The fundamental Variscan problem: high-temperature metamorphism at different depths and high-pressure metamorphism at different temperatures

Abstract

The evolution of the crystalline internal zone of the European Variscides (i.e. Moldanubian and Saxo-Thuringian) is best understood within a framework of two distinct subduction stages. An early, pre-Late Devonian (older than 380 Ma), subduction stage is recorded in medium-temperature eclogites and blueschists derived from low-pressure basaltic and gabbroic protoliths now found as minor relics in amphibolite facies meta-ophiolite or gneiss-metabasite nappe complexes. A second subduction and exhumation event produced further nappe complexes containing different types of mantle peridotites, along with their enclosed pyroxenites and high-temperature eclogites, associated with large volumes of high-T-high-P (900–1000°C, 15–20 kbar) felsic granulites. Abundant geochronological evidence points to a Carboniferous age (c. 340 Ma) for the high-P-high-T metamorphism as well as an extremely rapid exhumation because the fault-bounded, granulite-peridotite-bearing tectonic units are also cut by late Variscan granitic plutons (315–325 Ma). The massive heat energy for the characteristic, and most widespread feature of the Variscan event, the low-P-high-T metamorphism (750–800°C, 4–6 kbar) and voluminous granitoid magmatism (325–305 Ma), comes from three sources. An internal heat component comes from imbrication of crust with upper-crustal radiogenic heat production potential in the region parallel to the subduction zone; an external mantle heat component is undoubtedly contributing to the transformation of crust taken to mantle depths (i.e. the granulites); and a heat component advected to the middle and lower crust seems inescapable if the hot granulite-peridotite complexes were exhumed and cooled as rapidly as petrological and geochronological evidence seems to suggest. Major mantle delamination and asthenospheric upwelling as a cause of heating in Early Carboniferous times is not supported by geochemical, geophysical or petrological-geochronological studies, although slab break-off probably did occur.

Bottom to top lithosphere structure and evolution of western Eger Rift (Central Europe)

“…Bird 1978), in agreement with findings of O'Brien (2000) and Franke and Stein (2000), our data do not support any largescale detachment of the mantle lithosphere in the western BM and its substitution by a large volume of asthenospheric hot material, which would had been later transformed into a new lithosphere. Instead, we observe a collisional pattern of rigid blocks with a ready-made olivine fabric as a general feature of the European mantle lithosphere (Babuška and Plomerová 2006).…”

“…O'Brien (2000) reviewed one of fundamental Variscan problems related to the cause of the high-T and low-P metamorphism and associated magmatism dated mostly at 335-325 Ma. This first-order feature requires a large thermal anomaly (750-800°C, 4-6 kbar) on a lithosphere scale, and the author has suggested three possible sources: an internal heat component from radiogenic heat production, an external mantle heat component contributing to the transformation of crust taken to mantle depths (granulites) and a heat component from rapid exhumation of the hot granulite-peridotite complexes.…”

Eclogite relics in the Variscan orogenic belt of Bulgaria (SE Europe)

“…Hiberia-Ossa Morena, Armorica, French Central Massif, Alboran, Alpine Basements, Bohemian Massif, Moesia) preserve evidence of a complex geodynamic evolution involving pre-Ordovician microcontinents, likely to have originated at the Gondwanan margins (Haydoutov and Yanev 1997;Yanev 2000;von Raumer et al 2002 and references therein). Throughout the Variscan structures, eclogites and host rocks represent lithosphere that originated in a Cambro-Ordovician rifting that locally attained oceanization (von Raumer et al 2003 and references therein), followed by diachronous subduction events attaining a range of depths (O'Brien 2000). During the Variscan orogenic event, continental and oceanic blocks were accreted to the basement of southeastern Europe; the underthrusting has been reported from different segments at different times (e.g.…”

The Moslavačka Gora crystalline massif in Croatia: a Cretaceous heat dome within remnant Ordovician granitoid crust

“…Derived from the fact that LP/HT ultrametamorphic events are particularly typical for the Variscides (Zwart and Dornsiepen 1978;O'Brien 2000), it was the common opinion that the metamorphic rocks of the MGM represent a Variscan basement (Pamić 1998;Pamić et al 2000;Balen et al 2001;Pamić et al 2002a). Cretaceous K-Ar and Ar-Ar mica ages that were recorded in various parts of the massif (Lanphere and Pamić 1992;Palinkaš et al 2000;Balen et al 2001) were considered as dating a metamorphic overprint during the Alpine orogeny (Lanphere and Pamić 1992;Pamić 1998;Pamić and Jurković 2002).…”