Geochemical investigations on rocks from the Ophiolite Zone of ZermattSaas Fee (Western Alps, Switzerland) with special emphasis on the potential of Aitchison's logratio method
242 Seiten, Erscheinungsjahr: 2003
Preis: 40.50 EUR
Geochemie , Statistik , Zermatt , Ophiolit , Metabasite
This study is concerned with the geochemistry of the ZermattSaas Fee Zone
(ZSF hereafter). This ophiolitic suite marks the suture between the
European and the Adriatic continental plates, together with the other
ophiolites from the Western Alps. Its rocks, polymetamorphic mafic and
ultramafic extrusives and intrusives and minor metasediments, are
supposed to represent oceanic crust from the middle Jurassic to middle
Cretaceous LigurianPiemonte ocean, a small basin that opened up in response
to movements related to the opening of the central Atlantic ocean.
The three major rock types present in the ZSF, namely serpentinites,
metagabbros and metabasalts,are studied from a geochemical point of view.
The main focus is on the magmatic genesis of the ZSF rocks and on their
relationship. The peculiar difficulties of doing this kind of work in a
polymetamorphic terrain are dealt with. In particular it is tried to shed
some light on the geochemical effects of the metamorphic overprint. It is
considered whether statistical or other methods can help to evaluate the
relative mobility and immobility of diagnostic trace elements. In
particular, in view of the wellknown problems of doint statistics with
compositional data (closed data sets, where the components sum up to 100 %),
the logratio method of J. Aitchison is advocated and used extensively.
The serpentinites can be explained as residual mantle after extraction of
ca. 5 to 22 % melt in the spinel lherzolite field. The mode of melting seems
to have been closer to batch melting than to fractional melting. A
peculiarity are positive Ti and Nb anomalies and a negative Zr anomaly in
the mantlenormalized spidergram. The Ti and Nb anomalies are explained by
assuming that this mantle contains some exotic phase, perhaps
Ticlinohumite. As a potential explanation it may be assumed that the
serpentinites represent subcontinental mantle.
The metagabbros have been olivine + plagioclase cumulates, sometimes also
clinopyroxene + plagioclase cumulates. Some highly differentiated members
of this group are containing also FeTi minerals as cumulate phases. The
chemical data are compatible with the assumption that these rocks were
derived from those melts which belong to the restites now represented by
the serpentinites. In particular the gabbros are displaying Ti and Nb
anomalies complementary to the serpentinites. That they are also having a
negative Zr anomaly then would speak in favor of a primary lack of Zr in
the mantle source.
For the metabasalts, on the other hand, a magmatic relationship to either
the metagabbros or to the serpentinites can be excluded, arguing from the
lack of the aforementioned characteristic trace element anomalies.
Furthermore, the lack of a negative Eu anomaly precludes a relationship to
the gabbros as fractionated melt to cumulate.
The mantle source of the basalts must have been rather fertile, even more
fertile than a TMORB source, as the REE and HFSE are rather enriched. On
the other hand, the LILE are strongly depleted. This peculiar combination of
enrichment and depletion might again point to subcontinental mantle, but
this time lacking the exotic phase which has been present when the gabbroic
melts were extracted.
These findings are not compatible with a MOR genetic model. Instead they are
tentatively related to the framework of a simple shear rifting model as
proposed by Wernicke (1981, 1985), where a lower plate is unroofed through
extension along a detachment fault penetrating into the lithosphere. The
ZermattSaas Fee ophiolite zone then is an example of a lithospheric ocean,
as first proposed by Lemoine et al. (1987) for the Alpine ophiolites.
Wollen auch Sie Ihre Dissertation veröffentlichen?