Petrogenesis of Early Carboniferous adakitic dikes, Sawur region, northern West Junggar, NW China: Implications for geodynamic evolution

Abstract

Adakitic dikes are widespread in the Sawur region, northern West Junggar, northwestern China. Zircon U–Pb analyses of the dikes have yielded consistent ages of ca. 334Ma. The dikes are characterized by relatively high SiO2 (55.5–61.8wt.%), Al2O3 (14.5–16.3wt.%), and Sr (468–1005ppm), and low Y (10.1–14.1ppm) and Yb (0.93–1.39ppm) contents, with high Sr/Y (34–74) ratios and slight Eu anomalies, which are analogous to those of slab-derived adakites. In addition, the dikes are relatively MgO-rich (1.75–3.57wt.%; Mg#=44–56), with high Th/Yb ratios and positive εNd (334Ma) (+6.2 to +6.5) and εHf (334Ma) (+11.3 to +15.8) values. The data suggest that the dikes were generated by partial melting of subducted oceanic crust and overlying sediments. However, these Early Carboniferous adakitic rocks and associated I-type granites in the northern part of West Junggar are petrochemically distinct from the Late Carboniferous adakitic plutons from the southern part of West Junggar. The latter are associated with charnockites, A-type granites, tholeiites, and Nb-enriched, alkaline basalts, as well as magnesian dikes, which were generated by ridge subduction during the Late Carboniferous. The distinct lithological association of the Early Carboniferous adakites is consistent with flat slab subduction, which was characterized by anomalous heating of the slab and consequential partial melting to generate adakitic magma. After generation of the Early Carboniferous adakites, the subducting slab cooled, reverted to a more normal subduction angle in the late Early Carboniferous, and was associated with more typical I-type granite arc magmatism.