海南岛古特提斯洋壳的地层古生物判据及海西期构造演化

On the basis of the studies of lithogeochemistry and isotope chronology, the Chenxing-Bangxi ophiolite, the track of Paleotethys have been recognized from northern side of Changjian-Qionghai fracture belt, Hainan Island previously. However, the present evidences are not ample enough to support the conclusion on the presence of Paleotethyan oceanic crust on Hainan Island. It is short of such most important evidences as the depositional types of Late Paleozoic rocks，the Late Paleozoic faunas，and paleobiogeographic comparisons. Following studies in the Changjian-Qionghai fracture belt will be involved in the present proposed project. They are: to set up the Late Paleozoic stratigraphic sequence, lithologic characters, depositional environments, and petrofacies, and faunas; to gather the evidences including the deep-water deposits, open sea fossils; to made the paleobiogeographic comparisons between the study area and Qingzhou-Fangchen districts of Guangxi, and other areas in South China; and the further studies on the lithogeochemistry and isotope chronology of metabasites. In addition, the Late Paleozoic biostratigraphic studies such as to look for Early-Middle Devonian fossils in order to consummate the Upper Paleozoic column of the areas south to the fracture belt; and to gather the evidences on the Carboniferous-Permian glacioaqueous deposits, and Gondwana biotas at Nanlong area, Jiangbian Township, Dongfang City. In all, the research results of proposed project will offer the evidence of paleobiogeographic comparisons, the key evidence for further demonstration of Paleotethyan oceanic crust in Hainan Island, the Paleotethys extended eastwardly, and for the Hercynian tectonic evolution, and tectonic style of the Island.

根据岩石地球化学和同位素年代学研究辨认出的昌江-琼海断裂带北侧的邦溪-晨星蛇绿岩片，初步揭示了古特提斯洋在海南岛的形迹。然而，有关该岛古特提斯洋壳的论据目前尚不完善，尤其缺乏相关地层的沉积类型、古生物群特征、古生物地理区系对比方面的关键证据。本项目将建立昌江-屯昌断裂带内的晚古生代地层序列，研究各时代地层的岩性、岩相类型、沉积环境和古生物群特征；收集大洋深水沉积和放射虫等广海型化石证据；重点开展昌江-琼海断裂带晚古生代地层及古生物地理区系研究，并与邻区对比研究；对断裂带变基性岩的岩石地球化学及同位素年代学等作进一步研究。此外，开展断裂带南侧及其以南的琼中地区晚古生代生物地层的研究，进一步寻找早—中泥盆世化石、完善该地区晚古生代地层柱。本研究结果将为进一步论证海南岛的古特提斯洋壳提供古生物地理区系对比等决定性判据，并为探讨古特特斯东延、海南岛海西期构造演化和大地构造格局等问题提供科学依据。

海南岛大地构造位置独特，其形成演化一直是华南地区的重大基础地质问题之一。本项目主要从地层古生物角度探讨海南岛是否存在古特提斯大洋沉积，并进一步寻找和丰富海南地体泥盆纪的化石年代依据。我们的研究表明，海南岛晚古生代主体为滨浅海沉积环境，局部有裂陷槽沉积，但不存在深水洋壳。.海南岛泥盆系包括中晚泥盆世金波组，晚泥盆世弗拉期兰洋组、法门期昌江组。金波组为滨浅海碎屑岩沉积建造，岩性主要为砂岩、粉砂岩、含砾砂岩，局部夹粉砂质泥岩，厚度大于630m，其中部的炭质板岩夹层中含中泥盆世吉维特期几丁虫化石。兰洋组中、下部为碳酸盐台地沉积，厚度大于300m，含床板珊瑚、竹节石和藻类化石；上部以滨浅海碎屑岩沉积为主，夹酸性凝灰岩，厚度大于150m。昌江组为浅海、滨浅海灰岩、砂岩、泥质粉砂岩沉积，夹凝灰质砂岩，含牙形石、有孔虫、放射虫和藻类化石，厚度大于117m。.海南岛石炭系包括下石炭统鸡实组、上石炭统青天峡组。鸡实组厚度超过380m，主体为滨浅海碎屑岩沉积建造，局部夹灰岩，含腕足类、珊瑚和三叶虫化石。青天峡组厚度大于330m，底部为条带状灰岩，向上以砂泥质沉积为主，局部夹灰岩透镜体，含腕足类、双壳类、牙形石和海百合化石，为浅海、滨海沉积环境。.研究区二叠系为滨浅海相碎屑岩沉积和碳酸盐台地沉积，厚度大于1000m，含腕足类、牙形石、蜓类和海百合及蕨类植物化石。.除上述占主导地位的滨-浅海沉积建造外，在屯昌南棍园还有硅质岩、硅质泥岩和浊流沉积，代表水体相对较深的裂陷槽（裂谷）沉积环境。.本次工作在儋州市兰洋镇上番开村原划归石炭纪地层中发现晚泥盆世弗拉期的多筒竹节石（Polycylindrites sp.）和光壳节石（Styliolina sp），为海南岛首次发现竹节石化石，并在上覆岩层中发现酸性凝灰岩夹层，获锆石U-Pb年龄365±3Ma，代表海南岛近年来泥盆纪生物地层和年代地层研究的最重要进展。