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Harangi, Szabolcs and Downes, Hilary and Seghedi, I. (2006) TertiaryQuaternary subduction processes and related magmatism in the AlpineMediterranean region. In: Gee, D.G. and Stephenson, R. (eds.) European lithosphere dynamics. Geological Society of London Memoir

32. London: Geological Society of London, pp. 167-190. ISBN 978Downloaded from: http://eprints.bbk.ac.uk/160/

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Tertiary-Quaternary subduction processes and related magmatism in the Alpine-Mediterranean region Szabolcs Harangi1,*, Hilary Downes2 and Ioan Seghedi3 Department of Petrology and Geochemistry, Eötvös University, H-1117 Budapest, Pázmány Péter sétány 1/C, Hungary; e-mail: szabolcs.harangi@geology.elte.hu Birkbeck/UCL Research School of Earth Sciences, Birkbeck University of London, Malet St., London WC1E 7HX, UK Institute of Geodynamics, 19-21, str. Jean-Luis Calderon, Bucharest 70201, Romania * corresponding author number of word of abstract: 396 number of word of text: 12097 number of word of references: 9030 number of word of figure captions: 875

-1Abstract During Tertiary to Quaternary times, convergence between Eurasia and Africa resulted in a variety of collisional orogens and different styles of subduction in the Alpine-Mediterranean region. Characteristic features of this area include arcuate orogenic belts and extensional basins, both of which can be explained by roll-back of subducted slabs and retreating subduction zones. After cessation of active subduction, slab detachment and post-collisional gravitational collapse of the overthickened lithosphere took place. This complex tectonic history was accompanied by the generation of a wide variety of magmas. Most of these magmas (e.g. low-K tholeiitic, calc-alkaline, shoshonitic and ultrapotassic types) have trace element and isotopic fingerprints that are commonly interpreted to reflect enrichment of their source regions by subduction-related fluids. Thus, they can be considered as ‘subductionrelated’ magmas irrespective of their geodynamic relationships. Intraplate alkali basalts are also found in the region generally postdated the ‘subduction-related’ volcanism. These mantle-derived magmas have not been, or only slightly, influenced by subduction-related enrichment.

This paper summarises the geodynamic setting of the Tertiary-Quaternary “subductionrelated” magmatism in the different segments of the Alpine-Mediterranean region (BeticAlboran-Rif province, Central Mediterranean, the Alps, Carpathian-Pannonian region, Dinarides and Hellenides, Aegean and Western Anatolia), and discusses the main characteristics and compositional variation of the magmatic rocks. Radiogenic and stable isotope data indicate the importance of continental crustal material in the genesis of these magmas. Interaction with crustal material probably occurred both in the upper mantle during subduction (‘source contamination’) and in the continental crust during ascent of mantlederived magmas (either by mixing with crustal melts or by crustal contamination). The Sr/86Sr and 206Pb/204Pb isotope ratios indicate that an enriched mantle component, akin to the source of intraplate alkali mafic magmas along the Alpine foreland, played a key role in the petrogenesis of the ‘subduction-related’ magmas of the Alpine-Mediterranean region.

This enriched mantle component could be related to mantle plumes or to long-term pollution (deflection of the central Atlantic plume and recycling of crustal material during subduction) of the shallow mantle beneath Europe since the late Mesozoic. In the first case, subduction processes could have had an influence in generating asthenospheric flow by deflecting nearby mantle plumes due to slab roll-back or slab break-off. In the second case, the variation in the chemical composition of the volcanic rocks in the Mediterranean region can be explained by “statistical sampling” of the strongly inhomogeneous mantle followed by variable degrees of crustal contamination.

-2The Alpine-Mediterranean region is one of the most complex geodynamic settings on Earth. Subduction of oceanic plates, collision of continents, opening of extensional basins and possible upwelling of mantle plumes have all occurred associated with the formation of a wide variety of igneous rocks during the Tertiary and Quaternary. These processes are still active in some parts of this region. The geodynamic processes and volcanic activity have been the focus of researches for a long time. During the last decade a number of papers have been published using the results of new techniques such as seismic tomography and isotope geochemistry (see summary papers of Wilson & Bianchini 1999; Doglioni et al. 1999;

Lustrino 2000; Wortel & Spakman 2000).

Convergent margins are the sites where subduction of oceanic lithosphere occurs beneath oceanic or continental plates. The style of subduction depends upon various parameters, including the rate of convergence, the rate of subduction, the nature of the subducted lithosphere and the polarity of subduction (Jarrard 1986; Royden & Burchfiel 1989; Doglioni 1993). Tertiary-Quaternary subduction in the Alpine-Mediterranean region was governed by the convergence between Eurasia and Africa in an area where continental and oceanic microplates were trapped between the converging continental plates. This resulted in various styles of subduction and collision (Royden & Burchfiel 1989; Royden 1993; Doglioni et al. 1999). Royden & Burchfiel (1989) proposed that orogenic belts with high topographic elevation were formed where the rate of convergence exceeded the rate of subduction (advancing subduction; e.g., Alps). In contrast, low topographic relief and regional extension in the upper plate are considered to characterize subduction boundaries where the rate of subduction exceeded the rate of overall plate convergence (retreating subduction; e.g., Betics–Alboran-Rif, Apennines, Hellenic and Carpathian thrust belts).

Doglioni (1991; 1993) and Doglioni et al. (1999) emphasized the importance of subduction polarity. Westward-directed subduction zones oppose mantle flow and have similar features to retreating subduction boundaries, i.e. steep angle of subduction, slab roll-back, opening of extensional basins and termination of subduction when the buoyant continental lithosphere enters the trench. Eastward-directed subduction zones are reinforced by mantle flow and show a lower angle of subduction, together with a lack of extension in the overlying plate.

Following subduction of oceanic lithosphere, continent-continent collision occurs resulting in thickening of the continental crust and lithosphere. Detachment of the dense oceanic slab (Davies & von Blanckenburg 1995), delamination of the thick lithospheric mantle (Bird 1979), sometimes with the dense mafic lower crust (Lustrino et al. 2000) or convective removal of the lower lithosphere (Housman et al. 1981; Platt & Vissers 1989; Turner et al.

1999) could be responsible for post-collisional extension and related magmatism. The geochemistry of the magmas is dependant on the rheology of the continental plates involved in the collision, the extent of collision and the velocity of plate convergence (Wang et al.


The complex tectonic evolution of the Alpine-Mediterranean region has been associated with formation of a wide range of Tertiary to Quaternary and even recent igneous rocks (Fig. 1). Wilson & Bianchini (1999) divided the magmatic activity of this area into “orogenic” and “anorogenic” types. Alkali basaltic magmas of anorogenic type erupted mainly along the foreland of the Alps (see Wilson & Downes, this volume), but can be found also throughout the Mediterranean region. Orogenic calc-alkaline, potassic to ultrapotassic and silicic magmas erupted in the convergent margins of the Alpine-Mediterranean region.

These volcanic rocks show indeed a subduction-related geochemical composition. In this paper, we highlight the results of the most recent publications on the Tertiary to Quaternary volcanism and show that geodynamic and petrogenetic models are still highly controversial in spite of the emerging data. One of the aims of this paper is to present the competing tectonic and petrogenetic models, the volcanic histories of different areas of this region and finally to

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Generation of magmas with ‘subduction-related’ geochemistry Subduction of oceanic lithosphere often results in a linear chain of volcanoes along an island arc or active continental margin (Fig. 2A; Gill 1981; Thorpe 1982; Wilson 1989). Most of the volcanic rocks, which build up these volcanoes, are calc-alkaline in composition. In addition, low-K tholeiitic and potassic magmas can also be associated with active subduction.

The primary magmas are considered to form by melting of hydrous peridotite either in the asthenospheric mantle wedge or in the subcontinental lithospheric mantle (Gill 1981). The complex processes beneath active volcanic arcs seem to develop fairly quickly (Gill & Williams 1990; Gill et al. 1993; Reagan et al. 1994; Elliott et al. 1997). Dehydration of the descending slab, metasomatism of the mantle wedge by aqueous fluids, partial melting and eruption of magmas can take place in 30 kyr (Elliott et al. 1997; Turner et al. 2000).

Therefore, the presence of calc-alkaline magmatism is widely considered as evidence for the existence of an active subduction zone (e.g., Pearce & Cann 1973; Wood 1980). Most calcalkaline magmas are intermediate in composition (andesite to dacite) and true basalts are rare in most suites due to the differentiation processes in shallow level magma chambers.

Subduction-related magmas worldwide have trace element and isotopic fingerprints that are usually interpreted as reflecting fluid involvement in their genesis. Such fluids are either aqueous solutions or silicate melts released from the subducted oceanic lithosphere and its overlying sediments. Their effects are seen in high ratios of large ion lithophile elements (LILE; such as Cs, Rb, Ba, K, Sr) and Pb to high field strength elements (HFSE; e.g., Nb, Ta, Zr, Hf and Ti, Fig. 3). The LILE are soluble in aqueous fluids (Tatsumi et al. 1986), therefore they are enriched relative to the immobile HFSE and rare earth elements (REE; Gill 1981;

Pearce 1982; Ellam & Hawkesworth 1988; Hawkesworth et al. 1994; Pearce & Peate 1995).

This is reflected by negative anomalies in HFSE and the relative enrichment of LILE in trace element patterns in mantle-normalized multi-element diagrams (Fig. 3). Addition of pelagic or terrestrial sediment to the mantle has a fairly similar geochemical effect as aqueous fluid metasomatism (increase of LILE), however, it usually also results in a marked enrichment of Th relative to Nb (Elliott et al. 1997) and it strongly influences the radiogenic and stable isotope ratios. Subduction-related magmas often have relatively high 87Sr/86Sr, low Nd/144Nd and relatively high 207Pb/204Pb and 208Pb/204Pb isotope ratios compared with midocean ridge basalts, consistent with interaction with a continental crustal component. This component could be introduced to the mantle via subduction of sediment, or could enter the mantle-derived magmas as they pass through the continental crust.

Such ‘subduction-related’ geochemical fingerprints characterize, however, not only the calc-alkaline and low-K tholeiitic magmas in active subduction zones, but also the potassic and ultrapotassic rocks as well as silicic igneous rocks, which are formed in anorogenic or post-collisional settings. In this case, these trace element and isotopic signature were inherited from the mantle source region modified previously by fluids released from subducted slab (Johnson et al. 1978; Hawkesworth et al. 1995). Reactivation, i.e. partial melting of such metasomatized mantle, could occur due to decompression in a thinning lithosphere during extension (Fig. 2B) or due to the heat flux of upwelling hot asthenospheric mantle material.

Magmas with ‘subduction-related’ geochemical signatures can also be generated in syn- and post-collisional tectonic setting. A particular case is the generation of a slab-free region beneath a continental margin due to detachment of subducted oceanic lithosphere

-4slab break-off”; Fig.2C). This can occur when continental lithosphere enters the subduction zone (Davies & von Blanckenburg 1995; Wong et al. 1997). It result from the tensional force between the buoyant continental lithosphere and the denser oceanic slab, and can cause the formation of calc-alkaline and ultrapotassic magmas and crustal-derived silicic melts (Davies & von Blanckenburg, 1995; von Blanckenburg & Davies, 1995). Magmatism related to slab break-off is usually localised and instantaneous following the detachment. Melt generation occurs as a consequence of upwelling of hot asthenosphere into the void left between the separating lithospheres. The lithosphere of the overriding plate is conductively heated by the asthenospheric mantle flow and this can result in various degrees of melting within the metasomatized lithospheric mantle. Intrusion of mantle-derived mafic magmas into the thick continental crust could initiate crustal anatexis and formation of silicic magmas (“anatectic rhyolites or granites”).

Delamination or convective removal of the lithospheric mantle could also lead to generation of magmas with ‘subduction-related’ geochemical features (e.g., Turner et al.

1996; 1999; Chalot-Prat & Gîrbacea 2000; Fig. 2D). Removal of lower lithosphere results in upwelling of hot asthenosphere, which heats up the overlying cooler continental lithosphere.

Partial melting could occur first in the volatile-rich portions of the lithospheric mantle producing LILE-enriched magmas.

In summary, magmas with ‘subduction-related’ geochemical signatures could be formed in different tectonic settings. The common feature of these magmas is explained by the similar nature of their mantle source regions. Fluid-related metasomatism could take place just before the melt generation or be associated with an older subduction event.

Cenozoic subduction zones in the Alpine-Mediterranean region

Figure 4 shows the main Tertiary-Quaternary subduction systems in southern Europe.

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