Melilitites in the alkaline volcanics series on the Gorringe Bank (SW Portugal)

Chernysheva E.A., Kharin G.S.

P.P. Shirshov Institute of Oceanology, RAS, Atlantic Branch, Kaliningrad, Russia.


Underwater rise Gorringe Bank (GB) is situated 220 km to south-west from the cape Saint Vincent at the south of Portugal. It consists of two seamounts Gettysburg and Ormonde with a basement composed by uplifted and faulted fragment of mantle and oceanic crust of Late Jurrassic age which is overlapped by the series of alkaline volcanics of Paleocene age. Tectonic position of the GB is rather complicate. It is supposed that at the time of the Atlantic ocean opening around 120-112 Ma, the Iberia plate was attached to the Africa plate. The plate boundery was located in the Bay of Biscay and only after the Pyrenees formation it had jumped to the Azore-Gibraltar fracture zone, which had been separating Africa and Eurasia plates up to now. It was in the late Eocene (near 37 Ma) that Iberia became the part of Eurasia again. So, the history of Iberia and GB likely might be connected with the tectonic events in the north Africa 80-60 Ma ago. The main question is whether the alkaline magmatism of GB was of oceanic or continental origin.

The most detailed investigations of the alkaline rocks on the GB were carried out by G. Cornen (1982) and by V.V. Matveenkov and A.I. Al’mukhamedov (1996). The main series of rocks have been divided (lamprophyres, nephelinites, phonolites, trachytes). The whole rock analyses and some trace element contents were obtained. Using these data, we have chosen some samples for more detailed research from the stone collections dragged in 12 cruis of the scientific ship “Vityas’ ” and 16 cruis of š“Academic Mstislav Keldysh”.

Three samples of the dark coloured “lamprophyres” have been taken. They are represented by pebbles (2-5 cm) and poorly rounded boulders (10-20 cm), with biogenic limestone cover. All rocks have undergone strong alteration. Petrographic observation have shown that these lavas have porphyric texture and flow structure with some vesicles and small xenoliths of different rocks and minerals. Phenocrysts are represented by olivine (~ 10 %), fully replaced by carbonate, altered melilite (~ 30 %), and fresh clinopyroxene (~ 50 %). The groundmass is composed of clinopyroxene with few laths of melilite, difficult recognized interstitional analcime or nepheline, and great amount of tiny crystals of magnetite. Melilite forms typical lath-shaped phenocrysts, 1-2 mm length. It is replaced by unrecognized white finescaly substance, or by carbonate with brownish earthy material. Phenocrysts of clinopyroxene (augite), 1,5-2 mm size, sometimes contain relics of the other pyroxene, more green in colour and anhedral. This phenomenon have been described by Matveenkov and co-authors (1991) in the xenolith of pyroxenite from GB volcanics, and have been marked also in melilitites of Nizhnesayansky carbonatite complex, which we researched some years ago (Chernysheva, Belozerova, 2000). The “inner” pyroxenes possibly have been taken from the mantle rocks.

Thus, we classify this type of BG alkaline lavas as melilitites. Our conclusion is supported by chemical features of these rocks. On the classification diagram by Le Bas (1989): (SiO2+Al2O3) – (CaO+Na2O+K2O), - our samples take place in the field of melilitites. The rocks have high contents of MgO, CaO and TiO2, and low SiO2, and a remarkable trace elements composition: at the rather high Cr and Ni they have very high šSr, Ba, Rb, Zr and some other uncompatible elements – Nb, Ta, Th and LREE, - what represents “the family feature” of melilitites. On comparison with the melilitites of the other localities (Wilson et al., 1995), easy to see the most similarity of GB rocks with the Canary Island melilitites: (wt %) SiO2 - 38,94; 37,40; TiO2 - 2,94; 3,78; Al2O3 – 11,1; 9,35; Fe2O3 – 11,49; 12,40; MnO – 0,22; 0,20; MgO – 9,65; 14,02; CaO – 14,41; 14,27; Na2O – 3,53; 2,97; K2O – 1,93; 1,41; P2O5 – 0,80; 1,38; LOI - 5,62; 1,94; (ppm) V – 262; 296; Cr – 394; 373; Ni – 191; 337; Rb – 57,2; 32,7; Sr – 835; 1355; Ba – 866; 759; Y – 27; 37; Zr – 510; 317; Nb – 197; 90; U – 3,65; 2,52; Th – 11,52; 10,13; Pb – 9,38; 4,58; La – 102; 101; Ce – 188; 216; Nd – 77; 100; Sm – 12,1; 17,5; Eu – 3,37; 5,26; Yb – 1,95; 1,88; Lu – 0,27; 0,25.

It is known that the mafic alkaline volcanic rocks (melilitites, nephelinites, etc.), sometimes accompanied by carbonatites, are widespread in the continental magmatic provinces of the Europe and Africa, related with extension (“pre-rift”) and rifting of Late Tertiary-Recent age. In the central Spain (Iberia plate) there are many centres of alkaline mafic-ultramafic volcanism with carbonatite-melilitite eruption of that age (Bailey et al., 2005). Some facts let us to propose, that alkaline magmatism of the earlier Canary Islands, GB and North Africa could be connected with the African mantle plume activity on the early Paleocene, long before the European rifting occured.

All researchers of GB described many kinds of xenoliths founded in the lavas. There are pyroxenite, amphibole pyroxenite, melteigite, ijolite, syenite – the full range of intrusive rocks, usually occuring in carbonatite complexes. There are also fragments and aggregates of minerals: perovskite with sphene, amphibole, mica, apatite. So, we came to conclusion that the GB alkaline rocks belong to the typical continental carbonatite complex, destroyed and dislocated.

This study was financially supported by RFBR grant 06-05-64169-a.



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