Genetic Mechanism Of The Gneissic Structure In Motianling And Yuanbaoshan Granitic Pluton In North Guangxi

Based on the achievements of the geological investigation, this paper studied the geometry and kinematics of the gneissic structure in the Motianling and Yuanbaoshan Granitic pluton and revealed the formation mechanism of the gneissic structure by using the analysis approaches of modern structure geology. The main conclusions are as follows:1. Three kinds of gneissic structure are first identified in Motianling and Yuanbaoshan Granitic pluton: Primary peripheral position gneissic schistosity, Syn-tectonic gneissic schistosity and Epigenetic mylonite foliation.2. The occurrence of primary peripheral position gneissic schistosity is the same with plane of contact and varies according to the interface. The gneissic schistosity which is close to the interface is more developed while the further away from interface, the weaker gneissic schistosity is. The gneissic schistosity will be gradually disappeared beyond certain distance. Its genesis might be related to the stress of magmatic emplacement.3. Syn-tectonic gneissic schistosity is widely distributed in study area. This type of gneissic granite is composed of symmetrical feldspar phenocryst augens, symmetrical quartz phenocryst augens, quartz presented banding or filamentous, and biotite's directional alignment. The gneissic schistosity tends towards NNE and its dip angle is quite steep. This type of gneissic granite is coordinated with near W-E compression during Xuefeng orogeny. Its genesis might be related to the compression of syn-tectonic orogeny.4. The mylonite foliation is distributed in ductile shear zones, which dip to NWW-SWW as well as stretching lineations does. Judging from such shear sense indicator as unsymmetrical feldspar and quartz phenocryst augens and S-C fabric, ductile shear kinematics is majorly a normal ductile shearing. They might be related to post-orogenic extension of Caledonian orogeny.5. The long axis of the anamorphic tourmalite enclaves in Motianling and Yuanbaoshan Granitic pluton are oriented along with gneissic schistosity, which shows that the anamorphic of the tourmalite enclaves is related to the gneissic structure. The strain analysis result of tourmalite enclaves in Motianling and Yuanbaoshan Granitic pluton shows that there are three strain patterns of tourmalite enclaves: plane strain(K≈1), flatterning strain(0