MACROCRYSTAL POPULATIONS RETRIVED FROM LAMPROITES INDICATE ACCRETION OF YOUNG SSZ OCEANIC LITHOSPHERE IN THE ASSEMBLY OF SW. ANATOLIA, TURKEY

Western Anatolian lithosphere comprises several sutures and major continental fragments with Laurasian and Gondwanan affinities, assembled prior to the Oligocene. One of the major tectonic features – shaping its structure – is the Menderes metamorphic massif, which consists of several assembled and imbricated terrains exhumed in the Late Miocene during the extension that affected the entire Aegean province. The predominant opinion is that the Menderes is a core complex delineated by low-angle detachment faults. An alternative explanation associates the Menderes massif with shallow subduction of Neotethyan oceanic lithosphere that was later delaminated. To distinguish between these possibilities, direct evidence from the Western Anatolian mantle lithosphere would be ideal, because of the compositional differences between continental and oceanic mantle lithosphere. In the absence of mantle xenoliths, mantle-derived volcanics and their macrocrystal populations, offer the only information about the composition of the mantle-lithosphere beneath the Menderes.

In this contribution, we constrain the mineralogy of the lithospheric mantle based on a dataset for Western Anatolian lamproitic lavas and a systematical investigation of the major and trace element distribution in their macrocryst populations. The host lamproites are ultrapotassic, high-Mg mantle-derived rocks, which show geochemical resemblance with the rest of the Mediterranean lamproites. Two types of olivine macrocrysts are recognized: Phenocrystic olivine has high-Fo cores (Mg# up to 94), very high NiO contents (up to 0.8 wt.%), low Cr₂O₃ (<0.18 wt.%), and CaO (≤0.20 wt.%). They have high Li contents (up to 17 ppm) and relatively high Sc (up to 5 ppm) and Mn (up to 900 ppm) contents. This olivine hosts Mg-chromite inclusions with extremely high Cr# (as high as 0.84). Large (>1 mm) mantle xenocrystic olivine contains homogeneous cores with plateau-like compositions (Mg# around 0.92, but NiO and CaO contents <0.4 and <0.1% wt.%, respectively), which abruptly change into compositions similar to phenocrystic olivine, resulting in reversed zoning. Their Li contents, exceeds 2 ppm in all investigated grain profiles, indicating strong metasomatic imprint. Macrocrystal phlogopite also comprises two main populations analogous to olivine: Phlogopite phenocrysts typically have uniform core compositions with Mg# (atomic Mg/Mg+Fe) ≥ 90, Al₂O₃ 10.0-15.0 wt.%, and Cr₂O₃ less then 1.00 wt.% plotting in the field of lamproitic phlogopite core compositions. It has elevated Ba and Li contents ranging up to 15000 ppm and 70 ppm, respectively, and low Sr (<75 ppm) contents. Mantle xenocrystic phlogopite show core compositions characterized by high Mg#, Cr₂O₃ (up to 2.5 %), relatively high Al₂O₃ and F, and low TiO₂. Their high Cr₂O₃ and MgO contents closely overlap with mantle phlogopites from classical localities like Bear Paw and Finero. Their trace element composition is characterized by considerably lower Ba and Li contents around 1500 and 8 ppm, respectively, and sometimes elevated Sr and Nb concentrations up to 600 and 37 ppm, respectively.

The above complex macrocryst assemblages is used to constrain the lithospheric mantle beneath the Menderes Massif. Phlogopite xenocrysts document metasomatism of the lamproitic mantle source by hydrous K-enriched melts. This extreme K₂O and trace element enrichment is in strong contrast with the ultra-refractory character of olivine-spinel pairs, which reflect the pre-metasomatic ultra-depleted harzburgitic lithospheric mantle. High-Fo (up to 92% Fo) xenocrystic olivine in some samples indicates a depleted mantle source as well. This is a feature which can only be explained by a mantle that experienced an episode of strong depletion in melt components with subsequent K-enrichment. Our data support the interpretation that the mantle under the Menderes massif is a phlogopite bearing ultradepleted harzburgite, requiring a complex multistage geodynamic model to explain the origin of this lithospheric mantle. The first episode resulted in ultradepletion of the mantle in a SSZ environment during the closure of the Neotethyan Ocean in the Mesozoic. The second episode includes shallow subduction, accretion of forearc mantle and its interaction with subducted sediments, during which the observed complex composition of the lithospheric mantle was achieved. If our inferences about the W. Anatolian lithospheric mantle are correct, then the connection between accretion and/or shallowly subducted oceanic forearc lithosphere and uplift of the Menderes Massif may not be coincidental.

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Session No. 21--Booth# 21

Magmatism in evolving orogens: Collision to extension. Posters Part 2

Tuesday, 5 October 2010: 08:30-18:30

Exhibition Hall (METU Convention and Cultural Centre)

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