Growth Of An Accretionary Complex In The Southern Chinese Altai: Insights From The Palaeozoic Kekesentao Ophiolitic Melange And Surrounding Turbidites

As a record of oceanic subduction, an accretionary complex plays a key role in the reconstruction of plate tectonic history. The Kekesentao ophiolitic melange between the Chinese Altai orogen to the north and the Kazakhstan collage to the south played an important role in the subduction-accretion of the Chinese Altai in the Palaeozoic. This article provides new structural field relations, and petrographic, geochemical, and geochronologic data for the Kekesentao ophiolitic melange, and detrital zircon analyses of turbidites that are imbricated with the melange. The Kekesentao melange consists of ultramafic rocks, meta-gabbros, pillow basalts, cherts, and imbricated trench turbidites, tuffaceous sandstones and quartz schists (ocean plate stratigraphy). The melange shows a 'block in matrix' structure in the field, and the general imbricated dip of the thrust system is to the N-NE, indicating southward accretion. Pillow basalts and meta-gabbros have a typical N-MORB geochemical signature, and the U-Pb zircon age of a meta-gabbro, interpreted as the age of pre-subduction oceanic crust, is Early Devonian (399-393 Ma). The upper time limit of accretion of the melange is constrained by a Late Devonian (361 Ma) intrusive granitic pluton. Detrital zircon ages of surrounding turbidites range from 369 to 3,114 Ma, which is typical of magmatic 540-460 Ma zircons from the Chinese Altai, suggesting that the Chinese Altai arc was the source of zircons in the melange. The Kekesentao ophiolite was accreted to the accretionary complex before the Late Devonian. Our analysis of structures, ocean plate stratigraphy, and zircon isotopic provenance of the Kekesentao ophiolitic melange and turbidites provides a robust conclusion that southward growth of the accretionary complex in the Kekesentao area of the Chinese Altai was semi-continuous from the Early Palaeozoic to Devonian.