In situ Hf isotope analysis on

Fig. 9. Trace Indoximod discrimination diagrams of the Central Tianshan granitic gneisses. Nb–Y (a), Ta–Yb (b) and Rb–(Y + Nb) (c) diagrams are from Pearce et al. (1984) with Rb/30–Hf–Ta*3 diagram (d) from Harris et al. (1986). Trace element concentrations are quoted in ppm. Symbols are the same as those in Fig. 4. VAG — volcanic arc granites, COLG — collision granites, WPG — within plate granites, ORG — ocean ridge granites.Figure optionsDownload full-size imageDownload as PowerPoint slide
Characterized by euhedral to subhedral shapes, concentric oscillatory zoning and high Th/U ratios (0.30–2.05), magmatic-type zircons (ca. 72%) in mold study yield consistent ages of ca. 325–320 Ma, representing the best estimates for the crystallization ages of the granitic protoliths. Hence, our data suggest a continental arc system that developed along the northern margin of the Central Tianshan block during late Early to early Late Carboniferous time by the southward subduction of the Junggar oceanic plate (Fig. 10a). Therefore, the Junggar Ocean at least lasted to the early Late Carboniferous, consistent with previous findings such as ca. 325 Ma ophiolitic rocks and ca. 323 Ma arc-related volcanic rocks in the North Tianshan belt (Li et al., 2011 and Xu et al., 2006b).