Study On The Tectonic Evolution And The C-H-O Isotopes Of The Fluids Of Dabashan Foreland Belt

Several tectonic movements, of which the most important tectonic movements are NE-SW compression and NW-SE compression,have taken place in Dabashan Thrust-Fold belt and its foreland since Mesozoic period.. Moreover, widespread fluid flows have taken place during these two tectonic periods and calcite-veins (Cc-veins) filled in the fractures.In the NE-SW compressional fractures,the carbon, hydrogen, oxygen isotopes of veins filled in the fractures ranged from-7.0‰to+4.1‰,-88‰to-62‰,12.2‰to 28.5‰respectively. The carbon, oxygen isotopes of wallrock ranged from -1.6‰to+5.6‰,16.7‰to 25.2‰respectively.The carbon isotope of veins filled in the compressional fractures have larger range than the carbon isotope of wallrocks which indicated that the fluids mixed with other water. The oxygen isotope of veins filled in the compressional fractures have same range to the wallrocks indicated that the fluids mainly came from wall-rocks, some mixed with meteoric water.In the NW-SE compressional fractures,the carbon, hydrogen, oxygen isotopes of veins filled in the fractures ranged from -2.1‰to +4.8‰,-85‰to-74‰,18.1‰to 19.8‰respectively. The carbon, oxygen isotope of wallrock ranged from -0.8‰to +5.6‰,16.7‰to 25.2‰(most fluctuated at 19‰within 2.5‰)respectively.The carbon isotope of veins filled in the compressional fractures have the same range to the carbon isotope of wallrocks,and the oxygen isotope of veins filled in the compressional fractures have same range to wallrocks which indicated that the fluids mainly came from wall-rocks.On the diagram ofδ13C-δ18O of the veins filled in the NE-SW compressional fractures,the plots of veins display evident linear relationship,which also indicated that the fluid is mixture of two resources. On the diagram ofδ18OH2O-δDH2O, the plot shows that the fluid flowed in the fractures is originated from formation water and meteoric water. The fluids companied with this tectonic movement mainly came from wall-rocks, some mixed with meteoric water.On the diagram ofδ13C-δ18O of the veins filled in the NW-SE compressional fractures, the plots of Cc-veins and host-rocks didn't display evident linear tendency, moreover, theδ13C-δ18O isotopic compositions of the veins are similar to these of their host-rocks, which indicates that the fluids flowed in the fracture came from wall-rocks. Compared to those of the fluids formed during NE-SW compression, theδD of the fluids flowed during NW-SE compressional fractures show narrow range, which indicates the single origin of the fluid.On the diagram ofδ18OH2O-δD,most of the plots of the fluid fall inside the scale of formation water, indicated that the fluids flowed in the fractures mainly came from wall-rocks.Across dabashan tectonic belt, theδ13C increase and theδ18O decrease from backland to forland, which indicates that the backland was reduced environment. Moreover, the change ofδ13C andδ18O values are successive, indicating that the fluid flowed from backland to foreland.