J. Dostal (Y) 7 J. V. OwenDepartment of Geology, Saint Mary’s University, Halifax,Nova Scotia B3H3C3 Canada

R. CabyLaboratoire de Tectonophysique, Universite de Montpellier II,Sciences et Techniques, F-34095 Montpellier Cedex 05, France

C. DupuyCentre Geologique et Geophysique, Universite de MontpellierII, Sciences et Techniques, F-34095 Montpellier Cedex 05,France

C. MevelLaboratoire de Petrologie, Universite P. et M. Curie,4 place Jussieu, F-75252 Paris Cedex 05, France

Geol Rundsch (1996) 85 :619–631 Q Springer-Verlag 1996

ORIGINAL PAPER

J. Dostal 7 R. Caby 7 C. Dupuy 7 C. MevelJ. V. Owen

Inception and demise of a Neoproterozoic ocean basin:

evidence from the Ougda complex, western Hoggar (Algeria)

Received: 13 March 1995 / Accepted: 26 March 1996

Abstract The Neoproterozoic Ougda magmatic com-plex occurs within platformal carbonate rocks in thewestern part of the Pan-African fold belt of the Tuaregshield (NW Africa). It is composed of F800 Ma old,relatively high P–T (i.e., GrtcCpx-bearing: P`5 kbar;TF900 7C), tholeiitic mafic/ultramafic cumulates andrelated rocks intruded by intermediate to mafic calc-alkali plutons (e.g., CpxcHbl-bearing gabbro) anddikes. Apparent contrasts in structural level of crystalli-zation indicate that the calc-alkali rocks are significant-ly younger than the tholeiites, which temporally corre-late with a period of regional extension in this part ofAfrica. Intrusion of the calc-alkali rocks may have oc-curred during the formation of an arc after the tholeiit-ic rocks had been (diapirically?) emplaced within theshelf carbonates, and prior to (`630 Ma) the Pan-Afri-can orogeny. Data reported herein indicate that theOugda complex records the inception and demise of aNeoproterozoic ocean basin. Similar crustal sectionshave been described from collisional (e.g., Aleutian is-lands) and extensional (e.g., Ivrea-Verbano zone) set-tings, indicating that processes operating in both envi-ronments can generate nearly indistinguishable igneoussuites; the prevalence of shallow-level calc-alkali rocksin both settings may mask the presence of more mafic,tholeiitic rocks at depth.

Key words Pan-African 7 tectonics 7 geochemistry 7crustal extension 7 island arc tholeiite 7 calc-alkaligabbro

Introduction

The reconstruction of ancient tectonic settings relies ondetailed knowledge of the stratigraphy and lithology ofthe rocks in question, their geochemistry, and relativeand absolute age relationships. Much work has beendone on the tectonomagmatic significance of volcanicrocks (e.g., Pearce and Cann 1973) in particular. De-tailed investigations (e.g., Gass et al. 1984) of ophioliteshave shed light on the character of oceanic crust, andthe geochemistry of different types of mid-ocean ridgebasalt is now well established (e.g., Sun and McDon-ough 1989). Similarly, the calc-alkali character of inter-mediate igneous rocks formed at destructive plateboundaries is nearly a petrological axiom.

Crustal sections through obducted oceanic lithos-phere and magmatic island arcs are important for refin-ing the knowledge of petrogenetic processes at plateboundaries. Although the distinction between magmat-ic sequences characterizing destructive and constructiveplate boundaries is not always clear in ancient terranesdue to structural dismemberment and/or metamorphicoverprinting effects, lithological and geochemical data,coupled with regional tectonic constraints, are useful inevaluating the tectonic significance of igneous suites.

This paper presents these types of data for the Oug-da magmatic complex, in the western part of the Pan-African fold belt of the Tuareg shield (NW Africa).The belt in the Ougda region contains numerous dikes,sills, laccoliths, and diapirs of gabbros and ultramaficrocks which were emplaced into a collapsed and thin-ned continental area. This area was also the site of vo-luminous calc-alkali volcanism during the Neoprotero-zoic (Caby 1983, 1987). Careful field observations, cou-pled with lithological data, indicate that three genera-tions of rocks are present. The two youngest suites have

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calc-alkali affinities and are approximately coeval; theyapparently are significantly younger than the oldestsuite, which (re)crystallized under high P–T conditions,and has been dated, on the basis of its thermal meta-morphic effects, at ca. 793 Ma (Clauer 1976), corre-sponding to a time of regional extension and wide-spread magmatic activity in this part of Africa (Caby etal. 1980; Bertrand-Sarfati et al. 1987). The Ougda com-plex is therefore interpreted herein to record the incep-tion and demise of a Neoproterozoic ocean basin.

Geological setting

The study area belongs to the Neoproterozoic Pan-African belt (trans-Saharan segment; Fig. 1). It repre-sents one of several displaced terranes distinguished byBlack et al. (1994) in the Tuareg shield. This western-most terrane exposed east of the main Pan-African su-ture displays an active continental margin affinity, asexemplified by the thick accumulation (`10 km) ofpre- to synorogenic graywackes derived from penecon-temporaneous andesitic volcanoes (Caby et al. 1977;Chikhaoui et al. 1980). This evolution started approxi-mately 800 Ma ago, with the opening of a pre-Pan-Afri-can ocean to the west, and was followed by the devel-opment of an intraoceanic arc (Fig. 1) between 730 and630 Ma (Caby et al. 1989; Dostal et al. 1994) and by theformation of obduction/subduction-related ultra-high-pressure metamorphic rocks with preserved coesite(Caby 1994). Plate collision approximately 600 Ma agobetween the Tuareg shield and the west African cratonresulted in the emplacement of nappes at least one ofwhich (the Tassendjanet nappe; Caby 1983) is exposedin the study area. This nappe contains variably meta-morphosed rocks ranging from those which lack a slatycleavage to amphibolite facies rocks cut by synkinemat-ic plutons. The Tassendjanet nappe displays a well-pre-served inverted limb represented by shelf-type carbon-ates (`4000 m thick) of the “Série à stromatolites”(F1 Ga old) undetached from its `2.1 Ga old Ebur-nean basement.

The Ougda mafic-ultramafic plutonic complex(Fig. 2) is an elongated body (60!15 km) which in-truded the stromatolitic carbonates (Caby 1983). Con-tact metamorphism of metasediments close to thickgabbroic sills associated with the Ougda complex hasbeen dated at 793B32 Ma (Rb–Sr whole-rock age;Clauer 1976). This result probably represents the em-placement age of the complex.

The complex crops out on the eastern side of astrike-slip fault with approximately 45 km of left-lateraloffset which partly controlled the emplacement of theoverlying calc-alkali volcanic suite (Chikhaoui et al.1980) which rests unconformably on the eroded part ofthe plutonic complex. The lack of a positive gravimetricanomaly in the area (Bayer and Lesquer 1978) impliesthat the complex is a shallow structure (~2 km deep)and probably part of the Tassendjanet nappe. Large

Fig. 1 Geological map of the western part of the Tuareg shield.The darker field on the inset of Africa represents the Tuaregshield

pendants (100–1000 m) of limestones, quartzites, andshales are enclosed within the complex. Contact meta-morphism around ultramafic bodies produced high Tmetamorphic mineral phases such as diopside, amphi-boles, and forsterite in marbles over a distance of sev-eral meters. Superimposed regional Pan-African meta-morphism in the area is low (chlorite-) grade.

The Ougda complex is composed of three successivegenerations of magmatic rocks. The first generation in-cludes ultramafic rocks cut by dikes of garnet-bearinggabbro, a minor garnetiferous aplite, and quartz dioritesheets; the second generation encompasses undeformedgabbros and related rocks, whereas the third genera-tion consists of intermediate to basic dikes which wereductilly deformed during emplacement. The Late Pro-terozoic volcanic rocks in this part of the westernbranch of the Pan-African fold belt form two com-plexes: Tassendjanet and Gara Akofou. The Tassendja-net volcanic complex is up to 6000 m thick (Chikhaouiet al. 1978), and is exposed in an area of more than

621

Fig. 2 Geological map of the Tassendjanet-Ougda area. Thebasement refers to the Eburnian crystalline basement (`2.1 Gaold). (Modified after Caby 1970)

500 km2(Fig. 2). It consists of rocks which resemble re-cent calc-alkali suites of continental margins containingpredominantly andesitic types. The Gara Akofou vol-canic centre (Chikhaoui et al. 1980) outcrops ca.100 km south of the study area. The center, up to2000 m thick, is composed mainly of basaltic andesitesand andesites which also resemble modern continentalmargin calc-alkali suites (Chikhaoui et al. 1980).

Petrography

First generation

Although extensively serpentinized and carbonatized,the ultramafic rocks locally preserve relics of the prima-ry mineral assemblages; many of these rocks are cumu-lates. Cumulus olivine (FFo81) typically is rimmed byorthopyroxene (Wo1En81Fs18). Clinopyroxene and am-phibole are the intercumulus phases. In some samplescolorless to pale green amphibole is abundant(~40 vol.%). The amphibole (Figs. 3 and 4) variesfrom pargasite to tschermakite to magnesio-hornblende(Leake 1979). In some rocks the intercumulus clinopy-

Fig. 3 Tetrahedral Al (AlIV) vs (NacK) diagram (formula units;per 23 oxygens) for amphiboles from the rocks of the Ougda com-plex. Stars K 530 (ultramafic rock, first generation); full squares045D (garnet-bearing gabbro, first generation); empty squares4675 (gabbro, first generation); asterisks 4659 (garnet-bearinggabbro, first generation); full circles 4742 (metamafic rock, firstgeneration); triangles 4654 (amphiclasite, first generation); dia-monds 4744 (gabbro, second generation). Ts, Hb, and Pg respec-tive positions of tschermakite, hornblende, and pargasite endmembers

Fig. 4 Ca – (Fe*cMn) – Mg diagram (formula units) for the rep-resentative amphiboles (per 23 oxygens) and pyroxenes (per 6oxygens) of the rocks of the Ougda complex. Fe* total Fe asFe2c. Empty symbols amphiboles; Solid symbols clinopyroxene.Stars K 530 (ultramafic rock, first generation); squares 045D (gar-net-bearing gabbro, first generation); triangles 4675 (gabbro, firstgeneration); diamonds 4659 (garnet-bearing gabbro, first genera-tion); inverted triangles 4742 (metamafic rock, first generation);circles 4744 (gabbro, second generation); asterisk (amphiboleonly): 4654 (amphiclasite, first generation); cross (clinopyroxeneonly): 4656 (garnet-bearing gabbro, first generation). Solid curvedelineates the field of amphiboles from recent calc-alkali volcanicrocks (Ewart 1982), whereas the dashed curve outlines the field ofclinopyroxene from cumulate xenoliths of island arc lavas (Con-rad and Kay 1984)

622

roxene (I) with spinel exsolutions recrystallized partlyto granular and symplectitic assemblages includingdiopsidic clinopyroxene (II), colorless amphibole, andgreen Cr-poor spinel (XMgF0.59). This subsolidus re-equilibration occurred without deformation and was,according to petrographic observation, contempora-neous with the appearance of coronal garnet in the gab-broic dikes. Some serpentinites exhibit rare schlieren offine-grained Cr-poor spinel (~0.15% Cr2O3) which re-semble those from the late pyroxenite dikes of the Lan-zo peridotite massif (Boudier 1976; Bodinier et al.1986) and the ultramafic xenoliths from alkali basalticrocks (BVTP 1981; Dick et al. 1984).

Garnet-bearing gabbros and amphiclasites occur asirregular contorted dikes (up to 50 cm thick) within theultramafic rocks. Most of the garnet-bearing rocks con-sist of clinopyroxene, amphibole, plagioclase (labrado-rite-bytownite), pyrope-rich garnet, rutile and accesso-ry apatite, sphene and opaques. In some rocks no pri-mary clinopyroxene is present, whereas in others clino-pyroxene is enclosed by large amphibole crystals. Themodal composition of these rocks is highly variableeven within a single dike, and ranges from anorthositeto melanocratic amphiclasite. Some dikes are symmetri-cally zoned with a melanocratic border, whereas othersexhibit an agmatitic texture. A planar tectonic structureis also common, but many dikes do not show any pre-ferential mineral orientation. The gabbros and amphi-clasites are frequently cut by younger veinlets of low-temperature minerals (prehnite, clinozoisite, tremolite,chlorite) which likely formed during the Pan-Africanmetamorphic event.

The garnet is unevenly distributed in the maficrocks. It occurs usually as a rim at the contact betweenamphibole and plagioclase or as large poikilitic crystalsenclosing both plagioclase and amphibole. However,some garnetiferous rocks display an equigranular, uno-riented texture with no evidence of the garnet-formingreaction. In other rocks secondary clinopyroxene (II)occurs at the contact with garnet; texturally, bothphases appear to have equilibrated. Textural relation-ships indicate that the primary magmatic assemblage ofthe garnetiferous rocks contained plagioclase (An85),clinopyroxene (I), and amphibole; subsolidus recrystal-lization led to the formation of garnet, clinopyroxene(II), and possibly rutile.

It is unclear whether or not coexisting amphibole isa magmatic phase, or formed during a subsequent re-crystallization event. The amphibole is a high tempera-ture variety, as shown by its high AlIVand alkali con-tent (Fig. 3; cf., Veblen and Ribbe 1982). Garnet alsohas a variable composition with a pyrope componentranging between 28 and 58 mol.% (Fig. 5). Amphibole,garnet and clinopyroxene have variable Mg/Fe ratioswhich probably reflect the bulk rock composition. As-suming a pressure of 5–10 kbar, coexisting clinopyrox-ene (II) and garnet in gabbroic samples 4656 and 4659(see Appendix) give temperatures (860–920 7C) consis-tent with (re)crystallization in a high-grade environ-

Fig. 5 Composition of garnets from the rocks of the Ougda com-plex. Al-Sp almandinecspessartine; An-Gr andraditecgrossular-ite; Py pyrope end-members. Full circles 4656 (garnet-bearinggabbro, first generation); full stars 4659 (garnet-bearing gabbro,first generation); empty stars 045D (garnet-bearing gabbro, firstgeneration)

ment. A similar temperature (F860 7C) was deter-mined for a two-pyroxene metamafic rock (4742) fromthe complex.

Amphibole-rich rocks are associated with marbles.These rocks are composed mainly of fine-grained mag-nesio-hornblende. In some samples amphibole is em-bayed by plagioclase (An98) enclosing green spinel andopaques and large poikilitic olivine crystals (Fo71).

Quartz gabbro and diorite form elongated sheetswithin the complex. Most of the samples display cata-clastic texture and are composed of amphibole, plagio-clase, and subordinate amounts of quartz and minor K-feldspar. They may also contain relics of clinopyroxeneand orthopyroxene.

Second generation

The second generation rocks are composed of the un-deformed amphibole gabbro and diorite which makeup the bulk of the complex (Fig. 2). These rocks have across-cutting relationship with respect to the first gener-ation rocks. The rocks are coarse-grained, heterogene-ous, and display an agmatitic or ophitic texture. Thegabbro contains accessory opaques and apatite. Scarcebiotite is present in the dioritic rocks. Clinopyroxeneand amphibole analyzed in one gabbro sample (4744)are similar to those occurring in the rocks of the firstgeneration. The rocks of the second generation do notcontain, however, any high-grade minerals and were af-fected only by low-grade metamorphism.

623

Gabbro and diorite seem to pass gradually into morefelsic rocks characterized by a miarolitic porphyritictexture suggesting a high level of emplacement. Thesedifferentiated rocks contain antiperthitic albite and mi-crocline. Secondary minerals such as chlorite, calcite,and prehnite are ubiquitous.

Third generation

The third generation suite is represented by numerousdikes of foliated, amphibolitized intermediate to maficrocks which cut the first and second generation assem-blages. The rocks contain Ca-rich plagioclase and horn-blende phenocrysts. The groundmass consists of plagio-clase, biotite, Fe–Ti oxide and apatite. Some dikes dis-play an internal foliation oblique to that in the wallrocks. The petrography suggests that deformation inupper amphibolite facies conditions was restricted tothe dikes indicating a syn- to late-magmatic shearing ofa still not completely consolidated magma. No chilledmargin has been observed. These dikes probably repre-sent deeply exposed feeder dikes of a former volcanicedifice which has been eroded prior to extrusion of theyoungest andesites of the Tassendjanet zone (Chik-haoui et al. 1978, 1980).

Geochemistry

Analytical notes and alteration

The major elements were determined by rapid wetmethods. Cr, V, Ni, Co, Cu, Zn, Li, Rb, Sr, and Ba wereanalyzed by atomic absorption and Zr, Nb, and Y byX-ray fluorescence. Rare-earth elements (REE), Sc,Th, and Hf were analyzed by instrumental neutron acti-vation (Table 1). The precision and accuracy of theanalytical methods were reported by Dostal et al.(1986). In general, the precision of the trace elementdata is better than 5–10%. Mineral compositions weredetermined using a CAMECA microprobe at the La-boratoire de Petrologie, Universite de Paris VI usingnatural minerals as standards. Data were reduced usingthe EMPADR VII corrections.

The rocks were affected by secondary processes, in-cluding regional metamorphism, which led to selectivechemical modifications. In order to minimize the effectof the alteration processes, the discussion is basedmainly on trace elements, including high-field-strengthelements (HFSE) and REE, which are considered rela-tively immobile (e.g., Winchester and Floyd 1977).

Rocks of the first generation

The rocks of the first generation (with the exception ofplagioclase cumulates) display an iron enrichmenttrend on several geochemical plots (e.g., AFM diagram,

Fig. 6 A (Na2OcK2O)-F (FeOt)-M (MgO) diagram for the rocksof the Ougda complex. The dividing line between the tholeiiticand calc-alkali fields after Irvine and Baragar (1971). Stars rocksof the first generation; crosses plagioclase cumulates of the firstgeneration; solid diamonds rocks of the second generation; emptydiamonds rocks of the third generation

Fig. 6), as is characteristic of tholeiitic suites. The petro-graphy of most first generation rocks suggests that theyare cumulates. They are depleted in K, P, and mostlarge-ion-lithophile elements (LILE). The chemicalcomposition of the rocks (Table 1) correlates with thecumulus mineralogy. Three types of cumulates can berecognized according to petrography, Al2O3and MgOcontents and [Mg] values (Mg/[MgcFe2c] with Fe2c/[Fe3ccFe2c] assumed to be 0.85) :1. Ultramafic cumulates characterized by high [Mg],

MgO, Cr, and Ni and low Al2O3(Table 1)2. Plagioclase cumulates with high Al2O3, positive Eu

anomaly, and low MgO, FeOt, Cr, and Ni contents3. Clinopyroxene and/or amphibole cumulates which

contain nepheline in their norms, but differ from al-kali basalts by their lower K, Ti, P, and Zr, and high-er Al contents. Compositionally, they resemble theejected plutonic inclusions from volcanic arc rocks ofthe Lesser Antilles, which are considered to be cu-mulates crystallized under high water pressure froma fractionating basalt magma (Lewis 1973; Dostal etal. 1983), as well as the mafic/ultramafic complexfrom the Ivrea-Verbano zone, which contains cumu-lus peridotites and garnetiferous gabbros (e.g., Sinig-oi et al. 1994). All these Ougda cumulate rocks dis-play a wide range of transition element contentswhich vary according to their [Mg] values. With[Mg] decreasing from 0.87 to 0.70 in ultramafic andplagioclase cumulates, Cr and Ni decrease, whereasTi and V increase. Simultaneously, the Ni/Co andCr/V ratios drop from 8.3 to 1.3 and from 18 to 0.3,respectively. These variations indicate that the ear-liest cumulates precipitated from a liquid which wasprogressively depleted in Cr and Ni and enriched inTi and V, thus suggesting the fractionation of botholivine and clinopyroxene. Some plagioclase cumu-

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6010

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02.

460.

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451.

600.

25

UM

C –

ult

ram

afic

cum

ulat

e; P

LG

C –

pla

gioc

lase

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h cu

mul

ate;

CA

C –

clin

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d/or

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phib

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rich

cum

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625

lates have high [Mg], Cr/V, and Ni/Co ratios imply-ing that feldspar was also an early crystallizingphase.The clinopyroxene-amphibole gabbros have [Mg]

between 0.65 and 0.55 and may be divided into twogroups according to the Cr/V and Ti/V ratios. Thegroup with higher Cr/V and lower Ti/V ratios also hashigher V and SiO2contents due probably to the higherproportion of clinopyroxene and/or magnetite. Thelarge range of [Mg] values associated with regular var-iations of transition element contents suggests that thecumulates were formed at various stages of differentia-tion.

The cumulates display two distinct types of chon-drite-normalized REE patterns which resemble the pat-terns of mafic plutonic rocks of ophiolitic complexes(e.g., Suen et al. 1979; Coish and Rogers 1987; Harnoisand Morency 1989) and gabbroic rocks of modernoceanic environments (e.g., Dostal and Muecke 1978;Dietrich et al. 1978). The first type (e.g., samples 4675and 4651) is typical of plagioclase cumulates, character-ized by a distinct positive Eu anomaly and low REEabundances (Fig. 7). These compositional features cor-relate with the modal proportions of plagioclase. Thesecond type (e.g., sample 4666; Fig. 7) exhibits a lightREE (LREE) depletion relative to heavy REE(HREE) with La/Yb~1. The analyzed samples differfrom the oceanic and ophiolitic gabbros by their majorelement contents particularly by low SiO2. The lowSiO2and the shape of the REE patterns indicate thatthe rocks with the second type of REE pattern are am-phibole-rich cumulates.

The abundances of other incompatible trace ele-ments in the cumulate rocks are low. Some, such as Zr,increase with the decrease of [Mg] values and, in theevolved samples, Zr contents may attain high values(`300 ppm in sample 4677; Table 1) suggesting a lim-ited accumulation of zircon. Alkali and alkali-earth ele-ments do not show any obvious variation with the [Mg]value. However, the pronounced correlation among K,Rb, and Li, and their relatively high contents, particu-larly of Li, indicates that the abundances of these threeelements were modified by secondary processes.

The dioritic bodies of the first generation have high-er SiO2(F56–58%) concentrations and the lowest [Mg]values and Fe, Mg, and Ca contents. They are also lowin Cr and Ni, which indicates that they underwent ex-tensive fractional crystallization. Their REE patternsdisplay meager LREE enrichment with La/YbF3(Fig. 8). The mantle-normalized trace element patternsof the dioritic rocks (Fig. 9) show negative Nb and Tianomalies.

Rocks of the second generation

Most samples of the second generation have composi-tions typical of high Al basalts and basic andesites (Ta-ble 1). On the AFM (Fig. 6) and Al2O3–(FeOtcTiO2)–

Fig. 7A, B Chondrite-normalized rare-earth element abundancesin the cumulate rocks of the Ougda complex. A Clinopyroxene-amphibole gabbro of the first (solid line; samples 4659 and 4666)and second (dashed line; sample 4744) generations. B Plagioclase-rich gabbro of the first generation. Normalizing values after Sun(1982)

Fig. 8A–C Chondrite-normalized REE abundances in the noncu-mulate rocks of the Ougda complex. A Diorites of the first gener-ation; B Basaltic rocks of the second generation; dashed lines de-lineate the field of calc-alkali volcanic rocks of the Gara Akofoucomplex (Chikhaoui et al. 1980); C Basalts of the third genera-tion. Normalizing values after Sun (1982)

626

Fig. 9A–D Mantle-normalized trace element abundances in thenoncumulate rocks of the Ougda complex. A Diorites of the firstgeneration; B Basalts and andesites of the second generation; CBasaltic rocks of the third generation; D Late Proterozoic vol-canic rocks of the Tassendjanet and Gara Akofou zones (T aver-age of Tassendjanet andesites of Chikhaoui et al. 1978, 1980; B,Aaverages of basaltic and andesitic rocks of the Gara Akofou com-plex, respectively, after Chikhaoui et al. 1980). Normalizing val-ues after Sun and McDonough (1989)

MgO diagrams, these rocks plot in the calc-alkali field.Their calc-alkali character is also corroborated by thelow contents of Ni and Cr and the decrease of Fe, Ti,and V with increasing differentiation. The rocks havefractionated REE patterns with La/Yb ratios increasingfrom 6 in basalts up to 18 in andesites. In their REEpatterns and contents they resemble the continentalmargin calc-alkali rocks and the younger, overlying an-desitic suite outcropping in close proximity (Figs. 8 and9; Chikhaoui et al. 1978, 1980). These samples also haverelatively high contents of K and related trace elements

(Rb, Ba, and Sr) which are similar to those of calc-alkali volcanics emplaced on continental crust (Ewart1982). Because they appear to be fresh and are devoidof any high-grade minerals, the enrichment of LILEseems to be a primary magmatic feature. Their mantle-normalized trace element patterns (Fig. 9) also exhibitfeatures typical of continental margin calc-alkali vol-canic rocks, particularly a negative slope with progres-sive increase from Lu to LILE accompanied by a nega-tive Nb anomaly. The remaining rocks of the secondgeneration are cumulates (e.g., 4744; Fig. 7) with geo-chemical features of the clinopyroxene-amphibole gab-bros or plagioclase gabbros.

Rocks of the third generation

The cross-cutting dikes of the third generation havecompositions comparable to high Al basalts from orog-enic zones and display a calc-alkali trend in the AFMdiagram (Fig. 6). They have a slight LREE enrichmentwith La/Yb~6. Compared with equivalent rocks of thesecond generation, these samples have on average low-er contents of strongly incompatible trace elementssuch as light REE. Their mantle-normalized patterns(Fig. 9) are similar to those of the rocks of the secondgeneration and the calc-alkali rocks of the continentalmargins. They exhibit a gradual increase from Lu toLILE and a negative Nb anomaly.

Discussion

Among the noncumulate rocks, the first generation dio-rites with tholeiitic affinities have the lowest lithophileelement contents accompanied by a relatively flat REEpattern. These rocks resemble both modern island arctholeiites and within-plate continental tholeiites (BVTP1981). The rocks of the second and third generationshave significantly higher abundances of lithophile ele-ments and have typical calc-alkali characteristics(BVTP 1981). The large compositional differencesamong the three generations of the noncumulate rocks,particularly in the abundances of lithophile elements,imply that the tholeiitic and calc-alkali sequences werederived from different sources.

In addition to geological observations, several geo-chemical features suggest a petrogenetic relationshipbetween cumulate and noncumulate rocks in each ofthe studied suites. Both rock types of each generationtypically have similar element ratios such as Ti/V andZr/La. Furthermore, the nature of the magmatic miner-al phases present in the cumulates are in agreementwith the composition of the related noncumulate rocks.Olivine fractionation has frequently been invoked toexplain the low Ni and low Ni/Co ratios which charac-terize many mafic rocks. The presence in the Ougdacomplex of ultramafic cumulates rich in olivine is con-

627

sistent with such a process. Likewise, the pronouncedCr/V variation in noncumulate rocks suggests fractiona-tion of clinopyroxene, an abundant mineral in the cu-mulates. The fractionated HREE pattern in some non-cumulate rocks may result from the crystallization ofclinopyroxene. Amphibole probably also played a roleduring the evolution of these rocks.

The composition of the rocks of the second andthird generations is similar to that of the Proterozoiccalc-alkali volcanic suite which was extruded on top ofthe complex (Chikhaoui et al. 1980). For a given rocktype, the abundances of several trace elements and thecorresponding element ratios such as Zr/La, Ba/La, Ti/V, and K/Rb are about the same in both volcanic andsubvolcanic rocks. Likewise, they display similar man-tle-normalized trace element patterns (Fig. 9). The non-cumulate dikes of these two generations plausibly rep-resent feeders to an overlying, now-eroded volcanicpile which predated the younger Tassendjanet ande-sites. These eroded volcanics occur as volcanic gray-wackes (Fig. 2) which have composition comparable tothe Tassendjanet volcanic rocks (Caby et al. 1977).

The cumulates of the second generation are proba-bly related to the evolution of the parental magma forthese volcanic rocks suggesting that fractional crystalli-zation was an important process during the evolution ofthe volcanic suite. Least-squares mixing calculations in-dicate that the compositional variations observed in theNeoproterozoic andesite suite of Gara-Akofou (Chik-haoui et al. 1980) can be produced by the separation ofthe cumulate rocks. The dacites could have been de-rived from the andesites through fractionation of mi-neral phases which constitute the cumulate rocks in theproportions represented by the clinopyroxene-amphi-bole gabbros (Table 2). These results are consistentwith the abundances of trace elements such as transi-tion elements, Zr, and REE, and suggest that andesiticsuites of orogenic zones generate significant amounts ofgabbroic and pyroxenic cumulate rocks in the crust.

All of the analyzed Ougda complex rocks reportedhere display negative Nb anomalies on mantle-normal-ized plots (Fig. 9). This classical “subduction signature”does not contradict the interpretation (reported below)that the tholeiitic rocks of the first generation are re-lated to an extension. Instead, it simply suggests thatthese rocks formed very early in the history of theocean basin in which they formed, soon after thinningof continental lithosphere. The involvement of subcon-tinental lithosphere in the source can account for as-pects of their trace element signatures.

The sequences of rocks produced early and late inthe evolution of this ocean basin collectively resemblecross sections of both extension- (e.g., Ivrea-Verbanozone; Quick et al. 1994) and subduction- (e.g., Aleutianislands; Kay and Kay 1994) related environments. Forexample, deep structural levels of the Ivrea-Verbanozone consist of cumulus peridotites and garnetiferousgabbro with tholeiitic characteristics; these rocks dis-play evidence of synmagmatic deformation and under-

Table 2 Least-Squares Mass Balance Calculations: FractionalCrystallization Models Relating Andesite to Dacite

Parent Daughter CumulatesOugda complex

Obs Obs Calc C

SiO2(%)TiO2

Al2O3

FeOt

MnOMgOCaONa2OK2OP2O5

61.151.05

16.206.770.112.935.333.452.720.29

65.750.87

16.034.320.011.532.764.394.082.28

66.180.99

15.275.740.101.272.683.933.480.37

44.561.23

19.2710.160.188.39

14.091.870.200.05

Subtracted assemblage

CLiquidAR2

23.276.83.476

Least-squares calculations: ParentpDaughter (liquid)csub-tracted assemblage (the average composition of cumulates C).Obspobserved composition; Calcpcalculated composition; Sub-tracted Assemblagepcalculated proportions of the subtracted as-semblages (cumulates C); AR2psum of squared residuals; Li-quidpresidual liquid; Cpaverage composition of clinopyroxene-amphibole-rich gabbroic cumulates of the second generation with[Mg]~0.65, compositions of andesite (Parent-Obs) is the averageof this rock type from the Gara Akofou Complex (Chikhaoui etal. 1980), whereas the dacite composition (Daughter-Obs) is theaverage of dacites from the Tassendjanet Complex (Chikhaoui etal. 1978)

lie less deformed, coeval calc-alkali rocks. Although inthis case the trend toward calc-alkali compositions hasbeen attributed to contamination processes (Voshageet al. 1990; Sinigoi et al. 1994), rather than the diach-ronous evolution of an ocean basin, lithological, defor-mational, and geochemical similarities with the Ougdacomplex are striking. The possible role of contamina-tion in the Ougda complex is difficult to evaluate in theabsence of isotopic data; however, temporal differencesbetween the tholeiites and calc-alkali suites and similar-ities of the latter rocks to the nearby volcanic rocks sug-gest that the Ougda calc-alkali rocks are subduction-re-lated. Chikhaoui et al. (1978, 1980) also argued that theoverlying volcanic rocks were not significantly affectedby crustal contamination.

The Ougda magmatic sequence and overlying LateProterozoic volcanics also resemble Phanerozoic (Ton-sina: DeBari and Coleman 1989; central Mexico: La-pierre et al. 1992), and Proterozoic (Tilemsi: Dostal etal. 1994) island arc sections. These arcs evolved pro-gressively by repeated magmatic events. The older is-land arc tholeiites are superceded by younger and rela-tively evolved calc-alkali bodies with significantly high-er and more fractionated abundances of incompatibletrace elements. The transition reflects the increasinggrowth and maturity of the arc, which evolved throughtime from a relatively primitive to an evolved arc sys-

628

Fig. 10A–C Geodynamicalmodel for the evolution of theOugda complex and adjacentarea. Stage 1 :A-1 Continentalextension in the Tassendjanet(Ougda) area (ca. 800 Maago). Faulting of carbonates ofthe Série à stromatolites(F1 Ga old). Crustal thinningand associated high heat flowwas related to emplacementand evolution of tholeiitic bas-alt magma (first generation)derived from a mantle diapir.The magma produced gabbro-ic bodies, ultramafic cumu-lates, and volcanic complexes.A-2 Section showing the rela-tionship of the Série à stroma-tolites and deep-sea equival-ents in nearby subsiding areas(F100 km east of the Ougdacomplex; Moussine-Pouchkineet al. 1988). The deep-sea se-diments unconformably over-lie a Mesoproterozoic rift-re-lated sequence composed ofquartzites hosting 1.8-Ga-oldbasalts and rhyolites. B Tec-tonic emplacement of firstgeneration mafic/ultramaficrocks into the platformal car-bonate rocks. Stage 2 :C Es-tablishment of a west-dippingsubduction zone led to theconsumption of oceanic lithos-phere, and formation of arc-related mafic to intermediateplutons (second generation)and subvolcanic rocks andflows (third generation)

tem. The evolutionary trends are toward more incom-patible element-rich rocks.

Similarities with magmatic rocks from the Ivrea-Ver-bano zone and the Aleutian islands and other arc sys-tems imply that petrogenetic processes operating atboth extensional and convergent settings can generateremarkably similar sequences of rocks which may onlybe recognized if relatively complete sections are ex-posed. The prevalence of intermediate rocks at shallowcrustal levels above subduction zones may mask thepresence of related mafic rocks at depth.

Tectonic interpretation

Central to the interpretation of the Ougda complex isthe relationship between the different generations of

magmatic rocks. The second and third generation rocksare considered to be closely spaced in time, because an-desite dikes lack chilled margins next to their gabbroichosts. However, they both are probably significantlyyounger than the first generation rocks. The first gener-ation suite (re)crystallized in a high P–T (`5 kbar,F900 7C) environment, and is clearly crosscut by theyounger rocks. These observations suggest that the sec-ond and third generation rocks are temporally and tec-tonomagmatically unrelated to the first generation as-semblage. The tholeiitic character of the early rocks, to-gether with their formation during regional extensionat ca. 800 Ma, suggests that they are related to crustalthinning. In contrast, the calc-alkali character of theyounger rocks suggests that they formed at a destruc-tive plate boundary. These rocks are compositionallysimilar to voluminous Neoproterozoic calc-alkali vol-

629

canic rocks from this region. Although undated, thecalc-alkali rocks might have intruded at ~730 Ma agoduring formation of an arc, after the first generationrocks had been uplifted and (diapirically?) interleavedwith the shelf carbonates.

The data reported here, coupled with regional con-siderations (Black et al. 1994; Caby 1983; Moussine-Pouchkine et al. 1988), allow the establishment of athree-stage geodynamic interpretation of the Ougdacomplex and affiliated rocks (Fig. 10).

Stage 1

Stage 1 involved continental extension, thinning, andbreakup accompanied by a magmatic episode whichformed rocks of the first generation subsequently em-placed into the Série à stromatolites (Fig. 10A and B).This episode was probably a part of the widespread ex-tension-related magmatic activity in northwestern Afri-ca.

Stage 2

Stage 2 involved the subduction of oceanic lithosphere,accompanied by continental margin calc-alkali magma-tism (both intrusions and volcanic complexes). Themagmatic activity produced rocks of the second andthird generations (Fig. 10C) and their eroded counter-parts (i.e., thick sequences of volcanic graywackes;Fig. 2), as well as volcanic centers such as Tassendjanetand Gara Akofou.

Stage 3

Stage 3 involved the tectonic emplacement of the Tas-sendjanet nappe (not shown in Fig. 10).

Conclusions

The Ougda complex consists of a diachronous assem-blage of magmatic rocks of contrasting tectonic signifi-cance. The earliest rocks are tholeiitic; they includemafic/ultramafic cumulates which crystallized under re-latively high P–T (`5 kbar, F900 7C) conditions (e.g.,they locally contain Grt–Cpx and two-pyroxene assem-blages), and were emplaced into a platformal carbonatesequence at approximately 800 Ma, during a period ofregional extension in this part of Africa. The youngestrocks are calc-alkali; they include gabbroic/dioritic plu-tons and dikes which crosscut the high-grade, mafic/ul-tramafic cumulates, and are themselves intruded by an-desitic dikes. The calc-alkali rocks could be petrogene-tically linked to andesitic and dacitic flows as repre-sented by nearby volcanic graywackes. They are inter-preted to have formed at a destructive plate bound-ary.

Notwithstanding the fact that the earliest rocks ap-pear to be related to crustal extension, all rocks of theOugda complex have a “subduction” geochemical sig-nature (i.e., negative Nb anomalies). The tholeiites aretherefore interpreted to be derived from a source whichcontained a contribution from subcontinental lithos-phere, implying that they formed during initial stages ofcontinental fragmentation. Coupled with the subduc-tion-related characteristics of the younger, calc-alkalirocks, the Ougda complex therefore records the earlydevelopment and destruction of a Neoproterozoicocean basin.

Acknowledgements This study was supported by the CNRS ofFrance, Universite de Montpellier II, and the Natural Sciencesand Engineering Research Council of Canada. We thank RobertStern, Jim Quick, and Suzanne Kay for reviewing the manu-script.

Appendix Mineral Assemblages of Samples Analyzed by Microprobe

Rock type Generation Assemblage

Primary Secondary

K-5304675O45D46564659474246544744

Ultramafic rockGabbroGarnet-bearing gabbroGarnet-bearing gabbroGarnet-bearing gabbroMetamatif rockAmphiclasiteAmphibole gabbro

11111112

Ol, Opx, AmphPlg, Cpx, AmphCpx, Amph, PlgAmph, PlgAmph, Plg

AmphCpx, Amph, Plg

Cpx, green Sp, Amph

GrtCpx, Grt, RuCpx, Grt, RuOpx, Cpx, Amph, PlgPlg, green Sp, Ol

Primary Assemblage – considered to be magmatic. Secondary As-semblage – high T. All rocks contain low T secondary mineralssuch as serpentine (replacing olivine); prehnite (replacing plagio-

clase) etc. Ol – olivine; Opx – orthopyroxene; Cpx – clinopyrox-ene; Amph – amphibole; Sp – spinel; Plg – plagioclase; Grt – gar-net; Ru – rutile; Gen – generation

630

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