湘黔桂地区中上二叠统硅质岩的地球化学特征及沉积背景

Geochemistry and sedimentary background of the Middle-Upper Permian cherts in the Xiang-Qian-Gui region.

我国华南地区二叠系广泛发育硅质岩,但其沉积背景,如硅的来源和沉积环境,一直存在较大的争议。在对湘黔桂地区硅质岩进行大量的野外和室内岩石学研究的基础上,本文对研究区内黔桂盆地和湘桂盆地中14个不同产地的81件中上二叠统硅质岩(中二叠统33件、上二叠统48件)的主量和稀土元素进行了研究。中二叠世时期,Al2O3、TiO2、Al2O3/(Al2O3+Fe2O3)、∑REE、Ce/Ce*等整体上偏低,但具有比较一致的区域性差异:黔桂盆地相对偏低(0.09%~0.37%、0.00%~0.01%、0.22~0.47、2.47×10-6~14.59×10-6、0.23~0.69),而湘桂盆地(0.01%~5.73%、0.00%~0.23%、0.02~0.71、7.07×10-6~141.0×10-6、0.24~1.22)相对偏高。晚二叠世时期,Al2O3、TiO2、Al2O3/(Al2O3+Fe2O3)、∑REE、Ce/Ce*等整体上偏高,其中黔桂盆地分别为1.62%~10.67%、0.04%~0.65%、0.41~0.81、23.10×10-6~248.99×10-6、0.46~1.39,而湘桂盆地除来宾和柳州地区(0.00%~1.11%、0.00%~0.15%、0~0.42、0.55×10-6~36.59×10-6、0.27~0.83)偏低外,其他地区偏高(3.78%~21.37%、0.16%~0.91%、0.73~0.83、51.14×10-6~245.4×10-6、0.99~1.10),区域上呈东西两边高,中间低的分布趋势。中二叠统硅质岩自生铁W(Feauthig)百分含量整体偏高,仅少数低于50%。(La/Ce)N比值在黔桂盆地为1.61~5.04,湘桂盆地除来宾(2.63~4.90)外为0.82~1.94。而上二叠统硅质岩的W(Feauthig)除来宾、柳州和巴马地区大于70%外,其余地区的均小于50%。(La/Ce)N比值除来宾(2.42~4.50)外,整体上偏低,为0.69~2.47。研究区中上二叠统硅质岩Eu/Eu*基本没有明显的区域性差异,中二叠世时期,巴马地区一个样品为正异常(1.17),其余均为负异常,而晚二叠世时期,仅来宾和柳州地区显示了Eu/Eu*的正异常,其中柳州为1.07~1.22。结合Fe-Al-Mn、(La/Ce)N-Al2O3/(Al2O3+Fe2O3)、Fe2O3/TiO2-Al2O3/(Al2O3+Fe2O3)组合图解及岩石学研究,上述主量和稀土元素指标变化的结果表明:(1)中二叠统硅质岩的硅的来源相对复杂,黔桂盆地的硅质岩主要为热液,而湘桂盆地以生物来源为主,局部受到热液活动的影响;上二叠统硅质岩的硅的来源相对简单,主要为生物,仅巴马、来宾和柳州地区受到热液活动的影响。(2)中、晚二叠世热液活动范围和强度具有明显差异:中二叠世,研究区内热液活动较为广泛,但相对较弱;而晚二叠世热液活动较为局限却相对较强。(3)中二叠世黔桂盆地形成于远离陆源碎屑的边缘海盆环境,湘桂盆地为受到陆源碎屑影响的边缘海盆环境,而晚二叠世整个研究区的陆源碎屑输入增强,除来宾和柳州地区外,其余地区均为受陆源碎屑影响的边缘海盆环境。这一变化记录了中、晚二叠世时期研究区的构造-古地理变化信息,反映了以峨眉山玄武岩为代表的地幔柱引起的火山热液活动和区域性地壳抬升事件。

Permian cherts occured extensively in South China.However,their sedimentary backgrounds including their origins and depositinal environments remain controversial.Based on extensive field observations and lithofacies study of the cherts in the Xiang(Hunan)-Qian(Guizhou)-Gui(Guangxi) region,we analyzed and compared major and trace elements of 81 samples from the Middle-Upper Permian cherts outcropping at 14 localities in the Qiangui(Guizhou-Guangxi) basin and the Xianggui(Hunan-Guangxi) basin.Al2O3 contents,TiO2 contents,the ratios of Al2O3/(Al2O3+Fe2O3),∑REE and the values of Ce/Ce* of the Middle Permian cherts are totally low,and show a same distribution tendency in the studied region: They were lower(0.09%~0.37%,0.00%~0.01%,0.22~0.47,2.47×10-6~14.59×10-6,0.23~0.69) in the Qiangui basin and higher(0.01%~5.73%,0.00%~0.23%,0.02~0.71,7.07×10-6~141.0×10-6,0.24~1.22) in the Xianggui basin.Al2O3 contents,TiO2 contents,the ratios of Al2O3/(Al2O3+Fe2O3),∑REE and the values of Ce/Ce* of the Upper Permian cherts are higher than those of the Middle Permian,and they are different from localities,low in the middle area(0%~1.11%,0.00%~0.15%,0~0.42,0.55×10-6~36.59×10-6,0.27~0.83) and high in both eastern(3.78%~21.37%,0.16%~0.91%,0.73~0.83,51.14×10-6~245.4×10-6,0.99~1.10) and western(1.62%~10.67%,0.04%~0.65%,0.41~0.81,23.10×10-6~248.99×10-6 and 0.46~1.39) areas.The weight percentage of non-detrital Fe in total Fe,W(Feauthig),of the Middle Permian cherts are higher than 50% with a few exception.However,W(Feauthig) of the Upper Permian cherts are lower than 50% in most samples except for the samples at the localities of Laibin,Liuzhou and Bama where W(Feauthig) are higher than 50%.In the Middle Permian cherts,the values of(La/Ce)N are 1.61~5.04 in the Qiangui Basin and 0.82~1.94 in the Xianggui basin(2.63~4.90 in Laibin).However,(La/Ce)N of the most cherts in the Late Permian are 0.69~2.47 except for that in Laibin(2.42~4.50).The values of Eu/Eu* of the Middle and Upper Permian cherts both show no regional difference,with negative anomalies in most samples.A few positive Eu anomalies are found in Bama(1.17 in the Middle Permian) and Laibin and Liuzhou(1.04~1.22 in the Upper Permian).With plotting in diagrams of Fe-Al-Mn,(La/Ce)N vs.Al2O3/(Al2O3+Fe2O3),Fe2O3/TiO2 vs.Al2O3/(Al2O3+Fe2O3),the above data suggest that:(1) The Middle Permian cherts are different in origin.Those in the Qiangui Basin are of hydrothermal origin and those in Xianggui basin are biogenic with some hydrothermal influence.Most of cherts from the Upper Permian are of biogenic origin,except those in Bama,Laibin and Liuzhou with a hydrothermal origin.(2) The hydrothermal activity in the Middle Permian was weak,although extensively occurred,by the contract,it was strong but localized.(3) Although the Middle Permian cherts developed in continental marginal seas,the Xianggui basin received terrigenous input while the Qiangui basin did not.During the Late Permian,both the Xianggui basin and the Qiangui basin received significant terrigenous input.Such a dramatic change probably reflects the regional paleo-geographic evolution and implies that regional uplift and hydrothermal activity resulted from a mantle plume upwelling represented by the Emeishan basalt.