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| Titel |
Ultramafic lavas and pyroxene-spinifex high-Mg basaltic dykes from the Othris ophiolite complex, Greece |
| VerfasserIn |
Ioannis Baziotis, Maria Economou-Eliopoulos, Paul Asimow |
| Konferenz |
EGU General Assembly 2017
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 19 (2017) |
| Datensatznummer |
250137730
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| Publikation (Nr.) |
 EGU/EGU2017-535.pdf |
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| Zusammenfassung |
| This study aims to constrain the physico-chemical conditions and processes associated with
the origin of ultramafic lavas of the Agrilia formation and high-Mg basaltic dykes in
the Pournari area within the Othris ophiolite complex, a supra-subduction zone
ophiolite of Mesozoic age (Paraskevopoulos & Economou, 1986; Barth et al., 2008).
Hand-sample-scale spinifex texture is lacking from the ultramafic lavas and, despite
whole-rock MgO contents greater than 31 wt.%, we infer an upper bound of 17 wt.%
MgO for the erupted liquid, and thus identify these lavas as picrites containing
accumulated olivine. We use textural and compositional criteria to divide the crystals
within the Agrilia lavas between pre-eruptive and post-eruptive growth phases. The
high-Mg basaltic dyke margins display a distinctive thin-section-scale micro-spinifex
texture of skeletal and plumose Al- and Fe-rich clinopyroxene surrounded by large
crystals of orthopyroxene. Normally zoned clinopyroxene in the Agrilia lavas and
clinopyroxene of various textures (skeletal, needle- and dendritic-like) and sizes in the
Pournari dykes display anomalous enrichment in Al2O3 and FeO* with decreasing
MgO that require rapid, disequilibrium growth. Quantitative characteristics of the
micro-spinifex pyroxene textures (<10 μm in width and 50-100 μm in length) imply a
cooling rate for the marginal parts of the Pournari dykes of at least 25 ºK/hr and more
likely 45-55 ºK/hr (Faure et al., 2004) and rapid growth of clinopyroxene crystals
at a linear rate of about 10−6 m/s (Welsch et al., 2016). MELTS models of the
crystallization sequence of the Pournari dykes indicate that progressive low-pressure
(500 bar) fractional crystallization of the ultramafic dyke liquid occurred under
oxidized (QFM+2.0) and hydrous (at least 0.5 wt.% H2O) conditions. A hydrous
magmatic parent for the Othris ophiolite as a whole is further supported by preliminary
investigation of melt inclusions (5-20 μm in diameter) in fresh chromite from Agrilia
samples that reveals the presence of Na, K, S, Cl, and F in microcrystalline aggregates
of rhönite, clinopyroxene, amphibole, apatite, serpentine and chlorite. Ratios of
Platinum Group Elements and related metals are Pd/Ir=11.5-13.0, Cu/Pd=6000-7210,
Ti/Pd=22.78-31.97×103 for Agrilia lavas and Pd/Ir=4.5-14.0, Cu/Pd=3140-5550,
Ti/Pd=4.66-17.32×103 for Pournari dykes; all are very close to those reported for typical
komatiites (Barnes et al., 1988). Despite the absence of true komatiite lavas, a number of
geochemical features of the Othris suite, including the PGE contents and ratios and the
micro-spinifex, disequilibrium cpx growth, are similar to Mesozoic and Archaean
komatiites.
References
Barnes et al., 1988. Journal of Petrology 29, 305–331.
Barth et al., 2008. Lithos, 100(1), 234-254.
Faure et al., 2006. Journal of Petrology 47, 1591– 1610.
Paraskevopoulos, G., Economou, M., 1986. Ofioliti 11 (3), 293 - 304.
Roeder et al., 2001. The Canadian Mineralogist, 39(2), 397-416.
Welsch et al., 2016. Contributions to Mineralogy and Petrology 171(1), 1-19. |
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