Quartz features constrain the origin of terra rossa over dolomite on the Yunnan-Guizhou Plateau, China

摘要

The origin of terra rossa and the relationship between terra rossa and its underlying carbonate rocks pose long-standing and controversial questions. Little insoluble residue content in carbonate rocks makes it difficult to believe that the terra rossa derives only from their underlying carbonates. This study focuses on one 4.05 m thick Pingba Farm terra rossa profile over dolomite on the Yunnan-Guizhou Plateau, China. It is characterized by its ferromanganese concretion, gibbsite and terra rossa coexistence in profile. The A1_2O_3 contents in the terra rossa rise to 34%. The authors examined the quartz content, particle size distribution of quartz, surface micro-textures and quartz morphology, quartz oxygen isotope ratios, mineral composition, chemical composition, and other tracers in the terra rossa and dolomite to determine the derivation of the terra rossa. The results indicate that terra rossa in the Pingba Farm profile is an in situ weathering solum. The terra rossa comes partly, but not completely, from insoluble residues from the underlying dolomite. The debris minerals (quartz, feldspar, etc.) derive completely from the underlying dolomite. The clay minerals, and the Al, Fe and Mn oxides and hydroxides likely have three origins. One portion derives from insoluble residues of the underlying dolomite. A second portion transforms from primary minerals in the underlying dolomite. The final portion possibly comes from antigenic minerals which formed from aqueous solutes imported by the leaching water. The terra rossa is mainly composed of antigenic minerals.