摘要： 广泛发育于全球大洋中的新生代岛礁白云岩,具有埋藏浅、形成时间晚、后期改造弱等特点,被认为是研究经典“白云岩(石)问题”的天然实验室。文中系统梳理了岛礁白云岩的基本特征、时间演化、白云石化流体性质和成因模式,并探讨了控制该类白云岩形成的古环境和古气候因素,以期为解译“白云岩(石)问题”提供借鉴。新生代岛礁白云岩厚几十米至几百米,具有相似的岩石学(结构保存型和结构破坏型)、计量学(高钙白云石和低钙白云石)和地球化学(碳氧同位素、锶、铁、锰、钠元素)特征,发育时间上具有全球同步性(~10-0.5 Ma)。在单个岛礁内,受海平面变化和构造活动控制,白云石化时间由底部至顶部呈现出连续或断续变年轻的现象。结合氧同位素和团簇同位素数据分析显示,正常—轻微蒸发的海水是岛礁白云石化流体的主要来源, 其中高钙白云石主要与正常海水有关,而低钙白云石则受蒸发海水控制。单一的流体驱动模式(如渗透回流、地热对流等)难以普遍适用于岛礁白云岩的成因解释, 促进白云石化作用的岛礁内地下水流动机制可能是多重复合而成。新生代岛礁白云石化时期,单一的古环境、古气候因子(海水的Mg/Ca、温度、$\text{SO}_{4}^{2-}$浓度、氧化还原状态、pCO₂及pH等)并未呈现出特殊性,推测多种古环境、古气候因子的耦合作用可能共同控制了岛礁的白云石化事件。为了更好地理解岛礁白云岩的成因机制,未来应在白云石化微观过程、新兴探测技术应用和数值模拟方面进行拓展研究。

关键词: 白云岩, 新生代, 珊瑚礁, 成因机制, 研究进展

Abstract: Cenozoic island dolostones found throughout the global oceans are considered as ideal natural laboratories for deciphering the classic “dolomite problem”due to their shallow burial,eogenetic formation,and weak diagenetic modifications. This paper systematically summarizes the research progress on these dolostones in term of their basic characteristics,time evolution,fluid properties,and genetic models,and discusses the paleoenvironmental and paleoclimatic factors that influence their formation. The Cenozoic island dolostones(tens to hundreds of meters thick)show similar petrological(texture-retentive and texture-destructive),stoichiometric(high-calcium dolomite and low-calcium dolomite),and geochemical(e.g.,carbon and oxygen isotopes,strontium content)properties,and are formed contemporarily during the 10-0.5 Ma interval. The strontium isotopes of the dolostones indicate that the age of dolomitization generally becomes progressively younger from the bottom to the top of the dolostone sections in a continuous or semi-continuous way on individual islands(atolls,carbonate platforms)under the influence of eustatic change and tectonic subsidence. Carbonate clumped and oxygen isotopes indicate that the primary fluid source for the island dolomitization is normal to slightly evaporated seawater. Furthermore,the high-calcium dolomite is formed primarily from normal seawater,while the low-calcium dolomite is formed from slightly evaporated seawater. A single fluid flowing mechanism(e.g.,seepage,reflux,geothermal convection)is insufficient to explain the genesis of the Cenozoic island dolostones;and available data demonstrate that the formation of the island dolostones is promoted by multiple flow mechanisms. During the island dolomitization period(10-0.5 Ma),there is no single paleoenvironmental or paleoclimatic factor(e.g.,seawater Mg/Ca ratio,temperature,$\text{SO}_{4}^{2-}$ concentration,redox state,pCO₂,pH)that corresponds to significant dolomitization,implying that the Cenozoic island dolomitization event was probably controlled by multiple paleoenvironmental and paleoclimatic factors. To better understand the genetic mechanisms of the Cenozoic island dolostones,future research should focus on the micro-processes of dolomitization,the application of new geochemical methods and numerical simulation.

Key words: dolostone, Cenozoic, coral reef, genetic mechanism, research progress

中图分类号: 

• P581