Alkaline volcanism in the continent-ocean transitional zone, SW Iberia margin
Chernysheva E.A.*, Matveenkov V.V.**, Medvedev A.Ya.***
* P.P. Shirshov Institute of Oceanology RAS, Atlantic Branch, Kaliningrad, Russia;** P.P. Shirshov Institute of Oceanology RAS, Moscow, Russia; *** A.P. Vinogradov Institute of GeochemistrySBRAS, Irkutsk, Russia.
South-western margin of Iberian Peninsula and north-western Africa region represented passive oceanic margins since the North Atlantic opening in the late Cretaceous time. Alkaline volcanism manifestations are known on the numerous seamounts and islands of these regions as well as on the coasts. The region has complicate tectonic history with supposed “jumping” of the boundary between Eurasia and Africa from the Bay of Biscay to the Azore-Gibraltar Fracture Zone 37 Ma ago. As a result, the old (67-60 Ma) volcanic edifice Gorringe, created at the continental margin, was destroyed and dislocated to the ocean, and exists now as an underwater rise Gorringe Bank (GB). The continental nature of GB volcanism was supported by researching of (Bernard-Griffiths et al., 1997; and others) and by our investigations, which have shown the existence of melilitites among the volcanic rocks of the GB (Chernysheva, Kharin, 2009). This type of the mafic alkaline rocks is widespread in the continental volcanic provinces of the Europe, Africa, and others.
The nearest to the GB volcanoes in the ocean are seamounts Amper and Coral-Patch, located at the opposite side of the deep-sea depression, near 200 km to the south-west. Unlike to the most of researchers, who deal with dredged stone collections, we have got samples from the drilling cores, taken in exactly marked places of the region. The samples received in 1991 route of the experimental scientific ship “Bavenit” , when several shallow boreholes were drilled on the seamounts Amper, Josephine and Gorringe Bank – around Horse Shoe oceanic depression near SW Portugal (Matveenkov et al., 1993).
Volcanic rocks of the seamount Amper were sampled from the western and the eastern tops of the mountain (site 7 and 8, accordingly). They have some differences in composition and in the rate of the secondary alteration (palagonitization, albitization) and are represented mainly by porous amygdaloidal subalkalic basalts, varying from basanitoids to hawaiites. The rocks of the Josephine seamount are represented by picritic basalt and by subalkalic olivine basalt. The age of the Amper volcano is 31 Ma, age of Josephine – 12 Ma (Geldmacher, Hoernle, 2000). Therefore temporal evolution of the volcanic rock composition could be traced from 67 till 12 Ma in the transitional zone from continent to the ocean.
Variations of the bulk rock composition in oceanic samples are connected with moderate differentiation (with decreasing MgO content from 6,4 to 2,3 % and increasing of alkalinity in the same time, and so on), or with cumulative process (in pickrite of Josephine mount). The rate of enrichment of oceanic rocks in incompatible trace elements straightly correlated with K2O contents. The most depleted is pickritic basalt of Josephine seamount (0,63 % K2O). Primitive mantle-normalized trace elements patterns in this rock coincide with the same for the most tholeiites from seamounts and islands of Madeira region (Geldmacher, Hoernle, 2000). Subalkalic olivine basalt of Josephine mount (with 1,73 % K2O) is already enriched in incompatible trace elements to the level of the Amper seamount volcanic rocks and the similar rocks of the islands Porto Santo and Madeira. Volcanic rocks from the western top of Amper seamount (samples A 7) are more evolved and stronger altered than rocks from the eastern top (samples A 8). Rocks A 7 have higher contents of U, Th, Pb, Sr, Zr and HREE, and besides of that, are more enriched in Mo, Sn, Sb, W – probably, because of hydrothermal alteration.
The ratios of incompatible trace elements in the rocks can help us to compare the circumstances of their parental melts origin. Variations of the ratios Zr/Nb – Nb/U and Ce/Y – La/Yb, shown on the Fig. 1, evidence that continental melts of the GB have been formed at lower degree of mantle melting (low Zr/Nb, high La/Yb) and more deep in the mantle, under the thicker lithospheric plate (high Ce/Y), than the oceanic basalt melts (Ellam, 1992). Very low ratio Nb/U in volcanic rocks A 7 (10-20 against > 45 for the other rocks) reflects the crustal contamination of the melting source or influence of the core-forming processes on these rocks (Hofmann, 1997). It is possible to see (Fig. 1(2)) that relative enrichment in LREE rises from tholeiites of Josephine mount (J) to the more alkalic basalts, but it does not reach the level of the BG melilitites. It concerns also the other most incompatible trace elements: contents of Nb, Ta, Zr, Hf, U, Th, and some others in oceanic volcanic rocks are always lower than in continental alkaline series.
The origin of alkaline rocks at the Iberian margin (not only of GB volcanics, but of the dykes and intrusive rocks of the Serra de Monchique as well (Bernard-Griffiths et al., 1997)) is connected, most probably, with the African plume activity just before global tectonical events in the region in Paleogene. Deep generated small degree partial melts appeared when mantle plume ascended to the base of subcontinental lithosphere.
Volcanites of the seamounts, arised after uniting of Iberia and Eurasia (younger than 37 Ma), hardly could be generated under promotion of the same mantle plume, which gave rise to GB magmatism. It was shown above, that the depth of the melt generation and the mantle source of the oceanic volcanism are considerably different from continental ones. As a result, volcanic rocks of seamounts and islands in this region are represented exclusively by the rocks of basaltic family, moderately enriched in incompatible trace elements, similar to the most subalkalic oceanic basalts of the other regions of the North Atlantic (Chernysheva, Kharin, 2007).
The new information appeared not so far (Merle et al., 2006), concerning the more ancient seamounts along the
Fig. 1. Variations of the incompatible trace elements ratios in volcanites of the Gorringe Bank (BG),
seamounts Amper (A 7 and A 8) and Josephine (J) near SW Iberia margin.
continent-ocean transition zone of Iberia margin. Seamounts are situated in the nothern and central part of the Tore- Madeira Rise, along the oldest Atlantic magnetic anomaly in this region. The ages of five seamounts vary from 104 till 80 Ma. Dredged volcanites are represented by strongly evolved rocks of basaltic family, mainly trachytes and trachy-andesites, suffered the low-temperature alteration by seawater. On the isotopic characteristics these volcanites are distinct from the Atlantic MORB, and from the rocks of Madeira Archipelago and Azore Islands, but similar to the worldwide Ocean Island Basalts (OIB). It is supposed that the parental magma melting in transitional zone occured under oceanic lithosphere from the mantle domains with a small portion of continental material from Iberian plate.
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