Tectonic history of the Kolyvan-Tomsk folded zone (KTFZ), Russia: Insight from zircon U/Pb geochronology and Nd isotopes

Abstract

The Kolyvan-Tomsk folded zone (KTFZ) represents part of the Central Asian Orogenic Belt (CAOB). The KTFZ is mainly composed of detrital Late Palaeozoic sedimentary deposits, with minor intrusions. Detrital zircon geochronology on the Upper Devonian to Lower Permian sedimentary sequences of the KTFZ and the associated Gorlovo foreland basin yields four age peaks, reflecting the magmatic events in the source terranes. These events consist of (a) a minor Neoproterozoic peak (0.9-0.7 Ga), (b) a significant Early Palaeozoic peak (550-460 Ma), with a maximum at 500 Ma, and two well-defined Late Palaeozoic peaks during (c) the Middle-Late Devonian (385-360 Ma) and (d) the Carboniferous-Early Permian (360-280 Ma), with a maximum at 320 Ma. Older zircons (>1 Ga) are quite rare in the sampled sedimentary sequences. Slightly negative epsilon Nd values and associated relatively young Nd model ages were obtained (epsilon Nd(T) = -0.78, T (DM) ~1.1 Ga for Upper Devonian sandstones, epsilon Nd(T) = -1.1, T (DM) ~1.1 Ga for Lower Permian sandstones), suggesting only minor contribution of ancient continental crust to the main sedimentary units of the KTFZ. All intrusive and volcaniclastic rocks on the contrary are characterized by high positive epsilon Nd(T) values in the range of 3.78-6.86 and a Late Precambrian model age (T (DM) = 581-916 Ma), which corroborates its juvenile nature and an important depleted mantle component in their source. The oldest unit of the KTFZ, the Bugotak volcanic complex formed at the Givetian-Early Frasnian transition, at about 380 Ma. Upper Devonian detrital deposits of the KTFZ were formed in the Early Palaeozoic accretion belt of the Siberian continent and specifically in a passive continental margin environment. Deposits of the Gorlovo foreland basin, adjoining the KTFZ, were accumulated as a result of erosion of the Carboniferous-Early Permian volcanic rocks, which are now buried under the Meso-Cenozoic sedimentary cover of the West Siberian Basin. The magmatic events, recorded in the KTFZ zircon data, correspond to the most significant magmatic stages that affected the western part of the CAOB as a whole.