贺兰山地区晚三叠世延长组地球化学特征及其地质意义

沉积岩的微量和稀土元素对沉积环境变化有着较高的灵敏度，是研究古沉积环境以及沉积物源区构造背景的一种有效手段。本文主要利用地球化学方法对贺兰山晚三叠世延长组不同地区、不同层段的25件泥岩、粉砂岩以及细砂岩样品进行了主量、微量以及稀土元素测试分析，探讨了延长期沉积环境和物源区构造背景。结果显示：延长期整体处于氧化—还原过渡的淡水环境，古气候温暖潮湿；稀土元素球粒陨石标准化配分表现为轻重稀土分异明显，且轻稀土富集、重稀土相对亏损，Eu负异常，Ce异常不明显等特征；La/Yb-REE和La/Th-Hf源岩判别图及Gd/Yb的比值关系图显示延长组源岩主要为长英质岩石，部分为基性岩和沉积岩，且以后太古界为主要物源；La-Th-Sc，Th-Sc-Zr/10和Th-Co-Zr/10判别图解均显示研究区延长组物源区构造背景主要为大陆岛弧，晚期有活动陆缘构造背景的物源加入。

Geochemical characteristics of the Late Triassic Yanchang Formation in the Helan Mountain area and their geological significance

Analysis of trace elements and rare earth elements（REEs）in sedimentary rocks is an effective way to reconstruct the depositional environment and the tectonic settings, due to very high sensitivity of elements to the variation of depositing environment. In this study, twenty-five samples of mudstone, siltstone and fine-grained sandstone were collected from different parts and layers of the Late Triassic Yanchang Formation in the Helan Mountain. These samples were analyzed on major and trace elements and REEs in order to give clues of the depositional environment and tectonic settings. The results show that the Yanchang Formation generally deposited in an oxidation-reducing transitional fresh water environment under a warm-humid paleoclimate. Chondrite-normalized REE patterns show that the rocks are enriched in light REEs and depleted in heavy REEs, with negative Eu anomaly and weak Ce anomaly. Both La/Yb-ΣREE and La/Th-Hf discrimination diagrams and the Gd/Yb ratios indicate that the source rocks of the Yanchang Formation are mainly composed of post-Archean felsic rocks, with slight contribution of basic rocks and sedimentary rocks. According to discrimination diagrams of La-Th-Sc, Th-Sc-Zr/10 and Th-Co-Zr/10, source materials of the Yanchang Formation were likely deposited in a continental island arc setting and clastic materials from an active continental margin were partly added during the late depositional period.