华南沿海晚中生代地貌演化:闽西永安盆地沉积地球化学和Nd同位素分析

The Late Mesozoic Geomorphic Evolution of the Coast Area of South China:Geochemical and Nd Isotope Analysis of Sediments in the Yong’an Basin,Western Fujian

华南沿海中生代以来经历了强烈的构造和岩浆活动,地形地貌发生了巨大的反转,这一地貌演变过程在华南沿海晚中生代盆地和南海北缘新生代盆地沉积地层中得到了完整的记录。本文以华南沿海的闽西永安盆地白垩纪沉积岩为主要研究对象,通过岩石学、地球化学和Nd同位素分析,示踪盆地物源演化,结合台湾新生代地层沉积地球化学资料,推测出华南沿海晚中生代以来的地貌演化过程。野外调查及镜下鉴定发现,中侏罗世‒早白垩世早期样品成熟度相对晚白垩世样品较高。地球化学判别图显示,样品均落入中酸性长英质物源区域。中侏罗世‒晚白垩世地层的^(143)Nd/^(144)Nd值介于0.511772~0.512253之间,ε_(Nd)值为−16.9~−7.5。其中中侏罗统漳平组ε_(Nd)值最小,介于−16.9~−16.2之间,平均值为−16.6。下白垩统坂头组ε_(Nd)值增大为−14.8~−12.4,平均值为−13.3;而上白垩统沙县组和崇安组则具有较高的ε_(Nd)值,达到−9.3~−7.5,平均值为−8.7。从漳平组到崇安组,La/Sc、Th/Sc等微量元素比值与ε_(Nd)值表现出逐渐升高的趋势。研究结果显示,东南沿海中生代早期地貌较低,武夷山等华南内陆古老块体是永安盆地最重要的物质来源;晚白垩世,随着太平洋板块的持续俯冲,华南沿海山脉抬升,沿海花岗岩岩体近源快速剥蚀,成为永安盆地主要物源。相较于永安盆地晚白垩世地层,中国台湾新生代地层ε_(Nd)值明显偏低,且呈现出缓慢减小的趋势,对应于海岸山脉裂解及南海扩张。闽江等沿海水系向内陆袭夺,华南的古老基底成为南海北缘盆地及台湾新生代地层主要物质来源。

The coastal area of South China experienced strong tectonic and magmatic activities in Mesozoic,and the topography and geomorphology underwent a huge inversion.This geomorphic inversion is fully recorded in the sedimentary strata of the late Mesozoic basin along the South China coast and the Cenozoic basin on the northern margin of the South China Sea.Taking the Cretaceous sedimentary rocks of the Yong’an basin in Western Fujian as the main research object,this paper traces the provenance evolution of the basin through petrological,geochemical and Nd isotope analyses,combined with the Cenozoic stratigraphic sedimentary geochemical data of Taiwan,and proposes a scenario for the geomorphic evolution process along the coast of South China since the late Mesozoic.Field investigations and laboratory work have found that the maturity of sediments from the Middle Jurassic to the early stage of the Early Cretaceous is higher than that of the Late Cretaceous deposits,indicating a transformation of the Late Mesozoic provenance from the basement of Cathaysia to the arc igneous rocks.Geochemical data show that the source rocks are likely medium acid felsic rocks.The Middle Jurassic to Late Cretaceous strata have ^(143)Nd/^(144)Nd ratios of 0.511772 to 0.512253,andε_(Nd) values in range of−16.9 to−7.5.The rocks of the Middle Jurassic Zhangping Formation have the lowestε_(Nd) values,ranging from−16.9 to−16.2,with an average value of−16.6.Theε_(Nd) values of rocks from the Early Cretaceous Bantou Formation increase to−14.8 to−12.4,with an average value of-13.3.However,the rocks of the Late Cretaceous Shaxian Formation and Chong’an Formation have higherε_(Nd) values,varying from−9.3 to−7.5,with an average value of−8.7.From the Middle Jurassic Zhangping Formation to the Late Cretaceous Chong’an Formation,The La/Sc and Th/Sc ratios andε_(Nd) values of the rocks show a trend of gradual increasing.The landform of the southeast coast in the early Mesozoic was low,and the ancient blocks in the south China inland such as the Wuyi Mountain were the most important mass sources for the Yong’an Basin.With the continuous subduction of the Pacific Plate,the coastal mountains of South China uplifted in the Late Cretaceous,and the near-source coastal granites were rapidly eroded and became the main source for the Yong’an Basin.Compared with the late Cretaceous strata in the Yong’an basin,theε_(Nd) values of Taiwan’s Cenozoic strata are significantly lower than those of the Late Cretaceous strata in the Yong’an Basin,and show a slow decreasing trend,which corresponds to the breakup of the coastal mountains and the expansion of the South China Sea.The Minjiang and other water systems were captured inland,and the ancient basement in South China became the main material source for the Cenozoic strata in the northern margin of the South China Sea and Taiwan.