Uâ€“Pb LA-ICP-MS Detrital Zircon Ages from the Cambrian of Al

Journal of African Earth Sciences 79 (2013) 74–97

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Journal of African Earth Sciences

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U–Pb LA-ICP-MS detrital zircon ages from the Cambrian of Al Qarqaf Arch, central-western Libya: Provenance of the West Gondwanan sand sea at the dawn of the early Palaeozoic ⇑ Muftah Mahmud Altumi a, Olaf Elicki b, , Ulf Linnemann c, Mandy Hofmann c, Anja Sagawe c, Andreas Gärtner c a Libyan Petroleum Institute, Gergarsh Road, 6431 Tripoli, Libya b Freiberg University, Geological Institute, Bernhard-von-Cotta Street 2, 09599 Freiberg, Germany c Senckenberg Natural History Collections Dresden, Museum of Mineralogy and Geology, GeoPlasmaLab, Königsbrücker Landstraße 159, 01109 Dresden, Germany article info abstract

Article history: Detrital zircons from various stratigraphic levels of the sandstone-dominated Cambrian Hasawnah For- Received 4 April 2012 mation of the Al Qarqaf Arch type area (central-western Libya, Saharan Metacraton area) were geochro- Received in revised form 3 November 2012 nologically investigated for the ﬁrst time by LA-ICP-MS techniques for U, Th, and Pb isotopes. Of 720 Accepted 5 November 2012 analyzed grains, 329 were concordant. Of the total, about 60% of the U–Pb zircon ages are Neoproterozoic Available online 29 November 2012 and earliest Cambrian and cluster at c. 700–680, 670–650, 615–610, 590, 570–560, and c. 540–525 Ma. These zircon populations are interpreted as detrital material derived from the Pan-African and possibly Keywords: to a smaller proportion from the Cadomian orogen situated marginal to northwestern Gondwana. A Hasawnah Formation few slightly older Neoproterozoic ages (c. 950–750 Ma) point to rifting events related to the dispersal Libya Geochronology of the Rodinia supercontinent. A minority of zircons became formed during the conﬁguration of Rodinia Cambrian and cluster around the Mesoproterozoic–Neoproterozoic boundary (1039 ± 11, 1006 ± 12 and Gondwana 993 ± 13 Ma). Further, some early Mesoproterozoic zircon ages had been found (1592 ± 39 and Saharan Metacraton 1475 ± 20 Ma). The potential source area for the Mesoproterozoic zircons is interpreted to have been far distant from the Al Qarqaf Arch, probably concealed within the Arabian–Nubian Shield or situated in Chad, or in the Congo and Tanzania cratons. There is still no evidence for the existence of massive Mes- oproterozoic crust in the Saharan Metacraton area. A considerable proportion (28%) of zircons represents Palaeoproterozoic populations at c. 2.4–2.3 Ga, and c. 2.2–1.6 Ga. Less than 5% of all zircons are Archaean in age (c. 3.4–3.25 Ga, c. 2.95–2.7 Ga, c. 2.6–2.5 Ga). A potential source area for Palaeoproterozoic and Archaean zircon grains is the West African Craton and the western part of the Saharan Metacraton. The best candidates for the main source region for the sandstones of the Hasawnah Formation in the Al Qarqaf Arch type area are the Neoproterozoic–early Cambrian orogens of the Pan-African cycle in the Trans-Saharan Belt (Pharussian and Dahomeyean belts) and of the peri-Gondwanan terranes (Cado- mia). This conclusion is in accordance with published data from the Hoggar (Tassilis, Algeria) and from southwestern (eastern Murzuq Basin) and southeastern Libya (Al Kufrah Basin). In comparison to the strong input of Neoproterozoic zircon grains, input from the Palaeoproterozoic and Archaean sources of the cratonic basement (Saharan Metacraton and West African craton) is relatively limited. The exact source of the exotic Mesoproterozoic zircons remains problematic. The presented data lead to the conclus ion that the centre of early Palaeozoic thermal subsidence in central-northern Africa has to be located in the region of the Saharan Metacraton. The distinct unconformity at the base of the Cambrian Hasawnah Formation indicates major uplift and considerable denudation in the latest Neoproterozoic to early Cam- brian time interval. Because of the conspicuous maturity of the Hasawnah Formation siliciclastic deposits, a coeval intense chemical weathering under warm to humid climatic conditions in low to moderate southern latitudes and the formation of a Gondwanan peneplain is indicated. Ó 2012 Elsevier Ltd. All rights reserved.

⇑ Corresponding author. Tel.: +49 (0)3731 39 2435; fax: +49 (0)3731 39 12435. E-mail addresses: [email protected] (O. Elicki), [email protected] (U. Linnemann).

1. Introduction 2. Geological setting

The late Proterozoic to early Phanerozoic represents a period of The Neoproterozoic to earliest Palaeozoic structural evolution fundamental reorganization of geological processes and of sedi- of northeastern Africa is mainly characterized by two tectonic mentological, palaeogeographic, structural, climatic and evolution- phases following the breakup of the late Precambrian superconti- ary settings. One focus of these dramatic changes in Earth history is nents Rodinia and Pannotia: the Pan-African amalgamation and in the origination and early evolution of the northern margin of the the ‘‘Infracambrian’’ extension (Craig et al., 2008; Johnson et al., Gondwana palaeosupercontinent. With the aim of developing a 2011). During the first of these intervals a few palaeooceans had comprehensive model for the dynamic evolution of the northern been closed, among which the Trans-Saharan Ocean was most Gondwanan margin in the Ediacaran to early Palaeozoic, investiga- important with respect to present southern Libya (Schandelmeier tions have been significantly intensified during the last decade. and Wipfler, 1999). Whereas data from northwestern Africa, the Near East, western In Libya, early Palaeozoic rocks are mainly exposed at the mar- and central Europe and the Sinai Peninsula and Israel have already gins of large intracratonic basins in the central-western to south- been published, data from central-northern Africa is limited. western region of the country (Ghadamis Basin, Murzuq Basin), Hence, there is a significant knowledge gap to be filled in order in the southeast (Al Kufrah Basin), and in the Tibesti Mountains to create a consistent reconstruction of the entire palaeogeograph- in the south (Al Festawi, 2001; Tawadros, 2001). One of the most ic region. important outcrop areas is in the Jebel Hasawnah (also called Jebel In extensive areas of northern Africa the Precambrian igneous Fezzan or Jebel Al Qarqaf) of the Al Qarqaf Arch (AQA; Fig. 1), a and/or metamorphosed basement is nonconformably covered by prominent southwest-trending uplift structure in Libya, mainly a Cambrian–Ordovician sandstone blanket (Beuf et al., 1971; formed by Caledonian and Hercynian movements (Al Festawi, Tawadros, 2001, 2012; Hallett, 2002; Squire et al., 2006; Linne- 2001). The amplitude of the uplift is about 6000 m (up to 800 m mann et al., 2011). This phenomenon is also known from other elevation at present surface) in the AQA area relative to the base- time-equivalent regions of the world (e.g. Middle East and North ment level in the northern Ghadamis Basin area where this level America: Powell, 1989; Sharland et al., 2001; Khalifa et al., 2006; is about 5200 m below surface (Hallett, 2002). Following Al Fasa- Shinaq and Elicki, 2007; Hagadorn, 2011). Such regions offer an twi et al. (2003), the AQA was a topographic high already in late excellent opportunity to study such fundamental geological pro- Cambrian to early Ordovician time. Tectonic activity during the cesses as tectonic evolution, depositional history, erosion and Mesozoic and Cenozoic modified the region to various extents. transportation from source areas, palaeoclimate, and structuring The last tectonic uplift of significant amplitude was during the Al- of early life systems of this time window. In some regions there pine phase of deformation and is indicated by apatite fission track is also some economic aspect where included sediments, as in Li- data (Craig et al., 2008). The AQA largely separates the Ghadamis bya, are significant elements of source rock or reservoir architec- Basin in the northwest from the Murzuq Basin in the south ture or if they denote important aquifers (Binsariti and Saeed, (Fig. 1) and represents the northernmost basement outcrop of 2000; Hallett, 2002; Craig et al., 2008, 2009). the so-called Saharan Metacraton (Abdelsalam et al., 2002; East This paper is stratigraphically and regionally focussed on such a Saharan Craton sensu Bertrand and Caby, 1978) in this part of Afri- sedimentary cover: the Cambrian Hasawnah Formation of the Al ca (Conant and Goudarzi, 1967; Tawadros, 2001). Qarqaf Arch area of central-western Libya. The first detailed geo- Palaeogeographically, the study area was situated at the wes- logical investigation of the area was undertaken early in the second tern margin of the Gondwana palaeocontinent, which was in low half of the last century (Massa and Collomb, 1960; Collomb, 1962; to moderate southern latitudes at the beginning of the Cambrian Hecht et al., 1963; Goudarzi, 1970; Jurák, 1978) for mapping and and migrated southward during Cambrian and Ordovician time exploration of natural resources. A second, important increase in (e.g. Scotese, 2009; Torsvik and Cocks, 2009). knowledge has been published since the 1980s and has continued The first geochronological data for basement granitoids from until recent time mainly due to oil exploration activities in Palae- various regions of the AQA were published by Schürmann (1974), ozoic basins in the region (Salem and Busrewil, 1980; Salem and who presented ages of 640–549 Ma (Rb/Sr) and 541–491 Ma (K/ Belaid, 1991; Salem et al., 1991a,b,c, 2003, 2008a,b,c; Klitzsch Ar) for muscovite granites. These are consistent with additional and Thorweihe, 1999; Sola and Worsley, 2000; Tawadros, 2001; data (K/Ar ages of 554–520 Ma) later reported by Jurák (1978). Hallett, 2002; Salem and Oun, 2003; Salem and El-Hawat, 2008). These results indicate widespread igneous activity in the area at Despite excellent outcrop, geochronological investigation of the that time (Fullagar, 1980). Oun and Busrewil (1987) investigated Cambrian succession in Libya has been neglected and recent data some of the granitic basement from the Wadi Badran area of south- are practically unavailable until recently with very rare exceptions ern Jebel Hasawnah and came to the conclusion that the geochem- (see below). However, such information together with sedimento- istry of these rocks suggests an S-type nature for the granites, logical data is greatly needed for the reconstruction of the dynamic probably due to a ‘‘within-plate tectonic environment’’. Later on, evolution in the aftermath of the Pan-African Orogeny of this part Oun and Daly (1980) analyzed basement granites from the south- of Gondwana, and for correlation of this region to other areas of the ern (Wadi Badran; named Wadi Taráb by Jurák (1978)) and north- palaeocontinent. ern (Wadi Damran; named Wadi Darman by Jurák (1978)) areas of The aim of the present paper is to present the first geochrono- the Jebel Hasawnah, determining a late Pan-African age (c. logical data from detrital zircons of various lithostratigraphic levels 519 ± 34 Ma, Rb/Sr whole rock ages) and, in contrast to the former within the Hasawnah Formation from the northern, central and authors, an anorogenic A-type origin of the granites due to crustal southern Jebel Hasawnah mountain range of the Al Qarqaf Arch extension incorporating some early Pan-African roof metasedi- and to draw conclusions as to the Precambrian to Cambrian evolu- mentary rocks. tion of the palaeogeographic region. These geochronological data Subsequent sediments nonconformably overlie the Pan-African should contribute to filling the gap in available stratigraphic infor- granitic basement and were generated due to local erosion of this mation from this mainly non-marine and non-fossiliferous Cam- elevated bedrock. They are represented by localized thin, unfossil- brian succession. Furthermore, the data provide a sound basis for iferous sandstone packages (Mourizidie Formation) deposited in assessment of the tectonomagmatic evolution of the Precambrian ‘‘Infra-Cambrian’’ time within local palaeo-lows (Bellini and Massa, basement of the Saharan Metacraton and adjacent areas. 1980; Hallett, 2002; Benshati et al., 2009). To a much broader 76 M.M. Altumi et al. / Journal of African Earth Sciences 79 (2013) 74–97

Fig. 1. Geological map of Libya. (A) Overview map of major geological regions of Libya. (B) Enlarged detailed geological map of Al Qarqaf Arch (AQA) study area; numbers indicate colours for stratigraphic units (1 – Cambrian, 2 – Ordovician, 3 – Silurian, 4 – Lower Devonian, 5 – Middle and Upper Devonian, 6 – Lower Carboniferous, 7 – Middle Cretaceous, 8 – Upper Cretaceous, 9 – Palaeogene and Neogene, 10 – Cenozoic volcanic rocks); modified after Craig et al. (2008). (C) Satellite image of AQA corresponding to image B, with location of sampled sections (compare Fig. 2) in Jebel Hasawnah outcrop area (modified after Google Earth). (For interpretation of the references to colour in this figure, the reader is referred to the web version of this article.).

extent, the basement is nonconformably covered by a siliciclastic whether a stratigraphically complete section exists in the type area Cambro-Ordovician succession, the Qarqaf Group (Burollet, 1960; at all. No formal type section has been designated until today, so Oun and Daly, 1980; see Fig. 2). Although Klitzsch (1981) dis- that the formation is rather inadequately defined. claimed this grouping because of unconformities between the in- The thickness of the Hasawnah Formation is reported to be 250– cluded formations, the use of this term remains somewhat 300 m for the AQA area, but, may be up to 1000 m in the subsurface common. The lowest portion of the Qarqaf Group is represented of the Ghadamis Basin, 1150 m in the Al Kufrah Basin and 1700 m in by the Hasawnah Formation, the subject of this paper. central Libya (Klitzsch, 1970; Deunff and Massa, 1975; Hallett, The Hasawnah Formation was defined by Massa and Collomb 2002). Sedimentology and lithofacies of the formation in the AQA (1960) and outcrops widely in the Jebel Hasawnah type region. are poorly known. The formation overlies the Precambrian base- As well, beyond this type area the formation is proven both at sur- ment with distinct erosional contact and in the AQA region it shows face and in the subsurface, with a broad extent in western, central a tripartite subdivision (Cˇepek, 1980): the basal part of the lower and eastern Libya (Tawadros, 2001, 2012; Hallett, 2002). According unit is characterized by the occurrence of a distinctive basal con- to Hallett (2002), the Hasawnah Formation extends also into Egypt, glomeritic portion (total thickness up to 14 m) consisting of several Algeria and Chad, where it is identified under different names layers differing in pebble diameter and proportion of sandy matrix (Craig et al., 2008). However, regional correlation is somewhat and with a slight fining-upward tendency (Jurák, 1978; Cˇepek, problematic due to the lack of age-diagnostic fossils or other than 1980; personal observations). The pebbles are several centimeters lithostratigraphic data. For the same reason, it is also unclear in diameter and predominantly of well rounded quartz. This M.M. Altumi et al. / Journal of African Earth Sciences 79 (2013) 74–97 77

Fig. 2. Lithostratigraphy of Hasawnah Formation in AQA area with indication of stratigraphic span of sampled sections (Da2 – Wadi Damran-2, X – unnamed wadi ‘‘X’’, A1 – Wadi Al Abd-1, Ba1 – Wadi Badran-1, Ba2 – Wadi Badran-2) and position and names of sampled horizons (yellow rectangles). Generalized lithologic log redrawn and modiﬁed after Cˇepek (1980). (For interpretation of the references to colour in this ﬁgure legend, the reader is referred to the web version of this article.)

conglomeratic lithology grades upward into mainly medium- unit is interpreted as fluviodeltaic with some intertidal influence grained, high-angle cross-bedded sandstone showing fining-up- toward the top (Cˇepek, 1980; Hallett, 2002; personal observations). ward sets with truncated tops. The thickness of the complete lower The middle unit (about 70 m thick) is petrographically similar, but unit of the Hasawnah Formation in the AQA is about 100 m. The differs from the unit below in its greater diversity of sedimentary 78 M.M. Altumi et al. / Journal of African Earth Sciences 79 (2013) 74–97 structures (cross-bedding, planar lamination, convolute bedding, Table 1 ripples), by the intercalation of fine-grained sandstone and silt- Settings for the instruments used in the geochronological Laboratory (GeoPlasmaLab Dresden) of the Senckenberg Natural Hostory Collections Dresden (Excimer Laser, stone, and by the first, scarce occurrences of thin trace fossil hori- New Wave, UP 193) and (ICP-MS, Thermo Fisher, Element 2 XR). zons, dominated by Skolithos, arthropod and simple traces (personal observations). The environmental conditions for this ICP-MS Finnigan Element 2 XR middle unit are interpreted as transitional from marine intertidal Forward power 1390 W 1 to subtidal by Cˇepek (1980). The upper unit of the Hasawnah Forma- Gas flow rate 15.0 l min (plasma) 1.07 l min1 (aux) tion is about 80 m thick in the AQA and again consists of petro- Scan mode E-scan graphically similar sandstone. It differs from the middle unit by a Scanned masses 202, 204, 206, 207, distinct decrease in silty intercalations and change in the character 208, 232, 235, 238 of sedimentary structures (massive sandstone, sand bars with com- Mass resolution 300 plex internal structures, trough cross-bedding, synsedimentary Dead time 18 ns Oxide UO+/U+ <1% chaotic deformation), together indicating a dominantly subtidal Dwell time 4 ms depositional environment (Cˇepek, 1980). Settling time 61 ms/amu Although the general trend of lithofacies change within the Number of scans 1500 Hasawnah Formation suggests a transition from fluvial/deltaic Background 15 s Ablation time 30 s (lower unit) to tidally influenced (middle unit) to rather offshore Integration time 1.4 s (=25 scans) (upper unit) deposition, the inventory of sedimentary structures Laser system UP193 New Wave is somewhat ambiguous and does not permit more detailed con- 193 nm, excimer ˇ clusions with regard to depositional environment (Cepek, 1980). Nominal spot diameter 25–35 lm (unknowns) Stratigraphically, the Hasawnah Formation is interpreted as 35 lm (standard) Cambrian in age. Although the available fossil content (nonspecific Carrier gas 0.25 l min1 He 1 trace fossils) is not age diagnostic, the lithostratigraphic position of 1.1 l min Ar Laser settings 10 Hz, 55% LP the formation beneath the immediately overlaying Tremadocian Drill speed (DS)/Raster scan speed RSS) 0.5 lm/s (DS) Ash Shabiyat Formation (e.g. Collomb, 1962; Hecht et al., 1963; Cell volume c. 3 cm3 Bellini and Massa, 1980; Davidson et al., 2000; Tawadros, 2001; Sensitivity 6 106 counts/pg U Hallett, 2002), together with the suggested correlation with subsurface strata of the Ghadamis Basin and Sirt Basin containing middle to late Cambrian palynomorphs (corresponding to Cambrian series 3 and Furongian; Tawadros, 2001; Vecoli et al., 2003), and and Geology (GeoPlasma Lab, Senckenberg Natural History Collec- with the equivalent Sidi Toui Formation of Tunisia (with a rich pal- tions, Dresden) using a Thermo-Scientific Element 2 XR sector field ynomorph flora: Tawadros, 2012), makes such a conclusion rea- ICP-MS coupled to a New Wave UP-193 Excimer Laser System. sonable. Recently, some geochronological zircon data from Instrument settings for the Laser and the ICP-MS are given in sandstones of the Cambrian–Ordovician boundary interval have Table 1. A teardrop-shaped, low-volume laser cell constructed by been published for a few samples from the Dor el Gussa area (east- Ben Jähne (Dresden) and Axel Gerdes (Frankfurt am Main) was ern margin of the Murzuq Basin, approximately 350 km southeast used to enable sequential sampling of heterogeneous grains (e.g. of the Jebel Hasawnah; Meinhold et al., 2011): about two thirds of growth zones) during time-resolved data acquisition. Each analysis all investigated zircons yield Neoproterozoic dates with a majority consisted of approximately 15 s background acquisition followed in the range 720–530 Ma (maximum at 620–600 Ma). by 30 s data acquisition, using a laser spot size of 25 lm and The upper boundary of the Hasawnah Formation with the Ash 35 lm respectively. A common-Pb correction based on the inter- Shabiyat Formation is represented by an angular unconformity in ference- and background-corrected 204Pb signal and a model Pb the western AQA, but is transitional in the Ghat area (Tawadros, composition (Stacey and Kramers, 1975) was carried out if neces- 2012). The succeeding Ordovician portion of the Qarqaf Group con- sary. The necessity of the correction is judged on whether the cor- tains four formations: the early Ordovician marine Ash Shabiyat rected 207Pb/206Pb ratio lies outside the internal errors of the (=Achebyat) Formation, middle Ordovician shallow-marine to del- measured ratios. Discordant analyses were interpreted with care. taic Hawaz (=Haouaz) Formation, middle to late Ordovician marine Raw data were corrected for background signal, common Pb, and ?subglacial Melaz Shoqran (=Melez Chograne) Formation, and laser-induced elemental fractionation, instrumental mass discrim- late Ordovician proglacial fluvial to marginal marine Mamuniyat ination, and time-dependant elemental fractionation of Pb/Th and (=Memouniat) Formation (Fig. 2). The bundling of these formations Pb/U using an ExcelÒ spreadsheet program developed by Axel Ger- within the Qarqaf Group is not fully accepted by all authors be- des (Institute of Geosciences, Johann-Wolfgang-Goethe University cause of the observation of unconformities between them (Kli- Frankfurt, Frankfurt am Main, Germany). Reported uncertainties tzsch, 1981). For more detailed description of the Ordovician were propagated by quadratic addition of the external reproduc- portion of the Qarqaf Group see Tawadros (2001, 2012) and litera- ibility obtained from the standard zircon GJ-1 (0.6% and 0.5–1% ture cited therein. for 207Pb/206Pb and 206Pb/238U respectively) during individual analytical sessions and the within-run precision of each analysis. Con- 3. Methods and samples cordia diagrams (2r error ellipses) and concordia dates (95% confidence level) were produced using Isoplot/Ex 2.49 (Ludwig, Detrital zircon grains of six sandstone samples were investi- 2001), and frequency and relative probability plots using AgeDis- gated in the present study. Zircon concentrates were separated play (Sircombe, 2004). The 207Pb/206Pb date was taken for interpre- from 1 to 2 kg of each sample using standard methods at the Geo- tation for all zircons >1.0 Ga, and the 206Pb/238U dates for younger logical Institute of Freiberg University, Germany. Final selection of grains. Analyses were carried out using the procedures of Gerdes the zircon grains for U–Pb dating was achieved by hand-picking and Zeh (2006) and Frei and Gerdes (2009). For further details on under a binocular microscope. Zircon grains of all sizes and mor- analytical protocol and data processing, see those references. phological types were selected, mounted in resin blocks and pol- The uncertainty in the degree of concordance of Precambrian– ished to half their thickness. Zircons were analyzed for U, Th and Palaeozoic grains dated by the LA-ICP-MS method is relatively Pb isotopes by LA-ICP-MS techniques at the Museum of Mineralogy large and results obtained from just a single analysis have to be M.M. Altumi et al. / Journal of African Earth Sciences 79 (2013) 74–97 79 interpreted with care. A typical uncertainty of 2–3% (2r)in sult gives space for interpretation of concordance or slight discor- 207Pb/206Pb for a late Neoproterozoic grain (e.g. 560 Ma) relates dance. The latter could be caused by episodic lead loss, to an absolute error in the 207Pb/206Pb age of 45–65 Ma. Such a re- fractionation, or infiltration of Pb isotopes by a fluid or via micro-

Table 2 U–Th–Pb data of detrital zircon grains from sample X1-1 (61 of 120 measured grains which are concordant in the range of 90–110%) (unnamed Wadi ‘‘X’’ section X1, Cambrian, sandstone, lower part of the Hasawnah Formation, Qarqaf Group, Al Qarqaf Arch, Libya, coordinates: 28°23051.200N, 13°55043.400E).

Number 207Pba Ub Pbb Th/ 206Pb/ 206Pb/ 2r 207Pb/ 2r 207Pb/ 2r Rhod 206Pb/ 2r 207Pb/ 2r 207Pb/ 2r Conc. 238 235 206 (cps) (ppm) (ppm) Ub 204Pbc 238Uc (%) 235Uc (%) 206Pbc (%) U (Ma) U (Ma) Pb (Ma) % 2a61 10787 90 8 0.60 18342 0.0839 2.2 0.67 3.2 0.0580 2.3 0.69 520 11 522 13 531 51 98 a53 18614 192 16 0.64 31549 0.0845 3.2 0.67 3.7 0.0577 2.0 0.85 523 16 522 15 520 44 100 2a2 15307 354 33 0.73 9363 0.0850 2.2 0.69 2.6 0.0589 1.4 0.84 526 11 533 11 562 32 94 2a17 7527 170 15 0.46 12828 0.0851 2.3 0.68 3.2 0.0579 2.2 0.72 526 12 527 13 528 49 100 a43 8584 134 12 0.43 14608 0.0860 2.6 0.69 3.1 0.0580 1.8 0.81 532 13 532 13 531 40 100 a41 9097 135 12 0.45 2497 0.0863 2.6 0.70 5.7 0.0585 5.1 0.46 533 14 536 24 547 111 97 a50 11778 130 12 0.63 20004 0.0863 2.6 0.69 3.4 0.0581 2.1 0.77 534 13 534 14 535 46 100 a16 32388 738 157 0.52 14765 0.0863 2.5 0.69 2.8 0.0579 1.3 0.89 534 13 532 12 527 28 101 2a29 7202 167 15 0.34 5148 0.0866 2.2 0.69 3.4 0.0582 2.6 0.65 535 11 536 14 537 57 100 2a27 7213 166 15 0.35 12225 0.0867 2.2 0.70 3.4 0.0582 2.5 0.66 536 11 536 14 538 56 100 a19 66401 1355 206 0.26 16273 0.0869 2.5 0.69 2.7 0.0577 1.0 0.93 537 13 534 11 520 21 103 2a60 4575 36 4 0.88 7588 0.0875 2.3 0.72 3.5 0.0593 2.7 0.65 541 12 548 15 578 58 94 a49 8381 93 8 0.75 7409 0.0878 2.8 0.71 3.6 0.0586 2.2 0.80 542 15 545 15 554 47 98 2a32 6155 140 13 0.35 10475 0.0881 2.3 0.71 3.3 0.0582 2.5 0.67 544 12 543 14 537 54 101 2a63 19695 198 18 0.75 4946 0.0880 3.1 0.72 3.6 0.0596 1.7 0.87 544 16 552 15 588 38 92 2a52 2986 30 3 1.16 5035 0.0882 3.1 0.71 5.2 0.0586 4.2 0.59 545 16 546 22 551 92 99 a4 16323 234 23 0.64 510 0.0887 2.7 0.72 4.2 0.0592 3.2 0.65 548 14 553 18 575 69 95 a40 16889 298 28 0.44 28361 0.0892 2.5 0.72 2.9 0.0588 1.5 0.86 551 13 552 13 560 32 98 2a59 23511 215 19 0.29 35724 0.0893 4.0 0.71 4.3 0.0578 1.7 0.92 551 21 546 18 523 37 105 2a44 5568 82 8 0.79 9346 0.0898 2.0 0.73 3.8 0.0587 3.2 0.54 554 11 555 16 558 70 99 a3 13632 280 26 0.35 23321 0.0909 2.5 0.72 2.9 0.0577 1.5 0.85 561 13 552 12 519 33 108 a29 536 16 1 0.69 904 0.0909 4.4 0.74 8.2 0.0589 6.9 0.54 561 24 561 36 562 150 100 a42 15020 252 23 0.61 25007 0.0909 2.6 0.74 3.0 0.0594 1.5 0.87 561 14 565 13 581 32 97 2a42 13799 180 17 0.68 2848 0.0912 2.4 0.74 6.7 0.0588 6.3 0.36 562 13 562 30 560 137 100 a35 4027 87 8 0.53 2555 0.0914 3.1 0.74 7.8 0.0589 7.2 0.39 564 17 563 34 563 157 100 2a24 12879 310 29 0.55 21727 0.0928 2.0 0.76 2.9 0.0591 2.2 0.66 572 11 572 13 571 48 100 2a46 17942 260 29 1.23 30391 0.0931 2.0 0.75 2.6 0.0585 1.7 0.75 574 11 569 11 550 37 104 2a5 11489 222 21 0.31 19517 0.0941 2.0 0.76 2.7 0.0583 1.8 0.74 580 11 572 12 541 40 107 a18 11410 225 30 1.93 6547 0.0944 2.8 0.77 3.6 0.0592 2.3 0.77 582 16 580 16 575 50 101 a58 7429 59 6 0.61 12343 0.0954 3.0 0.78 4.0 0.0594 2.7 0.75 587 17 586 18 582 58 101 2a4 7025 137 13 0.44 7094 0.0955 2.1 0.77 2.9 0.0588 2.0 0.71 588 12 582 13 561 45 105 2a9 10332 204 20 0.34 8860 0.0964 2.1 0.79 3.1 0.0596 2.3 0.69 593 12 593 14 591 49 100 a37 9792 184 18 0.61 15814 0.0971 2.9 0.82 3.3 0.0612 1.6 0.88 597 16 607 15 645 34 93 2a14 9448 187 18 0.28 15745 0.0982 2.2 0.81 3.0 0.0595 2.0 0.75 604 13 600 13 585 42 103 a63 5445 31 3 0.69 8899 0.0988 2.7 0.82 3.3 0.0604 2.0 0.80 607 15 610 15 618 43 98 a13 10353 190 28 2.14 17335 0.1000 2.6 0.81 3.3 0.0590 2.0 0.80 614 15 605 15 568 44 108 2a37 12371 239 25 0.59 20273 0.1004 2.0 0.84 2.9 0.0605 2.1 0.69 617 12 618 13 620 45 100 2a16 5228 102 11 0.48 8756 0.1014 2.0 0.83 3.0 0.0590 2.2 0.67 623 12 611 14 568 49 110 a54 5999 27 3 0.29 1937 0.1022 6.0 0.85 15.4 0.0602 14.1 0.39 627 36 624 74 611 306 103 2a3 6286 118 12 0.53 10437 0.1021 2.3 0.84 3.1 0.0597 2.1 0.73 627 14 619 14 592 46 106 2a18 9718 190 19 0.25 15925 0.1021 2.0 0.85 3.3 0.0605 2.6 0.60 627 12 625 16 621 57 101 2a65 12933 77 8 0.25 20488 0.1029 2.4 0.88 2.9 0.0624 1.5 0.84 631 15 643 14 686 33 92 a17 9355 159 18 0.57 15262 0.1045 2.6 0.87 3.3 0.0605 2.1 0.77 641 16 636 16 620 46 103 a48 17751 172 22 0.97 28830 0.1059 2.6 0.89 3.0 0.0608 1.5 0.86 649 16 645 14 631 32 103 a11 10382 165 20 0.90 16907 0.1061 2.7 0.89 3.3 0.0605 2.0 0.81 650 17 644 16 622 42 104 2a7 13270 234 27 0.60 5862 0.1087 2.1 0.93 2.8 0.0619 1.8 0.75 665 13 667 14 672 39 99 2a11 3777 59 8 0.93 6025 0.1096 2.2 0.93 3.9 0.0616 3.2 0.58 671 14 668 19 661 68 102 a51 24578 137 22 0.33 13324 0.1575 3.1 1.51 3.4 0.0696 1.4 0.91 943 27 935 21 917 30 103 2a33 20024 99 31 0.49 2970 0.2818 2.3 4.05 3.0 0.1043 1.8 0.79 1600 33 1645 24 1702 34 94 a57 73290 131 40 0.72 16183 0.2965 5.0 4.41 5.2 0.1078 1.2 0.97 1674 75 1714 44 1763 23 95 2a25 45952 173 61 0.77 14167 0.3233 2.2 4.99 2.8 0.1119 1.7 0.79 1806 35 1817 24 1830 31 99 2a10 57178 154 56 0.58 49693 0.3242 2.2 5.09 2.6 0.1139 1.4 0.85 1810 35 1835 22 1863 24 97 a20 104333 248 95 1.68 895 0.3322 2.9 5.83 3.2 0.1273 1.4 0.90 1849 46 1951 28 2060 25 90 a27 54992 115 45 0.57 41482 0.3492 2.4 6.30 2.6 0.1309 0.9 0.94 1931 41 2019 23 2110 16 91 a5 50538 113 45 0.64 9268 0.3497 2.5 6.11 2.8 0.1268 1.2 0.91 1933 42 1992 24 2054 20 94 a30 1591 6 2 1.03 1368 0.3541 4.1 5.60 5.9 0.1148 4.3 0.68 1954 69 1916 53 1876 78 104 a15 51974 120 51 0.93 27499 0.3545 3.1 6.05 3.2 0.1238 0.9 0.96 1956 52 1984 28 2012 15 97 a21 56016 123 56 0.72 43153 0.4064 2.7 7.18 2.9 0.1281 1.1 0.93 2198 50 2134 26 2073 19 106 2a57 24310 14 9 1.21 3124 0.5033 2.6 12.43 3.7 0.1791 2.5 0.73 2628 57 2638 35 2645 42 99 2a15 273959 148 138 1.39 86227 0.6730 2.3 26.33 2.4 0.2838 0.9 0.93 3317 59 3359 24 3384 14 98

a Within-run background-corrected mean 207Pb signal in counts per second. b U and Pb content and Th/U ratio were calculated relative to GJ-1 and are accurate to approximately 10%. c Corrected for background, mass bias, laser induced U–Pb fractionation and common Pb (if detectable, see analytical method) using Stacey and Kramers (1975) model Pb composition. 207Pb/235U calculated using 207Pb/206Pb/(238U/206Pb 1/137.88). Errors are propagated by quadratic addition of within-run errors (2SE) and the reproducibility of GJ-1 (2SD). d Rho is the error correlation deﬁned as err206Pb/238U/err207Pb/235U. 80 M.M. Altumi et al. / Journal of African Earth Sciences 79 (2013) 74–97 cracks. Thus, zircons showing a degree of concordance in the range Sampling for zircon dating was done by two of the authors of 90–110% in this paper are classiﬁed as concordant because of the (M.M.A. and O.E.) in the course of measuring transitional sections overlap of the error ellipse with the concordia (e.g. Frei and Gerdes, from granitic basement to overlaying Hasawnah Formation. Six sam- 2009; Jeffries et al., 2003; Linnemann et al., 2007, 2011). Th/U ra- ples for geochronological analysis were taken from different strati- tios were obtained from the LA-ICP-MS measurements of investi- graphic levels (distance above the conglomeratic base of the gated zircon grains. U and Pb content and Th/U ratio were formation) from various geographic areas in the northern, central calculated relative to the GJ-1 zircon standard and are accurate and southern Jebel Hasawnah mountain range (Figs. 1 and 2). All to approximately 10%. six samples are from the Hasawnah Formation (Cambrian) of the

Table 3 U–Th–Pb data of detrital zircon grains from sample Ba2-10 (54 of 120 measured grains which are concordant in the range of 90–110%) (Wadi Badran, section Wadi Badran 2, Cambrian, sandstone, lower part of the Hasawnah Formation, Qarqaf Group, Al Qarqaf Arch, Libya, coordinates: 28°11025.300N, 13°58031.100E).

Number 207Pba Ub Pbb Th/ 206Pb/ 206Pb/ 2r 207Pb/ 2r 207Pb/ 2r Rhod 206Pb/ 2r 207Pb/ 2r 207Pb/ 2r Conc. b 238 235 206 (cps) (ppm) (ppm) U 204Pbc 238Uc (%) 235Uc (%) 206Pbc (%) U (Ma) U (Ma) Pb (Ma) % a44 19912 203 20 0.76 8051 0.0848 1.1 0.678 2.7 0.0580 2.5 0.42 525 6 526 11 528 54 99 a29 14202 190 17 0.56 24146 0.0850 1.6 0.678 2.2 0.0579 1.5 0.73 526 8 526 9 525 33 100 a54 18455 133 12 0.48 2943 0.0850 1.5 0.682 3.6 0.0582 3.2 0.41 526 7 528 15 536 71 98 3a30 28234 665 59 0.41 564 0.0864 4.2 0.700 6.7 0.0588 5.3 0.62 534 21 539 29 559 115 96 a64 16934 80 7 0.56 5014 0.0878 2.1 0.708 4.6 0.0585 4.0 0.47 542 11 544 19 549 88 99 3a13 13041 191 18 0.79 1326 0.0885 3.8 0.718 8.3 0.0589 7.4 0.46 547 20 550 36 562 161 97 3a26 12722 212 21 0.83 21697 0.0902 3.2 0.727 3.8 0.0584 2.0 0.85 557 17 555 16 546 44 102 a11 5476 67 7 1.02 1232 0.0916 1.2 0.744 3.1 0.0589 2.9 0.38 565 6 565 14 565 63 100 3a28 14820 230 22 0.35 24608 0.0915 3.1 0.757 3.4 0.0599 1.4 0.91 565 17 572 15 602 31 94 a3 5356 59 7 1.53 2258 0.0925 2.6 0.759 5.1 0.0595 4.4 0.50 571 14 573 23 584 97 98 a12 21091 255 22 0.05 35330 0.0946 1.0 0.766 1.7 0.0588 1.4 0.59 583 6 578 8 558 31 104 a8 17253 190 18 0.31 28253 0.0953 1.4 0.791 2.3 0.0602 1.7 0.64 587 8 592 10 610 37 96 3a15 15140 228 22 0.47 24553 0.0963 3.2 0.814 3.7 0.0614 2.0 0.85 593 18 605 17 652 43 91 3a52 15512 129 13 0.85 1929 0.0969 3.0 0.814 3.9 0.0610 2.5 0.78 596 17 605 18 639 53 93 a39 17243 167 17 0.54 1624 0.0977 1.2 0.814 4.6 0.0605 4.5 0.25 601 7 605 21 620 96 97 2a11 12052 174 18 0.52 19923 0.0982 4.0 0.812 5.0 0.0600 3.0 0.81 604 23 604 23 603 64 100 a4 22209 234 24 0.46 14304 0.1001 1.2 0.837 1.6 0.0607 1.1 0.72 615 7 617 8 628 24 98 3a64 12573 67 7 0.71 1113 0.1003 3.4 0.863 3.9 0.0624 1.8 0.88 616 20 632 18 687 39 90 3a49 3616 32 4 1.33 5981 0.1005 3.0 0.834 5.1 0.0602 4.1 0.59 617 18 616 24 611 88 101 a21 15559 196 21 0.58 16805 0.1015 1.5 0.848 3.1 0.0606 2.7 0.49 623 9 624 14 625 58 100 3a42 12398 139 15 0.52 7284 0.1023 3.2 0.865 3.9 0.0613 2.2 0.82 628 19 633 19 650 48 97 a36 63446 1063 110 0.12 237 0.1025 1.1 0.850 5.0 0.0601 4.9 0.22 629 7 625 24 609 105 103 a43 11671 137 15 0.62 2043 0.1033 1.1 0.871 4.3 0.0612 4.2 0.26 634 7 636 21 645 90 98 3a22 18631 229 28 1.23 3318 0.1041 3.4 0.881 5.0 0.0614 3.7 0.67 638 20 642 24 654 79 98 3a40 8929 107 12 0.66 9198 0.1060 3.3 0.901 4.0 0.0617 2.3 0.82 650 20 653 19 662 48 98 3a5 20993 289 33 0.74 629 0.1094 3.3 0.935 5.3 0.0620 4.1 0.62 669 21 670 26 673 89 99 a31 2133 23 3 0.53 3339 0.1096 1.6 0.953 6.8 0.0631 6.6 0.24 670 10 680 34 711 141 94 a16 2828 32 4 0.86 4487 0.1101 1.4 0.942 3.3 0.0621 2.9 0.42 673 9 674 16 677 63 99 3a6 9222 138 16 0.68 14762 0.1105 3.4 0.946 5.0 0.0621 3.6 0.68 675 22 676 25 679 78 100 3a18 9659 130 16 0.75 15310 0.1106 3.5 0.960 4.1 0.0629 2.2 0.85 676 23 683 21 706 46 96 3a62 4939 26 3 0.57 1213 0.1138 3.1 1.013 3.7 0.0645 2.0 0.84 695 20 710 19 759 42 92 3a11 5900 65 8 0.76 4142 0.1147 3.1 0.991 3.7 0.0627 2.1 0.83 700 21 699 19 697 44 100 a28 14812 191 24 1.28 1026 0.1151 1.6 0.995 3.7 0.0627 3.4 0.44 702 11 701 19 699 71 100 a20 4051 46 6 0.76 6257 0.1154 1.7 1.014 4.8 0.0637 4.5 0.36 704 11 711 25 733 95 96 a55 29805 183 22 0.80 725 0.1178 1.2 1.043 15.9 0.0642 15.8 0.07 718 8 725 86 748 335 96 a17 14165 126 17 0.83 21885 0.1194 1.3 1.049 2.4 0.0637 2.0 0.53 727 9 729 12 733 42 99 a38 15010 126 16 0.26 22451 0.1280 1.2 1.163 1.9 0.0659 1.5 0.63 777 9 783 11 802 32 97 3a55 10935 52 8 1.29 2619 0.1284 3.0 1.153 3.5 0.0651 1.9 0.85 779 22 779 19 778 40 100 a46 27795 187 27 0.82 1907 0.1293 1.4 1.165 2.2 0.0654 1.7 0.65 784 11 784 12 786 36 100 2a8 46912 431 65 0.71 1216 0.1429 4.3 1.345 5.2 0.0683 2.9 0.83 861 35 865 31 877 60 98 a35 30593 274 40 0.63 3265 0.1477 1.4 1.399 2.7 0.0687 2.3 0.53 888 12 888 16 890 48 100 a48 21540 198 32 0.79 865 0.1665 1.4 1.667 4.9 0.0726 4.6 0.30 993 13 996 31 1003 94 99 a9 33367 175 32 0.68 2326 0.1688 1.3 1.703 2.2 0.0732 1.8 0.58 1006 12 1010 14 1018 36 99 a7 34118 167 33 0.79 46253 0.1750 1.2 1.752 1.5 0.0726 0.9 0.81 1039 11 1028 10 1003 18 104 a30 31012 237 61 0.20 11490 0.2629 1.0 3.349 1.5 0.0924 1.1 0.70 1505 14 1493 12 1475 20 102 3a8 75663 207 76 1.34 5817 0.3088 3.1 4.981 3.2 0.1170 1.0 0.95 1735 47 1816 28 1910 19 91 a22 25153 52 20 1.19 4557 0.3149 1.4 4.663 2.0 0.1074 1.4 0.70 1765 21 1761 17 1756 26 100 a58 22040 21 8 1.01 5714 0.3190 1.7 4.884 2.6 0.1110 2.0 0.66 1785 27 1800 22 1817 36 98 a65 87049 50 20 0.60 19582 0.3632 1.3 6.407 1.6 0.1279 0.9 0.82 1997 23 2033 14 2070 17 96 a60 43130 32 14 1.33 2838 0.3634 1.2 6.176 2.1 0.1232 1.7 0.57 1998 20 2001 18 2004 30 100 a47 132646 206 84 0.57 1330 0.3846 1.6 6.638 1.9 0.1252 1.0 0.85 2098 29 2064 17 2031 17 103 3a14 108304 208 88 1.20 3181 0.4052 3.0 8.683 3.5 0.1554 1.7 0.86 2193 56 2305 32 2406 29 91 3a51 105574 54 33 1.09 13796 0.4983 3.2 12.018 3.4 0.1749 1.0 0.96 2606 70 2606 32 2605 17 100 3a65 352387 98 55 0.33 20003 0.5245 3.3 14.561 3.4 0.2013 0.8 0.97 2718 74 2787 33 2837 14 96 a52 256981 190 150 1.08 1397 0.6822 1.5 23.346 2.3 0.2482 1.8 0.64 3353 38 3241 23 3173 28 106

Qarqaf Group in the AQA area. Sample names, locations, coordinates, pebbles are 1–10 cm in size and almost exclusively represented lithology and stratigraphic levels are as follows (see also Fig. 2): by plutonic quartz. Stratigraphic level: lowermost Hasawnah Formation (1.5 m Sample no.: DA2-3. above base). Location: Wadi Damran (28°31054.400N, 13°57024.900E). Sample no.: A1-3. Lithology: medium- to fine-grained sandstone with thin coarse- Location: Wadi Al Abd (28°13039.900N, 13°53058.400E). grained intercalations, showing small-scale low-angle cross- Lithology: coarse-grained sandstone with thin, medium- to fine- bedding. grained intercalations showing small-scale low-angle cross- Stratigraphic level: middle portion of lower unit of Hasawnah bedding. Formation (38 m above base). Stratigraphic level: lower portion of lower unit of Hasawnah For- Sample no.: X1-1. mation (20 m above base). Location: unnamed wadi ‘‘X’’ (28°23051.200N, 13°55043.400E). Sample no.: Ba1-3. Lithology: stratified mixture of (1) detritus of the deeply Location: Wadi Badran (28°11052.600N, 13°57003.600E). weathered granitic basement, and (2) medium- to coarse- Lithology: medium-grained sandstone showing high-angle grained conglomerate to conglomeratic sandstone which trough cross-bedding.

Table 4 U–Th–Pb data of detrital zircon grains from sample Ba1-3 (46 of 120 measured grains which are concordant in the range of 90–110%) (Wadi Badran, section Wadi Badran 1, Cambrian, sandstone, lower part of the Hasawnah Formation, Qarqaf Group, Al Qarqaf Arch, Libya, coordinates: 28°11052.600 N, 13°57003.600 E).

207 a b b 206 206 207 207 d 206 207 207 Number Pb U Pb Th/Ub Pb/ Pb/ 2r Pb/ 2r Pb/ 2r Rho Pb/ 2r Pb/ 2r Pb/ 2r Conc. 238 c 238 235 206 (cps) (ppm) (ppm) 204Pbc U (%) 235Uc (%) 206Pbc (%) U (Ma) U (Ma) Pb (Ma) % a55 26169 177 16 0.50 27604 0.0863 1.8 0.70 2.5 0.0588 1.8 0.71 533 9 539 10 561 38 95 2a24 188895 2611 216 0.03 62549 0.0894 1.8 0.72 2.1 0.0586 1.1 0.86 552 10 552 9 552 24 100 a30 5498 79 8 0.67 9369 0.0905 2.7 0.73 3.8 0.0587 2.7 0.71 558 15 558 17 557 59 100 a9 11809 150 13 0.27 20040 0.0909 2.9 0.74 3.6 0.0588 2.2 0.80 561 16 561 16 561 47 100 a52 17205 107 9 0.11 28908 0.0922 2.3 0.76 2.9 0.0594 1.7 0.80 568 13 571 13 582 37 98 a66 9566 44 4 0.41 11113 0.0927 1.8 0.76 2.9 0.0592 2.2 0.65 571 10 572 13 573 47 100 2a41 21363 223 21 0.31 10837 0.0929 2.0 0.76 2.7 0.0592 1.8 0.75 572 11 573 12 574 39 100 2a65 13915 58 5 0.32 9912 0.0930 1.5 0.76 2.2 0.0592 1.6 0.70 573 8 573 10 574 34 100 2a53 10922 69 7 0.44 18460 0.0948 1.6 0.78 2.4 0.0595 1.8 0.66 584 9 584 11 584 39 100 a40 2717 30 3 0.43 4119 0.0958 2.0 0.79 4.4 0.0597 3.9 0.46 590 11 590 20 593 84 99 2a28 15985 206 20 0.23 15290 0.0963 1.3 0.79 2.3 0.0594 1.9 0.56 593 7 590 10 580 42 102 2a50 17842 158 16 0.49 30047 0.0966 1.8 0.80 3.6 0.0597 3.2 0.50 595 10 594 16 593 68 100 2a64 3544 15 2 0.80 807 0.0968 3.6 0.80 7.5 0.0597 6.6 0.48 596 21 595 35 592 143 101 2a43 18313 171 17 0.47 30665 0.0973 1.8 0.80 2.5 0.0599 1.7 0.72 599 10 599 11 600 37 100 2a61 15584 83 8 0.33 26215 0.0977 1.9 0.81 2.5 0.0599 1.6 0.76 601 11 600 11 599 35 100 2a33 14826 208 20 0.18 24910 0.0980 1.9 0.81 3.2 0.0598 2.5 0.61 603 11 601 15 596 55 101 a37 18494 245 23 0.24 12819 0.0984 1.5 0.82 2.1 0.0601 1.4 0.72 605 9 606 9 608 31 100 a60 9764 49 5 0.44 15936 0.1002 3.5 0.85 4.8 0.0614 3.2 0.74 616 21 624 23 652 69 94 2a55 12594 64 7 0.60 3489 0.1029 1.8 0.86 2.6 0.0607 1.9 0.68 631 11 631 12 630 41 100 a21 2184 25 3 0.96 2652 0.1051 1.9 0.89 4.8 0.0611 4.5 0.39 644 12 644 23 644 96 100 a53 8155 48 7 1.22 13283 0.1070 2.1 0.91 4.0 0.0615 3.4 0.54 656 13 656 19 657 72 100 a65 2639 11 1 1.07 4292 0.1072 2.0 0.91 4.0 0.0617 3.4 0.50 656 12 658 19 665 73 99 a16 4637 52 6 0.38 7691 0.1076 1.7 0.90 4.7 0.0605 4.3 0.37 659 11 650 23 621 93 106 2a8 14939 161 21 0.87 24188 0.1101 1.9 0.94 2.4 0.0621 1.5 0.78 673 12 674 12 676 33 100 a50 2493 15 2 1.16 4039 0.1106 2.1 0.94 4.9 0.0619 4.5 0.42 676 13 675 25 670 95 101 2a20 15623 161 22 0.96 24743 0.1164 1.7 1.02 2.3 0.0634 1.6 0.72 710 11 713 12 722 34 98 2a10 29107 203 32 0.49 9759 0.1484 2.0 1.41 2.3 0.0688 1.2 0.87 892 17 892 14 893 24 100 2a18 2301 15 3 0.68 3352 0.1501 2.0 1.43 4.9 0.0689 4.5 0.41 902 17 900 30 897 92 101 2a44 28307 120 23 0.84 39604 0.1651 1.5 1.63 2.0 0.0718 1.2 0.78 985 14 983 12 980 25 100 a61 82050 71 24 0.46 72696 0.3066 3.4 4.76 3.9 0.1125 1.8 0.88 1724 52 1777 33 1840 33 94 a44 39546 57 21 0.76 13047 0.3229 1.8 5.01 2.0 0.1125 0.9 0.90 1804 29 1821 17 1841 16 98 2a17 79388 155 62 1.05 66410 0.3312 1.5 5.48 2.0 0.1200 1.3 0.75 1844 24 1898 17 1956 23 94 a36 32688 68 27 1.16 3891 0.3315 2.2 5.21 2.5 0.1141 1.2 0.87 1846 35 1855 21 1865 22 99 2a26 91568 175 70 0.73 78985 0.3431 1.4 5.51 1.7 0.1165 1.0 0.80 1902 22 1902 15 1903 18 100 2a27 9176 17 8 1.49 7874 0.3441 1.3 5.56 2.6 0.1171 2.2 0.50 1906 21 1909 22 1913 40 100 2a21 58497 85 42 1.59 45406 0.3620 1.4 6.46 1.7 0.1294 1.0 0.81 1992 24 2040 15 2090 18 95 2a9 169807 237 101 0.43 10978 0.3915 1.5 7.02 2.1 0.1301 1.5 0.71 2130 27 2114 19 2099 26 101 a6 79302 117 51 0.42 55030 0.4014 2.0 7.45 3.1 0.1346 2.3 0.66 2175 38 2167 28 2158 40 101 a38 157105 230 101 0.49 125462 0.4060 1.5 7.01 1.7 0.1253 0.9 0.87 2197 28 2113 16 2033 15 108 a5 33741 38 19 0.71 15994 0.4384 1.8 8.36 2.1 0.1383 1.2 0.83 2344 35 2271 19 2206 21 106 2a63 123415 40 23 0.83 76559 0.4806 1.7 10.73 1.9 0.1619 1.0 0.87 2530 36 2500 18 2476 16 102 a49 288120 156 83 0.36 170584 0.4863 1.7 11.32 2.3 0.1689 1.4 0.77 2555 36 2550 21 2547 24 100 2a31 249640 232 125 0.29 41252 0.4876 3.3 11.57 3.5 0.1720 1.1 0.95 2560 70 2570 33 2577 18 99 2a15 121176 91 54 0.76 19192 0.5009 1.5 12.66 1.8 0.1833 1.1 0.81 2617 31 2655 17 2683 18 98 a11 557938 337 203 0.25 130229 0.5407 1.6 16.71 1.6 0.2241 0.4 0.97 2786 36 2918 16 3010 6 93 2a52 38730 11 9 1.43 18563 0.5733 1.7 16.58 2.3 0.2098 1.5 0.74 2922 40 2911 22 2904 25 101

Table 5 U–Th–Pb data of detrital zircon grains from sample A1-3 (61 of 120 measured grains which are concordant in the range of 90–110%) (Wadi Al Abd, section Wadi Al Abd 1, Cambrian, sandstone, lower part of the Hasawnah Formation, Qarqaf Group, Al Qarqaf Arch, Libya, coordinates: 28°13039.900N, 13°53058.400E).

Number 207Pb Ua Pba Th/ 206Pb/ 206Pb/ 2r 207Pb/ 2r 207Pb/ 2r Rho 206Pb/ 2r 207Pb/ 2r 207Pb/ 2r Conc. 204 238 d 235 d 206 d 238 235 206 (cps) (ppm) (ppm) Uc Pb U (%) U (%) Pb (%) U (Ma) U (Ma) Pb (Ma) % a15 10805 238 23 0.74 6882 0.0892 2.0 0.72 3.0 0.0584 2.2 0.68 551 11 550 13 546 48 101 a39 4492 90 9 1.00 2082 0.0896 2.6 0.72 5.5 0.0583 4.8 0.48 553 14 551 24 542 105 102 2a6 3313 59 7 1.85 5602 0.0900 1.0 0.73 3.1 0.0587 2.9 0.31 556 5 556 13 555 64 100 2a5 6245 110 12 1.16 10548 0.0903 1.2 0.73 2.5 0.0587 2.2 0.48 557 7 557 11 557 49 100 a48 11162 136 12 0.42 18440 0.0911 1.7 0.74 3.1 0.0587 2.6 0.54 562 9 561 14 556 58 101 2a42 6978 105 10 0.53 4310 0.0910 1.0 0.74 2.5 0.0587 2.3 0.39 562 5 560 11 554 50 101 a26 13428 297 30 0.68 11628 0.0912 2.1 0.74 3.9 0.0588 3.3 0.54 563 12 562 17 559 72 101 a19 10699 155 21 2.67 194 0.0926 3.6 0.76 6.1 0.0594 5.0 0.58 571 20 573 27 581 108 98 a11 7448 155 20 2.25 12136 0.0930 1.7 0.76 2.9 0.0595 2.4 0.57 573 9 576 13 587 51 98 2a10 5094 87 9 0.77 8472 0.0931 1.4 0.76 4.5 0.0594 4.3 0.32 574 8 575 20 580 93 99 2a31 7067 127 16 1.40 11748 0.0959 1.6 0.79 2.6 0.0597 2.0 0.63 590 9 591 12 592 44 100 2a14 16601 289 30 0.78 27480 0.0960 0.7 0.80 2.1 0.0600 1.9 0.35 591 4 594 9 605 42 98 a36 9713 232 32 2.38 2979 0.0972 1.9 0.80 4.1 0.0598 3.6 0.46 598 11 598 19 598 78 100 a42 162 3 0 1.14 261 0.0973 4.0 0.81 16.6 0.0603 16.1 0.24 598 23 602 78 616 347 97 a51 7383 74 9 1.68 4515 0.0972 1.9 0.80 3.7 0.0594 3.2 0.51 598 11 595 17 583 69 103 2a55 5799 46 5 1.18 2382 0.0971 1.4 0.80 2.9 0.0598 2.5 0.48 598 8 597 13 595 55 100 a61 15554 90 10 1.11 1030 0.0973 2.4 0.80 8.3 0.0598 7.9 0.29 599 14 598 38 598 171 100 a30 17381 350 42 1.20 19099 0.0985 2.1 0.82 3.0 0.0603 2.2 0.69 606 12 608 14 615 47 98 2a44 9889 133 16 1.51 1300 0.0988 1.3 0.84 3.4 0.0619 3.2 0.37 607 7 621 16 670 68 91 2a4 6590 110 12 0.84 1489 0.0992 1.5 0.82 2.7 0.0601 2.3 0.56 610 9 609 13 608 49 100 a28 19101 395 43 0.70 3858 0.1001 1.9 0.83 2.5 0.0603 1.6 0.77 615 11 615 12 615 34 100 a55 31775 117 16 1.36 165 0.1000 3.0 0.83 8.9 0.0604 8.4 0.34 615 18 615 42 617 181 100 2a39 6888 109 12 0.65 11317 0.1003 1.7 0.83 3.3 0.0603 2.8 0.53 616 10 616 15 615 61 100 2a8 7853 131 14 0.77 9952 0.1005 1.0 0.84 2.2 0.0603 1.9 0.47 618 6 617 10 614 42 101 a65 6183 34 4 0.91 10026 0.1008 2.0 0.83 3.1 0.0597 2.4 0.64 619 12 614 14 592 51 105 a32 10456 225 25 0.82 16894 0.1010 2.0 0.84 3.3 0.0603 2.6 0.60 620 12 619 15 613 57 101 2a3 14049 229 27 1.17 2153 0.1021 1.4 0.86 3.6 0.0610 3.4 0.39 627 8 629 17 639 72 98 2a32 20447 363 45 1.18 3331 0.1027 1.1 0.86 1.9 0.0607 1.5 0.59 630 7 630 9 628 33 100 a54 8198 64 7 0.87 13122 0.1031 1.7 0.86 3.0 0.0606 2.5 0.55 632 10 631 14 625 54 101 a35 4570 94 13 1.87 3869 0.1032 1.6 0.87 3.3 0.0609 2.9 0.50 633 10 634 16 636 61 100 2a35 2133 39 5 1.29 3459 0.1038 1.6 0.87 4.9 0.0611 4.7 0.33 637 10 638 24 642 100 99 2a60 5525 38 5 1.11 1763 0.1039 1.7 0.88 3.6 0.0611 3.2 0.46 637 10 639 17 644 69 99 2a29 8774 150 19 1.06 5933 0.1059 0.8 0.90 2.1 0.0613 2.0 0.37 649 5 649 10 651 42 100 2a65 13884 76 11 1.88 4565 0.1062 0.7 0.92 1.7 0.0628 1.5 0.42 651 4 662 8 702 32 93 a59 31722 104 15 0.98 312 0.1107 3.2 0.95 4.8 0.0623 3.6 0.66 677 21 679 24 685 77 99 a10 9755 148 18 0.87 1816 0.1118 2.0 0.96 3.3 0.0626 2.6 0.61 683 13 686 17 695 56 98 a18 8919 155 20 0.95 14058 0.1137 1.8 0.96 2.5 0.0615 1.7 0.72 694 12 685 12 657 36 106 2a21 9814 162 23 2.04 1554 0.1152 1.6 1.00 13.4 0.0629 13.3 0.12 703 11 704 70 705 282 100 a64 13834 63 9 1.02 3169 0.1297 2.2 1.19 2.9 0.0665 1.9 0.76 786 16 796 16 823 39 96 a66 20230 57 9 1.01 615 0.1370 2.8 1.27 5.1 0.0671 4.3 0.55 827 22 831 30 841 90 98 a53 25010 112 19 0.74 2469 0.1608 1.8 1.62 2.2 0.0728 1.3 0.80 961 16 976 14 1010 27 95 2a43 12000 34 12 1.44 4241 0.2813 1.2 3.81 2.4 0.0983 2.1 0.50 1598 17 1595 20 1592 39 100 a29 89326 349 127 1.21 7282 0.2942 2.1 4.09 2.6 0.1009 1.5 0.81 1662 31 1653 22 1641 29 101 2a48 36144 75 25 0.81 1803 0.2971 1.1 4.49 2.3 0.1096 2.1 0.45 1677 16 1729 19 1792 38 94 2a57 44923 57 20 0.63 39193 0.3026 2.6 4.74 3.1 0.1136 1.7 0.85 1704 40 1774 27 1857 30 92 a21 25120 90 34 1.12 12577 0.3167 1.7 4.55 2.1 0.1042 1.2 0.82 1774 27 1740 18 1701 22 104 a14 34340 101 38 0.90 29940 0.3262 1.8 5.00 2.1 0.1112 1.1 0.86 1820 29 1820 18 1819 19 100 2a33 45902 149 54 0.68 40633 0.3264 1.0 5.03 1.7 0.1118 1.3 0.61 1821 17 1825 15 1829 24 100 a4 105612 313 124 1.11 91839 0.3288 1.8 5.05 2.1 0.1113 1.1 0.86 1832 29 1827 18 1821 20 101 2a27 37014 109 41 0.76 32255 0.3307 1.3 5.19 1.7 0.1138 1.1 0.75 1842 21 1851 15 1860 21 99 2a11 85699 232 89 0.96 2817 0.3409 1.1 5.44 1.5 0.1158 1.1 0.70 1891 17 1891 13 1892 19 100 2a61 35729 36 14 0.91 2609 0.3436 1.1 5.55 1.7 0.1171 1.4 0.61 1904 17 1908 15 1913 24 100 2a22 18628 47 19 0.87 15745 0.3472 1.1 5.60 1.8 0.1170 1.5 0.59 1921 18 1916 16 1911 26 101 a40 21897 65 26 1.32 18918 0.3480 1.7 5.39 2.3 0.1124 1.5 0.74 1925 28 1883 20 1838 28 105 a3 20806 58 24 0.92 17480 0.3536 1.8 5.63 2.7 0.1154 2.0 0.67 1952 31 1920 24 1886 36 103 a7 14565 38 17 1.53 4756 0.3611 1.9 5.64 2.9 0.1133 2.2 0.65 1987 32 1923 25 1853 40 107 2a36 46301 136 62 1.39 39908 0.3623 1.5 5.74 1.8 0.1150 0.9 0.85 1993 26 1938 15 1879 17 106 a44 24505 57 23 0.80 20859 0.3802 1.9 6.02 2.7 0.1147 1.9 0.71 2077 34 1978 23 1876 34 111 a5 27939 48 25 1.03 17942 0.4430 1.6 9.23 2.3 0.1510 1.7 0.70 2364 32 2361 21 2358 28 100 a24 150896 225 164 2.00 14378 0.4984 2.5 11.75 2.8 0.1710 1.3 0.89 2607 53 2585 26 2567 22 102 2a7 118693 126 74 0.72 20527 0.5015 1.4 12.32 1.7 0.1782 0.9 0.84 2620 30 2629 16 2636 15 99 a25 184423 124 118 1.49 65773 0.6798 1.7 25.47 1.9 0.2717 0.7 0.92 3344 45 3326 18 3316 11 101

Table 6 U–Th–Pb data of detrital zircon grains from sample DA2-3 (61 of 120 measured grains which are concordant in the range of 90–110%) (Wadi Damran, section Wadi Damran 2, Cambrian, sandstone, lower part of the Hasawnah Formation, Qarqaf Group, Al Qarqaf Arch, Libya, coordinates: 28°31054.400 N, 13°57024.900E).

Number 207Pba Ub Pbb Th/ 206Pb/ 206Pb/ 2r 207Pb/ 2r 207Pb/ 2r Rhod 206Pb/ 2r 207Pb/ 2r 207Pb/ 2r Conc. 238 235 206 (cps) (ppm) (ppm) Ub 204Pbc 238Uc (%) 235Uc (%) 206Pbc (%) U (Ma) U (Ma) Pb (Ma) % a64 4932 30 3 1.17 5516 0.0829 2.5 0.659 3.6 0.0576 2.6 0.69 514 12 514 15 515 57 100 a65 2716 18 2 0.61 4288 0.0841 2.5 0.671 4.8 0.0578 4.1 0.53 521 13 521 20 523 90 100 2a24 7616 157 14 0.50 8079 0.0863 2.7 0.691 4.2 0.0581 3.2 0.66 534 14 534 17 532 69 100 2a25 24573 469 41 0.34 8212 0.0870 2.5 0.712 2.7 0.0593 1.2 0.90 538 13 546 12 579 26 93 2a37 10622 223 21 0.47 17712 0.0890 2.8 0.719 3.2 0.0585 1.6 0.86 550 15 550 14 550 35 100 a24 3589 69 6 0.00 2117 0.0906 1.9 0.729 3.8 0.0584 3.3 0.49 559 10 556 17 544 73 103 2a43 9363 135 17 1.82 15561 0.0912 2.6 0.739 3.0 0.0588 1.5 0.86 563 14 562 13 558 34 101 2a49 7413 79 8 0.69 4470 0.0921 2.6 0.748 3.9 0.0589 2.9 0.67 568 14 567 17 564 63 101 2a18 16797 292 28 0.34 1523 0.0923 3.0 0.758 8.3 0.0595 7.8 0.36 569 16 573 37 586 168 97 2a15 4992 95 10 0.75 2280 0.0927 2.6 0.754 3.4 0.0590 2.2 0.76 571 14 570 15 566 48 101 2a27 3451 70 7 0.46 5730 0.0927 2.7 0.753 4.3 0.0589 3.3 0.63 572 15 570 19 564 73 101 a15 31172 615 59 0.56 12073 0.0934 1.7 0.752 2.5 0.0584 1.7 0.71 575 10 570 11 546 38 105 2a4 7717 129 13 0.65 425 0.0939 2.7 0.776 4.7 0.0600 3.8 0.58 578 15 583 21 603 83 96 2a7 24645 449 43 0.40 5178 0.0938 2.4 0.781 2.8 0.0604 1.5 0.86 578 13 586 13 617 31 94 2a53 4945 43 5 1.40 8150 0.0949 3.2 0.775 4.0 0.0593 2.4 0.81 584 18 583 18 578 51 101 2a17 7886 124 14 1.18 6071 0.0982 2.9 0.814 3.9 0.0601 2.6 0.75 604 17 604 18 606 56 100 a12 18033 347 32 0.05 29379 0.0989 1.4 0.761 2.8 0.0558 2.4 0.50 608 8 574 12 444 54 137 2a62 13313 86 9 0.43 10359 0.0990 2.9 0.801 3.3 0.0587 1.4 0.90 608 17 597 15 556 31 109 2a51 3994 36 4 0.89 1724 0.0994 3.0 0.825 4.4 0.0602 3.2 0.68 611 17 611 20 609 69 100 a17 5763 100 10 0.35 3294 0.0997 1.8 0.824 8.9 0.0600 8.7 0.20 612 10 611 42 603 189 102 2a44 12666 161 18 0.90 6888 0.0999 2.4 0.830 3.1 0.0602 1.9 0.79 614 14 613 14 612 41 100 2a38 9445 160 23 1.84 6207 0.1044 2.6 0.878 3.2 0.0610 1.9 0.80 640 16 640 15 640 41 100 2a28 14083 231 26 0.56 779 0.1060 4.2 0.906 8.7 0.0620 7.7 0.48 650 26 655 43 674 164 96 2a10 14017 219 26 0.84 22276 0.1077 2.5 0.913 2.9 0.0615 1.5 0.86 660 16 659 14 655 31 101 2a21 853 17 2 1.48 1278 0.1082 2.9 0.921 9.7 0.0618 9.2 0.30 662 18 663 48 665 197 99 2a48 33296 278 31 0.37 51225 0.1088 2.6 0.952 3.1 0.0635 1.6 0.85 666 17 679 15 724 34 92 2a52 17808 243 26 0.28 2503 0.1107 4.8 0.943 5.8 0.0618 3.4 0.82 677 31 674 29 667 72 101 2a33 2283 40 6 1.64 3550 0.1117 2.7 0.944 4.2 0.0613 3.3 0.62 682 17 675 21 650 71 105 2a32 6554 185 23 0.94 1229 0.1136 2.5 0.959 3.2 0.0612 2.1 0.76 694 16 683 16 647 45 107 2a59 2662 17 2 0.71 4181 0.1229 3.7 1.076 5.2 0.0635 3.6 0.72 747 26 741 27 725 76 103 2a31 2289 27 4 1.00 3352 0.1378 2.8 1.269 5.4 0.0668 4.6 0.52 832 22 832 31 832 96 100 a42 167677 509 143 0.19 11762 0.2836 1.6 3.968 1.8 0.1015 1.0 0.85 1610 22 1628 15 1651 18 98 a26 36166 118 40 0.97 31942 0.2884 2.0 4.086 2.5 0.1027 1.5 0.81 1634 29 1652 20 1674 27 98 a2 106308 319 100 0.33 23151 0.2999 1.6 4.613 1.8 0.1116 0.8 0.90 1691 24 1752 15 1825 14 93 a13 146654 157 76 0.60 115 0.3054 2.5 4.365 3.6 0.1037 2.6 0.70 1718 38 1706 30 1691 47 102 a5 8891 24 10 1.68 3348 0.3133 1.7 4.579 2.9 0.1060 2.3 0.59 1757 26 1745 24 1732 43 101 a8 284078 736 253 0.59 18745 0.3133 1.4 4.741 1.6 0.1098 0.6 0.91 1757 22 1775 13 1796 12 98 a49 25322 42 16 1.09 22042 0.3155 2.0 4.550 2.2 0.1046 1.1 0.88 1768 31 1740 19 1707 20 104 a25 10106 31 15 2.63 8888 0.3256 1.5 4.636 2.4 0.1033 1.9 0.61 1817 24 1756 21 1684 36 108 a20 78424 184 69 0.73 56595 0.3306 1.5 5.742 1.7 0.1260 0.9 0.87 1841 24 1938 15 2042 15 90 a43 36334 73 28 0.93 9491 0.3305 1.3 4.840 2.0 0.1062 1.5 0.64 1841 21 1792 17 1736 28 106 2a35 7374 31 15 2.40 6911 0.3326 2.5 4.786 4.0 0.1044 3.1 0.62 1851 40 1782 34 1703 58 109 2a64 92942 97 34 0.50 31410 0.3326 3.2 5.258 3.3 0.1147 0.9 0.96 1851 51 1862 28 1874 16 99 a47 19301 33 16 1.97 5813 0.3358 1.7 4.952 2.3 0.1069 1.6 0.73 1867 28 1811 20 1748 30 107 2a16 63571 177 74 1.30 6055 0.3359 3.0 5.284 3.2 0.1141 1.2 0.93 1867 49 1866 28 1865 21 100 2a47 64044 108 43 1.03 54817 0.3374 2.6 5.309 2.8 0.1141 1.0 0.93 1874 43 1870 24 1866 18 100 2a3 27983 74 29 0.91 23943 0.3410 2.4 5.366 2.7 0.1141 1.2 0.89 1891 40 1879 24 1866 22 101 a58 47137 43 18 0.98 4551 0.3476 1.4 5.413 1.9 0.1129 1.2 0.78 1923 24 1887 16 1847 21 104 2a26 38527 93 44 1.56 20208 0.3612 2.6 6.056 2.9 0.1216 1.2 0.91 1988 45 1984 25 1980 22 100 a31 20421 50 27 2.54 16605 0.3636 1.6 5.606 2.2 0.1118 1.5 0.73 1999 28 1917 19 1829 28 109 a9 58380 122 49 0.49 43236 0.3646 1.3 6.174 1.6 0.1228 0.9 0.81 2004 22 2001 14 1997 17 100 a30 69441 201 82 0.74 54639 0.3687 1.3 5.876 1.5 0.1156 0.6 0.91 2023 23 1958 13 1889 11 107 a6 144906 331 130 0.41 114719 0.3717 1.4 5.889 1.6 0.1149 0.8 0.87 2038 24 1960 14 1878 14 108 2a13 138786 308 129 0.63 107518 0.3738 2.6 6.513 2.7 0.1264 0.8 0.95 2047 45 2048 24 2048 14 100 2a40 44412 86 41 1.28 7542 0.3797 2.7 6.634 3.0 0.1267 1.3 0.90 2075 48 2064 27 2053 23 101 2a30 171690 397 156 0.17 58024 0.3890 2.4 6.644 2.5 0.1239 0.9 0.94 2118 43 2065 23 2013 15 105 a29 175854 278 140 0.85 1030 0.4274 2.1 8.492 2.4 0.1441 1.3 0.85 2294 40 2285 23 2277 23 101 a22 252566 410 197 0.61 41130 0.4292 1.3 8.945 1.5 0.1512 0.7 0.87 2302 25 2332 14 2359 13 98 a35 147476 372 176 0.42 11857 0.4484 1.6 10.083 4.2 0.1631 3.9 0.37 2388 31 2442 40 2488 66 96 a16 108799 192 104 0.80 16170 0.4743 2.3 10.743 2.6 0.1643 1.2 0.89 2502 48 2501 24 2500 20 100 a4 45678 54 27 0.24 25590 0.4776 1.6 10.698 2.1 0.1624 1.4 0.77 2517 34 2497 20 2481 23 101 a14 199958 217 117 0.20 79545 0.5062 1.4 12.871 1.6 0.1844 0.8 0.87 2640 30 2670 15 2693 13 98 a33 591176 648 509 0.84 34296 0.6653 1.4 20.891 1.9 0.2278 1.2 0.76 3288 37 3134 19 3036 20 108

Stratigraphic level: basal portion of lower unit of Hasawnah For- 4. Results mation (7 m above base). Sample no.: Ba2-10. The U–Th–Pb data of all investigated detrital zircons are pre- Location: Wadi Badran (28°11025.300N, 13°58031.100E). sented in Tables 2–7. A total 120 detrital zircon grains were ana- Lithology: ﬁne-grained sandstone showing low-angle herring- lyzed in each sample. We discuss in that paper only zircons, bone cross-bedding. which are concordant in the range of 90–110%. In sample X1-1 (Ta- Stratigraphic level: basal portion of lower unit of Hasawnah For- ble 2), 61 grains are concordant (Fig. 3). The youngest concordant mation (4.5 m above base). grain yields a date of 520 ± 11 Ma; the oldest zircon yields a date Sample no.: Ba2-14. of 3384 ± 14 Ma. Two grains in this sample are Archaean in age Location: Wadi Badran (28°11025.300N, 13°58031.100E). (3384 ± 14 Ma, 2645 ± 42 Ma) (Table 2, Fig. 3). Some 17% of all Lithology: centimeter-scale alternation of (1) coarse-grained grains are Palaeoproterozoic, ranging from 2073 ± 19 Ma to and (2) medium- to ﬁne-grained sandstone, showing unidirec- 1702 ± 34 Ma. No Mesoproterozoic zircons were detected. The tional trough cross-bedding. majority, 60% of all grains, are Neoproterozoic and range from Stratigraphic level: middle portion of middle unit of Hasawnah 943 ± 27 Ma to 544 ± 12 Ma. Around 20% of all zircon grains yield Formation (136 m above base).

Table 7 U–Th–Pb data of detrital zircon grains from sample Ba2-14 (45 of 120 measured grains which are concordant in the range of 90–110%) (Wadi Badran, section Wadi Badran 2, Cambrian, sandstone, middle part of the Hasawnah Formation, Qarqaf Group, Al Qarqaf Arch, Libya, coordinates: 28°11025.300N, 13°58031.100 E).

Number 207Pba Ub Pbb Th/ 206Pb/ 206Pb/ 2r 207Pb/ 2r 207Pb/ 2r rhod 206Pb/ 2r 207Pb/ 2r 207Pb/ 2r Conc. 238 235 206 (cps) (ppm) (ppm) Ub 204Pbc 238Uc (%) 235Uc (%) 206Pbc (%) U (Ma) U (Ma) Pb (Ma) % a24 27632 298 29 0.44 397 0.0888 2.6 0.72 6.5 0.0589 6.0 0.40 549 14 551 28 563 130 98 2a11 4631 88 8 0.39 7808 0.0889 4.1 0.73 4.9 0.0592 2.7 0.83 549 22 554 21 575 60 96 3a50 16673 145 14 0.78 2314 0.0897 3.4 0.74 4.7 0.0595 3.3 0.72 554 18 560 20 587 71 94 a18 7165 106 12 1.33 12059 0.0900 2.6 0.73 3.7 0.0590 2.7 0.70 555 14 558 16 567 59 98 a35 13916 128 16 1.04 437 0.0901 2.6 0.73 6.9 0.0589 6.4 0.38 556 14 558 30 564 140 99 3a26 14212 293 27 0.41 4350 0.0908 4.1 0.75 4.5 0.0597 1.8 0.92 560 22 567 20 594 39 94 3a14 14511 222 25 1.04 10216 0.0909 3.4 0.75 3.8 0.0597 1.8 0.88 561 18 567 17 593 39 95 3a24 4658 77 8 0.94 4148 0.0916 3.3 0.76 4.9 0.0598 3.6 0.68 565 18 571 21 598 77 94 3a52 2599 19 2 1.59 4319 0.0930 3.4 0.77 5.3 0.0600 4.1 0.64 573 18 579 24 603 88 95 a48 10633 110 11 0.44 10328 0.0934 2.6 0.76 3.6 0.0594 2.6 0.71 575 14 577 16 582 55 99 3a3 8770 121 20 3.30 14553 0.0935 3.2 0.78 3.8 0.0602 2.1 0.84 576 18 583 17 610 45 94 a6 7148 90 9 0.59 11859 0.0964 2.6 0.80 3.4 0.0599 2.3 0.75 593 15 595 16 600 49 99 a36 9734 167 17 0.69 5793 0.0966 2.6 0.81 4.0 0.0605 3.1 0.65 594 15 600 18 622 66 96 a23 17447 250 23 0.05 17443 0.0981 2.4 0.82 2.7 0.0603 1.3 0.88 604 14 606 13 615 28 98 2a3 16585 300 32 0.52 24191 0.0985 4.0 0.81 4.3 0.0597 1.5 0.93 605 23 603 20 594 33 102 a5 62280 287 39 0.93 107 0.0994 2.6 0.83 7.6 0.0606 7.1 0.34 611 15 614 36 625 153 98 a42 20400 237 26 0.73 33461 0.1003 2.6 0.84 3.0 0.0605 1.5 0.86 616 15 618 14 623 33 99 a53 8687 56 6 0.52 2070 0.1004 2.8 0.84 5.4 0.0607 4.6 0.52 617 17 619 25 628 100 98 3a39 4388 53 7 1.48 7148 0.1008 3.4 0.85 4.6 0.0613 3.0 0.75 619 20 626 21 651 65 95 a17 7764 98 15 2.27 12579 0.1027 2.5 0.87 3.2 0.0612 2.0 0.78 630 15 633 15 645 44 98 3a44 54492 497 49 0.13 2195 0.1032 3.2 1.07 11.2 0.0751 10.8 0.28 633 19 738 61 1071 216 59 3a9 15090 230 27 0.80 24268 0.1054 3.3 0.90 3.6 0.0620 1.6 0.89 646 20 653 18 676 35 96 3a8 10187 159 18 0.76 1403 0.1059 3.3 0.91 5.2 0.0622 4.1 0.63 649 20 656 26 682 87 95 3a49 19477 174 20 0.42 4300 0.1126 3.2 0.98 4.1 0.0632 2.6 0.77 688 21 694 21 716 56 96 a39 40960 705 78 0.25 682 0.1130 2.9 0.99 4.6 0.0635 3.6 0.62 690 19 699 23 726 76 95 3a30 3049 43 5 0.88 4795 0.1131 3.3 0.99 4.9 0.0634 3.7 0.66 690 21 697 25 720 78 96 a26 18038 208 25 0.43 6593 0.1146 2.5 0.99 4.0 0.0629 3.2 0.62 699 17 700 21 704 67 99 a40 19301 72 22 0.56 14135 0.2923 2.8 4.12 3.3 0.1022 1.9 0.83 1653 40 1658 28 1665 35 99 3a37 108081 374 126 0.51 262 0.2962 3.9 4.26 5.1 0.1043 3.3 0.76 1672 57 1685 43 1701 61 98 a51 48044 61 21 1.04 2852 0.2995 4.8 4.41 5.1 0.1068 1.7 0.94 1689 71 1714 43 1745 32 97 a54 23121 35 12 0.61 8112 0.3000 2.7 4.63 3.6 0.1118 2.4 0.74 1691 40 1754 31 1829 44 92 a47 63671 133 45 0.63 7789 0.3115 2.8 4.66 2.9 0.1085 1.0 0.94 1748 42 1760 25 1774 19 99 a49 4480 4 2 1.19 3994 0.3126 4.4 4.65 5.9 0.1078 3.9 0.75 1754 68 1758 51 1763 72 99 3a22 17308 40 16 0.93 3032 0.3367 3.3 5.42 3.8 0.1168 1.8 0.88 1871 54 1889 33 1908 32 98 3a53 22641 21 8 0.68 2575 0.3407 3.3 5.66 3.9 0.1204 2.2 0.83 1890 54 1925 35 1962 39 96 a28 35315 79 32 0.76 27591 0.3423 2.8 5.52 3.5 0.1169 2.0 0.81 1898 47 1903 30 1910 36 99 2a10 110879 356 132 0.49 9463 0.3444 4.2 5.40 4.3 0.1137 1.1 0.97 1908 69 1885 38 1859 20 103 a41 64733 118 49 0.86 55442 0.3553 2.4 5.69 3.0 0.1161 1.9 0.79 1960 41 1930 27 1898 33 103 3a33 181042 301 124 0.22 1850 0.3726 3.6 6.57 3.8 0.1279 0.9 0.97 2042 64 2055 34 2069 16 99 a8 239357 674 269 0.37 4329 0.3867 2.5 7.57 2.6 0.1419 0.5 0.98 2108 46 2181 23 2250 8 94 3a21 55076 111 54 0.90 481 0.4029 3.4 7.92 5.8 0.1426 4.6 0.59 2183 64 2222 53 2259 80 97 a52 56533 43 21 0.73 38338 0.4278 2.6 8.64 2.9 0.1465 1.2 0.91 2296 50 2301 26 2306 21 100 a37 156116 157 89 0.48 87789 0.4967 2.7 12.09 3.0 0.1765 1.1 0.93 2600 59 2611 28 2621 18 99 2a5 105559 109 80 1.63 458 0.5779 4.2 16.19 4.5 0.2032 1.7 0.93 2940 99 2888 44 2852 28 103 a25 10011 2 3 0.23 24 0.5803 2.5 18.76 24.5 0.2345 24.3 0.10 2950 60 3030 268 3083 388 96

Fig. 3. U–Pb dates of detrital zircon grains from sample X1-1 (sandstone, Hasawnah Formation, Cambrian, unnamed wadi ‘‘X’’, AQA). (A) Concordia diagram. (B–C) Combined binned frequency and probability density distribution plots of detrital zircon grains. (B) 3500–400 Ma. (C) 1000–400 Ma. early Cambrian dates ranging from 542 ± 15 Ma to 520 ± 11 Ma age of sedimentation, a concordia date of 523 ± 7 Ma was calcu- (Table 2, Fig. 3). The probability plot shows distinct peaks at c. lated from the three youngest concordant zircons of sample X1-1 660, 625, 560 and 533 Ma (Fig. 3). To characterize the maximum (Fig. 4). 86 M.M. Altumi et al. / Journal of African Earth Sciences 79 (2013) 74–97

514 ± 12 Ma). The probability plot shows three major peaks, at c. 660, 610, and 570 Ma. Among the zircons of sample Ba2–14, grains are concordant (Table 7, Fig. 9) and the youngest grain yields a date of 549 ± 14 Ma. The oldest zircon is Archaean (2950 ± 60 Ma). Two additional Archaean zircons yield dates of 2621 ± 18 Ma and 2852 ± 28 Ma. A large proportion (33%) of Palaeoproterozoic grains are present in the sample; dates are in the range 2306 ± 21 Ma to 1665 ± 35 Ma. No Mesoproterozoic grains were found. Some 60% of all zircons in the sample are Neoproterozoic, in the range 699 ± 17 Ma to 549 ± 14 Ma. Cambrian zircons are not present among the analyzed grains. The probability plot shows distinct peaks at c. 680, 615, and 560 Ma.

5. Discussion

In the present study, U–Pb dates from detrital zircons provide

Fig. 4. U–Pb age (concordia age) of three youngest detrital zircon grains from much information concerning the geotectonic history of source sample X1-1 (sandstone, Hasawnah Formation, Cambrian, unnamed wadi ‘‘X’’, areas and timing of geotectonic events in the AQA region. From AQA). Cambrian in wadi ‘‘X’’ must be 523 ± 7 Ma or younger. 720 analyzed zircons, 329 grains were concordant and provide good constraints on Precambrian and Cambrian orogenic events in the source areas. Within the geological context of the AQA, silic- Among the zircons of sample Ba2-10 (Table 3), 54 grains went iclastic rocks of the Hasawnah Formation denote the basal portion concordant (Table 3, Fig. 5). The youngest concordant grain yields of an early Palaeozoic sequence overstepping the basement of the a date of 525 ± 6 Ma; the oldest zircon yields a date of Saharan Metacraton (Abdelsalam et al., 2002). In the central part of 3173 ± 28 Ma. Three grains are Archaean in age (3173 ± 28 Ma, the AQA, the basement of the metacraton crops out in a number of 2827 ± 14 Ma, 2605 ± 17 Ma). Some 13% of grains are Palaeoprote- localities as deeply weathered granitoids. Hitherto, radiometric rozoic, in the range 2406 ± 29 Ma to 1910 ± 19 Ma. Three Mesopro- dates were few, yet already pointed to Neoproterozoic to early terozoic zircons were detected in the sample (1018 ± 36 Ma, Cambrian intrusion ages: 640–549 Ma (Rb/Sr; Schürmann, 1974), 1003 ± 94 Ma, 1003 ± 18 Ma). Around 70% of all zircons in the sam- 541–491 Ma (K/Ar; Schürmann, 1974), 554–520 Ma (K/Ar; Jurák, ple are Neoproterozoic, in the range 888 ± 12 Ma to 547 ± 20 Ma 1978). (Table 3, Fig. 5). Five grains yield early Cambrian dates Because Cambrian strata of the Hasawnah Formation overlie the (542 ± 11 Ma to 525 ± 6 Ma). The probability plot is dominated deeply eroded granitoids nonconformably, strong uplift coupled by peaks at c. 670, 630, 525, and 560 Ma (Fig. 5). with deep erosion and weathering processes during latest Neopro- From the zircons of sample Ba1-3, 46 grains were concordant terozoic and early Cambrian time is indicated. High-maturity sand (Table 4, Fig. 6). The youngest and only Cambrian zircon grain deposits like the Hasawnah Formation imply intervals of intense yields a date of 533 ± 9 Ma. The oldest zircon is one of five Archae- chemical weathering and cleaning by streaming water at this time an grains with a date of 2904 ± 25 Ma. The other four Archaean (e.g., Linnemann et al., 2000, 2011; Dott, 2003; Avigad et al., 2005). dates of zircons from sample Ba1-3 are 3010 ± 6 Ma, This palaeogeographic area of Gondwana drifted into lower lati- 2683 ± 18 Ma, 2577 ± 18 Ma and 2547 ± 24 Ma. A remarkable pro- tudes during the lower and middle Cambrian (McKerrow et al., portion of 25% Palaeoproterozoic grains occurs in the sample. Such 1992; McKerrow and Cocks, 1995), where a warm to humid cli- dates range from 2476 ± 16 Ma to 1840 ± 33 Ma. No Mesoprotero- mate is suggested. Volcanism during late Neoproterozoic and early zoic zircons are present. Around 62% of all zircons in sample Ba1-3 Cambrian time created an unusually corrosive atmosphere with are Neoproterozoic, in the range from 985 ± 14 Ma to 552 ± 10 Ma very high atmospheric pCO2 that forced an extreme chemical (Fig. 6, Table 4). The probability plot shows distinct peaks at c. 660, weathering of the largely vegetation-free landscape under warm 590, 570, and 560 Ma (Fig. 6). to humid climatic conditions (Avigad et al., 2005). Intense erosion In sample A1-3, 61 grains are concordant (Table 5, Fig. 7) and and denudation of Pan-African orogens and cratonic basement led the youngest zircon grain yields a date of 551 ± 11 Ma; the old- to prevalent peneplanation. Cambrian–Ordovician rifting and est zircon yields a date of 3316 ± 11 Ma. Two more Archaean widespread thermal subsidence at the margins of Gondwana cre- grains are also present (2636 ± 15 Ma, 2567 ± 22 Ma). A greater ated shallow marine shelves where masses of the high-maturity proportion of Palaeoproterozoic zircons (27%) is in the range sands accumulated. The assumed aggressive weathering might 2358 ± 28 Ma to 1641 ± 29 Ma. Around 67% of all zircons in this also have had a great impact on the radiation of organisms, be- sample are Neoproterozoic, in the range 961 ± 16 Ma to cause abundant nutrients would have been transported into the 551 ± 11 Ma. No Cambrian zircons are present. The probability oceans by giant drainage systems in late Neoproterozoic–Cambrian plot shows distinct peaks at c. 650, 630, 615, 590, and 560 Ma time (Brasier, 1992; Tucker, 1992; Campbell et al., 2008; Squire (Fig. 7). et al., 2006). From sample DA2-3, 63 grains are classified as concordant Due to the absence of body fossils, a biostratigraphic age is not (Table 6, Fig. 8). The youngest grain yields a date of 514 ± 12 Ma. available for the Hasawnah Formation. However, our zircon data Only two zircons yield an Archaean age (3036 ± 20 Ma, support the onset of Cambrian sedimentation already in the Terre- 2693 ± 13 Ma). The majority of all zircon dates (44%) fall within neuvian (earliest Cambrian). Stratigraphically, sample X1-1 is our the Palaeoproterozoic, in the range 2500 ± 20 Ma to 1610 ± oldest sample from the five sections (Fig. 2). The youngest popula- 22 Ma. No Mesoproterozoic grains were detected. The second- tion of detrital zircons from that sample indicates a maximum age largest population is represented by Neoproterozoic zircons of sedimentation for Cambrian strata in the AQA area. A concordia (42%), in the range 832 ± 22 Ma to 550 ± 15 Ma. Four Cambrian zir- age of 523 ± 7 Ma from the three youngest zircon grains was calcu- cons are present (538 ± 13 Ma, 534 ± 14 Ma, 521 ± 13 Ma, lated (Fig. 4). Despite the large error, that age is good enough to M.M. Altumi et al. / Journal of African Earth Sciences 79 (2013) 74–97 87

Fig. 5. U–Pb dates of detrital zircon grains from sample Ba2-10 (sandstone, Hasawnah Formation, Cambrian, section Ba2 = Wadi Badran-2, AQA). (A) Concordia diagram. (B–C) Combined binned frequency and probability density distribution plots of detrital zircon grains. (B) 3500–400 Ma. (C) 1100–400 Ma. pinpoint the onset of Cambrian deposition in the AQA within the deposition of sediments in the AQA. In the AQA the formation of Terreneuvian (542–521 Ma) or slightly younger. Thus, during the the Gondwanan peneplain (Avigad et al., 2005) and early ﬁrst 10–12 Ma of the Cambrian period there is no evidence for Palaeozoic subsidence commenced in the early Cambrian (present 88 M.M. Altumi et al. / Journal of African Earth Sciences 79 (2013) 74–97

Fig. 6. U–Pb dates of detrital zircon grains from sample B1-3 (sandstone, Hasawnah Formation, Cambrian, section Ba1 = Wadi Badran-1, AQA). (A) Concordia diagram. (B–C) Combined binned frequency and probability density distribution plots of detrital zircon grains. (B) 3000–500 Ma. (C) 1100–500 Ma. study). On the contrary, related subsidence processes in the Tassilis means that the centre of early Palaeozoic thermal subsidence in area south of the Hoggar (Algerian Sahara) did not begin before the central-northern Africa must have been situated farther east, in latest Cambrian to early Ordovician (Linnemann et al., 2011). This the area of the Saharan Metacraton. M.M. Altumi et al. / Journal of African Earth Sciences 79 (2013) 74–97 89

Fig. 7. U–Pb dates of detrital zircon grains from sample A1-3 (sandstone, Hasawnah Formstion, Cambrian, section A1 = Wadi Al Abd-1, AQA). (A) Concordia diagram. (B–C) Combined binned frequency and probability density distribution plots of detrital zircon grains. (B) 3500–400 Ma. (C) 1000–400 Ma.

The overwhelming majority (56.53%) of detrital zircon U–Pb 700–680, 670–650, 615–610, 590, 570 and 560 Ma. For northwest- dates from all six investigated samples fall within the Neoprotero- ern Gondwana such dates are indicative of Pan-African and Cado- zoic (Fig. 10). Peaks in the probability plots (Figs. 3–9) cluster at c. mian orogenic events (see discussion in Linnemann et al., 2011 90 M.M. Altumi et al. / Journal of African Earth Sciences 79 (2013) 74–97

Fig. 8. U–Pb dates of detrital zircon grains from sample DA2-3 (sandstone, Hasawnah Formation, Cambrian, section Da2 = Wadi Damran-2, AQA). (A) Concordia diagram. (B– C) Combined binned frequency and probability density distribution plots of detrital zircon grains. (B) 3500–400 Ma. (C) 900–400 Ma. and references therein). A few Neoproterozoic dates scatter around et al., 2011). The clusters of Neoprotereozoic zircon populations are c. 950–750 Ma, which might be related to rifting and drifting dur- largely identical with those from the Cambro-Ordovician sand- ing dispersal of the Rodinia palaeosupercontinent (e.g. Linnemann stones of the Tassilis area of the Algerian Sahara (see Figs. 11 and M.M. Altumi et al. / Journal of African Earth Sciences 79 (2013) 74–97 91

Fig. 9. U–Pb dates of detrital zircon grains from sample Ba2-14 (sandstone, Hasawnah Formation, Cambrian, section Ba2 = Wadi Badran-2, AQA). (A) Concordia diagram. (B– C) Combined binned frequency and probability density distribution plots of detrital zircon grains. (B) 3200–400 Ma. (C) 1000–400 Ma.

12, and Linnemann et al., 2011). The only difference to the Tassilis Dates are somewhat scattered, with clusters at 1039 ± 11, area is in the occurrence of three zircons within the AQA formed 1006 ± 12 and 993 ± 13 Ma (Fig. 10, Tables 2–7). Only two further around the Mesoproterozoic–Neoproterozoic boundary at c. 1 Ga. Mesoproterozoic zircons, dated at 1592 ± 39 Ma and 92 M.M. Altumi et al. / Journal of African Earth Sciences 79 (2013) 74–97

zircons from the Hasawnah Formation sandstones yielded Meso- proterozoic dates, among which the majority clusters around 1 Ga. It seems that the eastern Murzuq Basin area investigated by Meinhold et al. (2011) was located closer to a source area that was largely composed of Mesoproterozoic crust. Because the proportion of Mesoproterozoic zircon grains is greater in the sample of these authors compared to that of the AQA (a phenomenon also observed in Ordovician to Carboniferous sedimentary rocks investigated in the same study), such a geological unit must have been located much farther to the east. A potential source area could be concealed within the Arabian–Nubian Shield (e.g. Kolodner et al., 2006) or situated in Chad, or much more distant, at the southeastern margins of the Congo and Tanzania cratons (Le Heron et al., 2009; Meinhold et al., 2011). Conversely, there is no evidence for the existence of massive Mesoproterozoic crust in the Saharan Metacraton area (see also discussion in Linnemann et al., 2011). The second-largest fraction of zircon dates in the present sample set is represented by Palaeoproterozoic zircon populations, derived from one or more cratonic basement units generated by magmatic events at c. 2.4–2.3 Ga and 2.2–1.6 Ga (Figs. 10 and 11). In addition, there are a number of Archaean zircons which scatter around c. 3.4–3.25 Ga, 2.97–2.95 Ga and 2.6–2.5 Ga (Figs. 10 and 11). Most of these seem to be derived from the West African Craton or a related cratonic source. The West African Craton was stabilized by orogenic events in the Archaean (Leonian, Liberian) and the Palaeoproterozoic (Eburnean). There, phases of major crustal growth occurred between 3.6 and 3.05 Ga (Kröner et al., 2001; Potrel et al., 1996, 1998), 3.0–2.7 Ga (Guerrot et al., 1989; Key et al., 2008) and 2.4–1.9 Ga (Abouchami et al., 1990; Boher et al., 1992; Rocci et al., 1991; Zhao et al., 2002a,b; Linnemann et al., 2011). Therefore, West Africa is clearly a potential source for most of the Palaeoproterozoic and Archaean zircons that cluster around 3.4–1.9 Ga. Palaeoproterozoic zircon populations in the range c. 1.85– 1.6 Ga seem to be representative for the western part of the Saha- ran Metacraton. Such dates are also reported by Abdelsalam et al. (2002) and Meinhold et al. (2011). Among the zircons in the present samples, grains of this age dominate among the Palaoprotero- zoic zircon populations. Palaeoproterozoic zircons derived from the West African Craton are less common (Fig. 11). However, in zircon populations from Cambrian–Ordovician sandstone in the Tass- ilis area south of the Hoggar (Algerian Sahara), the opposite is the case (Linnemann et al., 2011; see Fig. 11). In this more westerly area, zircons yielding dates of c. 2.2–1.9 Ga dominate the Palaeo- proterozoic zircon populations. Such zircons seem to be a good indicator for the proximity of the West African source. Southeast of the AQA, in the Murzuq Basin, the zircon population in the range 1.85–1.6 Ga dominates slightly among Palaeoproterozoic grains (Meinhold et al., 2011). Much further to the east and southeast, in the eastern Al Kufrah Basin, no zircon population of this age range has been found (Le Heron et al., 2009). Hence, in the Cam- Fig. 10. Tectonomagmatic and orogenic events combined with binned frequency brian of the AQA and eastern Murzuq Basin, zircon populations plots of all 329 U–Pb dates of analyzed zircon grains showing degree of concordance in range 90–110%. of such an age range were most probably derived from a local source. Thus, these zircon dates seem to be a good tracer of a source in the western part of the Saharan Metacraton and indicate 1475 ± 20 Ma (Figs. 10 and 11, Tables 2–7), were found. Taken as a a phase of major crustal growth in that area. For the Al Kufrah Ba- whole, among all the 329 zircon dates from the six investigated sin it is important to note, that the study of Le Heron et al. (2009) samples, only four are clearly Mesoproterozoic and the single zir- includes only zircons of one single sample from a Neoproterozoic con with a date of 993 ± 13 Ma overlaps the Mesoproterozoic–Neo- sandstone. No information about zircon ages from Palaeozoic sili- proterozoic boundary only in the error ellipse. Hence, in the ciclastics is available. Therefore our conclusions from the AQA Cambrian, a potential source area of Mesoproterozoic zircon grains and the eastern Murzuq Basin concerning Palaeoproterozoic zircon seems to have been situated far distant from the AQA. A similar populations should be used with care in view of the Al Kufrah occurrence pattern of Mesoproterozoic zircons was recently re- Basin. ported by Meinhold et al. (2011) from Cambrian strata of the east- In summary, it can be stated that the dominant source area for ern ﬂank of the Murzuq Basin, about 350 km to the southeast. In Cambrian sandstones of the Hasawnah Formation in the AQA was that study the authors report that about 20 of 202 measured Neoproterozoic crust of Pan-African basement. A good candidate is M.M. Altumi et al. / Journal of African Earth Sciences 79 (2013) 74–97 93

Fig. 11. Binned frequency plots of 329 U–Pb dates of analyzed zircon grains from Cambrian of AQA (present study) in comparison to 630 U–Pb dates from Cambro-Ordovician of Tassilis area south of Hoggar in Algerian Sahara (data from Linnemann et al., 2011).

the Pan-African orogen in the Trans-Saharan Belt (Pharussian and A similar data set like for the AQA (presented in this paper) is Dahomeyean belts, Fig. 13). It is relatively close to the AQA, and reported from the Cambrian–Ordovician of the Tassilis (Hoggar) the cluster of zircon populations is very similar to the ones of the to the west of the AQA (Linnemann et al., 2011; Fig. 11), as well Hasawnah Formation (Linnemann et al., 2011). Further, only few as from southeastern equivalents in the eastern Murzuq Basin area Mesoproterozoic zircon ages are known from there (Drost et al., (Meinhold et al., 2011). However, no Neoproterozoic zircons 2010). Other Pan-African belts to the east and south (East African related to Pan-African basement are reported from the Al Kufrah and Oubangouide belts, Fig. 13) and the Arabian–Nubian Shield Basin farther to the southeast (Le Heron et al., 2009). The latter, contain a signiﬁcant Mesoproterozoic zircon population (Avigad however, is to handle with care, because the zircon data came form et al., 2003). Therefore, we prefer an interpretation with dominant only one Neoproterozoic sample. Nevertheless, out data set sediment source in the Trans-Saharan belt. Another partial source strongly supports the interpretation, that the prevailing sediment area for the Cambrian sandstones of the Hasawnah Formation source area for the Hasawnah Formation sandstones must have could be represented by the Cadomian orogen of peri-Gondwana, been situated to the west. The source rock area must have been which was formed at the margin of northwest Gondwana during composed by Pan-African rocks and belong most probably to the Late Neoproterozoic to Early Cambrian times (Linnemann et al., Trans-Saharan Belt (Fig. 13). We cannot exclude an input from 2007) and which show zircon populations similar to the Pan-Afri- peri-Gondwanan terranes (Cadomian orogenic belt, see above). can one of the Trans-Saharan Belt (Linnemann et al., 2011). The few Mesoproterozoic zircons in our investigated samples indi- 94 M.M. Altumi et al. / Journal of African Earth Sciences 79 (2013) 74–97

Fig. 12. Detrital zircon date distributions of the six analyzed Cambrian sandstone samples of AQA (present study) in comparison to Baltica, Amazonia, West Africa and other adjacent parts of Gondwana (data compilation from Drost et al., 2010; Linnemann et al., 2004, 2011 and references therein).

cate only modest input from a distant source to the east-southeast. et al., 2011), was closer to a Mesoproterozoic source area, which The majority of Palaeoproterozoic zircons in the range 1.85–1.6 Ga must have been located much farther to the east and could be con- most probably came from a local source in the Saharan Metacraton. cealed within the Arabian–Nubian Shield or situated in Chad, or A smaller proportion of Palaeoproterozoic zircon grains (1.9– the Congo and Tanzania cratons. There is no evidence of the exis- 2.2 Ga) and most of the Archaean zircons seem to be derived from tence of massive Mesoproterozoic crust in the Saharan Metacraton the West African Craton or alternatively, from a portion of the area. Pan-African basement which has recycled older crust of the West The second-largest fraction of investigated zircons is repre- African Craton. sented by Palaeoproterozoic populations with ages of c. 2.4– 2.3 Ga and 2.2–1.6 Ga. There are also a number of Archaean zircons (c. 3.4–3.25 Ga, 2.95–2.7 Ga, 2.6–2.5 Ga) seemingly derived from 6. Conclusions the West African Craton or a related cratonic source. West Africa is the potential source for most of the Palaeoproterozoic and Ar- A total of 720 detrital zircons from the Cambrian Hasawnah For- chean zircons with ages of 3.4–1.9 Ga, whereas Palaeoproterozoic mation of the Al Qarqaf Arch (AQA) area (central-western Libya) zircons of c. 1.85–1.6 Ga seem to be representative of the western has been analyzed. Among these, 329 grains were concordant in part of the Saharan Metacraton. These zircon populations from the the range of 90–110%. Some 60% of the determined U–Pb dates Cambrian of the AQA and eastern Murzuq Basin most probably de- of all samples are Neoproterozoic. Peaks cluster at c. 700–680, rived from a local source. Thus, these zircon ages c. 1.85–1.6 Ga) 670–650, 615–610, 590, 570–560 Ma, and c. 540–525 Ma. Such indicate a phase of signiﬁcant crustal growth in the western part dates are characteristic of Pan-African and Cadomian orogenic of the Saharan Metacraton and seems to be represent a very good events in the northwestern Gondwana palaeogeographic region. tracer of this source area. A few zircons yielded older Neoproterozoic ages around 950– The unconformity between basement and Hasawnah Formation 750 Ma and point to rifting and drifting processes related to Rodi- sandstone indicates strong uplift and deep erosion during the lat- nia dispersal. est Neoproterozoic and early Cambrian (c. 550–530 Ma) in our Only three zircons became formed around the Mesoproterozo- investigated area. The high maturity of the sandstones implies ic–Neoproterozoic boundary, yielding U–Pb dates of intervals of coeval intense chemical weathering under warm to hu- 1039 ± 11 Ma, 1006 ± 12 Ma and 993 ± 13 Ma. Two further Meso- mid climatic conditions. These processes resulted in denudation proterozoic zircons show ages of 1592 ± 39 Ma and 1475 ± 20 Ma. and peneplain formation before the deposition of the Hasawnah Their potential source area seems to have been situated far distant Formation. from the AQA. The eastern Murzuq Basin region, from where some Because related subsidence in the Tassilis (Algerian Sahara) did geochronological data have already been published (Meinhold not begin before the latest Cambrian to early Ordovician (Linne- M.M. Altumi et al. / Journal of African Earth Sciences 79 (2013) 74–97 95

Fig. 13. Sources of Cambrian sandstones of AQA (black arrows) in framework of orogenic provinces of Africa. Constituent continental terranes shown in Gondwana conﬁguration (compiled by U. Linnemann from maps of Bahlburg et al., 2009; Begg et al., 2009; Linnemann et al., 2007, 2011 and references therein). (For colour version of this ﬁgure, the reader is referred to the web version of this article.)

mann et al., 2011), the centre of early Palaeozoic thermal subsi- of the East Saharan Craton (Libya): stratigraphy, facies, and pro- dence in central-northern Africa must have been located farther cess-correlation along the W-Gondwanan shelf’’. east (Saharan Metacraton). The main source area of the Hasawnah Formation Cambrian sandstones of the AQA was Neoproterozoic crust of the Pan-African References basement of the Trans-Saharan Belt, in accordance with data from the area of the Tassilis in the Algerian Sahara (Hoggar, west of the Abdelsalam, M.G., Liegeois, J.-P., Stern, R.J., 2002. The Saharan Metacraton. Journal of AQA; Linnemann et al., 2011), the eastern Murzuq Basin (Meinhold African Earth Sciences 34, 119–136. Abouchami, W., Boher, M., Michard, A., Albaréde, F., 1990. A major 2.1 Ga event of et al., 2011) and the much more southeasterly located Al Kufrah mafic magmatism in West Africa: an early stage of crustal accretion. Journal of Basin (Le Heron et al., 2009). A very limited input is indicated from Geophysical Research 95 (B11), 17605–17629. the far distant east-southeast (Mesoproterozoic dates), as well as Al Fasatwi, Y.A., Van Djik, P.M., Erren, J.W.M.G., 2003. Surface and subsurface characteristics of Al Qarqaf Arch and adjacent parts of Ghadamis and Murzuq some influence from a local source (Saharan Metacraton; Palaeo- Basins, West Libya: an integration of remote sensing, aeromagnetic and seismic proterozoic zircons). Some relationship is suggested with the West interpretation. In: Salem, M.J., Oun, K.M., Seddiq, H.M. (Eds.), The Geology of African Craton by some Palaeoproterozoic and Archaean zircon Northwest Libya, vol. 3. Earth Science Society of Libya, pp. 171–190. dates, although, e.g. a polyphase recycling of zircon grains from an- Al Festawi, Y.A.M., 2001. The Structural, Paleogeographical and Hydrocarbon System Analysis of the Ghadamis and Murzuq Basins, West Libya, with other area could be theoretically possible. Emphesis on their Relation to the Intervening Al Qarqaf Arch. Unpubl. Phd Thesis. Delft Technical University, 228 pp. Avigad, D., Kolodner, K., McWiliams, M., Persing, H., Weissbrod, T., 2003. Origin of northern Gondwana Cambrian sandstone revealed by detrital zircon SHRIMP Acknowledgements dating. Geology 31, 227–230. Avigad, D., Sandler, A., Kolodner, K., Stern, R.J., McWilliams, M., Miller, N., Beyth, M., The authors greatly acknowledge the Deutsche Forschungs- 2005. Mass-production of Cambro–Ordovician quartz-rich sandstone as a gemeinschaft (DFG) and the Libyan Petroleum Institute (Tripoli, Li- consequence of chemical weathering of Pan-African terranes: environmental implications. Earth and Planetary Science Letters 240, 818–826. bya) for support of fieldwork in Libya. Many thanks go to Adel M. Bahlburg, H., Vervoort, J.D., Du Frane, S.A., Bock, B., Augustsson, C., Reimann, C., Abdada (Tripoli, Libya) for important help during fieldwork and 2009. Timing of crust formation and recycling in accretionary orogens. Insights for logistic support, and to Dr. Michael Magnus (Freiberg Univer- learned from the western margin of South America. Earth-Science Reviews 97, 227–253. sity, Germany) for discussion on heavy-mineral microscopy and Begg, G.C., Griffin, W.L., Natapov, L.M., O´ Neill, C.J., Hronsky, J.M.A., Poudjom preparation. The authors are grateful to Dr. Peter Kruse (Adelaide, Djomani, Y., Swain, C.J., Deen, T., Bowden, P., 2009. The lithospheric architecture Australia) for helpful remarks and linguistic assistance. The work of Africa: seismic tomography, mantle petrology, and tectonic evolution. Geosphere 5, 23–50. reported here is embedded in the DFG research project of O.E. Bellini, E., Massa, D., 1980. A stratigraphic contribution to the Palaeozoic of the no. EL 144/19: ‘‘The start of the Phanerozoic at the northern edge Southern Basins of Libya. In: Salem, M.J., Busrewil, M.T. (Eds.), The Geology of 96 M.M. Altumi et al. / Journal of African Earth Sciences 79 (2013) 74–97

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