Emplacement Process And Structural Analysis Of Gupuoshan-Huashan Granitic Pluton, Guangxi

Attention has recently been paid to fruitful studies of Nanling granite belt that have provided important petrologic, geochemical, and other evidences for understanding the tectonic nature and evolution history and geodynamic setting of south China. The problem is, however, that these studies have only focused on the east and middle sections of the granite belt, and that few workers have made enough efforts on its west section, not even to say on the emplacement mechanisms and related structures of the plutons within the belt.This study thoroughly and systematically deals with the huge Gupuoshan-Huashan granite batholith located on the west section of Nanling granite belt and in Guangxi Province. The dissertation is based on intensive geologic mapping. Methods of finite strain measurement, modern structural analysis, petro-geochernistry, and magma dynamics have been employed. The structural style, deformational features, and emplacement process of Gupuoshan-Huashan batholith, and their relationships with tectonic environment are discussed, and the structural geometry, kinematics, and dynamics associated with granitic emplacement are analyzed. Conclusions are made as follows:1.Twenty-six individual intrusions have been identified within the Gupuoshan-Huanshan granitic batholith. They are classified into and named as Niumiao independent intrusion, Yangmeishan independent intrusion, and Guiling, Lisong, Wanggao, Xinlu, Baishuidai, and Huamei Units respectively. These granitic intrusions/units are grouped into three super-units: Niumiao Super-unit, Gupuoshan Super-unit, and Yinping Super-unit. Detailed study demonstrates that these three super-units are products of three periodic magmatic thermal events occurred during South China's Mesozoic tectonic-magmatic cycle, and that they were evolved sequentially from the same magma source.2. Study of magmatic dynamics of the Gupuoshan-Huashan pluton shows that the pluton crystallized within middle-deep crust with relatively high viscosity, slow ascend rate, and lower-than-its-country-rock density. The early sequences have lower viscosities and greater ascend rates, densities, temperature and pressure than late sequences.3. The major intrusive units were already subjected tocompressional force produced by ballooning when the magmas were at plastic and half-plastic states, and even at half-molten state. The ballooning also exerted a strong compression and a shear force to country rocks, suggesting an active emplacement mechanism. The unique "tail" shape and deformation features of the pluton, however, imply that a regional strike-slip wench faulting would also contribute to the opening of the space for the magma emplacement, therefore suggesting a passive emplacement mechanism as well.4. Strain analyses from 155 finite strain measurement stations within the pluton and its aureole also demonstrate that the strain is mainly characterized by a flattening pattern, and that the strain types and strain intensities are heterogeneously distributed from time to time. The early sequences such as Niumiao and Yangmeishan intrusions and Lisong and Wanggao Units show mostly flattening strain patterns, whereas the late sequences such as Xinlu Unit stretching strain patterns. Spatially, the flattening and planar strain patterns exist along the margins of each unit and independent intrusion, whereas the stretching patterns appear in their central area. From early sequences (units) to late sequences (units), the average strain intensity decreases. A strain gradient exists from the aureole to the contact due to an increasing strain intensity measured from the aureole to the contact. The strain heterogeneity and strain patterns imply that periodic pulsating emplacement could account for the deformation of the pluton and its aureole country rocks, suggesting an active emplacement mechanism for the pluton.5. The cooling and exhumation history of the pluton is re-constructed. It is found that the Mesozoic crust of the region has experienced three fast exhumation events...