New 40Ar–39Ar Dating of Lower Cretaceous Basalts at the Southern Front of the Central High Atlas, Morocco: Insights on Late Me
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International Journal of Earth Sciences (2018) 107:2491–2515 https://doi.org/10.1007/s00531-018-1609-7 ORIGINAL PAPER New 40Ar–39Ar dating of Lower Cretaceous basalts at the southern front of the Central High Atlas, Morocco: insights on late Mesozoic tectonics, sedimentation and magmatism G. Moratti1 · M. Benvenuti1,2 · A. P. Santo1,2 · M. A. Laurenzi3 · E. Braschi1 · S. Tommasini2 Received: 1 August 2017 / Accepted: 28 March 2018 / Published online: 9 April 2018 © Springer-Verlag GmbH Germany, part of Springer Nature 2018 Abstract This study is based upon a stratigraphic and structural revision of a Middle Jurassic–Upper Cretaceous mostly continental succession exposed between Boumalne Dades and Tinghir (Southern Morocco), and aims at reconstructing the relation among sedimentary, tectonic and magmatic processes that affected a portion of the Central High Atlas domains. Basalts interbed- ded in the continental deposits have been sampled in the two studied sites for petrographic, geochemical and radiogenic isotope analyses. The results of this study provide: (1) a robust support to the local stratigraphic revision and to a regional lithostratigraphic correlation based on new 40Ar–39Ar ages (ca. 120 Ma) of the intervening basalts; (2) clues for reconstruct- ing the relation between magma emplacement in a structural setting characterized by syn-depositional crustal shortening pre-dating the convergent tectonic inversion of the Atlasic rifted basins; (3) a new and intriguing scenario indicating that the Middle Jurassic–Lower Cretaceous basalts of the Central High Atlas could represent the first signal of the present-day Canary Islands mantle plume impinging, flattening, and delaminating the base of the Moroccan continental lithosphere since the Jurassic, and successively dragged passively by the Africa plate motion to NE. The tectono-sedimentary and magmatic events discussed in this paper are preliminarily extended from their local scale into a peculiar geodynamic setting of a continental plate margin flanked by the opening and spreading Central Atlantic and NW Tethys oceans. It is suggested that during the late Mesozoic this setting created an unprecedented condition of intraplate stress for concurrent crustal shortening, related mountain uplift, and thinning of continental lithosphere. Keywords Central High Atlas · Late Mesozoic basalts · 40Ar–39Ar dating · Geochemistry · Sr–Nd radiogenic isotopes · Tectono-sedimentary evolution Introduction The Atlas System, developed at the northern margin of the African plate (Piqué et al. 2002), is a well-studied case of structural inversion occurred in a tectono-sedimentary evo- lution from continental rifting to convergence of Europe- Electronic supplementary material The online version of this Africa plates. This dynamic setting was affected by regional article (https ://doi.org/10.1007/s0053 1-018-1609-7) contains supplementary material, which is available to authorized users. magmatic activity related to the evolution of the Central Atlantic Magmatic Province (CAMP). The CAMP, one of * G. Moratti the largest igneous provinces on Earth, originated from [email protected] mantle upwelling impinging at the base of the continental 1 CNR, Istituto di Geoscienze e Georisorse-Sede Secondaria lithosphere of the supercontinent Pangea at ca. 200 Ma: 7 2 Firenze, Via La Pira 4, 50121 Florence, Italy its products cover an area of some 10 km and outcrop in 2 Dipartimento di Scienze della Terra, Università degli Studi di North and South America, Africa, and Europe (McHone Firenze, Via La Pira 4, 50121 Florence, Italy 2003; Martins et al. 2008; Marzoli et al. 2011; Merle et al. 3 CNR, Istituto di Geoscienze e Georisorse, Via Moruzzi 1, 2011; Blackburn et al. 2013). This determined continental 56124 Pisa, Italy breakup and sea floor spreading with the formation of the Vol.:(0123456789)1 3 2492 International Journal of Earth Sciences (2018) 107:2491–2515 Atlantic Ocean basin (e.g., Marzoli et al. 1999, 2004; Hames inversion (Benvenuti et al. 2017). To support this hypothesis, et al. 2000; McHone 2003; Knight et al. 2004; Verati et al. two basalt outocrops, interbedded within the continental 2007; Frizon de Lamotte et al. 2008; Callegaro et al. 2014; deposits at the southern front of the CHA facing the eastern Whalen et al. 2015). Ouarzazate Basin, have been dated (40Ar–39Ar) and analysed Following the CAMP magmatism, transitional to alkaline for their geochemical and radiogenic isotope (Sr, Nd) sig- magmas emplaced all around the Central and South Atlan- nature. The results, along with stratigraphic and structural tic Oceans, forming the so-called Cretaceous Peri-Atlantic field analyses, are discussed for their implications at local Alkaline Pulse (PAAP) (Matton and Jébrak 2009). In the and regional scales. Central High Atlas (CHA), these transitional to alkaline magmas emplaced within thick poly-deformed sedimentary continental successions since the Middle Jurassic (Choubert Geological outline and Faure-Muret 1960–1962; Frizon de Lamotte et al. 2008; Ghorbal et al. 2008; Benvenuti et al. 2017). Previous studies Stratigraphic and structural setting on the Middle Jurassic–Early Cretaceous magmatic stage (Hailwood and Mitchell 1971; Westphal et al. 1979; Har- The tectono-sedimentary events that were involved in the mand and Laville 1983; Beraâouz et al. 1994; Amrhar et al. evolution of the Moroccan High and Middle Atlas have been 1997; Lachmi et al. 2001; Zayane et al. 2002; Charrière et al. subdivided into three main stages (Frizon de Lamotte et al. 2005; Bensalah et al. 2013) provided chronologic calibra- 2008 for a review): (1) the Late Permian–Early Jurassic con- tion and petrographic-geochemical characteristics of basalts tinental rifting stage, characterized by multiple rift activation occurring in the northern front of the CHA. The results of within a generalized process of supercontinent fragmentation these studies have been discussed in the context of a long followed by the Central Atlantic and North-Western Tethys post-rift phase characterized by tectonic quiescence punc- oceans opening; (2) the Middle Jurassic–Cretaceous post-rift tuated by magmatic activity (e.g., Frizon de Lamotte et al. stage during which the Atlantic domain of the Moroccan 2008). Nevertheless, low-temperature thermochronology Atlas underwent a classic passive margin evolution. Mean- studies carried out in the last decade in the High Atlas region while, the Tethyan Atlasic domains experienced a long phase (Ghorbal et al. 2008; Missenard et al. 2008; Teixell et al. of tectonic quiescence and eustatically controlled deposi- 2009; Ruiz et al. 2011; Bertotti and Gouiza 2012; Oukassou tional patterns; (3) the Cenozoic convergent stage, when et al. 2013; Gouiza et al. 2017; Sehrt et al. 2018), attest to Africa–Europe plate convergence, following to the North- major exhumation events that took place during the post- Western Tethys closure, propagated compressive stresses in rift phase, suggesting regional tectonic processes. The wide the Atlasic domains, and caused a progressive full tectonic distribution of continental middle-late Jurassic–Cretaceous inversion of the pre-existing rifted basins (Mattauer et al. successions along with the occurrence of syn-depositional 1977; Giese and Jacobshagen 1992; Laville and Piqué 1992; deformation had long been attributed to compressional/ Beauchamp et al. 1999). During the latest Paleozoic–Early transpressional events prior to the Cenozoic Africa–Eurasia Triassic, these rifted Atlasic basins were filled by a thick convergence (Choubert and Faure-Muret 1960–1962; Laville continental succession followed by mixed clastic-evaporite et al. 1977; Mattauer et al. 1977; Studer and du Dresnay deposits, shallow marine limestones and prominent basalt 1980; Jenny et al. 1981; Schaer 1987; Laville 2002). This flows emplaced at the Late Triassic–Early Jurassic transition Jurassic–Cretaceous deformation, however, was considered (El Arabi 2007; Frizon de Lamotte et al. 2008). In the Cen- of only local importance, and the effective High Atlas build- tral and Eastern sectors of the High Atlas, the Middle Juras- up was largely related to the Cenozoic Africa–Eurasia colli- sic–Cretaceous post-rift phase was marked by long periods sion [late Mesozoic “structural inversion” vs Cenozoic “oro- of continental deposition alternated to shorter transgressive genic inversion” of Laville and Piqué (1992)]. Successively, episodes (Choubert and Faure-Muret 1960–1962; Frizon de the High Atlas Chain has been related to the convergent tec- Lamotte et al. 2008; Fig. 1). These continental successions, tonics between the African and the Eurasian plates, and the widely exposed in the axial zone and in the northern front of documented early compressional tectonics was neglected the CHA (Couches Rouges Auctt.; Jenny et al. 1981; Had- (Fraissinet et al. 1988; Jacobshagen et al. 1988; Görler et al. doumi et al. 2002, 2008, 2010; Andreu et al. 2003; Charrière 1988; El Harfi et al. 1996, 2001; Beauchamp et al. 1999; et al. 2005; Mojon et al. 2009), include: (1) upper Bathonian Gomez et al. 2000, 2002; Teson and Teixell 2008; Frizon fluvial reddish fine-grained conglomerates, mostly sourced de Lamotte et al. 2008). from Lias and Dogger limestones, sandstones and mudstones Recent field studies on Middle Jurassic–Upper Creta- (Guettouia Formation); (2) Bathonian–Callovian/Hauteriv- ceous continental deposits and intervening basalts exposed ian lacustrine reddish mudstones with subordinate marls and along the southern front