Ordovician supra-subduction, oceanic and within-plate ocean island complexes in the Tekturmas ophiolite zone (Central Kazakhstan): age, geochemistry and tectonic implications

The Junggar-Balkhash region in the west of the Central Asian orogenic belt comprises Palaeozoic flysch, dismembered ophiolites, chert, basalt, and olistostrome within its accretionary wedges. The Tekturmas ophiolite zone in the north of the Junggar-Balkhash region hosts Ordovician to Early Silurian complexes, mostly formed with a convergent margin in front of the Baydaulet-Akbastau magmatic arc. The northern part of the Tekturmas zone comprises Middle to Late Ordovician (465-455 Ma) Bazarbai supra-subduction (SSZ) ophiolites, which were formed in the fore-arc settings. At the south of the Tekturmas zone Early Ordovician (similar to 470 Ma) Tortaul SSZ ophiolites, Middle Ordovician Karamurun oceanic island basalts (OIB) and Middle to Late Ordovician Tekturmas deep water cherts are present. There are also concurrent Middle Ordovician (similar to 460 Ma) high-temperature granulites, related to a partial melting of the subducted metasedimentary complexes. Since the second half of the Late Ordovician until the Early Silurian, the stratigraphic data constrain in front of the Baydaulet-Akbastau island arc the emergence of the Nura fore-arc trough. In the north of the Tekturmas zone, it is represented by the chert-tuffaceous Bazarbai Formation; southward, the zone can be interpreted as a concurrent accretionary wedge made up by the fragments of the Bazarbai and Tortaul ophiolites, Karamurun OIB basalts, and Tekturmas deep water cherts, accompanied by chaotic olistostrome. In the Early Silurian, growth of the accretionary wedge migrated to the south into the Uspensky zone. We propose that while subduction was steadily directed underneath the Baydaulet-Akbastau island arc, the near-surface thrusting OIB and deep water cherts was also directed northward onto the fore-arc in the Late Ordovician to the earliest Silurian, whereas from the middle Llandovery it reversed southwards. In the end of the Palaeozoic to the Mesozoic, the Tekturmas ophiolite zone was offset into a number of segments by the sinistral strike-slip faults.