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

作者：Shen, Ping^*; Pan, Hongdi; Seitmuratova, Eleonora; Yuan, Feng; Jakupova, Sholpan

来源：Lithos, 2015, 224: 61-77.

DOI：10.1016/j.lithos.2015.02.025

• Zircon U-Pb age
• Sr-Nd-Hf isotopes
• Nb-enriched basalt
• Adakitic rocks
• Bozshakol
• Kazakhstan

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 Sr-87/Sr-86 ratios (0.7026-0.7048), high zircon epsilon Hf(t) and whole-rock epsilon Nd(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 Sr-87/Sr-86 ratios (0.7040) and high epsilon Nd(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 Sr-87/Sr-86 ratios (0.7036-0.7039), high zircon epsilon Hf(t) values (+10.7 to +17.2) and whole-rock epsilon Nd(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 9.3-112.1 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.

• 出版日期2015-5
• 单位长安大学; 合肥工业大学; 中国科学院地质与地球物理研究所兰州油气资源研究中心; 中国科学院地质与地球物理研究所