Late Ordovician high-Mg adakitic andesite in the western South China block: evidence of oceanic subduction

The Early Palaeozoic was an important period in the geologic evolution of the South China block (SCB), marking the intracontinental orogen in the Wuyi-Yunkai region. One salient feature of the western SCB is the absence of Early Palaeozoic oceanic subduction-related magmatism. Here, we report the first known occurrence of Late Ordovician andesites from the Shimian area, western SCB. Zircon SHRIMP U-Pb dating reveals that the andesites formed at ca. 451Ma. They have geochemical features of high-Mg adakitic andesite (HMAA) and are characterized by low K2O (1.09-2.24wt.%) and Th (2.50-5.65 ppm) and high MgO (4.02-6.91wt.%) and Mg# (56-71). Furthermore, their zircon grains display positive epsilon(Hf)(t) (+11.4 to +19.6) and low O-18 (4.72-6.20 parts per thousand) values. The andesites are interpreted to have been derived from partial melting of a peridotitic mantle wedge in an oceanic subduction setting and subsequent fractional crystallization. Integrating previous studies of the Qinling-Dabie orogenic belt with the data presented in this contribution, we suggest that the SCB was probably involved in the Early Palaeozoic Andean-type orogeny along the Gondwanan proto-Tethyan margin. [GRAPHICS] .