地幔柱构造的沉积及环境响应——以峨眉地幔柱为例

Sedimentary and environmental responses to mantle plume: A case study of Emeishan mantle plume

通过对上扬子板块西北部中二叠统地层、岩性、古生物及地球化学特征,并结合前人研究资料分析认为,受峨眉地幔柱活动而形成的穹窿状地壳隆升影响,四川盆地中二叠世地貌特征表现为西南部较高,向东及东北方向平缓降低,其中不发育陡斜坡及斜坡转折带。该古地貌特征与区域海平面升降共同控制和影响了研究区古地理格局及演化,形成了受峨眉地幔柱活动控制的缓坡型碳酸盐岩台地。中二叠世岩性、古生物及地球化学特征对峨眉地幔柱活动均有相应的响应,根据穹窿状水下隆起不同部位沉积的岩性、古生物组合、沉积构造以及地球化学特征,四川盆地中二叠世总体表现为西南部环境开阔,水体动荡,发育开阔台地亚相,向东及北东方向逐渐过渡为浅缓坡亚相和深缓坡亚相发育区。开阔台地及浅缓坡环境中生物最为丰富,古串珠虫(Palaeotextularia)组合和翁格达藻(Ungdarella)组合分布于开阔台地,而浅缓坡环境的代表主要为米齐藻(Mizzia)-假蠕孔藻(Pseudovermoporella)组合、球旋虫(Glomospira)-始毛盘虫(Eolasiodiscus)组合及节房虫(Nodosarinae)组合等,向东及东北方向生物逐渐减少,至深缓坡环境,仅在其上部有少量分布。微量元素的丰度变化也反映了古环境特征,从开阔台地至深缓坡,沉积环境逐渐从氧化向还原环境变化,Ba、Sr、V、Cu等微量元素的平均丰度值逐渐增大。

According to the strata characters, lithological features, palaeontologic assemblages, geochemistry index and the result of predecessors＇ research in the northwest of the Upper Yangtze plate in the Middle Permian. Due to the crustal doming affected by Emeishan mantle plume activity in the Middle Permian, Sichuan Basin was characterized by high relief in the southwestern part of the Upper Yangtze platform, converted into fiat paleogeomorphic features eastward and northeastward, and poorly developed steep slope and distinct slope transition zone. The paleogeographical feature of this kind, and local sea level change directly controlled the paleogeographic pattern and evolution, forming a unique pattern of carbonate ramp platform. Emeishan mantle plume activity caused the sedimentary and environmental responses included lithology, paleontology and geochemistry characters. According to the sedimentary lithology, paleotogical assemblages, structure features and geochemistry characters in various parts of submarine doming, the study area in a broad environment with a high water energy is the subfacies open platform, located at the higher part of the submarine dome shaped area in the southwestern Upper Yangtze platform, and gradually transformed eastward and northeastward into developing area of the shallow and deep ramp facies. There are enriched of paleontological in the open platform and shallow ramp developing area. The Palaeotextularia assemblages, Ungdarella assemblages located in open platform. The Mizzia-Pseudovermoporella assemblages, C, lomospira-Eolasiodiscus assemblages and Nodosarinae assemblages, and so on, represent the shallow ramp environment, the palaeontology gradually decreased to eastward and northeastward. There are only a small amounts of paleontology distributed in the upper deep ramp facies developing area. Changes in the abundances of trace elements also reflect the paleogeographic characteristics. From the open platform to deep ramp facies developing area, the sedimentary environment also transform from oxidizing environment to reducing environment. In the process of environmental changes, the average abundances for Ba, Sr, V, Cu etc. , are gradually increased.