湘西、黔北地区志留系稀土元素地球化学特征及其地质意义

中国南方志留系是加里东运动末期沉积的一套以碎屑岩为主的地层,同时其底部广泛发育一套优质烃源岩,对该套地层沉积古环境和古构造背景的恢复有重要的地质意义。通过对中国南方湘西和黔北典型剖面系统的地球化学分析表明该区志留系泥岩的稀土总量较高,w(ΣLREE)/w(ΣHREE)及w(La)N/w(Yb)N等比值显示轻、重稀土分异明显,REE分布模式为明显的轻稀土元素富集、重稀土元素呈平坦状及显著的负Eu异常。以Ceanom值反映水体介质的氧化还原性,志留纪早期的海水还原性较强,更有利于有机质的保存并形成优质的烃源岩。将REE的分异程度作为一种指示剂来表征沉积物沉积速率,志留系剖面自下向上,w(La)n/w(Yb)n值有逐步降低的趋势,表明研究区志留系下部的沉积速率普遍比上部低,更有利于有机质聚集和保存。根据稀土元素组合及其比值特征,结合古地理研究成果,认为研究区的物源来自雪峰古隆起和黔中古隆起,源岩为花岗岩和沉积岩的混合。

REE Geochemical Characteristics and Their Geological Signification in Silurian,West of Hunan Province and North of Guizhou Province

The sediments of Silurian in South China are main detrital rocks,which were formed at the end of Caledonian.A set of high quality source rocks developed at the bottom of Silurian.It has an important signification to reveal the palaeo-environment and tectonic setting for this sequence.The systemic analysis of geochemistry indicates that the amount of REE of Silurian in West of Hunan Province and North of Guizhou Province is high.The ratios of Σ LREE/Σ HREE and LaN/YbN show that the differentiation of Σ LREE and Σ HREE is obvious.The REE distribution patterns are characterized by rich in LREE and obviously negative Eu abnormity.The Ceanom can be used to reveal redox of palaeo-ocean.The reduction in Early Silurian is strong with high Ceanom,which is advantageous for organic preservation and the formation of good quality source rocks.The differential degree of REE can also be used to reflect the sedimentary rates.The Lan/Ybn values decrease gradually from bottom to top in Silurian.The sedimentary rate at the bottom of Silurian is lower than that of upper sequence,so it is more favorable for organic accumulation and preservation.According to the characteristics of REE and their ratios,combined with achievements in palaeo-geography,it can be drawn that provenance of the study area comes from Xuefeng and Qianzhong palaeo-uplifts,and the source rocks are granites and sedimentary rocks.