Abstract:

The geochemical characteristics and depositional environment of Pengcuolin chert are studied based on the field investigations and analysis on major elements, trace elements and rare earth elements. The massive chert is green and brown, being high in SiO₂ content, locally enriched in  Fe and Mn, Al/(Al+Fe+Mn) ratio being low. In Fe-Mn-Al triangle diagrams, most chert samples fall into the hydrothermal region. Trace elements such as Sr,Zr,Cu,Zn and Ba are higher and ΣREE is lower, with Ce negative anomaly and Eu anomaly. Their NASC-normalized REE distribution patterns are slightly left-leaning, indicating their hydrothermal origin. In sedimentary environment discrimination diagrams such as 100×Fe₂O₃/SiO₂-100×Al₂O₃/SiO₂, Fe₂O₃/(100-SiO₂)-Al₂O₃/(100-SiO₂) and Fe₂O₃/TiO₂-Al₂O₃/(Al₂O₃+Fe₂O₃), chert samples fall into the areas of the mid-ocean ridges or deep-sea. In parameter variation diagrams, samples of PCL-1,PCL-5 and PCL-7 are closer to the hydrothermal center. It is shown that the Pengcuolin chert, southern Tibet was formed by the hydrothermal systems at convergence and subduction sites of plates.

Key words: chert, hydrothermal sedimentation, mid-ocean ridge, southern Tibet, Pengcuolin, geochemistry