The emplacement mechanisms and growth styles of the Guposhan-Huashan batholith in western Nanling Range, South China

Abstract

The Guposhan-Huashan batholith was emplaced during transition from Tethysian tectonic regime to Pacific tectonic regime in the Middle-Late Jurassic. The regional EW-striking Yishan-Quannan basement deep fault zone, associated with the Tethysian regime, is thought likely to control the formation of the batholith. The development of the batholith was, however, achieved when the Pacific tectonic extensional regime dominated. The batholith consists of seven individual intrusive units that were emplaced subsequently over a 15 m.y. span. Deformation structures and strain analysis of the intrusive units and their country rocks suggest the existence of active and passive emplacement mechanisms. For example, shape-preferred orientation of dioritic enclaves and megacrystic K-spars within earlier units and the development of rim synforms resulted from active mechanisms. It is estimated that active expansion and structural propagation each contributed approximately 27%, and stoping accounted for 46% of the space for the entire batholith. The active emplacement mechanisms produced earlier units at deeper levels of the crust and followed by passive emplacement mechanism at shallower crustal levels when the tectonic regime changed from compression to extension. Each intrusive unit shows a different growth mode. Niumiao and Yangmeishan intrusions show a polarized lateral growth mode, the main-phase Lisong and Wanggao intrusions show either a central growth mode or an eccentric growth mode, and the youngest Baishuidai and Huamei units have a polarized multi-point central growth mode. Both Guposhan and Huashan plutons show a concentric growth zoning pattern with the former built in an outward growth style and the latter an inward growth style. The well-preserved circular shape of both plutons is indicative of relatively stationary magma feeders, suggesting that any considerable horizontal displacement in the region was unlikely even if the area was in an intracontinental strike-slip tectonic setting during the last emplacement phases of the batholith.