花东海盆年龄及性质：锆石U-Pb定年及Hf-O同位素制约

The Western Pacific active continental margin located in the junction among the Eurasia Plate，the Pacific Plate and the India Plate．Under the effects of the India-Eurasia collision and roll-back subduction of the Pacific Plate，the Western Pacific active continental margin has a wide basin-arc-trench system which migrated and jumped eventually seaward and eastward．.The Huatung Basin is a small oceanic basin located east of Taiwan. It is bordered by the Luzon volcanic arc to the west and by the Gagua Ridge to the east. The history of plate motions in the Huatung Basin is critical to understand the growth of Eastern Asia and the origin of marginal basins at the eastern Eurasia margin, both from the point of view of the kinematics and the forces acting..Since no deep sea hole has been drilled in the Huatung Basin, the identification of the magnetic anomalies provide the most important constraint on the age of the seafloor. Previous age estimates from magnetic lineation studies indicated an Eocene age for the basin and formamed from the Central Basin Spreading Center of the West Philippine Basin during the last phase of spreading in Middle Eocene. Because the width of the basin is less than 150 km and the number of available N-S magnetic profiles is therefore restricted and not sufficient to determine reliable ages. .Ar/Ar ages obtained on gabbros dredged on oceanic basement highs of the Huatung Basin are Early Cretaceous and proposed that the basin is a fragment of the former `proto-south China Sea' or possibly the `New Guinea Basin' that has been trapped by the Philippine Sea Plate. However, the origin and age of the Huatung Basin are therefore still controversial..Zircon U-Pb dating and Hf-O isotopes of the rocks collected from the Huatung Basin and Gagua Ridge will be analyzed in this study. The purpose of the work is to improve the understanding of the age, origin and character of the the Huatung Basin and Gagua Ridge, and to contribute to future research into the tectonic evolution of the Huatung Basin, Philippine Sea Plate and the South China Sea. The work emphasizes the significance of approaches to geological problems, such as the eastern Eurasia margin from global view instead of local one of the South China Sea.

西太平洋地区发育有规模最宏伟的“沟弧盆”构造体系，是研究弧后扩张、洋壳俯冲及弧-陆碰撞造山的天然实验室。目前我国对东亚边缘海的研究大多局限于南海，对其它边缘海的构造演化研究非常少，极大地限制了对东亚大陆边缘构造及边缘海形成演化的整体认识。花东海盆处于欧亚大陆与太平洋板块相互作用的最前缘。海洋地质调查显示，花东海盆可能为晚中生代残留洋盆,代表新特提斯洋或古南海的残余部分，但还存在极大的争议。本项目以花东海盆及加瓜海脊为主要研究对象，对已获得的岩石样品进行锆石U-Pb 定年及Hf-O同位素分析：(1)确定花东海盆及加瓜海脊的年龄和性质;(2)分析花东海盆及加瓜海脊的构造演化历史;(3)探讨花东海盆与菲律宾海及南海的成因联系。本研究跳出南海，以更广的视角来研究东亚边缘海的构造演化问题，对深入理解东亚边缘海的整体构造演化格局、晚中生代洋盆的消亡及新生代边缘海的扩张等重要科学问题提供了新的思路。

花东海盆处于欧亚大陆与太平洋板块相互作用的最前缘，是研究东亚大陆边缘构造及边缘海形成演化的重要窗口。海洋地质调查显示，花东海盆可能为晚中生代残留洋盆，代表新特提斯洋或古南海的残余部分，但还存在极大的争议。本项目以花东海盆及加瓜海脊为主要研究对象，对已获得的岩石样品进行了地球化学、同位素、Ar-Ar、锆石U-Pb 定年及Hf-O同位素分析，取得了如下的主要成果和认识：.（1）花东海盆样品锆石分析结果显示，其U含量为46～291ppm，Th含量为20～167 ppm，Th/U比值为0.382～0.877，普通206Pb含量为0.21～3.35%，锆石成分非常均一。用207Pb/235U和206Pb/238U比值作U-Pb谐和图得到协和年龄为130.28 ± 0.95 Ma（N=18，MSWD=0.72），表明花东海盆的时代确实可追溯到早白垩世。该年龄与Deschamps et al. (2000) 对花东海盆综合研究得出的结论一致。.（2）花东海盆样品长石Ar-Ar年龄为71.1±2.0 Ma（MSWD= 2.19），等时线年龄为75.8±3.3 Ma（MSWD=0.38）。该年龄与锆石年龄相差很大，虽然锆石的结晶封闭温度比长石高，但不可能造成约60Ma的差别，因此长石Ar-Ar年龄应为长石矿物的重置年龄，暗示花东海盆在～70 Ma可能发生过岩浆侵入事件。.（3）花东海盆辉长岩RD20的全岩Rb/Sr值为0.022，初始87Sr/86Sr值为0.703175±6，初始143Nd/144Nd值为0.512883±6，εNd (t) = +8.0。辉长岩RD19的初始87Sr/86Sr值为0.703190±6，初始143Nd/144Nd值为0.512846±6，εNd(t) = +7.3，与RD20相近。花东海盆样品普遍具有LILE富集和Nb、Ta负异常特征，认为早白垩世花东海盆的本质属性为弧后盆地，在~70 Ma时该洋壳受到富含LREE和Nb、Ta元素的岩浆/热液的侵入。