广西那坡裂陷盆地晚古生代硅质岩地球化学特征及其地质意义

广西那坡裂陷盆地位于右江盆地南缘，晚古生代该盆地广泛分布包括硅质岩、泥岩和海相玄武岩在内的深水相沉积.对盆地内上泥盆统榴江组和中下二叠统四大寨组硅质岩地球化学特征研究表明，硅质岩SiO₂含量为88.55%~99.03%，PAAS组成含量小于20%，指示其含有较低的陆源碎屑组成.硅质岩的Al/(Al+Fe+Mn)值为0.45~0.94，Eu/Eu*值为0.51~0.95，为非热液成因硅质岩.除去SiO₂稀释作用的影响后，硅质岩具有较高的稀土元素含量(∑REE+Y含量相当于PAAS组成的2~5倍)，指示其形成于相对远离陆源供应的环境.岩信和鱼塘上泥盆统榴江组硅质岩具有中等的Ce负异常(Ce/Ce*值分别为0.37~0.72和0.58~0.89)以及较明显的Y正异常(Y/Ho值分别为39.05~83.74和34.33~36.70)，形成于远离陆源的开阔裂谷盆地环境.鱼塘中下二叠统四大寨组硅质岩具有明显的Ce负异常(Ce/Ce*值为0.12~0.33)，显示成熟洋盆的地球化学特征.结合右江其他地区硅质岩的地球化学特征认为，晚古生代硅质岩的地球化学特征记录了右江盆地从晚泥盆世裂谷盆地到早中二叠世扩张为开阔洋盆的过程. 

Depositional Chemistry of Cherts of the Late Paleozoic in Napo Rift Basin,Guangxi and Its Implication for the Tectonic Evolution

The Napo rift basin on the southern margin of the Youjiang basin was dominated by deep-water sediments including cherts, mudstones and marine basalts during the Late Paleozoic. Major and rare earth elements of the Upper Devonian Liujiang Formation and Lower-Middle Permian Sidazhai Formation cherts in Napo are analyzed in this study. The SiO₂ contents of samples range from 88.55% to 99.03%, and the PAAS components are less than 20%, which indicate few terrigenous clastic components in the Upper Paleozoic cherts. The high Al/(Al+Fe+Mn) ratios (0.45 to 0.94) and no positive Eu anomalies (0.51 to 0.95) indicate non-hydrothermal origins. Considering the effect of diagenetic SiO₂ dilution, their ∑REE+Y values are between twice and five times those of PAAS-like compositions, indicating that they were deposited in a basin far away from the terrigenous input environment. The Upper Devonian Liujiang Formation of Yanxin and Yutang have moderately negative Ce anomalies (0.37 to 0.72, 0.58 to 0.89, respectively) and higher Y/Ho ratios (39.05 to 83.74, 34.33 to 36.70, respectively), indicating these cherts were deposited in the open-rift basin far away from the terrigenous input. The Lower-Middle Permian Sidazhai Formation cherts of Yutang show similar geochemistry characteristics of the mature ocean basin cherts, with obviously negative Ce anomalies (Ce/Ce*=0.12 to 0.33). On the basis of our studies about cherts, we conclude that the depositional chemistry of the Upper Paleozoic cherts records the evolution process of the Youjiang basin from the rift basin during the Late Devonian into an open-ocean basin during the Early-Middle Permian.