Large Scale Variscan Granitoid Intrusion Throughout Europe: a Lateral Geochronological Trend? K
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Large scale Variscan granitoid intrusion throughout Europe: a lateral geochronological trend? K. Oud BSc Thesis, Faculty of Earth Sciences, Utrecht University, July 2006 Abstract Large-scale granitoid intrusion during the Variscan orogeny (370 to 250 Ma) in Europe above subduction zones of that time is assumed to be the result of a heat pulse due to slab detachment. This would show from a lateral trend in age of emplacement of all granitic bodies, migrating through the complete Variscan fold belt, and thus, a younging direction perpendicular to the former subduction zones. To test this, an inventory of intrusion ages (on the basis of U-Pb, Rb-Sr and K-Ar analysis), typology (I- or S-type) and location of 70 granitic bodies in the European Variscan fold belt, from the Armorican Massif to the Bohemian Massif, was made. From the collected data, no lateral trend in age is observed. However, a trend in age parallel to former subduction zones is shown in most massifs, with a younging direction towards the thrusting vergence. This is probably the result of nappe stacking. In the Armorican Massif and Massif Central, granites are all S-type, which is probably the cause of continental subduction. In central Europe, the typology is mixed S- and I-type, which can be caused by subduction of either continental or oceanic lithosphere. 1. Introduction result of the ‘usual’ heat and melt generation attributed to subduction The Variscan orogeny yielded a large processes. However, there is another number of granitoid intrusions throughout possibility of granite emplacement that all of the European Variscides. In all can be considered, which is not commonly orogenic events like subduction and mentioned in related studies. During the continental collision, the crust and mantle final stages of subduction, the subducted are subjected to extreme temperature and slab may break-off and sink into the pressure conditions; thermal pulses astenosphere. This is called slab caused by such conditions are abundant. detachment (fig. 1) [Wortel and Spakman, These often result in heating of the crust 2000]. during burial, or decompression during late-orogenic extension, and the subsequent melting of the crust and/or mantle which produces (granitic) magmas that intrude into the middle and upper crust. It has been shown that granitic magmas can have a rather varying ? composition, which means a derivation from differing source regions and source rocks [Chappell and White, 2001]. Therefore, the occurrence of granites in an orogenic belt is a reflection of its tectonothermal evolution and its deep structures and processes that are the least accessible and the worst exposed [Finger et al, 1997]. The massive occurrence of granites throughout the Variscan fold belt gives rise to the assumption of a large scale Fig. 1 . Slab detachment with possible thermal pulse along the entire Variscan granitic intrusion (indicated by the ‘?’). After suture. It is possible that this is just the Wortel and Spakman, 2000. 1 When a ‘cold’ slab detaches, it can be From previous studies concerning replaced by hot astenospheric material Variscan granites, I listed the ages of that creates a heat pulse underneath the intrusion from the granitic bodies, aiming overlying plate, which may cause to use the most reliable dating method magmatism [Wortel et al, 2003]. As is available. On the basis of geochemical shown by Wortel and Spakman [2000], information of the bodies involved, I slab detachment can take place during the determined whether they can be last stages of an orogeny, after ocean considered an I-type or S-type. All those closure and continent collision. With this data will be plotted in a geological map of process, the chance of breaking of the Europe and with this, it will be possible to slab along the entire plate boundary at see whether there is a trend in the age of once would be very small. Rather, the slab intrusion or not and thus, if the intrusion would start to break at a small area where of granites is the possible result of a heat applied stresses would overcome the pulse from slab detachment. strength of the plate segment. This fracture would then start to extend and migrate laterally along the plate 2. Geological and tectonothermal boundary. Consequently, one could expect background a similar lateral trend in the related heat pulse and granite emplacement, shown in The Variscan orogeny resulted from the crust above. Because it is shown by the collision of Euramerica (i.e. Laurentia, tomography [Bijwaard et al, 1998] that Baltica and Avalonia) and Gondwanaland the Variscan subducting slabs are no (Africa) which began in early Palaeozoic longer present under Europe, they must time (from 480 Ma, fig. 2). It meant the have detached. It is suggested by Franke subduction of a part of the latter continent [2000] that at least at some places in the under Euramerica. The suture was Variscan belt, subducting slabs broke off completed during the late with subsequent rising of astenospheric Carboniferous/early Permian (250 Ma) and material and formation of granitic melts. formed the supercontinent Pangaea This paper aims to test whether the [Stanley, 1999]. large scale granite intrusion during the The northern margin of Gondwanaland Variscan orogeny was caused by a heat was very fragmented and the collision pulse from slab detachment. For this area was therefore composed of purpose, I made an inventory of the ages continental microplates, separated by a and typology of the granitic intrusions in system of ridges and troughs, basins and the Variscan massifs in Europe, from the oceanic rifts [Pin, 1990b]. These terranes Armorican to the Bohemian Massif. were the origin of the European Fig. 2. Schematic map showing the world and its continents, microplates and oceans during Middle Silurian (from Scotese [2003]). 2 Variscan fold belt; they drifted northward southern margins of the Rheinisches from Gondwanaland, colliding with the Schiefergebirge and the Harz. The basin northern continents. This is shown in was closed during the Early Carboniferous. several zones by the occurrence of thick The Variscan collision yielded large sequences of marine sediment and small scale (thin-skinned) nappe-thrusting and but extensive occurrences of eclogites, (thick-skinned) crustal stacking and the originated from MOR-basalts which show Precambrian basement of the ridges fused that the rifting at least must have reached with thick layers of sediment and ocean a narrow-ocean stage [Franke, 1989]. floor of rift basins from Early Palaeozoic There are several hypotheses about time. The thrusting caused large scale the exact composition and evolution of the Barrovian-type (med-T/med-P) regional microplates involved in the suture, but metamorphism, most extensive in the geological and palaeomagnetic evidence interior of the Moldanubian zone. During indicates that the two largest terranes, the late Early Carboniferous the process of Avalonia and Armorica, were the nappe-stacking stopped because of foundations of the Variscan fold belt. dextral wrenching of Gondwana relative to These microplates are said to be Euramerica, which in turn led to dextral composed of several smaller terranes [Tait transpressional and transtensional et al, 1997] and are believed to be tectonics [Arthaud and Matte, 1977]. attached to Baltica and Laurentia before During this process, large parts of the the large collision between Gondwana and Moldanubian zone were affected by severe Euramerica. Also, two main oceanic basins high-T/low-P metamorphism. Also, the – each consisting of several smaller anti-parallel subducted mantle-slabs oceanic basins – are recognized: the possibly joined and became detached, Iapetus ocean between Avalonia, Baltica which caused the rise of astenospheric and Laurentia and the Rheic ocean material and therefore a large scale between Avalonia and Gondwana [Matte, intrusion of post-tectonic granites [Franke, 2001]. Closing of the first caused the 2000] (fig. 3, the red areas in the Caledonian orogeny in western and massifs). This hypothesis will be tested in northern Europe during middle Silurian to this paper. early Devonian (430 Ma to 370 Ma). The During the Late Carboniferous and Variscan orogeny was caused by the Permian, post-orogenic uplift and closure of the Rheic Ocean. extension of the Variscan crust occurred. The generally believed theory for the In the Late Carboniferous, along the composition of the European Variscan fold southern margins of the fold belt, there belt is that it consists of two large might have been some renewed subduction zones which originated before subduction of oceanic crust and accretion the collision in the ridge-trough system of terranes, which had sheared of from described above. A southern, northward Gondwanaland while this was drifting dipping system by which the Massif westward. However, it is not clear what Central and intra-Alpine oceans were their exact relationship was before these closed (the Moldanubian zone, fig. 3) and terranes were deformed by the Alpine more to the north a southward dipping orogeny in the Mesozoic and Tertiary system which led to the closure of the [Finger et al, 1997]. Saxothuringian and Rhenohercynian basins. This closure began during late Ordovician to early Devonian time, first 3. Granitoids involving oceanic crust, followed by subduction of continental crust during the 3.1 Intrusion Late Carboniferous. Contrasting with this Granite intrusions occur when heat or convergence is the opening of the water is added to the mantle or crust – Rhenohercynian basin during the having various geodynamic causes - such Devonian. This is possibly caused by back- that it melts and rises to intrude shallower arc spreading due to the northward crust. Granitic melts (magmas) can be subduction of oceanic crust further to the formed either through the addition of south [Franke, 1989]. Traces of the volatiles (i.e. water) in the mantle, opening of this basin can be found at the decompression of the crust or mantle or 3 Fig.