琼东南盆地上中新统—上新统中央峡谷沉积物来源

Provenance of Central Canyon in the Upper Miocene to the Pliocene in the Qiongdongnan Basin

应用元素地球化学、重矿物组成及碎屑锆石测年方法,分析了琼东南盆地上中新统—上新统中央峡谷沉积物来源及源区母岩类型。研究表明:中央峡谷稀土元素总量介于168.10×10^(-6)至218.79×10^(-6)之间,平均值为191.01×10^(-6),接近于PAAS而略高于UCC值。轻稀土(LREE)相对富集,La/Yb值较高,介于12.99~15.44之间,平均值为14.41。(La/Yb)N值变化范围为8.76~10.41,平均值为9.72,LREE/HREE值介于3.62~4.24之间,平均值为3.91。铕具有明显的负异常,呈"V"字型,δEu值变化范围为0.53~0.71,平均值为0.6。铈异常不明显,δCe值变化范围为1.03~1.10,平均值为1.06。重稀土(HREE)相对平坦,(Gd/Yb)N值变化范围为1.74~2.01,平均值为1.87,表明母岩为来自上地壳的酸性岩,这一结论与微量元素相关比值(Eu/Eu*、La/Sc、La/Co、Th/Sc以及Cr/Th)以及源区岩性图解Hf—La/Th、La/Sc—Co/Th以及V—Ni—Th×10相一致。对比三大潜在物源区(红河、海南岛及越南中部)沉积物的稀土元素配分模式,发现中央峡谷沉积物稀土元素球粒陨石标准化配分模式与越南中部最为接近;重矿物中锆石、赤褐铁矿以及白钛矿含量高这一特征与越南中部沉积物重矿物特征相吻合;结合前人有关中央峡谷碎屑锆石测年的相关研究,确定中央峡谷沉积物主要来自越南中部的昆嵩隆起带。

In this paper,trace and Rare Earth Elements（REE）,heavy mineral data and detrital zircon U-Pb ages were used to determine the provenance of Central Canyon in the Upper Miocene to the Pliocene in the Qiongdongnan Basin.The results show that the total REE contents range from 168.10×10^-6 to 218.79×10^-6,with a mean value of 191.01×10^-6,whereas the samples show values similar to those of PAAS and higher than that of UCC.These sandstone samples are characterized by the enrichment of Light Rare Earth Elements（LREE）,with high ratios of La/Yb=12.99-15.44（Av.14.41）,（La/Yb）N=8.76-10.41（Av.9.72）and ∑LREE/∑HREE=3.62-4.24（Av.3.91）,a negative Eu anomaly（δEu=Eu/Eu＊range from 0.53 to0.71,average 0.6）and normal amounts of Ce（δCe=Ce/Ce＊range from 1.03 to 1.10,average 1.06）.Heavy Rare Earth Elements（HREE）show an almost flat distribution,with（Gd/Yb）Nratios ranging from 1.74 to2.01（average 1.87）.The comparison of REE patterns suggest that the sandstones from Central Canyon System were derived from an old upper continental crust mainly composed of felsic igneous source rocks,as also shown by the ratios of Eu/Eu＊,La/Sc,La/Co,Th/Sc and Cr/Th and the diagram of La/Th-Hf,La/ScCo/Th and Ni-Th×10-V.These characteristics are similar to the felsic igneous source rocks in the Central Vietnam.Zircon,hematite and limonite,and leucoxene are the most frequent heavy mineral and are associated with the sediments in Central Vietnam.The zircon U-Pb ages of sandstone samples from the Central Canyon System have a range of 30-2 600 Ma,including four peak groups：100-160 Ma,230-250 Ma,420-450 Ma and 700-1 100 Ma.All these characteristics are associated with the zircon U-Pb ages of the felsic igneous source rocks in the Kontum massif in the eastern Indo-china Block.