Early Silurian tholeiitic-boninitic Mailisu ophiolite,South Tianshan,Kyrgyzstan: a geochemical record of subduction initiation |
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| Authors: | E Hegner DV Alexeiev M Willbold A Kröner G Topuz AV Mikolaichuk |
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| Institution: | 1. Department für Geo- und Umweltwissenschaften &2. GeoBioCenter, Universit?t München, Munich, Germanyhegner@lmu.de;4. Geological Institute (GIN), Russian Academy of Sciences, Moscow, Russia;5. Geowissenschaftliches Zentrum, Georg-August-Universit?t G?ttingen, G?ttingen, Germany;6. Beijing SHRIMP Centre, Institute of Geology, Chinese Academy of Geological Sciences, Beijing, China;7. Institute of Geosciences, University of Mainz, Mainz, Germany;8. Avrasya Yer Bilimleri Enstitüsü, Istanbul Teknik üniversitesi, Maslak, Istanbul, Turkey;9. Institute of Geology, National Academy of Sciences of Kyrgyzstan, Bishkek, Kyrgyzstan |
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| Abstract: | ABSTRACTOphiolite assemblages of the South Tianshan fold-and-thrust-belt (STS) track the sea floor dynamics of the late Cambrian to Carboniferous Turkestan Ocean in the western Central Asian Orogenic Belt. We interpret new geochronological and geochemical data for the Mailisu ophiolite from western Kyrgyzstan as evidence for an early Silurian incipient subduction zone outboard the South Chatkal Arc of the Middle Tianshan. Igneous zircons from a leucogabbro yielded a U-Pb age of 440 ± 6 Ma indicating one of the oldest sea floor fragments in the STS. Metabasalt samples have N- to E-MORB trace element patterns and initial εNd values of ~5 to 9 support melting of depleted asthenosphere comprising a minor OIB component. Cross-cutting diabase dykes show geochemical characteristics of high-Ca basaltic boninites. Their low REE abundances and highly fractionated middle-heavy REE patterns indicate sources that were more melt-depleted than those of the metabasalts. Concave upwards LREE patterns are compatible with a residual peridotite source, fertilised by small-degree OIB-like melts with εNd of ~5 to 6. Positive Ba, Sr, and Pb anomalies support the involvement of slab-derived hydrous fluids that probably facilitated the melting of the residual mantle. The association of MORB-like metabasalts with younger dykes of boninite affinity suggests subduction initiation for the origin of the Mailisu ophiolite. Our data elucidate one of the major yet little understood episodes in the Paleozoic history of the Turkestan Ocean. Future identification of similar rock assemblages will be helpful to understand the insufficiently constrained history of ocean floor subduction in the Paleozoic Oceans of the western CAOB. |
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| Keywords: | Ophiolite boninite geochemistry geochronology subduction initiation Central Asian Orogenic Belt South Tianshan Turkestan Ocean |
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