A Cambrian intra-oceanic subduction system in the Bozshakol area, Kazakhstan

• Volcanics erupted at 501.8 ± 3.2 Ma age and intrusive rocks emplaced at 489.5 ± 3.3 Ma.
• Arc volcanics and NEBs were formed by partial melting of mantle wedge and subducted slab.
• Arc granitoids and adakites were derived from the mantle wedge and slab melts.
• A Cambrian intra-oceanic subduction system with ridge signature occurred in the Bozshakol area.

The Bozshakol area is one of the most important copper resource concentrations in Central Kazakhstan. We report in situ zircon U–Pb age and Hf isotope data, whole rock geochemical and Sr–Nd isotopic data for the volcanics and intrusions from the Bozshakol area.Secondary ion mass spectrometry (SIMS) zircon U–Pb dating indicates that the volcanics erupted at 501.8 ± 3.2 Ma and the intrusions emplaced at 489.5 ± 3.3 Ma. The volcanics are subdivided into two types. Type I are tholeiitic to calc-alkaline basalt and calc-alkaline andesite and dacite, which are enriched in light rare earth elements (LREE) with a marked negative Nb anomaly and Th/Yb-enrichment. They also have low initial 87Sr/86Sr ratios (0.7026–0.7048), high zircon εHf(t) and whole-rock εNd(t) values (+ 9.7 to + 17.0 and + 5.4 to + 6.7, respectively). Type II are Nb-enriched basalts (NEBs, Nb = 6–7 ppm), which are sodium-rich (Na2O/K2O = 3–10) and differ from the vast majority of arc basalts in their higher Nb, Zr, and TiO2 contents and Nb/U ratio. NEBs also have low whole-rock initial 87Sr/86Sr ratios (0.7040) and high εNd(t) values (+ 5.6). Therefore Bozshakol volcanics were formed by partial melting of the mantle wedge and subducted slab.The Bozshakol ore-bearing intrusive rocks include the fine- and medium-grained tonalite porphyry. They belong to the medium-K calc-alkaline series and are strongly enriched in LREE with a marked negative Nb anomaly and Th/Yb-enrichment. The fine-grained tonalite porphyries exhibit element characteristics similar to normal arc granitoids. They have low initial 87Sr/86Sr ratios (0.7036–0.7039), high zircon εHf(t) values (+ 10.7 to + 17.2) and whole-rock εNd(t) values (+ 4.9 to + 5.7). Compared with the fine-grained tonalite porphyries, the medium-grained tonalite porphyries have high Al2O3 and Sr contents (16–17 wt.% and 565–569 ppm, respectively) and low Yb and Y concentrations (0.9–1.1 ppm and 9.3–12.1 ppm, respectively), showing a geochemical affinity to adakites. Therefore, Bozshakol intrusive rocks were also derived from the mantle wedge and minor slab melts. We propose a model of intra-oceanic subduction for the Middle to Late Cambrian magmatic evolution of magmatic arcs in northwestern central Kazakhstan.