Περίληψη:
On the island of Lesvos in the north-eastern Aegean Sea, thrust sheets
of mantle peridotite override a tectonic melange containing
metasediments, metabasalts and a few metagabbros. Previous workers have
collectively called the above rock types the “Lesvos ophiolite” and
suggested it represents Neotethyan (Jurassic) oceanic crust of N-MORB
and IAT affinities. However, the complete absence of a sheeted dyke
complex and of a layered cumulate sequence, in turn overlain by
isotropic gabbros does not support the ophiolite description. The
basalts of the melange define two distinct groups on the basis of Nb/Y
values: the Vatera (low Nb/Y) and the Melange (high Nb/Y) groups. The
Vatera basalts show multi-element profiles and REE patterns lying well
within the field of N-MORB with the exception of Y and HREE abundances
that are lower than those of N-MORB and match those of E-MORB. They
originated by similar to 10% melting of depleted MORB mantle that
carried a small amount (similar to 3%) of an enriched (OIB-like)
components corroborated by moderate initial epsilon Nd values
(+4.6-+6.2). The Melange basalts, in contrast, have multi-element
profiles and REE patterns typical of within-plate compositions,
transitional between E-MORB and OIB. They originated by <5% melting of
spinel (and occasionally garnet) Iherzolite that carried a small amount
(similar to 3%) of recycled crustal component, as supported by low
initial epsilon Nd values (+3.8-+5.1).
The Lesvos peridotites comprise both spinel Iherzolites and
harzburgites. Using mineral compositions and calculations of temperature
and oxygen fugacity, the Iherzolites plot in the fields of alpine-type
and continental peridotites. Trace element modelling suggests they have
experienced anhydrous melting in the stability field of spinel but also
garnet Iherzolite; calculated degrees of melting are generally small and
never exceed 14%. There is no evidence for refertilisation or hydrous
melting in a supra-subduction zone environment for the Iherzolites. The
harzburgites, despite being considerably more altered than the
Iherzolites, also display evidence of anhydrous melting in the stability
field of spinel. However, they also display spoon- and dish-shaped REE
patterns that can be produced by contamination of the harzburgites by a
percolating OIB-like component with an inherent recycled sedimentary
component, which is necessary to explain their isotopic shift towards
EM2 compositions.
SHRIMP dating of gabbro zircons revealed an Upper Permian (253 Ma)
crystallisation age and a Middle Triassic (236 Ma) resetting of the
zircon rims, compatible with the coeval and widespread rift-related
intrusion of A-type granites in the eastern Mediterranean. Gabbro zircon
ages of 777, 539 and 338 Ma strongly suggest inheritance from the
basement into which the gabbros were intruded and correspond to ages of
distinct terranes recently recognized in the Hellenides.
We propose that the Lesvos melange and ultramafic rocks best represent
an incipient continental rift setting of Triassic age, similar to what
has been described for the Italian Ligurides, that led to the subsequent
formation of a branch of Neotethys, and that the term “Lesvos
ophiolite” should be used with due caution or abandoned altogether.
(C) 2008 Elsevier B.V. All rights reserved.
Συγγραφείς:
Koglin, Nikola
Kostopoulos, Dimitrios
Reischmann, Thomas
