Variscan granitoids of central Europe: their typology, potential sources and tectonothermal relations E Finger 1, M. P. Roberts I , B. H a u n s c h m i d 1, A. Schermaier 1, and H. P. Steyrer 2 1 Institut for Mineralogie, Universit/it Salzburg, Salzburg, Austria 2 Institut ffir Geologic und Pal~iontologie, Universitfit Salzburg, Salzburg, Austria With 6 Figures Received November 12, 1996; accepted September 24, 1997
Summary During the Variscan orogenic cycle, central Europe was intruded by numerous granitoid plutons. Typological and age relationships show that the characteristics of the granitoid magmatism changed during the course of the Variscan orogeny. Five genetic groups of granitoids may be distinguished:
1. Late Devonian to early Carboniferous "Cordilleran" 1-type granitoids (ca. 370-340 Ma): These early Variscan granitoids are mainly tonalites and granodiorites. They often have hornblende and occur in association with diorites and gabbros. They form plutonic massifs in the Saxothuringian unit, in Central Bohemia and the intra-Alpine Variscides. In terms of existing models, they can be interpreted as volcanic arc granites, being related to the subduction of early Variscan oceans. Models involving mantle sources and AFC may be feasible. 2. Early Carboniferous, deformed S-type granite/migmatite associations (ca. 340 Ma): These occur in the footwall of a thick thrust in Southern Bohemia (Gf6hl nappe) and seem to represent a phase of water-present, syn-collisional crustal melting related to nappe stacking. 3. Late Visean and early Namurian S-type and high-K, I-type granitoids (ca. 340-310 Ma): These granitoids are mainly granitic in composition and particularly abundant along the central axis of the orogen (Moldanubian unit). This zone experienced a high heat flow at this time, probably as a consequence of post-collisional extension and magmatic underplating. Most of group 3 granitoids formed through high-T fluid-absent melting in the lower crust. Enriched mantle melts interacted with some crustal magmas on a local scale to form durbachites. Partial melting events in the middle
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F. Finger et al. crust produced a number of high-T/low-P, S- and I-type diatexites and some S-type granite magmas.
4. Post-collisional, epizonal 1-type granodiorites and tonalites (ca. 310-290 Ma): These plutons can be found throughout the Central European Variscides. However, most of them occur in the Alps (near the southern flank of the orogen). Such late I-type plutons could be related to renewed subduction along the southern fold belt flank, and/or to extensional decompression melting near the crust/mantle boundary. Post-collisional mantle or slab melting may have occurred in connection with remnant subduction zones below the orogen undergoing thermal relaxation and dehydration. 5. Late Carboniferous to Permian leucogranites (ca. 300-250 Ma): Many of these rocks are similar to sub-alkaline A-type granites. Potential sources for this final stage of plutonism could have been melt-depleted lower crust or lithospheric mantle.
Zusammenfassung Die variszischen Granitoide Mitteleuropas: Typologie, potentielle Quellen und tektonothermische Zusammenhiinge Im Verlauf der variszischen Orogenese intrudierten im mitteleuropfiischen Raum groBe Massen von Granitoiden. Eine Bewertung geochronologischer und granittypologischer Daten zeigt, dab sich die Magmencharakteristik mit der Zeit ver~indert hat. FiJnf Hauptgruppen von Granitoiden k6nnen unterschieden werden:
1.1-Typ Granitoide des spiiten Devon und friihen Karbon (ca. 370-340 Ma): Es handelt sich dabei durchwegs um I-Typ Tonalite und Granodiorite, welche h~iufig Hornblende fi,ihren. Typisch ffir diese Plutone ist die Pr~isenz gabbroischer oder dioritischer Endglieder. Eine Magmenentstehung aus Mantelquellen mit Modifikation durch AFC und eine genetische Verbindung zu friihvariszischen Subduktionszonen ist denkbar. 2. Syntektonische S-Typ Granite und Migmatite (ca. 340 Ma): GroBe Massen solcher Granitoide treten im Deckenstapel der stidlichen B6hmischen Masse auf. Sie repr~isentieren wasserges~ittigte, syn-kollisionale Krustenschmelzen, die sich in der N~ihe von tektonischen Uberschiebungsbahnen gebildet haben. 3. S-Typ und kalireiche l-Typ Granitoide des spiiten Vis~ und friihen Namur (ca. 340310 Ma): Diese Plutone haben in der Regel granitische Zusammensetzung und intrudierten vornehmlich in der moldanubischen Zentralzone des Orogens. Die dortige kontinentale Kruste war zu dieser Zeit einem extrem hohen W~irmefluB ausgesetzt, der vermutlich durch postkollisionale Extension mit rascher Krustenhebung und magmatischem ,,underplating" verursacht wurde. Die meisten dieser Granite bildeten sich durch Dehydratationsschmelzen der Unterkruste aus Paragneisen und eventuell auch intermedi~iren kaliumreichen Orthogneisen. Einige wenige Plutone zeigen Interaktionen mit mafischen Magmen, die aus einem angereicherten Lithosph~irenmantel stammen (Durbachite). Schmelzprozesse in der mittleren Kruste ftihrten weitr~iumig zur Bildung von Migmatiten mit groBen Anteilen an S-Typ und I-Typ Diatexiten. 4. Postkollisionale, epizonale l-Typ Granodiorite und Tonalite (ca. 310-290 Ma): Die Hauptverbreitung dieser Plutone liegt in den Alpen. Eine genetische Verbindung zu
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einer sp~itvariszischen Subduktionszone am Variszikums-Stidrand erscheint m6glich. Andererseits k6nnte auch die bloBe Reaktivierung und Dehydratation von alten (friihvariszischen) Subduktionszonen unter dem Orogen die Produktion entsprechender I-Typ Magmen bewirkt haben, ebenso wie ein postkollisionales Druckentlastungsschmelzen von I-Typ Quellen im Bereich der Krusten-Mantel Grenze ohne Subduktionzusammenhang.
5. Leukogranite des spiiten Karbon und Perm (ca. 300-250 Ma): Viele dieser Plutone zeigen Eigenschaften von A-Typ Graniten. Die entsprechenden Magmen sind vermutlich durch Schmelzprozesse in einer restitischen Unterkruste oder im lithosph~irischen Mantel entstanden. Introduction
The ascent of granitoid magmas and their emplacement in the middle and upper crust is a typical feature of orogenic events. Research has shown that these magmas may form from contrasting source regions, and that each source rock/source region is likely to produce fairly distinct granitoid types and granitoid suites (e.g. White and Chappell, 1983; Pitcher, 1983). A particularly high melt productivity is commonly attributed to lower crustal melting of prograde metamorphosed pelite and greywacke sources (Clemens and Vielzeuf, 1987; Conrad et al., 1988; Vielzeuf and Holloway, 1988; Pati~o-Douce and Johnston, 1991; Vielzeuf and Montel, 1994). Also, granitoid magmas may form through the partial melting of hydrated oceanic crust, e.g. slab melting in subduction zones (Peacock et al., 1993), lower crustal amphibolite melting (Clemens and Vielzeuf, 1987) or remelting of underplated basaltic crust (Atherton and Petford, 1993). If temperatures are high enough, felsic magmas may be produced by remelting of restitic lower crust (Collins et al., 1982). Furthermore, it is commonly believed that granitoid magmas may evolve through fractional crystallisation, or assimilation and fractional crystallisation (AFC) processes from mantle-derived basaltic magmas, associated predominantly with subduction zones and divergent plate boundaries (Brown, 1981; DePaolo, 1981; Brown et al., 1984; Pearce et al., 1984; Hildreth and Moorbath, 1988). The timing and localities of partial melt formation during an orogenic cycle depend on the crustal composition, as well as the thermal and tectonic state within the orogen (Thompson et al., 1984; England and Thompson, 1984; Harris et al., 1984; White and Chappell, 1983). Thus, the granite inventory of an orogenic belt may be seen as an important reflection of deep structures and processes that are commonly the least accessible and the worst exposed to direct geological observation. This paper is a preliminary attempt to establish a relationship between the very extensive Variscan plutonism of central Europe in terms of granitoid types, probable source rocks and source regions. The aim is to present a coherent largescale model that relates the timing of plutonism, and the regional distribution of different granitoid types, to the main geological structures and the geodynamic evolution of the Variscan orogen. An important basis is the compilation of high quality zircon and monazite ages that have become available over the past years from different areas. It is hoped that the presentation of this model will stimulate
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considerable discussion as to its viability and draw further attention to the significance of the abundant granitoid rocks distributed throughout Variscan central Europe.
Geological and tectonothermal background The Variscides of central Europe are commonly interpreted as a collision orogen that resulted from the docking of Gondwana and Laurasia in the late Devonian to early Carboniferous (Figs. 1 and 2). The pre-collisional framework may be visualised as a ridge-trough system of continental microplates, intervening basins and partly oceanic rifts, which evolved in the early Palaeozoic along the fragmented margin of northern Gondwana (e.g., Pin, 1990; Finger and Steyrer, 1995). It is commonly believed that two major pre-collisional subduction systems developed within this ridge-trough system (Franke, 1989; Matte, 1986). These were a northern, southward-dipping system (probably a tandem of two staggered subduction zones that led to the closing of the Saxothuringian and Rhenohercynian basins - Fig. 1), and a southern, northward-dipping system that led to the closing of a Massif Central - intra-Alpine ocean(s). During the Variscan collision, the Precambrian basement of the ridges amalgamated with the thick sedimentary filling and some ocean floor of the early
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Palaeozoic rift basins. Thrusting concomitant with crustal stacking was most intense in the late Devonian and early Carboniferous, leading to widespread Barrowian regional metamorphism, particularly in the internal Moldanubian unit. Parts of the Moldanubian and Saxothuringian realm were additionally affected by (subduction-zone related?) late Devonian/early Carboniferous eclogite/granulitefacies metamorphism (O'Brien and Carswell, 1993). Nappe stacking ceased in the late Visean and gave way to transpressional and transtensional tectonics, in response to the dextral wrenching of Gondwana relative to Laurasia (Arthaud and Matte, 1977; Schaltegger and Corfu, 1995). During this late Visean period, large parts of the Moldanubian unit experienced intense high-T/ low-P regional metamorphism and anatexis. In the late Carboniferous and Permian, the Variscan crust underwent postorogenic uplift and extension. However, renewed subduction of oceanic lithosphere and single terrane accretions might have occurred in the late Carboniferous along the southern flank of the fold belt (Fig. 2), after this had sheared off from the westward drifting Gondwana supercontinent (opening of a Pangea gulf - see Finger and Steyrer, 1990). Due to the involvement of these southern Variscan units in the Mesozoic to Tertiary Alpine orogen, their original relationships to each other before this later event are unclear. Granitoid groups and their geographical distribution The spatial distribution of Variscan (late Devonian to Permian) granitoids in central Europe is given in Fig. 3. This map shows that intrusions were concentrated along the central axis of the fold belt (the Moldanubian unit), where granitoid rocks make up more than half of the crystalline basement. The largest Moldanubian batholiths are the Southern Bohemian Batholith, the Central Bohemian Batholith, the Oberpfalz Batholith and the Schwarzwald batholithic terrain. Variscan plutons are very abundant in the intra-Alpine/Carpathian realm also, which represents the former southern flank of the orogen. About 20-30% of all prePermian rocks of these areas are recorded as "Variscan granitoids" on the official geological maps. Large Variscan batholiths occur in different tectonic positions of the Alpine nappe system. For instance, the Aar Batholith is part of the Helvetic unit, the Hohe Tauern Batholith belongs to the Penninic unit, and the Bernina, Schladming, Seckau/B6senstein and Semmering/Raabalpen Batholiths are part of the Austroalpine basement. The structural position of the Carpathian granitoid plutons corresponds to the Austroalpine nappe system also (Neubauer, 1994). Variscan plutons are present in the northern external zones of the fold belt as well. Several extensive plutonic massifs have been mapped in the Saxothuringian unit (Odenwald, Fichtelgebirge/Erzgebirge, Sudetic Batholith), and a few plutons occur in the Rhenohercynian unit (Harz). The principal purpose of this paper is to demonstrate that the Variscan granitoid rocks of central Europe can be loosely combined into five main groups based on age distribution, chemistry, mineralogy and structural relationships. The characteristics of the five granitoid groups are described below and outlined in Table 1. Their geographical distribution is shown in Fig. 3. The backbone of this tentative granite classification has resulted from studies on various granitoid occurrences in