Petrogenesis of Silurian ultramafic-mafic plutons in southern Jiangxi: implications for the Wuyi-Yunkai orogen, South China

The South China Block is one of the largest continental blocks located on the East Asian continent. The early Palaeozoic Wuyi-Yunkai orogen of the South China Block (known as the Caledonian orogen in Europe) is a major orogenic belt in East Asia and represents the first episode of extensive crustal reworking since Neoproterozoic time. Although this orogen is key to deciphering the formation and evolution of the South China Block, details about the orogen remain poorly defined. The Songshutang and Wushitou ultramafic-mafic units in southern Jiangxi Province, South China, have Pb-206-U-238 ages of c. 437 Ma, suggesting a Silurian formation age. All the Songshutang and Wushitou ultramafic-mafic rocks show relatively flat chondrite-normalized rare earth element patterns, depletions in Nb, Ta, Zr, Hf and Ti, and low epsilon(Nd)(t) values from -9.12 to -5.49 with negative zircon epsilon(Hf)(t) values from -10.84 to -2.58, resembling a typical arc magma affinity. Geochemical and isotopic data indicate that the newly identified ultramafic-mafic rocks, along with the reported Silurian mafic rocks in South China, possibly originated from the similar partial melting of an ancient subducted slab, fluid/sediment and metasomatized lithospheric mantle with varying degrees of fractional crystallization. In conjunction with other records of magmatism and metamorphism in South China, a late-orogenic extensional event led to the melting of the sub-continental lithospheric mantle in Silurian time and generated ultramafic-mafic rocks with a limited distribution along the Wuyi-Yunkai orogen and widespread late-orogenic granitic plutons in the South China Block.