Abstract:

Radiolarian cherts from the Luang Prabang tectonic zone contain radiolarian Entactinia vulgaris Won, Entactinosphaera palimpola Foreman and Belowea variabilis (Ormiston et Lane)from the Early Carboniferous Period, as it was first reported in Laos. The samples have high SiO2 content(95.29%98.17%). Most samples exhibit similar REE patterns, some show convex REE patterns with slight enrichment in MREEs. All samples have negative Ce anomalies(Ce/Ce*=0.640.74), whereas some display negative Eu anomalies(Eu/Eu*=0.580.68). The Y/Ho ratios range from 31.05 to 40.96, resembling those of Sasayama MiddleLate Permian pelagic cherts in Japan. Geochemical characteristics indicate an origination of the nonhydrothermal pelagic cherts with the involvement of acidic volcanoclastic rocks. These results prove the existence of an open Carboniferous backarc basin between the Simao Block and the Indochina Block.

Key words: radiolarian cherts, Luang Prabang tectonic zone, Carboniferous, PaleoTethys, NW Laos