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Journal of African Earth Sciences 125 (2017) 27e41

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Journal of African Earth Sciences

journal homepage: www.elsevier .com/locate/ jafrearsci

Geological Society of Africa Presidential Review

An overview on the origin of post-collisional Miocene magmatism inthe Kabylies (northern Algeria): Evidence for crustal stacking,delamination and slab detachment

Gilles Chazot a, *, Fatiha Abbassene a, b, Ren�e C. Maury a, Jacques D�everch�ere a,Herv�e Bellon a, c, Aziouz Ouabadi b, Delphine Bosch d

a Universit�e de Brest, CNRS, UMR 6538 Domaines Oc�eaniques, Institut Universitaire Europ�een de la Mer (IUEM) Place Nicolas Copernic, 29280 Plouzan�e,Franceb Universit�e des Sciences et de la Technologie Houari Boumedienne, Bab Ezzouar, Laboratoire de G�eodynamique, G�eologie de l’Ing�enieur et Plan�etologie(LGGIP/FSTGAT/USTHB), Algiers, Algeriac Universit�e de Brest, CNRS, UMR 6538 Domaines Oc�eaniques, 6 avenue Le Gorgeu, c.s. 93837, 29238 Brest, Franced Universit�e de Montpellier II, CNRS-UMR 5243, G�eosciences Montpellier, Place E. Bataillon, c.c. 060, 34095 Montpellier, France

a r t i c l e i n f o

Article history:Received 7 June 2016Received in revised form7 October 2016Accepted 19 October 2016Available online 19 October 2016

Keywords:Calc-alkaline magmasPost-collisionDelaminationSlab tearingAlgeria

* Corresponding author.E-mail address: Gilles.Chazot@univ-brest.fr (G. Ch

http://dx.doi.org/10.1016/j.jafrearsci.2016.10.0051464-343X/© 2016 Elsevier Ltd. All rights reserved.

a b s t r a c t

Miocene (17-11 Ma) magmatic activity in the Kabylies emplaced K-rich (and minor medium-K) calc-alkaline plutonic and volcanic rocks in five zones, delineating a ~450 km long EW trending strip locatedalong the northern coast of Algeria, between Annaba and Algiers. Their most likely source is the Kabyliansubcontinental lithospheric mantle previously metasomatized during the Paleogene subduction of theTethys oceanic lithosphere. Our preferred tectono-magmatic model involves a Tethyan slab detachmentcombined with African mantle delamination and crustal stacking, leading to the superimposition of theAfrican continental crust over the Kabylian metasomatized lithospheric mantle. At ca. 17 Ma, theasthenospheric upwelling arising from lithospheric delamination and Tethyan slab tear triggered thethermal erosion of the latter mantle, inducing its partial melting. The corresponding mafic medium-Kcalc-alkaline magmas interacted with the African basement units during their ascent, generating inter-mediate to felsic K-rich calc-alkaline melts that display a characteristic trace element and isotopic crustalsignature. Later on, slab tears propagated eastward and westward, promoting slab rollback perpendicularto plate convergence and inducing the emplacement of magmatic rocks of decreasing ages from central-eastern Algeria towards Tunisia and Morocco.

© 2016 Elsevier Ltd. All rights reserved.

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 282. The Maghrebides chain in the Kabylies (northeastern and central Algeria) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

2.1. The Kabylides within the circum-Mediterranean Alpine chains . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 282.2. The Maghrebides Cenozoic magmatic belt . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

3. Spatial and temporal distribution of Miocene magmatism in the Kabylides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 293.1. The Edough-Cap de Fer area (Zone 1) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 303.2. The Bougaroun-Beni Toufout area (Zone 2) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 303.3. The El Aouana area (Zone 3) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 313.4. The Bejaïa-Amizour area (Zone 4) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 323.5. The Th�enia-Dellys area (Zone 5) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

4. Petrologic and geochemical features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 324.1. Whole-rock geochemistry: major elements and petrographic types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

azot).

mailto:Gilles.Chazot@univ-brest.frhttp://crossmark.crossref.org/dialog/?doi=10.1016/j.jafrearsci.2016.10.005&domain=pdfwww.sciencedirect.com/science/journal/1464343Xwww.elsevier.com/locate/jafrearscihttp://dx.doi.org/10.1016/j.jafrearsci.2016.10.005http://dx.doi.org/10.1016/j.jafrearsci.2016.10.005http://dx.doi.org/10.1016/j.jafrearsci.2016.10.005

G. Chazot et al. / Journal of African Earth Sciences 125 (2017) 27e4128

4.2. Trace elements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 334.3. Sr and Nd isotopes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

5. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 335.1. A typical “subduction-related” geochemical signature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 335.2. Constraints on the origin of mafic magmas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 335.3. Intermediate and felsic magmas: evidence for crustal contamination by the African crust and local anatexis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 355.4. Tectonic framework of magma genesis and emplacement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 365.5. Tectono-magmatic model involving African slab breakoff/tearing and lithospheric mantle delamination . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

6. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

1. Introduction

The western Mediterranean domain experienced an especiallycomplex Cenozoic tectonic evolution (including the eastward andsoutheastward retreat of the Apennine-Calabria trench, and thesouthward and then westward retreat of the Betic trench) coupledwith an unusually diversified igneous activity (Savelli, 2002;Duggen et al., 2005; Lustrino and Wilson, 2007; Lustrino et al.,2011, 2013; Carminati et al., 2012). The origin of the latter isattributed to a complex interplay between geodynamic processesincluding active subduction, slab rollback and detachment, andassociated back-arc basin spreading and crustal thinning (R�ehaultet al., 1984; von Blanckenburg and Davies, 1995; Lonergan andWhite, 1997; Gueguen et al., 1998; Carminati et al., 1998a, b;2012; Jolivet and Faccenna, 2000; Rollet et al., 2002; Mauffretet al., 2004; Faccenna et al., 2004, 2014; Lustrino et al., 2011;Mancilla et al., 2015).

The Maghrebides segment of the circum-Mediterranean Alpinebelt forms a ~1200 km-long EW-trending magmatic lineament thatextends along the southern Mediterranean margin from La GaliteIsland in Tunisia to Ras Tarf in Morocco (Fig. 1). It includes mostlyMiocene K-rich calc-alkaline plutonic and volcanic rocks, andminoryounger (Messinian to Plio-Quaternary) alkali basalts and basanites(Maury et al., 2000; Savelli, 2002; Lustrino et al., 2011; Carminatiet al., 2012). These igneous rocks crosscut and/or overlie both theinner and the outer zones of the Maghrebides or their Africanforeland. Their geological position and ages as well as their petro-graphic and geochemical features are typical of post-collisionalmagmatism (Hernandez and Lepvrier, 1979; Maury et al., 2000).Indeed, the latter is characterized by a considerable geochemicalvariability and often displays temporal trends from calc-alkaline toK-rich calc-alkaline and shoshonitic compositions, usually followedby the emplacement of alkaline magmas (Harris et al., 1986; Turneret al., 1996; Lustrino and Wilson, 2007). The origin of the calc-alkaline signature is generally ascribed to mantle metasomatismby hydrous fluids or melts during an earlier subduction event(Miller et al., 1999; Wang et al., 2004; Chung et al., 2005). However,the tectonic processes triggering the partial melting of thissubduction-modified mantle after the collision event are debated.Complex combinations of processes involving slab rollback and/orslab detachment and/or lithospheric delamination have often beensuggested (Harris et al., 1986; Pearce et al., 1990;Mah�eo et al., 2002;Chung et al., 2005; Duggen et al., 2005). In the case of the Magh-rebides coastal magmatic belt, the nature of these processes as wellas the geological positions and origins of the mantle and crustalmagma sources are still a matter of debate (Maury et al., 2000;Lustrino et al., 2011; Carminati et al., 2012; Roure et al., 2012).

The aim of this work is to discuss these questions based on thestudy of the Miocene magmatic rocks in the Kabylies, located alongthe Mediterranean margin in Central and Eastern Algeria. This area

includes the oldest granitic plutons emplaced at ~17Ma (Abbasseneet al., 2016), that are also the largest magmatic bodies of the wholelineament (~200 km2 and a minimum of 500 km3 for the Bougar-oun granite; Bouillin, 1979, 1983). These intrusive bodies are un-equivocally post-collisional (Hernandez and Lepvrier, 1979; Mauryet al., 2000) because they crosscut either the high-grade meta-morphic basement of Lesser Kabylia that has been tectonicallyexhumed at ~17.8e17.4 Ma (Bruguier et al., 2009), its Oligo-Miocene cover and/or the flysch nappes emplaced during the Up-per Burdigalian at ~17.6 Ma. Finally, the Lesser Kabylia and east-ernmost Greater Kabylia overlie a present-day “no-slab” uppermantle region located between two segments of the Africanoceanic slab that have recently been identified around 200e300 kmdepth beneath the Algerian coast according to Fichtner andVillase~nor (2015).

2. The Maghrebides chain in the Kabylies (northeastern andcentral Algeria)

2.1. The Kabylides within the circum-Mediterranean Alpine chains

The Maghrebides (Rif and Tell alpine chains) represent thesouthern active margin of the Western Mediterranean domain(Fig. 1) and their structure is rather similar to that of their northernequivalent, i.e. the Betics (Wildi, 1983), with an opposite vergence.They include Internal Zones, in northern position, and ExternalZones, in southward position. In the Kabylides (Greater Kabylia andLesser Kabylia, Figs.1 and 2), the Internal Zones aremostly made upof old allochtonous Precambrian continental crust massifs thathave recorded multiple tectono-metamorphic events since theHercynian orogeny (granulite facies conditions at ~275e285 MainGreater Kabylia; Peucat et al., 1996; Hammor et al., 2006) and wereexhumed during the Miocene (~18-16 Ma in Edough, LesserKabylia; Moni�e et al., 1992; Hammor and Lancelot, 1998; Bruguieret al., 2009). These basement massifs originate, like those of theBetics, Sicily and Calabria, from the « AlKaPeCa » domain (Alboran-Kabylides-Peloritani-Calabria) dismantled by the opening of theMediterranean back-arc basins (Bouillin, 1986; Jolivet andFaccenna, 2000). In the Maghrebides, they are overlain to thesouth by a limestone cover (Fig. 2) preserved as tectonic slices(Coutelle, 1979).

The External (southern) Zones of the Kabylides are mostlycomposed of flysch nappes of Cretaceous or Paleogene (Numidianflyschs) ages derived from the Tethys Ocean (Fig. 2), thrust over theTellian units that merge into the autochtonous cover of the Africanbasement. In the northern part of the Kabylides, their basement isunconformably overlain by the Oligo-Miocene Kabylian (OMK)series, a molassic formation that includes Upper Oligocene con-glomerates overlain by metatuffs (silexites) dated at 19.4 Ma (K-Ar)in Greater Kabylia (Bellon,1976; Rivi�ere et al., 1977; El Azzouzi et al.,

NIberian foreland

Rif

Betics

Tell

Tanger

Tunis

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Menorca

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GR

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LKAlboran Sea

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1 High-K calc-alkaline magmatism; 2 Alkaline volcanism; 3-4-5 Inner Alpine chain zones 3 Basement; 4 Limestone cover;

RT

MG

10

4° 0° 4° 8°

4° 0° 4° 8°8°

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Fig. 1. General sketch map of the Southwestern Mediterranean margins showing the major paleogeographic domains of Southern Spain and Northern Africa and the mainallochtonous nappes, modified from Vila (1980) and Mahdjoub et al. (1997). The locations of the main Neogene magmatic outcrops are modified from Maury et al. (2000). Therectangle delineates the study area, detailed in Fig. 2.

G. Chazot et al. / Journal of African Earth Sciences 125 (2017) 27e41 29

2014). The OMK series are topped by tectonic olistostromes con-taining Cretaceous-Eocene flysch blocks. The latter are set into anUpper Aquitanian-Lower Burdigalian clay rich matrix (Bouillinet al., 1973). Numidian flysch nappes ranging in age from LateOligocene to Early Burdigalian age are thrust over these olistos-tromes. These nappes were emplaced during the Upper Burdigalian(at the boundary between N6 and N7 zones, ~17.6 Ma). All theallochtonous units mentioned above are unconformably overlainby autochtonous marls and sandstone sediments (Courme-Raultand Coutelle, 1982), usually termed “post-nappes Miocene de-posits” (Fig. 2) of Late Burdigalian-Early Langhian ages (N7-N8zones, ~17.6e15.1 Ma range). As shown below, Miocene magmaticrocks commonly crosscut or overlie most of these sedimentaryunits, even sometimes the youngest ones.

2.2. The Maghrebides Cenozoic magmatic belt

A ~1200 km-long EW-trending linear magmatic belt extendingfrom La Galite Island off the northern coast of Tunisia to Ras Tarf inMorocco is exposed in the Mediterranean coastal areas of theMaghrebides alpine belt (Fig. 1). It is mostly composed of Miocene(Langhian to Tortonian) shoshonitic, K-rich or medium-K calc-alkaline granites, granodiorites, andesites and rhyolites andyounger (Messinian to Plio-Quaternary) sodic alkali basalts andbasanites (Maury et al., 2000; Coulon et al., 2002; Savelli, 2002;Lustrino et al., 2011; Carminati et al., 2012). These magmaticrocks crosscut both the Internal and the External Zones of theMaghrebides or their African foreland (Fig. 1). According to Mauryet al. (2000), the ages of the calc-alkaline to shoshonitic volcanicand plutonic massifs roughly decrease from central-eastern Algeria(16-15 Ma) towards Tunisia (14e8 Ma) and Morocco (12e5 Ma).The oldest presently documented age is that of Bougaroun pluton inLesser Kabylia (17 Ma, Abbassene et al., 2016). The younger alkalinebasaltic to basanitic series do not occur in Central and EasternAlgeria (Fig. 1). They were emplaced in the Mogods, Tunisia,

between 8.4 and 5.0 Ma (Bellon, 1981) and until the Pleistocenenear Oran, western Algeria (Louni-Hacini et al., 1995; Coulon et al.,2002) and in Oujda and Guilliz, Morocco (El Azzouzi et al., 1999).

In the Kabylides, most igneous rocks show a strong subduction-related geochemical imprint that has been attributed to the partialmelting of a subduction-modified ‘orogenic’ mantle followed bycontamination of the corresponding mafic melts by the lower andupper continental crust (Maury et al., 2000; Fourcade et al., 2001;Laouar et al., 2005). The highly radiogenic 87Sr/86Sr(i) (up to0.7528), unradiogenic 143Nd/144Nd(i) (down to 0.51210) isotopicratios, high d18O (up to þ11‰) and relatively low d34S (downto �33‰) of Kabylian granitoids have been attributed to crustalcontamination of such mafic magmas (Semroud et al., 1994;Fourcade et al., 2001; Laouar et al., 2002, 2005), and less often topartial melting of metasediments (Filfila pluton).

3. Spatial and temporal distribution of Miocene magmatismin the Kabylides

Miocene magmatic rocks delineate a 450 km long EW trendingstrip located along the Mediterranean coast of Algeria, betweenAnnaba and Algiers (Fig. 3, upper left). We have distinguished fivemagmatic zones, and labeled them from East to West (Fig. 3). TheEdough-Cap de Fer area (Zone 1) that extends from the northwestof the EdoughMassif to Cap Takouch near Ch�etaïbi and Cap de Fer ismostly composed of diorites, microgranodiorites and andesites.Zone 2 (Bougaroun-Beni Toufout) located in Lesser Kabylia (Kabyliede Collo) includes the two large granitic/granodioritic massifs ofBougaroun and Beni Toufout, themuch smaller Filfila granite east ofSkikda, and the Collo and El Milia microgranodiorites as well as theCh�eraïa ignimbritic rhyolites. Zone 3 (El Aouana) is mostly andesiticand microdioritic and Zone 4 (B�ejaïa-Amizour) mainly granodio-ritic with subordinate andesites. Finally, magmatic activity in theTh�enia-Dellys area east of Algiers (Zone 5) emplaced a wide rangeof rocks ranging from basalts and andesites to rhyolites and

9

5°3°

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Béjaïa

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CCFBGBT

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EABAZm

DlDjEK

1 2 3 4 5 6 7

F AnnabaAB

Greater Kabylia

Lesser Kabylia

36°30’

Fig. 2. Map of Northern Central and Eastern Algeria (area delineated by the rectangle in Fig. 1) simplified after Vila (1980), showing the main structural elements of the Kabyliestogether with the locations of Miocene magmatic complexes.

G. Chazot et al. / Journal of African Earth Sciences 125 (2017) 27e4130

granites. As shown in Fig. 3 (upper left), Zones 1 to 4 roughly overliethe present-day “no-slab” region located between the two seg-ments of oceanic slabs recently identified at 300 km depth belownorthern Algeria according to Fichtner and Villase~nor (2015).

3.1. The Edough-Cap de Fer area (Zone 1)

The geology of the basement of the eastern part of LesserKabylia, known as the Edough-Cap de Fer massif (Fig. 3), is dis-cussed in a number of papers (Moni�e et al., 1992; Hammor andLancelot, 1998; Caby et al., 2001, 2014; Laouar et al., 2002, 2005;Bruguier et al., 2009; Bosch et al., 2014; Fernandez, 2015;Fernandez et al., 2016). Edough is a metamorphic dome of lower-crustal gneisses and migmatites containing slices of garnet am-phibolites and peridotites. In the north, this dome is overlain by atectonic m�elange unit containing relicts of diamond-bearing ultra-high pressure rocks, and then by the Kef Lakhal (“La Voile Noire”)massive amphibolites thrusted over the former unit at ~21 Ma(Fernandez et al., 2016). The latter display an oceanic (MORB-type)affinity, as well as the Bou Maïza gabbros located SW of Edough.They were first interpreted as fragments of back-arc basin crust(Bosch et al., 2014), but further studies lead to consider them asshallow subducted fragments of the Tethys oceanic crust(Fernandez, 2015; Fernandez et al., submitted). The whole Edoughmassif was finally exhumed as a metamorphic core complex at ~18-16 Ma (Moni�e et al., 1992; Hammor and Lancelot, 1998). The latterages were recently reevaluated to 17.8e17.4 Ma (U-Pb on mona-zites; Bruguier et al., 2009).

Comparatively, little attention has been paid to the Miocenemagmatic units that crop out west of the Edough basement (SidiBouguenna, Kef Bouassida, Sidi Sâadi, Aïn Barbar; Fig. 3) and aremore common between Ch�etaïbi and Cap de Fer (Hilly, 1957;Marignac and Zimmermann, 1983; Fougnot, 1990; Ahmed-Saidet al., 1993; Laouar et al., 2002, 2005). They include diorites, mi-nor gabbros, microgranodiorites, microdiorites and andesites thatcrosscut and/or overlie Edough basement, Numidian and Kabylianflyschs and Miocene sedimentary deposits. Diorites form severalsmall intrusions, e.g. near Ch�etaïbi. Microgranodiorites are themost common types. They form the relatively large laccolithic in-trusions of Siddi Akkacha-Cap de Fer (25 km2) and Djebel M'Zihla.Andesites occur as pyroclastic breccias and minor associated lava

flows crosscut by numerous andesitic, dacitic and rhyolitic dykes.They cover a 20 km2 area located between Ch�etaïbi and La Marsa(Fig. 3). Former studies demonstrated the K-rich calc-alkalinecharacter of these magmatic rocks (Ahmed-Said et al., 1993;Laouar et al., 2002, 2005). Abbassene (2016) provided a large setof new K-Ar ages onwhole rocks that range from 15.67 ± 0.41Ma to13.70 ± 0.35 Ma for diorites and microgranodiorites and from16.84 ± 0.58 Ma to 14.32 ± 0.33 Ma for volcanic rocks.

3.2. The Bougaroun-Beni Toufout area (Zone 2)

The Kabylie de Collo displays the largest amount of Miocenemagmatic rocks (that outcrop over ~350 km2) not only in Central/Eastern Algeria but in the whole Maghrebides belt (Fig. 1). Thevolcano-plutonic complex located in the Bougaroun Peninsula andsouth of it (Roubault, 1934; Bouillin, 1979; Ouabadi et al., 1992;Abbassene et al., 2016) include mainly peraluminous granitic plu-tons (Bougaroun in the north, Beni Toufout in the south and Filfilain the east; Fig. 3), minor amounts of gabbros in the northern andsouthern parts of the Bougaroun pluton (Fig. 3), microgranodioritesandmicrodiorites that crop out in the eastern part of the Bougarounpluton, in Collo and El Milia basins, and a few rhyolitic lavas.

The ~200 km2 Bougaroun magmatic complex (Fig. 3), mostlycomposed of peraluminous cordierite-bearing granites and grano-diorites, is the largest Tertiary plutonic massif in North Africa. Itdisplays a roughly elliptical shape, with an ENE-WSW trendingmajor axis. In its eastern part, it intrudes serpentinized peridotitesand kinzigitic gneiss from the Kabylian lower crust. It also intrudesthe Oligo-Miocene Kabylian (OMK) sediments, where it develops a~1 km wide contact metamorphic aureole, and deforms the post-nappe Miocene sediments (Bouillin, 1983), the deposition ofwhich started at 17.6 Ma. These geological features are consistentwith its 17 Ma U-Pb age obtained on zircons, as well as closelyrelated whole-rock K-Ar ages on granitic samples (17.23 ± 0.49 Ma,16.88 ± 0.43 Ma), and on associated granodioritic and dioritic(17.43 ± 0.53 Ma, 17.06 ± 0.41, 17.05 ± 0.43 Ma) samples (Abbasseneet al., 2016). The 17 Ma age (Upper Burdigalian) obtained on theBougaroun pluton is the oldest presently identified for K-rich calc-alkaline rocks in the whole 1200 km-long EW trending Maghre-bides magmatic belt. Younger K-Ar ages were, however, obtainedfor a fine-grained granitic dyke crosscutting the pluton

Fig. 3. Simplified geological maps of the five zones (labeled 1 to 5 from east to west) where Miocene post-collisional magmatic rocks crop out in the Kabylies. On the generallocation map, the light blue patterns delineate the areas underlain by oceanic slab segments at 300 km depth according to Fichtner and Villase~nor (2015). Zone 1: Edough-Cap de Fer(modified from Bosch et al., 2014; Fougnot, 1990; Vila, 1980). Zone 2: Bougaroun-Beni Toufout (modified from Abbassene et al., 2016; Bouillin, 1979). Zone 3: El Aouana (modifiedfrom Robin, 1970). Zone 4: B�ejaïa-Amizour (modified from Leikine et al., 1988; Semroud et al., 1994). Zone 5: Th�enia-Dellys (modified from Belanteur et al., 1995).

G. Chazot et al. / Journal of African Earth Sciences 125 (2017) 27e41 31

(15.09 ± 0.43 Ma), as well as for two microgranodiorites(13.59± 0.33Ma and 14.17± 0.35Ma), a doleritic dyke (10.90 ± 0.88Ma) and a rhyolitic dyke (10.72 ± 0.25 Ma) crosscutting gabbros(Abbassene et al., 2016). In addition, these authors measured older(Upper Oligocene) Ar-Ar hornblende ages of 27.0 ± 3.0 Ma and23.3 ± 3.2 Ma on LREE-depleted gabbros outcropping at Cap Bou-garoun sensu stricto, and considered them as related to the back-arccrust of the nearby Jijel basin.

The geology and petrology of Beni Toufout, El Milia and Filfilaintrusions has not been revised since the studies of Ouabadi (1994)and Fourcade et al. (2001). The Beni Toufout pluton (~80 km2) iscomposed of peraluminous cordierite-bearing granodiorites andmonzogranites dated at 15.4 ± 0.8 Ma and 15.2 ± 0.7 Ma by the K-Armethod on separated biotites (Penven and Zimmermann, 1986). Itintrudes (and develops a typical contact metamorphic aureolewithin) Berriasian to Lutetian formations of African origin that areexposed within the Beni Toufout tectonic window (Bouillin, 1979)opened in the Kabylian basement nappes. From a petrogeneticpoint of view it is therefore unrelated to the overlying Kabyliannappe materials. The neighbouring El Milia peraluminouscordierite-bearing microgranites form an arcuate set of intrusions

that develop contact metamorphic aureoles within their host rocksthat include Miocene (up to Langhian) deposits (Bouillin, 1979).One of these microgranites has been dated at 16.49 ± 0.81 Ma by K-Ar (Bellon, 1976 and references therein). The Filfila granite(Semroud, 1970; Semroud and Fabri�es, 1977), located ca 20 km eastof Skikda (Fig. 3) includes two small (

G. Chazot et al. / Journal of African Earth Sciences 125 (2017) 27e4132

1970) that crosscut and/or overlie the Numidian flyschs and thepost-nappe Miocene deposits. The central part of the massif con-sists of an up to 600 m thick pile of andesitic and dacitic lava flows(Port Maria unit) overlying palagonite-bearing pyroclastic deposits(Bou Soufa unit) emplaced in a shallow underwater environment(Villemaire, 1988). These central units are crosscut by numerous NStrending andesitic and dacitic dykes. They are surrounded by anumber of small (

G. Chazot et al. / Journal of African Earth Sciences 125 (2017) 27e41 33

along the differentiation trend, then alkali feldspar. They define asubalkaline series from basalts/gabbros to rhyolites/granites in theTAS diagram (Fig. 4a). In the K2O vs. SiO2 diagram (Fig. 4b), most ofthe intermediate and felsic rocks plot within the field of high-Kcalc-alkaline series. However, most rocks from El Aouana and afew ones from Edough-Cap de Fer, B�ejaïa-Amizour and Th�enia-Dellys have lower K contents and plot within the field of medium-Kcalc-alkaline series. Themafic rocks also show a large heterogeneityin K2O contents, from very low values up to ~2 wt %, and displayconsiderable variations within a single zone (e.g., Th�enia-Dellys).

4.2. Trace elements

Available rare earth and other trace element data are availablefor the five studied zones and cover a wide range of compositions.In the five studied regions, most of the samples share similar traceelement compositions. They have flat heavy rare earth elements(HREE) patterns around ten times chondrite, and strongly enrichedlight REE (LREE) compositions (Fig. 5a). All these samples have alsolow Nb and high Rb and Ba contents (Fig. 6a). These characteristicsare typical of K-rich arc magmas related either to an active sub-duction context or to a post-subduction or post-collision environ-ment. All these samples are characterized by high La contents andhigh La/Nb ratios (Fig. 7). In El Aouana and Th�enia-Dellys as well asin the Edough-Cap de Fer zones, some mafic or intermediatesamples have a lower LREE enrichment for similar HREE compo-sitions (lower La/Yb), and lower Rb and Ba contents consistent witha medium-K calc-alkaline affinity (Figs. 5b and 6b). Finally, in thetwo eastern provinces (Zones 1 and 2), some K-poor mafic samplesshow LREE-depleted compositions, while having among the high-est HREE contents. These samples are not enriched in Rb and Ba,and do not bear the subduction imprint shown by most of thesamples along the Algerianmargin (Figs. 5b and 6b). In the La vs. La/Nb diagram (Fig. 7), these samples display the lowest La contentsand among the lowest La/Nb ratios.

4.3. Sr and Nd isotopes

Except for El Aouana (Zone 3), Sr and Nd isotopic data areavailable for the different magmatic areas and show a very largescatter (Fig. 8a). 87Sr/86Sr ratios range from 0.7038 to values higherthan 0.752 while 143Nd/144Nd range from 0.51324 to 0.51205. Themost extreme isotopic values, especially the 87Sr/86Sr ratios higherthan 0.730, come from the Filfila granite in the Bougaroun-BeniToufout area (Zone 2), which has been described as a crustalmelting product (Fourcade et al., 2001). Low Sr and high Nd valuesare found in the most mafic samples from the Bougaroun-BeniToufout and Edough-Cap de Fer provinces while higher Sr andlower Nd isotopic compositions are recorded in the more differ-entiated samples from the four provinces. This observation isconsistent with the nice correlation between Sr isotopic ratios andsilica content, where the highest 87Sr/86Sr ratios are found in themost silica-rich samples (Fig. 8b). The Filfila granite recorded theisotopic composition of the local African crust, but the composi-tions of the other samples clearly indicate strong interactions be-tween mantle magmas and the continental crust during theirdifferentiation and emplacement.

5. Discussion

5.1. A typical “subduction-related” geochemical signature

The main magmatic phase occurred around 17 Ma ago along theKabylian margin and emplaced large volumes of magma in the fivestudied areas. Most of the magmatic rocks emplaced during this

period share similar geochemical characteristics. As summarizedabove, they show parallel LREE enriched patterns, high Rb, Ba, Thand U contents, and very low Nb contents. These rocks display thehighest La contents and the highest La/Nb ratios (Fig. 7). Except forsome samples from the Bejaïa-Amizour province, they are allpotassic rocks, despite important chemical variability. Most of theserocks are intermediate to evolved and have undergone importantcrustal interactions during their differentiation in crustal magmachambers. This is well illustrated by the large range of Sr-Nd iso-topic compositions depicted on Fig. 8a, with extreme values similarto those of the local crustal rocks. It is thus difficult to properlycharacterize their mantle source.

The presence of minor amounts of mafic rocks displayinggeochemical features similar to those of diorites, granites andrhyolites suggests the involvement of subduction-metasomatizedmantle in their genesis, as already proposed by Fourcade et al.(2001). These more mafic samples have low 143Nd/144Nd isotopicratios, not correlated with their SiO2 contents (Fig. 9) and moder-ately high d18O ratios (þ5.9 to þ9.0‰, Laouar et al., 2002, 2005).These features indicate that they also formed from an enrichedmantle source with a strong subduction imprint, as alreadydemonstrated for the Bougaroun-Beni Toufout samples (Abbasseneet al., 2016). Their high Th/Yb ratios are consistent with the pres-ence of a mantle modified by a subduction component (melt orfluid) extracted from a subducted oceanic lithosphere. The simi-larity in the shape of the REE patterns for all the concerned samplesimplies a similar process of magma formation all along the margin,and their differentiation within the continental crust, to producethe whole spectrum of compositions observed from major andtrace element data as well as for isotopes. This subduction-flavoured magmatism corresponds to the largest volumes ofigneous rocks emplaced along the Algerian margin, and thereforerepresents its main magmatic phase. This igneous activity startedsome 17 Ma ago mostly in the Bougaroun-Beni Toufout area andlasted for several millions years as ages ranging from 12 to 15 Mahave been determined in magmatic rocks from the five differentareas.

5.2. Constraints on the origin of mafic magmas

Intermediate and felsic rocks have frequently interacted withthe crust and their chemical and isotopic composition has some-times been strongly modified during this process, blurring theiroriginal mantle signature. The most mafic and thus less contami-nated rocks can bring valuable information about their mantlesources and their evolution through time. Such mafic volcanic andplutonic rocks exist all along the Kabylian coast and theirgeochemistry reveals interesting aspects of the geodynamic evo-lution of the Algerian margin.

In the TAS and the K2O vs. SiO2 diagrams (Fig. 4), some maficsamples appear to have very low alkalies contents, and especiallylow K2O contents. They are mainly gabbros and amphibolites fromEdough-Cap de Fer and Bougaroun-Beni Toufout areas, whichappear to be LREE depleted (Fig. 5b). They have high HREE, similarto the high-K rocks, and thus display the lowest La/Yb ratiosrecorded in Algeria. Extended patterns (Fig. 6b) show that thesesamples do not display the subduction-flavoured characteristics ofthe enriched samples as they have very low Ba, Rb and Sr content,and no negative Nb anomaly typical of geochemical subductionimprint. Their isotopic composition is also unique along themargin,as they have 143Nd/144Nd ratios higher than 0.5130 and among thelowest Sr isotopic ratios (Fig. 8a). Some of these samples havehigher Sr isotopic compositions probably due to sea-water in-teractions. These geochemical characteristics indicate that thesemafic samples come from a depletedmantle source and sharemany

35 45 55 65 75 850

2

4

6

8

10

12

14

16

SiO (wt%)2

Picro-basalt

Basalt

BasalticandesiteAndesite

Dacite Rhyolite

Trachyte

TrachydaciteTrachy-andesite

Basaltictrachy-andesite

Trachy-basaltN

a2O+K

2O (w

t%)

Thénia-Dellys area (1, 2, 3)Béjaïa-Amizour (3, 4, 5, 6)El Aouana (3, 6)Bougaroun-Beni Toufout (3, 6, 7, 8, 9, 10)Edough-Cap de Fer (9, 10, 11, 12, 13, 14, 15)

Subalkaline

Alkaline(a)

45 55 65 75 850

1

2

3

4

5

6

7

SiO (wt%)2

Low-K Calc-alkali

ne SeriesMedium

-K Calc-Alkaline

SeriesHigh-K C

alc-Alkalin

e Series

Shoshon

itic Seri

es

Basalt Bas Andesite Dacite Rhyolite And

K2O

(wt%

)

(b)

Fig. 4. Major element plots. (a) Total alkalies vs. silica (TAS) diagram (after Le Bas et al., 1986) for all the rocks from this study. (b) K2O vs. SiO2 diagram (after Peccerillo and Taylor,1976). The depleted mafic rocks have very low K contents while most of the intermediate and evolved samples belong to the high-K calc-alkaline series. Data: 1 Belanteur et al.(1995); 2 Belanteur (2001); 3 Unpublished data; 4 Semroud (1981); 5 Semroud et al. (1994); 6 Abbassene, 2016; 7 Abbassene et al. (2016); 8 Ouabadi (1994); 9 Fourcade et al.(2001); 10 Bellon (1976); 11 Abbassene (2016); 12 Laouar et al. (2005); 13 Bosch et al. (2014); 14 Ahmed-Said et al. (1993); 15 Fougnot (1990).

G. Chazot et al. / Journal of African Earth Sciences 125 (2017) 27e4134

characteristics with mid oceanic ridge basalts (MORB).Furthermore, field arguments as well as new Ar-Ar dating

(Abbassene et al., 2016) indicate that these mafic rocks are amongthe oldest magmatic rocks of these two provinces. They clearlycome from a depleted mantle source that could represent theambient asthenosphere still not modified by the subduction pro-cesses at the time of this early magmatism and could be related tothe Upper-Oligocene back-arc crust formation in nearby basins orto dyke systems or gabbroic intrusions crosscutting the stretchedKabylian continental crust (Bosch et al., 2014; Abbassene et al.,2016). Alternatively, they could represent exhumed tectonic slicesof the Tethys oceanic crust (Caby et al., 2014; Fernandez, 2015;Fernandez et al., 2016; Fernandez et al., submitted). These rocksare not genetically related to the other mafic, intermediate andfelsic rocks of the Algerian margin and have been juxtaposed toyounger magmatic rocks during the margin evolution.

Among the most mafic rocks, with SiO2 lower than 52 wt % formost of them, there are basalts, gabbros and some microdioritesand andesites with geochemical characteristics intermediate be-tween the depleted mafic samples and the subduction-flavoured K-rich rocks. These rocks are present in the Bougaroun-Beni Toufoutand Th�enia-Dellys areas, and a few samples are also present in theEl Aouana zone (Zone 3, Fig. 3). They have nearly flat REE patterns

(Fig. 5b) with a slight enrichment in LREE, and La/Yb ratios rangingfrom 2 to 6, higher than those of the depleted samples (Fig. 10).They have rather high Th and U contents, and low Nb concentra-tions. They also display intermediate Ba, Rb and Sr contents, with Baconcentrations lower than 200 ppm for most of the samples, whichare significantly higher than those of the depleted samples, butlower than most of the LREE-enriched subduction-flavoured rocks(Fig. 11). Only three of these samples have been analyzed for Sr andNd isotopes. They have lower Nd isotopic compositions than thedepleted samples for similar SiO2 content (Fig. 9), and Sr isotopiccompositions similar to the sea-water modified samples. Except afew intermediate samples, they are all mafic volcanic rocks (mostlybasalts) clearly not strongly affected by crustal contaminationprocesses as their isotopic compositions are not correlated withtheir SiO2 content. They have variable but sometimes high K2Ocontents. They obviously derived from a non-depleted mantlesource, but clearly different from the source of the main magma-tism of the Algerian margin (Figs. 5 and 6). Available K-Ar ages fromZones 2, 3 and 5 suggest that these moderately enriched rocks aresomewhat younger (16.5e11.6 Ma) than those emplaced during themain K-rich calc-alkaline magmatic event (17 Ma-old Bougaroungranite), with ages ranging from 16.5 to 11.6 Ma. The source of thisvolcanism is slightly modified by a subduction component, and is

Thénia-Dellys areaBéjaïa-AmizourEl AouanaBougaroun-Beni ToufoutEdough-Cap de Fer

La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Yb Lu

1

10

100

Roc

k/C

hond

rites

(a)

Thénia-Dellys areaEl AouanaBougaroun-Beni ToufoutEdough-Cap de Fer

1

10

100

La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Yb Lu

Roc

k/C

hond

rites

(b)

Fig. 5. Chondrite-normalized rare earth element patterns for (a) the highly LREE-enriched samples and (b) the moderately enriched and depleted samples from thefive different zones. Chondrite values from Anders and Grevesse (1989).

CsRb

BaTh

UNb

TaLa

CePb

PrSr

NdZr

HfSm

EuTi

DyY

YbLu

.01

.1

1

10

100

1000

Roc

k/Pr

imiti

ve M

antle

(a)

.01

.1

1

10

100

1000

CsRb

BaTh

UNb

TaLa

CePb

PrSr

NdZr

HfSm

EuTi

DyY

YbLu

Roc

k/Pr

imiti

ve M

antle

(b)

Fig. 6. Primitive mantle-normalized trace element patterns for the Kabylian magmaticrocks. (a) highly LREE-enriched samples and (b) moderately enriched and depletedsamples. Primitive mantle values from Sun and McDonough (1989).

G. Chazot et al. / Journal of African Earth Sciences 125 (2017) 27e41 35

present all along the Kabylian margin.

0 5 10 15 20 25 30 35 400

1

2

3

4

5

6

La/N

b

La

Fig. 7. La/Nb versus La diagram for the Kabylian Miocene magmatic rocks. Symbols asin Fig. 4. The red field highlights the LREE-depleted samples.

5.3. Intermediate and felsic magmas: evidence for crustalcontamination by the African crust and local anatexis

Crustal contamination can deeply alter the mantle signature ofmagmatic rocks. Indeed, the participation of the crust in the evo-lution of the magmas emplaced along the Algerian margin hasalready been evidenced. For example, Bougaroun intermediate andfelsic peraluminous (cordierite-bearing) rocks and the neighbour-ing granitoids mainly formed through incorporation of peliticmetasediments within mantle-derived mafic magmas (Fourcadeet al., 2001). Crustal anatexis of metasediments was evidencedonly in the case of the highly peraluminous small Filfila granite(Ouabadi, 1994; Fourcade et al., 2001) that displays the mostradiogenic Sr isotopic compositions (87Sr/86Sri ¼ 0.732e0.754)recorded along the Algerian margin (Fig. 8).

The continuum of major element compositions in all the fivestudied zones frommafic rocks to highly differentiated ones pointstowards crystal fractionation as the major process for forming thefelsic magmas instead of important crustal melting that would havegenerated more felsic rocks than mafic and intermediate ones.Most of the LREE enriched samples have parallel rare earth elementpatterns with an increase of the Eu negative anomaly from theintermediate to the most evolved rhyolites and granites, as a resultof feldspar fractionation. Nevertheless, crustal contaminationplayed an important role in the magma evolution along the

Kabylian margin. Isotopes are a very powerful tool to study suchcontamination processes. Few isotopic data have been published

(

Nd/

N

d)i

143

144

0.5120

0.5122

0.5124

0.5126

0.5128

0.5130

0.5132

0.702 0.712 0.722 0.732 0.742 0.752( Sr/ Sr)i87 86

(a)

0.702

0.712

0.722

0.732

0.742

0.752

45 50 55 60 65 70 75 80SiO (wt%)

2

( Sr

/ S

r)i

8786

(b)

Fig. 8. (a) Initial 143Nd/144Nd ratios versus initial 87Sr/86Sr ratios and (b) initial143Nd/144Nd ratios versus SiO2 contents (wt. %) for Kabylian Miocene magmatic rocks.Symbols as in Fig. 4. The red field highlights the LREE-depleted samples.

0.5120

0.5122

0.5124

0.5126

0.5128

0.5130

0.5132

(

Nd/

N

d)i

143

144

45 50 55 60 65 70 75 80SiO (wt%)

2

Fig. 9. Initial 143Nd/144Nd ratios versus SiO2 content for Kabylian Miocene magmaticrocks. Symbols as in Fig. 4. The red field highlights the LREE-depleted samples whilethe moderately LREE-enriched samples have lower Nd isotopic ratio for similar SiO2contents.

0 5 10 15La/Yb

0.5120

0.5122

0.5124

0.5126

0.5128

0.5130

0.5132

0.5134

(

Nd/

Nd)

i14

314

4

Fig. 10. Initial 143Nd/144Nd ratios versus La/Yb ratios for the Kabylian Miocenemagmatic rocks. Symbols as in Fig. 4. The red field highlights the LREE-depletedsamples.

G. Chazot et al. / Journal of African Earth Sciences 125 (2017) 27e4136

about the magmatism of the five studied zones, but available Sr andNd isotopic ratios arewell correlatedwith indexes of differentiationsuch as SiO2 (Fig. 8b) and also MgO contents. These correlations,

together with the high d18O (up to þ11‰) and relatively low d34S(down to �33‰) of Kabylian granitoids, are consistent with thehypothesis of strong interactions between metasomatized mantle-derived magmas and the African crustal rocks (Semroud et al.,1994; Fourcade et al., 2001; Laouar et al., 2002, 2005). The pres-ence of metamorphic minerals likely inherited from crustal xeno-liths in some granitic rocks confirms these observations. The largeamounts of mafic mantle magmas emplaced in the upper crust mayhave induced partial melting of the crustal rocks and hybridizationof these crustal melts with the mantle magmas in the course of thefractional crystallization processes. The overwhelming crustalsignature of many samples from the different magmatic provincesis unlikely to result exclusively from assimilation of metasedimentscoupled with fractional crystallization (AFC; DePaolo, 1981) ofmafic magmas: it might instead be a consequence of mixing be-tween mantle-derived magmas and minor amounts of crustal(anatectic) melts equivalent to the Filfila granite (Fourcade et al.,2001).

5.4. Tectonic framework of magma genesis and emplacement

The development of plate tectonics concepts led a number ofauthors (Dercourt, 1970; Le Pichon et al., 1971; Auzende et al., 1973;Bellon, 1976; Bouillin, 1979; Cohen, 1980; R�ehault et al., 1984; Ricouet al., 1986; Dewey et al., 1989) to consider that the Maghrebidesalpine belt resulted from the Cenozoic northward-dipping sub-duction of a Ligurian-Tethyan oceanic slab located north of theAfrican continental lithosphere beneath another continentaldomain termed « AlKaPeCa » (Alboran-Kabylides-Peloritani-Cala-bria; Bouillin, 1986; Jolivet and Faccenna, 2000) and later disman-tled by the opening of the Mediterranean oceanic basins formed inback-arc position during slab rollback (Lonergan and White, 1997;Gueguen et al., 1998). Cohen (1980) proposed that thisnorthward-dipping subduction ended through a slab detachmentprocess. This hypothesis was strongly supported by later tomo-graphic studies (Spakman, 1986a,b; Carminati et al., 1998a,b;Wortel and Spakman, 2000; Spakman and Wortel, 2004) and isstill a major ingredient of geodynamic reconstructions (e.g., Frizonde Lamotte et al., 2000; Rosenbaum et al., 2002a,b; Faccenna et al.,2004; 2014; van Hinsbergen et al., 2014). The recent tomographicstudy of Fichtner and Villase~nor (2015) identified possible rem-nants of the Tethys oceanic slab around 200e300 km depthbeneath the central and eastern Algerian coast. These authors

0 100 200 300 400 500 600 7000

5

10

15

20

25

30

35

40

La/Y

b

Ba

Fig. 11. La/Yb ratios versus Ba concentrations for the Kabylian Miocene magmaticrocks. Symbols as in Fig. 4. The red field highlights the LREE-depleted samples.

G. Chazot et al. / Journal of African Earth Sciences 125 (2017) 27e41 37

consider this slab as detached from the surface (>150 km) andbroken into two segments, presently located east and west of theKabylies, respectively (Fig. 3; Fichtner and Villase~nor, 2015; theirFig. 9).

Oceanic slab detachment/breakoff provides a rather attractiveexplanation for the origin of post-collisional magmatism (vonBlanckenburg and Davies, 1995; Davies and von Blanckenburg,1995). Indeed, the thermal flux originating from the deepasthenosphere and uprising through the slab tear is likely to pro-vide the excess heat triggering the partial melting of metasomat-ized lithospheric mantle and/or the continental crust overlying it.In the case of the Kabylides, an early Miocene slab breakoff is alsoconsistent with the rapid uplift and exhumation of high pressuremetamorphic rocks, regional metamorphism, hydrothermalismand mineralization indicating increased heat flow and followed byfast cooling (Carminati et al., 1998a,b; Caby et al., 2001). Theseprocesses are well documented in the Edough massif (Zone 1) at20.9 Ma for thrusting of Kef Lakhal oceanic complex, 17.7 Ma forexhumation of the Edough lower-crustal units and 17 Ma for fastcooling of their footwall rocks (Fernandez et al., 2015). They wereimmediately followed by the onset of K-rich calc-alkaline mag-matism at ~17 Ma in the neighbouring (Zone 2) Bougaroun-Collomassif (Abbassene et al., 2016). These results suggest that the ki-nematic reconstructions assuming an age younger than 17 Ma forthe collision between Africa and the Kabylian domain of AlKaPeCa(Carminati et al., 1998a, b; Frizon de Lamotte et al., 2000; Lustrinoet al., 2011; van Hinsbergen et al., 2014; Bouyahiaoui et al., 2015)need to be reconsidered.

The magmatic pulse at ~17 Ma in Bougaroun (Zone 2) is theoldest presently documented K-rich calc-alkaline event markingthe onset of the Neogene igneous history of the Maghrebides, andalso the one that emplaced the largest volumes of intermediate andevolved magmas, possibly up to 800e1000 km3 for Zone 2(Abbassene et al., 2016). However, according to available K-Ar agespresented in Section 3, magmatic activity started in the four otherzones between ~17 and ~15.5 Ma, and stopped between ~14 and~12 Ma (~11 Ma in Zone 2). Medium-K calc-alkaline rocks wereemplaced in minor amounts in Zones 2, 3 and 5 between 16.5 and11.6 Ma, i.e. almost during the whole span of magmatic activity inthe Kabylides. Available K-Ar ages for the single anatectic granite ofFilfila in Zone 2 (15.3 ± 0.8 Ma, Bellon, 1981) fall within the samerange. Therefore, no obvious spatial or temporal pattern ofemplacement of these various petrogenetic types within the

Kabylides can be ascertained.Assuming that the heat flux required for the partial melting of

lithospheric mantle and/or crustal units was provided by theasthenosphere uprising through the slab tear during a slabdetachment process, two problems (already pointed out by Mauryet al., 2000) arise concerning the genesis of Miocene calc-alkalinemagmatism in the Kabylies. First, a subduction-modified hydrouslithospheric mantle, potential source of the mafic and intermediatemagmas (section 5.2) was necessarily present beneath the studiedarea at depths consistent with the stability of pargasite (

G. Chazot et al. / Journal of African Earth Sciences 125 (2017) 27e4138

leading to the formation of the North Algerian basin (Schettino andTurco, 2006, 2011). LREE-depleted back-arc mafic magmas mighthave intruded the thinned continental crust of the Kabylides (Boschet al., 2014; Abbassene et al., 2016), although Cap Bougaroun (Zone2) and Bou Maiza (Zone 1) depleted gabbros and Kef Lakhal am-phibolites (Zone 1) may alternatively represent exhumed slices ofTethyan oceanic crust (Fernandez et al., 2016; Fernandez et al.,submitted).

The Burdigalian collision of the Kabylides with the Africanmargin induced delamination and slab breakoff process (Fig. 12B).Our interpretation (Fig. 12) involves a double “d�ecollement” ofcrustal and mantle units: (1) that of the metasomatized litho-spheric subcontinental mantle of the Kabylies with respect to histhinned continental crust, and (2) that of the edge of the Africanlithosphere (Tethyan passive margin), where slab sinking andbreakoff led to decoupling of the African continental crust from itsunderlying subcontinental mantle. Such processes have been

Fig. 12. Schematic cross-sections showing the possible positions of the lithospheric geochemsources during the Aquitanian-Burdigalian (A) and at 17e11 Ma (B), modified from Abbassevalues obtained for Lesser Kabylia (Mihoubi et al., 2014; Bouyahiaoui et al., 2015; Hamai etHinsbergen et al., 2014). Sediment thicknesses are only indicative. The model of delaminatioDuggen et al. (2005) and Roure et al. (2012). The extent of thermal erosion of the Kabylian litand delamination by Davies and von Blanckenburg (1995) and Bird (1979), respectively. Thealternative way to describe this evolution is to consider that the Kabylian metasomatized lithslab rollback after collision, as suggested by laboratory modelling (G€ogüs et al., 2011; BajoleOligo-Miocene); 3: Tethyan oceanic crust; 4: Back-arc oceanic crust (Algerian basin); 5: KabyTethyan suboceanic lithospheric mantle; 8: Back-arc lithospheric mantle (Algerian basin);mantle.

previously proposed for several sectors of the Maghrebides (Frizonde Lamotte et al., 2000; Roure et al., 2012; Mancilla et al., 2015;Villase~nor et al., 2015) but rarely with reference to theirmagmatic evolution (Duggen et al., 2005). Actually, both delami-nation and slab detachment processes trigger a fast rise ofasthenosphere to replace the sinking lithosphere (Bird, 1979;Wortel and Spakman, 2000). The sinking of the detached Tethysslab explains the progressive development of a “no-slab” regionbeneath the Kabylies owing to an east-west lithospheric tearing(Medaouri et al., 2014; Chertova et al., 2014; Rosenbaum, 2014),while remnants of this slab are still detectable by seismic tomog-raphy around 200e300 km depth below the Algerian coast(Fichtner and Villase~nor, 2015) east and west of the studied area,respectively (Fig. 3). In our tentative model, the thermal flux ofasthenospheric origin uprising through the slab tear induced animportant thinning of the Kabylian and African lithospheric man-tles by thermal erosion, similar to that envisioned in other cases of

ical reservoirs involved in the genesis of Kabylian magmas and/or their mantle/crustalne et al. (2016). Crustal and lithospheric thicknesses are interpolated from present-dayal., 2015; Carballo et al., 2015) and from the postulated ages of the oceanic slabs (vann of the edge of the African lithosphere is adapted from Frizon de Lamotte et al. (2000),hosphere following slab breakoff is qualitatively estimated frommodels of slab breakofftectonic and geodynamic arguments supporting this model are discussed in the text. Anospheric mantle was underthrusted below the African crust, owing to a continuation oft et al., 2012). 1: Sediments (undifferentiated). 2: Synrift sediments (2a: Mesozoic; 2b:lian continental crust; 6: Miocene magmatic bodies (BG Bougaroun, BT Beni Toufout); 7:9: Kabylian lithospheric mantle; 10: Subduction-metasomatized Kabylian lithospheric

G. Chazot et al. / Journal of African Earth Sciences 125 (2017) 27e41 39

mantle delamination or slab breakoff/lithospheric detachment(Bird, 1979; Davies and von Blanckenburg, 1995; von Blanckenburgand Davies, 1995; Duggen et al., 2005). We propose that this ther-mal erosion induced the onset of partial melting at 17 Ma of theKabylian lithospheric mantle metasomatized during the formersubduction event (Abbassene et al., 2016). Minor amounts of thecorresponding subduction-flavoured mafic and intermediate meltswere emplaced as metaluminous medium-K calc-alkaline magmasdevoid of major contamination effects by the African continentalcrust in Zones 2, 3 and 5. More often, however, massive incorpo-ration of pelitic metasediments from the African continental unitswithin those mafic magmas generated K-rich intermediate meltsand the majority of peraluminous cordierite-bearing graniticmagmas (e.g. Bougaroun and Beni Toufout) displaying a strong tooverwhelming crustal geochemical signature (Fourcade et al.,2001). Then, after the main magmatic event at 17-16 Ma,emplacement of smaller volumes of K-rich and occasional medium-K calc-alkaline melts occurred sporadically in Zones 1 to 5 until~14-11 Ma. The end of this magmatic activity might be related tothe progressive thermal reequilibration of the asthenospherebelow the Kabylides following slab detachment, as envisioned inother post-collision settings (Bird, 1979; Harris et al., 1986). Thisspatio-temporal evolution of magmatic activity is consistent withslab tear propagation eastward and westward from central-easternAlgeria, supporting an age decrease of calc-alkaline magmatic ac-tivity towards Tunisia and Morocco until the Pliocene (Maury et al.,2000).

6. Conclusions

1. Miocene (17-11 Ma) magmatic activity in the Kabylidesemplaced K-rich (and minor medium-K) calc-alkaline plutonicand volcanic rocks displaying a typical subduction-relatedgeochemical signature. Older LREE-depleted mafic rocks ofoceanic affinity (gabbros and amphibolites from Edough andBougaroun massifs) could represent either exhumed slices ofthe subducted Tethys oceanic crust or back-arc magmas relatedto the opening of the Algerian basin.

2. In the five studied zones, the peak of magmatic activity occurredbetween 17 and 16 Ma, emplacing large volumes of per-aluminous cordierite-bearing granitoids in Bougaroun-BeniToufout (Zone 2). Smaller amounts of K-rich (Zones 1 to 5) andoccasional medium-K (Zones 2, 3 and 5) calc-alkaline meltswere then emplaced sporadically until ~14-11 Ma toward theedges of the studied zones.

3. Medium-K mafic and intermediate magmas derived from thepartial melting of a subduction-modified mantle, combinedwith minor contamination by continental crustal materials.Conversely, the major, trace element and isotopic compositionsof K-rich lavas and granitoids are consistent with strong in-teractions between metasomatized mantle-derived magmasand the African crust. The most likely source of Miocene calc-alkaline magmas is the Kabylian subcontinental lithosphericmantle metasomatized during the Paleogene subduction of theLigurian Tethys oceanic lithosphere.

4. Our preferred tectono-magmatic model involves a Tethyan slabsouthwards roll-back, followed by final slab breakoff. This pro-cess was combined with crustal stacking and “d�ecollement” ofthe crust from its underlying lithospheric mantle at both edgesof the African and Kabylian continental lithospheres, leading tothe thrusting of the African continental crust over the Kabylianmetasomatized lithospheric mantle. At 17 Ma, the thermal fluxof asthenospheric origin upwelling through the Tethyan slabtear provoked the thermal erosion and thinning of the latterlithospheric mantle, and triggered its partial melting as well as

slab tear propagation. The corresponding mafic magmas wereoccasionally emplaced between in Zones 2, 3 and 5 as medium-K calc-alkaline lavas or gabbros/diorites devoid of importantcrustal contamination features. More often, complex in-teractions with the African basement during their uprise led tothe emplacement of intermediate to felsic K-rich calc-alkalinemelts with a strong to overwhelming trace element and iso-topic crustal imprint. The end of this magmatic activity at ~14-11Ma might be related to the progressive thermal reequilibrationof the asthenosphere following slab breakoff.

Acknowledgments

This study was supported by the Algerian-French researchProject SPIRAL (Structures Profondes et Investigations R�egionalesen ALg�erie), a cooperative program initiated in 2009 by DG-RSDT,CRAAG and Sonatrach in Algeria, and IUEM-UBO, Nice-SophiaAntipolis Universities and IFREMER in France. We thank Tim Hor-scroft for inviting us to submit this manuscript, and also GuillermoBooth-Rea and Dominique Frizon de Lamotte for their pertinentcomments on the tectonic and geodynamic evolution of the area.We also thank Marcaurelio Franzetti for his help designing Fig. 12,and Alain Coutelle for discussing with us the geology of NorthernAlgeria and the related literature. Members of the SPIRAL team arethanked for fruitful exchanges on the Algerian margin.

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