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| Titel |
Mineralogy and geochemistry of listwanite occurrences from the Othris ophiolite, Greece. |
| VerfasserIn |
Petros Koutsovitis, Andreas Magganas |
| Konferenz |
EGU General Assembly 2013
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 15 (2013) |
| Datensatznummer |
250083612
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| Zusammenfassung |
| Three small occurrences of extensively carbonate-altered serpentinite (listwanite) have been
identified in East Othris at the ophiolitic formation of Vrinena, and in South Othris at the
ophiolitic mélange formations of Agios Georgios and Paleokerasia. Their mineral assemblage
includes calcite + quartz + chlorite + spinel ± clinopyroxene, as well as accessory Fe-oxides,
titanite and apatite. Based upon their mineralogical composition they belong to the Type IB
listwanite, characterized by the predominance of calcite and the presence of Mg-rich chlorite,
mainly clinochlore and diabantite.
In the ophiolitic mélange formation of Agios Georgios listwanite was found in proximity
with serpentinite (former harzburgite), which could be considered as the precursor protolith.
Changes of major and minor elements between the listwanite and the protolith
have been calculated based on the method for mass-balance analyses[1]. Results
show that this listwanite resulted after metasomatic processes dominated by Ca
enrichment. Sr, Y and Pb contents were also significantly increased, whereas rather
moderate enrichments of Al, Mn, Cr and Cu also took place. Small reductions were
observed for Mg and Ni. Si, Ti and Fe remained relatively immobile. The chondrite
normalized REE patterns reveal significant enrichment of all analysed REE, and
especially of the LREE [(La/Yb)CN=20.4], and also with a negative Eu anomaly
(EuCN/Eu*=0.79).
Spinel grains from the Agios Georgios listwanite and the adjacent serpentinite are
in most elements compositionally similar. The listwanites from Agios Georgios,
Paleokerasia and Vrinena all contain spinel grains. Their mineral chemistry is respectively:
TiO2=0.18-0.25; 0.04-0.10; 0.22-0.54 wt%, Al2O3=23.13-25.03; 27.69-29.70; 5.69-7.35
wt%, FeO=18.24-22.98; 16.44-19.49; 21.47-24.61 wt%, CaO= 0.01-0.07; 0.03-0.15;
0.01-0.28 wt%, Cr#=52.28-54.93; 45.57-48.85; 83.58-87.59, Mg#=51.07-65.39; 56.68-65.62;
46.77-55.35. Their rims exhibit slightly higher FeO and CaO contents compared to cores.
Relict clinopyroxenes have been found in listwanite from Vrinena, classified as augites
(Mg#=84.55-85.91; Wo=42.50-44.34; TiO2=0.50-0.70 wt%).
The abundance of calcite and of REE enrichment indicate that the listwanite-forming
metasomatic event occurred with hydrothermal circulation of a CO2-rich fluid phase in a high
water/rock ratio. REE mobilized mainly as REE-carbonate complexes under mildly alkaline
conditions. Based upon the reciprocal slopes of the isocon method[2] the total mass gains are
restricted, indicating that this hydrothermal alteration event occurred isochemically under
mass preservation. Their formation is most likely associated with shallow level
ocean-floor metasomatism, observed also in listwanites from the Iti ophiolitic mélange
formation[3].
References: [1] Gresens 1967: Chemical Geol., 2, 47-65; [2] Grant 1986: Econ. Geol., 81,
1976-1982; [3] Tsikouras et al., 2006: Eur. J. Mineral. 18, 243-255. |
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