly lower ⁸⁷Sr/⁸⁶Sr and higher ¹⁴³Nd/¹⁴⁴Nd ratios than rocks from both Jordan and Yemen. Israeli intraplate volcanic rocks have lower ⁸⁷Sr/⁸⁶Sr than the majority of Jordanian rocks. Also shown in Figure 5a are possible mantle source compositions [e.g. Afar plume, Red Sea mid-ocean ridge basalt (MORB) and lithospheric mantle xenolith compositions]. Jordanian samples are isotopically more enriched (higher ⁸⁷Sr/⁸⁶Sr and lower ¹⁴³Nd/¹⁴⁴Nd) than Red Sea MORB and overlap with compositions that are representative of the Afar plume (e.g. uncontaminated basalts from Djibouti). Lithospheric mantle xenoliths from Yemen and Saudi Arabia have a wide range of compositions and encompass nearly all the Jordanian isotopic compositions.

View larger version (43K): [in this window] [in a new window]   Fig. 5. Nd–Sr isotopic compositions of the Jordan volcanic rocks. Plotted for comparison are: (a) fields for Red Sea MORB (Schilling et al., 1992; Volker et al., 1993; Haase et al., 2000), the Afar plume (Deniel et al., 1994), Arabian lithospheric mantle (Blusztajn et al., 1995; Baker et al., 1998), and Yemen, Saudi Arabian and Israeli intraplate volcanism (Altherr et al., 1990; Stein & Hofmann, 1992; Baker et al., 1997), and (b) crustal compositions for Late Proterozoic upper crust from Saudi Arabia (Duyverman et al., 1982; Hegner & Pallister, 1989), Late Proterozoic mafic granulites from Saudi Arabia (McGuire & Stern, 1993), Early Proterozoic mafic granulites from Tanzania (Cohen et al., 1984), and Early Proterozoic–Late Archaean upper and lower crust from Sudan (Davidson & Wilson, 1989). 87Sr/86Sr>>1 indicates that there are some extreme Late Proterozoic upper-crustal compositions (Duyverman et al., 1982) that plot off the diagram. Symbols are defined in Fig. 2.

 
Pb isotopic composition
The data for Harrat Ash Shaam basalts define a restricted range of compositions in Pb isotopic space (²⁰⁶Pb/²⁰⁴Pb = 18·83–19·24, ²⁰⁷Pb/²⁰⁴Pb = 15·56–15·65, ²⁰⁸Pb/²⁰⁴Pb = 38·65–39·01) (Fig. 6a and b). Jordanian rocks that have the most isotopically depleted Sr and Nd values lie close to the Northern Hemisphere Reference Line (NHRL) (Hart, 1984). More enriched samples (lower ¹⁴³Nd/¹⁴⁴Nd) extend to higher ²⁰⁷Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb compositions. In ²⁰⁶Pb/²⁰⁴Pb–²⁰⁷Pb/²⁰⁴Pb–²⁰⁸Pb/²⁰⁴Pb space Jordanian Pb isotope ratios overlap with the Afar plume, Saudi Arabian and Israel intraplate volcanic compositions, whereas the least radiogenic ²⁰⁷Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb compositions have similar values to the most radiogenic Red Sea MORB mantle compositions (Schilling et al., 1992; Volker et al., 1993). A large proportion of the Yemen intraplate volcanic rocks are offset to lower ²⁰⁶Pb/²⁰⁴Pb (Baker et al., 1997). However, a plot of 7/4 vs 8/4 (Fig. 6c) reveals subtle differences in composition between Jordanian volcanic rocks and Afar plume-related products. Afar plume-related rocks and Yemen volcanics have 7/4 values similar to Jordan volcanics, but the Jordan volcanics are offset to lower 8/4 values, and are similar to the lowest Red Sea MORB 8/4 ratios. The difference between Jordan intraplate volcanism and the Afar plume is not readily explained by errors resulting from mass fractionation during the analyses.

View larger version (42K): [in this window] [in a new window]   Fig. 6. (a, b) Pb isotopic compositions for Jordan intraplate volcanic rocks compared with Quaternary intraplate volcanism in Yemen (Baker et al., 1997), Saudi Arabia (Altherr et al., 1990) (excluding two samples with very high 7/4), and Israel (Stein & Hofmann, 1992). Also plotted for comparison are Red Sea MORB (Schilling et al., 1992; Volker et al., 1993) and the Afar plume (Deniel et al., 1994). Analytical uncertainty for Jordanian samples (2SD) is smaller than symbol size. (c) 7/4 vs 8/4 for Jordan volcanic rocks compared with Red Sea MORB, the Afar plume and Arabian intraplate volcanics. References are the same as for (a). (d) Pan-African Pb isotopic crustal compositions: Sudanese crust (Davidson & Wilson, 1989); Tanzanian crust (Cohen et al., 1984); Saudi Arabian upper crust (Hegner & Pallister, 1989); Zabargad gneisses (Brueckner et al., 1995). NHRL, Northern Hemisphere Reference Line (Hart, 1984). Symbols are defined in Fig. 2.

 

	DISCUSSION

TOP ABSTRACT INTRODUCTION GEOLOGICAL SETTING ANALYTICAL TECHNIQUES PETROGRAPHY MAJOR AND TRACE ELEMENT... Sr-Nd-Pb ISOTOPE CHEMISTRY DISCUSSION CONCLUSIONS SUPPLEMENTARY DATA REFERENCES

 
The Jordan intraplate volcanic rocks comprise alkaline basalts, basanites and hawaiites that have undergone fractionation of olivine ± plagioclase ± clinopyroxene and, although relatively mafic, are unlikely to be representative of primary magmas given their Mg numbers, and Ni and Cr contents (Table 1). There is considerable correlated variation in major and trace element abundances or ratios and isotopic ratios, but these variations are unlikely to be the product of fractional crystallization alone. There are no distinct compositional differences between the younger and older series. There are also Nd–Sr–Pb isotopic similarities between Jordan and other Arabian intraplate volcanic fields. However, the Afar plume and Yemen intraplate volcanic field appear to have higher 8/4 ratios, and Yemeni rocks are generally offset to lower ²⁰⁶Pb/²⁰⁴Pb ratios. We now examine the processes that may be responsible for these geochemical variations, but the focus of the discussion is on evaluating the chemical and isotopic variations in primitive samples that are difficult to explain by fractional crystallization or crustal contamination and are likely to reflect mantle processes.

Processes contributing to major and trace element and isotopic variation of Jordanian intraplate volcanism
The effects of alteration
The effects of alteration on the Jordanian intraplate volcanic rocks are difficult to assess. In thin section, the only mineral visibly affected by alteration is olivine, which is often completely or partially altered to iddingsite. However, in hand specimen caliche is present in some samples. Caliche is a common secondary growth product in arid environments, and as it occurs in cavities throughout the rock it is difficult to remove. Five samples have unusually high Ba concentrations (Ba 647–2493 ppm) (JS011, JS030, JS076, JS080 and JS082) and high incompatible trace element ratios involving Ba (e.g. Ba/Nb, Ba/La and Ba/Th). For example, Ba concentrations and ratios in samples JS011, JS030, JS076, JS080 and JS082 are similar to, or in excess of crustal values, e.g. SW Jordan average upper-crustal values for Ba/La are 43, and the highest Jordanian volcanic Ba/La ratio is 158. Three of these samples (JS030, JS076 and JS082) also have high ⁸⁷Sr/⁸⁶Sr ratios for their ¹⁴³Nd/¹⁴⁴Nd values (Fig. 5a). Pre-digestion leaching of the samples in hot 6 M HCl should have effectively removed any radiogenic Sr introduced during alteration. However, although the measured ⁸⁷Sr/⁸⁶Sr ratios on leached samples are likely to reflect magmatic values, we cannot rule out the possibility of potential alteration. Nevertheless, we have included the high ⁸⁷Sr/⁸⁶Sr samples in the discussion. The reason for the high Ba concentrations is more difficult to assess, given that it could be a source feature or the product of secondary alteration. High Ba concentrations are unlikely to be a source feature, because there is no correlation between Ba (and trace element ratios involving Ba) and major elements, trace elements and isotopes. Secondary alteration is considered more likely, because of the presence of caliche in these Jordanian volcanic rocks. High Sr contents in some of these samples are consistent with the presence of carbonate. In the light of probable alteration effects on Ba concentrations, we have avoided using this element in the subsequent discussion.

Fractional crystallization
The phenocryst assemblages observed in the Jordan intraplate volcanic rocks suggest that variable but, in most cases, fairly limited fractionation of olivine (ol) ± plagioclase (plag) ± clinopyroxene (cpx) has occurred. Fractional crystallization vectors in Figure 3 support ol ± cpx fractionation, as do strong decreases in Ni and Cr coupled with decreasing MgO. In thin section, plagioclase appears to be an important fractionating phase, particularly in the more evolved rocks. However, plagioclase fractionation is not supported by the general lack of correlation between Sr/Nd and SiO₂–MgO, and increasing Al₂O₃ with decreasing MgO. Furthermore, at high MgO contents (8·5–10 wt %) large variations in major element (e.g. SiO₂ varies from 41·6 to 48·5 wt %) (Fig. 3) and trace element abundances (e.g. Ce varies from 22·5 to 110 ppm) make it difficult to rigorously constrain the fractionating assemblage and proportions, and highlight the need to investigate other factors responsible for major and trace element variability.

Crustal contamination?
Here we investigate the role of crustal contamination in contributing to elemental isotopic variation during magma genesis. There have been no detailed studies of the age or the composition of the crust underlying the Jordan volcanic field, but studies of the basement from SW Jordan have yielded Rb–Sr ages of 540–639 Ma (Ibrahim & McCourt, 1995). This suggests that Late Proterozoic age crust may exist beneath the Harrat Ash Shaam volcanic field. There is a large range of Nd, Sr and Pb isotopic crustal compositions from Saudi Arabia and elsewhere, which provide an indication of possible crustal contaminants (Figs 5b and 6d). However, on the basis of studies in SW Jordan, it is likely that crust similar to the Arabian shield (of Late Proterozoic age and composition) is a more likely contaminant than isotopically more extreme Early Proterozoic–Late Archaean age crust found in Tanzania, Sudan and the southern Arabian shield (Yemen). On this basis, bulk mixing and assimilation–fractional crystallization (AFC) calculations were undertaken using compositions typical of this type of crust (upper crust—Duyverman et al., 1982; Hegner & Pallister, 1989; lower crust—McGuire & Stern, 1993). Bulk mixing and AFC trajectories in Nd–Sr isotopic space are illustrated in Figure 7a and b. The starting composition of the uncontaminated magma and the composition of the crustal contaminants are listed in Table 3. These calculations reveal the following.

1. Up to 20% bulk assimilation of a Late Proterozoic upper-crustal component (curves 1 and 2) can account for Nd–Sr variation in the ‘upper array’ (Fig. 7a), which is defined by three samples (JS030, JS076 and JS082) that are offset from the main Nd–Sr isotopic array to higher ⁸⁷Sr/⁸⁶Sr values. Modelling using AFC curves (DePaolo, 1981) (Fig. 7b) and the appropriate crustal contaminants do not differ markedly from simple bulk mixing curves in terms of the amount of crustal contaminant required to produce the observed data array. However, although bulk mixing and AFC modelling support crustal contamination of Jordanian intraplate volcanic rocks, we cannot rule out the possibility of secondary alteration having affected samples JS030, JS076 and JS082.
   
2. Assimilation of a Late Proterozoic lower-crustal component (3) can also reproduce the variation in the upper array but requires unrealistic amounts of contamination to reproduce the highest ⁸⁷Sr/⁸⁶Sr ratios (>>50%) (Fig. 7a), given the similarity of lower-crustal ⁸⁷Sr/⁸⁶Sr ratios to contaminated Jordanian rocks. Likewise, a mixture between the two end-member bulk mixing lower-crustal curves (3 and 4) could reproduce the steeper Sr–Nd isotopic array but also requires assimilation of >>50% crust. These features suggest that the lower Nd–Sr isotopic array is unlikely to be the result of significant amounts of assimilation of the locally available continental crust.
   
3. Archaean–Proterozoic silicic lower- and upper-crustal compositions from Sudan (Davidson & Wilson, 1989) have more extreme ⁸⁷Sr/⁸⁶Sr (0·76) and ¹⁴³Nd/¹⁴⁴Nd (0·51) ratios that could produce the Sr–Nd isotopic variations of the steeper array at more realistic amounts of contamination (not shown). However, this possibility can be excluded as: (a) there is no evidence for crust of this age underlying Jordan; (b) contamination by such an older crustal component would generate large changes in Pb isotopic ratios (Fig. 6d), which are not observed.
   

View larger version (27K): [in this window] [in a new window]   Fig. 7. (a) Bulk mixing curves illustrating the possible effects of crustal contamination on the Jordanian volcanic rocks. Continuous lines represent Late Proterozoic upper crust; dashed lines represent Late Proterozoic lower crust. (b) Assimilation–fractional crystallization calculations (DePaolo, 1981) illustrating the effects of crustal contamination. The uncontaminated basalt has the same composition as in (a); the composition of the crustal contaminant is the same as curve (1). The rate of assimilation to fractional crystallization was assumed to be 0·6. F, fraction of melt remaining; P, crust/magma ratio (Aitcheson & Forrest, 1994). Model parameters are given in Table 3. Symbols are as for Fig. 2.

 

View this table: [in this window] [in a new window]   Table 3: Compositions of the initial uncontaminated basalt and crustal contaminants used to calculate bulk mixing and AFC trajectories in Fig. 7

 
We therefore conclude that although contamination by Late Proterozoic crust may have contributed to some minor isotopic variation, it is not responsible for the major isotopic variations (e.g. the steep array in Nd–Sr isotopic space). It is important to note that there is no correlation between Mg number and Nd isotopic ratios, and that the complete range of ¹⁴³Nd/¹⁴⁴Nd is observed in samples with nearly identical and high Mg numbers. Instead, the data appear to require a heterogeneous mantle source. Strong correlations between isotopic data, major and trace element abundances and trace element ratios (next section) support this interpretation. For example, La concentrations decrease by a factor of more than five from SiO₂ of 40 to 50 wt % (Fig. 8c), and are accompanied by changing ¹⁴³Nd/¹⁴⁴Nd (0·51297–0·51284). The nature and magnitude of these correlations is not consistent with assimilation of upper or lower continental crust.

View larger version (32K): [in this window] [in a new window]   Fig. 8. (a) SiO2 vs FeO (total); (b) FeO (total) vs Na2O. Only Jordanian rocks with 8·5–10 wt % MgO are plotted, along with Yemen intraplate volcanic data (Baker et al., 1997) (regressed to 8·5 wt % MgO using least-squares linear regression). (c–e) La (ppm) and 143Nd/144Nd ratios show a negative correlation with SiO2, as does FeO vs Rb/Nb. (f) Red Sea MORB isotopic compositions are more depleted than Jordanian intraplate volcanics and the Afar plume. Data sources are the same as in Fig. 5a. Symbols are defined in Fig. 2.

 
Reasons for major and trace element and isotopic variation outside the control of fractional crystallization and crustal contamination
We can explore the possible mantle source characteristics and the effects of varying degrees and depths of partial melting on major element data by reducing the effects of fractional crystallization and crustal contamination in a manner similar to that of Klein & Langmuir (1987) and Turner & Hawkesworth (1995). For this exercise, the sample set was truncated and samples with <8·5 and >10 wt % MgO were excluded. Large scatter within the dataset prevents the use of least-squares regression techniques to eliminate the effects of fractional crystallization. The three samples that were recognized as having potentially assimilated 20% continental crust (JS030, JS076 and JS082) were also excluded from further discussion. Samples within this data range (8·5–10 wt % MgO) have chemical variations not consistent with fractional crystallization processes. For example, measured FeO decreases systematically with increasing SiO₂ and decreasing Na₂O, indicative of decreasing melting pressure (i.e. depth) coupled with increasing degrees of melting (Fig. 8a and b). Na₂O, TiO₂ and K₂O are moderately incompatible in peridotite and are concentrated in small melt fractions; as such, their concentrations in a melt decrease as the degree of melting (SiO₂) increases (Klein & Langmuir, 1987; Falloon et al., 1988). In contrast to Jordan, Yemen intraplate volcanic rocks (corrected to 8·5 wt % MgO by least-squares regression) are offset from the Jordan data to significantly lower FeO_(8·5) at similar SiO_2(8·5) and Na₂O_(8·5) contents. This suggests different melting and/or source conditions in Yemen and Jordan and supports evidence that Yemen intraplate volcanism is the result of melting rather shallow mantle enriched and hydrated by the Afar plume (Baker et al., 1997). Melting shallow hydrated mantle would result in lower FeO and higher SiO₂ contents than melting of a deeper source.

The Jordan data show strong correlations between major elements, trace elements and isotopes. SiO₂ and FeO exhibit good correlations with ¹⁴³Nd/¹⁴⁴Nd, ⁸⁷Sr/⁸⁶Sr and ²⁰⁷Pb/²⁰⁴Pb ratios, 7/4, incompatible element abundances such as La, and incompatible trace element ratios such as K/Nb and Rb/Nb (Fig. 8c–e). ¹⁴³Nd/¹⁴⁴Nd also correlates with ²⁰⁷Pb/²⁰⁴Pb and 7/4 (Fig. 8f). These major element, trace element and isotopic correlations cannot be accounted for by crustal contamination and fractional crystallization processes, and suggest contributions from a heterogeneous mantle source consisting of at least one depleted and one enriched component. The depleted component has high ¹⁴³Nd/¹⁴⁴Nd, and low ⁸⁷Sr/⁸⁶Sr, 7/4 and 8/4 ratios, with variable ²⁰⁶Pb/²⁰⁴Pb. Pb isotope compositions lie close to the NHRL (Fig. 6a and b). Although there are similarities with the Afar plume and Yemen intraplate volcanic rocks, Jordanian depleted samples have lower ²⁰⁸Pb/²⁰⁴Pb and 8/4 and encompass higher ¹⁴³Nd/¹⁴⁴Nd and lower ⁸⁷Sr/⁸⁶Sr compositions. None the less, incompatible trace element ratios and primitive-mantle-normalized multi-element patterns are comparable with HIMU OIB (St Helena) (Fig. 4c), and the samples are silica undersaturated (41·5–44·5 wt % SiO₂) with high MgO contents (9·5–12 wt %). Therefore, we suggest that the depleted component is more enriched than Red Sea MORB and subtly distinct from the Afar plume. For example, Figs 5a and 6c show the offset to lower ⁸⁷Sr/⁸⁶Sr and lower 8/4 than the Afar plume. Lower 8/4 might be explained by a long-term Th-depleted source more similar to MORB than a Th-enriched reservoir such as the continental lithosphere (Williams et al., 1992).

The enriched component has low ¹⁴³Nd/¹⁴⁴Nd and high 7/4, 8/4 and ⁸⁷Sr/⁸⁶Sr, with variable ²⁰⁶Pb/²⁰⁴Pb. High 7/4 and low ¹⁴³Nd/¹⁴⁴Nd are coupled with higher incompatible trace element ratios than the depleted component and incompatible trace element ratios are similar to enriched OIB, e.g. EMI (Gough): K/Nb 432 (Weaver, 1991); Jordan: K/Nb 408.

Although it is likely that the major element, trace element and isotopic correlations reflect a chemically and isotopically heterogeneous mantle source, the large variations in major and trace element abundances must also reflect the effects of partial melting processes. For example, a negative correlation between Sm/Nd and ¹⁴³Nd/¹⁴⁴Nd (Fig. 9a) places unusual constraints on the mantle source and melting processes in the petrogenesis of the Jordanian intraplate volcanic rocks. High ¹⁴³Nd/¹⁴⁴Nd ratios suggest a long-term depleted (high Sm/Nd) source. However, the most isotopically depleted Jordanian samples (e.g. JS078: ¹⁴³Nd/¹⁴⁴Nd 0·512970; ⁸⁷Sr/⁸⁶Sr 0·703120; ²⁰⁷Pb/²⁰⁴Pb 15·563) have the lowest Sm/Nd ratios. Low Sm/Nd ratios from a time-integrated high Sm/Nd source are likely to be the product of very low degrees of partial melting in the presence of garnet (as Nd is more incompatible than Sm). This is supported by a negative correlation between Sm/Nd and Dy/Yb and between Sm/Nd and La/Yb (Fig. 9b). Assuming a single starting composition, La/Yb or La/Y ratios reflect the degree of partial melting (La/Yb decreases with increasing degrees of melting), whereas changes in Dy/Yb ratios can be interpreted to reflect the presence of garnet in the source, as HREE are preferentially retained over the middle REE (MREE) during melting if garnet is present. These REE ratios also correlate with major elements, such as FeO and SiO₂, and with ¹⁴³Nd/¹⁴⁴Nd and 7/4 (Fig. 9c). Thus, mixing of small volume melts from a deep garnet-bearing depleted mantle source with larger volume melts from an enriched shallower source can produce the isotopic–elemental co-variations observed in the Jordan intraplate volcanic rocks and provides the simplest explanation for the unusual Sm/Nd–¹⁴³Nd/¹⁴⁴Nd correlations. Chen & Frey (1983) reported similar correlations in Hawaiian basalts and also explained this by mixing melts from an enriched mantle composition with low-degree partial melts from a MORB source.

View larger version (14K): [in this window] [in a new window]   Fig. 9. (a) Sm/Nd vs 143Nd/144Nd; (b) Sm/Nd vs La/Yb; (c) 143Nd/144Nd vs Dy/Yb. Negative correlations between Sm/Nd and 143Nd/144Nd, and Sm/Nd and La/Yb, as well as a positive correlation between Dy/Yb and 143Nd/144Nd, suggest mixing of melts from isotopically distinct mantle sources.

 
We note that the older and younger series are not isotopically distinct in ¹⁴³Nd/¹⁴⁴Nd–⁸⁷Sr/⁸⁶Sr space, nor are there significant differences in major elements or trace element ratios. However, there does appear to be some temporal control on melting sources. Figure 10d shows a systematic change in La/Y_(N) ratios with time, whereby the oldest rocks have low and restricted La/Y_(N) ratios, and the youngest rocks have a much larger range of La/Y_(N) values. Given that La/Y ratios reflect the degree and depth of partial melting, we suggest that the oldest volcanism in Jordan was produced by larger degrees of melting [low La/Y_(N) ratios] of a shallow source and, with time, younger volcanism also incorporated smaller degree melts from a progressively deeper source [high La/Y_(N) ratios].

View larger version (36K): [in this window] [in a new window]   Fig. 10. (a–c) Calculated partial melting curves assuming non-modal fractional melting of garnet and spinel lherzolite sources (garnet lherzolite: 0·598 ol, 0·211 opx, 0·076 cpx, 0·115 grt that melts in the proportions 0·05 ol, 0·2 opx, 0·3 cpx, 0·45 grt; spinel lherzolite: 0·578 ol, 0·27 opx, 0·119 cpx, 0·033 sp that melts in the proportions 0·1 ol, 0·27 opx, 0·5 cpx, 0·13 sp; Thirlwall et al., 1994). The composition of the primitive mantle is from Taylor & McLennan (1985). Modelling used the distribution coefficients from the compilation of McKenzie & O'Nions (1991). Stars annotated on the melting curves represent the following degrees of melting: 0·1, 1, 2, 5, 10, 15, 20 and 25%. Dashed lines represent mixing lines of partial melts from spinel- and garnet-facies mantle. (d) La/Y vs age (Ma). Yemen intraplate volcanic data from Baker et al. (1997). Symbols are the same as in Fig. 2.

 
Establishing the partial melting processes of the heterogeneous mantle source
Having established the nature of the mantle sources, and that partial melting was probably an important control on activating these sources, it is now possible to place some constraints on the conditions of partial melting. Rare earth element ratios are useful for constraining the extent and depth of partial melting along with the mineralogy and chemistry of the source. This is because the solid:melt partitioning of REE is different for garnet- and spinel-facies peridotite. Partial melting of either a garnet or spinel peridotite will preferentially enrich the LREE in the melt and produce La/Yb variations with variable degrees of melting, although the La/Yb variations will be much larger if melting garnet-facies rather than spinel-facies peridotite. Likewise, the degree of enrichment of MREE to HREE depends on whether garnet exists as a residual phase during melting, as HREE are preferentially retained by garnet during melting (high D_Yb 4·0–15) relative to MREE [e.g. using distribution coefficients from McKenzie & O'Nions (1991) and Van Westrenen et al. (2001)]. This produces large changes in MREE/HREE ratios in melts formed by partial melting of garnet-facies peridotite, and there are large differences between source and melt ratios. In contrast, melting of a spinel peridotite will produce little change in MREE/HREE ratios with melt fraction, and their melt and source ratios are similar. Correlations between LREE/HREE (e.g. La/Yb) and MREE/HREE (e.g. Dy/Yb) will be linear when two melts of different REE composition are mixed, as the same element forms the denominator of both ratios.

Figure 10a presents Dy/Yb_(N) vs La/Yb_(N) data for the Jordan intraplate volcanic field, along with trajectories for non-modal fractional melts of garnet and spinel peridotite sources. Source concentrations use primitive mantle values from Taylor & McLennan (1985), which is only a crude starting approximation of the likely source concentrations. A depleted MORB mantle source (not shown) does not have La/Yb ratios high enough to reproduce Jordanian La/Yb values, even at the smallest degrees of melting in the presence of garnet. Primitive-mantle-normalized diagrams and Nd isotopic data (Nd <6·5) also imply a source more enriched than depleted MORB mantle. We can deduce the following points from Fig. 10a–d.

1. Variable degrees of melting of two spinel-bearing sources cannot produce the observed variation in Dy/Yb_(N) and La/Yb_(N), as spinel peridotite partial melts do not have sufficient co-variation of Dy/Yb with La/Yb to produce the Jordanian data array, nor do they have sufficiently high Dy/Yb to reproduce the high Dy/Yb values of many samples (Fig. 10a).
   
2. Variable degrees of melting of two garnet peridotites cannot account for the observed La/Yb_(N)–Dy/Yb_(N) variation, as the lowest Dy/Yb_(N) ratios of the dataset require unrealistically large degrees of melting of garnet-facies mantle (>25%). Moreover, melting of garnet-facies mantle alone would produce no co-variation between La/Yb and Yb (Fig. 10b), as Yb is retained by garnet in the source.
   
3. The Jordanian rocks show small but significant co-variation between La/Yb and Yb, which suggests melting also involved a spinel-bearing source. The La/Yb_(N)–Dy/Yb_(N) linear array (Fig. 10c) is therefore consistent with mixing melts from spinel and garnet peridotite sources.
   
4. Melting of a primitive mantle source can reproduce the La/Yb_(N) ratio of the most LREE-enriched Jordanian sample (e.g. JS078). We can use this sample to place some constraints on the deeper mantle source. A minimum La/Yb_(N) ratio of the source can be determined by extrapolating the sample (JS078) downward to intersect the garnet melting curve (Fig. 10a). Although this value is dependent on source composition [as increasing the source La/Yb_(N) ratio stretches the melting curve parallel to the La/Yb_(N) axis], we estimate a minimum value of La/Yb_(N) 0·57 using a primitive mantle source. As this sample is unlikely to represent 0·01% melting, a source with La/Yb_(N) >0·57 is probable. This is more enriched than a depleted MORB source, which has La/Yb_(N) 0·4. Changing the garnet/clinopyroxene (grt/cpx) ratio also extends the melting curve parallel to the La/Yb_(N) axis. However, published mineral proportions of garnet peridotites (Johnson, 1998; Putirka, 1999) do not differ significantly from our model value of grt/cpx = 1·5. Our only upper constraint on the La/Yb_(N) ratio is from Nd isotopes of the depleted source end-member (¹⁴³Nd/¹⁴⁴Nd 0·51297), which suggest long-term LREE depletion relative to a chondritic mantle source [La/Yb_(N)<1], but not as extreme as depleted MORB mantle.
   
5. A primitive mantle source cannot produce the highest Dy/Yb_(N) ratio of the sample set. This requires a minimum source Dy/Yb_N ratio of 1·22 (Fig. 10c). This is unlikely to be coupled with corresponding La enrichment given that a long-term LREE-depleted source is likely. Increasing grt/cpx ratios also raises Dy/Yb_(N) ratios, but in this case maximizing the grt/cpx ratio using the given mineral proportions cannot raise the garnet melting curve to intersect the sample (JS078). Furthermore, modelling partial melting of garnet pyroxenites or eclogites that have much greater modal garnet cannot increase the Dy/Yb ratio, as grt/cpx ratios are often less than that of a garnet peridotite. A garnet distribution coefficient ratio (D_Dy/D_Yb) 0·2 will also produce the necessary Dy/Yb_(N) ratios, although garnet distribution coefficients are still poorly constrained. For example, garnet distribution coefficients from Johnson (1998) and Van Westrenen et al. (2001) are up to four times greater than those in the compilation reported by McKenzie & O'Nions (1991), and have D_Dy/D_Yb ratios of 0·33 and 0·39, respectively. This results in translation of the melting curve to lower Dy/Yb values.
   

In summary, melting appears to be polybaric, occurring over an extended depth range from garnet-bearing to garnet-absent mantle (i.e. spinel-facies mantle). For example, some samples appear to be derived totally from garnet-facies mantle whereas others are dominated (75%) by melts from the spinel field (Fig. 10c). Combining this information with temporal controls on volcanism, we conclude that earliest volcanism appears to involve predominantly shallower and larger degree melts (2–5%), from a spinel-bearing (and probably also garnet-bearing) isotopically enriched source. With time, volcanism incorporated a greater proportion of deeper, smaller degree melts (<1%) from an isotopically depleted garnet-bearing source. An older series sample (JS069) offset from the main trend to lower La/Yb_(N) can be accounted for by slightly larger degrees of melting of the garnet-bearing source (Fig. 10c).

Jordanian intraplate volcanism within the regional tectonic setting
Normal Arabian lithosphere is 100–120 km thick (Mooney et al., 1985). The upper lithosphere (or mechanical boundary layer) is presumed to comprise crust and spinel-facies (<80 km) and garnet-facies (>80 km) lithospheric mantle. The lower lithosphere will comprise garnet-facies mantle, which overlies and is influenced by the asthenosphere (1300°C). Consistent with the presence of 100–120 km thick lithosphere, surface heat flow measurements over western Arabia are lower than the world average (400–500°C at 40–50 km depth) (Gettings, 1981; Gettings et al., 1986). However, this is in contrast to temperatures calculated from mantle xenolith data, which suggest mantle temperatures of 900–1100°C at similar depths (Camp et al., 1992). The lack of agreement between surface and mantle temperatures was suggested as evidence for the presence of upwelling asthenosphere beneath western Arabia (McGuire & Bohannon, 1989). To what extent this mantle is normal temperature, or hotter than normal, may be influenced by the influx of hotter mantle originating as far south as Afar that is directed northward by ‘topography’ on the base of the lithosphere, as proposed by Ebinger & Sleep (1998).

In the context of the petrogenesis of the Jordanian intraplate volcanic rocks, the temporal and spatial evolution of the volcanic field can be summarized as follows. Volcanic rocks were initially derived from shallow, isotopically slightly enriched, spinel- and garnet-facies mantle by 2–5% melting. Given the thickness of the lithosphere (>100 km), the depth of melting and the distinct isotopic characteristics, we propose that the enriched source is located within the lithospheric mantle. With time, these melts mixed in varying proportions with melts predominantly derived from deeper, isotopically depleted, garnet-facies mantle generated by very small degrees of melting (<1%). Considering lithospheric thicknesses, we propose that the depleted source (>80 km) is likely to be located within the asthenosphere or the lowermost part of the lithospheric mantle.

This scenario is consistent with a model of progressive lithospheric rifting and thinning with time. The extended depth range over which melting occurs (from 100% garnet-facies derived melts to 100% spinel-facies derived melts) suggests a melting regime controlled by lithospheric extension. This is also supported by xenolith data, from which the asthenosphere–lithosphere boundary is estimated to be at a depth of 75 km (McGuire & Bohannon, 1989) compared with >100 km before thinning and rifting. The thermal boundary layer (i.e. the lower lithospheric mantle or the asthenosphere) is melted in response to thermal erosion of the lithosphere by upwelling asthenosphere driven by lithospheric extension. This allows small degree garnet-facies melts to mix with larger degree melts from spinel-facies (and garnet-facies) mantle as extension and volcanism proceeds.

Jordanian intraplate volcanism is isotopically similar in many respects to examples of Late Cenozoic volcanism throughout the Arabian peninsula (Israel, Saudi Arabia). Volcanism in Israel and Saudi Arabia is thought to have been derived from a shallow mantle source, perhaps enriched by an old plume head that periodically produced volcanism in response to lithospheric thinning (Altherr et al., 1990; Stein & Hofmann, 1992). It is likely that both Israel and Saudi Arabian volcanism are tapping similar mantle sources to the Jordanian volcanism. In contrast to Jordan, Yemen intraplate volcanism displays the influence of the Afar plume with shallower levels of melting of a hydrated (e.g. lower FeO and Dy/Yb) and a subtly distinct isotopic source (e.g. higher 8/4 than the Jordanian samples) (Baker et al., 1997). Given these chemical and isotopic contrasts, we conclude that the Afar plume has not been channelled northwards beneath the Arabian plate.

	CONCLUSIONS

TOP ABSTRACT INTRODUCTION GEOLOGICAL SETTING ANALYTICAL TECHNIQUES PETROGRAPHY MAJOR AND TRACE ELEMENT... Sr-Nd-Pb ISOTOPE CHEMISTRY DISCUSSION CONCLUSIONS SUPPLEMENTARY DATA REFERENCES

 
Miocene to Recent volcanism in northwestern Arabia produced the largest intraplate volcanic field on the Arabian plate (Harrat Ash Shaam, Jordan). The following conclusions can be gleaned from this study.

1. The chemically and isotopically diverse volcanic field comprises mafic alkali basalts and basanites. Most magmas have undergone only limited fractional crystallization of ol ± cpx ± plag, and rare samples have assimilated up to 20% of Late Proterozoic crust en route to the surface. Jordanian intraplate volcanic rocks lack the evolved compositions of intraplate volcanic rocks from Saudi Arabia and Yemen, as the extensional environment permitted ascent of more mafic magmas in Jordan, which did not undergo protracted fractionation.
   
2. Large coupled variations in major elements, trace elements and Sr–Nd–Pb isotopes (e.g. a negative correlation between Sm/Nd and ¹⁴³Nd/¹⁴⁴Nd) cannot be explained by fractional crystallization and crustal contamination alone and require polybaric melting of heterogeneous mantle sources. This involves 2–5% melting of a shallow spinel- and garnet-facies mantle source that represents Arabian lithosphere. Melts from this mix in variable proportions with smaller degree melts (<1%) from a deep garnet-bearing lithospheric or asthenospheric source. This source might be a minor ambient component of the asthenosphere preferentially tapped at small degrees of melting.
   
3. Volcanism in Jordan is consistent with a model in which polybaric melting of distinct sources occurred in response to lithospheric extension. Extension first triggered partial melting of shallow lithospheric mantle. As extension progressed and asthenosphere upwelled, these melts mixed with smaller degree melts from deep (asthenospheric?) mantle.
   
4. Jordanian intraplate volcanism is isotopically similar in many respects to examples of Late Cenozoic volcanism throughout the Arabian peninsula (Israel, Saudi Arabia). In contrast to Jordan, Yemen intraplate volcanism displays the influence of the Afar plume with shallower levels of melting of a hydrated and a subtly distinct isotopic source. We conclude that the Afar plume has not been channelled northwards beneath the Arabian plate and probably played no role in many of the northern Saudi Arabian, Israeli and Jordan intraplate volcanic fields.
   

	SUPPLEMENTARY DATA

TOP ABSTRACT INTRODUCTION GEOLOGICAL SETTING ANALYTICAL TECHNIQUES PETROGRAPHY MAJOR AND TRACE ELEMENT... Sr-Nd-Pb ISOTOPE CHEMISTRY DISCUSSION CONCLUSIONS SUPPLEMENTARY DATA REFERENCES

 
For supplementary data, please refer to Journal of Petrology online.

	ACKNOWLEDGEMENTS

 
We are grateful to the National Resources Authority of Jordan and the Geological Survey of Israel. Thoughtful and constructive reviews from David Peate, Nick Arndt, Karsten Haase and the editor Pamela Kempton improved the quality of the manuscript. The Danish Lithosphere Centre (DLC) and Royal Holloway University of London are thanked for the joint funding of a Ph.D. studentship that made this project possible. The DLC is supported by the Danish National Research Foundation. This paper is part of the doctoral research of J.E.S.

	REFERENCES

TOP ABSTRACT INTRODUCTION GEOLOGICAL SETTING ANALYTICAL TECHNIQUES PETROGRAPHY MAJOR AND TRACE ELEMENT... Sr-Nd-Pb ISOTOPE CHEMISTRY DISCUSSION CONCLUSIONS SUPPLEMENTARY DATA REFERENCES

 
Aitcheson, S. J. & Forrest, A. H. (1994). Quantification of crustal contamination in open magmatic systems. Journal of Petrology 35, 461–488.[Abstract/Free Full Text]

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