Composition and Sources of Silurian Terrigenous Rocks at the Periphery of the Tekturmas Ophiolite Zone (Central Kazakhstan)

The paper presents new petrographic and geochemical data from terrigenous rocks and first U–Pb ages of detrital zircons from sandstones of the Tekturmas zone of Junggar-Balkhash suture zone (central Kazakhstan). Petrographically and chemically the samples under study are greywacke sandstones. We performed U–Pb isotope dating of detrital zircons separated from two sandstones sampled in the northern (TK-17) and southern (TK-27) parts at the periphery of the Tekturmas zone. The distribution of U–Pb ages of zircons from sample TK-27 (northern Tekturmas zone) is unimodal with a major peak at 453 Ma. The distribution of U–Pb ages of detrital zircons from TK-17 sample (southern Tekturmas zone) has a bimodal character peaked at 510 and 450 Ma. Both samples contain a small amount of Precambrian zircons. The most probable sources of ca. 450 Ma zircons are andesitic to volcanic rocks of Tekturmas supra-subduction ophiolites. The ca. 500 Ma zircons are also low-rounded like those younger, but no igneous rocks of that age have been identified in the study area. The youngest U–Pb ages of zircons from both samples are similar (438–437 Ma), suggesting the beginning of sedimentation after the Llandovery. The greywacke composition and the poorly sorted, poorly rounded and immature characteristics of the sandstones under study and the clear major peaks of U–Pb detrital zircon ages at an extremely low amount of old zircons suggest their derivation from short-distant sources dominated by Late Cambrian-Early Ordovician and Middle-Late Ordovician supra-subduction igneous complexes. A very limited amount of oldest zircons could come from Precambrian complexes present at the periphery of both sedimentation basins.