Carbon and oxygen isotopic analysis of the carbonate rocks in the GarzeLitang suture zone,western Sichuan

The detailed field geological survey and laboratory carbon and oxygen isotope analysis are made for the carbonate rocks developed in the middle part of the Garze-Litang suture zone,western Sichuan. According to the modes of occurrence,two types of carbonate rocks are recognized:carbonate rocks overlain upon the basalts and cm-to m-grade irregular basalt-coated carbonate breccias. The carbon and oxygen isotopic data of the carbonate samples collected from the study area have disclosed that the carbonate rocks overlain upon the basalts display the δ¹³C values mostly ranging between 0.113‰ and 1.625‰,and δ¹⁸O values ranging between -10.263‰ and 5.745‰. However,the basalt-coated carbonate rocks display the δ¹³C values ranging between 4.254‰ and 3.786‰,which are higher than those of the carbonate rocks overlain upon the basalts,and the δ¹⁸O values ranging between -13.471‰ and -13.592‰,which are lower than those of the carbonate rocks overlain upon the basalts. These differences imply that the basalt-coated carbonate rocks once suffered from the paulopost diagenetic reworking. The Z values of more than 123 for all the collected samples indicate that the carbonate rocks in the study area should belong to the normal marine carbonate rocks. It is inferred from the modes of occurrence,lithology and geochemical signatures that the basalt-coated carbonate rocks may be deposited in the seamount sedimentary environments,and their ages are generally earlier than those of the carbonate rocks overlain upon the basalts.