广西南宁地区泥盆纪硅质岩稀土元素地球化学特征及沉积背景

广西南宁附近从早泥盆世埃姆斯期到晚泥盆世弗拉期发育了一套黑色层状硅质岩,常夹凝灰岩(或凝灰质硅质岩)或与之互层,在弗拉期晚期硅质岩变为浅黄色。本文对该区埃姆斯期、艾菲尔期、吉维特期到弗拉期硅质岩的稀土元素地球化学特征进行了系统研究,其∑REE平均值分别为:18.59×10^-6,12.28×10^-6,27.44×10^-6和43.07×10^-6;页岩标准化后Ce含量在这4个阶段均表现为弱-中等的负异常,Ce/Ce^*平均值分别为:0.85,0.72,0.85和0.67;Eu/Eu^*为弱的正异常,平均值分别为:1.08,1.21,1.46和1.04;(La/Ce)_N值中等,平均值分别为:1.42,1.50,1.35和1.73;(La/Yb)_N值变化较大,平均值分别为:1.19,1.06,1.24和0.45。这些结果表明该区硅质岩开始形成于陆内裂谷盆地,受陆源影响较大,在晚泥盆世迅速扩张为比较开阔的海盆环境,陆源影响明显减弱。硅质岩的形成虽受到一定热液活动的影响,但热液活动的影响普遍较弱。

REE GEOCHEMISTRY AND DEPOSITIONAL SETTINGS OF THE DEVONIAN CHERTS IN NANNING AREA, GUANGXI

Siliceous deposits(or chert),generally being dark grey to black and thin-bedded,widely occurred in the strata from the Emsian to Frasnian of the Devonian in Nanning area,Guangxi,which were intercalated commonly with brownish tuffites(or tuffaceous chert).This paper presented the rare earth element(REE) geochemistry of siliceous deposits there from which the depositional setting and basin evolution were elucidated.The total REE contents(∑REE) were generally lower in the cherts,with an average being 18.59?10^-6,12.28?10^-6,27.44?10^-6 and 43.07?10^-6 in the Emsian,Eifel,Givetian and Frasnian,respectively.The shale-normalized REE abundances in the cherts were featured with weak to moderate negative Ce contents in late Early Devonian to early Late Devonian,with Ce anomalies(avg.Ce/Ce^* values,being 0.85,0.72,0.85 and 0.67 respectively),generally lower positive Eu anomalies(avg.Eu/Eu^* values,being 1.08,1.21,1.46 and 1.04 respectively),intermediate(La/Ce)_N values(avg.1.42,1.50,1.35 and 1.73 respectively),and intermediate to lower(La/Yb)_N values(avg.1.19,1.19,1.24 and 0.45 respectively).These data suggested that these siliceous deposits were initially precipitated in an intracontinental rift basin with significant terrigenous influences in the Early Devonian,which was rapidly transformed into an open marine basin with a little terrigenous influences in the early Late Devonian(Frasnian) due to a rapid extension of the basin.Although the precipitation of the cherts was influenced by the hydrothermal venting,it was generally weak,probably away from hydrothermal vents in a distance.