福州凹陷中生代沉积源区示踪：剖析东海陆架区域基底属性

Whether the basement of continental shelf beneath the East China Sea (CSECS) is part of Cathaysia Block or is an exotic block during J3-K1 has remained controversial for decades, which directly affects the understanding of the tectonic evolution of the Western Pacific since the Mesozoic. Recent researches have demonstrated that the detrital zircon U-Pb age and Hf isotope provenance analysis can provide an effective indicator for probing the generation and reworking of continental crust and the occurrence of continental collision. Thus, the Fuzhou Sag, which is observed to have the most complete Lower Jurassic-Upper Cretaceous sedimentary sequences in the offshore area of South China by exploration wells, is an ideal place to solve the problem by provenance analysis. This research attempts to use a series of samples from two wells, which have been established the Mesozoic formation, to investigate the provenance of Mesozoic strata in this area by detrital zircon U-Pb chronology and Hf isotopic, as well as comparison with existing data in Cathaysia block. Furthermore, the exotic origin model can also be determined combining with the contemporary paleogeographic environment evolution of inland and offshore areas in South China. Based on the above analysis, the continental crust accretion can be deciphered by detrital zircons sourced from CSECS, which leads to better understanding of the relationship between the CSECS and the Cathaysia block. This study will provide key evidence to clarify the exotic origin model and explore the basement properties beneath the East China Sea.

东海陆架区基底是华南大陆向海域的延伸还是晚侏罗世-早白垩世期间的外来块体一直存在争议，直接影响到国内外学者对西太平洋中生代以来构造演化的认识。已有研究表明，碎屑锆石U-Pb定年及Hf同位素物源分析可为源区地壳演化以及是否发生块体拼贴提供直接证据。东海福州凹陷钻井揭示了华南近海海域最为完整的J1-K2沉积序列，区内物源演化是分析东海基底归属争议的最佳对象。本研究拟选取研究区两口中生代地层建组剖面井进行源区示踪分析，通过对各层位的系列样品进行碎屑锆石U-Pb年代学及Hf同位素分析，并与华夏块体已有数据进行详细对比，厘定其碎屑物质来源。在此基础上，结合同时期华南沿海及海域沉积古地理演化，确定该时期是否发生块体的拼贴，并针对来自东海基底的碎屑锆石分析其揭示的陆壳增生情况，探讨东海基底与华南基底在地质历史中的关系。本课题将为判别东海陆架是否为外来块体以及探索其基底属性提供关键证据。

东海陆架盆地是认识西太构造演化及长江演化的关键区域，其基底与华南的关系、新生代盆地成因及构造-沉积演化及长江的演化历史一直存在争议本次研究已东海福州凹陷中生代以来的沉积为基础，开展系统的碎屑锆石U-Pb定年及Hf同位素分析，并与邻近陆域进行了区域沉积及物源特征对比，结果显示，锆石石U-Pb定年主要分为3个主要年龄组分，其中，<150 Ma的年龄组分，其εHf(t)集中分布于-20至0，TDMC分布于2.3Ga-1.0Ga范围内，且与华南现代河流中同时期锆石特征完全一致。锆石石U-Pb定年为150-260 Ma的年龄，根据其εHf(t)值可细分为3组，其中，P1组分εHf(t) ( -26 to -17)及TDMC (~2800-2400 Ma)年龄相对集中；P2组分的εHf(t)更偏负，TDMC 集中分布于2.5-3.1Ga；P3显示出散布特征，与同时期长乐南澳断裂带沉积岩中的锆石特征一致，且与东海基底岩石的锆石特征存在明显差异，推测来自邻近的华南陆域。P1及P2组分的εHf(t)值及TDMC年龄与华南陆域明显不同，但与东海已钻遇的基底特征一致，因此，这两组年龄来自东海基底。锆石石U-Pb峰值年龄为1.8Ga的组分，其特征与华南陆域则基本一致，无法根据锆石年龄和Hf同位素进行区分，但该组年龄已在丽水灵峰凸起发现。综上，P1、P2及1.8Ga锆石组分代表了东海基底，东海基底演化与华南陆域明显不同，考虑到150Ma之后东海与华南一致的锆石特征，东海基底为外来块体，约150Ma拼贴至华夏东缘。这一认识，对东亚中生代构造及古太平洋板块俯冲具有重要意义。此外，本研究通过对盆地内部构造-沉积特征的系统梳理及区域对比，提出东海新生代盆地北部主要受控于俯冲体系，南部曾为南海被动大陆边缘的一部分。东海新生代碎屑锆石物源特征显示，具有现今长江特征的物源体系形成于晚中新世，并提出现今的长江形成于~12 Ma。