The Shimian ophiolite in the western Yangtze Block, SW China: Zircon SHRIMP U-Pb ages, geochemical and Hf-O isotopic characteristics, and tectonic implications

The Shimian mafic-ultramafic rocks in the western Yangtze Block is a structurally dismembered complex which is intruded by the Neoproterozoic granite and faulted against the Neoproterozoic Suxiong Group. In this study, we divide the Shimian mafic-ultramafic rocks into two suites including the Late Mesoproterozoic Shimian ophiolite and Early Neoproterozoic gabbros, based on differences in the field occurrence, age, and geochemical characteristics. The Late Mesoproterozoic Shimian ophiolite mainly consist of serpentinites, gabbros, basalts, and rodingites. Zircon U-Pb dating using a sensitive high-resolution ion microprobe (SHRIMP) on a gabbro sample yielded an age of 1066 ± 11 Ma, representing the formation age of the Shimian ophiolite. All basalt and gabbro samples are tholeiitic and have normal mid-ocean ridge basalt (N-MORB)-like immobile element patterns with negative Nb and Ti anomalies. The low zircon δ18O (4.6-5.6 ‰) and high positive zircon εHf(t) (+10.8 to +18.3) values indicate that these rocks were derived from a depleted mantle source. These features are comparable with those of the basaltic rocks in SSZ-type ophiolite. The Early Neoproterozoic gabbros intruded into the Shimian ophiolite at ca. 937 Ma. The geochemical and isotopic characteristics of these gabbros are similar to those of the island arc basalts. Integrating previous studies with the data presented in this contribution, we suggest that the northwestern Yangtze Block has probably witnessed the Late Mesoproterozoic-Early Neoproterozoic diachronous subduction.