Siliceous–volcanic associations of the Northern Balkhash ophiolite Zone (Central Kazakhstan): Biostratigraphy, sedimentation and tectonic evolution in the Middle-Late Ordovician

Abstract The Northern Balkhash Zone is the largest and the most complex ophiolite zone of the Junggar-Balkhash region in the west of the Central Asian Orogenic Belt. The zone encompasses serpentinite melange, containing fragments of dunite-harzburgite, dunite-verlite-pyroxenite complexes, layered and isotropic gabbro, sheeted dikes and plagiogranites, along with stacks of tectonic slivers composed of siliceous, siliceous-basaltic, and siliceous-volcanic-tuffaceous units. Conodont finds suggest the middle-late Darriwilian age of the siliceous-basaltic Itmurundy Fm., while the siliceous-basalt Tiuretai, siliceous Kazyk, and the siliceous-tuffaceous Uzuntobek formations are close in age, extend from the top of Darriwilian to the base of Katian. The siliceous-clastic-tuffaceous sediments of the Obaly Fm. are the Sandbian and Katian in age and the volcanic-sedimentary Dzhamanshuruk Fm. is broadly Upper Ordovician. Dating of these units has also permitted recognition of the units with intricate thrust-and-fold structure. Species of the genera Periodon, Pygodus, and Scabbardella dominate in the conodont assemblages in cherts. An uneven distribution of the conodonts in sections is caused by variable siliceous sedimentation rate that depends on productivity of radiolarians. The petrochemistry of the volcanics in the Itmurundy Fm. suggests that they correspond to OIB and E-MORB, being formed within seamounts or oceanic plateaus, while the Tiuretai basalts are close to N-MORB, erupted within a spreading center. The felsic volcanics in the Dzhamanshuruk Fm. have demonstrated an island arc affinity. The proposed evolution model suggests that during the Middle-Late Ordovician tectonic settings changed successively from the spreading center to island arc within the Northern Balkhash Zone of the Junggar-Balkhash region.