October 2009 LIP of the Month

Corresponds to event 41 in LIP record database

Late-Variscan Polymetallic Ore Deposits in Central Europe and Their Relationship to Lamprophyric Intrusions, a Large Igneous Province (LIP) and a Mantle Plume

Thomas Seifert
Technische Universität Bergakademie Freiberg, Institute of Mineralogy, Division of Economic Geology and Petrology, Brennhausgasse 14, D-09596 Freiberg, Germany
thomas.seifert@mineral.tu-freiberg.de

The link between hydrothermal ore deposits, Large Igneous Provinces (LIP) and mantle plumes has been discussed for more than 30 years (e.g., Burke and Dewey, 1973; Pirajno, 2000; Borisenko et al., 2006; Seifert, 1997, 2008a). In this context the Bohemian Massif in central Europe is an area of special interest (cf. Seifert, 2008a, 2009; Pavlova et al. 2009). The Permo-Carboniferous evolution of the Mid-European Variscides, especially the late-Variscan magmatism and mineralization in the Saxo-Thuringian zone at the northwestern rim of the Bohemian massif (cf. Seifert, 2008a), shows similarities to the Basin and Range Province in the western USA and to the Late Paleozoic-Early Mesozoic igneous rock complexes and Sn-W, Ni-Co-As, Mo-W(-Cu-Au), Ag-Sb, Ag-Pb, and Hg/Au-Hg mineralization in SE-Altai/Russia and NW-Mongolia which are interpreted to have  formed by mantle plume-related extensional tectonics at the southern rim of the central Siberian Large Igneous Province (cf. John, 2001; cf. Borisenko et al., 2006; Pavlova and Borisenko, 2009).

Late-orogenic extension of the Variscan Internides, from Iberia to eastern Europe, involved two main tectonic phases, Mid-Carboniferous (late Visean-Westphalian) and Late Carboniferous-Early Permian (cf. Burg et al., 1994; cf. Praeg, 2004): Geochronological and petrological data for large granite batholiths in the Variscan Orogen (e.g., late collisional granites of the South Bohemian Batholith, Schwarzwald, Bavarian Forest, Erzgebirge), indicate the production of large melt volumes in the late Viséan-Westphalian (330 – 320 Ma), shortly after the thermal peak of regional metamorphism.

At the end of the Variscan Orogeny the regional tectonic regime in Central Europe changed, indicating the beginning of the break-up of the supercontinent Pangaea. The Late Carboniferous-Early Permian in Europe was a period of widespread basin formation that was associated in many areas with mantle-derived magmatic activity (cf. Wilson et al. 2004 and references therein; Ziegler et al., 2004).

A number of geodynamic models have been proposed for the central Pangean, late Variscan tectono-magmatic province (cf. Perini et al., 2004 and references therein): (1) Intracontinental or oceanic subduction during the final evolution of the Variscan belt; (2) Strike-slip-movement related to the collision of Africa and Eurasia; (3) Extensional tectonics disrupting the over-thickened Variscan crustal belt; (4) Mantle upwelling resulting in regional uplift, profound thermal destabilization of the lithosphere and an extensive zone of hot upper mantle.

The most recent mantle upwelling hypothesis argues that the Permo-Carboniferous volcanic province of Europe and NW Africa might be interpreted in terms of a superplume impinging on the base of the lithosphere (cf. Perini et al., 2004 and references therein). Ernst and Buchan (1997) proposed that the c. 300 Ma-old dike swarms in NE England and the Scottish Midland Valley, the Oslo Graben and Scania, radiate from a triple junction in the northernmost part of Jutland (Denmark), and that this triple junction marked the axis of a deep-mantle plume centered in this area (cf. Kirstein et al., 2004; cf. Neumann et al., 2004; Figures 1 and 2). In this context it is important to note that this proposed mantle plume center is surrounded by significant numbers of lamprophyre intrusions (e.g., Rock, 1983; 1991; Leat et al., 1986; Thorpe et al., 1986; Seifert, 1994, 2008a; Von Seckendorf et al., 2004b; Figure 3) which show, in some districts, spatial-time relationships to Sn-polymetallic, Ag-base metal, and U mineralization (cf. Seifert, 2008a and references therein; Figure 4).

A key component of this Large Igneous Province is the Late Carboniferous and Early Permian volcanism developed within northern and central Europe, in an area stretching from southern Scandinavia, through the North Sea, into Northern Germany (cf. Neumann et al., 2004; cf. Wilson et al., 2004 and references therein). Peak magmatic activity of this Large Igneous Province was concentrated in a narrow time-span from 305 to 285 Ma (Figures 1 and 2). The most intense magmatism took place in the Oslo Graben (ca. 120,000 km3) and in the NE German Basin (48,000 km3), (cf. Neumann et al., 2004 and references therein). The dominant mantle source component for the mildly alkali basalts to subalkaline magmatism in the Oslo Graben and Scania is geochemically similar to the Prevalent Mantle (PREMA) component (cf. Neumann et al., 2004). The NE German Basin, which partly straddles the Variscan front, contains 302 – 277 Ma andesitic, rhyolitic and basaltic volcanic rocks with a maximum thickness of >2500 m as indicated by deep boreholes (cf. Benek et al., 1996; cf. Hoth et al., 2003; cf. Timmerman, 2004). According to Neumann et al. (2004) the lithosphere beneath the NE German Basin appears to be dominated by subduction-related chemical components that contribute significantly to the magma generation processes; the involvement of PREMA-like asthenospheric component is also permitted by the data. PREMA is a common source component in OIB petrogenesis, and recognition of PREMA involvement provides some of the strongest support that a mantle plume might have been involved in triggering the widespread Carboniferous-Permian magmatism within northern Europe (cf. Neumann et al., 2004). Early-Permian lavas in SW England comprise an early group of olivine basalts and a later potassium group of lamprophyric composition (290 Ma; cf. Timmerman, 2004). The latter are genetically related to the mica-minette dikes that occur throughout SW England (cf. Thorpe et al., 1986; Leat et al., 1987). Granitic intrusions in SW England show similar ages of c. 293 – 295 Ma and were accompanied by extrusion of lavas (cf. Timmerman, 2004). A large number of lamprophyre dikes intruded the Caledonian basement of the western Scottish Highlands and Isles (cf. Rock, 1991; Figures 2 and 3). According to Rock (1983, 1991), there are nine dike swarms with about 3000 dikes, most of them are Early Permian to late Viséan camptonites and monchiquites that indicate an intensive asthenosphere-derived magmatism.

In the Stepanian-Early Permian wrench tectonics affected not only the Variscan foreland, but also the entire Variscan orogen where large-scale dextral wrenching opened many fault-bounded basins (cf. Wilson et al., 2004 and references therein). These basins tend to be small but deep troughs, grabens and pull-apart basins. Magmatic underplating of the base of the crust by ascending mafic magmas is likely to have been widespread (e.g., Downes, 1993). Available seismic reflection data provide strong support for underplating of the crust by mafic magmas (cf. Wilson et al., 2004). Simultaneously in the Stephanian-Early Permian intensive bimodal magmatism associated with an intra-continental extensional setting occurs in the European Variscides (cf. Breitkreuz and Kennedy, 1999; cf. Perini et al., 2004; cf. Timmerman et al., 2004; cf. Von Seckendorf et al., 2004a, b; cf. Seifert, 2008a; cf. Wilson et al., 2004; cf. Zeh and Brätz, 2004; and references therein). Permo-Carboniferous volcanism in the Spanish Central System, Iberian Ranges, Cantabrian Chain, Pyrenees and the French Massif Central includes a range of mafic calc-alkaline and shoshonitic rock types, and lamprophyres (spessartites and camptonites) with ages between 295-270 Ma (Figure 3). The Mid-European Variscides show a large number of Permo-Carboniferous magmatic complexes with similar ages: 1) the Halle Volcanic Complex (trachybasalt, -andesite and –dacite lavas, and rhyolite laccoliths with ages of 294 to 307 Ma); 2) the Saar-Nahe Basin with andesite flows and pyroclastics that are associated with subvolcanic dacite and rhyolite domes, and diatremes of subalkaline basalt (tuff yielded a 297 ± 3.2 Ma U-Pb zircon age); and 3) the Ilfeld Basin (south Harz) with Permo-Carboniferous (295 – 300 Ma) tuffs, ignimbrites and latitic trachytic and rhyolithic flows. In the nearby Thuringian Forest and Harz areas granitoids, gabbros, rhyolites and intermediate dikes have 289-307 Ma U-Pb zircon ages. The basins are filled by an up to 2 km-thick sequence of Stephanian-Saxonian molasse sediments and trachyandesitic and rhyolitic volcanic rocks (295 - 290 Ma). The nearby Henneberg granite in the Fränkisch-Thüringisches Schiefergebirge shows a U-Pb zircon age of 299 Ma and is crosscut by lamprophyre dikes, from which one dike sample yields an age of 295 Ma (Von Seckendorf et al., 2004b).

To the east is intensive magmatism of the Northwest-Saxonian Volcanic Complexes with felsic and intermediate volcanic flows of up to 400 m in thickness, the bimodal volcanic rocks of the Sub-Erzgebirge basin and the Rhyolite Complex of Tharandt with ages between ~305 Ma and ~280 Ma. The Permo-Carboniferous shoshonitic/ultrapotassic-rhyolite volcanism in the Sub-Erzgebirge basin (305 – 290 Ma) is controlled by the Central Saxonian Lineament and shows similar mineralogical-geochemical and time characteristics to small intrusion Li-F granites (‘Sn-granites’) and lamprophyric intrusions in the Erzgebirge metallogenetic province (cf. Seifert, 2008a). The emplacement of lamprophyric (shoshonitic/ultrapotassic) melts at the base of the lower crust in the Erzgebirge and Sub-Erzgebirge basin is probably associated with asthenospheric doming; F-enriched rhyolitic rocks were probably produced by small amounts of partial melting of the lower crust, resulting from underplating of hot and fluid-enriched lamprophyric melts (Seifert, 2008a; Figures 4 to 7). It is important to note that the late-Variscan W-Mo, Sn-W-Mo, Ag-bearing Sn-In-base metal, Ag-Sb-base metal, and U mineralization at the northwestern rim of the Bohemian Massif (e.g., Erzgebirge metallogenetic province; Figures 4, 7 to 10) are spatially and temporal associated with intrusion centers of Permo-Carboniferous post-collisional mafic and rhyolitic (sub)volcanic magmatism along deep-rooted NW-SE fault zones, especially at the intersections with NE-SW and E-W major structural zones (Seifert, 1994, 2008a, b). These late-Variscan bimodal intrusion centers are of significant metallogenetic importance and indicate the high metallogenetic potential of the 305 to 285 Ma Large Igneous Province in Europe (Seifert, 2009).

References