Journal of African Earth Sciences 125 (2017) 27e41

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Journal of African Earth Sciences

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Geological Society of Presidential Review An overview on the origin of post-collisional Miocene in the Kabylies (northern ): Evidence for crustal stacking, delamination and slab detachment

* Gilles Chazot a, , Fatiha Abbassene a, b,Rene C. Maury a, Jacques Deverch ere a, Herve Bellon a, c, Aziouz Ouabadi b, Delphine Bosch d a Universite de Brest, CNRS, UMR 6538 Domaines Oceaniques, Institut Universitaire Europeen de la Mer (IUEM) Place Nicolas Copernic, 29280 Plouzane, b Universite des Sciences et de la Technologie Houari Boumedienne, Bab Ezzouar, Laboratoire de Geodynamique, Geologie de l’Ingenieur et Planetologie (LGGIP/FSTGAT/USTHB), , Algeria c Universite de Brest, CNRS, UMR 6538 Domaines Oceaniques, 6 avenue Le Gorgeu, c.s. 93837, 29238 Brest, France d Universite de Montpellier II, CNRS-UMR 5243, Geosciences Montpellier, Place E. Bataillon, c.c. 060, 34095 Montpellier, France article info abstract

Article history: Miocene (17-11 Ma) magmatic activity in the Kabylies emplaced K-rich (and minor medium-K) calc- Received 7 June 2016 alkaline plutonic and volcanic rocks in five zones, delineating a ~450 km long EW trending strip located Received in revised form along the northern coast of Algeria, between Annaba and Algiers. Their most likely source is the Kabylian 7 October 2016 subcontinental lithospheric mantle previously metasomatized during the Paleogene of the Accepted 19 October 2016 Tethys oceanic lithosphere. Our preferred tectono-magmatic model involves a Tethyan slab detachment Available online 19 October 2016 combined with African mantle delamination and crustal stacking, leading to the superimposition of the African over the Kabylian metasomatized lithospheric mantle. At ca. 17 Ma, the Keywords: Calc-alkaline magmas asthenospheric upwelling arising from lithospheric delamination and Tethyan slab tear triggered the fi Post-collision thermal erosion of the latter mantle, inducing its partial melting. The corresponding ma c medium-K Delamination calc-alkaline magmas interacted with the African basement units during their ascent, generating inter- Slab tearing mediate to felsic K-rich calc-alkaline melts that display a characteristic trace element and isotopic crustal Algeria signature. Later on, slab tears propagated eastward and westward, promoting slab rollback perpendicular to plate convergence and inducing the emplacement of magmatic rocks of decreasing ages from central- eastern Algeria towards and Morocco. © 2016 Elsevier Ltd. All rights reserved.

Contents

1. Introduction ...... 28 2. The Maghrebides chain in the Kabylies (northeastern and central Algeria) ...... 28 2.1. The Kabylides within the circum-Mediterranean Alpine chains ...... 28 2.2. The Maghrebides Cenozoic magmatic belt ...... 29 3. Spatial and temporal distribution of Miocene magmatism in the Kabylides ...... 29 3.1. The Edough-Cap de Fer area (Zone 1) ...... 30 3.2. The Bougaroun-Beni Toufout area (Zone 2) ...... 30 3.3. The El Aouana area (Zone 3) ...... 31 3.4. The Bejaïa-Amizour area (Zone 4) ...... 32 3.5. The Thenia- area (Zone 5) ...... 32 4. Petrologic and geochemical features ...... 32 4.1. Whole-rock geochemistry: major elements and petrographic types ...... 32

* Corresponding author. E-mail address: [email protected] (G. Chazot). http://dx.doi.org/10.1016/j.jafrearsci.2016.10.005 1464-343X/© 2016 Elsevier Ltd. All rights reserved. 28 G. Chazot et al. / Journal of African Earth Sciences 125 (2017) 27e41

4.2. Trace elements ...... 33 4.3. Sr and Nd isotopes ...... 33 5. Discussion...... 33 5.1. A typical “subduction-related” geochemical signature ...... 33 5.2. Constraints on the origin of magmas ...... 33 5.3. Intermediate and felsic magmas: evidence for crustal contamination by the African crust and local anatexis ...... 35 5.4. Tectonic framework of magma genesis and emplacement ...... 36 5.5. Tectono-magmatic model involving African slab breakoff/tearing and lithospheric mantle delamination ...... 37 6. Conclusions ...... 39 Acknowledgments ...... 39 References ...... 39

1. Introduction includes the oldest granitic plutons emplaced at ~17 Ma (Abbassene et al., 2016), that are also the largest magmatic bodies of the whole The western Mediterranean domain experienced an especially lineament (~200 km2 and a minimum of 500 km3 for the Bougar- complex Cenozoic tectonic evolution (including the eastward and oun granite; Bouillin, 1979, 1983). These intrusive bodies are un- southeastward retreat of the Apennine-Calabria trench, and the equivocally post-collisional (Hernandez and Lepvrier, 1979; Maury southward and then westward retreat of the Betic trench) coupled et al., 2000) because they crosscut either the high-grade meta- with an unusually diversified igneous activity (Savelli, 2002; morphic basement of Lesser that has been tectonically Duggen et al., 2005; Lustrino and Wilson, 2007; Lustrino et al., exhumed at ~17.8e17.4 Ma (Bruguier et al., 2009), its Oligo- 2011, 2013; Carminati et al., 2012). The origin of the latter is Miocene cover and/or the flysch nappes emplaced during the Up- attributed to a complex interplay between geodynamic processes per Burdigalian at ~17.6 Ma. Finally, the Lesser Kabylia and east- including active subduction, slab rollback and detachment, and ernmost Greater Kabylia overlie a present-day “no-slab” upper associated back-arc basin spreading and crustal thinning (Rehault mantle region located between two segments of the African et al., 1984; von Blanckenburg and Davies, 1995; Lonergan and oceanic slab that have recently been identified around 200e300 km White, 1997; Gueguen et al., 1998; Carminati et al., 1998a, b; depth beneath the Algerian coast according to Fichtner and 2012; Jolivet and Faccenna, 2000; Rollet et al., 2002; Mauffret Villasenor~ (2015). et al., 2004; Faccenna et al., 2004, 2014; Lustrino et al., 2011; Mancilla et al., 2015). 2. The Maghrebides chain in the Kabylies (northeastern and The Maghrebides segment of the circum-Mediterranean Alpine central Algeria) belt forms a ~1200 km-long EW-trending magmatic lineament that extends along the southern Mediterranean margin from La Galite 2.1. The Kabylides within the circum-Mediterranean Alpine chains Island in Tunisia to Ras Tarf in Morocco (Fig. 1). It includes mostly Miocene K-rich calc-alkaline plutonic and volcanic rocks, and minor The Maghrebides (Rif and Tell alpine chains) represent the younger (Messinian to Plio-Quaternary) alkali and basanites southern active margin of the Western Mediterranean domain (Maury et al., 2000; Savelli, 2002; Lustrino et al., 2011; Carminati (Fig. 1) and their structure is rather similar to that of their northern et al., 2012). These igneous rocks crosscut and/or overlie both the equivalent, i.e. the Betics (Wildi, 1983), with an opposite vergence. inner and the outer zones of the Maghrebides or their African They include Internal Zones, in northern position, and External foreland. Their geological position and ages as well as their petro- Zones, in southward position. In the Kabylides (Greater Kabylia and graphic and geochemical features are typical of post-collisional Lesser Kabylia, Figs. 1 and 2), the Internal Zones are mostly made up magmatism (Hernandez and Lepvrier, 1979; Maury et al., 2000). of old allochtonous Precambrian continental crust massifs that Indeed, the latter is characterized by a considerable geochemical have recorded multiple tectono-metamorphic events since the variability and often displays temporal trends from calc-alkaline to Hercynian orogeny (granulite facies conditions at ~275e285 Main K-rich calc-alkaline and shoshonitic compositions, usually followed Greater Kabylia; Peucat et al., 1996; Hammor et al., 2006) and were by the emplacement of alkaline magmas (Harris et al., 1986; Turner exhumed during the Miocene (~18-16 Ma in Edough, Lesser et al., 1996; Lustrino and Wilson, 2007). The origin of the calc- Kabylia; Monie et al., 1992; Hammor and Lancelot, 1998; Bruguier alkaline signature is generally ascribed to mantle metasomatism et al., 2009). These basement massifs originate, like those of the by hydrous fluids or melts during an earlier subduction event Betics, Sicily and Calabria, from the « AlKaPeCa » domain (Alboran- (Miller et al., 1999; Wang et al., 2004; Chung et al., 2005). However, Kabylides-Peloritani-Calabria) dismantled by the opening of the the tectonic processes triggering the partial melting of this Mediterranean back-arc basins (Bouillin, 1986; Jolivet and subduction-modified mantle after the collision event are debated. Faccenna, 2000). In the Maghrebides, they are overlain to the Complex combinations of processes involving slab rollback and/or south by a limestone cover (Fig. 2) preserved as tectonic slices slab detachment and/or lithospheric delamination have often been (Coutelle, 1979). suggested (Harris et al.,1986; Pearce et al.,1990; Maheo et al., 2002; The External (southern) Zones of the Kabylides are mostly Chung et al., 2005; Duggen et al., 2005). In the case of the Magh- composed of flysch nappes of Cretaceous or Paleogene (Numidian rebides coastal magmatic belt, the nature of these processes as well flyschs) ages derived from the Tethys Ocean (Fig. 2), thrust over the as the geological positions and origins of the mantle and crustal Tellian units that merge into the autochtonous cover of the African magma sources are still a matter of debate (Maury et al., 2000; basement. In the northern part of the Kabylides, their basement is Lustrino et al., 2011; Carminati et al., 2012; Roure et al., 2012). unconformably overlain by the Oligo-Miocene Kabylian (OMK) The aim of this work is to discuss these questions based on the series, a molassic formation that includes Upper Oligocene con- study of the Miocene magmatic rocks in the Kabylies, located along glomerates overlain by metatuffs (silexites) dated at 19.4 Ma (K-Ar) the Mediterranean margin in Central and Eastern Algeria. This area in Greater Kabylia (Bellon, 1976; Riviere et al.,1977; El Azzouzi et al., G. Chazot et al. / Journal of African Earth Sciences 125 (2017) 27e41 29

8° 4° 0° 4° 8°

40° N 40° Iberian foreland Menorca Sardinia Majorca 0 200 km Betics Mediterranean Sea

Tanger LG Alboran Sea Algiers MG GK LK Tunis RT OR Tell Rif GR OU 34° GL Atlasic foreland 34°

8° 4° 0° 4° 8°

1 2 3 4 5678 9 10

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

Fig. 1. General sketch map of the Southwestern Mediterranean margins showing the major paleogeographic domains of Southern Spain and Northern Africa and the main allochtonous 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). The rectangle delineates the study area, detailed in Fig. 2.

2014). The OMK series are topped by tectonic olistostromes con- between 8.4 and 5.0 Ma (Bellon, 1981) and until the Pleistocene taining Cretaceous-Eocene flysch blocks. The latter are set into an near Oran, western Algeria (Louni-Hacini et al., 1995; Coulon et al., Upper Aquitanian-Lower Burdigalian clay rich matrix (Bouillin 2002) and in Oujda and Guilliz, Morocco (El Azzouzi et al., 1999). et al., 1973). Numidian flysch nappes ranging in age from Late In the Kabylides, most igneous rocks show a strong subduction- Oligocene to Early Burdigalian age are thrust over these olistos- related geochemical imprint that has been attributed to the partial tromes. These nappes were emplaced during the Upper Burdigalian melting of a subduction-modified ‘orogenic’ mantle followed by (at the boundary between N6 and N7 zones, ~17.6 Ma). All the contamination of the corresponding mafic melts by the lower and allochtonous units mentioned above are unconformably overlain upper continental crust (Maury et al., 2000; Fourcade et al., 2001; by autochtonous marls and sandstone sediments (Courme-Rault Laouar et al., 2005). The highly radiogenic 87Sr/86Sr(i) (up to and Coutelle, 1982), usually termed “post-nappes Miocene de- 0.7528), unradiogenic 143Nd/144Nd(i) (down to 0.51210) isotopic posits” (Fig. 2) of Late Burdigalian-Early Langhian ages (N7-N8 ratios, high d18O (up to þ11‰) and relatively low d34S (down zones, ~17.6e15.1 Ma range). As shown below, Miocene magmatic to 33‰) of Kabylian granitoids have been attributed to crustal rocks commonly crosscut or overlie most of these sedimentary contamination of such mafic magmas (Semroud et al., 1994; units, even sometimes the youngest ones. Fourcade et al., 2001; Laouar et al., 2002, 2005), and less often to partial melting of metasediments (Filfila pluton).

2.2. The Maghrebides Cenozoic magmatic belt 3. Spatial and temporal distribution of Miocene magmatism A ~1200 km-long EW-trending linear magmatic belt extending in the Kabylides from La Galite Island off the northern coast of Tunisia to Ras Tarf in Morocco is exposed in the Mediterranean coastal areas of the Miocene magmatic rocks delineate a 450 km long EW trending Maghrebides alpine belt (Fig. 1). It is mostly composed of Miocene strip located along the Mediterranean coast of Algeria, between (Langhian to Tortonian) shoshonitic, K-rich or medium-K calc- Annaba and Algiers (Fig. 3, upper left). We have distinguished five alkaline granites, granodiorites, andesites and rhyolites and magmatic zones, and labeled them from East to West (Fig. 3). The younger (Messinian to Plio-Quaternary) sodic alkali basalts and Edough-Cap de Fer area (Zone 1) that extends from the northwest basanites (Maury et al., 2000; Coulon et al., 2002; Savelli, 2002; of the Edough Massif to Cap Takouch near Chetaïbi and Cap de Fer is Lustrino et al., 2011; Carminati et al., 2012). These magmatic mostly composed of diorites, microgranodiorites and andesites. rocks crosscut both the Internal and the External Zones of the Zone 2 (Bougaroun-Beni Toufout) located in Lesser Kabylia (Kabylie Maghrebides or their African foreland (Fig. 1). According to Maury de Collo) includes the two large granitic/granodioritic massifs of et al. (2000), the ages of the calc-alkaline to shoshonitic volcanic Bougaroun and Beni Toufout, the much smaller Filfila granite east of and plutonic massifs roughly decrease from central-eastern Algeria , and the Collo and El Milia microgranodiorites as well as the (16-15 Ma) towards Tunisia (14e8 Ma) and Morocco (12e5 Ma). Cheraïa ignimbritic rhyolites. Zone 3 (El Aouana) is mostly andesitic The oldest presently documented age is that of Bougaroun pluton in and microdioritic and Zone 4 (Bejaïa-Amizour) mainly granodio- Lesser Kabylia (17 Ma, Abbassene et al., 2016). The younger alkaline ritic with subordinate andesites. Finally, magmatic activity in the basaltic to basanitic series do not occur in Central and Eastern Thenia-Dellys area east of Algiers (Zone 5) emplaced a wide range Algeria (Fig. 1). They were emplaced in the Mogods, Tunisia, of rocks ranging from basalts and andesites to rhyolites and 30 G. Chazot et al. / Journal of African Earth Sciences 125 (2017) 27e41

3° 5° 7° 50 km Mediterranean Sea CCF BG Lesser Kabylia AB Dl BT Collo F Annaba Dj Greater Kabylia EM Algiers Zm EK BA EA Béjaïa

36°30’ 36°30’

3° 5° 7°

1234567 9

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 Kabylies together with the locations of Miocene magmatic complexes. granites. As shown in Fig. 3 (upper left), Zones 1 to 4 roughly overlie flows crosscut by numerous andesitic, dacitic and rhyolitic dykes. the present-day “no-slab” region located between the two seg- They cover a 20 km2 area located between Chetaïbi and La Marsa ments of oceanic slabs recently identified at 300 km depth below (Fig. 3). Former studies demonstrated the K-rich calc-alkaline northern Algeria according to Fichtner and Villasenor~ (2015). character of these magmatic rocks (Ahmed-Said et al., 1993; Laouar et al., 2002, 2005). Abbassene (2016) provided a large set ± 3.1. The Edough-Cap de Fer area (Zone 1) of new K-Ar ages on whole rocks that range from 15.67 0.41 Ma to 13.70 ± 0.35 Ma for diorites and microgranodiorites and from ± ± The geology of the basement of the eastern part of Lesser 16.84 0.58 Ma to 14.32 0.33 Ma for volcanic rocks. Kabylia, known as the Edough-Cap de Fer massif (Fig. 3), is dis- cussed in a number of papers (Monie et al., 1992; Hammor and 3.2. The Bougaroun-Beni Toufout area (Zone 2) Lancelot, 1998; Caby et al., 2001, 2014; Laouar et al., 2002, 2005; Bruguier et al., 2009; Bosch et al., 2014; Fernandez, 2015; The Kabylie de Collo displays the largest amount of Miocene Fernandez et al., 2016). Edough is a metamorphic dome of lower- magmatic rocks (that outcrop over ~350 km2) not only in Central/ crustal gneisses and migmatites containing slices of garnet am- Eastern Algeria but in the whole Maghrebides belt (Fig. 1). The phibolites and peridotites. In the north, this dome is overlain by a -plutonic complex located in the Bougaroun Peninsula and tectonic melange unit containing relicts of diamond-bearing ultra- south of it (Roubault, 1934; Bouillin, 1979; Ouabadi et al., 1992; high pressure rocks, and then by the Kef Lakhal (“La Voile Noire”) Abbassene et al., 2016) include mainly peraluminous granitic plu- massive amphibolites thrusted over the former unit at ~21 Ma tons (Bougaroun in the north, Beni Toufout in the south and Filfila (Fernandez et al., 2016). The latter display an oceanic (MORB-type) in the east; Fig. 3), minor amounts of gabbros in the northern and affinity, as well as the Bou Maïza gabbros located SW of Edough. southern parts of the Bougaroun pluton (Fig. 3), microgranodiorites They were first interpreted as fragments of back-arc basin crust and microdiorites that crop out in the eastern part of the Bougaroun (Bosch et al., 2014), but further studies lead to consider them as pluton, in Collo and El Milia basins, and a few rhyolitic . shallow subducted fragments of the Tethys The ~200 km2 Bougaroun magmatic complex (Fig. 3), mostly (Fernandez, 2015; Fernandez et al., submitted). The whole Edough composed of peraluminous cordierite-bearing granites and grano- massif was finally exhumed as a metamorphic core complex at ~18- diorites, is the largest Tertiary plutonic massif in North Africa. It 16 Ma (Monie et al., 1992; Hammor and Lancelot, 1998). The latter displays a roughly elliptical shape, with an ENE-WSW trending ages were recently reevaluated to 17.8e17.4 Ma (U-Pb on mona- major axis. In its eastern part, it intrudes serpentinized peridotites zites; Bruguier et al., 2009). and kinzigitic gneiss from the Kabylian lower crust. It also intrudes Comparatively, little attention has been paid to the Miocene the Oligo-Miocene Kabylian (OMK) sediments, where it develops a magmatic units that crop out west of the Edough basement (Sidi ~1 km wide contact metamorphic aureole, and deforms the post- Bouguenna, Kef Bouassida, Sidi Saadi,^ Aïn Barbar; Fig. 3) and are nappe Miocene sediments (Bouillin, 1983), the deposition of more common between Chetaïbi and Cap de Fer (Hilly, 1957; which started at 17.6 Ma. These geological features are consistent Marignac and Zimmermann, 1983; Fougnot, 1990; Ahmed-Said with its 17 Ma U-Pb age obtained on zircons, as well as closely et al., 1993; Laouar et al., 2002, 2005). They include diorites, mi- related whole-rock K-Ar ages on granitic samples (17.23 ± 0.49 Ma, nor gabbros, microgranodiorites, microdiorites and andesites that 16.88 ± 0.43 Ma), and on associated granodioritic and dioritic crosscut and/or overlie Edough basement, Numidian and Kabylian (17.43 ± 0.53 Ma, 17.06 ± 0.41, 17.05 ± 0.43 Ma) samples (Abbassene flyschs and Miocene sedimentary deposits. Diorites form several et al., 2016). The 17 Ma age (Upper Burdigalian) obtained on the small intrusions, e.g. near Chetaïbi. Microgranodiorites are the Bougaroun pluton is the oldest presently identified for K-rich calc- most common types. They form the relatively large laccolithic in- alkaline rocks in the whole 1200 km-long EW trending Maghre- trusions of Siddi Akkacha-Cap de Fer (25 km2) and Djebel M'Zihla. bides magmatic belt. Younger K-Ar ages were, however, obtained Andesites occur as pyroclastic breccias and minor associated for a fine-grained granitic dyke crosscutting the pluton G. Chazot et al. / Journal of African Earth Sciences 125 (2017) 27e41 31

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 general location map, the light blue patterns delineate the areas underlain by oceanic slab segments at 300 km depth according to Fichtner and Villasenor~ (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 (modified from Robin, 1970). Zone 4: Bejaïa-Amizour (modified from Leikine et al., 1988; Semroud et al., 1994). Zone 5: Thenia-Dellys (modified from Belanteur et al., 1995).

(15.09 ± 0.43 Ma), as well as for two microgranodiorites that develop contact metamorphic aureoles within their host rocks (13.59 ± 0.33 Ma and 14.17 ± 0.35 Ma), a doleritic dyke (10.90 ± 0.88 that include Miocene (up to Langhian) deposits (Bouillin, 1979). Ma) and a rhyolitic dyke (10.72 ± 0.25 Ma) crosscutting gabbros One of these microgranites has been dated at 16.49 ± 0.81 Ma by K- (Abbassene et al., 2016). In addition, these authors measured older Ar (Bellon, 1976 and references therein). The Filfila granite (Upper Oligocene) Ar-Ar hornblende ages of 27.0 ± 3.0 Ma and (Semroud, 1970; Semroud and Fabries, 1977), located ca 20 km east 23.3 ± 3.2 Ma on LREE-depleted gabbros outcropping at Cap Bou- of Skikda (Fig. 3) includes two small (<10 km2) zoned intrusions of garoun sensu stricto, and considered them as related to the back-arc peraluminous cordierite-bearing syenogranites and granites dated crust of the nearby Jijel basin. at 15.3 ± 0.8 Ma by K-Ar (Bellon, 1981). It contains numerous Al-rich The geology and petrology of Beni Toufout, El Milia and Filfila gneissic enclaves and is associated with many microgranitic, aplitic intrusions has not been revised since the studies of Ouabadi (1994) and pegmatitic dykes. Its geological position, within a small tec- and Fourcade et al. (2001). The Beni Toufout pluton (~80 km2)is tonic window opened in the Kabylian flysch nappes, is rather composed of peraluminous cordierite-bearing granodiorites and similar to that of Beni Toufout pluton, but its petrogenesis has been monzogranites dated at 15.4 ± 0.8 Ma and 15.2 ± 0.7 Ma by the K-Ar ascribed to partial melting of African metasediments (Fourcade method on separated biotites (Penven and Zimmermann, 1986). It et al., 2001). intrudes (and develops a typical contact metamorphic aureole within) Berriasian to Lutetian formations of African origin that are 3.3. The El Aouana area (Zone 3) exposed within the Beni Toufout tectonic window (Bouillin, 1979) opened in the Kabylian basement nappes. From a petrogenetic The El Aouana igneous complex has a subcircular shape and point of view it is therefore unrelated to the overlying Kabylian covers an area of ~42 km2 located 15 km southwest of Jijel (Fig. 3). It nappe materials. The neighbouring El Milia peraluminous is mostly composed of volcanic (andesites, dacites) and subvolcanic cordierite-bearing microgranites form an arcuate set of intrusions (microdioritic, microgranodioritic) metaluminous rocks (Robin, 32 G. Chazot et al. / Journal of African Earth Sciences 125 (2017) 27e41

1970) that crosscut and/or overlie the Numidian flyschs and the et al., 1995). post-nappe Miocene deposits. The central part of the massif con- Basaltic and andesitic lava flows are exposed in Dellys and Cap sists of an up to 600 m thick pile of andesitic and dacitic lava flows areas, where they are either interbedded within or overlie (Port Maria unit) overlying palagonite-bearing pyroclastic deposits Miocene deposits (Fig. 3). In Dellys, a lower folded unit includes (Bou Soufa unit) emplaced in a shallow underwater environment layered hyaloclastic basaltic breccias and a basaltic flow dated at (Villemaire, 1988). These central units are crosscut by numerous NS 15.6 ± 0.9 Ma, both interbedded within a Late Burdigalian molassic trending andesitic and dacitic dykes. They are surrounded by a sequence (Raymond, 1976). It is unconformably overlain by an number of small (<1 km) and shallow cryptodomes, sills and lac- upper sequence including pillowed basaltic flows and massive colites. These parasitic intrusions display variable textures ranging andesitic flows, one of which yielded a whole-rock K-Ar age of from microlitic (andesites, dacites) to microgranular (micro- 11.8 ± 0.4 Ma (Belanteur et al., 1995). Near Cap Djinet, a 300 m thick diorites) and granular (diorites). Two microdioritic samples yielded basaltic and andesitic lava sequence covers an area of ~25 km2.Itis K-Ar ages of 14.76 ± 0.71Ma and 15.47 ± 0.81 Ma, respectively interbedded within a Late Burdigalian to Langhian marl sequence (Bellon, 1976), that are consistent with their field relationships. (Degiovanni-Azizi, 1978). From base to top it includes a 15 m thick Hydrothermal alteration features are ubiquitous in the El Aouana columnar-jointed basaltic sill overlain successively by basaltic py- massif, that hosts polymetallic Cu-Pb-Zn ore veins, e.g. in Oued El roclastic deposits, pillowed and massive basaltic flows, and upper Kebir (Villemaire, 1988; Benali, 2007). basaltic andesitic flows. This sequence is crosscut by basaltic and andesitic necks and plugs (cryptodomes). Whole-rock K-Ar dating 3.4. The Bejaïa-Amizour area (Zone 4) of Cap Djinet basaltic lavas (Belanteur et al., 1995) yielded ages of 14.3 ± 0.4 Ma (neck), 13.6 ± 0.5 Ma (flow) and 12.0 ± 0.5 Ma The Bejaïa-Amizour volcanoplutonic complex (Semroud, 1981; (average value on a cryptodome). Semroud et al., 1994), located 250 km east of Algiers, crops out in a~140km2 area located at the easternmost suture zone between 4. Petrologic and geochemical features the Greater Kabylia and Tellian (African) units. It includes an upper andesitic to rhyolitic sequence, mostly made of altered pyroclastic In the five zones described above, magmatic rocks are repre- rocks, and seven plutonic bodies (Amjout, Ansifene, Adrar In’Z'eka, sented by plutonic and volcanic rocks emplaced mostly between 17 Nadjel, Takeraouit ou Medjour, Timnachine and Tizi Ouchen, Fig. 3) and 11 Ma, without any reliable west-east age trend. There is no intruding the Tellian units, Kabylian and Numidian flyschs as well consistent evolution of the magmas with time, as mafic and felsic as the Miocene deposits of the Algerian margin (Leikine et al., rocks were emplaced altogether within neighbouring areas during 1988). The post-nappe emplacement of these metaluminous dio- the whole period. Despite the large surfaces covered by these rocks, ritic to granodioritic intrusions is evidenced by the occurrence of accurate major, trace element and isotopic data are still scarce and contact metamorphic aureoles (hornblende hornfels) in the sur- most of them are scattered in Algerian and French theses and re- rounding sedimentary units and of flysch xenoliths within the ports. For the present study, we selected data either published in plutons. The latter were probably emplaced at very shallow levels recent papers or obtained during the 1990s in recorded homoge- beneath the kilometer-thick flysch nappe cover, as suggested by the neous analytical conditions [samples crushed and powdered in an common occurrence of microgranular rocks near their chilled agate mortar for whole-rock analysis; major and trace elements margins and as veins crosscutting them (Semroud, 1981). The determined by ICP-AES in Brest following procedures described in plutonic rocks show a rather wide range from diorites to granites Cotten et al. (1995); Sr, Nd isotopic data obtained in Rennes (SiO2 ¼ 55e69 wt %), granodioritic compositions being predomi- following procedures described in Fourcade et al. (2001); whole- nant. Considerable variations of their strontium initial isotopic ra- rock K-Ar ages measured in Brest according to the methods tios (0.7077e0.7116) are thought to indicate variable degrees of described in Bellon et al. (1981)]. Nevertheless, these data need to interaction with continental crust (Semroud et al., 1994). Whole- be interpreted with some caution because of the common occur- rock K-Ar ages measured on these plutonic rocks are 16.79 ± 0.81 rence of hydrothermal alteration, especially in the most silica-rich Ma for an Amjout microdiorite (Bellon, 1976); 15.56 ± 0.44 Ma for rocks. an Amjout microgranite and 14.08 ± 0.45 Ma for an Ansifene granodiorite, respectively (Abbassene, 2016). Hydrothermal alter- 4.1. Whole-rock geochemistry: major elements and petrographic ation features are common and polymetallic Cu-Pb-Zn ore veins do types occur (Benali, 2007). As a whole, the Algerian margin magmatic rocks display a large 3.5. The Thenia-Dellys area (Zone 5) range of major element compositions (Fig. 4), with silica contents as low as 48 wt % for the most mafic rocks and higher than 80 wt % for East of Algiers, Miocene magmatic rocks crop out within a the most differentiated ones. These very high silica contents are narrow coastal strip 80 km long and 20 km wide mostly composed likely related to hydrothermal fluid percolation in these rocks. of Plio-Quaternary deposits (Fig. 3). They include a single small Mafic rocks are mostly represented by gabbros, microgabbros and plutonic body, the Thenia granodiorite, in tectonic contact along doleritic dykes with subordinated basaltic lavas flows in the two EW trending faults with the Kabylian basement to the north Thenia-Dellys area. For the most differentiated rocks, granitoids are and the Miocene post-nappe deposits to the south (Courme-Rault dominant but often associated with rhyolitic flows and pyroclastic and Coutelle, 1982). The granodiorite has been dated by K-Ar at deposits. Primitive mafic magmas have not been identified, but 16.0 ± 0.4 Ma and 15.5 ± 0.4 Ma; it is crosscut by rhyolitic dykes and MgO contents can be as high as 14 wt % for some Edough-Cap de Fer overlain by rhyolitic flows dated at 13.4 ± 0.3 Ma (Belanteur et al., samples, and are of course very low in the felsic granites (MgO< 1995). The neighbouring small coastal outcrops of El 0.5 wt %). Available geochemical data cover the whole range of Bahri and El Kerma expose dacitic and rhyolitic pyroclastic flow compositions in the three main magmatic zones, while only in- deposits that contain xenoliths of the Thenia granodiorites. Whole- termediate and felsic rocks have been analyzed in Bejaïa-Amizour rock K-Ar dating of these formations yielded 15.4 ± 0.5 Ma for a and El Aouana smaller areas. Taken altogether, the available data rhyolitic pyroclastic flow, and 14.2 ± 0.3 Ma, 14.0 ± 0.3 Ma and define a roughly consistent trend of fractional crystallization, 13.9 ± 0.3 Ma for various xenoliths within this deposit (Belanteur involving the removal of , clinopyroxene and all G. Chazot et al. / Journal of African Earth Sciences 125 (2017) 27e41 33 along the differentiation trend, then alkali feldspar. They define a period share similar geochemical characteristics. As summarized subalkaline series from basalts/gabbros to rhyolites/granites in the above, they show parallel LREE enriched patterns, high Rb, Ba, Th TAS diagram (Fig. 4a). In the K2O vs. SiO2 diagram (Fig. 4b), most of and U contents, and very low Nb contents. These rocks display the the intermediate and felsic rocks plot within the field of high-K highest La contents and the highest La/Nb ratios (Fig. 7). Except for calc-alkaline series. However, most rocks from El Aouana and a some samples from the Bejaïa-Amizour province, they are all few ones from Edough-Cap de Fer, Bejaïa-Amizour and Thenia- potassic rocks, despite important chemical variability. Most of these Dellys have lower K contents and plot within the field of medium-K rocks are intermediate to evolved and have undergone important calc-alkaline series. The mafic rocks also show a large heterogeneity crustal interactions during their differentiation in crustal magma in K2O contents, from very low values up to ~2 wt %, and display chambers. This is well illustrated by the large range of Sr-Nd iso- considerable variations within a single zone (e.g., Thenia-Dellys). topic compositions depicted on Fig. 8a, with extreme values similar to those of the local crustal rocks. It is thus difficult to properly 4.2. Trace elements characterize their mantle source. The presence of minor amounts of mafic rocks displaying Available rare earth and other trace element data are available geochemical features similar to those of diorites, granites and for the five studied zones and cover a wide range of compositions. rhyolites suggests the involvement of subduction-metasomatized In the five studied regions, most of the samples share similar trace mantle in their genesis, as already proposed by Fourcade et al. element compositions. They have flat heavy rare earth elements (2001). These more mafic samples have low 143Nd/144Nd isotopic (HREE) patterns around ten times chondrite, and strongly enriched ratios, not correlated with their SiO2 contents (Fig. 9) and moder- light REE (LREE) compositions (Fig. 5a). All these samples have also ately high d18O ratios (þ5.9 to þ9.0‰, Laouar et al., 2002, 2005). low Nb and high Rb and Ba contents (Fig. 6a). These characteristics These features indicate that they also formed from an enriched are typical of K-rich arc magmas related either to an active sub- mantle source with a strong subduction imprint, as already duction context or to a post-subduction or post-collision environ- demonstrated for the Bougaroun-Beni Toufout samples (Abbassene ment. All these samples are characterized by high La contents and et al., 2016). Their high Th/Yb ratios are consistent with the pres- high La/Nb ratios (Fig. 7). In El Aouana and Thenia-Dellys as well as ence of a mantle modified by a subduction component (melt or in the Edough-Cap de Fer zones, some mafic or intermediate fluid) extracted from a subducted oceanic lithosphere. The simi- samples have a lower LREE enrichment for similar HREE compo- larity in the shape of the REE patterns for all the concerned samples sitions (lower La/Yb), and lower Rb and Ba contents consistent with implies a similar process of magma formation all along the margin, a medium-K calc-alkaline affinity (Figs. 5b and 6b). Finally, in the and their differentiation within the continental crust, to produce two eastern provinces (Zones 1 and 2), some K-poor mafic samples the whole spectrum of compositions observed from major and show LREE-depleted compositions, while having among the high- trace element data as well as for isotopes. This subduction- est HREE contents. These samples are not enriched in Rb and Ba, flavoured magmatism corresponds to the largest volumes of and do not bear the subduction imprint shown by most of the igneous rocks emplaced along the Algerian margin, and therefore samples along the Algerian margin (Figs. 5b and 6b). In the La vs. La/ represents its main magmatic phase. This igneous activity started Nb diagram (Fig. 7), these samples display the lowest La contents some 17 Ma ago mostly in the Bougaroun-Beni Toufout area and and among the lowest La/Nb ratios. lasted for several millions years as ages ranging from 12 to 15 Ma have been determined in magmatic rocks from the five different 4.3. Sr and Nd isotopes areas.

Except for El Aouana (Zone 3), Sr and Nd isotopic data are 5.2. Constraints on the origin of mafic magmas available for the different magmatic areas and show a very large scatter (Fig. 8a). 87Sr/86Sr ratios range from 0.7038 to values higher Intermediate and felsic rocks have frequently interacted with than 0.752 while 143Nd/144Nd range from 0.51324 to 0.51205. The the crust and their chemical and isotopic composition has some- most extreme isotopic values, especially the 87Sr/86Sr ratios higher times been strongly modified during this process, blurring their than 0.730, come from the Filfila granite in the Bougaroun-Beni original mantle signature. The most mafic and thus less contami- Toufout area (Zone 2), which has been described as a crustal nated rocks can bring valuable information about their mantle melting product (Fourcade et al., 2001). Low Sr and high Nd values sources and their evolution through time. Such mafic volcanic and are found in the most mafic samples from the Bougaroun-Beni plutonic rocks exist all along the Kabylian coast and their Toufout and Edough-Cap de Fer provinces while higher Sr and geochemistry reveals interesting aspects of the geodynamic evo- lower Nd isotopic compositions are recorded in the more differ- lution of the Algerian margin. entiated samples from the four provinces. This observation is In the TAS and the K2O vs. SiO2 diagrams (Fig. 4), some mafic consistent with the nice correlation between Sr isotopic ratios and samples appear to have very low alkalies contents, and especially 87 86 silica content, where the highest Sr/ Sr ratios are found in the low K2O contents. They are mainly gabbros and amphibolites from most silica-rich samples (Fig. 8b). The Filfila granite recorded the Edough-Cap de Fer and Bougaroun-Beni Toufout areas, which isotopic composition of the local African crust, but the composi- appear to be LREE depleted (Fig. 5b). They have high HREE, similar tions of the other samples clearly indicate strong interactions be- to the high-K rocks, and thus display the lowest La/Yb ratios tween mantle magmas and the continental crust during their recorded in Algeria. Extended patterns (Fig. 6b) show that these differentiation and emplacement. samples do not display the subduction-flavoured characteristics of the enriched samples as they have very low Ba, Rb and Sr content, 5. Discussion and no negative Nb anomaly typical of geochemical subduction imprint. Their isotopic composition is also unique along the margin, 5.1. A typical “subduction-related” geochemical signature as they have 143Nd/144Nd ratios higher than 0.5130 and among the lowest Sr isotopic ratios (Fig. 8a). Some of these samples have The main magmatic phase occurred around 17 Ma ago along the higher Sr isotopic compositions probably due to sea-water in- Kabylian margin and emplaced large volumes of magma in the five teractions. These geochemical characteristics indicate that these studied areas. Most of the magmatic rocks emplaced during this mafic samples come from a depleted mantle source and share many 34 G. Chazot et al. / Journal of African Earth Sciences 125 (2017) 27e41

16 (a) -Dellys area (1, 2, 3) 14 Béjaïa-Amizour (3, 4, 5, 6) Alkaline El Aouana (3, 6) 12 Trachyte Bougaroun-Beni Toufout (3, 6, 7, 8, 9, 10) 10 Edough-Cap de Fer (9, 10, 11, Trachy- 2 andesite Trachydacite 12, 13, 14, 15) O (wt%) 8 Basaltic

2 trachy-

O+K andesite 6 Trachy-

Na Rhyolite 4 Basalt Dacite Andesite Basaltic 2 Picro- andesite basalt Subalkaline 0 35 45 55 65 75 85 SiO (wt%) 2

7 (b) Basalt Bas Andesite Dacite Rhyolite 6 And

5

4

O (wt%) 3 2 Shoshonitic Series K

2 High-K Calc-Alkaline Series

Medium-K Calc-Alkaline Series 1

0 Low-K Calc-alkaline Series 45 55 65 75 85 SiO (wt%) 2

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);2Belanteur (2001); 3 Unpublished data; 4 Semroud (1981);5Semroud et al. (1994);6Abbassene, 2016;7Abbassene et al. (2016);8Ouabadi (1994);9Fourcade et al. (2001);10Bellon (1976);11Abbassene (2016);12Laouar et al. (2005);13Bosch et al. (2014);14Ahmed-Said et al. (1993);15Fougnot (1990). characteristics with mid oceanic ridge basalts (MORB). (Fig. 5b) with a slight enrichment in LREE, and La/Yb ratios ranging Furthermore, field arguments as well as new Ar-Ar dating from 2 to 6, higher than those of the depleted samples (Fig. 10). (Abbassene et al., 2016) indicate that these mafic rocks are among They have rather high Th and U contents, and low Nb concentra- the oldest magmatic rocks of these two provinces. They clearly tions. They also display intermediate Ba, Rb and Sr contents, with Ba come from a depleted mantle source that could represent the concentrations lower than 200 ppm for most of the samples, which ambient still not modified by the subduction pro- are significantly higher than those of the depleted samples, but cesses at the time of this early magmatism and could be related to lower than most of the LREE-enriched subduction-flavoured rocks the Upper-Oligocene back-arc crust formation in nearby basins or (Fig. 11). Only three of these samples have been analyzed for Sr and to dyke systems or gabbroic intrusions crosscutting the stretched Nd isotopes. They have lower Nd isotopic compositions than the Kabylian continental crust (Bosch et al., 2014; Abbassene et al., depleted samples for similar SiO2 content (Fig. 9), and Sr isotopic 2016). Alternatively, they could represent exhumed tectonic slices compositions similar to the sea-water modified samples. Except a of the Tethys oceanic crust (Caby et al., 2014; Fernandez, 2015; few intermediate samples, they are all mafic volcanic rocks (mostly Fernandez et al., 2016; Fernandez et al., submitted). These rocks basalts) clearly not strongly affected by crustal contamination are not genetically related to the other mafic, intermediate and processes as their isotopic compositions are not correlated with felsic rocks of the Algerian margin and have been juxtaposed to their SiO2 content. They have variable but sometimes high K2O younger magmatic rocks during the margin evolution. contents. They obviously derived from a non-depleted mantle Among the most mafic rocks, with SiO2 lower than 52 wt % for source, but clearly different from the source of the main magma- most of them, there are basalts, gabbros and some microdiorites tism of the Algerian margin (Figs. 5 and 6). Available K-Ar ages from and andesites with geochemical characteristics intermediate be- Zones 2, 3 and 5 suggest that these moderately enriched rocks are tween the depleted mafic samples and the subduction-flavoured K- somewhat younger (16.5e11.6 Ma) than those emplaced during the rich rocks. These rocks are present in the Bougaroun-Beni Toufout main K-rich calc-alkaline magmatic event (17 Ma-old Bougaroun and Thenia-Dellys areas, and a few samples are also present in the granite), with ages ranging from 16.5 to 11.6 Ma. The source of this El Aouana zone (Zone 3, Fig. 3). They have nearly flat REE patterns volcanism is slightly modified by a subduction component, and is G. Chazot et al. / Journal of African Earth Sciences 125 (2017) 27e41 35

1000 (a) Thénia-Dellys area 100 Béjaïa-Amizour (a) El Aouana 100 Bougaroun-Beni Toufout Edough-Cap de Fer 10 10

Rock/Chondrites 1 Rock/Primitive Mantle

1 .1

.01 La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Yb Lu Cs Ba U Ta Ce Pr Nd Hf Eu Dy Yb Rb Th Nb La Pb Sr Zr Sm Ti Y Lu

(b) Thénia-Dellys area El Aouana 1000 Bougaroun-Beni Toufout 100 Edough-Cap de Fer (b) 100

10 10 Rock/Chondrites

1 Rock/Primitive Mantle 1 .1 La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Yb Lu .01 Fig. 5. Chondrite-normalized rare earth element patterns for (a) the highly LREE- Cs Ba U Ta Ce Pr Nd Hf Eu Dy Yb enriched samples and (b) the moderately enriched and depleted samples from the Rb Th Nb La Pb Sr Zr Sm Ti Y Lu five different zones. Chondrite values from Anders and Grevesse (1989).

Fig. 6. Primitive mantle-normalized trace element patterns for the Kabylian magmatic rocks. (a) highly LREE-enriched samples and (b) moderately enriched and depleted present all along the Kabylian margin. samples. Primitive mantle values from Sun and McDonough (1989).

5.3. Intermediate and felsic magmas: evidence for crustal contamination by the African crust and local anatexis 6

Crustal contamination can deeply alter the mantle signature of 5 magmatic rocks. Indeed, the participation of the crust in the evo- lution of the magmas emplaced along the Algerian margin has already been evidenced. For example, Bougaroun intermediate and 4 felsic peraluminous (cordierite-bearing) rocks and the neighbour- ing granitoids mainly formed through incorporation of pelitic 3 fi metasediments within mantle-derived ma c magmas (Fourcade La/Nb et al., 2001). Crustal anatexis of metasediments was evidenced only in the case of the highly peraluminous small Filfila granite 2 (Ouabadi, 1994; Fourcade et al., 2001) that displays the most 87 86 ¼ e radiogenic Sr isotopic compositions ( Sr/ Sri 0.732 0.754) 1 recorded along the Algerian margin (Fig. 8). The continuum of major element compositions in all the five studied zones from mafic rocks to highly differentiated ones points 0 0 5 10 15 20 25 30 35 40 towards crystal fractionation as the major process for forming the La felsic magmas instead of important crustal melting that would have generated more felsic rocks than mafic and intermediate ones. Fig. 7. La/Nb versus La diagram for the Kabylian Miocene magmatic rocks. Symbols as Most of the LREE enriched samples have parallel rare earth element in Fig. 4. The red field highlights the LREE-depleted samples. patterns with an increase of the Eu negative anomaly from the intermediate to the most evolved rhyolites and granites, as a result of feldspar fractionation. Nevertheless, crustal contamination Kabylian margin. Isotopes are a very powerful tool to study such played an important role in the magma evolution along the contamination processes. Few isotopic data have been published 36 G. Chazot et al. / Journal of African Earth Sciences 125 (2017) 27e41

0.5134 0.5132 (a) 0.5132 0.5130 0.5130 0.5128

144 0.5128 0.5126 144 0.5126 143 ( Nd/ Nd)i 143

0.5124 ( Nd/ Nd)i 0.5124 0.5122 0.5122 0.5120 0.702 0.712 0.722 0.732 0.742 0.752 0.5120 87 86 051015 ( Sr/ Sr)i La/Yb

Fig. 10. Initial 143Nd/144Nd ratios versus La/Yb ratios for the Kabylian Miocene (b) 0.752 magmatic rocks. Symbols as in Fig. 4. The red field highlights the LREE-depleted samples.

0.742

d18 þ ‰ d34 0.732 together with the high O (up to 11 ) and relatively low S (down to 33‰) of Kabylian granitoids, are consistent with the 87 86

( Sr/ Sr)i hypothesis of strong interactions between metasomatized mantle- 0.722 derived magmas and the African crustal rocks (Semroud et al., 1994; Fourcade et al., 2001; Laouar et al., 2002, 2005). The pres- 0.712 ence of metamorphic minerals likely inherited from crustal xeno- liths in some granitic rocks confirms these observations. The large 0.702 amounts of mafic mantle magmas emplaced in the upper crust may 45 50 55 60 65 70 75 80 have induced partial melting of the crustal rocks and hybridization SiO (wt%) of these crustal melts with the mantle magmas in the course of the 2 fractional crystallization processes. The overwhelming crustal Fig. 8. (a) Initial 143Nd/144Nd ratios versus initial 87Sr/86Sr ratios and (b) initial signature of many samples from the different magmatic provinces 143 144 Nd/ Nd ratios versus SiO2 contents (wt. %) for Kabylian Miocene magmatic rocks. is unlikely to result exclusively from assimilation of metasediments Symbols as in Fig. 4. The red field highlights the LREE-depleted samples. coupled with fractional crystallization (AFC; DePaolo, 1981)of mafic 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). 0.5132 5.4. Tectonic framework of magma genesis and emplacement 0.5130 The development of concepts led a number of 0.5128 authors (Dercourt, 1970; Le Pichon et al., 1971; Auzende et al., 1973;

144 Bellon, 1976; Bouillin, 1979; Cohen, 1980; Rehault et al., 1984; Ricou 0.5126 et al., 1986; Dewey et al., 1989) to consider that the Maghrebides alpine belt resulted from the Cenozoic northward-dipping sub- 143

( Nd/ Nd)i 0.5124 duction of a Ligurian-Tethyan oceanic slab located north of the African continental lithosphere beneath another continental 0.5122 domain termed « AlKaPeCa » (Alboran-Kabylides-Peloritani-Cala- bria; Bouillin, 1986; Jolivet and Faccenna, 2000) and later disman- 0.5120 tled by the opening of the Mediterranean oceanic basins formed in 45 50 55 60 65 70 75 80 back-arc position during slab rollback (Lonergan and White, 1997; SiO (wt%) 2 Gueguen et al., 1998). Cohen (1980) proposed that this northward-dipping subduction ended through a slab detachment Fig. 9. Initial 143Nd/144Nd ratios versus SiO content for Kabylian Miocene magmatic 2 process. This hypothesis was strongly supported by later tomo- rocks. Symbols as in Fig. 4. The red field highlights the LREE-depleted samples while graphic studies (Spakman, 1986a,b; Carminati et al., 1998a,b; the moderately LREE-enriched samples have lower Nd isotopic ratio for similar SiO2 contents. Wortel and Spakman, 2000; Spakman and Wortel, 2004) and is still a major ingredient of geodynamic reconstructions (e.g., Frizon de Lamotte et al., 2000; Rosenbaum et al., 2002a,b; Faccenna et al., 2004; 2014; van Hinsbergen et al., 2014). The recent tomographic about the magmatism of the five studied zones, but available Sr and study of Fichtner and Villasenor~ (2015) identified possible rem- Nd isotopic ratios are well correlated with indexes of differentiation nants of the Tethys oceanic slab around 200e300 km depth such as SiO (Fig. 8b) and also MgO contents. These correlations, 2 beneath the central and eastern Algerian coast. These authors G. Chazot et al. / Journal of African Earth Sciences 125 (2017) 27e41 37

40 Kabylides can be ascertained. Assuming that the heat flux required for the partial melting of 35 lithospheric mantle and/or crustal units was provided by the asthenosphere uprising through the slab tear during a slab 30 detachment process, two problems (already pointed out by Maury et al., 2000) arise concerning the genesis of Miocene calc-alkaline 25 magmatism in the Kabylies. First, a subduction-modified hydrous lithospheric mantle, potential source of the mafic and intermediate La/Yb 20 magmas (section 5.2) was necessarily present beneath the studied 15 area at depths consistent with the stability of pargasite (<2GPa, ~65 km; Niida and Green, 1999). The most obvious candidate is the 10 AlKaPeCa lithospheric mantle metasomatized during the Tethys subduction event prior to the collision. However, the Kabylian 5 continental crust basement is probably only a few km thick ac- fi 0 cording to eld data (Bouillin, 1979). The underlying subcontinental 0 100 200 300 400 500 600 700 metasomatized mantle was therefore likely separated from the Ba crust during the collision process, as previously proposed for the Kabylides (Roure et al., 2012) and the Betics-Rif collision zone Fig. 11. La/Yb ratios versus Ba concentrations for the Kabylian Miocene magmatic rocks. Symbols as in Fig. 4. The red field highlights the LREE-depleted samples. (Frizon de Lamotte et al., 2000; Duggen et al., 2005; Mancilla et al., 2015; Villasenor~ et al., 2015). A second problem concerns the fact that Kabylian granitoids experienced strong interactions with the African crustal rocks consider this slab as detached from the surface (>150 km) and (Semroud et al., 1994; Fourcade et al., 2001; Laouar et al., 2002, broken into two segments, presently located east and west of the 2005), as discussed in Section 5.3. Beni Toufout and Filfila (Zone ~ Kabylies, respectively (Fig. 3; Fichtner and Villasenor, 2015; their 2) and Bejaïa-Amizour (Zone 4) massifs do not crosscut Kabylian Fig. 9). basement rocks and are therefore unlikely to be contaminated by Oceanic slab detachment/breakoff provides a rather attractive them (Bouillin, 1979). It is thus necessary to postulate that during explanation for the origin of post-collisional magmatism (von the collision process the African continental crust was either Blanckenburg and Davies, 1995; Davies and von Blanckenburg, thrusted northwards (relative to Kabylia) over the subduction- 1995). Indeed, the thermal flux originating from the deep modified Kabylian lithospheric mantle (Maury et al., 2000), or asthenosphere and uprising through the slab tear is likely to pro- alternatively that the Kabylian mantle flowed southwards vide the excess heat triggering the partial melting of metasomat- following the delaminated Maghrebian continental lithospheric ized lithospheric mantle and/or the continental crust overlying it. mantle. This continental crust was therefore necessarily peeled off In the case of the Kabylides, an early Miocene slab breakoff is also from the African subcontinental lithospheric mantle. In other consistent with the rapid uplift and exhumation of high pressure words, the African crust was sandwiched between the crust and the metamorphic rocks, regional metamorphism, hydrothermalism mantle of the Kabylian lithosphere (Fig. 12B). This interpretation and mineralization indicating increased heat flow and followed by agrees with the idea that at the post-collision stage (Fig. 12B), the fast cooling (Carminati et al., 1998a,b; Caby et al., 2001). These African lithospheric mantle was replaced by hotter Kabylian lith- processes are well documented in the Edough massif (Zone 1) at osphere, coevally with an asthenospheric rise beneath the Algerian 20.9 Ma for thrusting of Kef Lakhal oceanic complex, 17.7 Ma for basin and with slab detachment (Roure et al., 2012). The deep part exhumation of the Edough lower-crustal units and 17 Ma for fast of the present-day ~25e40 km thick crust of the Kabylies (Mihoubi cooling of their footwall rocks (Fernandez et al., 2015). They were et al., 2014; Bouyahiaoui et al., 2015; Hamai et al., 2015) would immediately followed by the onset of K-rich calc-alkaline mag- therefore be, according to our hypothesis, composed mostly of Af- matism at ~17 Ma in the neighbouring (Zone 2) Bougaroun-Collo rican basement units. massif (Abbassene et al., 2016). These results suggest that the ki- nematic reconstructions assuming an age younger than 17 Ma for 5.5. Tectono-magmatic model involving African slab breakoff/ the collision between Africa and the Kabylian domain of AlKaPeCa tearing and lithospheric mantle delamination (Carminati et al., 1998a, b; Frizon de Lamotte et al., 2000; Lustrino et al., 2011; van Hinsbergen et al., 2014; Bouyahiaoui et al., 2015) A schematic illustration of the genesis of Miocene magmatic need to be reconsidered. rocks from the Kabylides and their mantle/crustal sources is pro- The magmatic pulse at ~17 Ma in Bougaroun (Zone 2) is the posed in Fig. 12. The two lithospheric-scale cross-sections drawn at oldest presently documented K-rich calc-alkaline event marking the level of Zone 2 (Bougaroun-Beni Toufout, corresponding to the the onset of the Neogene igneous history of the Maghrebides, and maximal abundance of K-rich calc-alkaline granitoids and to the also the one that emplaced the largest volumes of intermediate and mean position of the “no-slab” region) depict the possible positions 3 evolved magmas, possibly up to 800e1000 km for Zone 2 of magmatic sources at the end of the Ligurian-Tethyan oceanic (Abbassene et al., 2016). However, according to available K-Ar ages subduction process (Aquitanian - Burdigalian; Fig. 12A) and at the presented in Section 3, magmatic activity started in the four other post-collision stage, i.e. during the emplacement of Bougaroun and zones between ~17 and ~15.5 Ma, and stopped between ~14 and neighbouring magmatic bodies at 17-11 Ma (Fig. 12B), respectively. ~12 Ma (~11 Ma in Zone 2). Medium-K calc-alkaline rocks were During Upper Oligocene (Fig. 12A), the Tethys oceanic slab was emplaced in minor amounts in Zones 2, 3 and 5 between 16.5 and still subducting towards the north below the Kabylides, meta- 11.6 Ma, i.e. almost during the whole span of magmatic activity in somatizing the adjacent parts of the Kabylian subcontinental lith- the Kabylides. Available K-Ar ages for the single anatectic granite of ospheric mantle. The rollback of this slab initiated backarc Filfila in Zone 2 (15.3 ± 0.8 Ma, Bellon, 1981) fall within the same extension and seafloor spreading (Gueguen et al., 1998; Jolivet and range. Therefore, no obvious spatial or temporal pattern of Faccenna, 2000; Faccenna et al., 2004; Booth-Rea et al., 2007; emplacement of these various petrogenetic types within the Carminati et al., 2012), dismembering the AlKaPeCa domain and 38 G. Chazot et al. / Journal of African Earth Sciences 125 (2017) 27e41 leading to the formation of the North Algerian basin (Schettino and previously proposed for several sectors of the Maghrebides (Frizon Turco, 2006, 2011). LREE-depleted back-arc mafic magmas might de Lamotte et al., 2000; Roure et al., 2012; Mancilla et al., 2015; have intruded the thinned continental crust of the Kabylides (Bosch Villasenor~ et al., 2015) but rarely with reference to their et al., 2014; Abbassene et al., 2016), although Cap Bougaroun (Zone magmatic evolution (Duggen et al., 2005). Actually, both delami- 2) and Bou Maiza (Zone 1) depleted gabbros and Kef Lakhal am- nation and slab detachment processes trigger a fast rise of phibolites (Zone 1) may alternatively represent exhumed slices of asthenosphere to replace the sinking lithosphere (Bird, 1979; Tethyan oceanic crust (Fernandez et al., 2016; Fernandez et al., Wortel and Spakman, 2000). The sinking of the detached Tethys submitted). slab explains the progressive development of a “no-slab” region The Burdigalian collision of the Kabylides with the African beneath the Kabylies owing to an east-west lithospheric tearing margin induced delamination and slab breakoff process (Fig. 12B). (Medaouri et al., 2014; Chertova et al., 2014; Rosenbaum, 2014), Our interpretation (Fig. 12) involves a double “decollement ” of while remnants of this slab are still detectable by seismic tomog- crustal and mantle units: (1) that of the metasomatized litho- raphy around 200e300 km depth below the Algerian coast spheric subcontinental mantle of the Kabylies with respect to his (Fichtner and Villasenor,~ 2015) east and west of the studied area, thinned continental crust, and (2) that of the edge of the African respectively (Fig. 3). In our tentative model, the thermal flux of lithosphere (Tethyan passive margin), where slab sinking and asthenospheric origin uprising through the slab tear induced an breakoff led to decoupling of the African continental crust from its important thinning of the Kabylian and African lithospheric man- underlying subcontinental mantle. Such processes have been tles by thermal erosion, similar to that envisioned in other cases of

Fig. 12. Schematic cross-sections showing the possible positions of the lithospheric geochemical reservoirs involved in the genesis of Kabylian magmas and/or their mantle/crustal sources during the Aquitanian-Burdigalian (A) and at 17e11 Ma (B), modified from Abbassene et al. (2016). Crustal and lithospheric thicknesses are interpolated from present-day values obtained for Lesser Kabylia (Mihoubi et al., 2014; Bouyahiaoui et al., 2015; Hamai et al., 2015; Carballo et al., 2015) and from the postulated ages of the oceanic slabs (van Hinsbergen et al., 2014). Sediment thicknesses are only indicative. The model of delamination of the edge of the African lithosphere is adapted from Frizon de Lamotte et al. (2000), Duggen et al. (2005) and Roure et al. (2012). The extent of thermal erosion of the Kabylian lithosphere following slab breakoff is qualitatively estimated from models of slab breakoff and delamination by Davies and von Blanckenburg (1995) and Bird (1979), respectively. The tectonic and geodynamic arguments supporting this model are discussed in the text. An alternative way to describe this evolution is to consider that the Kabylian metasomatized lithospheric mantle was underthrusted below the African crust, owing to a continuation of slab rollback after collision, as suggested by laboratory modelling (Gogüs€ et al., 2011; Bajolet et al., 2012). 1: Sediments (undifferentiated). 2: Synrift sediments (2a: Mesozoic; 2b: Oligo-Miocene); 3: Tethyan oceanic crust; 4: Back-arc oceanic crust (Algerian basin); 5: Kabylian continental crust; 6: Miocene magmatic bodies (BG Bougaroun, BT Beni Toufout); 7: Tethyan suboceanic lithospheric mantle; 8: Back-arc lithospheric mantle (Algerian basin); 9: Kabylian lithospheric mantle; 10: Subduction-metasomatized Kabylian lithospheric mantle. G. Chazot et al. / Journal of African Earth Sciences 125 (2017) 27e41 39 mantle delamination or slab breakoff/lithospheric detachment slab tear propagation. The corresponding mafic magmas were (Bird, 1979; Davies and von Blanckenburg, 1995; von Blanckenburg occasionally emplaced between in Zones 2, 3 and 5 as medium- and Davies, 1995; Duggen et al., 2005). We propose that this ther- K calc-alkaline lavas or gabbros/diorites devoid of important mal erosion induced the onset of partial melting at 17 Ma of the crustal contamination features. More often, complex in- Kabylian lithospheric mantle metasomatized during the former teractions with the African basement during their uprise led to subduction event (Abbassene et al., 2016). Minor amounts of the the emplacement of intermediate to felsic K-rich calc-alkaline corresponding subduction-flavoured mafic and intermediate melts melts with a strong to overwhelming trace element and iso- were emplaced as metaluminous medium-K calc-alkaline magmas topic crustal imprint. The end of this magmatic activity at ~14-11 devoid of major contamination effects by the African continental Ma might be related to the progressive thermal reequilibration crust in Zones 2, 3 and 5. More often, however, massive incorpo- of the asthenosphere following slab breakoff. ration of pelitic metasediments from the African continental units fi within those ma c magmas generated K-rich intermediate melts Acknowledgments and the majority of peraluminous cordierite-bearing granitic magmas (e.g. Bougaroun and Beni Toufout) displaying a strong to This study was supported by the Algerian-French research overwhelming crustal geochemical signature (Fourcade et al., Project SPIRAL (Structures Profondes et Investigations Regionales 2001). Then, after the main magmatic event at 17-16 Ma, en ALgerie), a cooperative program initiated in 2009 by DG-RSDT, emplacement of smaller volumes of K-rich and occasional medium- CRAAG and Sonatrach in Algeria, and IUEM-UBO, Nice-Sophia K calc-alkaline melts occurred sporadically in Zones 1 to 5 until Antipolis Universities and IFREMER in France. We thank Tim Hor- ~14-11 Ma. The end of this magmatic activity might be related to scroft for inviting us to submit this manuscript, and also Guillermo the progressive thermal reequilibration of the asthenosphere Booth-Rea and Dominique Frizon de Lamotte for their pertinent below the Kabylides following slab detachment, as envisioned in comments on the tectonic and geodynamic evolution of the area. other post-collision settings (Bird, 1979; Harris et al., 1986). This We also thank Marcaurelio Franzetti for his help designing Fig. 12, spatio-temporal evolution of magmatic activity is consistent with and Alain Coutelle for discussing with us the geology of Northern slab tear propagation eastward and westward from central-eastern Algeria and the related literature. Members of the SPIRAL team are Algeria, supporting an age decrease of calc-alkaline magmatic ac- thanked for fruitful exchanges on the Algerian margin. tivity towards Tunisia and Morocco until the Pliocene (Maury et al., 2000). References

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