Tectonic setting and coal accumulation patterns of coal-forming period of Machala Formation in Qamdo area, Qinghai-Tibet Plateau

In order to reconstruct the original coal accumulation, this paper analyzed the geochemical characteristics of Machala Formation. The results show that the source rocks of Machala Formation include sedimentary rock from passive continental margin and granite, alkali basalt from island arc (La/Yb=10.04~15.62 μg/g; La/Th=1.33~3.17 μg/g; Hf=4.58~13.90 μg/g). The site of samples in Zr-Th, La-Th-Sc, Th-Co-Zr/10 tectonic setting discrimination diagrams also show that Machala Formation sediments were deposited in the active continental margin, which is near the continental island arc, with the characteristics of both active continental margin and continental island arc (∑REE=193.96 μg/g, LREE/HREE=6.29; δEu=0.61; La/Yb=13.17; (La/Yb)_N=8.55). Machala Formation was deposited in continent margin with shallow sea and continental-oceanic environment. The coal seams were developed in TST, and are characterized by numerous thin layers, poor continuity, and coal-accumulating centre dispersion. By the analysis on sequence stratigraphy and provenance, when Machala Formation was deposited, Lancangjiang ocean basin had subducted. Coal accumulation pattern in Machala Formation was coal-forming in garc-island basin in active continental margin. The results can both detail the evolution of Tethysintheeraly Carboniferous and reveal the occurrence regularity of coal in Qamdo.