老挝西北部琅勃拉邦构造带放射虫硅质岩及其构造意义

琅勃拉邦构造带内放射虫硅质岩含有放射虫Entactinia vulgaris Won, Entactinosphaera palimpola Foreman和Belowea variabilis (Ormiston et Lane)，时代为早石炭世，为老挝境内首次报告。硅质岩样品具有很高的SiO2质量分数(95.29%~98.17%)，大部分样品表现出相类似的稀土配分模式，部分样品配分模式图表现为上凸状，具有中稀土富集，均具有明显的Ce负异常，Ce/Ce*值为0.64~0.74，其中部分样品具有Eu的负异常，为0.58~0.68。Y/Ho比值为31.05~40.96，类似日本Sasayama中-晚二叠世的远洋硅质岩。地球化学显示其为含酸性火山碎屑非热液成因的远洋硅质岩。这些研究证实了在思茅板块和印支板块之间存在一个开阔的石炭纪时期弧后盆地。

Radiolarian cherts from the Luang Prabang tectonic zone in NW Laos and their tectonic implications

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.