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Geochemical investigations on rocks from the Ophiolite Zone of Zermatt-Saas Fee (Western Alps, Switzerland) with special emphasis on the potential of Aitchison's log-ratio method
von Wendelin HimmelheberThis study is concerned with the geochemistry of the Zermatt-Saas Fee Zone
(ZSF hereafter). This ophiolitic suite marks the suture between the
Eu„ropean and the Adria“tic continental plates, together with the other
ophiolites from the Wes„tern Alps. Its rocks, polymetamorphic ma“fic and
ultramafic extrusives and intru„sives and mi“nor metasedi„ments, are
supposed to represent oceanic crust from the middle Juras“sic to middle
Cretaceous Ligurian-Piemonte ocean, a small basin that opened up in response
to movements related to the opening of the cen„tral At“lantic ocean.
The three major rock types present in the ZSF, namely serpentinites, meta„gab“bros and metabasalts, are studied from a geochemi„cal point of view.
The main focus is on the magmatic genesis of the ZSF rocks and on their re“lati„onship. The peculiar difficulties of doing this kind of work in a polymeta“morphic terrain are dealt with. In particular it is tried to shed some light on the geochemi„cal effects of the metamorphic over“print. It is considered whether statistical or other methods can help to evaluate the relative mobility and im„mobility of dia“gnos"„tic trace ele“ments. In particular, in view of the well-known problems of doint statistics with compositional data (closed data sets, where the components sum up to 100 %), the log-ratio method of J. Aitchison is advocated and used extensively.
The serpentinites can be ex„plained as resi“dual 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 melt„ing. A pe“cu"„liarity are positive Ti and Nb anomalies and a negative Zr anomaly in the mantle-normalized spidergram. The Ti and Nb ano“ma„lies are explained by assuming that this mantle contains some exotic phase, perhaps Ti-clinohumite. As a potential explanation it may be assumed that the serpen“tinites represent subcon„ti“nental mantle.
The metagabbros have been olivine + plagioclase cumulates, sometimes also clinopyroxene + plagioclase cumu„lates. Some highly differentiated members of this group are con“taining also Fe-Ti minerals as cumulate phases. The chemical data are compatible with the as„sumption that these rocks were derived from those melts which belong to the restites now repre“sented by the serpen„tinites. In particular the gabbros are displaying Ti and Nb ano“malies complementary to the serpentinites. That they are also having a nega„tive 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 anoma“lies. Furthermore, the lack of a negative Eu anomaly precludes a rela„tion“ship to the gabbros as frac„tionated melt to cumulate.
The mantle source of the basalts must have been rather fertile, even more fertile than a T-MORB source, as the REE and HFSE are rather enriched. On the other hand, the LILE are strongly depleted. This peculiar combination of en“richment and depletion might again point to subcontinen"tal 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 Zermatt-Saas Fee ophiolite zone then is an example of a lithospheric ocean, as first proposed by Lemoine et al. (1987) for the Alpine ophiolites.
The three major rock types present in the ZSF, namely serpentinites, meta„gab“bros and metabasalts, are studied from a geochemi„cal point of view.
The main focus is on the magmatic genesis of the ZSF rocks and on their re“lati„onship. The peculiar difficulties of doing this kind of work in a polymeta“morphic terrain are dealt with. In particular it is tried to shed some light on the geochemi„cal effects of the metamorphic over“print. It is considered whether statistical or other methods can help to evaluate the relative mobility and im„mobility of dia“gnos"„tic trace ele“ments. In particular, in view of the well-known problems of doint statistics with compositional data (closed data sets, where the components sum up to 100 %), the log-ratio method of J. Aitchison is advocated and used extensively.
The serpentinites can be ex„plained as resi“dual 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 melt„ing. A pe“cu"„liarity are positive Ti and Nb anomalies and a negative Zr anomaly in the mantle-normalized spidergram. The Ti and Nb ano“ma„lies are explained by assuming that this mantle contains some exotic phase, perhaps Ti-clinohumite. As a potential explanation it may be assumed that the serpen“tinites represent subcon„ti“nental mantle.
The metagabbros have been olivine + plagioclase cumulates, sometimes also clinopyroxene + plagioclase cumu„lates. Some highly differentiated members of this group are con“taining also Fe-Ti minerals as cumulate phases. The chemical data are compatible with the as„sumption that these rocks were derived from those melts which belong to the restites now repre“sented by the serpen„tinites. In particular the gabbros are displaying Ti and Nb ano“malies complementary to the serpentinites. That they are also having a nega„tive 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 anoma“lies. Furthermore, the lack of a negative Eu anomaly precludes a rela„tion“ship to the gabbros as frac„tionated melt to cumulate.
The mantle source of the basalts must have been rather fertile, even more fertile than a T-MORB source, as the REE and HFSE are rather enriched. On the other hand, the LILE are strongly depleted. This peculiar combination of en“richment and depletion might again point to subcontinen"tal 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 Zermatt-Saas Fee ophiolite zone then is an example of a lithospheric ocean, as first proposed by Lemoine et al. (1987) for the Alpine ophiolites.