A Volcanic District Between the Hoggar Uplift and the Tenere Rifts

Journal of African Earth Sciences 92 (2014) 14–20

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Journal of African Earth Sciences

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A volcanic district between the Hoggar uplift and the Tenere Rifts: Volcanology, geochemistry and age of the In-Ezzane lavas (Algerian Sahara) ⇑ Rachid Yahiaoui a,b, Jean-Marie Dautria b, Olivier Alard b, , Delphine Bosch b, Abla Azzouni-Sekkal c,d, Jean-Louis Bodinier b a Ecole Normale Supérieur – Vieux Kouba, Département des Sciences Naturelles, BP. 92, Vieux-Kouba, 16500 Alger, Algeria b Géosciences Montpellier (cc60) UMR5243, Université de Montpellier 2 – CNRS, Place Eugène Bataillon, 34095 Montpellier Cedex 5, France c Université Abou Bekr Belkaïd, FSNVSTU, BP 119, 13000 Tlemcen, Algeria d Laboratoire de métallogénie et magmatisme de l’Algérie, USTHB, BP 32, 16000 Alger, Algeria article info abstract

Article history: The In-Ezzane volcanic district (EZD), located at the triple junction of Algeria, Niger and Libya belongs to Received 4 March 2013 the Eastern Hoggar, covers 350 km2 and includes 9 volcanic ediﬁces that are probably aligned along NW– Received in revised form 29 November 2013 SE faults, parallel to the Tafassasset valley. The low volume (0.7 and 1 km3) of emitted lavas, the similar Accepted 2 December 2013 morphology of the monogenic cones and the lack of differentiated rocks indicate that the volcanic activity Available online 17 December 2013 of the EZD was restricted in time and volume. The new K–Ar age (i.e. 2.86 ± 0.07 Ma) indicates that the EZD is contemporaneous with the last alkali volcanism paroxysm in Hoggar and with the nearby Libyan Keywords: volcanics. The EZD alkali basalts (mainly basanite) show a remarkable homogeneous compositions both Basalts in major elements (44.8 6 SiO 6 45.8 wt.%; 5.2 6 (Na O+K O) 6 6.2 wt.%), trace elements (4.3 6 Mantle 2 2 2 6 6 6 Hoggar Th 5.5 ppm; 34.7 La 44.7 ppm; 16.1 < La/Yb(N) < 21.6) and radiogenic isotopes (0.70285 87 86 143 144 206 204 Tuareg shield < Sr/ Sr < 0.70303; 0.51298 < Nd/ Nd < 0.51301; (19.212 < Pb/ Pb < 19.340, 15.589 207 204 208 204 Algeria < Pb/ Pb < 15.602 and 38.834 < Pb/ Pb < 38.903). Relative to the Hoggar alkaly basalts the EZD basalts appear systematically impoverished in incompatible elements and show a depleted signature both in Sr and Nd isotopes (almost MORB-like). The Pb isotopes are relatively enriched and intermediate between tholeiites and alkali basalts of the Hoggar. This unusual geochemical signature, is uneasy to reconcile with the known characteristics of the Hoggar swell, and would rather ﬁngerprint a circum cratonic mantle lying beneath the west border of the Murzuq craton (Libya). Ó 2013 Elsevier Ltd. All rights reserved.

1. Introduction 2012), show that the EZD is situated both on the fringe of the Mur- zuk craton and around 300 km East to the inferred western bound- The In-Ezzane volcanic district (EZD), located at the triple junc- ary of the EAC craton (Fig. 1). tion of Algeria, Niger and Libya (Fig. 1) belongs to the Eastern Hog- This study combines ﬁeld observations, K–Ar age, and geochem- gar. It has never been studied as far as we know, in spite of its ical data (major and trace elements) as well as isotopic composi- particularly interesting position in the geodynamical setting of tions (Sr, Nd and Pb) on the lavas in order to replace the EZD North-Central Africa. Indeed the EZD is situated both on the south- within the frame of the recent magmatic activity of the Hoggar ern margin of the Hoggar swell (around 400 km far from the top) swell and, at a larger scale, within those of Northern Africa. and on the eastern shoulder of the Tafassasset basin which corre- sponds to the NW extension of the Tenere Rift system (Fig 1). Therefore, it may represent, with the Todgha district (Air, NE Niger; 2. Field observations Cantagrel and Karche, 1983), one of the two indicators of a recent 0 magmatic activity associated with the Tenere evolution. On the The main lava outcrops (between 10°40 and 11°E meridians, 0 2 other hand, recent works on the Hoggar basement (Bertrand and 23°15 and 23°N parallels) cover 350 km and draw roughly a Caby, 1978; Black et al., 1994; Fezaa et al., 2010; Liégeois et al., NNW-SSE ellipsis near 30 km long and 10 km large (Fig. 2). The volcanics lay horizontal sandstones and shales of Cambro-Ordovi- cian age discordant on the easternmost Hoggar terrane, the Edem- ⇑ Corresponding author. Tel.: +33 467149311; fax: +33 467143642. bo terrane (Fig. 1). This terrane of Archean to Paleaoproterozoic age E-mail address: [email protected] (O. Alard). reworked during the Panafrican has tectono-metamorphic

Fig. 1. Simplified geological and structural map of Hoggar (Algerian Sahara) and of its margins modified from Black et al. (1994), Fezaa et al. (2010) and Liégeois et al. (2012). (1) Anomalous mantle extension (Dautria and Lesquer, 1989); (2) major faults; (3) Eocene to Quaternary volcanic districts; (4) sedimentary cover; (5) oceanic Panafrican terranes (juvenile and reworked); (6) Archean and Palaeoproterozoic terranes strongly reworked at Panafrican; (7) Archean and Palaeoproterozoic terranes slightly reworked at Panafrican; (8) West African craton (WAC); (9) inferred boundary for the East African craton (EAC); E: Edembo terrane. characteristics resembling those of the terranes constituting the Latea Metacraton in the Central Hoggar (Liégeois et al., 2003; Fig. 1), which would constitute the Neoproterozoic cover of the Eastern African craton (Fezaa et al., 2010). It is noteworthy that there is another volcanic district located on the north-eastern margin of Hoggar and in the same structural setting as the EZD, the In- Teria district, near 500 km North (Fig. 1). The whole lavas from this district are highly SiO2 undersaturated and of melilitite composition (Megartsi, 1972; Bossière and Megartsi, 1982; Dautria et al., 1992). All EZD lavas have been emitted from only 9 volcanic edifices (noted EZ 1 to 9 on Fig. 2) that are probably aligned along NW- SE faults, parallel to the Tafassasset valley (Fig. 1). The large spread of the lava flows seems to be due mainly to the basement horizon- tality. Only the lava flows at the eastern and western margin of the district show elongation of near 10 km compatible with a run-off within paleovalleys. All edifices, except EZ2, are monogenetic cones, most often with Fig. 2. Schematic geological map of the In-Ezzane district. breached craters and diameters ranging between 500 and 1500 m and elevations between 50 and 100 m. The craters are always well marked and show evidence of protracted lava fountain activity. The cones EZ1 and EZ3 display several nested craters, suggesting a slight migration of the activity during eruption. The ejecta are typ- ically strombolian and are often crosscut by dykes with thickness varying between 0.5 and 3 m. Each lava flow shows relatively large extension (between 30 and 150 km2) but remains always thin (between 2 and 4 m) and roughly prismatic. Therefore each eruption has mobilized a small amount of magma (between 0.1 and 0.4 km3). Consequently the total volume of emitted magma can be roughly estimated between 0.7 and 1 km3. Several of these lava flows, as well as dykes, show embedded mantle and crustal xenoliths of small size (Ø < 7 cm). In contrast with the Hoggar and Air volcanic districts, evidence for phreatomagmatic activity is rather scarce. However, base surge deposits of weak thickness (<2 m), has been observed locally at the bottom of the EZ3 lava flow. Further, the EZ2 edifice shows a lava Fig. 3. Photo of one of the most representative In-Ezzane strombolian cone (EZ5). lake morphology filling probably an ancient tuff ring now fully eroded. sand (Fig. 3). Such morphology is commonly observed in Hoggar All cones have similar morphologies characterized by (i) the for- where it characterizes 1–3 Ma old volcanoes (e.g. in the Tahalgha mation of an external erosion cliff, 10 to 30 m high, carved in the volcanic district, Dautria et al., 2005). Further, the lava flow does ejecta (ii) the occurrence of a pediment, at the cliff base, c.a. not form marked inverted relief, suggesting a rather young age in 100 m wide, surrounding the edifice and often covered by eolian agreement with the cone morphology. Thus, morphologically 16 R. Yahiaoui et al. / Journal of African Earth Sciences 92 (2014) 14–20 speaking, the EZ district is extremely homogeneous, suggesting a Six lavas have been selected for this study and their analyses are short and a rather recent volcanic activity. given in Table 2. Except EZ1, these lavas display relatively homoge-

neous compositions [i.e. (Na2O+K2O) varies between 5.2 and 6.2 wt.% (with 0.36 < (K O/Na O) < 0.47) and SiO varies between 3. Analytical methods 2 2 2 44.8 and 45.8 wt.%]. They all belong to the alkaline series. Accord- ing to the classification of Le Bas and Streckeisen (1991) they are The rock samples were crushed and then pulverized in an agate mainly basanites with a relatively high degree of SiO -undersatu- mill. Whole-rock major elements were analyzed by X-ray fluores- 2 ration (7 < (Ne + Lc) < 14%). EZ1 is slightly more differentiated cence (XRF, Geoscience Laboratories, Ontario, Canada). Trace ele- norm [(Na O+K O) = 6.5 wt.%, SiO = 49.1 wt.%] and therefore it must ments and REE abundances were analyzed using a VG 2 2 2 be considered as a trachybasalt. The lavas as a whole have [mg] Plasmaquad II ICP-MS at the University of Montpellier II (Ionov (Mg/(Mg + Fe)) ranging from 0.56 to 0.61 (Table 2), suggesting that et al., 1992). they are all primary. The lack of highly differentiated lavas consti- Before undertaking the acid digestion for the Sr, Nd and Pb iso- tutes one major difference with the Hoggar and Todgha (Aïr) volca- topic analyses, all whole rock (WR) were leached for 30 min with nic districts (Cantagrel and Karche, 1983). These features suggest 6 N HCl at 80 °C. After the leaching steps, the residues were rinsed both low partial melting rates and the absence of magma chamber three times in purified milli-Q H O. The total blank contents for Pb, 2 beneath EZD, and are consistent with (i) the very low volume of the Sr and Nd were less than 35, 40 and 10 pg, respectively, for a emitted lavas and (ii) the occurrence of mantle xenoliths in several 100 mg sample. Pb and Nd isotopic compositions were measured flows and dykes. Thus, the weak major-element variations ob- on the Nu 500 MC-ICP-MS located at the Ecole Normale Supérieure served among the EZD lavas (including trachybasalt EZ1) can be in Lyon (France). The Pb isotopic compositions were measured simply explained by slight variations of the degree of partial melt- with an external precision lesser than 200 ppm for 206Pb/204Pb, ing and to a lesser extent by the extraction or accumulation of oliv- 207Pb/204Pb and 208Pb/204Pb ratios, using the Tl normalization ine/cpx crystals during the magma ascent. These observations are method. The NIST 981 standard was measured every two samples. corroborated by the trace-element data (see below). The Nd isotopic measurements were bracketed between the ‘‘Re- nnes’’ Nd standard every two samples with an average of 143 144 Nd/ Nd = 0.511964 ± 14(2r; n = 21). The Sr isotopic composi- 6. Geochemistry tions were measured on a Finnigan Triton TI mass spectrometer at the Laboratoire de Géochimie GIS of Nîmes (France). The results 6.1. Trace elements from the NBS 987 Sr standard yielded a mean value of 87 86 Sr/ Sr = 0.710251 ± 08 (2r; n = 12). All studied lavas have homogeneous trace element contents The K–Ar analyse has been performed at LSCE, CEA-CNRS, Gif- (4.3 < Th < 5.5 ppm; 34.7 < La < 44.7 ppm; Table 2) and almost con- sur-Yvette from phenocrysts free sample. The age calculation is stant inter-element ratios (6.1 < La/Yb(N) < 21.6): Therefore, the based on the decay and abundance constants of Steiger and Jäger normalized trace-element patterns (basanites and EZ1 trachyba- 10 1 10 1 40 (1977) kb = 4.962 10 a ; ke = 0.581 10 a ; K/ salt) are remarkably parallel (Figs. 4 and 5) and typical of alkali 4 K = 1.167 10 mol/mol. basalts requiring an enriched OIB-type mantle source. All display negative anomalies in Pb and Hf and a slight positive Sr anomaly. 4. Geochronology However we note that the fractionation of Nb and Ta relative to the large ion lithophile elements such as U or Th are more variable Given the short span of the volcanic activity inferred by the field (e.g., 0.53 < U/Nb(N) < 1.28). Relative to the alkali primitive lavas observations, only one lava flow from the EZ5 cone has been dated from the Hoggar swell, the EZD basalts appear systematically by K–Ar method. The measured age is 2.86 ± 0.07 Ma (Table 1). impoverished in REE (50 < La(ppm) < 120; Fig. 5) and lithophile This upper Pliocene age is in perfect agreement with the morpho- elements (i.e. 5 < Th(ppm) < 14; Dautria et al., 1988; Maza et al., logical observations presented above. Consequently, the EZD activ- 1998; Yahiaoui et al., unpublished; Azzouni-Sekkal et al., 2007; ity is contemporaneous with the most recent alkali volcanism in Kaczmarek et al., submitted for publication). Compared to the Hog- Hoggar (Aït-Hamou et al., 2000; Aït-Hamou, 2000; Dautria et al., gar tholeites (Aït-Hamou et al., 2000) the EZD lavas have an overall 2005) and Aïr (Cantagrel and Karche, 1983; Pouclet and Baubron, lower REE content, except for the most incompatible lithophiles 1988) as well as with the alkali volcanic districts around the near- elements, which show slight enrichment (Fig. 5a). by Murzuq craton (Vincent, 1970; Busrewil and Wadsworth, 1980; These differences can be explained by a combination of: (i) Bardintzeff et al., 2012). enrichment factor in the mantle source(s), (ii) extent of partial melting and (iii) proportions of garnet in the residue.

5. Petrology and major elements composition of lavas 6.2. Isotopes The lavas of EZD are basalts with ﬂuidal porphyritic microlitic to doleritic textures. The olivine phenocrysts, more or less The Sr, Nd and Pb isotopic compositions of the EZD basalts dis- iddingsitized, have sizes ranging between 0.5 and 1.5 mm and play very narrow variations: 0.702853 < 87Sr/86Sr < 0.703025; the titano-augites are often glomerophyric (around 1 mm in diam- 0.512978 < 143Nd/144Nd < 0.513011; 19.2123 < 206Pb/204Pb < eter). All lavas contain mantle olivine and spinel xenocrysts, often 19.3399, 15.5888 < 207Pb/204Pb < 15.6016 and 38.8339 < also peridotite inclusions, up to 2 mm. 208Pb/204Pb < 38.9032 (Table 2). This suggests that the EZD lava

Table 1 Unspiked K–Ar ages of the EZ5 basaltic ﬂow.

Sample i# K (wt.%) ± 2r Mass molten (g) 40Ar% 40Ar 1012 (mol/g) ± 1r Age (Ma) ± 2r EZ 5 1.507 ± 0.015 0.30787 24.328 7.491 ± 0.044 2.86 ± 0.07

10 1 10 1 40 4 1 Ages are calculated using the decay and abundance constants of Steiger and Jäger (1977): lb ¼ 4:962 10 a ; le = 0.581 10 a ; K/K = 1.167 10 mol mol . R. Yahiaoui et al. / Journal of African Earth Sciences 92 (2014) 14–20 17

Table 2 Localization and composition of In-Ezzane basalts (major. trace and isotopic elements).

EZ1 EZ2 EZ3 EZ4 EZ5-1 EZ5-2 23°0700800N23°0901900N23°1002500N23°1104700N23°0501600N23°0503400N 10°4704300E10°4801600E10°4904100E10°5102600N10°5104900E10°5005700E

SiO2 wt.% 49.11 44.81 45.21 45.65 45.76 46.53

Al2O3 14.84 13.78 13.5 14.07 14.23 14.53

Fe2O3 11.32 12.54 12.5 12.14 12.1 11.99 MnO 0.16 0.18 0.17 0.17 0.17 0.17 MgO 7.66 9.34 10.14 7.56 8.25 8.42 CaO 8.13 9.61 8.8 9.86 9.15 8.95

Na2O 4.97 3.77 4.53 4.47 3.66 3.77

K2O 1.51 1.76 1.67 1.59 1.55 1.75

TiO2 2.05 2.67 2.66 2.64 2.61 2.54

P2O5 0.52 0.89 0.86 0.74 0.71 0.66 LOI <0.05 1.54 <0.05 1.41 2.17 1.36 Total 100.2 100.89 100 100.3 100.37 100.66 Rb (ppm) 36.13 37.68 37.65 37 32.41 36.65 Sr 793 936.13 859.51 758.6 866.27 758.21 Y 23.944 26.27 26.073 27.583 26.511 26.162 Zr 284.5 293.97 286.67 209.98 276.36 225.14 Nb 68.78 66.61 78.9 54.83 Cs 0.51 0.55 0.55 0.62 0.35 0.5 Ba 417.1 476.46 468.92 475.53 435.76 450.68 La 34.69 44.73 43.42 36.66 36.95 34.81 Ce 66.73 86.7 84.81 72.38 71.57 67.46 Pr 7.01 9.3 9.12 7.86 7.71 7.31 Nd 29.56 39.74 39.23 34.19 33.47 31.85 Sm 6.29 8.15 8.05 7.39 7.27 6.96 Eu 2.31 2.88 2.83 2.64 2.62 2.51 Gd 6.24 7.65 7.55 7.34 7.22 6.94 Tb 0.88 1.04 1.03 1.03 1.01 0.97 Dy 4.95 5.62 5.54 5.69 5.58 5.42 Ho 0.84 0.93 0.93 0.97 0.93 0.92 Er 1.99 2.17 2.16 2.31 2.21 2.17 Tm 0.24 0.25 0.25 0.28 0.26 0.26 Yb 1.32 1.41 1.39 1.55 1.43 1.47 Lu 0.19 0.2 0.2 0.23 0.2 0.21 Hf 4.27 4.43 4.44 3.55 4.22 3.73 Ta 2.84 2.63 3.34 2.2 Pb 2.75 2.9 2.87 2.7 2.37 2.38 Th 5.47 5.43 5.32 4.5 4.47 4.33 U 1.14 1.33 1.31 1.13 1.12 1.03 87Sr/86Sr(i) 0.702908 ± 3 0.703025 ± 3 0.702853 ± 2 0.702862 ± 8 143Nd/144Nd(i) 0.513011 ± 7 0.512978 ± 4 0.513007 ± 5 0.512997 ± 4 206Pb/204Pb(i) 19.2404 ± 4 19.2123 ± 4 19.3399 ± 5 19.294 ± 5 207Pb/204Pb(i) 15.5888 ± 4 15.6016 ± 5 15.5895 ± 5 15.5915 ± 5 208Pb/204Pb(i) 38.8407 ± 14 38.9032 ± 14 38.8696 ± 16 38.8339 ± 14

Major and trace elements compositions of selected In-Ezzane basalts. Ne, nepheline normative content; [mg] = Mg/(Mg + Fe2+), with Fe3+ = 0.15 Fe2+. Note that sample EZ5-1 and EZ5-2 are from the same volcano. (i) Isotopic composition corrected for in situ decay, all samples have been considered to be 2.86 Ma old.

Globally, the Sr and Nd isotopic ratios from the EZD lavas differ significantly from the Hoggar tholeiites, lower (0.703630 < 87Sr/86Sr < 0.704592) and higher (0.512548 < 143Nd/144Nd < 0.512830), respectively (Table 2). In the 87Sr/86Sr vs. 143Nd/144Nd diagram (Fig. 6a), they plot within a restricted field constituting a possible extension of the Hoggar alkali basalt field where they would represent the less radiogenic in Sr and most radiogenic in Nd lavas. As noted above, the EZD samples plot closer to the MORB domain than the whole Hoggar lavas (Fig. 6a), this suggests a more long term depleted mantle source for EZD than for Hoggar. In the two Pb isotopic diagrams (Fig. 6b and c), the EZD samples plot slightly above (206Pb/204Pb vs. 207Pb/204Pb) or below (206Pb/204Pb vs. 208Pb/204Pb) the NHRL (Northern Hemisphere Line). In the 206Pb/204Pb vs. 207Pb/204Pb diagram (Fig. 6b), the EZD lavas define a limited domain located on the field of the worldwide OIB (e.g., 206Pb/204Pb > 19.2; 207Pb/204Pb > 15.58; 208Pb/204Pb Fig. 4. Extended trace elements patterns of In-Ezzane lavas normalised to Primitive > 38.84). Compared to the Hoggar alkali basalt, they are character- mantle, values are from Sun and McDonough (1989). ized by lesser radiogenic Pb signatures and appear as a possible unradiogenic extension of the Hoggar alkali-basalt field. Moreover, source is isotopically rather homogeneous and show unradiogenic the EZD basalts plot near the Hoggar tholeiitic field in the 206 204 207 204 206 204 Sr and Nd composition reminiscent of the MORB signature. Pb/ Pb vs. Pb/ Pb diagram (Fig. 6b) but in the Pb/ Pb 18 R. Yahiaoui et al. / Journal of African Earth Sciences 92 (2014) 14–20

Fig. 5. Chondrite-normalised Rare Earth Patterns of EZD lavas compared with (a) the Hoggar tholeiites (Taharaq district, Aït-Hamou et al., 2000; Aït-Hamou, 2000); (b) the Hoggar alkali-basalts (Dautria et al., 1988, 1992; Maza et al., 1998; Azzouni- Sekkal et al., 2007; Kaczmarek et al., 2013; Yiahiaoui et al., unpublished data) and (c) Libyan alkali basalt (Bardintzeff et al., 2012; Busrewil and Wadsworth, 1980). Normalizing values after Sun and McDonough (1989). vs. 208Pb/204Pb diagram (Fig. 6c), they plot off the tholeiitic area and show significantly lower 208Pb/204Pb ratios for a considered 206Pb/204Pb Fig. 6. 143Nd/144Nd(i) versus 87Sr/86Sr(i) (a), 207Pb/204Pb versus 206Pb/204Pb(i) (b), 208Pb/204Pb versus 206Pb/204Pb(i) (c) diagrams for the EZD basalts. The data for Hoggar tholeiites are from Aït-Hamou (2000); the data for Hoggar alkali basalts are 7. Discussion and conclusions from Allègre et al. (1980), Maza et al. (1998), Aït-Hamou (2000) and Kaczmarek et al. (unpublished data). The data for Libya (Bardintzeff et al., 2012) have been The similar morphology of the volcanoes, the low volume of reported for comparison. emitted lava, the homogeneity of lavas composition (mainly basanites) and the lack of significantly differentiated rocks suggest that produced by partial melting degree near 5%, would derive from the volcanic activity of the EZD was restricted in time and volume. an enriched source with Th and La content about 6 PM (Dautria The measured age (2.86 ± 0.07 Ma) indicates that this activity was et al., 1988). Therefore, these observations suggest that the mantle contemporaneous of the last paroxysmal alkali magmatic episode source of the EZD lavas could be close to the Hoggar tholeiites identified in the Hoggar (Aït-Hamou et al., 2000; Dautria et al., source, but is significantly impoverished with regard to the mantle 2005) and Aïr (Cantagrel et Karche, 1983; Pouclet and Baubron, source of the Hoggar alkali lavas. Within this scenario, the melili- 1988) provinces. Such ages have also been obtained in the alkali vol- tites from the In-Teria district (Fig. 1; Dautria et al., 1992; canic districts around the Murzuq craton (Busrewil and Wadsworth, Kaczmarek et al., submitted for publication) data would be pro- 1980; Bardintzeff et al., 2012; Wilson and Guiraud, 1992). duced by partial melting of the EZD source with rates around 2%, Trace elements abundances of the EZD lavas suggest that they in agreement with the peculiar petrology of these lavas. are the product of a low degree of partial melting. Further, the lack On the other hand, the Sr and Nd isotopic compositions of the of significant trace element variation indicates that they derived EZD lavas suggest that their mantle source is more depleted than from a rather homogeneous mantle source. According to the exper- the source of the Hoggar alkali basalts and close to MORB-like imental data of Green and Falloon (2005), the basanites are pro- composition. These isotopic compositions preclude a common duced by partial melting degrees near 5%. With this hypothesis source with the Hoggar tholeiites. Therefore at first sight, Sr and and using the simplified melting equation (Cl/C0 = 1/f, where Cl Nd isotopes in one part and trace element modelling and Pb iso- denotes the lava composition, C0 is the mantle source and f is the topes in another part are inconsistent. Trace element and Pb iso- degree of partial melting), we can then estimate that the EZD topes enrichment of the magma source must therefore be related source enrichment is about 3xPM, for the most incompatible trace to a more recent metasomatic event, while the MORB-like Sr and elements such as Th and La. In comparison, the tholeiites from the Nd isotopic signature would have been inherited from an ancient Hoggar, produced by partial melting degrees near 10% (Aït-Hamou, melting event. 2000), would derive from a source enriched between 3 and Recent publications (Liégeois et al., 2003; Fezaa et al., 2010) 4.5 PM. The Hoggar alkali basalts (mostly basanites), also point out the peculiar nature and geologic history of the basement R. Yahiaoui et al. / Journal of African Earth Sciences 92 (2014) 14–20 19 of the EZD district: the Edembo terrane. According to Liégeois et al. beneath the Hoggar (Aït-Hamou et al., 2000; Maza et al., 1998). (2012), this terrane would mark the west border of the Murzuq Such EM1 signature strongly contrasts with the isotopic signature craton. Whithin this framework, it is noteworthy that the basaltic of the EZD lavas. Therefore, given (i) the remote position of the EZD volcanism from Al-Harouj - Waw an Namous area (Libya), also relative to the main Hoggar volcanics (more than 400 km from the located on the border of the Murzuq craton (Fig. 1), show isotopic Hoggar swell top, 350 km from the nearest peripheral alkali dis- signatures close to the EZD lavas (Bardintzeff et al., 2012). Using trict, Adrar n’Ajjer, Fig. 1) and (ii) the depleted composition in Sr the trace elements data available for basalts and basanites from and Nd of the mantle source of the EZD relative to the Hoggar this area (Bardintzeff et al., 2012; Busrewil and Wadsworth, sources, we may then consider that the EZD mantle source escaped 1980) we derived a mantle source enrichment factor between 1 the Hoggar swell influence, and represent a distinct mantle and 2.5 PM, slightly lower (but within error) than the composi- domain: the circum cratonic mantle of the Murzuq craton. tion of the EZD source (3 PM). Such depleted isotopic signatures in Sr and Nd require a long term evolution of a low Rb/Sr and high Sm/Nd mantle section sug- Acknowledgements gesting a very ancient (Panafrican?) melting event and precludes any ‘‘ancient’’ metasomatism induced by an enriched component. Field-work has been supported by the Programme International Several hypotheses can be then put forward to explain the slight de Coopération Scientifique (Architecture lithosphérique et dynami- enrichment in the incompatible elements observed in the EZD lava que du manteau sous le Hoggar, 2008–2011) and by the Programme source: (i) lithosphere delamination proposed initially by Ashwal Hubert Curien –Tassili. We thanks Pr. K. Ouzegane (USTHB) and the and Burke (1989) to account for the fertility of the lithosphere be- Office du Parc National du Tassili for helping us with administra- neath the Panafrican belt and which could also account for the in- tive permits and logistic matters. We also would like to thanks tense Hoggar tardi-panafrican magmatism (Liégeois et al., 2003); the M-Zab tour guides and drivers fort their efficiency and under- (ii) upper Cretaceous to Eocene rifting, leading to the formation standing. The analytical cost was cover by the Institut National des of the Tenere Rift system which includes the Tafassasset valley Sciences de l’Univers (CNRS, Fr.) small grants to O.A. and by D.B. (Dautria and Lesquer, 1989); (iii) distal expression of the metaso- own funding resources for isotopic measurements, H. Guillou per- matism associated to the recent Hoggar doming (Dautria et al., formed K–Ar analysis at the LSCE (CEA, Gif s/Yvette, France). 1988; Dautria and Girod, 1991; Beccaluva et al., 2007). The first hypothesis (trace elements enrichment related to Pan- african post-collision lithospheric delamination) is inconsistent References with the observed EZ lavas Sr-Nd isotopic compositions, which Aït-Hamou, F., 2000. Un exemple de « point chaud » intra-continental en contexte show typical depleted mantle signatures. Indeed fractionation of de plaque quasi stationnaire : Etude pétrologique et géochimique du djebel the parent-daughter ratio (e.g. (Rb/Sr)PM = 2–3) would quickly pro- Taharaq et évolution du volcanisme cénozoïque de l’Ahaggar (Sahara Algérien). duce significant increase and decrease of the 87Sr/88Sr and Thèse. Univ. Montpellier, vol. 2, 250 pp. 143 144 Aït-Hamou, F., Dautria, J.M., Cantagrel, J.-M., Dostal, J., Briqueu, L., 2000. Nouvelles Nd/ Nd, respectively ruling out any ’’old’’ metasomatic events données géochronologiques et isotopiques sur le volcanisme cénozoïque de ago (hypothese 1). Further if we consider an isotopically enriched l’Ahaggar (Sahara algérien): des arguments en faveur d’un panache. Comptes metasomatic fluid/agent as usually expected and here suggested Rendus de l’Académie des Sciences de Paris – Série IIa 330, 829–836. 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