西藏岗巴-东亚地区永珠组沉积时代及沉积环境

前人对冈底斯成矿带的研究主要集中在新生代岩浆岩及其矿产方面，而关于其晚古生代沉积地层鲜有报道.近年来的冈底斯成矿带西段1:5万区域地质调查工作显示其出露大量的晚古生代永珠组的沉积地层.从野外地质调查、岩石学、典型沉积构造、孢粉化石、粒度分析、沉积地球化学等方面对永珠组展开了研究，结果表明，永珠组形成于晚石炭世（C₂）Bashikirian-Moscovian期（中、晚巴什基-莫斯科期），孢粉化石中裸子植物花粉占优势（80%）、蕨类植物孢子居从属地位（20%）.永珠组发育大量的水平层理、平行层理、粒序层理、浊积纹层、滑塌变形构造和鲍马序列等典型沉积构造.永珠组沉积岩表现出高Si（SiO₂为60.94%~76.24%）、Al（Al₂O₃为9.24%~13.20%）、K（K₂O为2.81%~5.47%）和低Mg（MgO为1.32%~2.82%）、Ca（CaO为1.19%~2.78%）、Na（Na₂O为0.2%~1.87%）的主量元素特征，且具有明显高的FeO含量（3.31%~6.67%）和明显低的Fe₂O₃含量（0.14%~0.65%）.稀土元素总量（174.24×10-6~278.39×10-6）较高，经北美页岩标准化的稀土元素配分模式不具Ce异常，Ce/Ce*=0.94~1.03.微量元素表现出Rb、Th、K等元素富集和Ba、U、Nb、Ta、Sr、P、Ti等元素亏损的特征.永珠组形成于大陆边缘环境的陆坡-深水盆地，为暖温气候、中盐度环境下的海退沉积产物，是古特提斯在冈底斯地区的沉积响应. 

Sedimentary Epoch and Depositional Environment of Yunzhug Formation in Gamba-East Asia Area,Tibet

The previous work is mainly concentrated in the Gangdise metallogenic belt in Cenozoic magmatite and mineral resources, but the study of Late Paleozoic sedimentary strata of the Gangdise belt is rarely reported.The recent 1:5 000 regional geological survey on western part of Gangdise metallogenic belt shows that there are a large number of exposed Late Paleozoic sedimentary strata of Yunzhug Formation.Taking the exposed Yunzhug Formation in Tibet area as the research object, the Yunzhug Formation sedimentary environment is analyzed through the geological survey, profile measurement, thin section identification, microfossils, particle size analysis and whole rock analysis firstly, and then the relationship with Gangdis is discussed in this study.It is found that Yunzhug Formation formed in Late Carboniferous (C₂) Bashikirian-Moscovian (Middle and Late Bashki-Moscow period), sporopollen fossils by gymnosperm pollen is predominant (80%) and fern spore in subordinate status (20%).Typical sedimentary structures such as horizontal bedding, parallel bedding, graded bedding, turbidite bedding, slump deformation structure and the Bauma sequence are developed in the Yunzhug Formation.Yunzhug Formation sedimentary rocks show high Si(SiO₂=60.94%-76.24%), Al(Al₂O₃=9.24%-13.20%), K(K₂O=2.81%-5.47%)and low Mg(MgO=1.32%-2.82%), Ca(CaO=1.19%-2.78%)and Na(Na₂O=0.2%-1.87%)compositions, with obvious high FeO content (3.31%-6.67%) and low Fe₂O₃ content (0.14%-0.65%).The ∑REE(174.24×10-6-278.39×10-6) is higher, and there is no Ce anomaly in the North American shale normalized REE pattern, and its Ce/Ce* is 0.94-1.03.Trace elements showed enriched elements such as Rb, Th, K, U, Nb, Ta, Sr, P, Ti and other elements.Yunzhug Formation formed in active continental margin environment, deposited on the continental slope and deep basin, as the product of warm climate, regression deposition in salinity environment, is in response to the Paleo-Tethys deposits in the Gangdise area of activity.