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 m & eacute;lange within the Chaglinka, Zhanatalap, and Chelkar regions. The m & eacute;lange 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 epsilon(Nd)(t)-epsilon(Hf)(t) values, and low Sr-87/Sr-86((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 epsilon(Nd)(t)-epsilon(Hf)(t) values, and low Sr-87/Sr-86((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 epsilon(Nd)(t) and epsilon(Hf)(t) values, implying they formed by melting of arc-related volcanogenic sediments in the m & eacute;lange. 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 m & eacute;lange zone and generated the juvenile crust of present-day northern Kazakhstan.