A comment on 'Tectonic evolution of the Hengshan-Wutai-Fuping complexes and its implication for the Trans-North China Orogen'

摘要

In a recent issue of Precambrian Research (vol. 170, pp. 73-87), Wang (2009) published a paper entitled 'Tectonic evolution of the Hengshan-Wutai-Fuping complexes and its implication for the Trans-North China Orogen, in which he proposes that (1) the formation of arc-related igneous rocks in the Hengshan and Wutai Complexes formed by northwest-directed subduction, not by(south)east-directed subduction; and (2) the Wutai granitoids were the products of an intra-oceanic island arc, not formed in a continent-derived arc. However, we consider both the structural and geochemical approaches Wang (2009) adopted to determine the polarity of the subduction and the tectonic setting of the Wutai granitoids are inappropriate or invalid. Firstly, all his structural observations used to determine the polarity of subduction are restricted to the Zhujiafang and Longquanguan ductitle shear zones, both of which developed at the late stage. It is unreliable to use kinematic indicators from these later structures to infer the polarity of the pre-collisional subduction. Secondly, Wang (2009) assumed that the initial positive epsilon(Nd) values (+0.5 to +4.5) of the Wutai granitoids indicate their generation directly from a mantle source, forming in an intra-oceanic arc. Such an interpretation is incorrect because the positive epsilon(Nd) values of the Wutai granitoids merely indicate that the igneous protoliths of these granitoids originated from a depleted mantle source, not the Wutai granitoids themselves. As the products of the earliest arc magmatism in the Wutai arc, the Wutai granitoids could not be generated directly from a mantle source but must have been derived by partial melting of juvenile crust.