泰国北部难河构造带二叠纪放射虫、硅质岩和玄武岩

Permian Radiolarians,Chert and Basalt from the Nan Suture Zone,Northern Thailand

在泰国北部难河构造带Pha Som变质杂岩中发现保存很好的放射虫硅质岩、玄武岩地层层序.层状硅质岩含放射虫化石Follicucullus porrectus,地质时代为中二叠世晚期至晚二叠世早期.其硅质岩Si O2含量均在92.5%以上,Al/(Al+Fe+Mn)平均比值为0.51,Ce/Ce*比值为1.14,为大陆边缘型硅质岩.玄武岩具有富集大离子亲石元素与高场强元素以及轻稀土富集等洋岛玄武岩的特点.说明难河构造带中-晚二叠世之交存在洋岛型火山岩和靠近大陆边缘的深海盆地硅质岩,代表了小洋盆的沉积组合.该构造带闭合时间应在晚二叠世与晚三叠世之间.

Well-preserved stratigraphic sequences composed of radiolarian chert and basalt were found in Pha Som metamorphic complex in Nan suture zone, northern Thailand. Bedded chert contains Follicucullus porrectus, fossils from late Middle Permian to early Late Permian. These cherts have high SiO2 （〉92.5%）, low A1/（Al＋Fe＋Mn） ratio （0. 51 on average）, and high Ce/Ce^＊ ratio （1.14 on average）. These geochemical characteristics mentioned above indicate that the cherts deposited on continental margins. The basalt has high abundances of large ion lithophile elements （LILE）, high field strength elements （HFSE）, and light rare earth lements （LREE）, suggesting characteristics of oceanic island basalt （OIB）. The OIB characteristics of basalts and deep-sea-basin chert imply that the Nan suture zone was a small oceanic basin, which is similar to modern Southwest Pacific. The Nan Ocean presumably closed during the period from Late Permian to Late Triassic.