Samples of volcanic rocks from Alboran Island, the Alboran Sea floor and
from the Gourougou volcanic centre in northern Morocco have been
analyzed for major and trace elements and Sr-Nd isotopes to test current
theories on the tectonic geodynamic evolution of the Alboran Sea. The
Alboran Island samples are low-K tholeiitic basaltic andesites whose
depleted contents of HFS elements (similar to0.5xN-MORB), especially Nb
(similar to0.2xN-MORB), show marked geochemical parallels with volcanics
from immature intra-oceanic arcs and back-arc basins. Several of the
submarine samples have similar compositions, one showing low-Ca boninite
affinity. Nd-143/Nd-144 ratios fall in the same range as many island-arc
and back-arc basin samples, whereas Sr-87/Sr-86 ratios (on leached
samples) are somewhat more radiogenic. Our data point to active
subduction taking place beneath the Alboran region in Miocene times, and
imply the presence of an associated back-arc spreading centre. Our sea
floor suite includes a few more evolved dacite and rhyolite samples with
(Sr-87/Sr-86)(0) up to 0.717 that probably represent varying degrees of
crustal melting. The shoshonite and high-K basaltic andesite lavas from
Gourougou have comparable normalized incompatible-element enrichment
diagrams and Ce/Y ratios to shoshonitic volcanics from oceanic island
arcs, though they have less pronounced Nb deficits. They are much less
LIL- and LREE-enriched than continental arc analogues and
post-collisional shoshonites from Tibet. The magmas probably originated
by melting in subcontinental lithospheric mantle that had experienced
negligible subduction input. Sr-Nd isotope compositions point to
significant crustal contamination which appears to account for the small
Nb anomalies.
The unmistakable supra-subduction zone (SSZ) signature shown by our
Alboran basalts and basaltic andesite samples refutes geodynamic models
that attribute all Neogene volcanism in the Alboran domain to
decompression melting of upwelling asthenosphere arising from convective
thinning of over-thickened lithosphere. Our data support recent models
in which subsidence is caused by westward rollback of an
eastward-dipping subduction zone beneath the westemmost Mediterranean.
Moreover, severance of the lithosphere at the edges of the rolling-back
slab provides opportunities for locally melting lithospheric mantle,
providing a possible explanation for the shoshonitic volcanism seen in
northern Morocco and more sporadically in SE Spain. (C) 2004 Elsevier
B.V. All rights reserved.