PEPI1993.Pdf

Physics of the Earth and Planetary Interiors, 79(1993)219—240 219 Elsevier Science Publishers By., Amsterdam Growth of the European lithospheric mantle—dependence of upper-mantle peridotite facies and chemical heterogeneity on tectonics and age Martin Adrian Menzies *,a,b Jean Louis Bodinier b a Department of Geology, Royal Holloway University ofLondon, Egham TW2O OEX, UK b Université de Montpellier II, CNRS, Montpellier, France (Received 8 February 1993; revision accepted 7 April 1993) ABSTRACT In Europe, during the Phanerozoic, collision of microplates caused the juxtaposition of disparate lithospheres of variable age and provenance. The complex prehistory of these plates, together with the present-day tectonic regime, generated considerable topography at the lithosphere—asthenosphere boundary. From north to south across Europe there exists a considerable variation in lithosphere thickness, seismic velocity and heat flow, with concomitant changes in the mantle helium flux, the extent, type and source of Cenozoic volcanism, and the age and origin of the lithospheric mantle protolith. Consideration of Moho depth and lithosphere thickness reveals that the lithospheric mantle should be dominated by garnet—diamond facies mantle beneath stable shield areas (e.g. the Baltic Shield) and young mountain belts (e.g. the Alps and Betics), spinel—garnet facies mantle beneath Variscan Europe and spinel—plagioclase facies mantle in the western Mediterranean, Pannonian Basin and Rhinegraben. However, consideration of mantle xenolith data reveals that garnet peridotites are rare beneath Variscan Europe and that plagioclase peridotites are unreported from the Pannonian Basin and the Rhinegraben. A tectonic dimension to lithosphere thickness, as well as a function that relates to the initial stabilisation age, is illustrated by the presence of thick lithosphere (diamond facies) beneath old tectonically stable areas such as the Baltic Shield, and young tectonically active regions such as the Alps. Thermo-tectonic processes have also produced lithosphere under the Archaean of NW Scotland that is as thin as that under parts of the Alpine Orogen (e.g. in western Mediterranean and the Pannonian Basin). Extreme chemical heterogeneity in the lithospheric mantle can result from both time-integrated effects over several billion years (e.g. in the North Atlantic craton) and mixing over several tens of millions of years along the tectonically active southern margin of Europe (e.g. in the Betics). Volcanic rocks provide a valuable probe of the lithospheric mantle and the asthenosphere, and it is apparent that throughout the Phanerozoic the lithospheric mantle evolved in response to repeated cycles of collision—subduction and intraplate extension. Volcanic rocks erupted during the Caledonian, Variscan and Alpine orogenies were derived from shallow lithospheric reservoirs containing a sedimentary component. In contrast, during syn- or post-orogenic extension volcanic rocks were derived from sub-lithospheric sources or by passive reactivation of young lithospheric mantle. 1. Introduction and stress, which is inextricably related to north to south changes in tectonic regime, crustal age, The European Geotraverse has provided earth volcanic activity, volcanic source region and the scientists with a wealth of information on the mantle helium flux. Unification of geochemical nature of the European lithosphere (Spakman, and geophysical data has led to a better under- 1986; Blundell et al., 1992). From north to south standing of other tectonically active continental across Europe a marked contrast exists in the regions with similar regional variations in heat depth to Moho, lithosphere thickness, heat flow flow, depth to Moho and lithosphere thickness (e.g. western USA and eastern China) (Zartman * Corresponding author. et al., 1991; Menzies et al., 1992). In the western 0031-9201/93/$06.00 © 1993 — Elsevier Science Publishers B.V. All rights reserved 220 M.A. MENZIES AND iL. BODINIER USA an integration of seismic tomographic and the Alpine orogeny has provided us, directly and heat flow data with information gleaned from the indirectly, with additional samples of the litho- widespread Cenozoic volcanic rocks and high spheric mantle, in which we can study mass trans- pressure mantle xenoliths made it possible to fer processes. These take the form of orogenic constrain the spatial geometry of the lithospheric (alpine) peridotite massifs in the young mountain mantle and the processes responsible for its for- belts and high-pressure mantle xenoliths en- mation (Menzies, 1989). In Europe, Tertiary vol- trained by volcanic rocks. canism is common in off-craton (younger than In a general sense, the geology of Europe can 2500 Ma. crust) areas, but little or no recent be thought of as successive terranes accreted volcanism has been reported from on-craton around the margins of older Precambrian crustal (older than 2500 Ma. crust) regions. Fortunately, remnants (Fig. 1). The age of the crust changes fl PRECAMBRIAN ~ EASTERN AVALONIAN TERRANES ~ VARISCAN SUSPECT TERRANES VARISCAN GONOWANA BALTIC o SHIELD ALPINE OROGEN U~ANIAN - BOHEMIANEMASSIf~ MASS~F- - - PANNONIAN ENTRAL~ BASIN yrenees ,~ : ~PEMNSULA~•~ Corsica~ ‘~JSardInia Betic Icily Fig. 1. Terrane map of Precambrian and Phanerozoic Europe. A destructive plate margin along the southern margin of Europe and a constructive plate margin in the mid-Atlantic dominate present-day tectonics (after Blundell et al., 1992). During the Caledonian, Variscan and Alpine orogenies successive episodes of collision (subduction) and extension (intraplate) were responsible for lithospheric accretion around the Precambrian Baltic Shield. GROWTh OF THE EUROPEAN LITHOSPHERIC MANTLE 221 dramatically from north to south, in concert with the Italian Peninsula, around the Aegean to major tectonic changes. Whereas Archaean crust Turkey, and, in a broad curve, around the Alps is found in NW Scotland (the Hebridean craton), and Carpathians. Greenland, Finland (the Baltic Shield) and Rus- The purpose of this paper is to revieW the sia, Proterozoic crust is present in most of Eu- geochemistry of the European mantle and to rope from Scandinavia and the UK to France and integrate these data with geophysical information Spain. However, much of our discussion will cen- on the European lithosphere. We have chosen to tre on the Phanerozoic evolution of the litho- limit much of our discussion to Cenozoic volcanic spheric mantle, as it is during this time that the rocks, orogenic (alpine) peridotite massifs and record is the most complete. During the Phanero- mantle xenoliths, in an assessment of the role of zoic (Ricou et al., 1986), lithospheric accretion sub-lithospheric processes and subduction-related around Precambrian protoliths occurred in re- processes in the genesis of the European litho- sponse to the Caledonian (Ordovician—Devonian), spheric mantle. We have explored the relation- Variscan (Carboniferous) and Alpine (Creta- ship between chemical heterogeneity, mantle ceous—Recent) orogenies (i.e. collision—subduc- peridotite facies variation, crustal thickness, litho- tion). Lithospheric extension tended to pre-date sphere thickness, heat flow, crustal age and man- and post-date these orogenies such that Phanero- tie helium flux. It is beyond the scope of this zoic lithosphere also grew in response to sub-lith- paper to review and list all publications pertinent ospheric processes (i.e. intraplate). For example, to the geochemistry and geophysics of the Euro- the Alpine orogeny was associated with pre-oro- pean mantle. We have attempted to provide a genic extension in the Jurassic to early Creta- representative source bibliography of published ceous (Neo-Tethys) and post-orogenic extension work on mantle geochemistry, and accept that it in the Neogene—Quaternary (Pannonian Basin is by no means a complete documentation of all and Po Plain). papers written on rocks of mantle derivation. The The collision of continental microplates of dis- reader is directed toward recent texts or special parate origin during the Phanerozoic ensured the volumes on general aspects of mantle geochem- juxtaposition of lithospheres of different age and istiy (Nixon, 1987; Menzies and Hawkesworth, provenance (Fig. 1). Equally important was the 1987; Wilson, 1989) and others with a strong formation of oceanic lithosphere (Neo-Tethys) in European dimension (Menzies et al., 1991). post-Variscan pre-Alpine Europe. Jurassic oceanic lithosphere formed in Tethys and the Liguro-Piemontaise basins during seafloor 2. Lithospheric mantle formation and transfor- spreading in the Central and South Atlantic. In mation the Cretaceous a transition occurred from passive to convergent margin processes, and the northern We are uniquely placed in Europe to integrate edge of Tethys was characterised by an active data from Cenozoic volcanic rocks with informa- margin associated with the final emplacement of tion from orogenic (or alpine) massifs and arc lithosphere in Cyprus and ridge lithosphere in basalt-borne high-pressure mantle xenoliths. Oro- the Betics, Sesia—Lanzo and Liguria. Palaeogene genic massifs offer the largest exposures of sub- extension created the Rhine—Rhône graben continental (and sub-oceanic) mantle rocks at the (Germany—France) and the Egergraben (Czecho- Earth’s surface and they exist in profusion in the slovakia), and Neogene collision between the mountain belts of Europe. Representatives can Eurasia and African plates was inextricably linked be found of lithospheric

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