Cambrian-Ordovician mid-ocean ridge magmatism in the Kyrgyz Middle Tianshan and origin of the Karaterek ophiolite

• Published in: Lithos 2022-02, Vol.410-411, p.106576, Article 106576
• Main Authors: Hegner, E., Alexeiev, D.V., Messling, N., Tolmacheva, T.Yu, Willbold, M.
• Format: Article
• Language: English
• Subjects: Central Asian Orogenic Belt; Conodont; MORB; Nd isotopes; Ophiolite; Trace elements
• ISSN: 0024-4937; 1872-6143
• DOI: 10.1016/j.lithos.2021.106576

The Karaterek ophiolite represents a remnant of a previously unknown early Paleozoic ocean basin in the Kyrgyz Middle Tianshan (MTS) and sheds new light on the accretionary history of the western Central Asian Orogenic Belt (CAOB). Late Cambrian (~495–485 Ma, this study) and Middle Ordovician (~470–460 Ma) conodont ages support more than 30 Ma of seafloor spreading. The ophiolite comprises predominantly LREE-depleted mid-ocean ridge basalts (N-MORB) with chemically evolved compositions (Mg# 57–45). La-Th-Nb systematics and Ti/Zr and Ti/V ratios corroborate MORB sources. Al2O3 and FeO variations support plagioclase-controlled and tholeiitic magma fractionation processes, respectively. Initial εNd (i) values of +6 to +8 overlap the range of the contemporaneous global sub-ridge mantle. A section of OIB and N-MORB pillows is interpreted as near-ridge seamount tapping small-scale little (εNd (i) + 1) and highly depleted (εNd (i) + 8) mantle, respectively. Coinciding MORB and arc-features in subordinate basaltic dike samples suggest melting of hydrous residual peridotite (εNd (i) ~ 9) during subduction initiation. We propose a geodynamic model of rifting of the MTS prior to the late Cambrian (~490 Ma), open-ocean seafloor spreading during the late Cambrian to Middle Ordovician, and basin closure and plate collision in the Late Ordovician at ~450 Ma. [Display omitted] •Karaterek ophiolite reveals an early Paleozoic ocean in the western CAOB.•Conodont ages support seafloor spreading over more than 30 Ma.•Geochemical and radiogenic isotope data indicate normal mid-ocean ridge magmatism.•Subordinate samples suggest subduction initiation and seamounts.