IS THE EARLY PALEOZOIC OROGENY IN SOUTH CHINA AN INTRAPLATE OR A COLLISIONAL ONE?

The ca. 460-420 Ma (late Ordovician-Silurian) early Paleozoic Wuyi-Yunkai/Kwangsian orogeny in South China, mostly affecting the West Cathaysia terrane, is characterized by HT/HP (up to >1.0 GPa) metamorphism and extensive magmatism. The magmatism is characterized by S-type granites that formed due to partial melting of the crust. Lack of coeval arcs and ophiolite in South China led to a suggestion that the orogeny was an intraplate one, post-dating and unrelated to Gondwana assembly. However, intraplate orogeny is generally believed to be a far-field response to plate convergence (most likely collision) elsewhere, but no adequate candidate for such a collision has been recognized. In addition, intraplate orogeny is difficult to explain the high-pressure metamorphism. We propose here that the orogeny is a result of promontory/microcontinent collision.

We suggest that West Cathaysia in South China was a microcontinent or (part of) a promontory on the Yangtze-West Cathaysia continent. The microcontinent or promontory, situated on the lower plate, collided with another continent (most likely Australia) in late Cambrian-Ordovician, leading to loading of the lower plate and formation of a foreland basin on West Cathaysia. The collision and the resulting slower subduction rate turned off arc magmatism at the site of collision and potentially elsewhere as well. Subduction of the remaining oceanic lithosphere led to continued convergence between the two plates and subduction and progressive burial of the West Cathaysia continental crust at the site of collision. The buried West Cathaysia crust reached upper amphibolite-granulite facies metamorphic conditions with partial melting occurring in late Ordovician-Silurian, generating S-type granites. Since conductive heating of large slabs of cold crust buried by thrusting is a slow process and heating up to upper amphibolite and granulite conditions can take tens of millions of years, the model readily explains that peak metamorphism and partial melting took place tens of millions of years after onset of collision. In this model, the late Ordovician-Silurian (Wuyi-Yunkai/Kwangsian) orogeny was a continuation of the Cambrian-Ordovician (Kunngan/Yu’nan?) collisional orogeny that took place at the late stage of Gondwana assembly.