S AND Pb ISOTOPES OF SEDIMENTARY PYRITES IN THE NORTHERN DABA MOUNTAINS AND THEIR TECTONIC SIGNIFICANCE

From the S and Pb isotopic data of sedirnentary pyrites in the Sinian·Silurian strata in thenorthern Daba Mountains, the following two ideas are got about the formation evolution of thepassive continental margin of the northern part of the Yangtze plate :（1）The average￣（206）pb/￣（204）pbratio in the early stage of rifting in the Sinian rifting was 17. 66,which indicates that rifting inthis period attained the lower crust;the average ratio was 19. 03 in the late stage of rifting andthe early stage of migration of the plate in the Cambrian,indicating that rifting went down to adepth near the upper and lower crust;the average ratio increased to 20. 08 in the late stage ofmigration in the Silurian,suggesting that rifting remained within the upper crust. （2） Theδ￣（34）Svalues of pyrite had a trend of gradual increase frorn + 8 .25‰to +17. 49‰from the earlystage of rifting （Z） to the late stage of migration（S₁）,which implies that this area represents aclosed paleo-ocean basin environment.