A Study Of The Late Paleozoic Stratigraphy In Nankunyuan, Tunchang County, Hainan Island, And Its Tectonic Significance

The study area is located in the joint terrain of the Eurasia, India and Pacific blocks, with aunique tectonic location and particular evolution history, In tectonics it belongs to the easternbranch of the Hei River and Hong River suture zone, regarded as a "window"to study the easternextension of the Paleo-Tethys, the crustal growth of East Asia, the evolution of continentalmargins and the formation of South China Sea. The Hainan Island has a Paleoproterozoic metamorphic basement.and is divieded into thesouthern Sanya Terrain and the central-northern Wuzhishan Terrain by the Jiusuo-Lingshui Fault.They have different basements in composition and characters, The Sanya Terrain might be a partof the South China Sea-Idochina Terrain while the basment of the Wuzhishan Terrain consists ofthe Baoban Group and some granitic rocks. The Jiusuo-Lingshui fault controls the struture andevolution of the basements. Based on the study of the strata in Nankunyuan, Tunchang, including the lithostratigraphy,biostratigraphy, chronostratigraphy and geochemistry, the following results have been achieved:1. During the regional geological survey (scale at 1:250000), a section covering the Lower and Middle Permian strata was measured and described bed by bed and 32 fossil samples and 18 geochemical samples are collected systematically.2. The strata were metamorphised at a low grade and some radiolarian chert beds are intercateted in the upper and middle parts. The study indicates that the original rocks of the low-grade metamorphic strata are a series of granule clastic rocks, which were metamorphised through a low greenschist metamorphic series by regional metamorphism. The total deformation is weak but strong in local.3. The composition, structure, basic sequence and lithology of the rocks have been intensively studied, and the depositional facies and environments of retrieved and discussed. They are believed a flysch facies formed in a bathypelagic or half-bathypelagic environment.4. Some of radiolarian (Pseudoalbaillella) and sporopollen fossils have been found in the strata for the first time through a series of patient analyses. The radiolarian Pseudoalbaillella indicates an age of the Early-Middle Permian, which provides a unique evidence for the geological age of the strata.5. The protolith of the metamorphic rocks includes mudstone,sandstone and quartz siltstone (fine sandstone), and their geochemical characters are clearly different. The quartz siltstone(fine sandstone) has a high compositional maturity and a low content of ? REE, Nb and Y, indicating that the source came from the continental basement. The mudstone has low values of SiO2, La/Th and Ti/Zr, high MgO, Fe2O3 and ? REE, and the compatibleelements such as Cr, V, Sc, Ni are ricker than those in the upper crust, showing that the source from the region of rick ferromagnesian components. The geochemical characters of the sanstone are between the quartz siltstone (fine sandstone) and the mudstone. In conclusion, the source of the metamorphic sendimentary was partly from a matured continental crust and partly from the basic volcanic rocks in an island arc. The sediments deposited in an arc basin close to the continent. This result is consistent with the study on the sedimentary facies and environoments.6. The intercalated chert beds are characterized by having average SiO2=88%, MnO/TiO2<0.1, Al2O3/(Al2O3+Fe2O3)=0.84, Sr/Ba<1, (La/Ce)N ranging from 1.03 to 1.38, dCe of 0.53 to 0.98, (on an average 0.77). The elemental geochemistry characteristics shows that the tectonic setting yielding the chert beds is the edge of the oceanic basin close to continental margin instead of a typicalcontinetal margin.7. Central-northern Hainan Terrain (Wuzhishan terrain) was tectonically controled by the Tethyan tectonic domain. The extension and depression of the oceanic crust resulted in a certain taphrogenic basin. The Early-Middle Permian abyssa...