三水盆地及周边地区古近纪火山-沉积组合与南海早期演化

By using the Paleogene volcano-sedimentary records collected from the Sanshui basin and its neighboring basins lying at the north margin of the SCS (South China Sea), the researches of lithofacies, element and isotope geochemistry, tectono-sedimentology and geochronology will be integratedly carried out in this project. The project will help us recognize the spatial and temporal distribution and evolutionary tendency of the magmatic process, analyze the mechanism of volcanic eruption characterized by trychytic rocks and its relation to the tectonic background, and explore the pattern of continent rupturing and rift type in the SCS's initial evolution. According to the results from the above works, we will better understand whether the geological records from the study areas may mirror intracontinental rifting and response the initial tectonic evolution and set up a significant model. On the basis of the previous research, we give a hypothetical model in which there was a process of mantle upwelling induced by the Proto-SCS's subduction and a stage of active rifting and it was strongly held down by its surrounding tectonic movement since the late Eocene (about 40Ma). Consequently no upwelling turned up at the stage of the SCS's spreading and the oceanic crust of the SCS was just from partial melting of the lithospheric mantle under powerful extension. This new model will be strictly examined by the project.

应用广东三水盆地及周边盆地群的古近纪火山-沉积记录，开展详细的岩相学、元素与同位素地球化学、构造沉积学和地质年代学研究，确立南海北缘早新生代岩浆活动的时、空特征与演化趋向，分析以粗面岩为典型代表的火山喷发机制与相关构造背景的关联，探讨南海早期演化过程中的大陆破裂样式与裂谷性质。本项目通过上述工作，明确陆区地质记录是否能够反映陆内裂谷作用及其在南海早期演化中所做出的响应，构建相关模式。在以往研究工作的基础上，本项目预设南海演化进程中曾存在“古南海”俯冲活动诱发的地幔上涌作用和主动裂谷阶段，但在晚始新世（约40Ma）受到周边构造活动的制约，未能在南海扩张阶段进一步发挥作用。这一新的演化模式将通过本项目的实施进行严格检验。

项目以三水盆地为主要研究目标，对盆地及其邻区的晚白垩世以来的火山-沉积记录进行了系统研究。在收集并产出地质年代学、地层学、岩石地球化学、矿物学、构造沉积学、岩相古地理学等学科资料的基础上，创新性地确定了盆地性质和演化框架。项目组经过研究认为，新生代三水盆地是在晚中生代安第斯型陆緣小型弧后前陆盆地群基础上发育而来的统一张裂盆地，挤压过程大约结束于大塱山期早期（约80-75Ma），并在大塱山期-莘庄村期即中新生代之交（约65Ma）进入区域裂陷阶段。莘庄村组仅代表古新统下部，该层位内虽吹响板内火山喷发的哨音，但多属零星的壳源型酸性岩类，裂陷过程由构造拉张所主导，显现明显的被动裂谷特征。㘵心组和宝月组经厘定后被划为古新统中上部，所有证据指出这是一个地幔上涌、整个盆地发生大面积热沉降的时代，表现为基性幔源型岩浆大量喷发，从被动裂谷转为主动裂谷。始新世（55Ma）起岩浆活动更加强烈，但转为以粗面岩为主、碱性玄武岩为辅的喷发模式。项目组确认，粗面岩是地幔上涌后基性岩浆在浅部岩浆房结晶分异的产物，它们与研究区碱性玄武岩演化路径不同，但同样是地幔上涌深度卷入区域构造的关键性证据。华涌组是三水盆地最上部地层单元，大体代表早始新世，以滨湖沼泽相、曲流河相和扇三角洲相为主，指示盆地进入最后的萎缩充填阶段。三水盆地在主动裂谷火山喷发的高峰期内停止发育，随后大陆破裂中心向南迁移，与渐新世初的南海扩张实际上已无关联。项目首次提出三水盆地在中、新生代的两阶段演化模式及其在华南陆緣体系中的代表意义，揭示了白垩纪末期以来该盆地独特的从被动裂谷到主动裂谷并过早结束发育的地质过程，实现了预定研究目标和研究内容。