Kinematics And Dynamics Of Pre-Devonian Tectonic Evolution At South Margin Of Yangtze Block In North Guangxi

Pre-Devonian crust evolution in North Guangxi at the west part of south margin of Yangtze block is less studied than in Zhejiang, Anhui, Jiangxi and Hunan at the east part. Especially, the geometry, kinematics, dynamics and chronology of Proterozoic-Early Paleozoic strata and igneous rocks are under-explored. On the basis of hard field work and abundant first-hand data, the deformation structures of Pre-Devonian strata, the geometry and kinematics of ductile shear zones in Proterozoic-Early Paleozoic granites and mafic-ultramafic rocks, mineralogy and microstructures of mylonites in the ductile shear zones, geochemical characteristics and tectonic environments of Pre-Devonian strata and igneous rocks, paleo-magnetic properties of Pre-Devonian strata, chronology and dynamics of Pre-Devonian crust evolution are extensively studied by using techniques such as modern structure analysis, microstructures, petrology, petrological geochemistry, structural geochemistry, paleo-magnetics, tectonic chronology, tectonics, Geographic information system(GIS), Global positioning system(GPS) and Remote sensing(RS). Achievements are made as follows:1. "Folding layers" are first widely discovered in Neoproterozoic Danzhou Group at Mahai of Longsheng, Sirong and Antai of Rongshui, Baotan of Luocheng. The "folding layers" are bedding solid rheological structures, which are composed of bedding recumbent folds, bedding cleavages, viscous boudins, lens-like structures, syn-tectonic secretion veins and bedding ductile shear zones. The bedding solid rheological structure, which indicates bedding shearing under extensional circumstance at middle-deep crust, is likely to be a special structure formed by Xuefeng orogeny in North Guangxi. The Xuefeng orogeny, a crust thinning process under extensional environment, may not be a crust thickening process under compressional environment as previously suggested.2. Large scale extensional ductile shear zones are first discovered and identified in Middle-proterozoic Bendong granodiorites, Neoproterozoic Motianling and Yuanbaoshan granites and Caledonian Yuechengling granites. Major ductile shear zones host mylonitic gneisses, which develop foliations striking to NNE and dipping to NWW at dip angle from 40° to 80° , stretching lineations plunging to SWW or NWW. Shear senseindicators, including S-C fabric, unsymmetrical feldspar and quartz phenocryst augens, indicate a normal ductile shearing. The 40Ar/39Ar ages of new micas in mylonites of Bendong, Motianling and Yuanbaoshan ductile shear zones are 404.3+ 6.2Ma, 425.67±0.91V^ 324.82±0.58Ma, respectively, which is from late Caledonian to post- Caledonian.3. Ductile shear zones and mafic-ultramafic mylonites are first discovered in Proterozoic altered mafic-ultramafic igneous rocks in North Guangxi. The major ductile shear zones, striking to NNE, are parallel to Caledonian structural trends. In Jiuwandashan area, mafic-ultramafic mylonites, with foliations gentle dipping at less than 40° and stretching lineations plunging at less than 35° , indicate a normal shearing. The 40Ar/39Ar age of actinolite in ultramafic mylonite is 339±36.4Ma, which demonstrates that it might be related to post-Caledonian extension. Mafic-ultramafic mylonites, with foliations steeply dipping at 50-80° , show a thrust shearing in Longsheng. They might be related to near W-E compression during Caledonian orogeny.4. Geochemical studies show that mafic-ultramafic igneous rocks in Jiuwandashan terrene are volcanic arc calc-alkaline basalts, which are generated by magmatism along convergent plate margin, not mantle plume. Mafic-ultramafic igneous rocks in Longsheng terrene are characterized by magmatic arc, MORB and in-plate basalt, which reflects a complicated tectonic environment. Two types of granites, biotite granites and granodiorite granites, are formed in different tectonic environment in Jiuwandashan terrene. Biotite granites are from syn-collision to post-orogenic granites, while granodiorites are mainly volcanic arc granites. Sandstone of Sibao group reflects an active continental margin and sandstone of Danzhou group reflects a transforming continental margin from active to passive in Jiuwandashan terrene, while sandstone of Danzhou group reflects an active continental margin in Longsheng terrene.5. Paleomagnetic analyses show that paleomagnetic pole of Sibao group in Yangmeiao area was located at Vlat=49.4, Vlong= 127.0, Plat=13.6. Paleomagnetic pole of Sibao group in Xingdongkou area was located at Vlat=48.1, Vlong=109.6, Plat=16.6. Sibao fracture was unlikely to be a collision zone between two plates or blocks because two blocks at each side of Sibao fracture were not significantly split and displaced. Paleomagnetic pole of Danzhou group in Sirong area was located atVlat=57.1, Vlong=354.4, Plat=9.0. Paleomagnetic pole of Danzhou group in Piaoli area was located at Vlat=18.2, Vlong=28.8, Plat=15.6. Sanjiang fracture was likely to be a collision zone between Jiuwandashan and Longsheng terrene because the two blocks were located in separate terrenes. Paleomagnetic pole of Cambrian system in Shoucheng area was located at Vlat=-70.9, Vlong=114.0, Plat=-6.1. Paleomagnetic pole of Cambrian system in Laochang area was located at Vlat—64.8, Vlong=168.2, Plat=-10.1. Paleomagnetic pole of Cambrian system in Jinxiu area was located at Vlat=-42.2, Vlong=157.0, Plat=10.8. Lipu fracture was likely to be a collision zone between Yangtze and Cathaysia plate because Jinxiu block at the south side of Lipu fracture was displaced 2000km to north and rotated 11° anti-clockwise from Laochang block at the north side of Lipu fracture.6. Axial planes of Caledonian folds are generally dipping to NWW at high or vertical dip angle and hinges trending NNE at low or horizontal plunging angle in North Guangxi, which shows that the Caledonian folds were formed under NWW-SEE compressional environment. From late Caledonian to post-Caledonian extensional stage, thickened crust was the most important dynamic constraints on extensional tectonics. Large-scale extensional ductile shear zones were controlled by gravitational collapse mechanism.