Geochemical characteristics and sedimentary environment of siliceous rocks in Yalongri area, western section of Bangong-Nujiang suture zone

Abstract：Based on the systematic studies of petrology and geochemical characteristics of major, trace and rare earth elements of the siliceous rock in Nawucuo ophiolitic mélange belt, the genesis and sedimentary enviornment were discussed.The results showed that: (1) The siliceous rock in this area appeared in the form of rock slices on outcrops and contacted with adjacent geologic units by faults; (2) The SiO₂mass fraction of siliceous rock was 83.32%～94.37%, andw(SiO₂) /w(Al₂O₃) varied from 14.65 to 49.02,w(Al₂O₃) /w(Al₂O₃+Fe₂O₃+MnO) varied in the range of 0.58～0.79.It indicated that the formation of genesis of the siliceous rock was affected by terrigenous material, and the mass fraction of terrigenous argillaceous sediment was relatively high; (3) Al-Fe-Mn genetic discrimination diagram showed biogenic chert rather than hydrothermal chert; (4)w(Al₂O₃) /w(Al₂O₃+Fe₂O₃) varied in the range of 0.59～0.80, combined with the 100×w(Fe₂O₃) /w(SiO₂) -100×w(Al₂O₃) /w(SiO₂) and thew(Fe₂O₃) /w(TiO₂) -w(Al₂O₃) /w( Al₂O₃+ Fe₂O₃) discrimination diagrams, and the rare earth distriburin patterns of rare earth elements after North America Shale Standardization (NASS) showed the siliceous rock with Ce-negative anomalies, (w(La) /w(Ce) )_Nvalues were 1.67～2.15,w(Ce) /w(Ce*) values were 0.46～0.61, all these features showed that the siliceous rock was formed in a tectonic environment from oceanic basin to continental margin.Comprehensive studies showed that the genesis of the siliceous rock in Nawucuo ophiolitic mélange belt was biogenic, which was formed in a sedimentary environment from oceanic basin to continental margin.

Keyword：geochemistry;sedimentary environment;siliceous rock;Bangong-Nujiang suture zone;