Early Ordovician to early Silurian forearc accretionary processes in front of the Selety–Stepnyak arc in the Kokchetav area, northern Kazakhstan: implications for continental growth in the northwestern Altaids

In the Kokchetav area, a late Neoproterozoic to early Palaeozoic metamorphic, ophiolitic, and volcanic–sedimentary complex provides important insights into crustal accretion in the northwestern Altaids. We carried out a systematic study of the Northern Complex in the mélange within the Chaglinka, Zhanatalap, and Chelkar regions. The mélange includes fragments of mafic igneous rocks, pelagic cherts, volcaniclastic rocks, and turbidites. The mafic rocks have high Mg#, Cr, and Ni values, small negative Nb–Ta anomalies, juvenile ε Nd (t)–ε Hf (t) values, and low 87 Sr/ 86 Sr (i) ratios, suggesting they were derived from a depleted mantle source in a supra-subduction zone setting. The volcaniclastic rocks exhibit moderate light rare earth element enrichment, small negative Nb–Ta anomalies, juvenile ε Nd (t)–ε Hf (t) values, and low 87 Sr/ 86 Sr (i) ratios, indicating they were derived from arc-related igneous rocks. The granites have features consistent with high-Sr/Y or A-type granites and variable ε Nd (t) and ε Hf (t) values, implying they formed by melting of arc-related volcanogenic sediments in the mélange. Zircon U–Pb ages for the volcaniclastic rocks vary from 540 to 423 Ma (Early Cambrian to late Silurian). These ages are consistent with those of volcanic rocks exposed in the Stepnyak–Selety arc. Subduction of the oceanic lithosphere beneath the Stepnyak–Selety arc may have occurred during the Early Cambrian and was followed by roll-back of the subducting slab during the Early Ordovician and continued subduction in the early Silurian. The successive subduction and accretion resulted in gradual maturation of the Stepnyak–Selety arc, which produced the compositionally and isotopically variable igneous rocks in the Kokchetav mélange zone and generated the juvenile crust of present-day northern Kazakhstan. Graphical abstract