Petrogenesis and geochemical characteristics of Early Carboniferous sanukitic high-Mg andesite from Atengtao Mountain, Yili Block: Implications for the tectonic setting during Late Palaeozoic in Chinese West Tianshan

摘要

As an important part of the Central Asian Orogenic Belt (CAOB), Chinese western Tianshan contains voluminous Late Palaeozoic volcanic rocks. Here, we present geochronological, geochemical, and in situ Lu-Hf isotopic data from the Atengtao sanukitic high-Mg andesites (HMAs) of the western Yili Block. LA-ICP-MS zircon U-Pb analyses indicate that the Atengtao high-Mg andesites were generated in the Early Carboniferous (346 +/- 2 Ma). All the studied rock samples are characterized by high MgO (3.30-5.45 wt.%), SiO2 (53.25-60.10 wt.%), Cr (90.3-157 ppm), and Ni (31.0-46.7 ppm) contents, with geochemical characteristics analogous to those of sanukitoids of the Setouchi Volcanic Belt (SVB), SW Japan, and Bieluagaxi area, West Junggar. The Atengtao sanukitic HMAs are enriched in LILE and LREE but depleted in HFSE, similar to those of typical arc-related magmatic rocks. They are also characterized by high Nb (11.4-15.8 ppm) concentrations, high Th/La (0.3) and Th/Yb (2.6-4.0) ratios, and varying zircon epsilon Hf(t) values (-2.3 to +4.3). These features are suggested to result from the partial melting of a relatively enriched mantle wedge metasomatized by subducting oceanic slab-derived sediments. Considering the occurrence of sanukitic HMAs, combined with the contemporaneous special rock assemblages (such as slab-derived adakite, Nb-enrich basalt, A-type granite/rhyolite, and bimodal volcanic rocks) found in the western part of Yili Block, we propose that the generation of the Carboniferous sanukitic HMAs in Atengtao Mountain were most likely related to the slab break-off following the southern Tianshan Ocean slab northward subduction beneath the Yili-Central Tianshan Block. This mechanism not only produces intense magmatism with compositional diversity but also gives rise to a local extension in the western part of Yili Block. The slab break-off model may play an important role in the crustal growth of the CAOB in Phanerozoic time.