Abstract:

Based on comprehensive analysis of field work, petrological and geochemical characteristics, this paper studies the characteristics and origin of giant patch dolomite of the Lower-Middle Ordovician HangGuletag Formation in Wuligezitag area, NE Tarim Basin, China. It can be seen in the field that the grayish yellow giant patch dolomite partially develops in the black thin-bedded micritic limestone, with an irregular boundary not controlled by sedimentation. The giant patches are several meters in size (the largest >10 m) which is larger than and quite different from the several-centimeter (decimeter) burrow-related dolomite or leopard fur dolomite. Microscopically, it consists of micro-finely and planare(s) dolomite crystals with a size near 100 μm. Besides, it is also characterized by its unique geochemical features: 1) δ¹³C_PDB (−1.34‰-−0.62‰) accords with the range of Early Ordovician seawater while δ¹⁸O_PDB (−8.01‰ -−4.79‰) is higher; 2) ⁸⁷Sr/⁸⁶Sr  (0.711000-0.711535, AVG=0.710863) is much higher than that of limestone and contemporaneous seawater; 3) average high Fe and Mn content (2001.32 and 601.73 μg/g, respectively) and low Sr and Ba (33.14 and 8.27 μg/g, respectively) content versus that of limestone; 4) dolomite has slightly negative Eu anomaly and similar REE composition and distribution patterns compared with limestone; 5) low order degree value (AVG=0.6). This study reveals that the dolomitization was related to fault and occurred in low-temperature and shallow-burial environment in Early-Middle Ordovician. Dolomitizing fluid was generated from the concentrated formation fluid in lower-middle member of HangGuletag Formation and underlying Tursaktag Group. Most Mg²⁺ came from the stabilization of quasi-stable carbonate minerals, and the transformation of clay minerals of (calcareous) mud in the limestone strata not only provided a little Mg²⁺ but caused the rise of ⁸⁷Sr/⁸⁶Sr. Dolomitizing fluid was likely to be richened in Xingdi-related normal fault and fractures which also acted as the main fluid pathways, and it finally caused the partial dolomitization of nearby limestone strata.

Key words: Wuligezitag, HangGuletag Formation, giant patch dolomite, geochemical characteristics, origin of dolomite

摘要：

通过详细的野外观测, 结合岩石学和地球化学等方法, 对塔里木盆地东北缘乌里格孜塔格中-下奥陶统巨斑状白云岩特征及成因进行研究。野外观测表明, 黄灰色粉-细晶白云岩呈巨斑状(巨斑直径可超过10 m)发育于灰黑色薄层泥晶灰岩中, 界面形态不规则, 且不受沉积控制。显微镜下观察, 白云石粒径多在100 μm 左右, 直面为主, 半自形-自形。巨斑状白云岩具有独特的地球化学特征: 碳同位素(δ¹³C_PDB: −1.34‰~−0.62‰)位于早奥陶世海水值范围内, 氧同位素(δ¹⁸O_PDB: −8.01‰~−4.79‰)比同期海水值高; ⁸⁷Sr/⁸⁶Sr 比值(0.711000~0.711535, AVG=0.710863)远大于泥晶灰岩与同期海水比值; Fe (2001.32 μg/g)和Mn (601.73 μg/g)含量高于泥晶灰岩, Sr (33.14 μg/g)和Ba (8.27 μg/g)含量相对较低; 稀土元素组成及配分特征与泥晶灰岩相似, 弱的Eu 负异常; 有序度(AVG=0.6)较低。综合研究认为, 白云岩化发生于早?中奥陶世浅埋藏、较低温环境, 属于构造?断裂相关的局部白云岩化。白云岩化流体主要来源于巷古勒塔格组中下段及突尔沙克塔格群封存的地层流体, 准稳定碳酸盐矿物稳定化提供主要的Mg²⁺, 灰岩地层中泥质沉积中的黏土矿物转化不仅提供少量Mg²⁺, 更提供放射性⁸⁷Sr。兴地同沉积断裂相关的伴生断裂富集自源白云岩化流体, 充当其运移通道, 使断裂系统附近灰岩优先白云岩化, 形成巨斑状构造。

关键词: 乌里格孜塔格, 巷古勒塔格组, 巨斑状白云岩, 地球化学特征, 白云岩成因