Melilitites in the alkaline volcanics
series on the Gorringe Bank (SW
Chernysheva E.A., Kharin
P.P. Shirshov Institute of Oceanology,
RAS, Atlantic Branch, Kaliningrad,
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
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,
length. It is replaced by unrecognized white finescaly
substance, or by carbonate with brownish earthy material. Phenocrysts
of clinopyroxene (augite),
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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