班公湖-怒江缝合带西段改则硅质岩地球化学特征及沉积环境

Geochemical characteristics and sedimentary environment of siliceous rocks of Gerze area in western Bangong Co-Nujiang suture zone,Tibet

班公湖-怒江缝合带是西藏第二条规模巨大的基性-超基性岩带。文章通过对该带西段改则地区硅质岩的系统地球化学研究,探讨了硅质岩形成的成因和构造背景。研究表明：①硅质岩的w（SiO2）变化范围在85.23%~93.73%之间,Si/Al比值为34.08~56.92,表明它们含有较高比例的陆源泥质沉积物;②Hf、Pb、Nb、Ta、Th和U等不相容元素亏损较明显;③Al/（Fe＋Mn＋Al）比值为0.41~0.69,具有生物成因特征,Ba/Sr比值和U/Th比值均小于1,Al-Fe-Mn三角图和Zr-Cr协变图上均为非热水成因的特征;④Al2O3/（Al2O3＋Fe2O3）比值为0.50~0.77,稀土元素球粒陨石标准化和北美页岩标准化模式图显示出LREE富集、弱Ce正异常和无明显Ce异常的特征,（La/Ce）SN、Ce/Ce＊和（La/Lu）SN比值分别为1.11~1.17、0.90~0.92和1.10~1.62,w（V）〈23μg/g,V/Y〈2.8,以上特征均说明改则地区硅质岩当时形成的环境海底热液活动不强烈,而陆源物质输入较多,沉积环境以在大陆边缘形成为主;⑤从Si和O同位素值看出,其属于典型的放射虫硅质岩类型,δ30Siδ-18O呈正相关趋势,说明SiO2从溶液中晶出石英时,硅、氧同位素表现出相似的动力学分馏过程;⑥该区放射虫硅质岩地球化学特征表明其沉积时可能形成于相对稳定的洋盆闭合大陆边缘环境。

The Bangong Co-Nujiang suture zone is the second huge ultramafic rock belt in Tibet.Based on detailed geochemical research,this paper discusses the genesis and tectonic setting of siliceous rocks in Gerze area,Tibet.Some conclusions have been reached： ① The SiO2 content range is 85.23%～93.73%,and Si/Al ratios vary in the range of 34.08～56.92,indicating that they contain a relatively high proportion of terrigenous muddy sediments;② Incompatible elements such as Hf,Pb,Nb,Ta,Th and U are obviously depleted;③ Al/（Fe＋Mn＋Al） ratios are 0.41～0.69,suggesting bio-genesis.Ba/Sr and U/Th ratios are lower than 1,and Al-Fe-Mn triangular diagram and Zr-Cr diagram show non-hydrothermal characteristics of siliceous rocks;④ Al2O3/（Al2O3＋Fe2O3） ratios are 0.50～0.77,and chondrite-normalized as well as North American shale-normalized REE patterns show that siliceous rocks are enriched with LREE with weak positive Ce anomaly and no obvious Ce anomalies;（La/Ce）SN,Ce/Ce＊ and（La/Lu）SN values are 1.11～1.17,0.90～0.92 and 1.10～1.62,V23 μg/g,and V/Y2.8.All these features suggest that the siliceous rocks were formed in an environment of relatively weak hydrothermal activity but fairly abundant terrigenous material inputting.In addition,the deposition was dominated by a continental margin environment;⑤ Si and O isotopes indicate typical characteristics of radiolarian chert.Besides,δ30Si-δ18O assumes positive correlation,indicating that Si and O isotopes had similar kinetic fractionation processes;⑥ Geochemical characteristics of chert implies that it was mainly deposited in a continental margin environment during the closure of the Tethys Ocean.