Petrogenesis of Early-Permian sanukitoids from West Junggar, Northwest China: Implications for Late Paleozoic crustal growth in Central Asia

Abstract

Sanukitoids and their equivalents are rare subduction-related rock types that have been found in modern arc settings and in Late Archean sequences. The investigation of sanukitoids is of particular importance to a better understanding of crust-forming processes and continental growth. In this paper we report zircon U–Pb and Ar–Ar ages and major element, trace element, and Sr–Nd–Hf isotope data for the Bieluagaxi dioritic pluton and dikes from the southern part of the West Junggar, NW China. These rocks formed in the Early Permian and show a remarkable geochemical affinity with Cenozoic sanukitoids of the Setouchi Volcanic Belt of SW Japan with high Mg# (48–73) values, Cr (54–539ppm), Ni (21–197ppm) contents and Th/La (0.15–0.37) ratios and low Sr/Y ratios (16–27) and Sr (263–442ppm) contents. They may be generated by the partial melting of subducting sediments, and subsequent melt–mantle interaction. Additionally, the sanukitoids are also widespread in the Karamay–Baogutu area of West Junggar accompanied by high Sr (average 713ppm) contents and Sr/Y (50–130) ratios, and low Y (6.9–12.6ppm) contents. The difference in petrochemical characteristics between the Baogutu–Karamay and Bieluagaxi sanukitoids can be explained by the difference in depth of initial melting, origin composition and fractional crystallization. The Baogutu–Karamay sanukitoids were probably formed under eclogitic conditions, while the Bieluagaxi sanukitoids were at a shallower depth. Moreover, the compositional similarity between continental-crust forming rocks and the Bieluagaxi sanukitoids suggests that the sanukitoids genesis could be closely related to the process of continental crust formation. The Late Carboniferous–Early Permian sanukitoids in the West Junggar may be an indicator of anomalous thermal activity. Ridge subduction may play a crucial role in the evolution and growth of the continental crust in Central Asia.