西沙群岛晚第四纪碳酸盐岩淡水成岩作用——来自永兴岛SSZK1钻孔的地球化学响应证据

Geochemical Signatures in the Late Quaternary Meteoric Diagenetic Carbonate Succession,Xisha Islands,South China Sea

基于西沙永兴岛上最新钻孔(SSZK1)取得的55.92 m岩芯的U-Th定年、矿物、薄片、主微量元素及碳氧稳定同位素等资料,开展了西沙群岛晚第四纪碳酸盐岩沉积相和淡水成岩作用的研究。根据不同的矿物组成特征,可将SSZK1钻孔岩芯分为上、中、下三段:下段(33.89~55.92 m,主要为低镁方解石)、中段(18.39~33.89 m,主要为文石和低镁方解石)、上段(0~18.39 m,主要为文石、高镁方解石和低镁方解石)。由于下段碳酸盐岩几乎全为稳定的低镁方解石组成,碳氧同位素值的严重负偏和小幅度变化,可推断其经历了程度较大的淡水成岩作用。中段和上段还存在不稳定的文石和高镁方解石,碳氧同位素值相对下段正偏和高幅高频变化,推测其淡水成岩作用的程度比下段要小。中段碳氧同位素值高幅高频变化同时也说明该段的矿物纵向变化较复杂。这种矿物组成的复杂变化可能是由于晚第四纪海平面频繁变化,该段被大气水渗流带和潜流带交替占据引起的。主微量元素的变化同时受到淡水成岩作用和沉积环境的影响。在中段、下段中可识别出sq1、sq2、sq3、sq4四个完整的相旋回。Na2O,S,Sr和碳氧同位素受到的淡水成岩作用而被消耗和负偏,且由于老一期的旋回经历了更长时间的淡水成岩作用,新、老旋回间的Na2O,S,Sr含量值和碳氧同位素值有明显差异。利用新、老时期形成的旋回间淡水成岩作用剩余Na2O,S,Sr含量和碳氧同位素值的差别可以将新、老两个旋回区分开来。

Petrographic,mineralogical,stable isotopic and elemental data from the late Quaternary carbonates at Xisha Islands indicate that the sequences were calcitized in a near-surface meteoric diagenetic environment.The variation in the distribution of aragonite,high-Mg calcite(HMC)and low-Mg calcite(LMC)divides the shallow-water carbonates in core SSZK1 into three intervals:unitⅠ(33.89-55.92 m;LMC),unitⅡ(18.39-33.89 m;aragonite and LMC)and unit III(upper 18.39 m of core;aragonite,LMC and HMC).The lowermost unitⅠ,which consists of Pleistocene shallow-water carbonate,has suffered almost complete freshwater diagenesis,whereas the overlying unitsⅡandⅢhave undergone incomplete meteoric freshwater diagenesis,in which the shallow-water carbonates feature more positive and variousδ18O andδ13C values.Especially in unitⅡ,high-frequency and high-amplitude fluctuations of stable isotopic values suggest a more complex mineralogical composition,which may be explained by frequently alternating vadose and phreatic paleoenvironments.Elemental concentration has been influenced by the different degrees of diagenetic effects as well as the facies conditions.Approximately four intact facies/water-depth cycles were recognized.The study demonstrates that the cumulative depletion of elements such as Sr,Na and S caused by multiple episodes of meteoric diagenesis distinguishes the old reef sequences from those of the young reef.