云南中三叠世罗平生物群产出地层的地球化学特征和沉积环境

Geochemical Characteristics and Sedimentary Environments of the Fossiliferous Layers of Middle Triassic Luoping Biota,Yunnan Province

为了研究安尼期罗平生物群产出地层的沉积环境，从云南罗平地区关岭组二段大凹子精测剖面采集50件样品进行了主量、稀土、微量元素分析。分析结果表明，所有灰岩样品的K、Na含量较低，Mg／ca比值也较低，反映罗平地区在中三叠世安尼期为温暖潮湿的气候条件；Sr／Ba比值（1．22—68．96，平均值为24．35）较高，m值（=100×MgO／Al_2O_3）为27．22—27400，平均值达到1432，稀土元素较低的Y／Ho比值，表现出平缓或者微弱的LREE亏损，以及与近岸河口沉积物相似的稀土配分曲线特征，指示其属于近岸的碳酸盐台地内部盆地沉积。从剖面底部到顶部，Al、K、V、Ti和P等元素含量以及Co／Ti、Ni／Ti比值具有多个变化旋回，反映海平面升降变化频繁，总体发生了两次较大规模的海平面相对上升活动。所有样品的V／（V＋M）、V／Cr、Ni／Co、U／Th等元素比值、δU、lg（Ce／Ce‘）等参数值都较高，反映沉积水体为相对低能、缺氧的环境。氧化还原敏感元素V、U、Co、Ni、Zn和cr等呈现出锯齿状曲线特征，表明水体并不完全处于缺氧环境，而是随着海平面周期性升降活动，底部水体表现为间歇性缺氧。结合地球化学及地层学资料，认为中三叠世安尼期罗平地区处于温暖潮湿、还原、低能的离岸较近台内盆地，这一时期海平面升降频繁，底部水体为间歇性缺氧环境。

In order to study the geochemical and sedimentological record of environmental changes in the the Anisian Luoping Biota, 50 fresh samples were collected from the Member II of the Middle Triassic Guanling Formation, Dawazi section, and analyzed for major and trace elements. The low K, Na concentration and Mg/Ca ratio, indicating a warm and humid paleoclimate. The high Sr/Ba ratio （ 1.22 ～ 68.96, averaged 24.35 ） and magnesium/aluminium ratio （m = 100 × MgO/A1203, 27.22 ～ 27400, averaged 1432）, moderate or weak LREE depletion, and low Y/Ho ratio, similar to the characteristics of REE patterns of coastal and estuarial deposits, indicating that the Luoping area is located in a intraplatform basin nearby the land. From bottom to top of the section, A1, K, V, Ti and P concentrations and Co/Ti, Ni/Ti ratios, show repeated fluctuations, reflecting frequent change of the sea-level. It is confirmed that two large-scale sea-level rise events occurred during the deposit time of the Luoping area. The high V/（V ＋Ni）, V/Cr, Ni/Co, and U/Th ratios, and δU, lg（Ce/Ce^＊ ） values, reflect low-energy, anoxia conditions in this area. Redox sensitive elements such as V, U, Co, Ni, Zn and Cr change in multiple cycles in the section, indicating overall anoxia of the bottom water with intermittent reoxgenation related to the sea-level change. Based on the stratigraphical and geochemical data, it is concluded that the Luoping area is located in an intraplatform basin during the Anisian, Middle Triassic. The climate was warm and humid. The bottom water was anoxic in general with periodical reoxgenation associated with sea-level change.