鄂尔多斯盆地中部上三叠统延长组物源方向分析与三角洲沉积体系

首次利用稀土元素和微量化学元素,分析鄂尔多斯盆地中部上三叠统延长组母岩类型和物源方向。研究表明盆地中部延长组砂岩和泥岩的稀土元素地球化学特征、REE分配模式及微量化学元素蜘蛛图解相同,表现为轻稀土富集,重稀土亏损,铕、铈元素亏损;REE分配模式均呈“右倾斜、LREE富集、HREE平坦”型;微量化学元素蜘蛛图解具“四峰三谷一平台”型,曲线呈阶梯状分布;砂岩和泥岩的地球化学特征、REE分配模式及微量化学元素蜘蛛图解与盆地东北缘的太古代、元古代变质岩相一致,而与火成岩的重稀土富集、轻稀土亏损、“V”字型REE分配模式和“三峰两谷”型、曲线呈“W”字型分布的蜘蛛图解的特征有显著的差别。说明延长组母岩是盆地东北缘的太古代、元古代变质岩,主要物源方向为北东向。依据物源方向、沉积环境、砂岩岩石学特征、砂岩稀土元素和微量化学元素的地球化学特征,将鄂尔多斯盆地中部上三叠统延长组沉积划分为两大三角洲沉积体系,即北东向的安塞三角洲沉积体系、志靖三角洲沉积体系、安边三角洲沉积体系和北西向的盐定三角洲沉积体系。在三角洲沉积体系划分的基础上,详细阐述了不同三角洲沉积体系的岩石学特征、稀土元素和微量化学元素的地球化学特征及延长组长6-长2期三角洲沉积体系的演化过程。

SOURCE DIRECTION ANALYSIS AND DELTA DEPOSITIONAL SYSTEMS OF YANCHANG FORMATION OF THE UPPER TRIASSIC IN THE CENTRAL ORDOS BASIN

Rare earth elements(REE) and trace elements were used to analyse source rock and source direction of Yanchang Formation of the Upper Triassic in Ordos Basin. Research shows that the sandstone and mudstone of the Yanchang Formation in the central Ordos Basin have same geochemical characteristics, the REE distribution, and trace elements spider diagrams. The light rare earth elements are enriched, the heavy ones, Eu and Ce are to the bad. The REE distribution is a type which inclined to the right, LREE enriched, and HREE flat. The trace elements spider diagram shows a type of 4 peaks, 3 valleys, and 1flat, the curve is distributed like a ladder. The geochemical characteristics, the REE distribution, and trace elements spider diagram of sandstone and mudstone in the Yanchang Formation are identical with metamorphic rocks of the Archean and Proterozoic from the northeast basin, but different remarkably from igneous rocks with the heavy rare earth elements(HREE) enriched, light elements lost, V-shape REE ditribution mode, and the spider graph of 3 peaks and 2 valleys, the curve distributed like "W". Therefore, the metamorphic rocks, from the Archean and Proterozoic are the dominant source rocks of the Yanchang Age, and the main source is from eastnorth. In the light of source direction, depositional enviroment, sandstone petrography, geochemistry marks of rare and trace elements, the delta deposition of the Yanchang Formation can be divided into two systems. One is Ansai-Zhijing-Anbian delta depositional system from eastnorth, and the other is Yanding delta depositional system from westnorth. On the basis of the delta depositional systems, petrographic characteristics, geochemical characteristics of the rare earth and trace elements of different delta depositional systems, and the evolution process of the delta depositional systems of the Member 6-2 in the Yanchang Formation are elaborated in detail.