| Summary: | 碩士 === 國立臺灣大學 === 地質科學研究所 === 105 === Ophiolites from SE Turkey have been recognized as SSZ-type and interpreted as remnants of the Neotethyan oceanic lithosphere. In this study we provide constraints from geochemistry of peridotites in Kizildağ and Guleman ophiolites. The Guleman harzburgites are fresh with L.O.I. from 0.3~2.7% and show porhpyroblastic textures with granulation and kinking structures. Kizildağ harzburgites obtain equigranular textures with a larger degree of serpentinization (L.O.I.~10%). From depletion trends observed from mineral chemistry, the Kizildağ harzburgites are as depleted as the most depleted abyssal peridotites, while the Guleman harzburgites are even more depleted, in a degree similar to those fore-arc peridotites from the Izu-Bonin-Mariana (IBM) arc system. Trace element modeling not only suggests an approximately 20% melting of a refractory source that has already undergone around 16% melting of DMM, but also show enrichments in LREEs and LILEs. This 2-stage melting scenario is consistent with the modern petrogenetic model for boninites, hence we interpret these harzburgites as near pure melting residues after 2 stages of partial melting, with the latter possibly related to the generation of boninites in a fore-arc system. No significant metasomatism is documented in mineral chemistry, and only cryptic metasomatism was observed from trace element and PGE geochemistry. One of the Kizildağ harzbugite (KZD1301) are much more enriched in terms of trace elements than other samples, and combined with lines of evidence from petrography, spinel chemistry and trace element geochemistry, KZD1301 is intepreted in this study as residue that underwent a similar degree of melting with other KZD harzburgites but experienced more intense melt-rock interaction.
Their Re-Os isotope ratios range from 0.1168 to 0.1271 for 187Os/188Os and from 0.0581 to 0.4819 for 187Re/188Os, the latter indicating recent Re enrichment. GI harzburgites obtain moderately subchondritic 187Os/188Os (~0.126) with flat PGE patterns slightly more depleted than the primitive mantle, while the 187Os/188Os of GII are more unradiogenic (~0.1234) and are subtly more enriched than primitive mantle in terms of PGE patterns. KZD harzburgite on the other hand, the least unradiogenic 187Os/188Os ratio (0.1168) from one Kizildağ peridotite with low 187Re/188Os of 0.0816, and refractory PGE pattern (Pd/IrN < 1) yields tRD age of 1.59 Ga. Compiling with available Os data of Guleman peridotites (Wang, K.-L., unpublished data), these peridotites yield TRD model age clusters of ~300 Ma, 650-700, ~800 Ma and 1600 Ma. Correlated with local crust forming events, the TRD age is coherent with rifting of the Neotethys (~250Ma). The remaining ancient Os model ages yielded may indicate other magmatic events related to juvenile crust formation documented in remnant lithospheric fragments isolated by disruption of the ancient continental regions during rifting (O’Reilly et al., 2009), or may just be reflecting mantle heterogeneity in Os isotopes.
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