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Journal of African Earth Sciences 61 (2011) 308–330

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Journal of African Earth Sciences

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A heavy mineral study of sandstones from the eastern Murzuq Basin, Libya:Constraints on provenance and stratigraphic correlation

Andrew C. Morton a,b, Guido Meinhold a,c,⇑, James P. Howard a, Richard J. Phillips a,d, Dominic Strogen a,e,Yousef Abutarruma f, Mohamed Elgadry g, Bindra Thusu h, Andrew G. Whitham a

a CASP, University of Cambridge, West Building, 181A Huntingdon Road, Cambridge CB3 0DH, United Kingdomb HM Research Associates, 2 Clive Road, Balsall Common CV7 7DW, United Kingdomc Department of Sedimentology & Environmental Geology, Geoscience Centre, University of Göttingen, Goldschmidtstrabe 3, 37077 Göttingen, Germanyd School of Earth and Environment, University of Leeds, Leeds LS2 9JT, United Kingdome GNS Science, P.O. Box 30368, Lower Hutt 5010, New Zealandf Earth Science Society of Libya, Tripoli, Libyag Libyan Petroleum Institute, Gergarish Road, P.O. Box 6431, Tripoli, Libyah Maghreb Petroleum Research Group, Department of Earth Sciences, University College London, Gower Street, London WC1E 6BT, United Kingdom

a r t i c l e i n f o a b s t r a c t

Article history:Received 19 May 2011Received in revised form 23 August 2011Accepted 25 August 2011Available online 16 September 2011

Keywords:Sediment provenanceHeavy mineral analysisNorth GondwanaMurzuq BasinLibya

1464-343X/$ - see front matter � 2011 Elsevier Ltd.doi:10.1016/j.jafrearsci.2011.08.005

⇑ Corresponding author at: Department of SedimGeology, Geoscience Centre, University of Göttingen,Göttingen, Germany. Tel.: +49 551 393455; fax: +49

E-mail address: [email protected]

This paper presents the results of an integrated heavy mineral and mineral chemical study of Precam-brian–Mesozoic clastic sediments from the eastern Murzuq Basin. The purpose of this study was to con-strain the provenance of sediment and to assess the value of heavy minerals as a stratigraphic tool.Conventional heavy mineral analysis was carried out on 64 samples, tourmaline geochemical analysison 25 samples, garnet geochemical analysis on four samples, rutile geochemical analysis on 21 samples,and clinopyroxene geochemical analysis on two samples.

The study indicates that heavy mineral analysis is a valuable tool for understanding the provenance ofPalaeozoic and Mesozoic clastic sediments in Libya, despite the intense weathering that surface sampleshave undergone. Based on heavy mineral ratios and mineral chemical data, there appear to be three keyevents when the provenance signature changed within the Palaeozoic–Mesozoic sedimentary successionat the eastern Murzuq Basin: (i) at the base of the Tanezzuft Formation (early Silurian), (ii) at the base ofthe Tadrart Formation (Early Devonian), and (iii) at the base of the Mrar Formation (Carboniferous), sub-dividing the succession into four intervals (Hasawnah–Mamuniyat, Tanezzuft–Akakus, Tadrart–AwaynatWanin, and Mrar–Nubian). There is probably also an event between the Precambrian and the HasawnahFormation (Cambrian), but there are currently insufficient data to prove this unequivocally.

Comparing data of the present study with results from the previous work in the Kufra Basin it is evidentthat heavy mineral data provide useful evidence for differences in provenance both regionally and strati-graphically in basins of the central Sahara.

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1. Introduction

The Murzuq Basin of southern Libya (Fig. 1) is an importantpetroleum province with proven major petroleum reservoirs inCambrian, Ordovician and Devonian sandstones (e.g., Boote et al.,1998; Davidson et al., 2000; Craig et al., 2008). The Palaeozoic sed-imentary succession of the Murzuq Basin contains substantialthicknesses of clastic, often poorly dated marine and non-marinesediments (the exception is the Carboniferous Mrar Formation).In the absence of biostratigraphic data, the correlation of forma-

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entology & EnvironmentalGoldschmidtstrabe 3, 37077

551 397996.n.de (G. Meinhold).

tions relies solely on lithostratigraphic characteristics. In marineunits, macrofaunas are sparse and dating relies on palynomorphsand other microfossils. Although biostratigraphic studies are possi-ble on fossil material recovered from boreholes, extreme surfaceweathering ensures that such studies are impossible on materialfrom outcrop. The lack of biostratigraphic data means that othertools have to be sought to test the correlation of units within andbetween basins.

Heavy mineral studies have been used with considerable suc-cess in other basins where conventional biostratigraphic correla-tion of units is problematic. Examples include the Devonian–Carboniferous succession in the Clair Field, west of Shetland, UK(Allen and Mange-Rajetzky, 1992; Morton et al., 2010), and Perm-ian–Triassic successions on the UK continental shelf (Mange et al.,1999; Morton et al., 2007). Here, subdivision and correlation of

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Fig. 1. Map of Libya showing surface outcrops with Palaeozoic rocks (dark greycolour) and the location of Dor el Gussa at the eastern margin of the Murzuq Basin.The sampling localities of samples collected north of Dor el Gussa are indicated.Basin outlines are after Boote et al. (1998). Abbreviations: A – Algeria, C – Chad, E –Egypt, N – Niger, S – Sudan, T – Tunisia. Basin outlines are shown as dashed lines.

A.C. Morton et al. / Journal of African Earth Sciences 61 (2011) 308–330 309

clastic successions are based on changes in heavy mineral charac-teristics related to changes in provenance, climate or sedimenttransport history, and are recognised using a range of prove-nance-sensitive parameters, including ratios of the abundance ofstable, hydraulically-equivalent minerals, mineral chemistry, grainmorphology and mineral geochronology (Morton et al., 2002). Hea-vy mineral studies also provide crucial information on sedimentprovenance, which is a vital component in palaeotectonic recon-structions (e.g., Meinhold et al., 2010, 2011), and on the diageneticevolution of sandstones (e.g., Morton et al., 1989; Turner and Mor-ton, 2007).

This paper presents the results of a heavy mineral study of Pre-cambrian–Mesozoic sandstones collected during the course ofCASP fieldwork in Dor el Gussa on the eastern margin of the Mur-zuq Basin (Figs. 1 and 2). The main aims of the study were (i) toprovide constraints on the provenance of the sandstones, (ii) toinvestigate the potential value of heavy mineral analysis as a strati-graphic tool, and (iii) to compare the provenance of sandstones inDor el Gussa with equivalent sandstones in the Kufra Basin, re-ported by Morton et al. (in press).

The data set of Dor el Gussa comprises results from 64 sand-stone samples. Most of them are quartz arenites; the Carboniferoussamples are arkosic arenites. Stratigraphic information is given inTable 1. Conventional (petrographic) heavy mineral data were col-lected from all these samples, with rutile, garnet, tourmaline andclinopyroxene geochemical data on a subset, as given in the Sup-plementary data (see Appendix A), with tourmaline geochemicalanalysis on 25 samples, garnet geochemical analysis on four sam-ples, rutile geochemical analysis on 21 samples, and clinopyroxenegeochemical analysis on two samples.

2. Regional setting

The intracratonic Murzuq Basin is an erosional remnant of alarge Palaeozoic and Mesozoic sedimentary basin that originally

extended over much of North Africa (Boote et al., 1998; Davidsonet al., 2000). It covers a vast area (�350,000 km2) and is locatedin the southwest of Libya and continues into northwestern Chad,northern Niger and eastern Algeria (Fig. 1). Its present-day sedi-mentary fill reaches a maximum thickness of about 4000 m inthe basin centre (Davidson et al., 2000). The present-day basinmargins are formed by the basement highs of the Tibesti to the east(with Dor el Gussa being the continuation toward the NW), Tihe-mboka to the west, and Gargaf to the north. The centre of the Mur-zuq Basin has been relatively well investigated by drilling andseismic profiles (e.g., Craig et al., 2008). The basin margins, how-ever, are still lacking detailed geological investigation. The base-ment high of Dor el Gussa on the eastern margin of the MurzuqBasin is dominated by a clastic sedimentary sequence of Cambrianto Carboniferous age (Klitzsch, 1964; Klitzsch and Ziegert, 2000).Precambrian basement rocks can be found to the north and south.

Sandstones samples from across Dor el Gussa were collected fora heavy mineral provenance study from most parts of the stratig-raphy, with the majority coming from the Cambrian to Carbonifer-ous interval. The stratigraphy and sedimentology of theformations/units investigated is briefly given below. A strati-graphic column of the Murzuq Basin is shown in Fig. 3. Further de-tails can be found in Klitzsch (1964), Klitzsch and Ziegert (2000)and Hallett (2002).

2.1. Basement metasediments (Precambrian)

The oldest sedimentary rocks investigated are low-grade meta-morphic rocks of dark greyish quartzite described by Klitzsch(1964), which may be part of the basement rather than the Infra-cambrian succession. The quartzites crop out in the vicinity ofbasement granites north of Dor el Gussa. Equivalent rocks to thesegranites were penetrated in two wells, Sabha-2 (in Sabha) and WA-T2 (100 km NE of Sabha) (Baumann et al., 1992). Rb–Sr mineral–whole-rock isochron age dating yielded 554 Ma and 551 Ma,respectively, interpreted by Baumann et al. (1992) as intrusionages. The low-grade metamorphic rocks are older than the gran-ites, and thus they are probably older than 550 Ma. Two quartziteswere selected for heavy mineral analysis.

2.2. Hasawnah Formation (Cambrian)

Within Dor el Gussa, the Hasawnah Formation is exposed in thenorth and along its eastern flank where it unconformably overliesbasement rocks (Klitzsch, 1964; Klitzsch and Ziegert, 2000). Theformation consists of thick-bedded and massive sandstones, med-ium- to very coarse-grained, often cross-laminated, feldspar-bear-ing, and pale to brownish in colour (Klitzsch, 1964). The basal�600 m of the succession is conglomeratic. At the northern marginof Dor el Gussa, the Hasawnah Formation dips 20–45� to the SWand is transgressively overlain by Ordovician sediments, whichdip 5� to the SW (Klitzsch, 1964). The thickness of the HasawnahFormation varies from 600–1700 m in the northern part of Dor elGussa (Klitzsch, 1964; Hallett, 2002) to 300–400 m to the southof Dor el Gussa, in the Mourizidie area (Hallett, 2002). The Hasaw-nah Formation lithofacies has been interpreted as reflecting depo-sition in a sand-dominated braided fluvial to shallow marineenvironment (Cepak, 1980). Based on field relationships with theunderlying Precambrian rocks, a Cambrian age is generally sug-gested for the Hasawnah Formation. Six samples were selectedfrom the Hasawnah Formation for heavy mineral analysis.

2.3. Hawaz Formation (Middle Ordovician)

The Hawaz Formation consists of very fine- to medium-grainedsandstone, thin- to thick-bedded and well sorted (Aziz, 2000;

Fig. 2. Simplified geological map of the Dor el Gussa region (modified from Collomb, 1962) showing the sampling localities.

310 A.C. Morton et al. / Journal of African Earth Sciences 61 (2011) 308–330

Klitzsch and Ziegert, 2000; Echikh and Sola, 2000). Ramos et al.(2003, 2006) also described minor intercalations of silty shaleand several K-bentonite beds from the western part of Gargaf.Ramos et al. (2006) suggested that the lower part of the HawazFormation was deposited in a nearshore to inner-platform setting,the middle part in a shoreface storm-reworked to beach settingwith a maximum of volcanic ash contribution, and the upper partmainly in a subtidal setting. The Hawaz Formation conformablyoverlies the Ash Shabiyat Formation (Hallett, 2002). However,where the Ash Shabiyat Formation (see Hallett, 2002 for descrip-tion) has not yet been differentiated as a distinct unit, e.g., on the

eastern flank of the Murzuq Basin (Hallett, 2002), the Hawaz For-mation unconformably overlies the Hasawnah Formation. The Me-lez Shuqran Formation, Mamuniyat Formation or the TanezzuftFormation unconformably overlies the Hawaz Formation (e.g.,Hallett, 2002; Ramos et al., 2006). The thickness of the Hawaz For-mation varies considerably from <1 to 150 m. In Dor el Gussa, it isless than 50 m thick. The Hawaz Formation is Middle Ordovician inage, based on acritarch assemblages (Aziz, 2000). Some authorsgive a more precise age of Darriwilian (Llanvirnian–Llandeilian),also based on acritarchs (Ramos et al., 2006). Eight samples wereselected from the Hawaz Formation for heavy mineral analysis.

Table 1Details of sandstone samples analysed during this study. Samples are sorted by stratigraphic age youngest to oldest. DEG – Dor el Gussa.

Sample name Stratigraphy Age Area Latitude Longitude

H6039 Nubian Sandstone Triassic–early Cretaceous Southern DEG 25�11050.600 15�4702.2200


H6062 Nubian Sandstone Triassic–early Cretaceous Central DEG 25�54057.3700 16�13029.8200


H6037 Mrar Formation Carboniferous Northern DEG 25�32035.9900 16�21046.1100

H6041 Mrar Formation Carboniferous Southern DEG 25�3047.4400 15�4909.6900

H6042 Mrar Formation Carboniferous Southern DEG 25�7025.7800 15�57037.9700

H6061 Mrar Formation Carboniferous Central DEG 25�43051.500 16�23023.0900

H6034 Awaynat Wanin Group Middle–Late Devonian Central DEG 25�38014.3900 16�27043.7500



S5334 Ouan Kasa Formation Middle–Late Devonian Southern DEG 24�41023.7500 16�703.3700

S5346 Ouan Kasa Formation Middle–Late Devonian Southern DEG 24�55033.4100 16�15034.1800

S5347 Ouan Kasa Formation Middle–Late Devonian Southern DEG 24�5503600 16�15031.8700

S5348 Ouan Kasa Formation Middle–Late Devonian Southern DEG 24�5503600 16�15031.8700

S5415 Tadrart Formation Middle–Late Devonian Northern DEG 25�54057.5400 16�45017.1400

S5434 Tadrart Formation Middle–Late Devonian Northern DEG 25�58011.8500 16�45022.0600

P5878 Tadrart Formation Middle–Late Devonian Northern DEG 25�57050.1100 16�46025.6800

P5881 Tadrart Formation Middle–Late Devonian Northern DEG 25�55025.4300 16�43052.7500

P5893 Tadrart Formation Middle–Late Devonian Southern DEG 24�38024.9600 16�5030.5400

P5895 Tadrart Formation Middle–Late Devonian Southern DEG 24�39024.100 16�5039.6100

H6032 Tadrart Formation Middle–Late Devonian Central DEG 25�36020.3500 16�30040.6700









H6059 Tadrart Formation Middle–Late Devonian Central DEG 25�41027.4100 16�3700 8.400

S5316 Akakus Formation early–late Silurian Northern DEG 25�55015.4200 16�44046.0800

S5317 Akakus Formation early–late Silurian Northern DEG 25�55012.5600 16�43025.5800

SCDEG-2a Tanezzuft Formation early Silurian Central DEG 25�36019.2700 16�30047.7500

S5289 Tanezzuft Formation early Silurian Northern DEG 25�5500.0900 16�45032.2900

S5292 Tanezzuft Formation early Silurian Northern DEG 25�5500.0900 16�45032.2900

H6053 Tanezzuft Formation early Silurian Central DEG 25�4000.2900 16�35057.9400

H6058 Tanezzuft Formation early Silurian Central DEG 25�41027.4100 16�3708.400

S5296 Mamuniyat Formation Late Ordovician Northern DEG 25�56010.9800 16�46046.600


S5339 Mamuniyat Formation Late Ordovician Southern DEG 24�40016.7300 16�8019.7900


S5369 Mamuniyat Formation Late Ordovician Central DEG 25�34028.5600 16�33041.9300





S5324 Melez Shuqran Formation Late Ordovician Northern DEG 25�57054.6300 16�45056.8700

S5310 Hawaz Formation Middle Ordovician Northern DEG 25�53029.2500 16�48050.8700

P5877 Hawaz Formation Middle Ordovician Northern DEG 25�55049.800 16�48033.4100



H6048 Hawaz Formation Middle Ordovician Central DEG 25�38032.2600 16�34015.1500




W5206 Hasawnah Formation Cambrian Central DEG 25�31038.1200 16�3708.9700

W5207 Hasawnah Formation Cambrian Central DEG 25�32012.9400 16�35053.1300

S5398 Hasawnah Formation Cambrian Northern DEG 24�37023.7800 16�12010.4700

P5876 Hasawnah Formation Cambrian Northern DEG 25�55049.800 16�48033.4100



P5885 Precambrian basement Precambrian Northern DEG 26�17017.9900 16�50035.6300

P5887 Precambrian basement Precambrian Northern DEG 26�29031.3100 16�50021.4800


2.4. Melez Shuqran Formation (Late Ordovician)

The Melez Shuqran Formation consists of silty to sandy, greyand red shale, with many intercalated fine-grained horizontalsandstone beds (Klitzsch and Ziegert, 2000; McDougal and Martin,2000). McDougal and Martin (2000) mentioned the occurrence ofdropstones (see also Hallett, 2002). In the section north of Dor el

Gussa, the Melez Shugran Formation is characterised by shale withferruginous oolites (Aziz, 2000). The succession shows evidence fordeposition by sediment density flow processes and post-deposi-tional sediment remobilisation and liquefaction. It represents amarine deposit, accumulated during a period of transgression(highstand) reflecting the first major deglaciation event (Hallett,2002). The Melez Shuqran Formation, if present, is overlain by

Fig. 3. Stratigraphic chart for the Dor el Gussa region and major tectonic events recorded in the Murzuq Basin (compiled from Klitzsch, 1964; Klitzsch and Ziegert, 2000;Ramos et al., 2006).


the Mamuniyat Formation, probably with a stratigraphic break(Klitzsch, 1964). The Melez Shuqran Formation has a thickness of625 m in western Dor el Gussa (Klitzsch, 1964; Klitzsch and Zieg-ert, 2000), with much greater thicknesses in the centre of the Mur-zuq Basin (e.g., Aziz, 2000). In some areas, the formation iscompletely missing. Its stratigraphic age is thought to be Carado-cian, based on brachiopods (Havlicek and Massa, 1973) and paly-nological analyses (Aziz, 2000), although more recent analysis ofbrachiopod fauna by Sutcliffe et al. (2001) suggests a Hirnantianage. One sample was selected from the Melez Shuqran Formationfor heavy mineral analysis.

2.5. Mamuniyat Formation (Late Ordovician)

The Mamuniyat Formation is predominantly composed of mas-sive, fine- to coarse-grained, in parts conglomeratic, often tabularcross-bedded sandstone (e.g., Klitzsch, 1964; Aziz, 2000; Klitzschand Ziegert, 2000). In the eastern part of the Murzuq Basin (atnorthern Dor el Gussa) the base of the Mamuniyat Formation com-prises a highly ferruginous sandstone bed (�2 m thick), whichpasses laterally into nearly pure haematite (Klitzsch, 1964), indi-cating the presence of a major hiatus (Aziz, 2000). Ghienne et al.

(2003) and Le Heron et al. (2004) described palaeovalley-fill depos-its and local half-graben depocentres filled with fluvial and shal-low-marine sandstone from the Gargaf area. Soft-sedimentdeformation structures within the Mamuniyat Formation areattributed to glaciotectonic processes (e.g., Le Heron et al., 2005).The Mamuniyat Formation unconformably overlies the MelezShuqran Formation (Fig. 3). Towards the southeast, in southeasternDor el Gussa, the Mamuniyat Formation lies directly on the Hasaw-nah Formation with an angular unconformity of 25–40� (Klitzsch,1964; Klitzsch and Ziegert, 2000). The Mamuniyat Formation istransgressively overlain by the Tanezzuft Formation (or by theBir Tlacsin Formation depending on the location within the Mur-zuq Basin; e.g., Echikh and Sola, 2000). The Mamuniyat Formationis 30 m thick in Dor el Gussa (Klitzsch, 1964; Klitzsch and Ziegert,2000). Analysis of acritarchs suggests an Ashgillian age for theMamuniyat Formation (Aziz, 2000). A late Asghillian age (Hirnan-tian; extraordinarius/persculptus biozone) is suggested based onbiostratigraphic data for the youngest preglacial deposits and theoldest postglacial strata from time and facies equivalent succes-sions of West Gondwana (see Le Heron et al., 2005 and referencestherein). Nine samples were selected from the Mamuniyat Forma-tion for heavy mineral analysis.


2.6. Tanezzuft Formation (early Silurian)

The Tanezzuft Formation when unweathered consists of darkgrey shale, very compact, commonly with mica and pyrite, some-times with silty and fine-grained sandy interlaminae and beds (Kli-tzsch, 1964; Aziz, 2000). In the Murzuq Basin, coarse clasticmaterial is observed to increase towards the south and southeast(Echikh and Sola, 2000). However, Echikh and Sola (2000) notedthat fine dark shale, with graptolites and sideritic lenses, occursclose to the present-day, NNW–SSE-striking pinchout line recogni-sed in the NE Murzuq Basin in the subsurface map. This suggestserosion from these areas during the late Silurian rather than pri-mary depositional patterns. The Tanezzuft Formation unconform-ably overlies the Mamuniyat Formation, and in the Mourizidiearea, the Tanezzuft Formation directly overlies basement rocks(Hallett, 2002). The transition from the Tanezzuft Formation intothe overlying Akakus Formation is gradual (e.g., Klitzsch, 1964).Near the track from central Dor el Gussa to Waw al Kabir the Tadr-art Formation rests with an unconformity of 3–5� on the middlepart of the Tanezzuft Formation. In general, the Tanezzuft Forma-tion is of variable thickness. In northern Dor el Gussa, it has a thick-ness of 160 m with a progressive reduction due to subsequent‘‘Caledonian-aged’’ erosion as one moves south (Klitzsch, 1964; Kli-tzsch and Ziegert, 2000). Analysis of graptolites from Gargaf andfrom the Ghat area suggests a Rhuddanian to Aeronian age forthe Tanezzuft Formation (Štorch and Massa, 2006). Four samplesfrom outcrop and one from the shallow borehole CDEG-2a were se-lected from the Tanezzuft Formation for heavy mineral analysis.

2.7. Akakus Formation (early–late Silurian)

The Akakus Formation consists of fine- to medium-grained siltysandstones, white to grey, thinly bedded, commonly cross-bedded,with ripple and flute marks (Klitzsch, 1964). The top is marked byferruginous sandstone with convolute bedding and stromatoliticstructures, which represents a period of subaerial exposure (Hal-lett, 2002). There is no obvious break in the succession from theTanezzuft Formation into the Akakus Formation, which would pro-vide a clear boundary; the transition is gradual with an increase inthe sandy component (Klitzsch, 1964). The formation is interpretedto be a prograding deltaic system that built northwestward intothe Ghadames Basin. The Akakus Formation is unconformablyoverlain by the Tadrart Formation (Fig. 3). The thickness of theAkakus Formation varies largely depending on its location withinthe Murzuq Basin. The formation is 6465 m thick in northernDor el Gussa and absent in central Dor el Gussa (Klitzsch, 1964; Kli-tzsch and Ziegert, 2000). It is important to note that the AkakusFormation is diachronous, meaning the Tanezzuft and Akakus for-

Table 2Summary of the heavy mineral analytical data set for sandstones from Dor el Gussa, easte

Stratigraphy Age Conventional heavmineral analysis

Nubian Sandstone Triassic–early Cretaceous 4Mrar Formation Carboniferous 4Awaynat Wanin Group Middle–Late Devonian 3Ouan Kasa Formation Middle–Late Devonian 4Tadrart Formation Middle–Late Devonian 16Akakus Formation early–late Silurian 2Tanezzuft Formation early Silurian 5Mamuniyat Formation Late Ordovician 9Melez Shuqran Formation Late Ordovician 1Hawaz Formation Middle Ordovician 8Hasawnah Formation Cambrian 6Precambrian basement Neoproterozoic 2Total 64

mations are at least partially time-equivalent (Hallett, 2002). In theGhat area, the Akakus Formation is late Llandovian and Ludlovian(and possibly Pridolian) in age (Hallett, 2002), although this lacksdirect biostratigraphic proof. In the subsurface of the Murzuq Ba-sin, in wells A1-NC58 and E1-NC58, palynological data indicate aWenlockian to Ludlovian age (Hallett, 2002). In general, there is atrend for the age of the base of the Akakus Formation to becomeyounger northwestward. Two samples were selected from the Aka-kus Formation for heavy mineral analysis.

2.8. Tadrart Formation (Middle–Late Devonian)

The Tadrart Formation consists of massive sandstone, light todark brown, fine-grained to slightly conglomeratic, thick-bedded,cross-stratified, highly ferruginous in some beds, but normallywithout matrix and very friable (Klitzsch, 1964). The depositionalsequence is interpreted to represent a braided-fluvial to braided-deltaic environment that builds out into tidal-dominated shorefacefacies (Sutcliffe et al., 2000). The Tadrart Formation unconformablyoverlies the Akakus Formation (Fig. 3). In central Dor el Gussa, atapproximately 25�330N and 16�300E, the Tadrart Formation restswith a ‘‘Caledonian’’ unconformity of 3–5� on the middle part ofthe Tanezzuft Formation. The Tadrart Formation is conformablyoverlain basinward by the Ouan Kasa Formation. The Tadrart For-mation varies in thickness from 415 m in the centre of the northernDor el Gussa (Klitzsch, 1964; Klitzsch and Ziegert, 2000) to 100 min parts of southern Dor el Gussa (Klitzsch, 1964; Klitzsch and Zieg-ert, 2000). It thins onto the Al Haruj uplift to the east, and is absentin the central Murzuq Basin (Klitzsch and Ziegert, 2000). The vari-ations in thickness and its distribution reflect regional tectonismduring the Devonian (mid-Eifelian uplift). No direct age determina-tion is possible for the formation because of the lack of diagnosticfossils (Klitzsch, 1964; Hallett, 2002). Nonetheless, the Tadrart For-mation is commonly suggested to be Early Devonian (Lochkovian/Pragian) in age in the Gargaf area (Mergl and Massa, 2000). In cen-tral and northern Dor el Gussa, recent observations made duringCASP fieldwork and palaeobotanical studies (unpublished CASPdata) suggest that Tadrart Formation sediments may be of Middleto early Late Devonian age. Sixteen samples were selected from theTadrart Formation for heavy mineral analysis.

2.9. Ouan Kasa Formation (Middle–Late Devonian)

The Ouan Kasa Formation consists of shale and silty shale alter-nating with fine-grained sandstone and calcareous siltstone (Kli-tzsch, 1964; Klitzsch and Ziegert, 2000). Mergl and Massa (2000)noted that the percentage of primary carbonate beds varies from15% to 25%. The siltstones and sandstones coarsen upwards and

rn Murzuq Basin.

y Rutilechemistry

Tourmalinechemistry

Garnetchemistry

Clinopyroxenechemistry

2 11 2 31 22 26 42 2

1 14 3

12 53 2

21 25 4 2


show wavy and flaser-bedding (Hallett, 2002). The depositional se-quence is interpreted to show a gradational change through timefrom a brackish lagoon to offshore bars. There is a gradual transi-

Table 3Relative abundance of non-opaque detrital heavy minerals in the 63–125 lm fraction� of saby stratigraphic age youngest to oldest. � excludes SCDEG-2a, analysis on 32–63 lm fract

Samplename

Stratigraphy At Ap Br Ca Cp Ct Ep

H6039 Nubian Sandstone 0.6 2.4 10.7 4.2 1.2H6040 Nubian Sandstone 0.5H6062 Nubian Sandstone 0.5H6064 Nubian Sandstone 0.5H6037 Mrar Formation 1.2 0.6 4.8 0.6H6041 Mrar Formation 2.5 1.5 1.5H6042 Mrar Formation 3 8.5 1 1H6061 Mrar Formation 1 RH6034 Awaynat Wanin Group 3.7 0.7H6043 Awaynat Wanin Group 1.5 0.5H6044 Awaynat Wanin Group 1.5 0.5S5334 Ouan Kasa Formation 2S5346 Ouan Kasa Formation 2.5S5347 Ouan Kasa Formation 1 R 0.5S5348 Ouan Kasa FormationS5415 Tadrart Formation 0.5S5434 Tadrart Formation 1.5P5878 Tadrart Formation 1.5P5881 Tadrart Formation 1P5893 Tadrart Formation 1 RP5895 Tadrart Formation 2 RH6032 Tadrart Formation 1H6033 Tadrart FormationH6045 Tadrart Formation 1H6047 Tadrart Formation 0.5H6049 Tadrart Formation 1H6051 Tadrart Formation 0.5 RH6054 Tadrart Formation 0.5 0.5H6055 Tadrart Formation 1 R 0.5H6057 Tadrart Formation 0.5 0.5 0.5 23.5 2.5H6059 Tadrart Formation 0.5 R 6 24.5 7.5S5316 Akakus Formation 3.5S5317 Akakus Formation 3 RSCDEG-2a Tanezzuft Formation 2.5 1.5S5289 Tanezzuft Formation 7 RS5292 Tanezzuft Formation 0.5 0.5 6 74 2.5H6053 Tanezzuft Formation 3.5 0.5 0.5 1H6058 Tanezzuft Formation 0.5 3 25 3.5S5296 Mamuniyat FormationS5322 Mamuniyat Formation 1.5S5339 Mamuniyat Formation 0.5S5341 Mamuniyat Formation RS5369 Mamuniyat Formation 0.5S5400 Mamuniyat Formation 3 R RS5408 Mamuniyat Formation 1.5S5409 Mamuniyat Formation 3.5 R 0.5S5418 Mamuniyat Formation 0.5S5324 Melez Shuqran

Formation12 45.5 10

S5310 Hawaz Formation 4.5 0.5P5877 Hawaz FormationP5879 Hawaz FormationP5880 Hawaz Formation 0.5H6048 Hawaz Formation 0.5H6050 Hawaz Formation 1H6060 Hawaz FormationH6063 Hawaz Formation 2.5W5206 Hasawnah Formation 0.5 0.5 0.5W5207 Hasawnah FormationS5398 Hasawnah Formation 2P5876 Hasawnah Formation 2P5889 Hasawnah Formation 2.5 8.8P5890 Hasawnah Formation 1 1 5 1.5P5885 Precambrian basement 7.1P5887 Precambrian basement 1 3 78 3

At – anatase, Ap – apatite, Br – brookite, Ca – calcic amphibole, Cp – clinopyroxene, Ct – colivine, Op – orthopyroxene, Ru – rutile, Sl – sillimanite, Sp – titanite, St – staurolite, ToR – rare (< 0.5%)

tion from the Tadrart Formation to the Ouan Kasa Formation,which itself is unconformably overlain by the Awaynat WaninGroup (Fig. 3). The Ouan Kasa Formation is suggested to be up to

ndstones from the eastern Murzuq Basin, expressed as frequency %. Samples are sortedion.

Gh Gt Ky Mo Ol Op Ru Sl Sp St To Zr Total

11.3 6.5 7.7 1.2 1.8 37.5 14.9 1682.5 15 2 7 73 2001 19 1.5 7 71 200

69.5 0.5 9 1 2 8.5 9 2008.9 0.6 22 8.3 53 16849.5 0.5 15 0.5 8.5 20.5 20048.5 1 19 8 10 20031 R 19.5 R R 31.5 17 2000.7 26.9 3 65 134R 10.5 34 53.5 200

7.5 9.5 81 20025 41.5 31.5 200

2 23 20 52.5 2001 5.5 R 29 63 2007 4.5 15 73.5 200

3.5 10 86 20028.5 30 40 200

1 7 11 79.5 2000.5 0.5 12.5 9 76.5 200R R 9 R 4 86 200

R R 17 28.5 52.5 20010 2.5 86.5 20010 5.5 84.5 200

0.5 13 26.5 59 20014.5 3 82 200

0.5 10.5 8 80 200R 0.5 9.5 2 87.5 200

6.5 5 87.5 2000.5 10 6 82 200

1.5 8 5.5 6 51.5 2002.5 6.5 2 1.5 5 44 200

20.5 30 46 2000.5 39.5 2.5 54.5 200

1.5 0.5 32.5 21 40.5 20011 16.5 53 12.5 2002 R R 3 2 0.5 0.5 4 4.5 2004 0.5 23.5 63 3.5 2001.5 6.5 1 0.5 4 54.5 200

1 2.5 1 95.5 200R 12.5 23 63 200

6.5 13 80 20012.5 11.5 76 20013 0.5 86 200

0.5 R 3 14 R 0.5 18.5 60.5 20020 25 53.5 200

0.5 1 13.5 20.5 60.5 2001.5 21.5 26.5 50 200

2.5 3 5.5 1 0.5 12.5 7.5 200

12.5 16 66.5 2000.5 11 22.5 66 2003.3 7.4 9.8 79.5 1220.5 15 9 75 200

10 0.5 89 2000.5 13.5 3.5 81.5 200

14 4.5 81.5 20036 17 44.5 200

R 8.5 17.5 72.5 2007 3 90 2007.5 9.5 81 200

6 6 10.5 75.5 2001.3 2.5 6.3 5 73.6 80

1 11 0.5 3.5 25 50.5 2001.8 10.7 17.9 62.5 562.5 1 0.5 1.5 2.5 7 200

hloritoid, Ep – epidote, Gh – gahnite, Gt – garnet, Ky – kyanite, Mo – monazite, Ol –– tourmaline, Zr – zircon.


120 m thick in western Dor el Gussa (Klitzsch and Ziegert, 2000).Analysis of brachiopod faunas suggested an uppermost Pragianor Emsian age for the Ouan Kasa Formation in the Gargaf area(Mergl and Massa, 2000). For central and northern Dor el Gussa re-

Table 4Provenance-sensitive ratio parameters from sandstones of the eastern Murzuq Basin. Data iby stratigraphic age youngest to oldest.

Sample name Stratigraphy ATi Total GZi Total

H6039 Nubian Sandstone 5.9 67 43.2 44H6040 Nubian Sandstone 0 100 0 201H6062 Nubian Sandstone 0 100 0 247H6064 Nubian Sandstone 0 100 88.2 238H6037 Mrar Formation (1/15) 14.4 104H6041 Mrar Formation 13.3 30 71 221H6042 Mrar Formation 51.4 111 83.3 287H6061 Mrar Formation 0.4 259 64.4 385H6034 Awaynat Wanin Group (0/4) 1.1 88H6043 Awaynat Wanin Group 0 200 0.4 227H6044 Awaynat Wanin Group 0 100 0 200S5334 Ouan Kasa Formation 0 200 0 200S5346 Ouan Kasa Formation 0 200 0 200S5347 Ouan Kasa Formation 0 200 0 200S5348 Ouan Kasa Formation 0 200 0 200S5415 Tadrart Formation 0 200 0 200S5434 Tadrart Formation 0 200 0 200P5878 Tadrart Formation 0 62 0 200P5881 Tadrart Formation 0 100 0 200P5893 Tadrart Formation 0 200 0 200P5895 Tadrart Formation 0 200 0.5 201H6032 Tadrart Formation 0 100 0 220H6033 Tadrart Formation 0 36 0 205H6045 Tadrart Formation 0 107 0 240H6047 Tadrart Formation 0 100 0 230H6049 Tadrart Formation 0 100 0 200H6051 Tadrart Formation 1.2 84 0 200H6054 Tadrart Formation 0 100 0 250H6055 Tadrart Formation 0 100 0 200H6057 Tadrart Formation 7.7 136 2.9 104H6059 Tadrart Formation (1/18) 0 197S5316 Akakus Formation 0 200 0 200S5317 Akakus Formation 0 41 0 200SCDEG-2a Tanezzuft Formation 0 200 4.8 210S5289 Tanezzuft Formation 0 200 40 200S5292 Tanezzuft Formation 6.2 113 20.2 198H6053 Tanezzuft Formation 0 220 34 47H6058 Tanezzuft Formation (0/16) 2.4 205S5324 Mamuniyat Formation 0 25 25 20S5296 Mamuniyat Formation 0 50 0 200S5322 Mamuniyat Formation 0 200 0 200S5339 Mamuniyat Formation 0 200 0 200S5341 Mamuniyat Formation 0 200 0 200S5369 Mamuniyat Formation 0 50 0 200S5400 Mamuniyat Formation 0 200 0.5 201S5408 Mamuniyat Formation 0 200 0 200S5409 Mamuniyat Formation 0 200 0.5 201S5418 Melez Shuqran Formation 0 200 0 200S5310 Hawaz Formation 0 200 0 200P5877 Hawaz Formation 0 100 0 202P5879 Hawaz Formation (0/12) 0 97P5880 Hawaz Formation 0 100 0 202H6048 Hawaz Formation 0 100 0 100H6050 Hawaz Formation 0 100 0 200H6060 Hawaz Formation 0 100 0 200H6063 Hawaz Formation 0 117 0 309W5206 Hasawnah Formation 1 200 0 200W5207 Hasawnah Formation 0 100 0 200S5398 Hasawnah Formation 0 200 0 200P5876 Hasawnah Formation 0 45 0 212P5889 Hasawnah Formation (0/3) 1.7 60P5890 Hasawnah Formation 0 91 0 161P5885 Precambrian basement (0/6) 4.5 22P5887 Precambrian basement (0/5) 25 20

ATi – apatite:tourmaline index, GZi – garnet:zircon index, RZi – TiO2 minerals-zircospinel:zircon index.

cent observations during CASP fieldwork and palaeobotanical stud-ies suggest that the Ouan Kasa Formation sediments may be ofMiddle to early Late Devonian age. Four samples were selectedfrom the Ouan Kasa Formation for heavy mineral analysis.

n parentheses are raw grain counts for samples with poor recovery. Samples are sorted

RZi Total RuZi Total MZi Total CZi Total

35.9 39 34.2 38 10.7 28 0 2516.9 242 16.9 242 3.4 208 0 20120.8 312 20.8 312 1.2 250 0 24761 400 59.8 388 1.4 222 0 21930.5 128 29.4 126 1.1 90 0 8946.2 119 42.3 111 3 66 0 6471.6 236 69 216 5.6 71 0 6756.7 379 55.4 368 1 202 0.5 20132 128 29.3 123 0 87 0 8718.4 277 16.6 271 0 226 0 2269.3 441 8 435 0 200 0 20044.2 269 41.6 257 0.5 201 0 20035.1 308 34 303 3.4 207 0 20010.7 224 9.1 220 2 204 0 2005.2 211 5.2 211 8.3 218 0 2004.8 210 4.3 209 0 200 0 20042 345 41 339 0 200 0 2008.3 218 8.3 218 1.5 203 0 20013.8 232 13.8 232 0.5 201 0 20011.1 225 9.9 222 0 200 0 20023.4 261 22.2 257 0.5 201 0 20011.6 249 10.6 246 0 220 0 22010.5 229 10.5 229 0 205 0 20519.2 297 18.1 293 0.8 242 0 24015.1 271 14.8 270 0 230 0 23012.7 229 11.5 226 0.5 201 0 20010.3 223 9.9 222 0 200 0 2007.1 269 6.7 268 0 250 0 25011.9 227 11.1 225 0.5 201 0 20014.4 118 13.7 117 0 101 0 1019.2 217 7.9 214 6.2 210 0 19732.4 296 32.4 296 0 200 0 20041.9 258 40 250 0.5 201 0 20046.5 241 43.7 229 0.5 201 0 20063.1 271 58.5 241 0 200 0 20029.2 212 26.8 205 0.6 159 0 15882.4 176 80.1 156 6 33 0 3110.3 223 9.9 222 0 200 0 20042.3 26 42.3 26 (0/15) (0/15)2.4 205 2.4 205 1 202 0 20018.4 245 16.7 240 0.5 201 0 2007 215 6.5 214 0 200 0 20020.3 251 20 250 0 200 0 20014.5 234 14.2 233 0 200 0 20016.3 239 13.8 232 3.4 207 0 20025.4 268 24.2 264 0 200 0 20023.1 260 20.3 251 1 202 0 20027.3 275 25.7 269 1.5 203 0 20018.4 245 14.5 234 0 200 0 20014.4 236 14.4 236 1 204 0 2028.5 106 8.5 106 3.9 101 0 9717.6 245 16.9 243 0.5 201 0 20210.2 334 9.9 333 0 300 0 30014.9 235 14.2 233 0.5 200 0 20014.9 235 14.9 235 0 200 0 20046.4 576 44.7 559 0 309 0 30910.7 224 10.3 223 0 200 0 2007 215 7 215 0 200 0 20010.3 223 8.3 218 0 200 0 2007.4 229 7.4 229 7.4 229 0 2123.3 61 3.3 61 0 59 0 5921.4 206 20.6 204 1.8 165 0 16112.5 24 12.5 24 0 21 0 21(4/19) (2/17) (0/15) (0/15)

n index; RuZi – rutile:zircon index, MZi – monazite:zircon index, CZi – chrome


2.10. Awaynat Wanin Group (Middle–Late Devonian)

The Awaynat Wanin Group (also spelt ‘‘Aouinet OuenineGroup’’) consists of ochre-coloured sandstone, which is fine-grained, thin- to partly thick bedded and intercalated with siltyshale layers up to 10 m thick. The Awaynat Wanin Group uncon-formably overlies the Ouan Kasa Formation and is unconformablyoverlain by Carboniferous sediments. In the eastern Murzuq Basin,the upper part of the Awaynat Wanin Group is missing (Klitzschand Ziegert, 2000). In general, the Devonian succession thickensnorthwards, from 140 to 310 m along the western flank of Dor elGussa (Klitzsch and Ziegert, 2000) to 900 m in the centre of theGhadames Basin (Mergl and Massa, 2000). Analysis of brachiopodfauna suggests a Middle to Late Devonian (Eifelian to Famennian)age for the Awaynat Wanin Group in its type area (Mergl and Mas-sa, 2000). Three samples were selected from the Awaynat WaninGroup for heavy mineral analysis.

2.11. Mrar Formation (Carboniferous)

The Mrar Formation, seen as the stratigraphic equivalent to theDalma Formation in the Kufra Basin, overlies the Devonian stratawith a hiatus. In many places along the western flank of Dor elGussa the base of the Mrar Formation is marked by a 1-m-thickconglomeratic sandstone bed overlain by <120 m of partly silty,mainly grey–green shale (Klitzsch, 1964; Klitzsch and Ziegert,2000). The succession is overlain by a further 250 m of interbeddedsandstone and shale, with an increase in the sandstone componenttoward northwest Dor el Gussa. Numerous fossils have been foundsuggesting a Carboniferous (Mississippian) age (Klitzsch and

Fig. 4. Stratigraphic variations in ZTR (zircon + tourmaline + rutile), GZi, clinopyroxen

Ziegert, 2000). Four samples were selected from the Carboniferoussuccession for heavy mineral analysis.

2.12. Nubian Sandstone (Mesozoic)

West of Dor el Gussa, the Nubian Sandstone has been subdi-vided by Klitzsch and Ziegert (2000) into two formations (UnarFormation and Messak Formation). However, owing to uncertain-ties in assigning our samples to one or other of these subdivisions,we retain the general term Nubian Sandstone. The Nubian Sand-stone lies unconformably on late Carboniferous and older strata.It consists of varicoloured sediments typically reddish, yellowish,purplish and brownish. The sediments consist of quartzose,cross-bedded sandstone and pebbly sandstone with interbeds ofconglomerate, siltstone and mudstone. Fossils are extremely rareand restricted to plant fragments or silicified wood. The NubianSandstone is Triassic–early Cretaceous in age (Klitzsch and Ziegert,2000). Four samples were selected from the Nubian Sandstone forheavy mineral analysis.

3. Methodology

3.1. Introduction

Sandstone samples were collected from surface exposure; onesample (SCDEG-2a) was recovered from core of the shallow bore-hole CDEG-2a (geographic coordinates: 25�36019.2700N,16�30047.7500E) drilled by CASP in Dor el Gussa at the eastern mar-gin of the Murzuq Basin, southern Libya, in spring 2008. Samplesfrom surface exposure show weathering features, whereas the sub-surface sample SCDEG-2a is free of weathering. Various analytical

e (Cpx) abundance, RuZi and MZi in sandstones from the eastern Murzuq Basin.


techniques have been adopted in this study. Conventional (petro-graphic) heavy mineral data have been acquired on all samples,supplemented by mineral chemical data on rutile, tourmaline, gar-net and clinopyroxene.

3.2. Sample preparation

Samples were gently disaggregated by use of a pestle and mor-tar, avoiding grinding action. Chemical pre-treatment was avoidedas far as possible, thus preventing the possibility of modifyingassemblages in the laboratory. Following disaggregation, the sam-ples were immersed in water and cleaned by ultrasonic probe toremove and disperse any clay might have been adhering to grain

Fig. 5. Rutile compositions in Mrar, Ouan Kasa and Tadrart formations sandstones. Thrutiles. Sample and formation names are given above each histogram. Number in bracketof metamafic and metapelitic rutile grains for each analysed sample. Key as in Fig. 7.

surfaces. The samples were then washed through a 63-lm sieveand re-subjected to ultrasonic treatment until no more clay passedinto suspension. At this stage, the samples were wet sievedthrough the 125 and 63 lm sieves, and the resulting >125 lmand 63–125 lm fractions were dried in an oven at 80 �C. The 63–125 lm fractions were placed in bromoform with a measured spe-cific gravity of 2.8. Heavy minerals were allowed to separate undergravity, with frequent stirring to ensure complete separation. Theheavy mineral residues were mounted under Canada Balsam foroptical study using a polarising microscope. Where possible, a splitwas retained for mineral chemical study.

One sample (SCDEG-2a, Tanezzuft Formation) proved to be veryfine-grained and heavily carbonate-cemented, and as a result, the

e black dotted line marks the boundary between amphibolite and granulite faciess is the number of analysed rutile grains. Histograms and pie charts give the amount


sample yielded only carbonate. In order to acquire data from thissample, a finer fraction (32–63 lm) was analysed, and carbonatewas removed by acid treatment. Acetic acid proved unsuitablefor this purpose, and hence dilute HCl was used. The lack of apatitein this sample may therefore be the result of laboratoryprocessing.

3.3. Conventional analysis and ratio determination

Heavy mineral proportions were estimated by counting 200non-opaque detrital grains using the ribbon method described byGalehouse (1971). Identification was made based on optical prop-erties, as described for grain mounts by Mange and Maurer (1992).

Fig. 6. Rutile compositions in Tadrart, Akakus and Mamuniyat formations sandstones. Trutiles. Sample and formation names are given above each histogram. Number in bracketof metamafic and metapelitic rutile grains for each analysed sample. Key as in Fig. 7.

A qualitative assessment was also made of other components, suchas diagenetic minerals, opaque minerals and mica. Provenance-sensitive mineral ratios were also determined using the ribboncounting method, ideally based on a 200-grain count. It was not al-ways possible to achieve the optimum 200-grain count because ofthe scarcity of some of the mineral phases or because of small sam-ple sizes. Heavy mineral data and provenance-sensitive ratios aregiven in Tables 3 and 4 respectively.

3.4. Garnet, tourmaline and clinopyroxene geochemistry by EMP

Samples for geochemical analysis using the electron microprobe(EMP) were selected based on the results of the conventional

he black dotted line marks the boundary between amphibolite and granulite faciess is the number of analysed rutile grains. Histograms and pie charts give the amount


optical analysis. Grains were picked with a needle from the dry res-idues during optical examination under the polarising microscope,mounted on glass slides, sectioned and polished, coated with car-bon, and analysed at the British Geological Survey using either aCameca SX50 electron microprobe or a Cambridge InstrumentsS360 electron microscope. The Cameca SX50 electron microprobeis fitted with three wavelength-dispersive spectrometers.Throughout the procedure an accelerating voltage of 15 kV, a sam-ple current of 20 nA and a beam focussed to �1 lm were used. Amixture of natural minerals and synthetic materials were usedfor calibration and raw data were processed using the Quantiviewsoftware provided by Cameca. The Cambridge Instruments S360electron microscope is fitted with an Oxford Instruments X-rayanalysis system (EDXA) consisting of a germanium (GEM) detectorand Oxford INCA microanalysis software. Analyses were obtainedusing an accelerating voltage of 15 kV, a probe current of 500 pAand a count time of 30 s.

For garnet, the quality of each result was monitored to ensurethat the stoichiometrically determined garnet formula wasapproximately that of an ideal garnet. Studies of North Sea detritalgarnets have shown that intra-grain variations are usually negligi-ble in grains between 63 and 125 lm diameter (Morton, 1985;Morton et al., 1989). Therefore, it is unlikely that compositionalzoning has a significant effect on the overall range of garnet com-positions in any individual sample. Garnet compositions are ex-

Fig. 7. Rutile compositions in Hawaz Formation and Hasawnah Formation sandstones. Trutiles. Sample and formation names are given above each histogram. Number in bracketof metamafic and metapelitic rutile grains for each analysed sample.

pressed in terms of the relative abundance of the Mg, Fe2+, Caand Mn end-members. The compositions of garnet assemblagesare shown using ternary diagrams with molecular proportions ofFe2++Mn, Mg and Ca as poles, calculated assuming all Fe is presentas Fe2+, as recommended by Droop and Harte (1995). Tourmalinecompositions were plotted on the source discriminant ternary dia-gram of Henry and Guidotti (1985), and compared using ternaryplots of types B + C (principally of granitic origin), type D (of Al-richmetasedimentary origin), and types E and F (principally of Al-poormetasedimentary origin). Clinopyroxenes were classified using thePoldervaart and Hess (1951) scheme, and their host magma com-positions were assessed following LeBas (1962) and Leterrieret al. (1982). The mineral chemical data are given as Supplemen-tary data (see Appendix A).

3.5. Rutile geochemistry by LA–ICP–MS

Rutile geochemical analyses were carried out in the School ofEarth, Ocean and Planetary Sciences at Cardiff University, using aThermo Elemental X(7) series ICP–MS coupled to a New Wave Re-search UP213 Nd:YAG 213 nm UV laser (LA–ICP–MS). The laserbeam diameter was 30 lm and laser repetition rate set at 4 Hz. He-lium gas was used for ablation initial transport from the laser celland this was combined with argon outside the cell as the samplewas transported to the ICP–MS. Thermo Elemental Plasmalab

he black dotted line marks the boundary between amphibolite and granulite faciess is the number of analysed rutile grains. Histograms and pie charts give the amount


time-resolved analysis (TRA) data acquisition software was usedwith a total acquisition time of 60 s per analysis, allowing about30 s for background followed by 25 s for laser ablation. Plasmalabwas used for initial data reduction with post-processing in Excel.The calibration employed BIR-1G, BHVO-2G and BCR-2G (USGS ba-salt glass standards) to produce a 4 point (including the origin) cal-ibration curve. Data were normalised to Ti (98% TiO2) and adjustedaccordingly. Instrumental drift was monitored by repeat analysisof BHVO-2G after every 25–30 grains. The composition of the rutilesource rocks has been assessed following criteria proposed byMeinhold et al. (2008), with metamorphic temperatures calculatedaccording to Watson et al. (2006), as suggested by Meinhold(2010). The mineral chemical data are given as Supplementary data(see Appendix A).

4. Results

4.1. General observations

Most of the heavy mineral assemblages present in the Dor elGussa region are limited in diversity, being dominated by rutile,tourmaline and zircon (ZTR, as defined by Hubert, 1962). ZTR val-ues range from 11 to 100, with the vast majority of samples havingZTR > 90 (Fig. 4). Exceptions to this pattern are largely due either tothe abundance of garnet, which is present in significant amounts inone Precambrian sample, in the Melez Shuqran and Tanezzuft

Fig. 8. (a and b) Comparison of rutile compositions in sandstones from the Murzuq BasiKufra basins.

Formations, and in the Mrar–Nubian interval (Fig. 4), or to theabundance of clinopyroxene, which is abundant in the samePrecambrian sample, in the Melez Shuqran Formation sample,some Tanezzuft Formation samples and two Tadrart Formationsamples.

4.2. Stratigraphic variations in heavy mineral parameters

Provenance-sensitive parameters, as defined by Morton andHallsworth (1994), are rutile:zircon (RuZi), garnet:zircon (GZi),monazite:zircon (MZi) and CZi (chrome spinel:zircon). These ratioscompare the relative abundance of minerals that are stable in thediagenetic context of the study, and have comparable hydrody-namic behaviour (principally governed by grain size, grain density,and to a lesser extent grain shape). Hence, the parameters cannotbe significantly modified by variations in diagenesis or by differ-ences in sedimentary facies.

There are significant variations in RuZi throughout the succes-sion (Fig. 4). RuZi is consistently low in the Precambrian–Mam-uniyat interval, values being �20 or below, except for a singleHawaz Formation sample (H6063) and the Melez Shuqran Forma-tion sample (S5324); the latter however may be suspect owing tothe low grain count. RuZi is significantly higher in most TanezzuftFormation and Akakus Formation samples, many of which haveRuZi > 40. The Tadrart Formation shows a reversion to low RuZi(less than �20), again with a single exception (sample S5434). RuZi

n. (c and d) Comparison of rutile compositions in sandstones from the Murzuq and


is more variable in the Ouan Kasa Formation and Awaynat WaninFormation, and is generally high in the Mrar Formation and NubianSandstone.

Garnet:zircon (GZi) values are at their highest in the youngest(Mrar Formation and Nubian Sandstone) parts of the succession,

Fig. 9. Tourmaline compositions from Nubian, Mrar, Awaynat Wanin, Ouan Kasa and Tadof Henry and Guidotti (1985).

although high values are also seen in the older clinopyroxene-bear-ing sandstones. Apart from one Precambrian sample (P5887), allthe samples with high clinopyroxene contents (>20%) occur inthe Melez Shuqran Formation, the Tanezzuft Formation and, moresporadically, in the Tadrart Formation.

rart formations sandstones of the eastern Murzuq Basin plotted on the ternary plot


There are variations in monazite:zircon (MZi), as shown inFig. 4. These appear to lack any consistent pattern in the lowerparts of the succession (Hasawnah–Tadrart formations). However,they seem to have more structure towards the top, with monazite

Fig. 10. Tourmaline compositions from the Tadrart, Akakus, Tanezzuft and MamuniyatHenry and Guidotti (1985).

being consistently present in the Ouan Kasa Formation, absent inthe Awaynat Wanin Group, and then present again through theMrar Formation and Nubian Sandstone. Finally, CZi is consistentlyzero, implying the absence of ultramafic rocks in the source area.

formations sandstones of the eastern Murzuq Basin plotted on the ternary plot of


4.3. Rutile compositions

Rutile geochemical data have been acquired on 21 samples(Table 2). Populations have been characterised in terms of both

Fig. 11. Tourmaline compositions from the Hawaz Formation and Hasawnah FormationGuidotti (1985).

lithology and metamorphic grade of their source rocks, using theCr–Nb discrimination diagram of Meinhold et al. (2008) and theZr-in-rutile thermometer of Watson et al. (2006) respectively, fol-lowing Meinhold et al. (2011). Further details about the application

sandstones of the eastern Murzuq Basin plotted on the ternary plot of Henry and


of rutile geochemistry and thermometry can be found in Meinhold(2010).

Rutile compositions are shown in Figs. 5–7. There are distinctvariations in the relative abundance of metamafic (15–47%) andmetapelitic (53–85%) rutiles, and in metamorphic temperature.Granulite facies rutiles (>750 �C) are comparatively minor (7–36%, with a mean of 20%), and greenschist facies rutiles are veryscarce, with the majority being formed under amphibolite faciesconditions. Within the amphibolite facies group, there are varia-tions in relative abundance of lower amphibolite facies (<650 �C)and upper amphibolite facies types, with lower amphibolite faciesrutiles forming between 16% and 62% of the rutile populations. Ru-tile compositions are compared using the relative abundance ofgranulite, upper amphibolite and lower amphibolite facies types,and a crossplot of abundance of metamafic types against the pro-portion of lower amphibolite facies types (Fig. 8a and b). Theseplots show that for the most part, there is as much heterogeneitywithin individual formations as there is between formations. Someformations show less heterogeneity than others do: for example,

Fig. 12. (a and b) Comparison of tourmaline compositions in sandstones of the eastern M(1985). (c and d) Comparison of Murzuq Basin and Kufra Basin tourmaline compositions.line separates the two groups seen in the eastern Murzuq Basin.

the four Mamuniyat Formation samples have comparatively uni-form rutile compositions (Fig. 8a and b), whereas the HasawnahFormation samples are highly variable.

The two youngest samples analysed (Mrar Formation and Nu-bian Sandstone) have the highest proportions of lower amphibolitefacies rutiles, and have a relatively high content of metamaficgrains. This supports the evidence from other parameters (suchas RuZi, MZi and GZi) for a difference in provenance at this level.

4.4. Tourmaline compositions

Tourmaline compositions have been acquired on 25 samples(Table 2). They are displayed using the ternary discriminant plotof Henry and Guidotti (1985), as shown in Figs. 9–11. There aremarked variations in the relative abundance of tourmalines thatfall in field B (principally derived from Li-poor granitoids), fieldD (principally derived from Al-rich metasedimentary rocks) andfields E and F (most of which are derived from Al-poor

urzuq Basin. Tourmaline types B, C, D, E and F are as defined by Henry and GuidottiTourmaline types B, C, D, E and F are as defined by Henry and Guidotti (1985). Blue

Fig. 13. Garnet compositions in sandstones from the eastern Murzuq Basin.


metasediments. Ternary diagrams compare the tourmalinepopulations (Fig. 12a and b).

4.5. Garnet compositions

Garnet data have been acquired from four samples (Table 2),three from the Mrar Formation and one from the Nubian Sand-stone. The four samples have similar assemblages, all being richin high-Mg, low-Ca garnets (Type A, in the terminology of Mortonet al., 2004) and containing subordinate high-Mg, high-Ca (Type C)and low-Mg, variable-Ca (Type B) garnets (Fig. 13).

4.6. Clinopyroxene compositions

Clinopyroxenes have been analysed from the Melez ShuqranFormation sample S5324 and from the Tanezzuft Formation sam-ple S5292. The two populations have closely comparable composi-tions (Figs. 14 and 15), being classified as salite–augite according tothe Poldervaart and Hess (1951) scheme, and falling predomi-nantly in the alkaline field on the SiO2–Al2O3 plot of LeBas(1962). According to the discriminant diagrams proposed by

Leterrier et al. (1982), both populations were derived from alkaline,non-orogenic mafic sources (Fig. 16).

5. Discussion

5.1. General observations

The highly stable nature of the heavy mineral assemblages inthe analysed sandstone samples is considered to be largely dueto surficial weathering, since apatite (which is highly sensitive toweathering) is extremely scarce (0.0–8.5%, mean 0.2%). The generalscarcity of garnet, which is also sensitive to weathering (althoughless so than apatite), is probably at least partly due to the sameprocess. It is noticeable that samples with abundant clinopyroxenegenerally also contain garnet (Fig. 4), suggesting that samples withabundant clinopyroxene (considered to imply an important volca-nic provenance) may be less susceptible to weathering (possiblybecause of poor porosity/permeability characteristics). However,it is also noticeable that garnet is more consistently present inthe Mrar–Nubian interval, in this case in the absence of abundant

Fig. 14. Classification of detrital pyroxenes from the Tanezzuft Formation and Melez Shuqran Formation sandstones, after Poldervaart and Hess (1951).


clinopyroxene, and it is possible that this marks a significantchange in provenance.

Burial diagenesis has probably also played a role in reducingassemblage diversity, since staurolite (which is stable in weather-ing, but unstable during burial diagenesis) is present onlysporadically.

5.2. Stratigraphic variations in heavy mineral parameters

Because of the evidence for extensive modification of heavymineral assemblages through surficial weathering and, to a lesserdegree, burial diagenesis, the only ratio parameters that can be re-garded as truly provenance-sensitive are rutile:zircon (RuZi), mon-azite:zircon (MZi) and CZi (chrome spinel:zircon). Of these, CZi isconsistently zero (implying the absence of ultramafic rocks in thesource area).

The appearance of sandstones with high RuZi in the Tanezzuftand Akakus formations is analogous to the situation seen in the Ku-fra Basin (Morton et al., in press), although values in the MurzuqBasin are generally lower than those found in the Kufra Basin. Nev-

ertheless, it seems likely that the palaeogeographic change respon-sible for the increase in RuZi in the Kufra Basin also affected theMurzuq Basin area, with the more muted response in the MurzuqBasin being interpreted as due to dilution by local sources withlower RuZi.

In the Kufra Basin, the Tadrart Formation is characterised byvariable but generally high RuZi values, similar to those in the Aka-kus Formation, but in the Murzuq Basin, values are low, suggestinga reversion to the same low RuZi provenance that characterised thepre-Tanezzuft Formation succession. The appearance of higherRuZi values in the Ouan Kasa Formation and younger sandstonesis, however, similar to the situation in the Kufra Basin, where theBinem and Dalma formations (=equivalents units to the AwaynatWanin Group and Mrar Formation respectively) and Nubian Sand-stone show a large range in RuZi.

Although clinopyroxene abundances may have been modifiedto some degree by weathering, variations in clinopyroxene abun-dance appear to have stratigraphic significance. Apart from onePrecambrian sample (P5887), all the samples with high clinopyrox-ene contents (>20%) occur in the Melez Shuqran Formation, the

Fig. 15. Detrital pyroxenes from the Tanezzuft Formation and Melez ShuqranFormation sandstones plotted on the SiO2–Al2O3 covariation diagram of LeBas(1962).


Tanezzuft Formation and, more sporadically, in the Tadrart Forma-tion. In the Kufra Basin, the highest clinopyroxene contents are alsoseen in the Tanezzuft Formation, although here, abundances aresignificantly lower. It therefore appears that both basins record abroadly equivalent phase of input from volcanic sources, althoughthe present evidence indicates that the effects were more pro-nounced in the Murzuq Basin.

As discussed above, GZi values are at their highest in the youn-gest (Mrar Formation and Nubian Sandstone) parts of the succes-sion, although high values are also seen in the olderclinopyroxene-bearing sandstones. It is possible that the generalincrease in GZi in the Mrar Formation and Nubian Sandstonemay be related to provenance, but without access to subsurfacesamples, the possibility that the pattern is due to variable weath-ering cannot be entirely discounted.

Finally, there are variations in monazite:zircon (MZi), as shownin Fig. 4. These appear to lack any consistent pattern in the lowerparts of the succession (Hasawnah–Tadrart formations). However,they seem to have more structure towards the top, with monazitebeing consistently present in the Ouan Kasa Formation, absent inthe Awaynat Wanin Group, and then present again through theMrar Formation and Nubian Sandstone.

5.3. Rutile compositions

Crossplot of abundance of metamafic rutile types against theproportion of lower amphibolite facies rutile types (Fig. 8a and b)show that for the most part, there is as much heterogeneity withinindividual formations as there is between formations. This sug-

gests that there are significant variations in rutile provenancewithin each unit, but that the same (or similar) sources continuedto supply sediment for the duration of the analysed succession.Some formations show less heterogeneity than others do: forexample, the four Mamuniyat Formation samples have compara-tively uniform rutile compositions (Fig. 8a and b), whereas theHasawnah Formation samples are highly variable.

The two youngest samples analysed (Mrar Formation and Nu-bian Sandstone) have the highest proportions of lower amphibolitefacies rutiles, and have relatively high contents of metamaficgrains. This supports the evidence from other parameters (suchas RuZi, MZi and GZi) for a difference in provenance at this level.This change in provenance may be related to the Late Devonian–Early Carboniferous glaciation, recorded, for example, in Niger,Egypt, Sudan and the Central African Republic (e.g., Isaacsonet al., 2008, and references therein), which caused erosion and sed-iment supply of lower amphibolite facies material from centralGondwana.

There is virtually no evidence for derivation of rutile from orebodies, with none of the rutiles having elevated levels of Sn orSb. A small number have elevated W contents (>1000 ppm), butin the absence of elevated Sn or Sb, it seems more likely that theseindicate derivation from granitic pegmatites (see Meinhold, 2010).

The rutiles from the eastern Murzuq Basin are compared withthose from the Kufra Basin in Fig. 8c and d. The comparison is re-stricted to the Hasawnah–Akakus formations interval since rutilecompositions have not yet been determined from the Tadrart For-mation and younger units in the Kufra Basin. Although both areashave similar ranges in metamafic and metapelitic rutile abun-dances, the Kufra Basin has significantly higher proportions of ru-tiles from lower amphibolite-facies metamorphic rocks. Themetamorphic basement rocks that supplied the Kufra Basin (eitherdirectly or indirectly, through reworking) were therefore generallylower grade compared with those supplying the Murzuq Basinregion.

5.4. Tourmaline compositions

Tourmaline compositions are shown in Figs. 9–11. Granitictypes vary in abundance from 2–38%, Al-rich metasedimentarytypes from 36–80%, and Al-poor metasedimentary types from10–42%. Ternary diagrams comparing tourmaline populations(Fig. 12a and b) suggest the involvement of two tourmalinesources, distinguished principally on the abundance of the TypeD (Al-rich metasedimentary) group. However, there is no obviousstratigraphic significance to the distribution of these two groups,and, as with the rutile data, there appears to be as much heteroge-neity in provenance within formations as there is within the suc-cession as a whole. The tourmaline data, however, show theexistence of differences in provenance between the Kufra and Mur-zuq basins (Fig. 12c and d). None of the tourmaline populationsanalysed to date from the Kufra Basin match the group with highType D contents seen in the Murzuq Basin, and it appears that thissource did not contribute sediment to the Kufra Basin. The tourma-line data therefore support the evidence from rutile compositionsfor a difference in provenance between the two areas.

5.5. Garnet compositions

Garnet data from the Mrar Formation and the Nubian Sandstonehave similar assemblages (Fig. 13). A diverse provenance, includingmetasediments from a range of metamorphic environments(amphibolite and granulite facies) and metamafic rocks, is diag-nosed, supporting the evidence from rutile geochemistry. Garnetswith similar compositions were identified in the Kufra Basin

Fig. 16. Detrital pyroxenes from the Tanezzuft Formation and Melez Shuqran Formation sandstones plotted on discriminate diagrams of Leterrier et al. (1982).


(Morton et al., in press), although there is evidence for greater het-erogeneity in the Kufra Basin area.

5.6. Clinopyroxene compositions

Clinopyroxenes from the Melez Shuqran Formation sampleS5324 and from the Tanezzuft Formation sample S5292 have clo-sely comparable compositions (Figs. 14 and 15) and, according tothe discriminant diagrams proposed by Leterrier et al. (1982), werederived from alkaline, non-orogenic mafic sources (Fig. 16). How-ever, a felsic source may also be possible since salite–augite is alsopresent in felsic alkaline rocks (e.g., Njonfang and Nono, 2003, andreferences therein). For example, El Makhrouf (1988) describedalkaline granites from the Tibesti Massif but these rocks lack clino-pyroxene. Early Palaeozoic rocks related to post-orogenic alkalinemagmatism (Veevers, 2007) may also potential source lithologiesbut they have not yet been observed in and around Dor el Gussa.One of the Precambrian samples (P5887) also has common clino-pyroxene, and it is recommended for future studies that theseare analysed in order to determine whether the Precambrian base-ment could have been a source for the Melez Shuqran and Tane-zzuft Formations.

6. Conclusions

Despite evidence for significant intraformational variability, andextensive modification of assemblages, especially by surficialweathering, heavy mineral data identify a number of significantstratigraphic changes in provenance within the analysed succes-sion in Dor el Gussa.

Rutile:zircon (RuZi) shows a distinct increase between theMamuniyat and Tanezzuft Formation (with the Melez Shuqran For-mation sample being suspect owing to the low grain count) and asubsequent decrease between the Akakus and Tadrart formations.The Mrar Formation and Nubian Sandstone have generally highRuZi, with the Ouan Kasa Formation and Awaynat Wanin Grouphaving variable values.

Clinopyroxene contents are high in the Melez Shuqran Forma-tion and in some Tanezzuft Formation samples, as well as in onePrecambrian basement sample. Clinopyroxene is present sporadi-cally and in smaller amounts elsewhere in the succession, notablyin the Hasawnah and Tadrart formations.

Garnet:zircon values are variable. Although this variation maybe partly due to surficial weathering, the Mrar Formation and Nu-bian Sandstone have consistently higher GZi than underlying units,and a change in provenance is therefore considered a strongpossibility.

Monazite:zircon (MZi) is variable in the lower parts of the suc-cession, but is consistently relatively high in the Mrar Formationand Nubian Sandstone, indicating a change in provenance at baseof the Mrar Formation. The Ouan Kasa Formation may also be rec-ognisable based on consistently high MZi but more analyses are re-quired to test this possibility.

Rutile compositions are relatively heterogeneous through theanalysed succession, but the Mrar Formation and Nubian Sand-stone appear to have higher abundances of lower amphibolite-fa-cies types than the rest of the interval, and metamafic types arealso common in these units.

Taking all the above evidence into consideration, there appearsto be three key provenance events within the succession: (i) at thebase of the Tanezzuft Formation, (ii) at the base of the Tadrart


Formation, and (iii) at the base of the Mrar Formation, subdividingthe succession into four intervals (Hasawnah–Mamuniyat, Tane-zzuft–Akakus, Tadrart–Awaynat Wanin, and Mrar–Nubian). Thereis probably also an event between the Precambrian basementand the Hasawnah Formation, but only two Precambrian basementsamples have been analysed to date and these have contrastingmineralogy.

In addition to these key events, there is significant variability inprovenance within individual units, expressed both by conven-tional heavy mineral data and by mineral chemical data from rutileand tourmaline populations. These variations may indicate thatheavy minerals could provide a high-resolution basis for correla-tion within individual formations, but this possibility cannot betested at present. Ideally, this should be approached using subsur-face material from below the weathering zone (deeper than �35 mbelow the surface), since the samples can be placed in stratigraphiccontext to one another and the effects of surficial weathering canbe eliminated.

There are both similarities and differences between the heavymineral assemblages in the Murzuq Basin succession and thosepreviously reported from the Kufra Basin. The Kufra Basin showsa similar increase in RuZi between the Mamuniyat and the Tane-zzuft Formations, believed to represent a change in sedimenttransport direction (Morton et al., in press). This change is less wellmarked in the Murzuq Basin, suggesting this area was more distalto the supply of high-RuZi detritus, and was diluted by locally-sourced low-RuZi material. This may also account for the differ-ence in rutile compositions between the Akakus Formation of thetwo areas, although the rutile data set from the Akakus Formation(especially in the Kufra Basin) is small and requires substantiation.The Dalma Formation and Nubian Sandstone samples in the KufraBasin also have similar RuZi values, but the Tadrart Formation hasgenerally lower RuZi in the Murzuq Basin than in the Kufra Basin.

The influx of clinopyroxene (derived from alkaline, non-oro-genic mafic or felsic sources) seen in the Melez Shuqran and Tane-zzuft Formations in the Murzuq Basin is also present in the KufraBasin, although here, clinopyroxene abundances are lower andgeochemical data are not available. Although it is possible thatlocal reworking from the Precambrian was responsible for the in-flux of clinopyroxene, the close coincidence of the appearance ofcommon clinopyroxene and the increase in RuZi at or close tothe base of the Tanezzuft Formation suggests both events mayhave regional significance.

Rutile compositions in the Hasawnah–Akakus formationsinterval indicate that although both regions contained similarabundances of metapelitic and metamafic lithologies, the meta-morphic grade of the Kufra Basin source was somewhat lower thanthat supplying the Murzuq Basin, since lower amphibolite faciesrutiles are more abundant in the former region. Tourmaline dataalso indicate differences in source, with the Murzuq Basin contain-ing populations richer in the Type D component (sourced from Al-rich metasediments). It is therefore evident that heavy mineraldata provide useful evidence for differences in provenance bothregionally and stratigraphically.

Finally, we may speculate about the cause of the three key prov-enance events identified within the Palaeozoic–Mesozoic succes-sion of the eastern margin of the Murzuq Basin. The change inprovenance at the base of the Tanezzuft Formation may be relatedto the final pulse of the Late Ordovician–Early Silurian Gondwanaglaciation. Glaciers could have cut deeply into the substrate ofthe hinterland, thereby accessing new material. Alternatively, gla-ciers and melt water could have brought new material from otherareas of central Gondwana toward the North Gondwana margin. Anorthwestward-prograding sandy deltaic system (‘‘Akakus Forma-tion’’) (e.g., Craig et al., 2008) brought material from the SE of theMurzuq and Kufra basins during the Silurian. The change in prov-

enance at the base of the Tadrart Formation coincides with theestablishment of the ‘‘Caledonian’’ unconformity (Fig. 3), whichwas caused by a major eustatic see-level fall during the latest Silu-rian/Early Devonian (Craig et al., 2008). Pre-Tadrart Formation sed-iments have locally been eroded. Furthermore, river systems mayhave changed their location in the hinterland, accessing new mate-rial that was then transported toward the North Gondwana mar-gin. The change in provenance at the base of the Mrar Formationmay be related to the Late Devonian and Early Carboniferous glaci-ation, which caused erosion and sediment supply of lower amphib-olite facies material from central Gondwana.

Acknowledgments

We are very grateful to Ramadan Aburawi, Ahmed I. Asbali,Bourima Belgasem and Faraj Said for their scientific support andguidance during the project work in Libya. We also would like tothank the logistics team, provided by Bashir Grenat, for assistancein the field. The administrative and logistical support of staff at theLibyan Petroleum Institute is gratefully acknowledged. The consor-tium of subscribing oil and gas companies is thanked for its finan-cial support to the CASP Southern Basins of Libya Project.

Appendix A. Supplementary material

Supplementary data associated with this article can be found, inthe online version, at doi:10.1016/j.jafrearsci.2011.08.005.

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