Geochemical Characteristics and Geological Implications for the Neoproterozoic Meta-Sedimentary Rocks of the Cathaysia Block in Zhejiang and Fujian Provinces
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摘要: 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.Abstract: 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.
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图 1 夏地块前寒武纪基底变质岩系分布(据Li et al., 2005修改)
Fig. 1. Sketch map showing the distribution of Precambrian metamorphic basement of the Cathaysia block
图 2 西南、闽西北地区上部变质岩系变沉积岩照片
a.片岩, N26°55’11.5”, E117°00’01.6”; b.变粒岩。N27°27’11.6”, Ell8°21’48.2”; c大理岩, N26°50’23.6”E118°23’42.3”; d.条带状含磁铁石英岩, N27°51’02.1”, E118°58’38.6”, 己大理岩与变粒岩呈互层状产出, N26°50’23.6”, Ell8°23’42.3”; L片岩夹石英脉和伟晶岩脉, 沿裂隙发生绿帘石化现象。N26°55’11.5”, E117°00’01.6, ,
Fig. 2. Representative geologieal images of upper metamorphic basement from southwestern Zhejiang and northwestern Fujian areas
图 3 )龙泉群、马面山群和万全群变沉积岩西蒙南图解(Simonen, 1953), 其中坐标值为Niggli值; (b)A-C-FM判别图解(谢缅年科, 1966, 转引自王仁民等, 1987)
Fig. 3. The Simonen diagram(a) and the A-C-FM diagram(b)for schists and leptynites of the Longquan, Mamianshan, and Wanquan groups
图 4 泉群、马面山群和万全群变沉积岩SiO2-Ca/Mg和SiO2-Fe/(Ca+Mg)图解(CaO≤5%)(据米什金, 1981, 转王仁民等, 1987)
Ⅰ.蒙脱石粘土; Ⅱ.水云母粘土; Ⅲ.高岭石粘土; Ⅳ.长石砂岩和石英长石质砂岩。左图包括斜长石长石砂岩, Ⅴ.其他岩石(Ⅴ1.石英岩, 硅质岩; Ⅴ21.亚杂砂岩, 右图包括斜长石长石砂岩; Ⅴ22.亚杂砂岩、复矿物砂岩, 长石砂岩; Ⅴ3.杂砂岩; Ⅴ4.铝土矿, 红土)
Fig. 4. Diagrams of SiO2 vs Ca/Mg and SiO2 vs Fe/(Ca+Mg)for meta-sedimentary rocks of the Longquan, Mamianshan, and Wanquan groups
图 5 龙泉群、马面山群和万全群变沉积岩稀土元素球粒陨石标准化曲线(球粒陨石REE含量引自Sun and McDonough, 1989)
Fig. 5. hondrite-norrmlized REE patterrm for meta-sedinmatary rocks of the Longquan, Mamiamhan, and Wanquan groups
图 6 龙泉群、马面山群、万全群片岩类和变粒岩类主量元素相关性图解(Bhatia, 1983)
A. 大洋岛弧杂砂岩; B. 大陆岛弧杂砂岩; C. 活动大陆边缘杂砂岩I D. 被动大陆边缘杂砂岩
Fig. 6. Corelation diagram of bulk composition of schists and leptynites for the Longquan, Mamianshan, and Wanquan groups
图 7 龙泉群、马面山群和万全群片岩类和变粒岩类La/Se-Ti/Zr图解(A、B、C、D图例同图 6)
Fig. 7. La/Sc vs Ti/Zr diagram for schists and leptynites of the Longquan, Mamianshan, and Wanquan groups
图 8 (a) 龙泉群、马面山群、万全群片岩类和变粒岩类Hf-La/Th图解(Floyd and Leveridge, 1987; Gu et al., 2002); (b)浙闽地区华夏地块前寒武纪变质基底岩石稀土元素球粒陨石标准化曲线对比图解
球粒陨石REE含量引自Sun and McDonough, 1989; 其中龙泉群、马面山群、万全群样品为片岩类和变粒岩类; 天井坪组、八都群、陈蔡群、麻源群样品为斜长角闪岩类、片麻岩类和变粒岩类等, 数据为作者未发表资料, n为样品数
Fig. 8. (a)Diagram of Hf-La/Th for schists and leptynites of the Longquan, Mamianshan, and Wanquan groups; (b)Comparative diagrams of chondrite-normalized REE patterns for Precarnbrian metamorphic basement rocks of the Cathaysia block in Zhejiang and Fujian Provinces
表 1 浙闽地区华夏地块新元古代变沉积岩样品主量元素(%)和微量元素(10-6)分析结果
Table 1. Maior and trace element compositions for Noproterozoic netasedimentary rocks of the Cathaysia block in Zhejiang and Fujian provinces
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