Late Devonian and Triassic Basalts from the Southern Continental Margin of the East European Platform, Tracers of a Single Heterogeneous Lithospheric Mantle Source
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Late Devonian and Triassic basalts from the southern continental margin of the East European Platform, tracers of a single heterogeneous lithospheric mantle source Franc¸oise Chalot-Prat1,∗, Petr Tikhomirov2 and Aline Saintot3,4 1Centre de Recherches Petrographiques et Geochimiques – Nancy University, BP20, 15 rue Notre Dame des Pauvres, F-54501 Vandoeuvre-les-Nancy Cedex, France. 2Geological Faculty of Moscow State University; Vorobiovy Gory, 117311 Moscow, Russia. 3Vrije Universiteit, Instituut voor Aardwetenschappen, Tektoniek afdeling, De Boelelaan 1085, 1081 HV Amsterdam, Netherlands. 4Geological Survey of Norway, N-7491 Trondheim, Norway. ∗e-mail: [email protected] In Late Devonian and Early-to-Late Triassic times, the southern continental margin of the Eastern European Platform was the site of a basaltic volcanism in the Donbas and Fore-Caucasus areas respectively. Both volcanic piles rest unconformably upon Paleoproterozoic and Late Paleozoic units respectively, and emplaced during continental rifting periods some 600 km away from expected locations of active oceanic subduction zones. This paper reports a comparative geochemical study of the basaltic rocks, and views them as the best tracers of the involved mantle below the Eastern European Platform. The Late Devonian alkaline basic rocks differ from the calc-alkaline Triassic basic rocks by their higher alkali-silica ratio, their higher TiO2,K2O, P2O5 and FeO contents, their higher trace element contents, a higher degree of fractionation between the most and the least incompatible ele- ments and the absence of Ta-Nb negative anomalies. These general features, clearly dis- tinct from those of partial melting and fractional crystallization, are due to mantle source effects. With similar Nd and Sr isotopic signatures indicating mantle-crust mixing, both suites would originate from the melting of a same but heterogeneous continental mantle lithosphere (refertilized depleted mantle). Accordingly the Nd model ages, the youngest major event associated with mantle metasomatism occurred during Early Neoproterozoic times (∼650 Ma). 1. Introduction Episodes of rifting destabilized the subsequent platform regime of the East European Craton, the The southern margin of the Eastern European first during the Riphean (Meso- to Neoprotero- Platform is structurally divided into two main zoic) and the second during the Late Devonian. parts: the Sarmatia segment of the East European The latter created the large Pripyat-Dniepr-Donets Craton and the Scythian Platform lying south of intra-cratonic rift basin (Bogdanova et al 1996; it (figure 1). Artemieva 2003). The Sarmatia cratonic area is formed by four or The origin and nature of the Scythian Plat- five Archean terranes, welded together at 2.3 and form basement (figure 1) remain unclear. It is 2.1 Ga (see Bogdanova et al 1996). overlain by a quite thick Phanerozoic sedimentary Keywords. Basalts; East European Platform; Late Devonian; Triassic; Donbas; Fore-Caucasus; geochemistry; geodynamics; geology. J. Earth Syst. Sci. 116, No. 6, December 2007, pp. 469–495 © Printed in India. 469 470 Fran¸coise Chalot-Prat et al Figure 1. Late Paleozoic tectonic map of the southern East European craton and adjacent areas (modified after Tikhomirov et al 2004) in the hypothesis of a Variscan basement for the Scythian Platform (SP) (as for example in Zonenshain et al 1990). 1. Early Permian basins within the East European Craton; 2. Areas of Precambrian consolidation within the Late Paleozoic orogen; 3. Areas of Late Paleozoic consolidation; 4. Donets and Karpinsky basins, thought to be inverted during the Permian time; 5. Late Paleozoic and possibly younger thrusts; 6. Presumed shelf margin for the end of Permian; 7. Gissar-Donets fault zone; 8. Location of the Late Devonian (D) Donbas and Triassic (T) Fore Caucasus areas. A. Agrakhan–Guriev fault. cover (locally more than 10 km thick). Geophys- by episodes of rifting during the Late Devonian, ical data only allow characterizing the basement the Permo(?)-Triassic (Gaetani 2000), and further as few drill-holes reached at the deepest the Late southwards, Jurassic (Lordkipanidze et al 1989; Paleozoic succession. A classical model suggests Nikishin et al 1998a, b) and Cretaceous (Nikishin that the Scythian Platform is part of the Late et al 1998a, b). Variscan orogenic belt, between Western Europe Among the rift basins which formed on the and Urals (Arthaud and Matte 1977; Zonenshain Scythian Platform during Phanerozoic times et al 1990; Nikishin et al 1996 and presented as (figure 1), five were accompanied by significant such on figure 1). However, no strong penetrative eruptions and/or sub-surface intrusions of mantle- Late Paleozoic deformation is actually observed on derived magma: the Scythian Platform. Another hypothesis to test • the Late Devonian Dniepr-Donets-Donbas basin is that the Scythian Platform was part of the Late (Chekunov and Naumenko 1982; Wilson and Proterozoic Baikalian belt fringing Baltica (e.g., Lyashkevich 1996; McCann et al 2003); Saintot et al 2006). This Late Proterozoic orogenic • the Triassic Eastern Fore Caucasus basin event might have resulted from the amalgama- (Dubinski and Matsenko 1965; Burshtar et al tion/accretion of subduction-related arc/oceanic 1973; Nazarevich et al 1986; Tikhomirov et al complexes and micro-continental blocks (as the 2004); Beloretzk terrane along the Southern Proto-Urals, • the Jurassic Western Fore Caucasus basin (Lord- Glasmacher et al 1999; the Pechora-Barentsia ter- kipanidze et al 1989); ranes on the Timan-Pechora foldbelt, Nikishin et al • the Cretaceous Karkinit rift basin (western part 1996; see Torsvik and Rehnstrom 2001) along the of Scythian Platform) (Muratov 1969; Chekunov margin of the present-day northern and eastern et al 1976; Grigorieva et al 1981; Leschukh 1992; Baltica, without involving collision of large conti- Nikishin et al 1998a, b, 2001); nental masses. The Scythian Platform, considered • the Neogene Minvody basin (Stavropol High) herein as part of the Late Proterozoic Baikalian (Polovinkina 1960; Adamia and Lordkipanidze belt, became a renewed passive margin affected 1989). Late Devonian Triassic basalts East European Platform Margin 471 The two volcanic suites and associated sediments The Donbas rift belongs to the Prypiat-Dniepr- of interest herein, of the Donbas and of the East- Donets (PDD) mega-rift (figure 1). This Late ern Fore-Caucasus, were deposited in quite simi- Devonian structure was contemporaneous with lar tectonic settings. The Late Devonian event of (1) widespread peri- and intra-cratonic magmatism the Donbas and the Triassic event of the Eastern elsewhere on the whole Eastern European Platform Fore Caucasus are both related to continental rift- (about 4 million km2) from the Timan–Pechora ing of a previously deformed and peneplaned base- and East Barents Sea rift systems northwards to ment. The rifting was due to tensional lithospheric the Peri-Caspian Basin eastwards (Stephenson et al stresses in the Donbas (Saintot et al 2003), and 2006), (2) the accretion of an intra-oceanic vol- more likely transtensional in the Eastern Fore canic arc following the eastward subduction, there- Caucasus (Gaetani 2000). fore away from the Eastern European Platform of At Late Devonian times, subaerial basaltic lava (or part of) the Paleo-Ural Ocean (Puchkov 1997; flows were emplaced at the beginning of the Brown et al 1998, 2002; Brown and Spadea 1999), Donbas rift formation (figure 1) (McCann et al and (3) development of extensional basins on the 2003 and ref. therein), well after the final Protero- southern margin of the Scythian Platform (on the zoic consolidation of the Sarmatia basement (at Greater Caucasus area, Khain 1975). The back-arc 1500 Ma, according to Milanovsky 1996 and ref- position of such remains uncertain despite the fact erences therein, or at 2000–1800 Ma according to that a northward subduction zone of the Proto- Bogdanova et al 1996). Tethys Ocean has very often been adopted in From Early to Late Triassic times, three mafic plate kinematic models (e.g., Adamia et al 1981; volcanic suites, including some rhyolitic materi- Gamkrelidze 1986; Ziegler 1990; Zonenshain et al als, were emplaced successively in continental and 1990; Stampfli et al 2001; Stampfli and Borel 2002; marine sedimentary environments on the Eastern von Raumer et al 2003). The closest Paleozoic Fore Caucasus (figure 1) (Tikhomirov et al 2004 subduction zone, if any, could have been far to and ref. therein), some tens of millions years after the south, at 600 km minimum along the Greater the basement consolidation of the Scythian Plat- Caucasus, remnants of which are mafic and ultra- form if Variscan or some hundreds of millions years mafic rocks interpreted as an ophiolitic complex after, if Baikalian. Devonian in age (Khain 1975; Adamia et al 1981) Remark: The time gap between the last orogeny and emplaced prior to the late Visean (Khain 1975; and continental rifting is important to specify in as Adamia et al 1981). Whereas Zonenshain et al much as the petrotectonic setting of these continen- (1990) considered this ophiolitic series as the suture tal rifting-related eruptions is usually described as of a wide ocean basin, that is, “proto-Tethys” or “anorogenic or continental intra-plate”, and “post- a branch of the Iapetus–Tornquist oceanic system, orogenic or post-collisional” after either a long or Adamia et al (1981) suggested the closure of a a short time gap respectively. Apriori,thistime small back-arc oceanic basin with the