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

Abstract

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.