Early Paleozoic juvenile crustal growth in the Paleo-Asian Ocean: A contribution from the Zasur'ya accretionary complex of NW Altai

Reconstruction of proportions between juvenile and recycled crust remains challengeable because a big part of juvenile source magmatic rocks formed at intra-oceanic arcs, can be destroyed by surface and tectonic erosion leaving, at best, greywacke sandstones. Such sandstones are typically hosted by accretionary complexes, which study, therefore, is of crucial importance. In this paper we review available geological and micropaleontological data and present first U-Pb detrital zircon ages, geochemical data and isotope (Sm-Nd, Lu-Hf) data from sandstones of the late Cambrian-early Ordovician Zasur'ya accretionary complex (AC) of NW Altai. No island-arc units have ever been reported there. The Zasur'ya AC includes ocean plate stratigraphy (OPS) magmatic and sedimentary rocks of three formations, Listvenny (l. Cambrian), Talitsa (Tremadocian) and Marcheta (Floian), which age was constrained by microfossils. The OPS assemblage includes basalt, pelagic chert and hemipelagic siliceous mudstone and siltstone, and sandstone. We analyzed zircons in five samples. The distributions of U-Pb ages of detrital zircons from five samples of sandstones are all unimodal with main peaks at ca. 488 Ma (Listvenny Fm.), 491 Ma (Talitsa Fm.) and 485 Ma (Marcheta Fm.). The Precambrian ages are very few. The maximum deposition ages inferred from the youngest populations of zircon ages are ca. 464 Ma. Compositionally, the sandstones are greywackes or feldspar litharenites. The Listvenny and Talitsa sandstones are higher silicic (SiO2av. = 68.5 wt%) compared to Marcheta samples (SiO2av. = 60.5 wt%). The major and trace element features of all sandstones are similar to supra-subduction intermediate-felsic (Listvenny, Talitsa) and mafic-intermediate (Marcheta) magmatic rocks. All samples yielded positive values of zircon epsilon Hf(t) (+4.3 to +20.1) and bulk-rock epsilon Nd(t) (+0.6 to +4.8) indicating juvenile magmatic rocks in the provenance. However, the Listvenny and Talitsa samples show lower epsilon Nd(t) (1.3 and 0.8, respectively) than those of the Marcheta Fm. (4.7). Probably the Zasur'ya sandstones were deposited in a back-arc basin (Listvenny, Talitsa), and fore-arc basin (Marcheta). All Zasur'ya greywacke sandstones were derived by destruction of a single late Cambrian -Early Ordovician intra-oceanic arc. The Zasur'ya intra-oceanic arc was identified in NW Altai for the first time and it represents a new site of juvenile crustal growth in the CAOB.