Evidence from Bougaroun (Northeastern Algeria) and Geodynamic Implications
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Tectonophysics 674 (2016) 114–134 Contents lists available at ScienceDirect Tectonophysics journal homepage: www.elsevier.com/locate/tecto A 17 Ma onset for the post-collisional K-rich calc-alkaline magmatism in the Maghrebides: Evidence from Bougaroun (northeastern Algeria) and geodynamic implications Fatiha Abbassene a,b,⁎, Gilles Chazot a, Hervé Bellon a,c, Olivier Bruguier d, Aziouz Ouabadi b,RenéC.Maurya, Jacques Déverchére a, Delphine Bosch d, Patrick Monié d a Université de Brest, CNRS, UMR 6538 Domaines Océaniques, Institut Universitaire Européen de la Mer (IUEM) Place Nicolas Copernic, 29280 Plouzané, France b Université des Sciences et de la Technologie Houari Boumedienne, Bab Ezzouar, Laboratoire de Géodynamique, Géologie de l'Ingénieur et Planétologie (LGGIP/FSTGAT/USTHB), Alger, Algeria c Université de Brest, CNRS, UMR 6538 Domaines Océaniques, 6 avenue Le Gorgeu, c.s. 93837, 29238 Brest, France d Université de Montpellier II, CNRS-UMR 5243, Géosciences Montpellier, Place E. Bataillon, c.c. 060, 34095 Montpellier, France article info abstract Article history: Bougaroun is the largest pluton (~200 km2) in the 1200 km-long Neogene magmatic belt located along the Received 16 July 2015 Mediterranean coast of Maghreb. New U–Pb dating on zircons and K–Ar ages on whole rocks and separated min- Received in revised form 1 February 2016 erals document its emplacement at 17 Ma within the Lesser Kabylian basement, a continental block that collided Accepted 5 February 2016 with the African margin during the Neogene. This Upper Burdigalian intrusion is therefore the oldest presently Available online 17 February 2016 identified K-rich calc-alkaline massif in the whole Maghrebides magmatic lineament and marks the onset of its activity. The Bougaroun peraluminous felsic rocks display a very strong crustal imprint. Associated maficrocks Keywords: “ ” Calc-alkaline magmas (LREE-enriched gabbros) have preserved the orogenic (subduction-related) geochemical signature of their Geochronology mantle source. Older depleted gabbros cropping out at Cap Bougaroun are devoid of clear subduction-related Geochemistry imprint and yielded Ar–Ar hornblende ages of 27.0 ± 3.0 Ma and 23.3 ± 3.2 Ma. We suggest that they are related Post-collision to the Upper Oligocene back-arc rifted margin and Early Miocene oceanic crust formation of the nearby Jijel basin, Kabylia an extension of the Algerian basin developed during the African (Tethyan) slab rollback. The fact that the Maghreb Bougaroun pluton intrudes exhumed Kabylian lower crustal units, mantle slices and flysch nappes indicates that the Kabylian margin was already stretched and in a post-collisional setting at 17 Ma. We propose a tectono-magmatic model involving an Early Miocene Tethyan slab breakoff combined with delamination of the edges of the African and Kabylian continental lithospheres. At 17 Ma, the asthenospheric thermal flux upwell- ing through the slab tear induced the thermal erosion of the Kabylian lithospheric mantle metasomatized during the previous subduction event and triggered its partial melting. We attribute the strong trace element and isoto- pic crustal signature of Bougaroun felsic rocks to extensive interactions between ascending maficmeltsandthe African crust underthrust beneath the Kabylie de Collo basement. © 2016 Elsevier B.V. All rights reserved. 1. Introduction the remobilization of this subduction-modified mantle during or after the collision event are widely debated. Complex processes involving Post-collisional magmatism is characterized by a considerable slab rollback and/or slab breakoff and/or lithospheric delamination geochemical diversity and temporal trends from calc-alkaline to K-rich have often been envisioned (Harris et al., 1986; Pearce et al., 1990; calc-alkaline and shoshonitic compositions, sometimes followed by Mahéo et al., 2002; Chung et al., 2005; Duggen et al., 2005). intraplate-type alkaline magmas (Harris et al., 1986; Turner et al., The Maghrebides segment of the peri-Mediterranean Alpine Belt in- 1996; Lustrino et al., 2007). The origin of the calc-alkaline signature is cludes a ~1200 km-long EW-trending magmatic lineament that extends generally ascribed to mantle metasomatism by hydrous fluids or melts from La Galite Island off the northern coast of Tunisia to Ras Tarf in during an earlier subduction event (Miller et al., 1999; Wang et al., Morocco (Fig. 1a). It is mostly made up of Miocene (Langhian to 2004; Chung et al., 2005), but the tectonic processes responsible for Tortonian) potassic calc-alkaline granites, granodiorites, andesites and rhyolites and younger (Messinian to Plio-Quaternary) alkali basalts and basanites (Maury et al., 2000; Savelli, 2002; Lustrino et al., 2011; ⁎ Corresponding author at: Université de Brest, CNRS, UMR 6538 Domaines Océaniques, Carminati et al., 2012). These magmatic rocks crosscut either the inner Institut Universitaire Européen de la Mer (IUEM) Place Nicolas Copernic, 29280 Plouzané, France. Tel.: +33 213561386874. or the outer zones of the Maghrebides or even their African foreland E-mail address: [email protected] (F. Abbassene). (Fig. 1a). Of special interest are the Cap Bougaroun, Beni Toufout and http://dx.doi.org/10.1016/j.tecto.2016.02.013 0040-1951/© 2016 Elsevier B.V. All rights reserved. F. Abbassene et al. / Tectonophysics 674 (2016) 114–134 115 Fig. 1. Geological sketch maps of North Algeria. (a) General map showing the three paleogeographic domains of Southern Spain and Northern Africa and the different allochtonous nappes, modified from Mahdjoub et al. (1997). The main magmatic outcrops are also shown. (b) Map of North Algeria showing the locations of magmatic complexes simplified after Vila (1980). The area in the rectangle represents the Cap Bougaroun region for which we present new geochronological and geochemical data. Edough-Cap de Fer occurrences in Petite Kabylie (eastern Algeria; element analyses, Sr and Nd isotopic compositions as well as U–Th– Fig. 1b) because: (1) according to K–Ar dating, they are the oldest Pb, Ar–Ar and K–Ar dating, we aim to reconstruct the succession of known rocks of the whole lineament (Burdigalian: 16.4 Ma in tectono-magmatic events that shaped this key area since Early Miocene Bougaroun and 16.2 Ma in Beni Toufout; Maury et al., 2000); (2) they and to highlight their geodynamic significance in the frame of the include rocks ranging from K-rich calc-alkaline gabbros to granites, convergence between the African and European plates. the petrogenesis of which has previously been investigated (Ouabadi, 1994; Fourcade et al., 2001; Laouar et al., 2002, 2005); (3) these granitic 2. Geological framework plutons represent by far the largest magmatic bodies of the whole lineament (~200 km2 and a minimum of 500 km3 for the Bougaroun 2.1. Magmatism in the southwestern Mediterranean domain granite; Bouillin, 1979, 1983); and finally (4) they crosscut either the high-grade metamorphic basement of the Petite Kabylie, which has The western Mediterranean domain experienced a widespread and been tectonically exhumed at ~17.8–17.4 Ma (Bruguier et al., 2009), geochemically diverse igneous activity during the Cenozoic (Savelli, the Oligo–Miocene cover or the flysch nappes emplaced during the 2002; Duggen et al., 2005; Lustrino and Wilson, 2007; Lustrino et al., Upper Burdigalian at ~17.6 Ma. 2011, 2013; Carminati et al., 2012). Its origin is attributed to a combina- Thus, studying these magmatic rocks allows us to document an tion of geodynamic mechanisms involving slab rollback and slab tearing unusually fast succession of geodynamic events leading to exhumation under the continental margins, producing concomitant back-arc basin of the lower crust (Caby et al., 2001), partial melting of mantle and spreading and crustal thinning (Réhault et al., 1984; von Blanckenburg crust, and emplacement of magmas in the inner and outer zones of an and Davies, 1995; Gueguen et al., 1998; Carminati et al., 1998a, b, Alpine Belt. Using new field studies and sampling, major and trace 2012; Jolivet and Faccenna, 2000; Faccenna et al., 2004; Facenna et al., 116 F. Abbassene et al. / Tectonophysics 674 (2016) 114–134 2014; Lustrino et al., 2011; Mancilla et al., 2015). The complexity of the 2) The Precambrian Kabylian basement crops out widely in the south- western Mediterranean tectonic framework (e.g., the eastward and ern part of the studied zone and comprises two major units. The southeastward retreat of the Apennine–Calabria trench, and the lowermost basement is mainly made up of high-grade gneisses southward and then westward retreat of the Betic trench) has led metamorphosed in granulite facies conditions (Djellit, 1987). In the many authors to emphasize the difficulty to deal with the distribution “Grande Kabylie” (Greater Kabylia) Massif, granulite facies condi- of western Mediterranean igneous activity (Lustrino et al., 2011; El tions have been dated at ~275–285 Ma (Peucat et al., 1996; Azzouzi et al., 2014). Hammor et al., 2006). The uppermost unit is formed by marbles, The ~1200 km-long magmatic belt outcropping along the northern micaschists and phyllites affected by low-grade metamorphism coast of Maghreb (Fig. 1a) is made of volcanic and plutonic massifs (greenschist facies; Mahdjoub et al., 1997). This Kabylian basement belonging to two temporally and geochemically distinct Cenozoic is unconformably overlain by the Oligo–Miocene Kabylian (OMK) magmatic suites (Maury et al., 2000): (1) an older calc-alkaline to series, a molassic formation that presents basal Upper Oligocene shoshonitic volcanic and plutonic belt, the ages of which roughly conglomerates and is topped by metatuffs (silexites) dated at decrease from central-eastern Algeria (16–15 Ma) towards Tunisia 19.4 Ma (K–Ar) in Greater Kabylia (Bellon, 1976; Rivière et al., (14–8 Ma) and Morocco (12–5 Ma); (2) a younger alkaline basaltic to 1977; Gélard et al., 1973; Géry et al., 1981; El Azzouzi et al., 2014). basanitic series emplaced since 8 Ma in Tunisia and Morocco (Bellon, Olistostromes form the upper part of the OMK series. These tectonic 1981) and until the Pleistocene in western Algeria (Louni-Hacini et al., units are made of Cretaceous–Eocene flysch blocks set into an Upper 1995)andinMorocco(El Azzouzi et al., 1999).