西藏札达县夏浦沟的放射虫硅质岩和岛弧火山岩：新特提斯洋内俯冲体系的记录？

Radiolarian Chert and Island-Arc Volcanic Rocks in Xiapugou Tibet:Records of Neo-Tethys Intra-Oceanic Subduction System?

在西藏阿里地区夏浦沟野外调查发现放射虫硅质岩、熔岩组合.放射虫硅质岩SiO2含量在89.47%～92.94%之间,Si/Al在43～67之间,Al/(Al+Fe+Mn)比值在0.68～0.74之间,MnO/TiO2平均比值为0.60,Ce/Ce*平均值为0.89,LaN/CeN平均值1.09,指示它位于洋盆和大陆边缘过渡的环境,总体更接近大陆边缘环境.伴生的火山熔岩以安山质为主,具低TiO2(0.75%～0.98%)、轻稀土弱富集和源自板片流体的易溶元素如Ba、U、Pb相对轻稀土富集,亏损高场强元素(HFSE)Nb、Ta、Ti等,指示形成于岛弧环境,很可能是中生代新特提斯洋洋内俯冲系统的组分.放射虫动物群主要包括Alievium cf.regulare、Alievium cf.fatuum、Archaeospon goprunum cf.patricki、Archaeodictyi mitra mitra Dumitrica等早白垩世分子,其提供了岛弧活动时间上限的约束.

The stratigraphic units composed of radiolarian chert and lava were found in Xiapugou in western of Yarlung Zangbo Suture Zone during the 1/250 000 field geology survey. The radiolaxian cherts are characterized by SiO2 ranging from 89. 47 to 92. 94% , Si/ Al ratio ranging from 43 to 67, Al/（Al＋Fe＋Mn） ratio from 0. 68 to 0. 74, The average MnO/TiO2 , Ce/Ce^＋ and LaN/CeN ratio is 0. 60, 0. 89 and 1. 09, which indicates the radiolarian chert was formed in transitional environment between continental margin basin and the oceanic basin, and had much more close relationship with the former one. The lava mainly belongs to andesite and is characterized by （1） low TiO2 varying from 0. 75 to 0. 98% ; （2） gentle enrichment in LREE ; （3） fluid-soluble elements, such as Ba, U and Pb, are extremely enriched compared with LREE; （4） depleted in high-field strength dements （HFSE） such as Nb, Ta, Ti）. All these characters imply that the stratigraphic units are consistent with a juvenile intra-oceanic arc, which maybe represents the remains of New-Tethys intra-oceanic subduction system. The early Cretaceous fauna of radiolarian fossils from the chert including Alievium cf. regulate, Alievium el. fatuum, Archaeoslongoprunum cf. patricki, Archaeodictyimitra mitra Dumitrica et al. have been identified, yielding a upper time restrain of this juvenile intra-oeeanic arc.