Study on Isotopes and Organic Inclusions of Fluid Flowing during the Mesozoic Tectonic Movements in Dabashan Foreland Belt
|School||Central South University|
|Keywords||dabashan Sm-Nd Rb-Sr isotope tracing organicinclusions|
Dabashan forland belt is situated in the transfer zone between Qinling orogen and Sichuan basin. According to different characters of structural deformation, from NE to SW, Dabashan can be divided into4different tectonic units:the thrust-nappe structural belt, the basement decoupling structural belt, the cap rock decollement structural belt and the forland depressional belt. The styles of structural defomation are different in these belts. From NE to SW, the pattens of deformation are from ductile to brittle. The strength and depth of deformation decrease gradually.Several tectonic movements have taken place since Mesozoic period in this area. Of which the most important tectonic movements are NE-SW compression and NW-SE compression. Moreover, widespread fluid flows have taken place during these two tectonic periods and calcite-veins (Cc-veins) filled in the fractures. Two kinds of veins, bedding-parallel and fracture-filled veins, were collected for analyses of rare-earth elements (REE), samarium-neodymium and strontium isotopes. The total REE contents of the calcites from the bedding-parallel vein are much higher than those from fracture-filled veins and are rich in light REE (LREE). The chondrite-normalized patterns of REE curves of the calcites from fracture-filled veins are flatten. The87Sr/86Sr and143Nd/144Nd ratios of calcites from bedding-parallel vein are from0.71068to0.71115and from0.51192to0.512058, respectively. On the147Sm/144Nd vs143Nd/144Nd diagram, the six samples from the bedding-parallel vein give a Sm-Nd isochron age of357±230Ma(2σ)(MSWD=24) with an initial143Nd/144Nd ratio of0.51168±0.00019. However, the plots of the four samples from fracture-filled veins are rather scatter either on147Sm/144Nd vs143Nd/144Nd or on87Sr/86Sr vs143Nd/144Nd diagram. The fluids, from which the two kinds of veins precipitated, are different in origin and flowing.Characteristics of organic compositions of the inclusions and carbon-hydrogen-oxygen isotopes of fluids indicate that the fluids flow not only along faults and fractures, but also in penetration manner in the formation during tectonic movements. It’s of great significance for the secondary migration and accumulation of oil and gas afer extraction from the formation.