浙闽地区华夏地块新元古代变沉积岩地球化学特征及其地质意义

浙闽地区华夏地块前寒武纪基底上部主要由新元古代龙泉群、马面山群和万全群组成.上述3个岩群的变沉积岩具有相似的岩石组合及主、微量元素地球化学特征, 它们是华夏地块统一基底的重要组成部分.它们主要由片岩类、变粒岩类、石英岩类和大理岩类组成, 其中片岩类和变粒岩类的SiO₂变化于52.89~75.03, TiO₂为0.48~1.05, Al₂O₃为9.19~20.3, ∑REE为(149~323)×10-6, δEu为0.34~0.80, (La/Yb)_N为7.96~15.6, 具中等-强烈铕负异常, 轻、重稀土分异明显; 石英岩类的SiO₂变化于95.49~97.44, ∑REE很低, 在(8.89~15.1)×10-6, δEu=0.63~0.81.原岩性质及构造环境分析表明, 片岩类和变粒岩类的原岩主要是杂砂岩和粘土岩等, 石英岩的原岩为硅铁质沉积岩.所有变沉积岩的原岩均以长英质成分为主, 具有上地壳岩石的地球化学特征, 部分样品显示有古老沉积物的加入; 它们形成于岛弧-活动陆缘环境, 成熟度不高, 属于近源沉积, 与本地区华夏地块基底下部变质岩系岩石(古元古代的天井坪组、八都群、陈蔡群、麻源群)具有相似的物源. 

Geochemical Characteristics and Geological Implications for the Neoproterozoic Meta-Sedimentary Rocks of the Cathaysia Block in Zhejiang and Fujian Provinces

The upper basement of the Cathaysia block in Zhejiang and Fujian provinces consists mainly of the Neoproterozoic Longquan Group, Mamiansban Group, and Wanquan Group. These three groups exhibit similarity in rock assemblage as well as major and trace elements geochemistry. They are the main components of the uniform upper basement of the Cathaysia block, consisting mainly of schists, leptynites, quaterizes and marbles. Schists and the leptynites have variable composi-tions, with SiO₂ contents ranging from 52.89% to 75.03%, TiO₂ from 0.48% to 1.05%, Al₂O₃ from 9.19% to 20.3%, ∑REE between 149×10-6 and 323×10-6, δEu values between 0.34 and 0.80, and (La/Yb)_N values between 7.96 and 15.6. The REE patterns display medium to high negative Eu anomaly with obvious fractionation between LREE and HREE. The quartzites have different compositions, with SiO₂ contents varying from 95.49% to 97.44%, and relatively low ∑REE contents (8.89× 10-6 to 15.1 × 10-6). It is proposed that protoliths of the schists and the leptynites are wackes and clay rocks, whereas protoliths of the quartzites are ferruginous silica rocks, mainly quartz-feldspathic; and some protoliths show geochemical characteristics of ancient allochthonous sediments. The meta-sedimentary rocks of the Longquan Group, Mamianshan Group, and Wanquan Group were probably formed in an environment of arc-active continental margin. The lower maturity implies that they are near field sediments, similar to the metamorphic rock system of the lower basement in this area (Paleoproterozoic Tianjingping Formation, Badu Group, Chencai Group and Mayuan Group), analogous to the upper crustal rocks.